Noasaurus leali Bonaparte and Powell 1980
Creators: name: Hendrickx, Christophe
name: Cerroni, Mauricio A.
name: Agnolín, Federico L.
name: Catalano, Santiago
name: Ribeiro, Cátia F.
name: Delcourt, Rafael
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Noasauridae
Noasaurus
Noasaurus leali

description: Species Noasaurus leali Bonaparte and Powell, 1980
Derivation of generic name: From NOA, the acronym for north-western Argentina (‘ Noroeste Argentino’ in Spanish), a geographic and historical region of Argentina composed of the provinces of Catamarca, Jujuy, La Rioja, Salta, Santiago del Estero, and Tucumán (NB—this is the first acronym to be used in the genus of a non-avian theropod; Molina-Pérez and Larramendi 2019), and sauros (σΑύΡΑ) meaning ‘lizard’ or ‘reptile’. Etymologically, Noasaurus translates to ‘reptile from north-western Argentina’.
Derivation of specific name: From the family name ‘Leal’ to honour lab technician and discoverer of the El Brete fossil site Juan Carlos Leal (Fig. 1A).
Holotype: PVL 4061, a left maxilla with five erupted crowns, a right quadrate, a mid-posterior cervical vertebral arch, a mid-cervical rib, a posterior cervical rib, an indeterminate postcervical vertebra, a manual phalanx of digit III, manual unguals possibly belonging to the left and right digit I, and a right metatarsal II (Figs 1E, 3A, B; Tables 2, 3).
Occurrence and age: El Brete Estancia, Department of Candelaria, southern Salta Province, Northeastern Argentina (Fig. 2A, B); Lecho Formation, Balbuena Subgroup, Salta Group; early or mid Maastrichtian, Upper Cretaceous (Bonaparte et al. 1977, Bonaparte and Powell 1980, Marquillas et al. 2005, Montano et al. 2022; Fig. 2C).
Diagnosis: Ceratosaur theropod with the following autapomorphies: (i) a concave alveolar margin of the maxilla; (ii) a diagonally oriented ridge along the dorsal portion of the maxillary fossa; (iii) a strongly arched quadrate body with a dorsoventrally low anterior margin of the pterygoid flange; (iv) maxillary crowns with minute mesial denticles and a low number (<30) of comparatively large distal denticles not diminishing in size basally; (v) a cervical neural arch with anterior epipophyseal prongs (modified from: Bonaparte and Powell 1980); (vi) a well-defined and acute middle longitudinal crest on the ventral surface and at midlength of the manual ungual blade (modified from: Agnolín and Chiarelli 2010); (vii) a strongly ventrally curved manual ungual forming an angle of about 90°; (viii) a manual ungual with a deep C-shaped articular surface in lateral view; and (ix) a deep subtriangular flexor fossa delimited distally by two ridges forming a V on the manual ungual (modified from: Bonaparte and Powell 1980, Agnolín and Chiarelli 2010).
Body length, body mass, and age: As early as its first mention in the literature, Noasaurus was seen as a small theropod not exceeding 2 m (Bonaparte et al. 1977). The total length of this theropod was later estimated to be <1 m (Bonaparte 1996b, 2007), <1.5 m (Bonaparte 1996a), 1.5 m (Novas 2009, Paul 2010, 2016, 2024), 2.4 m (Lambert 1983, 1990, 1993, Holtz 2007), and 3 m (Benton 1984, Molina-Pérez and Larramendi 2019) in subsequent work. None of these authors, however, applied a formula to estimate the body length of Noasaurus. Using our dataset on skull, metatarsal, and body lengths in the most complete theropods (Supporting Information, S1.6), Noasaurus is estimated to be around 2 m in length using an estimated cranial length of 185 mm, and 1.6 m using the proximodistal height (115 mm) of metatarsal II. Our formula relies on metatarsal III, which is the tallest metatarsal of the foot and whose measurements are typically given in the literature. Because metatarsal II is slightly proximodistally shorter than metatarsal III in noasaurids (Noas aurinae + Elaphrosaurinae), the body length of Noasaurus probably ranged between 1.7 and 2 m (Fig. 3A, C).
No femur or tibia is preserved to calculate the body mass of Noasaurus using Benson et al. ’s (2014), Campione et al.’s (2014), or Campione and Evans’ (2020) formulas. Noasaurus ’ body mass was estimated to be around 6 kg by Paul (2024), 15 kg by the same author (1988, 2010, 2016), and 38 kg by Molina-Pérez and Larramendi (2019). Likewise, the age of Noasaurus could not be assessed as histological analysis on the holotype is currently forbidden. Stiegler (2019), however, suggested that the Noasaurus material may belong to an immature individual based on the strongly arched quadrate, a condition restricted to immature individuals of Limusaurus.
DESCRIPTION
Cranium
Preserved cranial elements
The cranial elements of Noasaurus include a fairly complete left maxilla preserving five partially to almost complete erupted teeth, as well as a relatively complete right quadrate (Figs 3A, B, 4–6; Tables 2, 3). The squamosal described by Bonaparte and Powell (1980: fig. 7D, E; Fig. 1E) was later identified as a cervical rib by Novas (1989) in his Ph.D. dissertation (Bonaparte 1991) and is consequently described in the section ‘Axial skeleton’.
Maxilla
Most of the left maxilla is undistorted and only the posterior extremity of the jugal ramus is out of place (Fig. 4). Multiple fractures are, however, present throughout the bone, the two main ones crossing the middle parts of the ascending and jugal rami horizontally and vertically, respectively. A small piece of bone directly ventral to the ascending ramus was also glued back. The maxilla has been damaged since 2012 (when it was examined the first time by one of us), with an additional fracture at the base of the ascending ramus, while the first maxillary tooth is now loose and kept separately (Fig. 6O, P). The distalmost portion of the ascending ramus illustrated by Bonaparte and Powell (1980) is also missing and most likely lost. A photo of the maxilla provided by Candeiro (2007) shows that this portion of the ascending ramus was already missing before 2007. However, an illustration of the Noasaurus material published by Bonaparte (2007), as well as a movie showing some of the specimens shortly after their preparation (Fig. 1D; Supporting Information, S1.2), confirm that the missing part was originally present.
