Milleretta rubidgei
Creators: name: Jenkins, Xavier A.
name: Benson Maya Elliot, Roger B. J.
name: Jeppson, Gabriel
name: Dollman, Kathleen
name: Fernandez, Vincent
name: Browning, Claire
name: Ford, David P.
name: Choiniere, Jonah
name: Peecook, Brandon R.
Publisher: name: Zenodo
Year: 2025
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Millerettidae
Milleretta
Milleretta rubidgei

description: Milleretta rubidgei (Broom, 1938)
Synonymy
Millerina rubidgei Broom, 1938
Milleretoides platyceps Broom, 1948
Milleretops kitchingi Broom, 1948
Material studied: BP/1/3818, a nearly complete skull lacking the antorbital snout anterior to the prefrontals. BP/1/3822, a nearly complete skull of a juvenile individual lacking the dorsal surface of the antorbital snout. SAM-PK-K5212, a weathered skull lacking the palate. SAM-PK-K7366, juvenile skull lacking the dorsal surface of the antorbital skull and left prefrontal. SAM-PK-K10581, a dorsoventrally crushed but complete skull with a closed lower temporal fenestra that is likely to pertain to a subadult or adult individual.
Locality and horizon: Upper Cistecephalus Assemblage Zone of the Karoo Basin from Doornplass, Eastern Cape, South Africa (BP /1/3818 and BP/1/3822). Daptocephalus Assemblage Zone, Zuurplaats 114 Graaff-Reinet, South Africa (SAM-PK-K5212 and SAM-PK-K7366). Daptocephalus Assemblage Zone, Wilgerbosch, Graaff-Reinet, South Africa (SAM-PK - K10581).
Revised diagnosis: Millereta is a small-bodied millerettid reptile with the following unique combination of characters: cranial osteoderms extending from the skull roof onto the lateral skull including the jugal, quadratojugal, squamosal, postorbital, lacrimal, and maxilla*; 15 or fewer maxillary teeth*; an anteroposteriorly shortened antorbital region including the maxilla, lacrimal, and nasal*; a lacrimal that does not contribute to the external naris; posterior border of jugal with crenulations in juvenile specimens*; a posterior development of the posterior process of the jugal that secondarily closes the lower temporal opening through ontogeny*; a dorsal process of the quadratojugal visible in medial view; a tympanic emargination between the quadrate, quadratojugal, and squamosal; the presence of enlarged vomerine teeth anteriorly on the vomer; a transversely narrow parabasiphenoid body; a prootic that bears a notch instead of a foramen for cranial nerve (CN) VII; the loss of a dorsal process of stapes; a distally expanded stapedial shaft; anteroposteriorly expanded ribs that overlap the succeeding rib distally late in ontogeny*; and unfused distal tarsals IV and V (distinguishes from Milleropsis). Asterisks represent autapomorphies that distinguish Millereta from other millerettids based on comparisons primarily with Broomia perplexa and Milleropsis pricei.
Description
Skull
The cranial descriptions provided for Millereta rubidgei here are based on SRµCTµCT scans of BP/1/3818 (subadult) and BP/1/3822 (juvenile, sensu Gow 1972). BP/1/3818 preserves nearly the entire skull roof and most bones anterior to the occiput, although the temporal region is poorly preserved (Figs 1C, D, 2C, D.) BP/1/3822 preserves most of the skull, although the surface of the exposed elements in this specimen is less well preserved compared with BP/1/3818 owing to damage during manual preparation (Figs 1A, B, 2A, B). When possible, comparisons are made with the holotype specimen (R.C. 14) and R.C. 70, the most mature individuals of Millereta known (Gow 1972), in addition to the referred specimens below (Fig. 3).
Our descriptions of Millereta are supplemented further by referred specimens SAM-PK-K5212, SAM-PK-K7366, and SAM-PK-K10581 (Fig. 3), which we refer to Millereta based on the following features unique to Millereta among millerettids: the presence of cranial osteoderms that extend onto the temporal region, a short antorbital skull similar in length to the postorbital skull, a crenulated posterior border of the jugal, the closure of the lower temporal fenestra through ontogeny (SAM-PK - K10581), and the presence of 15 or fewer maxillary and dentary teeth.
Theskullof Millereta isproportionallyshorterthanthatofother millerettids, with the snout (nasal, prefrontal, and premaxilla) being anteroposteriorly reduced in comparison to the relatively moregracileskullof Milleropsispricei.Thisisaccentuatedfurtherby the increased dorsoventral height of the antorbital skull, best seen in BP/1/3822 and SAM-PK-K7366 here, but also the holotype specimen, R.C. 14. There is therefore room for fewer alveoli, and Millereta therefore bears the lowest number of marginal dentition amongst millerettids. Unfortunately, cranial anatomy (particularly the sutural relationships between antorbital bones) of Millereta, especially in osteologically mature individuals, such as the holotype (R.C. 14) and R.C. 70, is complicated by historical preparation methods and cranial osteoderms that obscure antorbital sutures in mature individuals. Furthermore, the temporal region develops drastically during ontogeny, with the lower temporal fenestra remaining open in juveniles (such as BP/1/3822 here) and there being a lack of postorbital–supratemporal contact, whereas in mature individuals (e.g. R.C. 14), the lower temporal fenestra is closed and the posterior extent of the postorbital increases, contacting the supratemporal. A careful consideration of the influence of cranial osteoderms and ontogeny in classical interpretations of Millereta is therefore necessary.
