Latest Cretaceous megaraptorid theropod dinosaur sheds light on megaraptoran evolution and palaeobiology

Domain: earth_science
Published: 2025-09-23
Editorial outcome: minor_revision

Abstract
Recent fossil discoveries have cast considerable light on the palaeobiology of Megaraptora, a group of large-clawed carnivorous theropod dinosaurs known from Cretaceous deposits in Asia, Australia, and especially South America. Nevertheless, many important aspects of megaraptoran morphology and evolution remain poorly understood, due in large part to the fragmentary nature of most fossils of these theropods and the scarcity of anatomically overlapping skeletal elements among the known taxa. Here we report a previously unknown megaraptoran genus and species represented by a partially articulated partial skeleton recovered from an uppermost Cretaceous stratum of the Lago Colhué Huapi Formation of south-central Chubut Province in central Patagonia, Argentina. Pertaining to the derived megaraptoran subclade Megaraptoridae, the taxon is among the most completely represented and latest-surviving megaraptorans. Its stratigraphic occurrence indicates that these dinosaurs likely persisted to the Cretaceous/Palaeogene boundary; moreover, the preservation of a crocodyliform humerus between the dentaries of the new theropod may provide information on megaraptoran dietary preferences and feeding strategies. Megaraptorids appear to have been the apex predators in central and southern Patagonian palaeoecosystems approaching the end of the Cretaceous, in contrast to more northerly areas of South America where these niches were occupied by other non-avian theropod groups.

Introduction
Among the most mysterious of the many predominantly Gondwanan Cretaceous non-avian theropod dinosaur groups is Megaraptora, a tetanuran clade characterized by (at least in derived members of the group) elongate skulls with low-crowned teeth, extensively pneumatized bones, and, most strikingly, powerfully developed forelimbs equipped with hypertrophied unguals on the first and second digits 1–9. Although the stratigraphically oldest and earliest diverging representatives of this clade come from Asia 2,10–13 and Australia 14–19, megaraptorans are particularly abundant in South America, especially in the Late Cretaceous of Patagonia (e.g., refs. 1,3,4,6–8,20–32; see also Supplementary Notes 1 and 2). The evolutionary relationships of Megaraptora within Tetanurae—as either carcharodontosaurian allosauroids or early branching coelurosaurs, potentially basal tyrannosauroids—are controversial (see refs. 6,8,21,25,33–36, among others), due in large part to the preponderance of fragmentary, highly incomplete fossils of the clade and the scarcity of anatomically overlapping skeletal elements among the various megaraptoran taxa.

Here we report a new megaraptoran genus and species based on a well-preserved, partly articulated partial skeleton that includes cranial, postcranial axial, and appendicular elements, recovered from the Upper Cretaceous (Coniacian–Maastrichtian) Lago Colhué Huapi Formation of south-central Chubut Province, central Patagonia, Argentina (see Supplementary Note 3 for additional locality and stratigraphic data). Specifically, the new taxon, a member of the derived megaraptoran subclade Megaraptoridae, comes from a stratigraphic horizon only a few meters below of the top of this formation, establishing it as latest Cretaceous (Maastrichtian, probably late Maastrichtian) in age, and therefore one of the geologically youngest (if not the youngest) megaraptorans yet discovered. Moreover, because this taxon is also one of the most completely represented megaraptorans, it adds significantly to our understanding of the morphology, phylogenetic relationships, and palaeobiology of this enigmatic large-clawed theropod group. Furthermore, the unusually close association of a crocodyliform humerus with the type specimen of this theropod may provide information on the dietary preferences and feeding strategies of Megaraptoridae.

Results
Systematic palaeontology
Saurischia Seeley, 1887 37
Theropoda Marsh, 1881 38
Tetanurae Gauthier, 1986 39
Megaraptora Benson, Carrano, and Brusatte, 2010 33
Megaraptoridae Novas, Agnolín, Ezcurra, Porfiri, and Canale, 2013 34
Joaquinraptor casali gen. et sp. nov.

Etymology. Joaquín, in tribute to the son of the first author (L.M.I.) and the informal name given to the locality when the skeleton of the taxon was discovered (Valle Joaquín); Latin, raptor, thief. Specific epithet casali in recognition of Dr. Gabriel Andrés Casal for his many contributions to knowledge of the Cretaceous palaeontology and geology of central Patagonia (including the formal recognition and naming of the formation from which this megaraptorid was recovered).

Holotype. UNPSJB-PV 1112, a partially articulated partial skeleton of a single individual that includes the mostly disarticulated partial skull (right maxilla, skull roof and braincase, probable right postorbital, right quadrate, both dentaries, in situ and isolated teeth), complete or nearly complete elements of the postcranial axial (atlantal intercentrum, three caudal vertebrae, dorsal ribs, other rib and probable gastralium fragments, haemal arch) and appendicular (left scapulocoracoid, humerus, radius, and ulna, right manual ungual II, left femur, much of the right tibia, the distal end of the probable right pedal phalanx III-3, the right pedal ungual III, and the possible partial left pedal phalanx IV-1) skeletons, and numerous indeterminate fragments.

Locality and horizon. Valle Joaquín, headwaters of the Río Chico, east of the southeastern shore of Lago Colhué Huapi, Chubut Province, central Patagonia, Argentina. Upper (Maastrichtian, probably upper Maastrichtian) stratum of the Upper Cretaceous Lago Colhué Huapi Formation (see below; also Fig. 1 and Supplementary Figs. 1 and 2).

Diagnosis. The holotype of Joaquinraptor casali is considered to represent a sexually but possibly not somatically mature individual based on the fusion of braincase sutures and the results of our osteohistological analysis, which reveal a reduction in the distance between growth marks in the outer bony cortex of limb bones (see below). Joaquinraptor is regarded as a member of Megaraptoridae based on its possession of the following suite of synapomorphies of the clade: (1) reduced anteriormost dentary alveolus 35; (2) absence of mesial denticles from tooth crowns 34,35,40,41; (3) olecranon process of ulna mediolaterally compressed, blade-like, and subtriangular in lateral view; and (4) ulnar shaft with well-developed, proximodistally oriented lateral crest or tuberosity 21,34–36. Joaquinraptor is diagnosed by the following autapomorphies (see also Supplementary Note 4 and Supplementary Figs. 3–7): (1) parietals strongly compressed transversely, narrower than frontals; (2) anterior margin of supratemporal fossa with sigmoid contour; (3) braincase lacking median septum in pneumatic recess (probably, but see below); (4) rugose atlantal intercentrum with well-marked lips and rectangular ventral contour; (5) middle or posterior caudal vertebral centra with well-marked elliptical lateral fossae that occupy more than half the lateral surfaces; (6) humerus with subrectangular deltopectoral crest; ulna with (7) proximodistally enlarged olecranon process, (8) vertical ridge on medial surface of proximal end, (9) nearly straight shaft (but see below), and (10) medially oriented, anteroposteriorly compressed distal end; and (11) distal end of radius crescent-shaped. Differs from the only other named Maastrichtian (probably early Maastrichtian and therefore stratigraphically older; see refs. 42–44) megaraptorid species Maip macrothorax in having a coracoid with a subglenoid ridge, posteroventral fossa, and hooked posteroventral process. The preserved dorsal ribs of Joaquinraptor and Maip also exhibit morphological differences, though these could potentially be due to non-equivalence in serial position rather than taxonomic distinction.

This published work and the nomenclatural acts it contains have been registered in ZooBank, the proposed online registration system for the International Code of Zoological Nomenclature (ICZN). The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix “http://zoobank.org/”. The LSIDs for this publication are: urn:lsid:zoobank.org:pub:468D27F1-2076-4187-911C-E70028218787; urn:lsid:zoobank.org:act:AED2BB3B-C081-445C-A081-C357E7CEC580; urn:lsid:zoobank.org:act:3AA601F2-5DFC-452B-9609-E83C913F8C2D.

Age. Geological studies of the uppermost part of the Lago Colhué Huapi Formation, particularly those of palynomorphs, support a late Maastrichtian age for these strata, probably close to the Cretaceous/Palaeogene boundary 45,46. The palaeoclimate of central Patagonia during deposition of this part of the formation is suggested to be warm with at least seasonal rainfall 45. On the other hand, palaeoclimatic studies of the middle (Santonian–?lower Maastrichtian) section of the Lago Colhué Huapi Formation indicate a seasonally dry or semiarid climate 47. Recent analysis of clays from the Joaquinraptor quarry indicates a composition similar to that of the top of the formation (see Supplementary Note 3). Therefore, this megaraptorid was found at a stratigraphic level that is more related to those of the top of this unit than those of the middle section, supporting a Maastrichtian (and probably late Maastrichtian) age for this specimen.

The oldest megaraptorans are Phuwiangvenator yaemniyomi and Vayuraptor nongbualamphuensis from Thailand 11–13, both probably late Valanginian–early Hauterivian in age 48. The youngest records of the clade are from the Campanian–Maastrichtian of Argentina, Chile, and possibly Brazil 8,22,23,49–53. Nevertheless, these latter megaraptorans probably date to the Campanian–early Maastrichtian interval 8,42–44,52–54 and are thus similar in age to the fragmentary megaraptorid fossils (mostly manual unguals) recovered from the middle section of the Lago Colhué Huapi Formation 7,31,55. Therefore, Joaquinraptor seemingly constitutes the stratigraphically youngest representative of Megaraptora yet discovered anywhere in the world. Interestingly, what are probably both the oldest 7 and youngest (Joaquinraptor) definitive South American megaraptorids known at present come from strata of the Golfo San Jorge Basin of central Patagonia.

Osteological description. The megaraptoran fossil record is generally depauperate; therefore, not many specimens within the group preserve overlapping skeletal elements. In this context, Joaquinraptor casali is important because it is represented by a well-preserved partial skeleton that includes cranial, postcranial axial, and appendicular elements (Fig. 2a and Tables 1, 2), allowing relatively extensive comparisons with other megaraptoran taxa (see Supplementary Notes 5 and 6 and Supplementary Figs. 8–18). This is the case for the subtriangular maxilla (Fig. 2b, c), which is otherwise preserved only in a juvenile specimen of Megaraptor namunhuaiquii (MUCPv 595; see ref. 6). In Joaquinraptor, the anterior part of the antorbital fossa is perforated by a poorly delimited, fossa-like structure that may represent the confluent maxillary fenestra and promaxillary foramen. The morphology and configuration of these probable pneumatic excavations closely resembles that in Megaraptor, supporting this condition as a synapomorphy of Megaraptora or one of its subclades instead of an autapomorphy of the latter genus (contra 6). Similarly, in both these megaraptorids, the medial maxillary shelf bifurcates posteriorly, forming a deep, “horizontal V”-shaped, well-defined antrum (=posterior anteromaxillary fenestra; see ref. 56), which may represent another megaraptoran synapomorphy. The maxilla of Joaquinraptor differs from that of Megaraptor in having a substantially taller and blunter anterior process (=anterior ramus), an anteroposteriorly narrower anterior part of the antorbital fossa, an acute anterior margin of the antorbital fenestra, and a dorsoventrally thicker ascending process, although whether these distinctions are of taxonomic significance rather than reflective of the differing ontogenetic stages of UNPSJB-PV 1112 and MUCPv 595 is unknown at present. Interestingly, in being low, elongate, and subtriangular, the maxilla of Joaquinraptor is also somewhat similar to that of the penecontemporaneous large-bodied unenlagiine theropod Austroraptor cabazai from northern Patagonia 57. Nevertheless, the maxilla of the new megaraptorid differs from that of Austroraptor in, among other features, the degree of elongation of the antorbital region (which is pronounced in the unenlagiine), the shape of the antorbital fossa, and the number of alveoli (which is substantially higher in the unenlagiine). Although the surfaces of the maxillary interdental plates are slightly eroded in the Joaquinraptor holotype, they appear to be unfused, unlike those of Fukuiraptor kitadaniensis 2. The interdental plates are triangular in shape and subequal in size, as in the juvenile Megaraptor (MUCPv 595). On the other hand, allosauroids more derived than Sinraptor dongi have larger interdental plates 6.

The braincase (Fig. 2d, e) is anteroposteriorly longer and more transversely compressed than that of the reportedly subadult holotype of the megaraptorid Murusraptor barrosaensis (MCF-PVPH 411; see ref. 25)—which is, at present, the only other almost complete megaraptoran braincase known—and still more narrow and elongate than the partial braincase of the aforementioned juvenile Megaraptor (MUCPv 595; see ref. 6). No sutures are visible (except for the interparietal suture, see immediately below), which accords with osteohistological evidence that indicates that the Joaquinraptor holotype was sexually and possibly somatically mature at the time of death (minimum age of 19 years; see below). Nevertheless, although it has possibly been exaggerated by potential taphonomic artifacts, a well-marked, non-interdigitated, open interparietal suture is evident, as is also the case in Murusraptor; this feature may therefore characterize Megaraptoridae 27. The parietals are more transversely constricted than in Murusraptor, closely resembling the condition in tyrannosauroids 36; this feature is therefore considered an autapomorphy of Joaquinraptor within Megaraptora (Supplementary Fig. 3). The frontals are subquadrangular in shape, as in other megaraptorans 6,18,24,25. In lateral view, the posterior region of the ventrolaterally oriented frontals occupies a more dorsal position than the anterior portion, conferring a step-like morphology similar to that of Murusraptor (see ref. 27) and the probable megaraptorid MCF-PVPH 320 (see ref. 24); this feature may therefore represent another synapomorphy of Megaraptora or a clade therein. Anteriorly, there is a wide, anteriorly projected supratemporal fossa similar to that seen in Megaraptor and Murusraptor, although it differs from those of these other megaraptorids in having a sigmoid contour in dorsal view. Conversely, in allosauroids such as Acrocanthosaurus atokensis, Allosaurus fragilis, and Carcharodontosaurus saharicus, the supratemporal fossa is anteroposteriorly short 36 and its anterior margin exhibits a more ovoid aspect. The ventrolateral inclination of the frontals from the midline of the skull roof, forming a triangular cross-section, has been regarded as an autapomorphy of Murusraptor (see ref. 27), but may instead represent a synapomorphy of Megaraptoridae as it is also present in Joaquinraptor. Ventrally, the median septum that divides the basisphenoid recess that is described in Megaraptor 6 and Murusraptor 27 appears to be absent in Joaquinraptor, although this could potentially be an artifact of preservation. The more complete of the two basipterygoid processes is robust and hooks posteriorly in lateral view, unlike those of Murusraptor that are shorter and mostly ventrally oriented.