The maxillary bone is fairly complete, missing the dorsal part of the ascending ramus, the anterior portion of the anteromedial process, the medial wall from the posterior extremity of the jugal ramus, small pieces of the alveolar margin and the antorbital ridge, as well as several maxillary teeth (Fig. 4). In lateral view, the maxilla consists of a subtriangular bone with a dorsoventrally tall maxilla body, a short ascending ramus, an anteroposteriorly short pre-antorbital body and long jugal ramus (Fig. 4A). The preserved maxilla is dorsoventrally lower than anteroposteriorly long and the complete bone (Fig. 1E) was longer than tall. The anterior body occupies two-fifths of the maxillary length and is shorter than the jugal ramus. Its anteriormost part is particularly short and dorsoventrally lower than the jugal ramus (Fig. 4A). The anterior margin of the anterior body is weakly concave and posterodorsally inclined at an angle of 53° from the long axis of the alveolar margin. Both the anterior ramus and pre-antorbital body of the maxilla are roughly subtriangular in outline and the angles of the anterior corner of the anterior ramus and pre-antorbital body are approximately 43° and 48°, respectively (Fig. 4A). The anterior ramus occupies one-half of the anterior body and three-fourths of the antorbital body. The anterior margin of the maxilla is weakly sigmoid, with the margin of the anterior body and the ventral one-third of the preserved ascending ramus being slightly concave while the dorsal two-thirds of the remaining ascending ramus is weakly convex. The alveolar margin of the maxilla is gently concave, with the apex of the concavity located around half the length of the bone (Fig. 4A). The jugal ramus, which accounts for ~67% of the maxillary body, corresponds to an elongated subtriangular projection pointing posteriorly and delimited by two weakly sigmoid, almost straight ventral and dorsal margins. The jugal contact at the posterior end of the jugal ramus forms an almost right-angled isosceles triangle pointing anteriorly in lateral view. Based on Bonaparte and Powell (1980: fig. 7A, B; Fig. 1E), the ventral two-thirds of the ascending ramus mainly projects dorsally, perpendicular to the alveolar margin of the maxilla, whereas the dorsal third curves posterodorsally at an angle of ~45° from the long axis of the alveolar margin. The dorsal extremity of the ascending ramus was forked, with the two short dorsal and ventral subtriangular extremities projecting posterodorsally and posteriorly, respectively, as revealed by Bonaparte and Powell’s (1980) figure 7A, B (Fig. 1E; NB—the oldest available photo of the maxilla provided by Bonaparte (2007) reveals that the posteriormost extremity of the ascending ramus was already missing before 2007; Supporting Information, S1.1, Fig. SA3). The forked extremity of the ascending ramus was probably articulating with the anterior process of the lacrimal. If we cannot exclude the fact that the ascending ramus illustrated byBonaparte and Powell (1980) was incomplete and the articulation with the lacrimal, which may have then occurred more posteriorly, had a different shape, a posterior U-shaped fork of the ascending process is the typical morphology for the lacrimal contact of the maxilla in theropods (Rauhut, pers. comm. January 2024), supporting the fact that the ascending ramus figured by Bonaparte and Powell (1980) was complete.
A row of maxillary alveolar foramina is present, directly dorsal to the alveolar margin, parallel to the latter. The foramina are difficult to discern as most of them are obscured by matrix or correspond to a very shallow depression, but they can be seen with razing light. A second row of foramina, the maxillary circumfenestra foramina, is present directly ventral to the antorbital ridge and the most visible foramina are present at the level of mx6-8 (Fig. 4A).
A well-delimited antorbital fossa is present laterally, extending over the posterior one-third of the anterior body, the dorsal one-third of the jugal ramus, and the anterodorsal two-thirds of the ascending process (Fig. 4A). This fossa is ventrally delimited by a low and thick (i.e. dorsoventrally tall) antorbital ridge extending along the dorsal third of the maxillary body ventral to the antorbital fossa. The antorbital ridge narrows and fades anteriorly up to an angular corner formed by the vertically and horizontally orientated borders of the antorbital fossa, at the level of the fourth alveolus. Conversely, the antorbital ridge only slightly narrows posteriorly and remains prominent in its posteriormost part where it curves ventrally to reach the alveolar margin at the level of the tenth alveolus. The jugal contact consists of a rugose and poorly delimited surface located within the posteriormost part of the antorbital fossa, directly dorsal to the posterior extremity of the antorbital ridges. This contact roughly covers the posterior one-fourth of the jugal ramus; however, it is possible that it extended further posteriorly. There is no clear anterior limit of the jugal contact, which is mainly distinguished by its rugose surface and the presence of parallel ridges and grooves directed anteroposteriorly along the posterior half of the articulation surface.
A maxillary fossa is visible in the anterior corner of the lateral antorbital fossa (Fig. 4H, I). This pneumatic structure, interpreted by Stiegler (2019) as the pneumatic recess of the ascending ramus (e.g. but see Discussion below), can be described as a shallow and poorly delimited oval depression on the lateral wall of the antorbital fossa. The maxillary fossa is taller than long and occupies the same height as the dorsoventrally oriented and anteriorly inclined margin of the antorbital fossa. The anterior part of the maxillary fossa is bounded medially by the anterolateral wall of the maxillary body and cannot be seen in lateral view. In the visible posterior half, the fossa is ventrally and dorsally delimited by faint, rounded ridges, which become more prominent and increase in dorsoventral height anteriorly up to the anterior border of the antorbital fossa. A faint and low anteroposteriorly directed and anterodorsally inclined ridge crosses the maxillary fossa at two-thirds of its height (Fig. 4H, I). The maxillary fossa is filled with sediment in its anteriormost part and it is, therefore, unknown whether it extends more anteriorly within the maxillary body. This pneumatic structure does not at least communicate laterally, medially, or anteriorly by any opening. No maxillary, promaxillary or pneumatic fenestra piercing the maxilla are, consequently, present in the maxilla of Noasaurus. Likewise, there is no pneumatic excavation within the lateral wall of the ascending ramus.
In medial view, the maxilla is characterized by a tall medial wall dorsally, a short interdental wall ventrally, a prominent anteromedial process in the anteroventral corner of the bone, and faint palatal contact along the posterior two-thirds of the maxilla, directly dorsal to the dorsal wall (Fig. 4B). The medial wall covers the whole jugal ramus and most of the maxillary body, whereas the interdental wall is restricted to the ventral fourth of the maxillary body. The surface of both medial and interdental walls is smooth (i.e. there are no rugosities or ridges) and relatively uniform. The interdental wall is separated from the medial wall by a well-marked step-like nutrient groove sloping ventrally posteriorly and running along the whole maxilla length. The interdental wall, which results from the fusion of the interdental plates, reaches its tallest dorsoventral height at the level of the anterior margin of the maxilla, and gradually diminishes in height dorsally (Fig. 4B). It, however, continuously occupies the ventral third of the jugal ramus all along its length. The ventral margin of the interdental wall extends slightly more dorsally than the alveolar margin of the maxilla. A low and poorly delimited anteroposteriorly directed ridge is visible on the ventralmost surface of the medial wall, directly dorsal to the nutrient groove. This faint ridge is parallel to the interdental wall and shares the same dorsoventral height than the interdental wall along the median wall. The ridge increases in lateromedial thickness before reaching the anteromedial process anteriorly. The latter is located in the anteroventral corner of the medial wall (Fig. 4J). The anteromedial process consists of a prominent and anteroposteriorly elongated protuberance comprised of two anteroposteriorly oriented ridges separated by a shallow groove. The ventral ridge is more prominent, dorsoventrally taller and lateromedially thicker than the dorsal ridge. It runs anteroposteriorly parallel to the long axis of the nutrient groove. Conversely, the dorsal ridge corresponds to a faint and diagonally oriented prominence curving ventrally towards the ventral ridge up to the anterior margin of the maxilla. The ventral ridge also increases in dorsoventral height and lateromedial thickness anteriorly. The medial process is incomplete and extends further anteriorly beyond the anterior margin of the maxilla (Fig. 4J). The palatal contact of the maxilla is made of two faint and anteroposteriorly directed ridges running along the medial wall of the maxilla, directly dorsal to the nutrient groove, and subparallel to the later (Fig. 4B). The two ridges are faint and badly preserved posteriorly, where they converge, and their preserved parts extend over 14 mm along the central part of the jugal ramus. They become more prominent anteriorly, where they are separated by a distance of around 1 mm, and the dorsal ridge of the palatal contact is prominent enough to delimit a medial antorbital fossa in this part of the maxilla (Fig. 4B).