Premaxilla: The premaxillae of all described Millereta skulls are incomplete. Of the scanned specimens, only BP/1/3822 preserves portions of the paired premaxillae, represented by two subnarial processes with part of their palatal flanges preserved (Fig. 4). The premaxillae of referred specimens are likewise poorly preserved, except for SAM-PK-K10581, which bears both premaxillae, although the left premaxilla is slightly displaced owing to the dorsoventral distortion in this specimen, and weathering has removed the dentition in this specimen.
The preserved portions of the subnarial process in BP/1/3818 and BP/1/3822 confirm the observations of Gow (1972) in that there are at least three premaxillary teeth in Millereta, although there is room for an additional tooth position, giving an inferred premaxillary tooth count of four. In contrast, Milleropsis pricei has five premaxillary teeth (Jenkins et al. 2025). Posteriorly, the palatal flange of the premaxilla of Millereta is weakly incised by the choana or internal naris (Fig. 4). A facet on the palatal flange marks the articulation with the vomer, although this contact is not present owing to the dorsal displacement of the vomers in BP/1/3822.
The supranarial process of the premaxilla is best preserved in SAM-PK-K10581 (Fig. 3C). The supranarial process of Millereta terminates at the level of the first or second maxillary tooth position immediately posterior to the midlength of the external naris. The supranarial process angles anterodorsally, forming a small ‘rostral process’thatextendsbeyondtheanteriormostpointofthepremaxilla tooth row, as in Milleropsis (Jenkins et al. 2025) and in Eunotosaurus (Gow, 1997). The body of the supranarial process is thick in comparisontotherelativelygracilesupranarialprocessof Milleropsis pricei (X.A.Jpers. obv. BP/1/720) and is more similar in proportions to that of Eunotosaurus (Gow, 1997). The external naris of Millereta, as in other described millerettids (e.g. Milleropsis pricei, Watson 1957), is enlarged, deeply emarginating the maxilla and nasal.
Maxilla: The maxillae of BP/1/3818 are incomplete anterior to the orbit (Fig. 1C), and the following description is based on the complete right maxilla of BP/1/3822 (Fig. 1B). The maxilla of Millereta extends from the posterior margin of the orbit to the external naris and is proportionally shorter anteroposteriorly than that of Milleropsis (Jenkins et al. 2025). The maxilla of Millereta comprises an attenuating subnarial process that contributes to the external naris, a dorsal process forming much of the lateral surface of the snout, and a suborbital ramus that contacts the jugal and contributes to the orbit. It contacts the premaxilla and nasal anteriorly, the palatine medially, the lacrimal dorsally, and the jugal posteriorly (Fig. 1).
The subnarial process of the maxilla of Millereta forms the posteroventral margin of the external naris, where it narrowly overlaps the subnarial process of the premaxilla (Fig. 4A). The subnarial process of the maxilla is mediolaterally thin where it contributes to the external naris (Fig. 5), therefore lacking a narial shelf or maxillary depression present in ankyramorphan stem reptiles, including acleistorhinids and procolophonians (Debraga and Reisz 1996).
The alveolar shelf of the maxilla of Millereta (BP/1/3822) bears 15 alveoli, although only 13 teeth are present (Figs 2B, 3A). The maxillary dentition Millereta can be categorized broadly as very weakly ‘subpleurodont’, in that the labial alveolar shelf extends further ventrally than the lingual alveolar shelf. However, in contrast to classically pleurodont taxa, each shallow alveolus is separated from the succeeding alveolus by a low and thin interdental plate, a condition that is very similar to Orovenator (Ford et al. 2021; Fig. 6). The teeth of Millereta are subconical, with circular bases, and are weakly recurved posteriorly at their apex. The marginal dentition of Millereta is extremely large in comparison to the more narrow and strongly recurved teeth in Milleropsis and Eunotosaurus (Gow, 1972, 1997b). Plicidentine grooves are restricted to the base of each tooth (Fig. 6C; Gow 1972), similar to Milleropsis pricei (Jenkins et al. 2025) and the early diverging neodiapsid Youngina capensis Broom, 1914 (Hunt et al. 2023). Tooth replacement in the left maxilla of BP/1/3822 is alternating (with exceptions posteriorly), but replacement is not occurring in the right maxilla of this specimen. A distinct caniniform region or tooth is absent, similar to most Permian neoreptiles (e.g. the neodiapsid Acerosodontosaurus piveteaui Currie, 1980; Bickelmann et al. 2009) but in contrast to the millerettid Broomia perplexa (Cisneros et al., 2008), which bears slightly enlarged maxillary teeth near the anterior end of the maxilla. The depth of the alveolar shelf of Millereta thickens dorsoventrally immediately posterior to the external naris in medial view. (Fig. 5C).
The canal for the maxillary artery of Millereta is visible in individual slices from the scan data throughout the length of the maxilla, and it is similar to that of many other stem reptiles. It bears a distinct conical cavity, and little to no distinct dorsal branching of the alveolar canals is visible, although the contrast in this region is relatively poor (Fig. 5). The maxilla of Millereta contributes to the ventrolateral margin of the foramen orbitonasale, as in all other stem reptiles in which this feature is known, such as the bolosaurid Belebey vegrandis Ivakhnenko, 1973 (Reisz et al., 2007) and the early diverging neodiapsid Youngina capensis (X.A.J. pers. obv. BP/1/2817; Fig. 5).