The bone tentatively interpreted as the right postorbital (Fig. 2f) is almost complete, missing only parts of the anterior and posterior processes, though it has probably also experienced some plastic deformation. It is anteroposteriorly and mediolaterally narrower than the postorbitals of Aerosteon riocoloradensis 4,30, Murusraptor 25, and Orkoraptor burkei 22. Compared with these megaraptorids, the anterior process appears shorter and the ventral process straighter (i.e., the latter is not ‘cradle-shaped’). Joaquinraptor shares with Aerosteon and Murusraptor the presence of a small tuberosity on the anterior margin of the ventral process that is well exposed in lateral and anterior views (see ref. 30, these authors’ Fig. 4a, d, “rugose bump”). Furthermore, both Joaquinraptor and Aerosteon possess a well-marked foramen on the dorsal surface of the postorbital, situated relatively close to the frontal contact.

The condylar (ventral) portion of the right quadrate is preserved (Fig. 2g). In overall morphology, it resembles the quadrate of Aerosteon more than that of Murusraptor, which is generally more gracile. There is a well-marked, enclosed posterior pneumatic foramen (sensu 58) on the posterior surface, as in other megaraptorids and many other coelurosaurs 58. In Joaquinraptor, however, this foramen is placed on the lateral part of the quadrate adjacent to the quadratojugal contact, whereas in Aerosteon and Murusraptor it is more centrally placed.

An isolated maxillary tooth of Joaquinraptor (Supplementary Fig. 17) is proportionally apicobasally taller than those of Murusraptor. As in other megaraptorids, the tooth is labiolingually compressed, the distal margin of the crown is strongly concave in labial and lingual views, and the distal denticles are subrectangular to rectangular. The mesial margin lacks a serrated carina, as in most other South American megaraptorids 40.

The Joaquinraptor dentaries (Fig. 2h, i) are elongate, mediolaterally compressed, and shallow in lateral view, as in Australovenator wintonensis (AODF 604) 59. Furthermore, both these megaraptorids and the early branching megaraptoran Fukuiraptor 2 share a reduced symphyseal facet and an anterior dentary margin that trends anterodorsally–posteroventrally. However, in overall morphology, the dentary of Joaquinraptor is more robust than that of Australovenator. Slightly pronounced primary and secondary neurovascular foramina are evident on the lateral surface. White et al. 59 suggested that the absence of these foramina may be autapomorphic of Australovenator, contrasting with the original description of this taxon (see ref. 5). Similar neurovascular openings were described and figured in dentary fragments of Fukuiraptor 2; therefore, the presence of slightly marked neurovascular foramina that are not enclosed or delimited by grooves may represent a synapomorphy of Megaraptora or one of its subclades (e.g., Megaraptoridae). Both Joaquinraptor dentaries preserve 17 alveoli—two fewer than in Australovenator—although this could be a taphonomic artifact caused by erosion of the posteriormost portions of these bones. Unlike those of Fukuiraptor and Australovenator, the dentary interdental plates are unfused.

The atlantal intercentrum is articulated with the occipital condyle (Fig. 2e and Supplementary Fig. 3b), such that most of its anterior features cannot be described in detail; however, it is rugose and disc-shaped, differing from its more crescentic counterparts in the megaraptorans Aerosteon 4,30, Orkoraptor 22, and Phuwiangvenator 11. Ventrally, the intercentrum exhibits well-marked rims and is rectangular in shape, unlike other megaraptorans in which this element is preserved.

The three preserved caudal vertebrae belong to the middle or posterior section of the tail, being substantially longer anteroposteriorly than tall dorsoventrally (see Table 1). The centra (Fig. 2j) are hourglass-shaped in ventral view, and their lateral surfaces are excavated by well-developed, elliptical fossae, as in many other definitive or possible megaraptorans (e.g., 4,7,22,30,49,50,60,61); however, the fossae seen in Joaquinraptor are proportionally very large, encompassing more than half the length of the lateral surfaces, a unique feature of this megaraptorid (Supplementary Fig. 4). Dorsal ribs (Fig. 2k) are pneumatized and generally resemble those of other megaraptorids, such as Australovenator 5, Maip macrothorax 8, and Murusraptor 25. Nevertheless, and although this could potentially be due to differences in serial position, the preserved ribs of Joaquinraptor appear more gracile than those of Maip, and also do not exhibit the prominent, subtriangular medial flange present in the first dorsal rib of that southern Patagonian taxon.

The anterior haemal arch (Fig. 2l) is long and slender, features shared with the enigmatic megaraptoran Aoniraptor libertatem 26,62 and Megaraptor 3. The articular surface is saddle-shaped, as in Aoniraptor. The haemal canal is ovoid in posterior view, and the blade of the haemal arch curves slightly posteriorly as in Aoniraptor, Megaraptor, and Orkoraptor.

The partially preserved left scapula and coracoid are fused (Fig. 2m), forming a scapulocoracoid. In overall morphology, the partial coracoid resembles the corresponding region of this bone in Aerosteon 4,30 and Maip 8, although the posteroventral (=anteroventral, if the long axis of the scapulocoracoid is held horizontally) process of Joaquinraptor appears more hooked than in these taxa; moreover, unlike Maip, the new form possesses a subglenoid ridge and fossa sensu 8,30. As in Aerosteon and Megaraptor 3, the scapular blade of Joaquinraptor has sharp dorsal and ventral edges and maintains a near-constant dorsoventral width throughout its length, except at its slightly expanded posterior end. In lateral view, the blade is proportionally wide relative to its length, as in other megaraptorids but contrasting with the condition in the phylogenetically controversial Patagonian theropod Gualicho shinyae, which has a narrower scapular blade 63.

Although all described megaraptoran humeri are robust, that of Joaquinraptor is notably more massive than that of any other member of the group (Fig. 2n, o, p). The shaft is bowed anteriorly as in Megaraptor 6,64, but in both these genera, the degree of bowing is less marked than in the early diverging megaraptoran Fukuiraptor and the Australian megaraptorid Australovenator, suggesting that a relatively straight humeral shaft may be a synapomorphy uniting these two Patagonian megaraptorids. Unlike in Australovenator, Fukuiraptor, and Megaraptor, the deltopectoral crest is subrectangular in lateral view (Supplementary Fig. 5). Although, as in these taxa, the crest projects prominently anteriorly and tapers proximally and distally, these features are less marked in Joaquinraptor, with the exception of the juvenile Megaraptor (MUCPv 595) in which the deltopectoral crest is small (see ref. 6; these authors’ Fig. 10).

The left radius (Fig. 2q, r) differs from those of other megaraptorids, particularly in the distal end, which is crescent-shaped (Supplementary Fig. 7), unlike the more ovoid and subtriangular shapes seen in Australovenator and Megaraptor, respectively. The similarly diagnostic left ulna (Fig. 2s, t) exhibits a well-developed, proximodistally elongate olecranon process that is proportionally longer than that of any other megaraptoran (Supplementary Fig. 6). The bone narrows distally and its shaft is almost straight in lateral and medial views, closely resembling the condition in the Megaraptor holotype (MCF-PVPH 79) 1 but not in one of the specimens referred to this taxon (MUCPv 341) 3; therefore, this feature represents either an autapomorphy of Joaquinraptor, a synapomorphy of the former taxon plus Megaraptor, or an intraspecifically variable character. There is a relatively marked fossa on the posterolateral surface of the ulnar shaft of Joaquinraptor, as in other megaraptorids and, interestingly, some spinosaurids as well 16. Moreover, there is a notable longitudinal ridge on the medial surface of the proximal end that is not described or illustrated in other megaraptorans, suggesting that it may represent another unique feature of Joaquinraptor (Supplementary Fig. 6). The distal end of the ulna is medially expanded and anteroposteriorly compressed, unlike the rounded or triangular shape seen in other megaraptorids 1,3,14,65.

The right manual ungual II (=manual phalanx II-3; Fig. 2u) resembles those of the Argentinean megaraptorids Megaraptor and MCNA-PV 3109 28 (L.M.I. pers. obs.), differing from those of Australovenator and early diverging megaraptorans such as Fukuiraptor and Phuwiangvenator. It is slightly shorter and thicker than in Australovenator, a feature that may unite the Argentinean forms to the exclusion of this Australian taxon. Moreover, in Fukuiraptor, manual ungual II is much more strongly curved than in megaraptorids (Supplementary Fig. 14).

The nearly complete left femur (Fig. 2v, w) is much more robust than those of Australovenator and Fukuiraptor, more closely resembling that of the generically unidentified Bajo Barreal Formation megaraptorid UNPSJB-PV 9587 in this aspect. The posterior flange of the femoral head (sensu 5) present in Australovenator and Fukuiraptor is lacking in Joaquinraptor; moreover, although this may be a taphonomic artifact, the anterior (=lesser) trochanter appears more separated from the remainder of the bone than in these genera. The Joaquinraptor femur exhibits an ovoid, projected fourth trochanter that, although slightly more distally placed, closely resembles that of Australovenator, differing from the ridge-like structure described in Fukuiraptor 2. The femoral shaft is bowed laterally, unlike the straighter femora of these more ancient megaraptorans.

Much of the right tibia was recovered, albeit surface-collected and broken into several pieces. The shaft is hollow and anteroposteriorly narrow, appearing flat anteriorly but rounded posteriorly. The most striking feature of the distal end (Fig. 2x) is the presence on the anterior surface of a tall, well-marked facet for reception of the ascending process of the astragalus. A tall ascending process of the astragalus that covers most of the anterodistal surface of the tibia is a diagnostic feature of Coelurosauria 66.

The distal end of a pedal non-ungual phalanx, probably the right III-3 (Supplementary Fig. 18a–d), has a deep extensor fossa on the dorsal surface, as in Australovenator 15. The medial condyle is slightly taller than the lateral condyle. On both the medial and lateral sides, there are well-marked collateral ligament pits that differ in depth. The right pedal ungual III (=pedal phalanx III-4; Fig. 2y) closely resembles that of Australovenator. Another possible partial pedal phalanx, possibly the left IV-1 (Supplementary Fig. 18e–g), is potentially pathological, having an unusually rounded and ‘inflated-looking’ proximal end in comparison to the same element in Australovenator.

Histological description. The osteohistological sample of the tibia includes approximately half the complete midshaft section (Supplementary Note 7 and Supplementary Fig. 19a). Despite its incompleteness, the section reveals that the element is formed by a thick cortex surrounding a free medullary cavity. A thick layer of secondarily formed lamellar bone tissue surrounds the medullary cavity (Supplementary Fig. 19b–d). This layer, interpreted as an inner circumferential layer (ICL), contains several radially oriented simple vascular canals, which are open to the medullary cavity (Supplementary Fig. 19b, c). Highly vascularized primary bone tissue predominates in the compacta. The spatial arrangement of intrinsic fibers of the matrix grades from low (i.e., woven-fibered bone) to high (i.e., parallel-fibered bone) (Supplementary Fig. 19e–h). Parallel-fibered bone predominates in the outer portion of the compacta. This tissue is also formed in other areas of the cortex, including the middle and inner regions (Supplementary Fig. 19h). Vascularization is profuse in the entire compacta. Primary osteons are mostly organized in a plexiform pattern (Supplementary Fig. 19d, e, g). This pattern, however, changes in some areas. For example, a distinct layer of disorganized oblique and longitudinal canals is observed in some areas of the middle cortex (Supplementary Fig. 19i). Both the density and the sizes of the lumina of the vascular canals tend to decrease toward the outer edge of the bone. Cyclical growth marks (CGMs) in the form of single and double lines of arrested growth (LAGs) are recorded from the inner to the outer cortex of the sample (Supplementary Fig. 19d, g, j). A minimum number of 19 LAGs can be distinguished. A strong reduction in the spacing between LAGs occurs from the fifth to the 16th and from the 17th to the 19th growth marks (counting from the inner to the outer cortex). Sharpey’s fibers are distinct in the outer cortex, in some instances reaching the inner region (Supplementary Fig. 19k). These extrinsic fibers are almost perpendicularly oriented with regard to the subperiosteal surface. Regarding secondary bone, Haversian osteons are mostly formed in the inner cortex (Supplementary Fig. 19b, l). Two particular features are observed with regard to the distribution of secondary osteons: first, concentric patches of these osteons are formed in different levels of the cortex (Supplementary Fig. 19m); and second, despite the fact that these osteons predominate in the inner cortex, they are not formed in the ICL.