In ventral and dorsal views, the lateral margin of the maxilla and the medial surface of the medial wall are weakly anteroposteriorly convex and concave, respectively (Fig. 4C, D). The medial process of the maxilla, which strongly protrudes medially from the medial wall, is subtriangular in outline, increasing in lateromedial thickness anteriorly. The posterior extremity of the maxilla where the jugal and lacrimal articulated, is slightly oriented laterally (Fig. 4C, D) but it is unknown if this is due to taphonomic deformations or from the fact that the skull widened at this level of the maxilla. In ventral view, the nutrient groove is filled with sediment and its lateromedial thickness slightly decreases posteriorly to become particularly narrow along the posterior fourth of the medial wall. The maxilla includes 13 alveoli gradually diminishing in size posteriorly (Figs 4D, 6A–C). The alveoli are oval to subrectangular in outline, with weakly to strongly convex medial margins and flatter lateral borders. In dorsal view, the anteromedial process forms a pointy structure directed anteromedially (Fig. 4C). The groove presents on the dorsal surface of the anteromedial process and separates the ventral ridge from the dorsal one, diminishing in lateromedial width anteriorly. The maxillary fossa is lateromedially narrow and delimited laterally by the antorbital ridge. The ascending ramus projects vertically from the maxillary body. The nasal contact, which is present on the anterior border of the ramus, corresponds to a smooth surface with no asperities.
In anterior view, the contact with the premaxilla consists of a relatively uniform and flat surface with no rugosities (Fig. 4E, G). A faint, dorsoventrally elongated, and diagonally oriented ridge extending lateroventrally is present on the dorsal two-thirds of the premaxillary contact (Fig. 4E, G). This ridge extends dorsally from the anteromedial process to the lateral surface of the maxilla ventrally. A dorsoventrally elongated and diagonally oriented depression is present directly medial to this faint ridge and parallel to the later. This depression is shallow ventrally and increases in depth dorsally up to the ventral limit of the anteromedial process. In posterior view, the medial wall delimiting the lateral antorbital fossa is dorsoventrally convex and particularly thin, and the lateral margin of the maxillary fossa forms a symmetrically convex parabola (Fig. 4F). The posterior extremity of the jugal ramus is slightly lateromedially wider in its central part and the lateral border of the jugal ramus is asymmetrically convex anterior to the jugal contact (Fig. 4F).
Quadrate
The fairly complete right quadrate of Noasaurus is missing a large portion of the lateral process, most of the entocondyle (i.e. the medial condyle of the mandibular articulation; sensu Hendrickx et al. 2015a), a central piece of the quadrate ridge, and parts of the pterygoid flange (Fig. 5). The ventral portion of the latter has been strongly reconstructed. This is particularly the case for the medial surface in which the reconstructed portion occupies a large part of the ventral half of the flange (Fig. 5A–D). The pterygoid flange has also suffered multiple damages since 2012. Although the quadrate is not affected by distortion, the bone is broken in several places, with fractures running lateromedially or diagonally along the medial and posterior surfaces. The posterior surface is the least well-preserved of the quadrate, with some sediment glued to the central portion of the bone. The anterior and dorsal sides of the quadrate are, however, relatively intact.
In posterior view, the Noasaurus quadrate is a dorsoventrally elongated bone flaring out ventrally from a lateromedially narrow quadrate head to a wider mandibular articulation (Fig. 5C). It is characterized by a lateromedially narrow, sub-vertical and poorly delimited, rod-shaped quadrate ridge running along the medial edge of the quadrate body. This ridge emerges ventrally at one-third of the body, well-dorsal to the mandibular articulation, and terminates dorsally directly ventral to the quadrate head. The quadrate body is delimited medially by two surfaces meeting at an obtuse angle at two-fifths of the bone height. The surface occupying the ventral two-fifths of the quadrate is flat surface and strongly slopes ventrally. Conversely, the surface along the remaining dorsal three-fifths of the bone is slightly convex and corresponds to the medial margin of the quadrate ridge. An incomplete lateral process is visible in posterior view, projecting strictly laterally from the quadrate body (Fig. 5C). The dorsal margin of the lateral process strongly slopes ventrally and extends from the quadrate body well-ventral to the quadrate head. The ventral margin of the lateral process is sub-horizontal, mediolaterally convex in its preserved part and connects to the quadrate body at the level of the ectocondyle. Although Bonaparte and Powell (1980) described a small quadrate foramen in Noasaurus and Carrano and Sampson (2008) scored it as absent in their datamatrix, the presence of a quadrate foramen cannot be determined given that the lateral border of the lateral process contacting the quadratojugal is unpreserved (Fig. 5A, C). It is also unknown whether the quadrate and quadratojugal were fused to each other or not (contra Bonaparte 1991; see Discussion). The surface along the central two-thirds of the quadrate and running from the quadrate ridge to the lateral process is lateromedially and dorsoventrally concave. No pneumatic fossa or foramina are visible on this posterior concavity but the surface is too badly preserved to rule out their presence. The articular surface of the quadrate head is convex and restricted to the posteriormost surface of the quadrate body. The ectocondyle extends only slightly posteriorly beneath the quadrate body and its dorsal margin forms a widely convex parabola (Fig. 5C). Based on the preserved portion of the entocondyle, and given that more than a half of this condyle is missing, the mandibular articulation must have protruded strongly medially from the rest of the quadrate body, forming a prominent and pointy medial projection in posterior view. With the entocondyle and the lateral process complete, the quadrate likely had the shape of an Eifel-tower in posterior view.
In medial and lateral views, the posterior margin of the quadrate body is concave and strongly arched, forming an almost perfectly and symmetrically curved parabola (Fig. 5D). Although it is unknown how the quadrate articulated within the cranium, the quadrate head probably strongly projected posterodorsally and was probably positioned well-posterior from the mandibular articulation. The anteroposteriorly short quadrate ridge is only visible along the central two-fourths of the quadrate, being delimited anteriorly by a shallow furrow running dorsoventrally. A subtriangular pterygoid flange projecting anteriorly is visible in medial and lateral views (Fig. 5B, D). The ratio between the anteroposterior length of the flange and the dorsoventral height of quadrate body is 0.45, corresponding to a moderately extended pterygoid flange. The flange projects from the quadrate head dorsally and reaches the quadrate around one-fifth of the quadrate body ventrally, well-dorsal to the mandibular articulation. Although figured as a triangular projection by Bonaparte and Powell (1980: fig. 7C), the pterygoid flange is in fact subtrapezoidal, with a short subvertical anterior margin (Fig. 5D). The dorsal margin of the flange is almost straight, whereas the ventral border is slightly convex. The medial fossa of the pterygoid flange is particularly shallow and no medial pneumatic foramen is present on the preserved part of the flange. A notch corresponding to the intercondylar sulcus between the ento- and ectocondyles is visible on the anterior portion of the mandibular articulation. It is, however, unknown whether this notch was visible when the entocondyle was completely preserved.
In anterior view, the pterygoid flange is subvertical and parallel to the long axis of the quadrate body (Fig. 5A). The flange is straight along most of its lateral and medial surfaces and convex along the ventralmost margin from which projects a lateromedially narrow ventral shelf medially. A concave surface is visible on the lateroventral surface of the mandibular articulation. This concavity extends laterally from the lateral process to the lateral half of the ectocondyle and its dorsal margin forms a lateromedially convex parabola.
In lateral view, the lateral process forms an anteroposteriorly thin and dorsoventrally convex bony projection with a parabolic curvature similar to that of the posterior surface of the quadrate body. The pterygoid flange is devoid of any pneumatic aperture or recess (Fig. 5B). The ventral limit of the ectocondyle is convex, and anterior and posterior pointy projections are visible along the medial portion of the mandibular articulation.