The lateral exposure of the anterior end of the dorsal process of the maxilla in Millereta increases in dorsoventral height, forming an apex where the dorsal process attains its maximum height at the level of the third maxillary alveolus (Fig. 1B). The dorsal process of the maxilla contacts the nasal at this level, therefore excluding the lacrimal from the narial margin in all specimens available for study (Fig. 1). This contrasts with historical descriptions of Millereta, such as those of Broom (1938) and Gow (1972), although Gow (1972: fig. 13) correctly observed this feature in BP/1/3822. The left lacrimal of SAM-PK-K7366 also falls short of the external naris, although the right element artificially appears to approach the naris owing to the missing nasal in this specimen (Fig. 4A).
The absence of a lacrimal contribution to the external naris is similar to the condition in other millerettids, including Milleropsis (Carroll, 1988; Jenkins et al. 2025), in that the lacrimal likewise fails to extend to the external naris. We note that historical preparation of these specimens, paired with the presence of osteoderms on the lateral surface of the anterior snout, often obliterate sutures in lateral view, and both Watson (1957) and Gow (1972) acknowledged uncertainty in this region in their descriptions and reconstructions. The posterior margin of the external naris of Millereta is dorsoventrally expanded, similar to that of Eunotosaurus aficanus (Bever et al., 2015). A large anterior maxillary foramen pierces the lateral surface of the maxilla of Millereta at the junction of the subnarial and dorsal processes (Fig. 1A), a feature widespread in crownward stem amniotes (the captorhinid Saurorictus Modesto & Smith, 2001) and early stem reptiles (e.g. Orovenator; Ford and Benson 2019). A row of foramina that continues posteriorly from the anterior maxillary foramen is present in R.C. 14 and BP/1/3822 but is not discernable in our segmented reconstruction.
The suborbital process of the maxilla of Millereta contributes weakly to the orbit, preventing the lacrimal–jugal contact from being visible in lateral view (Fig. 1B). The suborbital process continues posteriorly, where it attenuates at the level posterior to the middle margin of the orbit; here, the maxilla bears a medial groove for the suborbital process of the jugal, which is best visible in dorsolateral view (Fig. 2B). The posterior end of the suborbital process of the maxilla of Millereta is weakly ‘twisted’, where the lateral surface becomes increasingly inclined dorsolaterally, weakly overhanging the tooth row. There is no contact between the suborbital process of the maxilla and the quadratojugal, unlike the condition in some pareiasauromorphs (Tsuji 2006) and varanopid synapsids (Botha-Brink and Modesto 2009).
Septomaxilla: The left septomaxilla is present in BP/1/3822, visible through the left external naris (Fig. 1B). As preserved, the septomaxilla of Millereta is positioned centrally within the narial cavity, but it is loosely disarticulated with the surrounding elements and is rotated by 10°–20° medially. The septomaxilla of Millereta is a complex bone, possessing a ventral plate that contacts the vomer posteromedially, a dorsal process that contacts the premaxilla dorsally, and contacts the contralateral septomaxilla medially, similar to the description of R.C. 70 by Gow (1972: fig. 2D). It partitions the external naris into anterior and posterior portions, as visible in dorsolateral view.
The anterior surface of the ventral plate of the septomaxilla rises weakly anterodorsally, forming an anterior process that contributes to the ventromedial margin to the septomaxillary canal (Fig. 7). A blade-like dorsal process of the septomaxilla rises dorsally at the level of the anterior process. The anterior surface of the dorsal process is rounded, whereas the posterior end attenuates (Fig. 7). The medial margin of the dorsal process of the septomaxillawouldhavecontactedthecontralateralseptomaxilla (Gow 1972), although the left and right septomaxilla are weakly disarticulated in BP/1/3822. The dorsal margin of the dorsal process would have contacted the supranarial process of the premaxilla, although this contact is missing in scanned specimens here owing to the damaged or missing premaxillae (Fig. 7). The anterior margin of the dorsal process bears an anterior extension and is extremely similar to the internarial process of the septomaxilla described in synapsids (Hillenius 2000).
Nasals: The nasals are poorly preserved in BP/1/3822 and are dorsoventrally crushed in BP/1/3818 (Fig. 1). Preparation of BP/1/3822 has damaged much of the outer surface of the bone, such that the sutures of the nasals with surrounding elements of the snout are difficult to reconstruct in lateral view. Fortunately, the internal sutures of the nasal with surrounding elements (except the premaxilla) are readily visible in the tomography data. The nasals are nearly complete in R.C. 14 and R.C. 70 but damaged by preparation, whereas only skims of the nasals are present in SAM-PK-K10581 and SAM-PK-K-5212 (Fig. 3A, C). The nasal of Millereta is anteroposteriorly short and sheet-like in dorsal view. The nasal of Millereta contacts the frontal posteriorly, forming a weakly posterolaterally directed suture with the frontal, whereas the nasal is overlapped laterally by the prefrontal and lacrimal posteriorly (Fig. 2).
The anterior margin of the nasal of Millereta forms the posterodorsal margin of the external naris and extends ventrally, contributing to the lateral surface of the snout. Here, the nasal contacts the dorsal process of the maxilla, excluding a lacrimal contribution to the nares in lateral view. The nasal of Milleropsis forms a posterolateral suture with the anterior process of the frontal and a sagittal suture with the prefrontal (Gow 1972). The nasal of BP/1/3822 is devoid of the dermal sculpturing that extends onto the nasal in mature individuals of Millereta (R.C. 14 and R.C. 70; Gow 1972).