The histologically sampled dorsal rib exhibits an anteroposteriorly elongate cross-section. The medial side is mostly flat, but the lateral side is strongly convex (Supplementary Fig. 20a). Whereas one of the margins (it is not possible to determine if this is the anterior or the posterior margin) is broken, the other is preserved and exhibits a groove. The shaft is formed by a distinct cortex of compact bone that surrounds a medullary region occupied by a large pneumatic cavity (Supplementary Fig. 20a). The medial cortex is thicker than the lateral cortex. The compact bone is mostly formed by dense Haversian bone (Supplementary Fig. 20b, c). Secondary osteons occupy almost the entire medial cortex and most of the lateral cortex. The relative sizes and shapes of the secondary osteons are variable. More than one Haversian canal is observed in several secondary osteons. Primary bone tissue is preserved in the outer portion of the lateral cortex and is formed by well-vascularized fibrolamellar and parallel-fibered bone tissue (Supplementary Fig. 20d–g). Vascular canals are mainly longitudinally and obliquely oriented (Supplementary Fig. 20d–g). Radial canals are also scattered in the compacta. As observed in the tibia, the vascular canal density and the relative size of the canal lumina tend to decrease toward the subperiosteal margin. At least 13 LAGs are recorded in this portion of the cortex (Supplementary Fig. 20d–f). There is no clear pattern regarding the spacing between LAGs. Sharpey’s fibers are observed in different areas of the outer cortex, being more abundant in an area located in the lateral portion of the rib (Supplementary Fig. 20h). These extrinsic fibers are also recorded in the thin layer of subperiosteal tissue preserved in the medial side of the rib cortex. The pneumatic cavity is surrounded by a thin layer of secondarily formed lamellar bone tissue, which corresponds to an ICL (Supplementary Fig. 20c, i). Most of this ICL has been remodeled. A distinct patch with Sharpey’s fibers is observed in the ICL (Supplementary Fig. 20j, k). These extrinsic fibers are loosely grouped and exhibit two main orientations, which are oblique to the ICL surface.

Interpretation of ontogenetic stage and minimum age. Bone tissues preserve records of important events in the life history of a given vertebrate individual 67–69. In this regard, discrete microstructural changes occur when individuals reach both somatic (i.e., asymptotic growth) and sexual maturity 67,68. Somatic maturity is usually inferred on the basis of the presence of an external fundamental system (EFS) in the subperiosteal cortex 67,68,70. An EFS is a form of bone microstructure sometimes observed in the subperiosteal cortex within a transverse bone cross-section, particularly within long bones. This structure is poorly vascularized (or avascular) and is composed of slowly deposited parallel-fibered or lamellar tissue 67,71. Regarding sexual maturity, it is broadly assumed that this occurred well before somatic maturity in non-avian dinosaurs (e.g., 72–74). Sexual maturity is usually inferred on the basis of a distinct change in the spatial organization of intrinsic collagenous fibers (from poorly to well-organized) and/or a noticeable reduction in the spacing between successive CGMs (e.g., 75,76). Since there is no evidence for a distinct EFS in the tibia and dorsal rib of the Joaquinraptor holotype, we infer that this individual died before the attainment of somatic maturity. It is worth noting, however, that this ontogenetic stage estimate is tentative. In this regard, although post-mortem destruction of the EFS in both skeletal elements seems improbable, this possibility cannot be entirely ruled out. On the other hand, the noticeable reduction in the spacing between LAGs in the tibia suggests that the individual had attained sexual maturity. Assuming that LAGs correspond to annually deposited structures, a minimum age of 19 years is inferred for this Joaquinraptor specimen.

The histological analysis of two different skeletal elements of Joaquinraptor is important to determine the ontogenetic stage and minimum age of the individual; moreover, the results also provide data regarding which bones are most reliable for minimum age estimation in non-avian dinosaurs. Although the ontogenetic stages of non-avian dinosaurs have commonly been inferred from long bone histology (e.g., 77–79), dorsal ribs have recently been demonstrated to be useful for at least minimum age estimation when long bones are not available (i.e., 80–82). In megaraptorans, the long bones (in the case of Joaquinraptor, the tibia) appear to preserve a better growth record than the dorsal ribs. The lower number of CGMs in the dorsal rib is related to two main factors. First, this element shows a prominent pneumatic cavity in the sampled area. In addition, the dorsal rib of Joaquinraptor exhibits a higher degree of secondary remodeling than does the tibia. The presence of a pneumatic cavity and profuse secondary remodeling results in poor preservation of primary bone tissue and, hence, of the growth record in the cortex. A similar condition has been reported in other non-avian dinosaurs, including abelisaurid theropods and titanosaurian sauropods 83–85.

Histological evidence for postcranial pneumaticity. Postcranial skeletal pneumaticity (PSP) has been recognized in pterosaurs and diverse clades of saurischian dinosaurs (e.g., refs. 86–89). Among non-avian theropods, PSP is particularly extensive in megaraptorans (e.g., refs. 4,7,8,21,30,62), in which pneumatic cavities reach the mid-caudal vertebrae and selected appendicular elements (e.g., furcula, ilium; see refs. 4,21). Aranciaga Rolando et al. 62 showed that, in Aoniraptor, anatomical features related to PSP (i.e., presence of cortical foramina connected to internal cavities) co-occur with histological structures that have also been proposed as evidence for PSP. Specifically, the endosteal bone that forms the internal pneumatic cavities of a caudal centrum and a haemal arch of Aoniraptor contains tissue identified as pneumosteon (sensu 90), a bone tissue described in pneumatic structures of some groups of saurischian dinosaurs that is composed of abundant, delicate extrinsic fibers.

The histological analysis conducted on the Joaquinraptor dorsal rib provides data regarding the possible correlation of PSP and bone microstructure. As recorded in other saurischians (e.g., refs. 62,90,91), the internal pneumatic cavities are encased by secondarily deposited lamellar bone tissue. Whereas a portion of the secondary lamellar bone tissue does not exhibit any trace of pneumosteon-like tissue (Supplementary Fig. 20i), extrinsic fibers are distinct in other parts of the sample. However, the tissue described here for Joaquinraptor (Supplementary Fig. 20j, k) is not identical to the pneumosteon described in other dinosaurs (e.g., 62,90). Pneumosteonal tissue has been characterized as densely packed, thin, parallel fibers 90. Conversely, the extrinsic fibers recorded in the dorsal rib of Joaquinraptor are loosely packed and exhibit variable orientations, which raises doubts regarding their possible homology with pneumosteon. We conclude that: (1) at least in megaraptorans, PSP is not always correlated with the presence of pneumosteon, as revealed by the absence of this tissue from most of the secondary lamellar bone that surrounds the pneumatic cavity of the rib; and (2) if the extrinsic fibers observed in the endosteal bone of Joaquinraptor are homologous to pneumosteonal tissue, then this tissue exhibits a higher histological diversity than has previously been proposed.

Phylogenetic analysis and results. Two iterative phylogenetic analyses were performed to test the systematic position of Joaquinraptor. Because the primary aim of these analyses was to evaluate the systematic position of the new Lago Colhué Huapi Formation taxon within Megaraptoridae—as opposed to resolving the higher-level affinities of Megaraptora among Tetanurae (see, for example 25)—we added Joaquinraptor to the data matrix published by Aranciaga Rolando et al. 8, which includes the most comprehensive megaraptoran sample of any phylogenetic data set yet published (Supplementary Notes 8 and 9). Herein, and following Novas et al. 34, Megaraptora is defined as a stem-based clade containing all taxa closer to Megaraptor namunhuaiquii than to Chilantaisaurus tashuikouensis, Neovenator salerii, Carcharodontosaurus saharicus, Allosaurus fragilis, Baryonyx walkeri, Tyrannosaurus rex, and Passer domesticus. Megaraptoridae is defined as the most inclusive clade containing Megaraptor namunhuaiquii but not Fukuiraptor kitadaniensis, following Kotevski et al. 18 and Novas et al. 34.

Phylogenetic analysis 1 included all operational taxonomic units within the matrix, whereas analysis 2 excluded the fragmentary taxon Aoniraptor. Both analyses postulated Megaraptora as well nested within Coelurosauria as the sister group of Tyrannosauroidea (Supplementary Note 10 and Supplementary Fig. 21). Analysis 1 (Fig. 3a) found 102 most parsimonious trees (MPTs) of 1477 steps, with a Consistency Index (CI) of 0.303 and a Retention Index (RI) of 0.594. In the strict consensus of these MPTs, the Thai taxon Phuwiangvenator is placed at the base of Megaraptora, followed by a polytomy that includes the Asian forms Vayuraptor and Fukuiraptor, Australovenator and the fragmentary specimen LRF 100–106 16 from Australia, Orkoraptor and Aoniraptor from Argentine Patagonia, and a clade containing all remaining South American megaraptorans included in the analysis (=Megaraptoridae following the phylogenetic definition employed herein). Relationships among this latter clade are generally well resolved, albeit with low Bremer support values (=1). More specifically, the generically unidentified megaraptorid from the Bajo Barreal Formation (represented by specimens UNPSJB-PV 944 and UNPSJB-PV 958; see ref. 7) is recovered as the sister taxon of Murusraptor. Relative to those of other Patagonian Upper Cretaceous stratigraphic units, the dinosaur fauna of the Bajo Barreal Formation is frequently regarded as being comprised by comparatively plesiomorphic representatives of its constituent clades 31,92; therefore, the early branching position of the Bajo Barreal taxon within Megaraptoridae is not unexpected. Both these Patagonian forms in turn constitute the sister taxon of a diverse clade of derived South American megaraptorids. This clade, herein informally termed Megaraptorid Clade 1 (=MC1) for ease of discussion, is united by the ambiguous synapomorphy of the parietals being narrow between the supratemporal fossae, with each reduced to about 10–30% the mediolateral width of the fossa (character 265, state 1; present in Joaquinraptor and Megaraptor). Within MC1, Joaquinraptor is placed as an early diverging form.

Analysis 2 (Fig. 3b), conducted after Aoniraptor was pruned, yielded only 12 MPTs of 1476 steps, with a CI of 0.307 and an RI of 0.602. The resulting strict consensus tree shows improved resolution, particularly at the base of Megaraptora. Phuwiangvenator is recovered in the same position as in analysis 1, but the above mentioned polytomy is largely resolved, leaving only Vayuraptor and Fukuiraptor as the next-latest diverging megaraptorans. The Australian forms LRF 100–106 and Australovenator are postulated as early diverging megaraptorids and successive sister taxa to a clade comprising all South American megaraptorans in the analysis. This latter group is united by the absence of mesial denticles (character 2, state 2). The recovery of this clade—consisting entirely of Argentinean megaraptorids reaching or exceeding approximately 7 m in total body length—supports the hypothesis that Megaraptoridae experienced a substantial increase in body size upon their arrival in South America during the mid-Cretaceous 7 (see also below). The results of analysis 2 postulate Orkoraptor as the earliest diverging member of the Argentinean clade. As in analysis 1, the remaining South American megaraptorids are situated within a clade that is itself divided into two subclades, UNPSJB-PV 944 & 958 + Murusraptor and MC1. As in analysis 1, Joaquinraptor is recovered as the earliest diverging member of MC1.

In order to down-weight characters that exhibit widespread homoplasy, we applied extended implied weighting (k=3) to both analyses. Similar results were obtained for analysis 1, but (after pruning Aoniraptor) analysis 2 yielded better resolution within Megaraptoridae (see Supplementary Fig. 21c). Among derived megaraptorids, both the clades UNPSJB-PV 944 & 958 + Murusraptor and MC1 were again recovered; however, there is improved resolution among derived members of MC1, with Maip being postulated as the sister taxon of Tratayenia rosalesi + Aerosteon.

In accordance with the study of Aranciaga Rolando et al. 8, both our phylogenetic analyses (analysis 1 and analysis 2) recovered nested clades of South American megaraptorids, one more inclusive than the other. Here, we compare the results of our analysis 2 (i.e., our analysis that excluded Aoniraptor) to those of the analysis without Aoniraptor and the Bajo Barreal Formation megaraptorid (the latter represented by specimens UNPSJB-PV 944 and UNPSJB-PV 9587) presented by Aranciaga Rolando et al. 8. The topologies of the strict consensus trees recovered by these two analyses differ with regard to the hypothesized placements of selected Patagonian megaraptorids. In our analysis, Megaraptor occupies a relatively derived position, whereas Orkoraptor is supported as an earlier diverging form. Therefore, instead of the clade Megaraptor + Murusraptor + “Clade B” recovered by Aranciaga Rolando et al. 8 (i.e., these authors’ “Clade A”), we postulate Orkoraptor as the outgroup to an unnamed clade that itself includes the sister taxa UNPSJB-PV 944 & 958 + Murusraptor and MC1 (the latter of which encompasses all megaraptorid taxa within Aranciaga Rolando et al.’s 8 “Clade B”, with the addition of Joaquinraptor and the aforementioned exception of Orkoraptor).

In sum, our phylogenetic analyses yielded several notable results that may be pertinent to future discussions of megaraptoran systematics. First, and unsurprisingly given the data matrix we used, the analyses support the placement of Megaraptora within Coelurosauria, as the sister group of Tyrannosauroidea. Moreover, Joaquinraptor is recovered as the earliest diverging member of a newly identified clade of derived South American megaraptorids, herein termed Megaraptorid Clade 1 (=MC1). Nevertheless, although the position of Joaquinraptor within Megaraptoridae on the whole is well supported, MC1 is presently supported by only a single synapomorphy; furthermore, two putative members of the clade (Maip and Tratayenia) are relatively incompletely preserved, being presently known only from postcranial (mainly postcranial axial skeletal) bones. As such, future analyses should evaluate the proposed monophyly and taxonomic content of MC1 and, therefore, its potential systematic validity, as well as the specific relationships of taxa within the clade.