In dorsal view, the quadrate head forms a single, semispherical condyle with a strongly convex posterior margin and a more widely convex, almost subrectangular, anterior margin (Fig. 5E). The pterygoid flange is straight and only projects anteriorly, perpendicular to the laterally projected lateral process. Both pterygoid flange and lateral process are thin bony laminae increasing in width towards the quadrate body.
In ventral view, the mandibular articulation is made of two condyles delimited by a shallow and poorly delimited intercondylar sulcus whose orientation cannot be determined (Fig. 5F, G). Although incomplete, the entocondyle was almost certainly larger than the ectocondyle (Fig. 5F). The latter forms an oval to oblong condyle whose long axis is strongly diagonally oriented from that of the mandibular articulation. The shape of the entocondyle is unknown but its long axis also appears to have been diagonally oriented from the long axis of the mandibular articulation. A lateromedially wide notch is visible on the posterior surface of the mandibular articulation, between the ento- and ectocondyles, whereas the anterior margin of the articulation forms a wide convexity along its preserved portion (Fig. 5F). The lateral process connects to the ectocondyle anteriorly and the ventral shelf of the pterygoid flange is lateromedially narrow (Fig. 5G).
Dentition
Only the left maxillary dentition of Noasaurus is preserved. Four isolated theropod teeth (PVL 4062) referred by Bonaparte and Powell (1980) to an indeterminate ‘Carnosauria’ were also recovered from the El Brete fossil site but their larger size (CH> 10 mm) and distinct crown and denticle morphologies strongly suggest that they belong to a non-noasaurid theropod, most probably an abelisaurid (C.H. pers. obs.). The maxillary dentition of Noasaurus preserves five fully erupted crowns from the first, third, sixth, eighth, and 10th alveoli (Fig. 6A). Bonaparte and Powell (1980) and Novas (2009) suggested that the maxilla had 10 or 11 teeth but 13 maxillary alveoli can be recognized in ventral view (Fig. 6B, C). Only the Lmx8 crown is almost complete as the others are missing a small (Lmx1, Lmx6) or large portion (Lmx3, Lmx10) of the apex. A picture of the Noasaurus maxilla illustrated by Bonaparte (2007; Supporting Information, S1.1, Fig. SA3) shows that all the preserved teeth were complete or fairly complete after their preparation. The maxillary dentition currently shows some damages typically consisting of mesiodistally oriented fractures running along the crown-base or the mid-crown. Lmx6 is the least well-preserved, with a large diagonally oriented fracture on the crown and most of the apicodistal surface missing. The denticles present on the best-preserved crowns are also incomplete and sometimes difficult to distinguish, while the enamel is often missing on some portions of the crown surfaces.
The maxillary dentition of Noasaurus is ziphodont, i.e. it includes labiolingually compressed, distally recurved, and bladeshaped crowns bearing denticulated carinae. In situ teeth of the maxilla are also decumbent (i.e. teeth with no inclination, pointing vertically from the tooth bearing bone; Hendrickx et al. 2019, also known as ‘orthodont’ for the incisors of rodents; Thomas 1919) as it is the case in most theropods. Little variation exists in the morphology of the preserved maxillary teeth, which mainly differ in their compression and elongation. They are all slightly distally curved, with a weakly concave distal margin, so that the apex of the tooth lies at the same level as the distal margin of the crown at the cervix (Fig. 6A). Although incomplete, none of the maxillary crowns appear to have exceeded 10 mm in height, suggesting that the maxillary dentition was particularly short compared to the cranium’s height (Fig. 3B). The best-preserved crown (Lmx8) is almost 5 mm in height, whereas the largest one Lmx3 (CH> 3.12 mm) may have been 5 mm possibly 6 mm in height. Based on the crown–base length (CBL) of the available crowns and the size of the maxillary alveoli, the largest and tallest teeth were probably from the third, fourth, and fifth alveoli, which is typically the case in ziphodont theropods (C.H. pers. obs.). Little variation in size, however, occurred in the maxillary dentition of Noasaurus as CBL only varies less than 1 mm, from 2.83 mm in Lmx10 to 3.54 mm in Lmx3. The first maxillary crown (CH> 2.67 mm), which is only missing the apicalmost part of the crown (Fig. 6E, F), was shorter than Lmx3, Lmx6, and Lmx8, and the smallest teeth were probably present in the distalmost maxillary alveoli. With a CBR ranging from 0.54 to 0.65, the maxillary crowns were weakly to moderately compressed, and Lmx1 was the thickest. The crown compression does not seem to follow a trend along the maxilla but the most compressed teeth appear to have been present in the distalmost portion of the maxilla (Table 3). Little is known of the elongation of the maxillary crowns. The only complete tooth Lmx8 (Fig. 6I, J) has a normal elongation (CHR = 1.64) and the other partially preserved crowns seem to have had similar elongation. A cross-section of mx3, which is missing the apex, reveals that the cross-sectional outline of this crown was lenticular, with almost symmetrically convex labial and lingual margins (Fig. 6B). The labial side is only slightly more angular in its central part than the lingual one. All the preserved maxillary teeth are believed to have a lenticular or lanceolate cross-sectional outline at the base and at mid-crown.
The maxillary crowns all have straight or weakly apicobasally arched mesial and distal carinae (Fig. 6N). The mesial carina is centrally positioned on the mesial surface in all preserved crowns other than Lmx 8 in which it slightly twists on to the mesiolingual surface basally (Fig. 6C). The distal carina is also either centrally positioned or slightly labially displaced, as seen in Lmx1 (Fig. 6N) and Lmx8. A denticulated distal carina is present in all teeth, whereas mesial denticles can be observed in at least Lmx3 and Lmx8 (Fig. 6H–J). As noted by Candeiro (2007), the mesial denticles are significantly smaller than those of the distal carina. We calculated a denticle density of 20 to 25 distal denticles and 32.5 mesial denticles per 5 mm at mid-crown (Fig. 6S), giving a DSDI of 1.3 to 1.44 for the maxillary crowns. The distal denticles can be seen along the whole crown height in all preserved crowns. Mesial denticles are more difficult to discern but the denticulated mesial carina also appears to extend to the cervix in Lmx3, Lmx8, and Lmx10, and possibly Lmx1. The mesial carina of Lmx10 is devoid of denticles suggesting the fact that mesial denticles were probably absent in the distalmost maxillary teeth (Fig. 6M). With an estimation of less than 30 denticles along the whole distal carina, the distal denticles are particularly large compared to the crown height (Fig. 6G). We counted 10 distal denticles on the basal half of Lmx3, indicating that the number of denticles on the distal carina was probably close to 20. The distal denticles do not diminish in size basally (Fig. 6G). When complete, the distal denticles project perpendicularly from the distal carina and are subquadrangular in outline, with a parabolic and symmetrically convex external margin (Fig. 6Q, R). The interdenticular space is narrow, and short and diagonally oriented interdenticular sulci are present on the labial side of Lmx8, between the baso- and centrodistal denticles. Interdenticular sulci, however, appear to be absent between the distal denticles of the other preserved crowns. The mesial denticles are minute and difficult to distinguish in Lmx3 and Lmx8, whereas their presence is unknown in Lmx1. The most complete and best-preserved mesial denticles are seen in Lmx3 at mid-crown where they are short, poorly defined, apicobasally subrectangular in outline, and with a symmetrically convex external margin (Fig. 6S). No interdenticular sulci are present between the mesial denticles.