Lacrimal: The lacrimals are present in both scanned specimens, but the most complete is the right lacrimal of BP/1/3822 and is the basis of the following description (Fig. 5). The lacrimals are also preserved in referred specimens, but sutures are visible only in SAM-PK-K7366 (Fig. 3). The ventral part of the lacrimal is overlapped by the maxilla throughout its length and contacts the nasal and prefrontal dorsally and the jugal posteriorly.
Understanding of the morphology of the anterior surface of the lacrimal in R.C. 14 and R.C. 70 is complicated by the presence of cranial osteoderms that extend onto the antorbital snout. Previous reconstructions of Millereta (Broom, 1938; Watson, 1957; Gow, 1972) have figured the lacrimal of Millereta as an anteroposteriorly long element that extends far anteriorly, forming the posterior margin of the external naris. Although the lacrimal of Millereta is considerably more elongate than those of other millerettids, such as Milleropsis (Carroll, 1988), it does not contact the external naris as visible in lateral view, owing to contact between the nasal and maxilla, nor does it extend to the lacrimal in medial view, as in acleistorhinids (MacDougall and Reisz 2012).
The lacrimal contributes weakly to the anteroventral margin of the orbit, forming only ~5% of the orbit margin (Fig. 5B). Here, it is mediolaterally thin, lacking the broad dorsomedial process that forms much of the anterior margin of the orbit in crownward stem amniotes, such as captorhinids (e.g. Labidosaurikos meachami Stovall 1950; Dodick and Modesto 1995) and recumbirostran microsaurs (e.g. Huskerpeton englehorni Huttenlocker et al., 2013; Huttenlocker et al. 2013, Gee et al. 2019). The lacrimal of Millereta does not contribute to the antorbital buttress or the foramen orbitonasale, unlike in some parareptiles, such as bolosaurids (Reisz et al. 2007) and procolophonians (e.g. Leptopleuron lacertinum Owen, 1851; Säilä 2010). In Millereta, two large foramina for the nasolacrimal duct pierce the lacrimal in this region (Fig. 5A). A third, smaller foramen marks the position of a canal that eventually converges with the nasolacrimal duct anteriorly (Fig. 5). As in Milleropsis (Jenkins et al. 2025), the lacrimal of Millereta bears a medial expansion over the anterior pathway of the nasolacrimal duct, roofing this canal for most of its length (Fig. 5C). This medial expansion is most strongly developed at the level of the sixth maxillary tooth, where the nasolacrimal duct exits the lacrimal and continues anteriorly along the medial maxilla–nasal suture (Fig. 5C).
The posteroventral process of the lacrimal becomes acuminate posteriorly where it overlaps the suborbital ramus of the jugal at the level 10th maxillary alveolus, although this contact is hidden in lateral view by a maxillary contribution to the orbit (Fig. 1B).
Prefontal: BP/1/3822 and BP/1/3818 preserve both prefrontals, with the right prefrontal being the most complete in both specimens. The prefrontals are best preserved in BP/1/3818, with undamaged dorsolateral surfaces. The prefrontal is triangular in lateral view and bears a thin posterodorsal process that contributes to the anterior margin of the orbit, a dorsoventrally broad and laterally swelling anterior process, and a ventromedial process (Fig. 5). The prefrontal contacts the nasal anteriorly, the palatine and lacrimal ventrally, and the frontal posterodorsally.
The anterior process of the prefrontal extends anteriorly to the fifth alveolus, ending in a squared tip that is overlapped by the nasal anteriorly and the lacrimal anteroventrally (Fig. 1B). The lateral surface of the anterior process in BP/1/3822 is smoothly finished, lacking the dermal sculpting or cranial osteoderms present in more mature individuals of Millereta (including BP/1/3818; Fig. 2C). The anterior process of the prefrontal bears its greatest dorsal exposure at the level immediately anterior to the orbit. Here, the lateral surface of the anterior process swells laterally, overhanging the lacrimal (Fig. 5A). A similar swelling of the prefrontal is present in other millerettids, including Milleropsis pricei (Jenkins et al. 2025). However, the swelling in Millereta is more strongly developed, and almost tubercle-like in lateral view (Fig. 5B). A pair of foramina is present on the dorsal surface of the anterior process above this expansion in BP/1/3818 (Fig. 1C)
A ventromedial process of the prefrontal of Millereta contacts the ascending process of the palatine ventrally, although this process is not mediolaterally expansive in comparison to those of classical parareptiles, such as procolophonians (e.g. Saurodektes kitchingorum; Reisz and Scott 2002). The ventromedial process of the prefrontal of Millereta lacks the tonguelike medial process present in Milleropsis (Jenkins et al. 2025). The dorsal margin of the foramen orbitonasale is expressed as a notch on the ventromedial process of the prefrontal of Millereta, with ventral contributions by the palatine and maxilla (Fig. 5A).
The posterodorsal process of the prefrontal attenuates posteriorly and fits into an anteriorly extending groove on the dorsolateral surface of the frontal. There is no posterior contact with the postfrontal.
Frontal: The paired frontals of BP/1/3818 and BP/1/3822 are almost completely preserved and are missing only their anterior margins owing to weathering. The external surfaces of the frontals are best preserved in BP/1/3822, although there are cracks throughout the specimen, particularly in the right frontal. The frontal of Millereta is extremely elongated and is the longest bone of the dermal skull roof, almost half the anteroposterior length of the skull (Fig. 2). The frontal contacts the nasal anteriorly, the prefrontal anterolaterally, the postfrontal posterolaterally, and the frontal posteriorly, and forms most of the dorsal orbit margin (Figs 2, 8).