Implications for Patagonian theropod faunal provincialism and megaraptorid palaeobiology. The post-Turonian central (Fig. 4a) and southern Patagonian fossil record of large-bodied non-avian theropod dinosaurs is, at the moment, numerically dominated by megaraptorids 7,8,22,23,31,53. Nevertheless, to date, all described central and southern Patagonian megaraptorid specimens have been fragmentary; therefore, Joaquinraptor is important in being, by a substantial margin, the most complete representative of the clade yet found from this geographic area. Moreover, this taxon was recovered from only a few meters below the top of the Lago Colhué Huapi Formation, and is therefore Maastrichtian (probably late Maastrichtian) in age, rendering it the latest surviving megaraptoran yet known from central Patagonia and one of the stratigraphically youngest (quite possibly the youngest) members of the group found anywhere in the world. Although more evidence is needed, Patagonian megaraptorids present a suite of putative synapomorphies, such as the absence of mesial denticles from the tooth crowns, a manual ungual I that is subequal in length to the ulna, and a well-developed “mediolateral platform” on the flexor tubercle of this ungual 8,35, that suggest that these theropods may have been evolutionarily and geographically differentiated from earlier diverging members of the group (e.g., Australovenator).

Furthermore, the discovery of Joaquinraptor supports the hypothesis that megaraptorids were the apex predators and the most abundant large-bodied theropods in the latest Cretaceous (Campanian–Maastrichtian) of central and southern Patagonia, contrasting with penecontemporaneous faunas from more northerly areas of South America that appear to have been dominated by abelisaurids and large unenlagiines (Fig. 4b; see also refs. 7,8,23,93). Novas et al. 23 suggested that this putative theropod provincialism may have been related to palaeoenvironmental differences. Notably, however, the sedimentology of the middle (i.e., Santonian–?lower Maastrichtian) section of the Lago Colhué Huapi Formation, which has yielded several other megaraptorid fossils 31,55; Fig. 4a), is characterized by fluvial channel morphologies that indicate that the fauna it preserves lived inland, well away from the palaeoshoreline; moreover, clay analyses support a seasonally dry or semiarid climate for this part of the formation (see ref. 46 and references therein). Conversely, in the uppermost section of the formation, at a stratigraphic level close to that from which Joaquinraptor was collected, sedimentological and palynological inferences support the existence of a warm, humid floodplain palaeoenvironment close to the sea (see Supplementary Note 3). Consequently, it appears that, at least in central Patagonia, megaraptorids endured significant palaeoclimatic changes and successfully adapted to fluctuating palaeoenvironmental conditions through Late Cretaceous time, raising the possibility that these theropods were able to thrive in a broad range of habitats. As such, alongside putative palaeoenvironmental distinctions, additional factors, such as the potential palaeogeographic isolation of central and southern Patagonia from more northern areas of South America by epicontinental seas (Fig. 4b; see also, e.g., Reguero and Goin 94 [these authors’ Fig. 8]) and/or pronounced floristic differences (see, for example, the reconstructed Late Cretaceous palynological bioprovinces proposed by Vajda and Bercovici 95 [these authors’ Fig. 3b]) may also have driven the differentiation of large-bodied theropod faunas in southern South America at the end of the Mesozoic.

The distinctive megaraptorid forelimb may have played an important role in the apparent adaptability of these theropods, perhaps allowing them to exploit resources in varying ways and/or to feed on an array of vertebrate taxa. For example, megaraptorids may have used their hypertrophied manus with its enlarged, trenchant unguals to access soft tissues 96 and/or to aid in prey capture and manipulation 9,20,25,97,98. In addition, several features of the megaraptorid antebrachium, such as the proximodistally elongate olecranon process (the insertion area of M. triceps brachii) and the vertical ridge on the proximal lateral surface of the ulna (a probable osteological correlate for the M. anconeus insertion 9,99,100), may have conferred enhanced extension and flexion capabilities.

As mentioned above, in addition to the Joaquinraptor skeleton, a crocodyliform right humerus was recovered from the type quarry of the new megaraptorid. Interestingly, this bone was situated between the closely associated left and right dentaries of the Joaquinraptor holotype, with the apices of several tooth crowns of the megaraptorid in direct contact with the humeral shaft, which also shows potential tooth marks (Fig. 5); moreover, this humerus is the only non-megaraptorid vertebrate fossil yet identified from the site. As such, this discovery may constitute direct evidence regarding prey selection within Megaraptoridae (although isolated megaraptorid teeth have been found in association with other saurischian dinosaur specimens, potentially also indicating feeding behavior [e.g., refs. 41,101]). Nevertheless, this megaraptorid/crocodyliform co-occurrence could conceivably be instead related to other behaviors, such as interspecific combat or competition for food, or even just a coincidental association (e.g., the humerus was moved to a position between the Joaquinraptor dentaries by water currents). Detailed taphonomic studies are currently underway to assess the probable cause and potential significance of this association.

In sum, the singular morphology of the megaraptorid forelimb may have played an important role in the evolutionary success of these theropods in the Late Cretaceous of central and southern Patagonia. The specialized development and capabilities of the forelimb were almost certainly adaptations related to prey capture 9,97,98, and, in the case of Joaquinraptor, these prey items may have at least occasionally included crocodyliforms (Fig. 5).

Megaraptorid palaeobiogeography and body size evolution. Based on the fossil record of Megaraptoridae as it is currently understood, Lamanna et al. 7 hypothesized that South American members of the clade underwent a substantial increase in body size with respect to their earlier diverging and mostly stratigraphically older Australian relatives. Specifically, megaraptorids are proposed to have experienced an increase in size from approximately 5 m in total body length and 300 kg in body mass to more than 7 m in length and 1000 kg in mass not long after their dispersal to South America from Australia, presumably via Antarctica, during the mid-Cretaceous 7. Moreover, South American megaraptorids are hypothesized to have maintained this elevated body size until their extinction at or near the end of the Cretaceous 7,8.

The nearly complete (missing only the medial condyle of the distal end) left femur of the holotype of the Maastrichtian megaraptorid Joaquinraptor has a total proximodistal length of 685 mm and a circumference (measured immediately distal to the fourth trochanter) of 275 mm (Table 2). Based on the scaling equation presented by Campione and Evans 102 and employed in many subsequent works (e.g., refs. 7,103), this circumference yields a mass estimate of 1036 kg for this individual. Approximately the same circumference (280 mm) was obtained from the fragmentary femur of another large-bodied megaraptorid specimen from a nearby exposure of the stratigraphically older (Cenomanian–Turonian) Bajo Barreal Formation (UNPSJB-PV 958), from which an estimated body mass of 1089 kg was derived 7. Furthermore, a mass estimate that is precisely the same as that generated for Joaquinraptor (1036 kg) was proposed for MUCPv 1353, an undescribed partial megaraptorid skeleton (probably referable to Megaraptor) from the Turonian–Coniacian Portezuelo Formation of northwestern Patagonia 7. On the other hand, the type specimen of the early diverging Australian megaraptorid Australovenator has a femur length of 578 mm 15, whereas that of the holotype of the non-megaraptorid megaraptoran Fukuiraptor is 507 mm 2. Likewise, the estimated body mass of the former taxon (314 kg 103) is approximately 3.3–3.5 times less than that of Joaquinraptor, MUCPv 1353, and UNPSJB-PV 9587. Aranciaga Rolando et al. 8 proposed a body length of more than 9 m for Maip, a megaraptorid from the Maastrichtian Chorrillo Formation of southern Patagonia, which would likely establish this taxon as the largest-bodied megaraptorid yet known.

In sum, Joaquinraptor accords with Lamanna et al.’s 7 hypothesis of body size evolution in South American megaraptorids in that it was probably longer than 7 m and heavier than 1000 kg; in other words, approximately the same size as MUCPv 1353 and UNPSJB-PV 958. Furthermore, Joaquinraptor is from the Maastrichtian—probably the late Maastrichtian—supporting the hypothesis that Megaraptoridae maintained this elevated body size until their extinction (Fig. 6).

Recently, Kotevski et al. 19 proposed that selected megaraptorid postcranial elements (a set of two articulated caudal vertebrae and their corresponding haemal arches [NMV P257414], an isolated partial manual ungual II [NMV P186153], and the distal end of a tibia [NMV P257415]) from the upper Barremian–lower Aptian upper Strzelecki Group of Victoria, Australia suggest that ca. 6 m individuals of this theropod clade inhabited that continent during the Early Cretaceous. The upper Strzelecki Group bone that implies the largest individual—a megaraptorid perhaps slightly greater than 6 m in body length—is the fragmentary manual ungual II NMV P186153, which was also described and/or analysed at length by Benson et al. 104, Poropat et al. 17, and White and Campione 105. That said, this claw is isolated and incomplete, and as such, the hypothesized body size of the individual to which it pertained—though it may well be correct—cannot be considered to be robustly documented given the fragmentary nature of the currently available evidence. Consequently, we consider the hypothesis that >6 m megaraptorids were present in Australia to be only tentatively supported at present. Moreover, even if NMV P186153 does indeed represent a megaraptorid marginally greater than 6 m in body length, the likelihood remains that all definitive South American megaraptorid records indicate animals that are still larger (ca. 7 m [+] in length and ca. 1000 kg [+] in mass). As such, in our view, Lamanna et al.’s 7 hypothesis that megaraptorids experienced a substantial increase in body size shortly after their arrival in South America remains consistent with the presently available data (Fig. 6). That said, it may potentially be only a matter of time before definitive evidence of large-bodied (≥7 m, ≥1000 kg) megaraptorids is discovered in the Early or mid-Cretaceous of Australia.

To conclude, as proposed by Lamanna et al. 7 and supported by more recent discoveries (e.g., 8,106), including Joaquinraptor (Fig. 6), South American megaraptorids appear to have attained larger body sizes (~7 m or greater in total body length) than their generally more ancient relatives from Australia (~5–6 m 19). Patagonian megaraptorids may have continued to increase in size through the remainder of their evolutionary history—perhaps spurred at least in part by the extinction of carcharodontosaurids during the Turonian 8,106—with some latest Cretaceous forms (e.g., Maip) potentially reaching body lengths of 9 m or more 8. Among other benefits, this may have allowed megaraptorids to occupy the apex predator niches in their respective habitats. Moreover, and as evidenced by the Maastrichtian (probably late Maastrichtian) stratigraphic occurrence of Joaquinraptor, these theropods may well have survived to the Cretaceous/Palaeogene boundary, only to ultimately perish alongside all other non-avian dinosaurs.

Methods
Field permits and permissions
The field research that recovered the holotypic specimen of Joaquinraptor casali (UNPSJB-PV 1112) was carried out at the informally named Valle Joaquín site in south-central Chubut Province, central Patagonia, Argentina. Personnel from the Laboratorio de Paleontología de Vertebrados Dr. Rubén Martínez of the Universidad Nacional de la Patagonia San Juan Bosco obtained all necessary permissions from the Subsecretaría de Cultura de la Provincia del Chubut and the relevant landowner to conduct this fieldwork.

Palaeohistology
We conducted a histological analysis to assess the ontogenetic stage and other osteohistological attributes of UNPSJB-PV 1112. Transverse sections were obtained from the midshaft of the right tibia and the proximal shaft of a dorsal rib. Histological thin sections were prepared at the petrographic laboratory of the Universidad Nacional de San Luis (San Luis, Argentina). The thin sections were prepared using standard petrographic methods (e.g., 107) and studied using a petrographic polarizing microscope (Leica DM750P). The nomenclature and definitions of structures used in this study are derived from Francillon-Vieillot et al. 108 and de Buffrénil and Quilhac 69.

Phylogenetic analysis
Morphological characters 1–351 used in our phylogenetic analysis were sourced from Aranciaga Rolando et al. 36 and references therein, with their descriptions as edited and presented by Lamanna et al. 7 (these authors’ Supplementary Appendix A). In a few cases, these character descriptions were slightly further modified (without changing their meaning) from those published by Lamanna et al. 7. Characters 352–357 were subtly edited (also without changing their meaning) from Aranciaga Rolando et al. 8.