The maxillary crowns do not show any ornamentation such as marginal or transverse undulations, flutes, longitudinal ridges or grooves, basal striations, concave surfaces adjacent to the carinae, or labial and lingual depressions on the crown surface. The enamel surface texture also does not have any particular pattern and is, therefore, described as irregular (sensu Hendrickx et al. 2015d). An extensive spalled surface is, nevertheless, visible on the lingual surface of Lmx1 (Fig. 6F, O, P). This surface is oval in shape in its basal part, diagonally oriented from the long axis of the crown, and extends on the mesial half and apical two-thirds of the crown. No other spalled surface or wear facets are neither present on the labial side of the maxillary teeth nor on the lingual surface of the other crowns.
Axial skeleton
Cervical vertebral arch
The cervical vertebral arch of Noasaurus was briefly described by Bonaparte and Powell (1980), whereas Novas (1989) expanded the description of this element in his Ph.D. dissertation. The neural arch is virtually complete, lacking some structures such as the left diapophysis, the neural spine, which is broken at its base, the anterior tip of the right epipophysis, and a small area of the dorsomedial aspect of the left prezygapophysis. Novas (2009) identified this element as being presumably from the sixth cervical vertebra. We agree with this author that this neural arch probably belongs to a mid-posterior cervical element, probably to the sixth or seventh cervical. The presumed position is based on the inclination of the transverse processes, which are lateroventrally projected, forming an angle of 53° from the mid-sagittal plane, as well as the slightly ventral bending of the prezygapophyseal–epipophyseal lamina, the wide distance between pre- and postzygapophyses (this distance is short in more caudal cervical vertebrae), the epipophyses surpassing posteriorly the postzygapophyses, and the resemblance with mid-posterior cervical vertebrae of other abelisauroids (Carrano et al. 2002, 2011, O’Connor 2007, Langer et al. 2019). This change is evidenced by the prezygapophyses and centroprezygapophyseal fossae, which are more dorsally placed relative to the neural canal.
Distinctive features of this element (Fig. 7) include an enlargement of the neural arch mainly resulting from the strong elongation of epipophyses, which occupies nearly 65% of the maximum length of the arch, a markedly concave surface of the transverse processes, an anteroposteriorly short neural spine, distinctly separated dorsal and lateral aspects of the neural arch due the development of the epipophyseal prezygapophyseal lamina, and numerous pneumatic features represented by several laminae and fossae.
In dorsal view, the general bone surface is well preserved, except for the missing neural spine (Fig. 7E, K). The dorsal surface of the neural arch has a rectangular profile due to the enlarged lateral epipophyses and the slightly divergent prezygapophyses. The anterior and posterior sides are characterized by the V-shaped incisions inwards the arch, formed by the zygapophyses and the spinopre- and spinopostzygapophyseal fossae. The preserved base of the neural spine is transversely compressed and anteroposteriorly short. The anterior extent of the spine base does not meet the spinoprezygapophyseal fossa, whereas it hardly contacts the spinopostzygapophyseal fossa posteriorly.
The dorsal surface of the neural arch is separated from the lateral sides by a well-developed epipophyseal–prezygapophyseal lamina (eprl) (Fig. 7K), as occurs in all abelisauroids (Sereno et al. 2004, Langer et al. 2019). The epipophyses are particularly hypertrophied, with anterior and posterior prongs (‘spikelike structures’ sensu Bonaparte and Powell 1980) that are autapomorphic of Noasaurus within noasaurids. Although the epipophyses are almost subparallel to each other, their main axis is oriented anteromedially to posterolaterally, so that they slightly diverge posteriorly.
The anterior projections of the epipophyses flare slightly medially and extend anteriorly to the level of the diapophysis; both anterior and posterior projections of the epipophyses are acuminate on their ends. The surface between the epipophyses and the base of the neural spine is roughly flat and smooth. The prezygapophyses are divergent from each other, each one having a lobate shape. Both spinopre- and spinopostzygapophyseal fossae are broad and V-shaped, and the spinopostzygapophyseal fossa is superficially broader. The angle of aperture of the spinoprezygapophyseal fossa is about of 60°, whereas the spinopostzygapophyseal fossa reaches 46°.
In lateral view, the epipophyseal–prezygapophyseal lamina has a faintly undulating profile (Fig. 7A, B, G, H), posterior to the prezygapophyses; it, however, does not form a markedly concave margin as in the mid-cervicals of other noasaurids (e.g. Laevisuchus and Masiakasaurus). The anterior prong of the epipophysis forms a marked and deep notch on the dorsolateral side of the arch. The epipophysis is well differentiated from the rest of the lateral surface of the transverse process and is particularly marked on its posterior half towards the posterior prong. The surface between the epipophysis and the transverse process is mostly smooth (Fig. 7A, G). As for the prezygapophyses, they are well below the level of the epipophyses.
The prezygodiapophyseal lamina (prdl) has a lightly curved anterior margin reaching the prezygapophyses at an angle of 29°. Its dorsoventral development completely obscures the anterior centrodiapophyseal lamina (acdl), as well as the lateral centroprezygapophyseal lamina (lat.cprl) partially in lateral view (Fig. 7A, B, G, H). The prezygapophyseal centrodiapophyseal fossa (prcdf) is also partially visible laterally. As for the postzygodiapophyseal lamina (podl), the margin with its contact with the postzygapophyses is strongly curved. This lamina overhangs the postzygapophyseal centrodiapophyseal fossa (pocdf), which is superficially broad in lateral view (Fig. 7B, H). The subfossae within are, however, hidden by the postzygodiapophyseal lamina. The posterior centrodiapophyseal lamina (pcdl) extends from the posterior surface of the transverse process to the posterior pedicles, describing a curved shape. The lateral centropostzygapophyseal lamina (lat.cpol) has slightly curved and vertically directed margins on its contact with the dorsal aspect of pedicles. The diapophysis is triangular, ventrally projected with a rounded distal end.
In anterior view, the dorsal surface of the neural arch is almost flat, with the anterior projections of the epipophyses flaring medially (Fig. 7C, I). The articular surfaces of the prezygapophyses are dorsomedially oriented at an angle of approximately 40° relative to the horizontal plane. The spinoprezygapophyseal fossa is mediolaterally wide and separates both centroprezygapophyseal fossae (cprf) on its dorsal sector by the junction of the intraprezygapophyseal laminae (tprl). The latter join together at an angle of 140°, forming the ventral limit of the spinoprezygapophyseal fossa. The centroprezygapophyseal fossae are located lateroventrally to the spinoprezygapophyseal fossa and dorsolaterally to the neural canal, forming large ovoidal excavations on the anterior side of the neural arch; it is unclear whether these fossae are blind or pierced by foramina as they are partially filled with sediment. Each centroprezygapophyseal fossa is delimited by the lateral (lat.cprl) and medial centroprezygapophyseal laminae (med.cprl). Both laminae meet at the neural canal mid-height at an angle of almost 90°. The medial centroprezygapophyseal lamina contacts the intraprezygapophyseal lamina at an angle of 71°, whereas the lateral centroprezygapophyseal lamina joins the prezygodiapophyseal lamina at 52°. The prezygapophyseal centrodiapophyseal fossa (prcdf) is broadly exposed and subtriangular [the ‘cavidad antediapofisial’ of Novas (1989)]. The centrodiapophyseal fossa (cdf) has a minimal expression when seen in anterior view due to the fact that it is hidden by the anterior centrodiapophyseal lamina (acdl). The latter has a slight ventral curvature towards its contact with the diapophyses. Because all described fossae are currently filled with sediment, it remains unclear whether they lead to pneumatic camerae, as in the cervical vertebrae of other noasaurids (Brum et al. 2018, Smyth et al. 2020).