The anterior process of the frontal extends to approximately the midlength of the snout, where it contacts the nasal, forming an anterolateral suture, best visible in the holotype specimen R.C. 14 owing to damage in this region in the imaged specimens and SAM-PK-K7366 (Fig. 2C). The frontal contacts the prefrontal anterolaterally, forming a sagittal suture in a short, tongue-in-groove joint. The dorsal surface of the anterior process in BP/1/3818 bears cranial osteoderms in the form of low bosses; these bosses are restricted to the frontoparietal suture in BP/1/3822 (Fig. 9).
The frontal of Millereta contributes broadly to the orbit, lacking the lateral lappet present in several synapsids (e.g. Edaphosaurus boanerges Romer & Price, 1940; Romer and Price 1940) and acleistorhinid stem reptiles (e.g. Delorhynchus cifelli Reisz et al., 2014; Reisz et al. 2014). The frontal forms more than half of the dorsal margin of the anteroposteriorly expanded orbit, similar to other millerettids, including Milleropsis (Gow, 1972). Posterior to the orbit, a short posterolateral process of the frontal of Millereta is received by a groove by the parietal, forming an obtuse suture in dorsal view (Fig. 2B). The frontal makes a short, abutting contact with the postfrontals laterally, which is strongly constricted by a triangular anterior process of the parietal (Fig. 2).
The left frontal of BP/1/3818 underlaps the right frontal ventrally, forming a scarf joint (Fig. 2). The ventral surfaces of the frontals also bear moderately developed crista cranii, although ventral flanges for articulation with an ossified sphenethmoid are absent (Fig. 8B), unlike in captorhinids, where such flanges are present(Heaton 1979). The ventral surfaces of the frontals are concave medial to the crista cranii, and the anterior portion of this concavity expands, giving the frontals a somewhat hourglass shape in dorsal and ventral view (Fig. 8A).
Parietal: The parietal of Millereta is a rectangular bone, approximately two-thirds the length of the frontal in dorsal view (Fig. 2). The parietal of Millereta contacts the frontal anteriorly, the postfrontal anterolaterally, the postorbital and squamosal laterally, and the supratemporal, tabular, and postparietals at the occipital margin. In both scanned specimens, the parietals are weakly displaced from one another along the midline, although both elements remain in articulation in BP/1/3818 (Fig. 2). The dorsal surface of the parietal is covered in the low ‘cranial osteoderms’ (Watson 1957) that are more strongly developed in BP/1/3818 than in BP/1/3822.
The anterior end of the parietal contacts the frontal via a weakly interdigitating suture with the posterolateral process of the frontal, which overlaps the parietal ventral to this suture (Fig. 2). The parietal bears a distinct anterolateral process that extends a short distance onto the lateral surface of the skull, partly separating the postorbital from the postfrontal, similar to other millerettids (Romer 1956), although this feature is also variably present in early amniotes, including mesosaurids (Modesto 2006) and the early captorhinid Euconcordia (Müller and Reisz 2005).
The parietal bears a ventral flange (Fig. 9) immediately posterior to the anterolateral process that is received by dorsal facets of the postorbital and squamosal, a feature that Watson (1957) noted was diagnostic of ‘Millerosauria’ and that is also present in Eunotosaurus aficanus (Gow, 1997b). The loose contact between the ventral flange and the postorbital and squamosal appears to develop during ontogeny (Fig. 10). In less mature individuals of Millereta, such as BP/1/3822 and SAM-PK-K7366 (Fig. 10), small unossified ‘gaps’ are present between the ventral flange and the lateral skull, but these are absent in R.C. 14, and the contact of the ventral flange and postorbital and squamosal is essentially transverse (Gow, 1972: fig. 4). A similar projection has been noted in the Eunotosaurus (Gow, 1997), although damage to this region in juvenile specimens of Eunotosaurus (e.g., SAM-PK-7909) has been interpreted as an upper temporal fenestra. Our observation of SAM-PK-7909 suggests instead that this ‘fenestra’ represents a damaged portion of the ventral flange of the parietal, given the presence of this flange in all other known skulls of Eunotosaurus (e.g. BP/1/7852 and CM-777; Gow 1997b). This is corroborated by our observations of Millereta specimen SAM-PK-K7366, which bears a complete ventral flange in the left parietal although damage to the right parietal in this region gives the appearance of an upper temporal fenestra between the supratemporal, postorbital, and parietal, and SAM-PK-K10581, which bears an ‘upper temporal fenestra’ dorsal to the postorbital that is certainly preparation damage (Fig. 3A, C).
A short, triangular posterolateral process of the parietal wedges between the medial margin of the supratemporal and the lateral margin of the tabular in Millereta, similar to that present in Eunotosaurus specimen CM-777 (Gow 1997b). This posterolateral process of millerettids is much shorter than that present in Orovenator (Ford and Benson 2019) and early neodiapsids more broadly (e.g. Youngina; Gow 1974). A posterolateral process of the parietal is generally absent in non-neodiapsid amniotes, in that the posterolateral margin of the parietal is unexpanded and forms a transverse contact with the supratemporal or tabular, when present (e.g. the eureptile Captorhinus Cope, 1896; Heaton 1979).