We used the software TNT (Tree analysis using New Technology) version 1.5 109 to evaluate the phylogenetic position of Joaquinraptor as well as to test the topology within Megaraptora after the addition of the new theropod. We employed the traditional search function using the tree bisection-reconnection (TBR) swapping algorithm and a random seed of 1. The traditional search included 10,000 replications with ten trees saved per replication. We performed two analyses (analysis 1 and analysis 2) under equally weighted parsimony. Analysis 1 included all megaraptoran taxa analysed by Aranciaga Rolando et al. 8, but Aoniraptor libertatem was pruned from analysis 2. Aoniraptor was excluded from analysis 2 because this theropod was the least stable megaraptoran taxon in analysis 1, in part due to its fragmentary preservation (i.e., it is presently known from less than 15% of the skeleton). Bremer support values within Megaraptora were low (=1) in the strict consensus trees generated by both analyses. Subsequently, we applied extended implied weighting (k=3) to both analyses to down-weight particularly homoplastic characters.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "General\nThe manuscript is a technically sound piece of scientific research. The data of the study supports the conclusions. The\nstatistical analysis has been performed appropriately and rigorously. All data underlying the findings in the manuscript are\nfully available (as supplementary material). The manuscript is presented in an intelligible fashion and written in standard\nEnglish.\nComments to the Authors\n1) This work adds the number of megaraptorids to Gondwana and globally. This is especially important because this\ntheropod clade is rare and highly fragmented, and their status is still debated.\n2) I do agree with the authors overall. This theropod shows megaraptorid characters as the authors showed, e.g., on the\nteeth, humerus, ulna, maxilla, and tibia.\n4) On the diagnosis, I have some questions about its autapomorphies, i.e.,\n4.1) autapomorphy number 2: “anterior margin of supratemporal fossa with sigmoid contour”. Is it possible that this is due to\npreservation? Are they present on both sides? Since the supp. Fig 3A and supp. Fig. 9K are not quite similar (the supp. fig.\n9K is not quite clear for this character). How is this character in other closely related theropods (e.g., carcharodontosaurs,\nbasal coelurosaurs)?\n4.2) same as 4.1) is the autapomorphy number 3 “braincase lacking median septum in\npneumatic recess” due to preservation? Maybe CT-scan (in further work?) will reveal this.\n4.3) autapomorphy number 3, “middle or posterior caudal vertebral centra with well-marked elliptical lateral fossae that\noccupy more than half the lateral surfaces”, is quite clear and interesting, since Aerosteon middle caudal vertebra has lateral\nfossa occupy almost half the centrum lateral surface (Aranciaga Rolando et al. 2021). However, you may need to compare\nwith the Brazilian caudal vertebrae MPMA 08-003-94 (Mendez et al. 2012) and CPPLIP 1324 (Martinelli et al. 2013),\nwhether they exhibit this feature or not. Thus, this character is unique to the new taxon or not.\n4.4) autapomorphy number 7, the “ulna with proximodistally enlarged olecranon process” is probably the same as in\nMagaraptor (MCF-PVPH70 and MUCPv 341) (based on supp. Fig. 13 in the MS). Please check.\nIs the enlarged olecranon process also present in Australovenator? But maybe it is due to a different angle (the ulna shaft in\nthe Australovenator is slightly anteriorly curved)? Please check.\nOther autapomorphies are quite clear.\n5) The maxilla of the new megaraptorid is subtriangular, low, and elongated, somewhat similar to, but also different from, the\nunenlagiine Austroraptor (from neighboring province, which also stated in the introduction that unenlagiines are dominated\nin this area). So, a short paragraph on the comparison/ differentiation of their maxillae might be good for clarifying that they\nare not the same taxon/ clade.\n6) Focusing on the maxilla interdental plates of Joaquinraptor vs Megaraptor and the dentary interdental plates of\nJoaquinraptor vs Australovenator vs Fukuiraptor. How are they different or similar from each other? Please provide\ninformation. Is there any pattern or distribution of this character in Megaraptora?\n7) The scapula blade ratio: is it different from that of Megaraptor and Aerosteon? Compared with Gualicho (a probable\ncarcharodontosaur closely related to Megaraptora or an elaphrosaurine) which also exhibits fused scapulocoracoid, how are\nthey different or related?\n8) Humerus deltopectoral crest: from supp. Fig. 5C, the deltopectoral crest looks somewhat trapezoidal-shaped as in other\nmegaraptorids (instead of subrectangular) (Supp. fig. 12) (please check). But I agree that it is more robust in the new\nmegaraptorid.\n9) Even though the new taxon clearly belongs to Megaraptoridae, some material could be compared with other theropod\nclades, such as carcharodontosaurs, basal coelurosaurs, tyrannosauroids, and some problematic such as Chilantaisaurus\nand Gualicho, for more understanding of the evolution of Megaraptora.\n10) Line 393-396 “Patagonian megaraptorids present a suite of putative synapomorphies, such as an elongated rostrum and\na considerably enlarged manus, that suggest that these theropods may have been evolutionarily and geographically\ndifferentiated from earlier diverging members of the group (e.g., Australovenator).” – The elongated rostrum (at least dentary)\nand enlarged manus are present in Australovenator (compare measurement of each element in Australovenator and\nJoaquinraptor). So, this statement might be incorrect, or at least, must include Australovenator.\n11) Why Joaquinraptor is not the same taxon as Maip or Tratayenia? It was stated in the MS that the coracoids and ribs of\nJoaquinraptor resemble those of Maip. If not much overlapped material can be compared. Even though the phylogenetic\nanalysis groups them into a single clade, the different positions on the resulting tree(s) may be due to the artifact of, or\nbiased by, non-overlapped material. Please explain or state the reason or give evidence (apart from the fact that they are\nfrom different provinces and rock strata) that they might be, or not be, the same taxon.\n12) Line 453-454 You stated that crocodyliforms could be one of the prey items for megaraptorids. Could it be that these two\nanimals just fight each other, like competition for food while scavenging some carcass? Or, the other way around, the croc\nmight come to eat that ‘dying’ megaraptorid, and they fought and got stuck in the mud. How or what concrete evidence\ndo you have to conclude this?\n13) Fig. 4. and Line 397 onward – Why was Megaraptoridae absent in the north of Golfo San Jorge Basin? Apart from the\npaleoenvironment, is there any evidence of island biogeography in this area? For example, was Austral-Magallanes an\nisland, that why megaraptorids were here but not in the north of Golfo San Jorge Basin? If so, had the island influent to the\ngigantism in Megaraptoridae?\n14) Line 39 - a member of the derived megaraptoran[s] subclade Megaraptoridae. (add ‘s’)", "reviewer_2": "Please see attached pdf", "reviewer_3": "Dear authors,\nI have reviewed the manuscript by Dr. Ibiricu and colleagues. The manuscript describes a partial skeleton of a megaraptoran\ntheropod dinosaur, which is substantially more complete than known megaraptorans. Accordingly, the insights from the\nspecimen provide new light on the anatomy of megaraptoran dinosaurs, and the geological context of the fossil extends the\nrecord of this clade into the latest Cretaceous.\nThe manuscript is high quality, the figures are excellent, and the writing is clear and succinct. I have no major qualms with\nthe research conclusions or methodologies. I feel that because of the rarity and charisma of this clade, the manuscript will\ngarner significant media attention. Scientifically, this manuscript has a high likelihood of becoming well-cited, as its detailed\nanatomical notes will be important for future work on these animals and their relatives.\nI have a few minor suggestions for improvement, which are generally optional and at minimum involve only small changes to\nthe text.\nIn the section on phylogenetics, it is argued that the position of Joaquinraptor in the tree supports a tyrannosauroid affinity for\nmegaraptorans (over an allosauroid one). This is presently one of the major debates about these animals, and so I suspect\nthis statement may draw some flak because the specimen was included in a matrix which already found megaraptorans as\ntyrannosauroid relatives. To me, the new analyses with this taxon do not necessarily improve the support for the case, but\nthey show that this new specimen is consistent with the hypothesis. This is as we would expect: a late-branching form is\nunlikely to produce major changes in root nodes, but this likewise means that its addition is not a very strong test of the\nformer hypothesis.\nIf the authors choose, I feel that this potential weakness could be strengthened in one (or all) of three ways:\neither 1) a brief statement and revision to the wording explaining this debate in a little more detail in the phylogenetic\nsection, and explaining why this particular matrix was chosen when others exist. It would be important here to mention that\nthis phylogenetic approach is not intended to test the position of megaraptorans within the tree, but of Joaquinraptor within\nMegaraptora.\n2) a broader discussion of the tyrannosauroid-like features of the anatomy of Joaquinraptor, which might further strengthen\nthe tyrannosauroid argument with characters outside of the phylogenetic matrix (some of these characters might also be\nworth adding to the matrix, either here or in future work);\nor 3) adding Joaquinraptor to a matrix that finds megaraptorans as allosauroids, to test whether its inclusion in that matrix\nchanges patterns.\nMy only other substantial comment regards the palaeohistological interpretations, and I feel that these are a bit less optional.\nThese are in two veins:\n1) Age estimation should be done from the number of cyclical growth marks, which themselves can comprise multiple LAGs.\nIn this case, it appears that you have double and triple LAGs, and the current interpretation of those features is that they\ntogether represent a single annual growth mark (see Cullen et al. 2021 in Biology Letters). Anywhere that LAGs are paired\nclosely together, but this group is further away from other groups, they should be considered as being formed in the same\nyear and belonging to the same CGM. Revising these counts will not change your overall interpretations of the maturity\nestimation, but they will put a younger age on the specimen. In some ways, this might add more support to a coelurosaur\nidentity, as differences in growth patterns between tyrannosauroids and carcharodontosaurids are known (see another\nCullen paper: Cullen et al. 2020 in Proc B).\n2) I am not convinced that the preservation of the ICL in the rib is good enough to make many\ncomments on pneumosteon, and these, to me, are a weak part of the paper that doesn’t contribute to the major conclusions.\nI would at the very least consider taking them out of the main text, and if you wish to keep them, have them exclusively in the supplement.\nI am happy to be contacted by the authors, should they have any questions about my suggestions.\nSincerely,\nGreg Funston,\nVisiting Assistant Professor\nUniversity of California, Davis\ngfunston@ucdavis.edu"}, "author_response": "Responses to review comments on Nature Communications manuscript NCOMMS-24-72004-T:\nLatest-surviving megaraptorid theropod dinosaur sheds light on megaraptoran\nevolution and palaeobiology\nLucio M. Ibiricu, Matthew C. Lamanna, Bruno N. Alvarez, Ignacio A. Cerda, Julieta L.\nCaglianone, Noelia V. Cardozo, Marcelo Luna, and Rubén D. Martínez\nReviewer #1 (Remarks to the Author):\nGeneral\nThe manuscript is a technically sound piece of scientific research. The data of the study\nsupports the conclusions. The statistical analysis has been performed appropriately and\nrigorously. All data underlying the findings in the manuscript are fully available (as\nsupplementary material). The manuscript is presented in an intelligible fashion and written in\nstandard English.\nResponse: We appreciate the reviewer’s acknowledgment that our manuscript meets these\ncriteria and are grateful to this person for their thorough and helpful review.\nComments to the Authors\n1) This work adds the number of megaraptorids to Gondwana and globally. This is especially\nimportant because this theropod clade is rare and highly fragmented, and their status is still\ndebated.\nResponse: We appreciate the reviewer’s comment here, and naturally, we agree.\n2) I do agree with the authors overall. This theropod shows megaraptorid characters as the\nauthors showed, e.g., on the teeth, humerus, ulna, maxilla, and tibia.\nResponse: See our previous response.\n4) On the diagnosis, I have some questions about its autapomorphies, i.e.,\n4.1) autapomorphy number 2: “anterior margin of supratemporal fossa with sigmoid contour”. Is\nit possible that this is due to preservation? Are they present on both sides? Since the supp. Fig\n3A and supp. Fig. 9K are not quite similar (the supp. fig. 9K is not quite clear for this character).\nHow is this character in other closely related theropods (e.g., carcharodontosaurs, basal\ncoelurosaurs)?\nResponse: We agree that the photograph of the skull roof, braincase, and atlas that comprises\nour Suppl. Fig. 3a and the interpretive line drawing of this bone complex shown in our Suppl.\nFig. 9k do look slightly different (especially in that the anterior margin of the supratemporal fossa\nis not well-defined in the latter). However, in our view, both these images effectively serve the\npurposes for which they were created, the former for illustrating potential autapomorphies of the\nnew megaraptorid taxon Joaquinraptor casali in this part of the skeleton and the latter for\nproviding general morphological comparisons to other megaraptorid specimens. Although, in the\nJoaquinraptor holotype UNPSJB-PV 1112, this bone complex does exhibit some taphonomic\nalteration (especially in the posterior region of the skull roof), we believe that the shape of the\nsupratemporal fossa has not been significantly altered; in other words, we consider the\nsigmoid contour of the anterior margin of this fossa to represent original, authentic morphology\nrather than being a taphonomic artefact. Among other attributes, this sigmoid anterior contour is\nbilaterally symmetrical; i.e., it is seen on both the left and right sides of the skull. That said, and\nfollowing the reviewer’s recommendation, we have added information on the shape of the\nanterior margin of the supratemporal fossa in some other, potentially phylogenetically\nproximal large-bodied theropod taxa (please see the attached, revised version of the\nmanuscript, in which all proposed changes to our original submission are tracked). Namely, we\nhave added, “Conversely, in allosauroids such as Acrocanthosaurus atokensis, Allosaurus\nfragilis, and Carcharodontosaurus saharicus, the supratemporal fossa is anteroposteriorly short\n(Aranciaga Rolando et al., 2019), and its anterior margin exhibits a more ovoid aspect.”