In posterior view, the spinopostzygapophyseal fossa (spof) is mediolaterally broad, deep, and wider than tall (Fig. 7F, L). The intrapostzygapophyseal laminae (tpol) join at an angle of 110°. The centropostzygapophyseal fossae (cpof) appear below these laminae and dorsolaterally to the neural canal. These fossae are deep but relatively small in area when compared with the centroprezygapophyseal counterpart. The centropostzygapophyseal fossae have a scalene triangle contour and are delimited by the lateral (lat.cpol) and medial (med.cpol) centropostzygapophyseal laminae. The medial centropostzygapophyseal lamina is transversally thinner than the lateral one, which becomes thicker towards its contact with the postzygapophysis. These laminae contact each other at an angle of 41°. The medial centropostzygapophyseal lamina joins the intrapostzygapophyseal lamina forming an angle of 108°. The articular surfaces of the postzygapophyses are oriented ventrolaterally at an angle of 28°, which is less than the inclination of the prezygapophyses. The posterior projection of the epipophysis is laterally offset from the postzygapophysis and a shallow groove running mediolaterally separates the posterior surface of both structures. The postzygodiapohyseal lamina is also strongly curved in posterior view, contacting dorsally the postzygapophysis at an angle of 46°, and ventrally joins the posterior centrodiapophyseal lamina at an angle of 36°. The postzygapophyseal centrodiapophyseal fossa (pocdf) is located on the posterior surface of the transverse process. A peculiarity is that two smaller subfossae are differentiated by a median lamina that runs mediolaterally within this fossa. Both subfossae were previously identified by Novas (1989) as the ‘postdiapophysial cavities’ (‘cavidades postdiapofisiales’; Novas 1989). The subfossae are circular and one is positioned below the other. They appear to be deep but it is unclear if they lead to pneumatic cavities. These subfossae of the postzygapophyseal centrodiapophyseal fossa are present on both sides of the neural arch but they are less apparent on the right as both excavations remain filled with sediment.
In ventral view, the neurocentral suture is well discernible along the ventral surface of the neural arch (Fig. 7D, J). The centroprezygapophyseal fossae and lateral centroprezygapophyseal laminae are partially visible. The latter meet the prezygodiapophyseal lamina at an angle of almost 56° in that view. The anterior centrodiapophyseal lamina, which is located anteriorly to the diapophysis, is obliquely oriented and meets the prezygodiapophyseal lamina at an angle of 62°. These laminae form the lateral and posterior walls of the prezygapophyseal centrodiapophyseal fossa, which is bean-shaped, deep, and larger than wide. The centrodiapophyseal fossa, which is clearly visible in ventral view, is wider than long, being much larger that the previously described fossae of the anterior aspect of the neural arch. The centrodiapophyseal fossa is subdivided by a median swelling of bone rather than a lamina, delimiting a relatively large subfossa posteromedially and a smaller one anterolaterally, close to the anterior centrodiapophyseal lamina. The postzygapophyseal centrodiapophyseal fossa, which is only partially visible, shows that the larger subfossa is anteroposteriorly long and almost reaches the postzygapophysis. The posterior projections of the epipophyses project far posteriorly the articular facets of the postzygapophyses.
Mid-cervical rib
ThiselementwascorrectlyidentifiedasacervicalribbyBonaparte and Powell (1980: fig. 8I, J) who, however, did not specify its position within the cervical series. Novas (2009) stated that this bone may pertain to more anterior cervical ribs based on its morphological similarity with other abelisauroids. Carrano et al. (2011), however, suggested that this element may pertain to a fourth cervical rib, resembling those of Masiakasaurus. We agree with the latter authors in the fact that this bone most likely represents a mid-cervical rib, probably a fourth or fifth cervical element, based on the short distance between the capitulum and tuberculum, a thin capitulotubercular web, and the short neck of the capitulum (when compared with the longer neck of the posterior cervical rib of Noasaurus). The cervical rib corresponds to a left proximal end (Fig. 8), with some fractures on the anterolateral process and lacking the styliform process.
In medial view, the tuberculum is ellipsoidal, anteroposteriorly longer than the capitulum, and slightly exceeds the horizontal plane of the main rib body (Fig. 8B). The capitulum is less ellipsoidal and is not at the same level than the tuberculum, which is displaced posterodorsally relative to the former. Both structures are linked by the transversally thin and oblique capitulotubercular web. This lamina gets thicker towards its connection with the capitulum and tuberculum. The anterolateral process of the cervical rib tapers anteriorly into a blunt end and connects with the capitulum through a concave lamina. In lateral view, the rib has a ridge-like buttress with a nearly constant transverse section except anteriorly where it gradually diminishes after the tuberculum (Fig. 8A). The lateral surface of the rib is mostly smooth but some faint striations are visible. Due to the poor preservation of the bone, it is unclear whether a bifurcated process was present in the rib of Noasaurus, as commonly present in other abelisauroids (O’Connor 2007, Carrano et al. 2011).
In ventral view, the lateral buttress of the rib extends from the anterolateral to the posterolateral processes (Fig. 8D). The angle that the capitulum forms with the main axis of the rib [‘neckshaft angle’ sensu O’Connor (2007)] is approximately 105°. It is clear that the tuberculum is strongly concave and the capitulum is irregularly convex. In dorsal view, the lateral buttress also forms a slightly ornamented keel (Fig. 8E).
In anterior view, the rib has a wide fossa between the capitulotubercular web and anterolateral process (Fig. 8C); however, this fossa is non-invasive. The lateral surface of the rib body is strongly concave in this view. Conversely, a broad pneumatic fossa with two large foramina piercing the surface can be seen in posterior view (Fig. 8F). The larger (5 mm) is located close to the capitulum whereas the smaller (~ 2 mm) is present near the tuberculum.
Posterior cervical rib
Bonaparte and Powell (1980) originally identified the cervical rib as a right squamosal (Novas, 1989, 2009). The cervical rib of Noasaurus represents a well-preserved proximal left element only lacking some areas of the capitulum surface and the whole rib shaft (Fig. 9). This element corresponds to a caudal cervical rib, probably from the ninth or 10th cervical vertebra, based on the wide distance of the capitulotubercular web, the presence of the sharp anterolateral process, a capitulotubercular web pierced by a pneumatic foramen on tis anterior surface, and the obtuse angle (128°) formed between the main axis of the rib body and the capitulum (Fig. 9A, E; see: O’Connor 2007). Carrano et al. (2011) argued that this element may pertain to a tenth cervical or even the first dorsal element based on a comparison with Masiakasaurus.
In dorsal view, the dorsolateral process (dlp) is mediolaterally wide posterior to the contact with the tuberculum and becomes less expanded towards the anterolateral process (alp; Fig. 9C). The latter is developed as a pointy and blunt end with slightly concave and convex medial and lateral margins, respectively. The anterolateral process is marked on its dorsal and ventral surfaces by a lateral buttress that extends posteriorly as a faintly ornamented ridge toward the dorsolateral process. The capitulum is robust and projects anterolaterally (in this view) from the main shaft at an angle of 128°; both anterior and posterior margins are straight and ornamented with rugosities, whereas the distal end is eroded and it is not possible to observe whether it was convex as in other abelisauroids. The tuberculum is also robust, dorsomedially projected, and has a short neck when compared with the tuberculum. Its distal end, which is damaged, is circular in cross-section. The capitulotubercular web is hidden by the tuberculum in this view.
In ventral view, the tuberculum and capitulotubercular web are not observable due to the strong development of the capitulum (Fig. 9F). The overall surface between the capitulum and the main rib body is smooth, except for the lateral buttress (which is continuous from the anterolateral to the dorsolateral processes), which exhibits some slight rugosities. The margin that connects the anterolateral process and tuberculum has a strongly concave contour, almost forming a notch in this area. On the other hand, the central area of the dorsolateral process is pierced by a small and distinct pneumatic foramen (~ 2mm).