The dorsal surface of the parietal is weakly convex, corresponding to large fossae along the ventral surface of this bone (Fig. 8). A large pineal foramen is present at the midlength of the dorsal surface of the parietal in BP/1/3818 and more anteriorly in BP/1/3822 (Fig. 2). The posterior margin of the dorsal surface of the parietal deflects ventrally (~80° relative to the dorsal surface) where it contributes to the occipital margin and bears a facet for the tabular laterally and the postparietal medially (Fig. 11). The postparietal facet is restricted to the dorsal surface of the parietal, contrasting with the condition in Captorhinus laticeps Williston, 1909, in that the postparietal is overlapped by the parietal, such that facets for the postparietal are on the ventral surface (Heaton 1979).
The ventral surface of the parietal is surprisingly complex (Fig. 8B). Near the midline of the parietal and the pineal foramen, the ventral surface is flat. There is no ridge or concavity surrounding the pineal foramen. A large, parabolic fossa is positioned lateral to this, which corresponds to the ventral surface of the ventral flange of the parietal (Fig. 8B). Striae are present in this fossa and are directed anteromedially. This fossa is likely to mark the insertion of the m. adductor mandibulae externus. The subdivisions of this muscle described by Heaton (1979) for Captorhinus laticeps are not present, nor is the temporoparietal artery foramen, although these features also cannot be found in Milleropsis (Jenkins et al. 2025) or Youngina (X.A.J. pers. obv. BP/1/2871). Facets marking the articulation of the squamosal and postorbital are present on the ventral surface of the parietal lateral to the fossa for the m. adductor mandibulae externus.
Cranial osteoderms: The skull roof of Millereta has been described as bearing cranial osteoderms, which take the form of raised bosses or tubercles on the skull roof and lateral skull (Fig. 8; Watson 1957). These structures are most prominent in osteologically mature individuals, such as R.C. 14, and obscure many aspects of the sutural morphology of the posterior skull roof and anterior snout (Gow 1972). For example, osteoderms on the antorbital snout of the holotype of Millereta cross and obscure the sutures of the maxilla, lacrimal, prefrontal, and nasal, whereas osteoderms in the postorbital region pass from the jugal onto the postorbital (Watson 1957). These osteoderms have complicated previous interpretations of the anatomy of Millereta, especially in historically prepared specimens (see description of the lacrimal above). To quote Gow (1972: 356), referencing these cranial osteoderms: ‘Accurate determination of suture lines is made difficult by this roughening, so that it was always gratifying to be able to separate off individual bones to be absolutely certain’. The tomography data here demonstrate that these osteoderms consist of additional layers of dorsal trabecular bone on the skull roof and lateral skull (Fig. 8C). Individual osteoderms (or at least layers of ossification) pass from one bone to another and are almost indistinguishably fused to the underlying bone. These osteoderms form subcircular tubercles on the dermal skull roof, and they decrease in both size and depth as they approach the lateral skull (Fig. 8). The osteoderms present on the lateral surface skull are more irregular and are oval to rectilinear in shape (Gow 1972). The cranial osteoderms of Millereta are remarkably similar to those of some extant squamates (e.g. cordylids, gerrhosaurids, and helodermatids), especially xenosaurids, which also bear cranial osteoderms that are fused to the underlying bone (Bhullar 2011, Dubke et al. 2018).
Postfontal: The postfrontal of Millereta is a triradiate bone, bearing an attenuating anterior process that forms the posterodorsal margin of the orbit, a ventral process that overlaps the postorbital anteriorly and is medially expanded at the orbit, and a short posterodorsal process that fits onto a facet on the parietal (Fig. 1). The postfrontals are best preserved in BP/1/3822, owing to damage in the circumorbital and temporal regions of BP/1/3818 (Fig. 1).
The posterodorsal process of the postfrontal is broadly subtriangular in dorsal view and contributes to the dorsolateral surface of the skull. The posterodorsal process of the postfrontal is more strongly developed than that of Milleropsis pricei (Jenkins et al. 2025) that bears a more crescentic postfrontal. Dermal sculpting or ‘cranial osteoderms’ are present on the postfrontal BP/1/3818 and the holotype, R.C. 14 (Gow 1972: plate 1), but are absent in BP/1/3822, corroborating observations by Gow (1972) that dermal sculpting of the skull roof becomes more prominent through ontogeny in Millereta (Fig. 10).
Postorbital: The postorbital of Millereta is a triradiate bone in lateral view. It bears a short, anterodorsal process that contacts the ventral process of the postfrontal, a ventral process that contributes to the posterior margin of the orbit and contacts the jugal, and a broad posterior process that forms the dorsal margin of the lower temporal opening and makes dorsal contact with the parietal (Fig. 9). A single, fragmentary left postorbital is present in BP/1/3818, whereas both postorbitals are preserved in BP/1/3822. The following description is based on the right postorbital of BP/1/3822, which remains in close articulation with the other temporal elements.
The anterodorsal process of the postorbital is subtriangular and is received by a facet on the posterior surface of the ventral process of the postfrontal (Figs 9, 10). The ventral process of the postorbital is concave where it contributes to the posterior margin of the orbit and becomes acuminate anteroventrally. The anterior surface of the ventral process is mediolaterally expanded at the orbit. The posterior surface of the ventral process bears a facet for the jugal.