\n4.2) same as 4.1) is the autapomorphy number 3 “braincase lacking median septum in\npneumatic recess” due to preservation? Maybe CT-scan (in further work?) will reveal this.\nResponse: Upon reflection, and taking the reviewer’s comment into account, we agree that\nthere is some possibility that the lack of a median septum in the pneumatic recess on the ventral\naspect of the UNPSJB-PV 1112 braincase could represent a taphonomic artefact. That said, and\nas we have previously argued, the absence of this septum could also be an authentic,\nautapomorphic feature of the new megaraptorid taxon Joaquinraptor. However, we have\nrevised the section of our manuscript in which this feature is described to mention that it\ncould also be due to taphonomic processes (specifically, the manuscript now states,\n“Ventrally, the median septum that divides the basisphenoid recess that is described in\nMegaraptor (Porfiri et al., 2014) and Murusraptor (Paulina-Carabajal and Currie, 2017) appears\nto be absent in Joaquinraptor, although this could potentially be an artefact of preservation.”).\nPresumably, the future discovery of additional Joaquinraptor braincase material that preserves\nthe pneumatic recess could support or refute this putative autapomorphy. Similarly, and in\naccordance with the reviewer’s suggestion, in the near future, our team plans to conduct a\ndetailed, computed tomography (CT)-based anatomical study of the UNPSJB-PV 1112\ncraniomandibular elements, including the skull roof/braincase/atlas complex. This future\nanalysis—which is beyond the scope of the present study—may also aid in the determination of\nthis feature as a natural character versus a preservational artefact.\n4.3) autapomorphy number 3, “middle or posterior caudal vertebral centra with well-marked\nelliptical lateral fossae that occupy more than half the lateral surfaces”, is quite clear and\ninteresting, since Aerosteon middle caudal vertebra has lateral fossa occupy almost half the\ncentrum lateral surface (Aranciaga Rolando et al. 2021). However, you may need to compare\nwith the Brazilian caudal vertebrae MPMA 08-003-94 (Mendez et al. 2012) and CPPLIP 1324\n(Martinelli et al. 2013), whether they exhibit this feature or not. Thus, this character is unique to\nthe new taxon or not.\nResponse: As requested, we have compared this proposed autapomorphy of Joaquinraptor to\nthe conditions in the isolated vertebral centra MPMA 08-003-94 and CPPLIP 1324 from the Late\nCretaceous of Brazil, which have been proposed to belong to Megaraptora by Méndez et al.\n(2012) and Martinelli et al. (2013), respectively. However, it is difficult to precisely determine the\nserial positions of these Brazilian vertebrae relative to the three preserved caudal vertebrae of\nJoaquinraptor. We interpret those of Joaquinraptor to correspond to posteriormost middle or\nanteriormost posterior caudals; whether MPMA 08-003-94 and/or CPPLIP 1324 also belong to\nthis region of the caudal series—and therefore whether they are directly anatomically\ncomparable to the preserved caudals of Joaquinraptor—is, again, challenging to ascertain. (In\nthis context, it is worth mentioning that Motta et al. [2016] suggested that these two Brazilian\nvertebrae may instead pertain to the sacral rather than the caudal series.) That said, and\nirrespective of their exact serial positions, based on the figures published by Méndez et al.\n(2012) and Martinelli et al. (2013), the lateral fossae of MPMA 08-003-94 and CPPLIP 1324,\nthough deep and well-defined, appear to be anteroposteriorly (and dorsoventrally) less\nextensive than those of Joaquinraptor (i.e., in both these Brazilian vertebrae—which, again,\nmay potentially be sacrals rather than caudals—the lateral fossa occupies less than half the\nlength of the lateral surface of the centrum).\nIn the case of the large-bodied megaraptorid Aerosteon riocoloradensis from Mendoza\nProvince, Argentina, based on the proportions of the middle caudal vertebra in question (e.g.,\ncentrum anteroposterior length versus dorsoventral height), this element is very probably\nsubstantially more anterior in position than the preserved caudals of Joaquinraptor. Regardless,\nthe lateral fossae of this Aerosteon vertebra are less anteroposteriorly extensive than\nthose of Joaquinraptor, occupying less than half the lateral surface of the centrum (see, for\nexample, Aranciaga Rolando et al., 2022b:fig. 33).\nIn sum, although future discoveries of more complete megaraptorid caudal vertebral sequences\ncould theoretically prompt a reassessment of this feature, given current evidence, we maintain\nthe inclusion of “middle or posterior caudal vertebral centra with well-marked elliptical\nlateral fossae that occupy more than half the lateral surfaces” as an autapomorphy of\nJoaquinraptor.\n4.4) autapomorphy number 7, the “ulna with proximodistally enlarged olecranon process” is\nprobably the same as in Magaraptor (MCF-PVPH70 and MUCPv 341) (based on supp. Fig. 13\nin the MS). Please check.\nResponse: We appreciate the reviewer’s observation here, but we respectfully disagree.\nRelative to the overall length of the ulna, the olecranon process of Joaquinraptor is more\nproximodistally elongate than that of all known specimens of Megaraptor namunhuaiquii,\nAustralovenator wintonensis, and every other presently recognized megaraptoran taxon.\nThis becomes clear when one conducts first-hand examinations of the ulnae in question, as well\nas when one calculates the proportional proximodistal length of the olecranon process\n(considered to comprise the entire region of the ulna proximal to the anterior [= cranial or\ncoronoid of some authors] process; see the horizontal line in our Suppl. Fig. 6a) relative to that\nof the bone as a whole. In Joaquinraptor, the ulna is 271 mm in overall proximodistal length,\nwhereas the olecranon process is 55 mm, yielding a ratio of 20.3% (see Suppl. Fig. 6a).\nUnfortunately, measurements of the length of the olecranon process are rare in the\nmegaraptorid literature, so, to demonstrate the uniqueness of the condition in the new\nPatagonian megaraptorid, we had to estimate the proportional length of the olecranon process\nin various megaraptoran specimens using published images. Doing so using our Suppl. Fig. 13\nyielded the following estimated ratios of olecranon process length to total ulna length for various\nmegaraptoran taxa and specimens:\nTaxon Specimen Holotype? Ratio (approximate)\nFukuiraptor kitadaniensis FPDM 9712210 Yes 11.5%\nMegaraptoridae indet. NMV P186076 N/A 12.7%\nMegaraptoridae indet. LRF 100–106 N/A ? (not estimated due to incompleteness)\nAustralovenator wintonensis AODF 604 Yes 18.8%\nMegaraptor namunhuaiquii MCF-PVPH 79 Yes 14.0%\nMegaraptor namunhuaiquii MUCPv 341 No 15.3%\nJoaquinraptor casali UNPSJB-PV 1112 Yes 20.3%\n(The marked-up PDF version of our Suppl. Fig. 13 from which these ratios were estimated is\navailable upon request.)\nAs such, although, at an estimated 18.8%, the holotype of Australovenator comes close, at\n20.3% the total length of the ulna, the olecranon process of Joaquinraptor is indeed\nproportionally longer relative to the overall length of the bone than that of any other\nmegaraptoran. As such, we prefer to maintain “ulna with proximodistally enlarged olecranon\nprocess” as a hypothesized autapomorphy of the new taxon. If the editors and/or reviewers\nwould mandate this, however, we could make this more specific by altering it to “ulna with\nolecranon process that is 20% the proximodistal length of the bone.”\nIs the enlarged olecranon process also present in Australovenator? But maybe it is due to a\ndifferent angle (the ulna shaft in the Australovenator is slightly anteriorly curved)? Please check.\nResponse: As explained in our previous response, the olecranon process of Joaquinraptor\n(measuring from the proximodistal level of the anterior process of the ulna to the proximal end of\nthe olecranon process [and, therefore, the element as a whole]) is proportionally longer\nrelative to the remainder of the bone than that of Australovenator, Megaraptor, and every\nother known megaraptoran (although, admittedly, it is only slightly longer than that of\nAustralovenator).\nOther autapomorphies are quite clear.\nResponse: We appreciate the reviewer’s comment here, and naturally, we agree.\nAs a related aside, it is important to mention that the discovery of new fossils can sometimes\nalter the status of what were previously considered autapomorphies of a given species or genus\ninto synapomorphies of a higher-level clade. This is particularly true for groups whose fossil\nrecord is depauperate, as is presently the case for Megaraptora. As such, we acknowledge that\nfuture discoveries may ‘transform’ some of the proposed autapomorphies of Joaquinraptor into\nsynapomorphies of Megaraptora or, more probably, clades within it.\n5) The maxilla of the new megaraptorid is subtriangular, low, and elongated, somewhat similar\nto, but also different from, the unenlagiine Austroraptor (from neighboring province, which also\nstated in the introduction that unenlagiines are dominated in this area). So, a short paragraph on\nthe comparison/ differentiation of their maxillae might be good for clarifying that they are not the\nsame taxon/ clade.\nResponse: The general similarities of known skull elements of Joaquinraptor to those of the\nlarge-bodied, penecontemporaneous unenlagiine maniraptoran Austroraptor cabazai are indeed\ninteresting, and are something that we hadn’t extensively considered before. As the reviewer\npoints out, both Joaquinraptor (and the juvenile specimen of Megaraptor MUCPv 595) share\nwith Austroraptor (and, probably, the large Brazilian unenlagiine Ypupiara lopai) a proportionally\nlow, elongate, and subtriangular maxilla that (we would add) houses low-crowned teeth. Though\nthis is, at this point, speculative given our incomplete understanding of the remainder of the\nskeletons of these theropods, perhaps this hints at some degree of ecomorphological\nconvergence between the two forms, which might also explain why they seem to occur in\ndifferent areas of South America during the latest Cretaceous (large-bodied unenlagiines in\nBrazil and northern Patagonia, megaraptorids in central and southern Patagonia). (In other\nwords, and although this is necessarily very tentative, perhaps large unenlagiines and\nmegaraptorids occupied similar ecological niches and/or exploited similar resources, but in\ndifferent geographic areas.)\nThat said, however, although the maxillae of Joaquinraptor and Austroraptor are similar in some\nways, they exhibit significant differences as well. For instance, the antorbital region is more\nelongate in the unenlagiine, with the antorbital fossa being extremely attenuated, tapering\nanteriorly to an acute point. Moreover, the maxilla of Austroraptor contains 24 teeth (Novas et\nal., 2008b); the maxillary tooth count of Joaquinraptor cannot be definitively determined but it\nwas certainly substantially lower (approximately 15–17 based on the preserved morphology and\ncomparisons to the better-preserved right maxilla of MUCPv 595; Porfiri et al., 2014).\nIn sum, and following the reviewer’s suggestion, we have added text to the relevant part of\nour osteological description section to briefly compare the maxillae of Joaquinraptor and\nAustroraptor (see tracked changes in the attached revision). If the editors and/or reviewers\nwould like, we could add further information, although given the already considerable length of\nour manuscript it may not be advisable to significantly increase the amount of text here.\n6) Focusing on the maxilla interdental plates of Joaquinraptor vs Megaraptor and the dentary\ninterdental plates of Joaquinraptor vs Australovenator vs Fukuiraptor. How are they different or\nsimilar from each other? Please provide information. Is there any pattern or distribution of this\ncharacter in Megaraptora?\nResponse: Following the reviewer’s suggestion, we have added text (to the appropriate parts\nof the description) comparing the maxillary interdental plates of Joaquinraptor to those of\nFukuiraptor kitadaniensis and Megaraptor and the dentary interdental plates of the new taxon to\nthose of Fukuiraptor and Australovenator. In short, at least as far as can be ascertained, the\ninterdental plates of Joaquinraptor and Megaraptor are unfused, whereas those of Fukuiraptor\nand Australovenator seem to be fused. As such, and although this is unresolved at present,\nunfused interdental plates could conceivably represent a synapomorphy of South American\nmegaraptorids or a subset thereof.\n7) The scapula blade ratio: is it different from that of Megaraptor and Aerosteon and Aerosteon? Compared with\nGualicho (a probable carcharodontosaur closely related to Megaraptora or an elaphrosaurine)\nwhich also exhibits fused scapulocoracoid, how are they different or related?\nResponse: We have searched the non-avian theropod literature, and unfortunately, we cannot\nbe absolutely sure what the reviewer is referring to by “scapula blade ratio.” Do they\npotentially mean the anteroposterior (= craniocaudal or ‘proximodistal’) length of the scapular\nblade divided by its minimum dorsoventral depth? If so, this is encapsulated by character 119 of\nour phylogenetic analysis: “Scapula, blade length compared with minimum width of scapula\n(Forster, 1999): less than 7.5 (0); more than 7.5 (1).” Joaquinraptor exhibits state 0, i.e., less\nthan 7.5, as do Megaraptor and Aerosteon. Gualicho shinyae, by contrast, exhibits state 1,\nmeaning that this enigmatic northern Patagonian theropod has a scapular blade that is\ndorsoventrally narrower relative to its length than those of unquestioned megaraptorids. We\nhave added a sentence to this effect at the appropriate point of our osteological description.\n8) Humerus deltopectoral crest: from supp. Fig. 5C, the deltopectoral crest looks somewhat\ntrapezoidal-shaped as in other megaraptorids (instead of subrectangular) (Supp. fig. 12) (please\ncheck). But I agree that it is more robust in the new megaraptorid.\nResponse: As mentioned above, the primary objective of our Suppl. Figs. 8–16 is to compare\nthe overall appearance of various skeletal elements within Megaraptora (and in the case of\nSuppl. Fig. 12, specifically the humerus). In our view, the deltopectoral crest of Joaquinraptor is\nindeed subrectangular (as opposed to more trapezoidal) in shape, and that this morphology\nmay be readily observed in, among other figures, Suppl. Fig. 5C. As stated in our main text,\n“Unlike in Australovenator, Fukuiraptor, and Megaraptor, the deltopectoral crest is\nsubrectangular in lateral view (Supplementary Fig. 5). Although, as in these taxa, the crest\nprojects prominently anteriorly and tapers proximally and distally, these features are less\nmarked in Joaquinraptor, with the exception of the juvenile Megaraptor (MUCPv 595) in which\nthe deltopectoral crest is small...” We therefore maintain “humerus with subrectangular\ndeltopectoral crest” as a proposed autapomorphy of Joaquinraptor, though we will attempt\nto describe this same morphology in a different way (e.g., “anterodistal corner of humeral\ndeltopectoral crest pointed instead of rounded in lateral view”) if the reviewers and/or editors\ndeem this necessary.