In anterior view, the tuberculum and capitulum meet at an angle of 84° (Fig. 9D). The capitulotubercular web is mostly missing but its preserved portion proves that it was broad and transversally thin near the tuberculum. The surface between the tuberculum and the anterolateral process is concave; the same condition occurs for the ventral surface that joins the capitulum and the anterolateral process. The pointy anterior end of the anterolateral process flares laterally and its distal tip has a circular cross-section. A conspicuous pneumatic foramen (~ 2 mm) is present within a small fossa. This foramen pierces the anterior surface between the capituloturbecular web and the anterolateral process and enters into the main rib body.
In posterior view, a smooth and elliptical fossa characterizes the capitulotubercular web (Fig. 9E). This fossa bears a large and teardrop-shaped pneumatic foramen (~ 5 mm) adjacent to the tuberculum. This foramen also enters the rib body and is probably connected internally with the aforementioned pneumatic structures. The tuberculum is transversally thicker than the capitulum in this view.
Indeterminate postcervical vertebra
This element was originally mentioned by Bonaparte and Powell (1980) as part of the holotype but was neither described nor figured. This isolated vertebral body is poorly preserved, with largely eroded areas reconstructed with plaster (Fig.10). The centrum probably belongs to the series between the middle and posterior trunk vertebrae owing to the absence of parapophyses and a blind excavation on its lateral surface (O’Connor 2007, Langer et al. 2019). However, we cannot reject the possibility that it may come from the caudal series based on the presence of a lateromedial constriction at mid-length and articular surfaces that are taller than wide, as seen in Masiakasaurus (Carrano et al. 2011). Nevertheless, although being damaged, the ventral surface does not show clear evidence of haemal facets while a shallow sulcus is partially visible (see description below).
In lateral view, the centrum, which is a relatively simple element, is elongated (1.7× longer than deep) and weakly amphicoelous, despite both articular surfaces being slightly eroded (Fig. 10A, B). The lateral surface is characterized by smooth and poorly-developed pleurocentral fossae, lacking other distinctive features. Although not completely observable in this view, the constriction of the centrum on its midlength is visible by the concave ventral margin connecting both articular surfaces. The articular surfaces are ovoid (Fig. 10C, F), taller than wide, with the posterior articular surface projecting slightly more ventrally relative to the anterior one. In ventral view, the centrum is spool-shaped due to its transversally compressed nature. A shallow and elongate sulcus characterizes the anterior half of the centrum surface (Fig. 10D). This sulcus is bounded laterally by thin ridges that gradually fade at the midlength of the centrum.
Appendicular skeleton
Manual phalanges
In the original description of the Noasaurus material, Bonaparte and Powell (1980) concluded that the available phalanges came from the foot and given its trenchant aspect, the ungual may probably correspond to a sickle-like claw, similar to, but convergently acquired with that of deinonychosaur theropods (Bonaparte 1991). The claw of Noasaurus was found as very different from that of other theropods in being notably curved and in having an excavated ventral surface devoid of flexor tubercles. The morphology of the Noasaurus pedal claw remained a unique trait of this carnivorous theropod. The raptorial claw of Noasaurus pes is, however, regarded as being from the manus by several authors, an opinion we here follow (Agnolín et al. 2004, Carrano et al. 2004, Carrano and Sampson 2008, Agnolín and Chiarelli 2010).
The manual phalanges of Noasaurus leali are represented by a non-ungual phalanx probably from digit III, as well as a partial and a complete ungual possibly from digit I (Figs 11–13). Because the manus anatomy of noasaurid is still poorly known (Langer et al. 2019) and abelisaurid digits are strongly modified, the homology and position of Noasaurus ’ manual elements is uncertain. In contrast to the original interpretation, and following Agnolín and Chiarelli (2010), the non-ungual and ungual phalanges are not considered as being consecutive. In fact, when put in articulation, their mobility is strongly reduced, precluding important flexor or extensor mobility.
Non-ungual manual phalanx
A single non-ungual phalanx was recovered (Fig.11). In contrast to recently published foot phalanges of noasaurids (e.g. Velocisaurus and Vespersaurus; Brissón Egli et al. 2016, Langer et al. 2019), the phalanx of Noasaurus resembles those of the manus in having the collateral ligamental pit dorsally displaced and the distal articular condyles ventrally located with respect to the main axis of the phalanx shaft (Agnolín and Chiarelli 2010). Based on its size and the well-differentiated proximal and distal articular surfaces (Fig. 11A, B), this phalanx probably does not belong to manual digit IV. Phalanx IV-1 is indeed nub-shaped, with a rounded distal end, as seen in other ceratosaurs such as Ceratosaurus, Aucasaurus, and Majungasaurus (Gilmore, 1920; Coria et al., 2002; Burch and Carrano, 2012; Carrano and Choiniere, 2016). Likewise, it cannot belong to digit I as the manual phalanges of the first manual finger are strongly asymmetrical in ceratosaurs (Coria et al. 2002, Burch and Carrano 2012, Langer et al. 2019). Comparison with the corresponding phalanx of Vespersaurus suggests that the non-ungual phalanx of Noasaurus probably belongs to the left hand. As seen in the Noasaurus phalanx, the lateral surface of phalanx III-1 of Vespersaurus is straighter than the medial one, which is more deeply excavated and shows a more prominent bony edge in the proximal end (Langer et al. 2019). We, therefore, identify the non-ungual phalanx of Noasaurus as a left manual phalanx III-1.
The phalanx is proportionally short, dorsoventrally compressed and robust (Fig. 11A, B). In dorsal and ventral views, it is roughly subtriangular in outline (Fig. 11D, E). In side view, the proximal and distal articular surfaces are separated by a well-developed phalangeal neck (Fig. 11A, B). The proximal articular surface shows nearly straight to subparallel dorsal and ventral surfaces (Fig. 11F). This results in an inverted trapezoidal shape. The proximal end shows an extensor process that is dorsally flat and strongly transversely expanded. In contrast, the flexor process is subquadrangular in outline and transversely narrow, representing less than one-third of the proximal phalangeal width. The medial surface of the extensor process is thicker and forms a bump absent on its lateral surface.
The proximal articular surface is strongly excavated and subdivided by a deep and subvertically oriented keel (Fig. 11F). Both surfaces are well-excavated and are subequal in size and shape, the medial one being slightly narrower than the lateral one on its ventral surface. This results in the fact that the proximal articular surface is more exposed in medial than in lateral view.
The distal ginglymoid is notably expanded and shows very narrow and acute articular condyles. The latter are separated by a deep and wide groove (Fig. 11E), forming a pulley-like distal articular surface. In distal view, the distal end of the phalanx is subquadrangular in outline (Fig. 11C). The articular condyles are subparallel to each other and slightly medially oriented. Both condyles are subequal in shape, the medial one being only slightly larger than the lateral one. Both flexor and extensor fossae are deep and well defined, resulting in a deep separation between the proximal and distal articular surfaces. Collateral ligamental pits are deep and ellipsoidal in contour and the medial one is wider but shallower than the lateral one.