The posterior process of the postorbital of Millereta increases in absolute length and relative dorsoventral height through ontogeny, similar to other elements of the temporal region. In immature individuals, the short (but dorsoventrally thin) posterior process is restricted to the anterodorsal portion of the lower temporal opening (SAM-PK-K7366; Figs 3, 10). In more mature but still juvenile specimens, such as BP/1/3822, the posterior process is well developed and forms much of the dorsal margin of the lower temporal opening but falls slightly short of making a posterior contact with the supratemporal (Figs 9, 10). In contrast, the more developed posterior process in BP/1/3818 and the holotype specimen, R.C. 14, contacts the supratemporal and is dorsoventrally tall, approximately half the total dorsoventral height of the element (Fig. 10C). The ‘unossified gap’ (described above) is present between the dorsal surface of the posterior process and the ventral flange of the parietal (Figs 9, 10). This ‘unossified gap’ can be mistaken for the presence of an upper temporal fenestra and probably influenced the accidental preparation of a ‘fenestra’ in this region in various millerettid specimens (e.g. SAM-PK-K10581; Fig. 3C).
Postparietal: The postparietal of Millereta is broadly rectangular in posterior view, although it decreases in dorsoventral exposure and attenuates laterally (Fig. 11A). The paired postparietals of Millereta are located on the midline of the skull in both BP/1/3822 and BP/1/3818 (Figs 2B–D, 6A). In both specimens, the postparietals are moderately disarticulated from their respective articular facets on the posterodorsal surface of the parietal owing to the weak, overlapping suture between these two elements (Fig. 11A). The postparietal contacts the parietal anteriorly, the tabular laterally, and the supratemporal ventrally (Figs 2, 11).
The postparietal is a relatively small, approximately equal to one-third of the mediolateral length of a single parietal (Fig. 2B). The lateral margin of the postparietal contacts the tabular at approximately the level of the posterolateral process of the parietal (Fig. 11A). The posterior end of the postparietal overlies the supraoccipital, which bears facets for the reception of both postparietals. The median contact of the two is uninterrupted by the supraoccipital (Fig. 2B), as in most neoreptiles, such as the mesosaurid Saurodektes kitchingorum (Reisz and Scott 2002) and the early diverging neodiapsid Youngina (Carroll, 1981). This contrasts with the supraoccipital of crownward stem amniotes, such as recumbirostran ‘microsaurs’ (e.g. Brachydectes Cope, 1868; Pardo and Anderson 2016) and captorhinid ‘eureptiles’ (Heaton 1979), in which the postparietals are partly separated along their midline by an ascending process of the supraoccipital, although we note that a cartilaginous ascending process has been described in definitive crown amniotes, including synapsids (Romer and Price 1940).
Tabular: The tabular of Millereta is long and narrow bone that extends posteroventrally from a facet on the medial margin of the posterolateral process of the parietal and approaches the contact between the supratemporal and paroccipital process of the opisthotic (Fig. 11A). Both tabulars are preserved in BP/1/3822, but they are missing in BP/1/3818.
The tabular of Millereta does not contact the squamosal ventrally, similar to other millerettids (e.g. Milleropsis pricei; Jenkins et al. 2025) and neodiapsids (e.g. Youngina capensis; Gow 1974), but unlike early stem reptiles, such as araeoscelidians (e.g. Petrolacosaurus kansensis Lane, 1945; Reisz 1981), in which contact between these elements is visible in posterior view. The ventromedial surface of the tabular of Millereta is weakly concave where it contributes to the dorsolateral margin of a large posttemporal fenestra (Fig. 11A, B). Previously, Millereta was reconstructed as possessing small posttemporal fenestra, although this observation was based on the dorsoventrally crushed holotype, R.C. 14, in which the postparietals are ventrally displaced (Gow 1972). However, Millereta bears relatively large posttemporal fenestrae, approximately as mediolaterally large as the paroccipital processes of the opisthotic (Fig. 11).
Supratemporal: The supratemporal of Millereta is narrow and ogival and is confined to the lateral occipital margin juvenile individuals but with increased dorsal exposure in more mature individuals. Both supratemporals are present in BP/1/3822, and a single, left supratemporal is present in BP/1/3818. The supratemporal of Millereta contacts the postorbital and parietal anteriorly, the squamosal ventrally, and the tabular medially (Figs 1, 10, 11).
The dorsal exposure of the supratemporal is reduced in BP/1/3822. In this specimen, the supratemporal is not completely supported by the ventral flange of the parietal, and instead an unossified gap is present in this region (Fig. 9). However, in more mature individuals the ventral flange of the parietal underlies the supratemporal, supporting it with a facet. Therefore, this unossified gap becomes closed through ontogeny (Fig. 10). In R.C. 14 and R.C. 70, the largest known individuals of Millereta, a more elongate supratemporal extends across the temporal region and contacts the posterior process of the postorbital ventrally, restricting the dorsal exposure of the ventral flange of the parietal, although not to the same extent as in Eunotosaurus (Gow, 1997b; Fig. 10).