\n9) Even though the new taxon clearly belongs to Megaraptoridae, some material could be\ncompared with other theropod clades, such as carcharodontosaurs, basal coelurosaurs,\ntyrannosauroids, and some problematic such as Chilantaisaurus and Gualicho, for more\nunderstanding of the evolution of Megaraptora.\nResponse: We agree that the Joaquinraptor fossils could (and potentially should) be compared\nwith the corresponding skeletal elements in selected non-megaraptoran theropods, such as\nthose mentioned by Reviewer 1, and indeed, we do already provide a few such comparisons.\nThat said, given the limited number of words that Nature Communications allows within the\nmain text, we chose to focus such comparisons on other indisputable and probable members of\nMegaraptora, in order to provide a detailed assessment of the morphology of Joaquinraptor\nrelative to other representatives of this poorly known clade, and, in so doing and in that sense,\nincrease understanding of its evolution. As such, we would prefer to save extensive\ncomparisons of Joaquinraptor to non-megaraptoran theropods for a future, more\ncomprehensive osteology of the new megaraptorid.\n10) Line 3939-396 “Patagonian megaraptorids present a suite of putative synapomorphies, such\nas an elongated rostrum and a considerably enlarged manus, that suggest that these theropods\nmay have been evolutionarily and geographically differentiated from earlier diverging members\nof the group (e.g., Australovenator).” – The elongated rostrum (at least dentary) and enlarged\nmanus are present in Australovenator (compare measurement of each element in\nAustralovenator and Joaquinraptor). So, this statement might be incorrect, or at least, must\ninclude Australovenator.\nResponse: Although the reviewer is potentially correct in suggesting that the rostrum of\nAustralovenator may have been as proportionately elongated as is that of its South American\ncousins (this is difficult to ascertain given that the only skull element yet known for\nAustralovenator is the dentary), the manus, or at least portions thereof, was undoubtedly\nenlarged in Patagonian megaraptorids relative to, for example, the ulna (e.g., in the Megaraptor\nholotype MCF-PVPH 79, the ulna measures 332 mm and manual ungual I measures 339 mm in\nproximodistal length [Novas, 1998]; these same measurements are 267 and 187 mm,\nrespectively, in the Australovenator type specimen AODF 604 [White et al., 2012]). As such, we\nmaintain that South American megaraptorids exhibit synapomorphies that suggest that\nthese animals may have been phylogenetically and geographically differentiated from\ntheir mostly more stratigraphically ancient Australian relatives. We have rephrased the\nsentence as follows: “Although more evidence is needed, Patagonian megaraptorids present a\nsuite of putative synapomorphies, such as the absence of mesial denticles from the tooth\ncrowns, a manual ungual I that is subequal in length to the ulna, and a well-developed\n‘mediolateral platform’ on the flexor tubercle of this ungual… that suggest that these theropods\nmay have been evolutionarily and geographically differentiated from earlier diverging members\nof the group (e.g., Australovenator).” We will, however, simply delete the sentence if the editors\nor reviewers mandate this.\n11) Why Joaquinraptor is not the same taxon as Maip or Tratayenia? It was stated in the MS\nthat the coracoids and ribs of Joaquinraptor resemble those of Maip. If not much overlapped\nmaterial can be compared. Even though the phylogenetic analysis groups them into a single\nclade, the different positions on the resulting tree(s) may be due to the artifact of, or biased by,\nnon-overlapped material. Please explain or state the reason or give evidence (apart from the\nfact that they are from different provinces and rock strata) that they might be, or not be, the\nsame taxon.\nResponse: Here, we will begin by reiterating the fact that, as the reviewer pointed out,\nJoaquinraptor, Maip macrothorax, and Tratayenia rosalesi all come from different\nArgentinian provinces, regions of Patagonia, and rock formations (exposures of which are\nmany hundreds of kilometres apart from one another). They also come from different basins\nand, most importantly, Tratayenia is certainly (much) geologically older than Joaquinraptor,\nand Maip probably is as well. The geographic and stratigraphic provenance of each species is\nsummarized in the table below (data for Maip and Tratayenia drawn from Aranciaga Rolando et\nal., 2022a and Porfiri et al., 2018, respectively).\nTaxon Region Basin Province Distance of Formation Age Temporal\n(Patagonia) (Argentina) type locality distance from\nfrom Joaquinraptor\nJoaquinraptor\nJoaquinraptor Central Golfo San Chubut N/A Lago Maastrichtian N/A\ncasali Jorge Colhué (middle or,\nHuapi more probably,\nlate)\nMaip Southern Austral- Santa Cruz ~600 km Chorrillo Maastrichtian Uncertain, but\nmacrothorax Magallanes (unspecified) probably 2–6\nMa (older)\nTratayenia Northern Neuquén Neuquén ~750 km Bajo de la Santonian 15–19 Ma\nrosalesi Carpa (older)\nAs such, despite the fact that, with the exception of dorsal ribs (one complete rib and many\nfragments of others in Joaquinraptor, two fragments in Tratayenia), there is no osteological\noverlap among definitive specimens of Joaquinraptor and Tratayenia, it is a virtual certainty\nthat the two forms cannot represent the same taxon on the basis of geographic and\nespecially stratigraphic and temporal distance (i.e., at least 15 million years apart).\nThe stratigraphic and temporal correspondence of Joaquinraptor and Maip is much closer,\nthough also very probably not the same. Both these megaraptorids likely date to the\nMaastrichtian, the final stage of the Cretaceous; within that stage, Joaquinraptor is at least\nmiddle and more probably late Maastrichtian. The age of Maip is less clear, with most sources\n(e.g., Aranciaga Rolando et al., 2022a) listing it as simply Maastrichtian without further\nspecification, which is unfortunate because the Maastrichtian is 6.1 million years in length\n(72.1–66.0 Ma; see Walker and Geissman, 2022). Some recent publications (e.g., Novas et al.,\n2019) even list the age of the Chorrillo Formation—the unit that yielded the Maip holotype—as\npotentially extending into the late Campanian, i.e., late Campanian–early Maastrichtian. As\nsuch, Maip is probably a few million years older than Joaquinraptor, although this cannot\npresently be stated with absolute certainty (see also our response to Reviewer 2’s first point\nbelow).\nFortunately, there is slightly more anatomical overlap between the holotypic specimens of\nJoaquinraptor and Maip than there is between the former taxon and Tratayenia. Both\nJoaquinraptor and Maip preserve dorsal ribs and the coracoid. As pointed out by Reviewer 2\n(see below), the preserved dorsal ribs of Maip (identified as the first, second, and sixth; see\nAranciaga Rolando et al., 2022a) have features that are not present in the preserved dorsal ribs\nof Joaquinraptor, namely multiple pneumatopores and a prominent, subtriangular medial flange\non the proximal half of dorsal rib 1 and seemingly more robust shafts in all preserved ribs.\nNevertheless, the absence of these features in Joaquinraptor could conceivably be due to\npotential discrepancies in serial position of the dorsal ribs preserved.\nWith regard to the coracoids of Joaquinraptor and Maip, according to Aranciaga Rolando et al.\n(2022a), the coracoid of the latter genus exhibits several diagnostic features, with hypothesized\nautapomorphies indicated with an asterisk:\n“… (4) prominent anterior projection on the coracoid*; (5) coracoid without a subglenoid ridge*;\n(6) coracoid without a posteroventral fossa; and (7) and coracoid with ventromedial margin forming a\ndorsoventrally deep articular surface for the sternum*.”\nUnfortunately, the anterior margin of the coracoid of Joaquinraptor is broken away, so it is\nimpossible to determine whether characters (4) and (7) are present in the new taxon. That said,\ncharacters (5) and (6) appear to be absent in Joaquinraptor (including a hypothesized\nautapomorphy, character [5]), in that the coracoid of this taxon possesses a subglenoid ridge\nand a posteroventral fossa. Please see the comparative image of the Joaquinraptor and Maip\ncoracoids in ventral or posterior (depending on how one orients this bone) view below:\nMoreover, the posteroventral process of the Joaquinraptor coracoid is more strongly hooked\nthan that of Maip, as illustrated by the comparative image below:\nIn sum, based on the morphologies of the preserved bones, detailed anatomical comparisons to\nother megaraptorans, and the results of our phylogenetic analysis, we consider Joaquinraptor\ncasali to be well supported as a new genus and species of megaraptorid. Despite the near-\ntotal absence of overlapping bones, Joaquinraptor differs from Tratayenia in being very\nsubstantially (i.e., at least 15 million years) geologically younger. The new taxon is also probably\nstratigraphically younger than Maip, and moreover the coracoids and dorsal ribs of the two\nforms exhibit multiple differences. We have added text to the diagnosis and descriptive\nsections of our manuscript to highlight the morphological discrepancies between\nJoaquinraptor and Maip.\n12) Line 453-454 You stated that crocodyliforms could be one of the prey items for\nmegaraptorids. Could it be that these two animals just fight each other, like competition for food\nwhile scavenging some carcass? Or, the other way around, the croc might come to eat that\n‘dying’ megaraptorid, and they fought and got stuck in the mud. How or what concrete evidence\ndo you have to conclude this?\nResponse: In our view, the most likely explanation for this intriguing fossil association is\nthat the holotypic Joaquinraptor individual was in the process of feeding on (i.e., either\nscavenging a carcass or eating a prey item it had killed) the crocodyliform when the theropod\nperished. After all, the crocodyliform humerus was found between the near-articulated dentaries\nof the megaraptorid, with some in situ tooth crowns of the theropod actually in contact with the\nhumeral shaft. However, following Reviewer 1’s comments, we have modified our text to\nhighlight other potential explanations for this association (see the Track Changes version of our\nrevised manuscript).\nAdditionally, it is important to note that a detailed taphonomic study of the Joaquinraptor quarry\nand the fossils preserved within it is currently underway. Among other aspects, this study may\npotentially support or refute whether the presence of this crocodyliform humerus at the site\nconstitutes evidence of dietary preferences in Megaraptoridae. (Interestingly, and perhaps not\ncoincidentally, Fiorelli [2010] described a peirosaurid crocodyliform with probable theropod bite\nmarks from the Bajo de la Carpa Formation of northwestern Patagonia. The largest known\ntheropod in the Bajo de la Carpa palaeoecosystem is the megaraptorid Tratayenia [Porfiri et al.,\n2018].)\n13) Fig. 4. and Line 397 onward – Why was Megaraptoridae absent in the north of Golfo San\nJorge Basin? Apart from the paleoenvironment, is there any evidence of island biogeography in\nthis area? For example, was Austral-Magallanes an island, that why megaraptorids were here\nbut not in the north of Golfo San Jorge Basin? If so, had the island influent to the\ngigantism in Megaraptoridae?\nResponse: The reviewer raises an interesting point here; we had not sufficiently considered the\npotential role that the latest Cretaceous palaeogeography of southern South America might\nhave played in the apparent provincialism of large-bodied theropods observed in central and\nsouthern Patagonia (where megaraptorids appear to have been the apex carnivores) on one\nhand and more northerly areas (in which abelisaurids and large unenlagiines seem to have filled\nthis niche) on the other. Interestingly, a recent palaeogeographic reconstruction of the region\nduring the Campanian (see Reguero and Goin, 2021:fig. 8) shows the depositional areas of the\nGolfo San Jorge and Austral-Magallanes basins (i.e., central and southern Patagonia) as being\nlargely isolated from more northern areas by shallow seas and deltaic palaeoenvironments.\nAlongside palaeoenvironmental and/or palaeoecological differences (see, e.g., the map of\nproposed palynological bioprovinces in Vajda and Bercovici, 2014:fig. 3b), this could indeed\nhave been a potential catalyst for the differentiation of continental vertebrate faunas, including\nnon-avian theropods. We have accordingly added text to this effect at the appropriate\npoint in our manuscript and citations of Reguero and Goin (2021) and Vajda and Bercovici\n(2014) to our bibliography.\n14) Line 39 - a member of the derived megaraptoran[s] subclade Megaraptoridae. (add ‘s’)\nResponse: The phrase “derived megaraptoran subclade” is actually correct here (certainly more\ncorrect than “derived megaraptorans subclade”) so we have not made this change.\nReviewer #2 (Remarks to the Author):\nPlease see attached pdf\nResponse: We are grateful to Reviewer 2 for taking the time to review our manuscript, and for\ntheir helpful comments.\nI think that \"Latest\" should be replaced by \"Late\", because Maip comes also from lae\nMaastrichtian beds from Chorrillo Formation....\nResponse: According to our review of the recent, relevant literature (e.g., Novas et al., 2019,\n2024; Ghiglione et al., 2021; Aranciaga Rolando et al., 2022a; Moyano-Paz et al., 2022;\nPerez Loinaze et al., 2023; Novas pers. comm. to MCL, 2024), the Chorrillo Formation of southern\nPatagonia, Argentina—the geologic unit that yielded the exceptionally large-bodied\nmegaraptorid Maip macrothorax—is widely considered to be late Campanian–early\nMaastrichtian, early Maastrichtian, or simply Maastrichtian in age (the latter without further\nspecification, which is unfortunate because the Maastrichtian is currently thought to be 6.1\nmillion years in length [i.e., 72.1–66.0 Ma; Walker and Geissman, 2022]).\n(For completeness’ sake, a paper published very recently, subsequent to our original\nsubmission, by Perez Loinaze et al. [2025] states that “… palynological study of samples the\nupper levels of [the Chorrillo Formation] supports an upper Maastrichtian age (not reaching the\nK/Pg boundary)” and also “an upper Maastrichtian age for the Chorrillo Formation is proposed\nsupported by its palynological content.” As such, it is unclear whether Perez Loinaze et al.\n[2025] are arguing that the entire Chorrillo Formation dates to the late Maastrichtian, or whether\nonly its uppermost levels are of this age. This is an important distinction to make because the\noldest/lowest Chorrillo horizons that Perez Loinaze et al. [2025] sampled are at roughly the 440\nm level of the formation, whereas the stratum that yielded the Maip holotype is much lower in\nsection, ca. 170 m [see Perez Loinaze et al., 2025:fig. 2]. Consequently, Maip came from a\nstratigraphic horizon that is 270 m below the oldest “upper Maastrichtian” level sampled by\nPerez Loinaze et al. [2025]. How much time is represented by this 270 m span remains\nunknown, but based on the conclusions of other recent authors [e.g., Ghiglione et al., 2021;\nMoyano-Paz et al., 2022; Novas et al., 2024] we consider that the Chorrillo Formation\nstratum that yielded Maip is most likely early Maastrichtian in age [or simply\n“Maastrichtian,” if one prefers to be more conservative and less specific, as did Perez Loinaze\net al. [2023] in their earlier paper on the palynology and age of this unit].)