Manual unguals
The Noasaurus holotype includes two manual unguals (Figs 12, 13). Unguals of Noasaurus were originally interpreted as belonging to the foot (Bonaparte and Powell 1980, Bonaparte 1991, 1996a). The available unguals can, however, be referred to the manus based on the following combination of traits: notably curved blade with a narrow cross-section, a deep and transversely narrow proximal articular surface, with a prominent and well-defined median keel (Agnolín and Chiarelli 2010). The particular morphology of the unguals of Noasaurus, which strongly differ from those of other ceratosaur claws, makes a referral to any digit of the hand particularly challenging. However, the available complete ungual is tentatively referred to digit I based on its strong blade curvature, as well as a nearly symmetrical proximal articular surface. Indeed, ungual I is often the most curved claw of the hands in non-avian theropods (e.g. Afrovenator, Allosaurus, Balaur, Buitreraptor, Coelophysis, Herrerasaurus, Megaraptor, Nqwebasaurus, therizinosaurs, Suchomimus, troodontids, and Yutyrannus; Sereno 1994, 2017, Zanno 2006, Barta et al. 2018, Chinzorig et al. 2018, Kubota et al. 2024; C.H. pers. obs.), especially those with enlarged ungual I. The ungual is also tentatively assigned to the right digit based on the slightly medial/inner curvature of the distal extremity of the claw in ventral, dorsal, proximal, and distal views (Fig. 12C– E, G). The other claw, despite only preserving the proximal half (Fig. 13), is tentatively identified as ungual I from the left manus owing to the fact that it shares with the complete ungual the same size and curvature, and the slight medial curvature of the claw in proximal view (Fig. 13F).
The manual claw of Noasaurus shows a strongly curved blade that forms an arch of about 90° along its ventral margin (Fig. 12). The proximal end of the dorsal margin of the blade is nearly straight, with a curvature starting approximately at one-third of the length of the bone (Fig. 12A, B). The ungual blade is strongly laterally compressed and shows a roughly ovoidal to subtriangular cross-sectional outline. One surface of the blade is nearly flat, whereas the other is slightly convex, suggesting that the former probably corresponds to the medial aspect of the ungual, whereas the latter is the lateral surface (Figs 12E, 13D). There is a single collateral vascular groove extending along the dorsal third of the blade. The medial collateral vascular groove appears to be deeper than the lateral one; this may, however, be an artefact of preservation.
The ventral surface of the manual ungual is notably complex and lacks any sign of flexor tubercle and flexor facets (Figs 12A, B, 13A, B). It, however, shows a very deep and subtriangular fossa which is delimited by a poorly raised posteroventral process proximally and by two well-defined ridges distally. These two ridges contact each other distally to form a ‘V’-shape (Fig. 12F) whose anteriorly directed apex extends as a midline ridge up to the distal point of the claw. Such peculiar morphology of the ventral surface of the manual ungual is unknown in any other theropod, including other noasaurids such as Vespersaurus (Langer et al. 2019), and is here considered as an autapomorphy of Noasaurus.
The proximal articular surface of the ungual is subrectangular in proximal view (Figs 12C, 13F). The subvertical midline keel separates two deep and subrectangular concave cotyles for the articulation with the condyles of the non-ungual phalanx. The cotyles are well defined, particularly deep, and result in a ‘C’-shaped articular surface when viewed from the sides (Figs 13A, B, 14A, B). The lateral cotyle is transversely narrower and dorsoventrally taller than the medial one. Both cotyles are delimited at their sides by narrow and acute ridges. The midline keel is notably prominent and forms a small wall separating the cotyles in side view. This keel is well-separated from the flexor process [the ‘proximoventral process’ of Agnolín and Chiarelli (2010)] in this same view. The extensor process [the ‘proximodorsal process’ of Agnolín and Chiarelli (2010)] is prominent and well defined and no depression is present distal to this process.
Only the proximal third of the left? manual ungual is preserved (Fig. 13). The ungual blade shows a subtriangular outline in cross-section (Fig. 13C), being strongly laterally compressed and showing a flat ventral surface delimited by the medial and lateral ridges. The medial collateral vascular groove appears to be deeper than the lateral one, although this might be a preservation artefact. The ventral surface is poorly preserved (Fig. 13D) and lacks a flexor tubercle. Parts of the ridges are preserved and form the ‘V’ delimiting a prominent ventral fossa, as seen in the complete manual ungual. The proximal articular surfaces of both unguals are almost identical.
Metatarsal II
The second right metatarsal (here abbreviated Mt) of Noasaurus is the only preserved element of the hind limb (Fig. 14). It is a long, gracile, and particularly well-preserved bone with only some fractures over the diaphysis (Fig. 14A–D). The proximal end of MtII is ovoidal and mediolaterally narrow (Fig. 14E), with a flat lateral surface for the contact with MtIII. The medial surface is convex and the posterior and anterior margins end in a rounded tip. A relatively small, flat facet is present on the proximolateral area of the shaft and may correspond to the attachment site for some pedal muscle. The metatarsal shaft, which is anteroposteriorly deep, becomes mediolaterally compressed over two-thirds of its length distal to the proximal end. The transverse section is D-shaped at two-thirds of the shaft due to the flat lateral facet that received MtIII and the convex lateral margin which extends distally. The distal one-third of MtII expands mediolaterally to form a slightly convex anterior surface, resulting in a subquadrangular transverse profile (Fig. 14A, C). In medial/lateral views, a faint inflexion point with a very low angle (i.e. <5°) is seen shortly after the widening of the shaft (Fig. 14A, C).
The flat lateral surface of the shaft becomes slightly concave (Fig. 14C) forming a wide groove towards the distal end, though it is unclear if this is a taphonomical artefact. A barely visible and subquadrangular hyperextensor fossa is present on the anterior surface of the distal end (Fig. 14B). The most conspicuous feature of the medial surface of MtII (Fig. 14A) is a low elliptical bulge ornamented by small tubercles corresponding to a muscle attachment and where some of the short extensors of digit II (m. extensor digiti II?) probably originated. In posterior view, the mediolaterally compressed shaft is represented by a sharp edge expanding mediolaterally on the distal half (Fig. 14D). An elongate and faintly sculptured muscle scar, which probably represents one of the insertions of the mm. gastrocnemii, is present at the midlength of MtII (Carrano and Hutchinson 2002). A few muscle striations are similarly present on the posterior surface of the distal half, proximal to the ginglymus. Both collateral pits [‘ligamentous fossae’ in Bonaparte and Powell (1980)] are subcircular and subequal in size. While the medial collateral pit is shallow (Fig. 14G), the lateral pit is deep and contained within a proximodistally large fossa that is continuous with the ‘groove’ formed by the lateral surface (Fig. 14H).
The distal end of the metatarsal has a roughly quadrangular shape, being only slightly longer anteroposteriorly than mediolaterally (Fig. 14F). The ginglymus is well defined on the plantar aspect and both condyles are well-developed posteriorly. They are, however, strongly asymmetrical, with the lateral condyle being lateromedially buttressed and more posteriorly projected than the small and blunt medial condyle. The deep U-shaped sulcus that separates both condyles is relatively shallow and more developed medially. This sulcus probably corresponds to the passage of the main flexor muscles of the second toe.
description: Published as part of Hendrickx, Christophe, Cerroni, Mauricio A., Agnolín, Federico L., Catalano, Santiago, Ribeiro, Cátia F. & Delcourt, Rafael, 2024, Osteology, relationship, and feeding ecology of the theropod dinosaur Noasaurus leali Bonaparte and Powell, 1980, from the Late Cretaceous of North-Western Argentina, pp. 1-51 in Zoological Journal of the Linnean Society (zlae 150) (zlae 150) 202 (4) on pages 12-26, DOI: 10.1093/zoolinnean/zlae150, http://zenodo.org/record/14824224