\nThe age interpreted for the stratum from which Joaquinraptor casali was recovered is, at the\nvery least, middle Maastrichtian, and moreover, this horizon is probably situated near the top of\nthe Lago Colhué Huapi Formation, for which a late Maastrichtian age is well supported.\nTherefore, Maip is probably a few (ca. 2–6) million years older than Joaquinraptor (see\nalso our response to Reviewer 1’s point 11 above). As such, we have chosen to retain\n“Latest” within our manuscript’s title. However, we will gladly change this word to “Late” if\nthe editors and/or reviewers insist that we do so, as this revised title would remain accurate.\nYou should include here citations of previous works!\nResponse: Yes, we can certainly do so. We have added citations of older literature here, as\nfollows: Novas, 1998; Azuma and Currie, 2000; Calvo et al., 2004; Sereno et al., 2008; Hocknull\net al., 2009; and Porfiri et al., 2014. There are many other works that we could cite here, too, if\nthe editors and/or reviewers would mandate this.\nThese features are already present in other megaraptorids, particularly Megaraptor. I think\nshould be excluded from the diagnosis...\nResponse: This comment refers to the following features (numbered as in our diagnosis of\nJoaquinraptor casali): ulna with (7) proximodistally enlarged olecranon process, (8) vertical ridge\non medial surface of proximal end, [and] (9) nearly straight shaft (but see below). Though we\nappreciate Reviewer 2’s attention to detail here, we respectfully disagree with most of their\npoints. Again, the proximodistally enlarged olecranon process of the ulna is in fact unique to\nJoaquinraptor, at least in terms of the degree to which it is developed proximal to the anterior\nand lateral processes (see our response to Reviewer 1’s query 4.4 above). As for the vertical\nridge on the medial surface of the proximal end, to our knowledge this too is unique to\nJoaquinraptor and not found in other megaraptoran ulnae (see Suppl. Fig. 6). Lastly, as we point\nout, an ulna with a straight shaft is indeed found in Megaraptor namunhuaiquii, but only in the\nholotype of this species, MCF-PVPH-79. The ulna of the referred specimen MUCPv 341 is\nnoticeably curved, which suggests that either MUCPv 341 is not referable to M. namunhuaiquii\nor that this character varies between individuals of the same taxon. As such, we have elected\nto retain characters (7)–(9) as proposed diagnostic features (specifically,\nautapomorphies) of Joaquinraptor.\nHere you should specify features that distinguish the new taxon from the coeval Maip. If you do\nnot find distinctive characters (in the case of Maip there are some bumps in the ribs, which are\nabsent in your taxon), you should make this explicit....\nResponse: As noted above (see our response to Reviewer 1’s point 11), it is difficult to\ncompare Joaquinraptor to Maip due to the incompleteness of known specimens of both taxa,\nbut especially the latter. That said, as Reviewer 2 points out, the dorsal ribs of the two taxa differ\nin that the shaft of at least the first dorsal rib of Maip possesses a large flange or tubercle\nwhereas the ribs of Joaquinraptor do not. That said, however, this difference could be due to a\nnon-equivalence in the serial positions of the preserved ribs of these genera rather than\nconstituting legitimate evidence of their taxonomic distinction. More importantly, though, the\ncoracoids of Joaquinraptor and Maip differ in several respects, namely the presence of a hook-\nlike posteroventral process and a subglenoid ridge and posteroventral fossa in the former taxon\n(see also above). Again, we have added text to better differentiate the two taxa in our\nmanuscript.\n(Incidentally, and as also noted above, Maip and Joaquinraptor probably aren’t “coeval”; rather,\nthe former is most likely early Maastrichtian in age whereas the latter is probably late\nMaastrichtian. Given that the Maastrichtian extended for 6.1 million years [72.1–66.0 Ma per\nWalker and Geissman, 2022], the temporal distance between the two forms is likely very\nconsiderable [ca. 2 Ma at minimum].)\n[Delete “local”]\nResponse: We don’t see any issue with including the word “local” in this context, but we have\ndeleted it in accordance with Reviewer 2’s preference.\nThis bone is really different from that of other megaraptorids (and several theropods known to\nme). Are you sure that is a postorbital? I think that if you are not certain, it can be safely\nexcluded from the description...\nResponse: We will concede that we have debated the identity of this bone ever since it was\ndiscovered among the skeletal elements from the Valle Joaquín locality. Indeed, of all the bones\npreserved in the quarry, this was perhaps the most difficult to identify, in part because it is\nincompletely preserved but also because, whatever it is, it appears to be of somewhat peculiar\nmorphology. Accordingly, in our text, we make clear that our identification of this bone as a\npostorbital is tentative (e.g., “The bone tentatively interpreted as the right postorbital...”). That\nsaid, however, based on our comparisons of this bone to cranial elements of other megaraptorid\ntaxa, particularly Aerosteon riocoloradensis (Sereno et al., 2008; Aranciaga Rolando et al.,\n2022b), Murusraptor barrosaensis (Coria and Currie, 2016), and Orkoraptor burkei (Novas et\nal., 2008a), we have maintained the provisional identification of this bone as a postorbital.\nI am not sure, but a work of Aranciaga Rolando et al. also discuss the pneumaticity in\nmegaraptorids (Aranciaga Rolando, M., Garcia Marsà, J., & Novas, F. (2020). Histology and\npneumaticity of Aoniraptor libertatem (Dinosauria, Theropoda), an enigmatic mid‐sized\nmegaraptoran from Patagonia. Journal of Anatomy, 237(4), 741-756.).\nResponse: Yes, the reviewer is correct, the cited paper by Aranciaga Rolando et al. (2020)\nexplores vertebral pneumaticity in the fragmentary and enigmatic Patagonian theropod\nAoniraptor libertatem (see also our Supplementary Information), which, contrary to what\nReviewer 2 states, is most frequently regarded as a non-megaraptorid megaraptoran as\nopposed to a megaraptorid proper (see, e.g., Motta et al., 2016; Aranciaga Rolando et al.,\n2020). Regardless, following the suggestion of Reviewer 3 below, we have deleted the\nparagraph in question, rendering Reviewer 2’s comment here moot.\nReviewer #3 (Remarks to the Author):\nDear authors,\nI have reviewed the manuscript by Dr. Ibiricu and colleagues. The manuscript describes a partial\nskeleton of a megaraptoran theropod dinosaur, which is substantially more complete than\nknown megaraptorans. Accordingly, the insights from the specimen provide new light on the\nanatomy of megaraptoran dinosaurs, and the geological context of the fossil extends the\nrecord of this clade into the latest Cretaceous.\nThe manuscript is high quality, the figures are excellent, and the writing is clear and succinct. I\nhave no major qualms with the research conclusions or methodologies. I feel that because of\nthe rarity and charisma of this clade, the manuscript will garner significant media attention.\nScientifically, this manuscript has a high likelihood of becoming well-cited, as its detailed\nanatomical notes will be important for future work on these animals and their relatives.\nResponse: We appreciate Reviewer 3’s (Greg Funston’s) encouraging words here, and as with\nthe other two reviewers, we are grateful for the time he took to review our work, as well as his\nhelpful comments.\nI have a few minor suggestions for improvement, which are generally optional and at minimum\ninvolve only small changes to the text.\nIn the section on phylogenetics, it is argued that the position of Joaquinraptor in the tree\nsupports a tyrannosauroid affinity for megaraptorans (over an allosauroid one). This is presently\none of the major debates about these animals, and so I suspect this statement may draw some\nflak because the specimen was included in a matrix which already found megaraptorans as\ntyrannosauroid relatives. To me, the new analyses with this taxon do not necessarily improve\nthe support for the case, but they show that this new specimen is consistent with the hypothesis.\nThis is as we would expect: a late-branching form is unlikely to produce major changes in root\nnodes, but this likewise means that its addition is not a very strong test of the former hypothesis.\nIf the authors choose, I feel that this potential weakness could be strengthened in one (or all) of\nthree ways:\neither 1) a brief statement and revision to the wording explaining this debate in a little more\ndetail in the phylogenetic section, and explaining why this particular matrix was chosen when\nothers exist. It would be important here to mention that this phylogenetic approach is not\nintended to test the position of megaraptorans within the tree, but of Joaquinraptor within\nMegaraptora.\n2) a broader discussion of the tyrannosauroid-like features of the anatomy of Joaquinraptor,\nwhich might further strengthen the tyrannosauroid argument with characters outside of the\nphylogenetic matrix (some of these characters might also be worth adding to the matrix, either\nhere or in future work);\nor 3) adding Joaquinraptor to a matrix that finds megaraptorans as allosauroids, to test whether\nits inclusion in that matrix changes patterns.\nResponse: As Dr. Funston surmised, the primary aim of our phylogenetic analysis was to\nresolve the systematic position of Joaquinraptor within Megaraptora, as opposed to\nevaluating the higher-level affinities of Megaraptora among Tetanurae. With that goal in mind,\nwe chose a phylogenetic data matrix that we felt was best suited to that purpose, namely the\nmatrix that includes (to our knowledge) the most comprehensive sample of megaraptoran\nspecies and specimens of any published dataset. We have added language to clarify this\npoint; in other words, we have opted to follow his first recommendation above.\nAs a related aside, we agree that the higher-level phylogenetic affinities of Megaraptora\nunquestionably merit further investigation, as do the interrelationships of the members of the\nclade itself. With that in mind, some of us are currently involved in the description of new\nPatagonian megaraptorid fossils that will also include a revised phylogenetic analysis of\nmegaraptoran affinities. Because that work (1) is being led by one of the present coauthors (as\nopposed to the first author) and (2) entails a great deal of additional effort, we would prefer to\nsave it for a future publication.\nMy only other substantial comment regards the palaeohistological interpretations, and I feel that\nthese are a bit less optional. These are in two veins:\n1) Age estimation should be done from the number of cyclical growth marks, which themselves\ncan comprise multiple LAGs. In this case, it appears that you have double and triple LAGs, and\nthe current interpretation of those features is that they together represent a single annual growth\nmark (see Cullen et al. 2021 in Biology Letters). Anywhere that LAGs are paired closely\ntogether, but this group is further away from other groups, they should be considered as being\nformed in the same year and belonging to the same CGM. Revising these counts will not\nchange your overall interpretations of the maturity estimation, but they will put a younger age on\nthe specimen. In some ways, this might add more support to a coelurosaur identity, as\ndifferences in growth patterns between tyrannosauroids and carcharodontosaurids are known\n(see another Cullen paper: Cullen et al. 2020 in Proc B).\nResponse: The counting of growth marks and the establishment of minimum ages is always a\nproblematic issue, even more so in light of recent studies of extant taxa that show that the\nnumber of Lines of Arrested Growth (LAGs) does not always represent the actual age of the\nindividual (e.g., Schucht et al., 2021). Therefore, the age estimation is necessarily tentative. In\naccordance with what was stated by the reviewer, we have considered the multiple LAGs as\nrepresentative of a single growth cycle. For example, by enlarging our Suppl. Fig. 19D, the\nreviewer will see that the arrowheads that indicate the position of LAGs 5, 7, and 9 (counting\nfrom the perimedullary cortex to the outer cortex) are indeed double LAGs that have been\nconsidered as a single growth cycle. These particular LAGs are very close together (around 40\nmicrons or less) and resemble other double LAGs described in extant species. The reviewer\nindicates that other groups of LAGs should also be considered as multiple LAGs, which would\nreduce the estimated minimum age of the Joaquinraptor individual. Unfortunately, the criterion\nproposed by Cullen et al. (2020) to determine whether or not they are multiple LAGs cannot be\napplied here, since we do not have a complete section that allows us to determine whether the\ngroups actually originate from the split of a single line. Thus, since there is no criterion that\nallows us to objectively determine whether the groups of LAGs indicated by the reviewer are\nLAGs formed in different growth cycles (single LAGs) or LAGs formed in a single growth cycle\n(i.e., multiple LAGs), we prefer to maintain the original age estimate.\n2) I am not convinced that the preservation of the ICL in the rib is good enough to make many\ncomments on pneumosteon, and these, to me, are a weak part of the paper that doesn’t\ncontribute to the major conclusions. I would at the very least consider taking them out of the\nmain text, and if you wish to keep them, have them exclusively in the supplement.\nResponse: Following the reviewer’s suggestion, we have deleted this part of the discussion\n(and the citation of Lambertz et al., 2018) from the main text.\nI am happy to be contacted by the authors, should they have any questions about my\nsuggestions.\nSincerely,\nGreg Funston,\nVisiting Assistant Professor\nUniversity of California, Davis\ngfunston@ucdavis.edu\nResponse: We are grateful to Dr. Funston for his review and for making himself available to\nanswer any questions we may have."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The described new megaraptoran skeleton provides valuable insights into the anatomy and evolution of this theropod group,\nwhich has limited information, and much more research and exploration are still needed.\nThe authors have adequately addressed the majority of the concerns raised in the previous round of review. I appreciate the\nthoughtful revisions and clarifications provided. The manuscript has improved in clarity and quality, and I am satisfied with\nthe current version.", "reviewer_2": "I congratulate the authors for their complete response to all my previous requests. The manuscript should be published as it\nis,\nAll the best,", "reviewer_3": "Dear Dr Ibiricu and colleagues,\nThank you for your time and effort in revising your manuscript. I am pleased to see that you engaged with all of my\nsuggestions and have either taken them on board or justified why you prefer not to. I have no further suggestions for\nimprovement of your manuscript.\nSincerely,\nGregory F. Funston, PhD\nVisiting Assistant Professor\nUniversity of California, Davis"}, "author_response": ""}]

Decision letter