The broad contribution of the supratemporal to the skull roof is similar to that of Eunotosaurus aficanus (Gow 1997b) among taxa previously interpreted as millerettids, whereas in the more basal Milleropsis pricei, the supratemporal is restricted to the occipital margin. The supratemporal of Millereta extends further posteroventrally compared with the tabular (Fig. 11) and mildly overhangs the posterior skull in lateral view. However, the tympanic fossa does not extend onto the supratemporal, similar to other ‘millerosaurs’, including the millerettid Milleropsis pricei (Watson 1957), in addition to neodiapsids (e.g. SAM-PK-K7710), in which the tympanic fossa is restricted to the posterolateral surface of the skull. The lack of a supratemporal contribution to the tympanic emargination in millerettids contrasts with the condition in procolophonian stem reptiles, in which the supratemporal posteriorly overhangs the skull and contributes to the tympanic recess, bearing an emargination and a crest that extends onto the supratemporal from the squamosal (e.g. Saurodektes kitchingorum; Reisz and Scott 2002). The supratemporal is absent in recumbirostrans (e.g. Rhynchonkos; Szostakiwskyj et al. 2015) and bolosaurid parareptiles (e.g. Eudibamus cursoris Berman et al. 2000).
Jugal: The jugals are poorly preserved in BP/1/3818, represented by only a fragmentary right jugal, whereas both are preserved in BP/1/3822 (Fig. 1). The jugal of Millereta contacts the lacrimal and maxilla anteriorly, the ectopterygoid medially, the postorbital dorsally, and the squamosal and quadratojugal posteriorly.
The jugal in immature individuals of Millereta is boomerang-shaped, consisting of a narrow and elongate suborbital process and a dorsoventrally tall posterior process (Fig. 10). The suborbital process contacts the lacrimal medial to the maxillary contribution to the orbit, and the medial surface of this process contacts the posterolateral process of the ectopterygoid (Fig. 2B). The posterior process of the jugal of Millereta increased in length through ontogeny (Fig. 10). In BP/1/39 (previously the holotype of Millerosaurus nuffieldi; Watson 1957), a lower temporal emargination is present, and the jugal and the quadratojugal do not contact (Fig. 10A). In more mature juveniles (BP/1/3822 and SAM-PK-K7366), the posterior process extends further posteriorly, contacting the quadratojugal, hence closing the ventral margin of the lower temporal emargination to form instead a lower temporal fenestra. This increasing posterior extent of the posterior process gives the jugal a more quadrangular appearance in lateral view. In specimens of Millereta more ontogenetically mature than SAM-PK-K7366 (e.g. BP/1/3822 and R.C. 14), the posterior process of the jugal contacts the anteroventral process of the squamosal, excluding the quadratojugal from the lower temporal fenestra. In R.C. 14, the jugal forms an interdigitating suture with the squamosal and postorbital, leaving a small fenestra between these three elements of the temporal region. The lower temporal fenestra is then entirely absent in Millereta SAM-PK-K10581 and referred specimen, R.C. 70 (Gow 1972: fig. 2).
Millereta is the only millerettid known to lose its temporal opening throughout ontogeny (Fig. 10); all other millerettids possess a lower temporal emargination, including Broomia (Thomassen and Caroll, 1981) and Milleropsis (Gow, 1972). The closure of the lower temporal opening through ontogeny in Millereta is similar to the acleistorhinid Delorhynchus cifelli, although in Delorhynchus the lower temporal opening is never fully closed and only the jugal contributes to this partial closure (Haridy et al. 2016). The posterior process of the jugal also increases in length through ontogeny in neodiapsids, including early members of Sauria (Ezcurra and Butler 2016).
The dermal sculpting (or cranial osteoderms) of Millereta extends onto the lateral surface of the jugal, in contrast to earlier branching millerettids, such as Milleropsis (Jenkins et al. 2025) and Broomia (Cisneros et al. 2008), in which dermal sculpting is restricted to the skull roof.
Squamosal: Both squamosals are preserved in BP/1/3822. The squamosal is a complex bone, consisting of an anteroventral process that overlaps the quadratojugal, excluding it from the lower temporal opening (Figs 1B, 6), an anterodorsal process that forms the posterodorsal margin of the lower temporal opening in early stages of ontogeny (or makes sutural contact with postorbital and jugal in late ontogeny, e.g. R.C. 70; Gow 1972: fig. 2), and a short posterodorsal process that is overlapped by the supratemporal and lies on the dorsal surface of the quadrate (Fig. 1B).
The anteroventral process of the squamosal forms the posteroventral margin of the lower temporal opening. Contact of the squamosal with the posterior process of the jugal prevents a contribution of the quadratojugal to the lower temporal opening in all but the least mature individuals of Millereta (e.g. BP/1 /39, former holotype of Millerosaurus nuffieldi).
The squamosal of Millereta, as in other millerettids (e.g. Milleropsis pricei; Gow 1972), lacks an occipital shelf, allowing the quadrate to be exposed in both occipital and lateral views. This lack of an occipital shelf is a feature that millerettids share with neodiapsids, including crown group reptiles (e.g. Youngina capensis; Gow 1974; although see Claudiosaurus Carroll, 1981; Carroll 1981). In contrast, crownward stem amniotes, such as protorothyridids (Clark and Carroll 1973), captorhinids (Heaton 1979) (Clark and Carroll 1973), and definitive early reptiles, such as araeoscelidians (e.g. Petrolacosaurus; Reisz 1981) and ankyramorphs (e.g. Delorhynchus; Reisz et al. 2014), possess a rectilinear squamosal with a broad contribution to the occiput by the occipital shelf, entirely sheathing the quadrate in lateral and occipital views (Pritchard and Nesbitt 2017). The posterolateral surface of the squamosal of Millereta bears a laterally oriented posterolateral process that contributes to a posterolaterally oriented tympanic emargination (Fig. 12). The tympanic emargination of the squamosal is supported by medial and lateral tympanic crests, identical in shape to those of Milleropsis pricei (Jenkins et al. 2025). The medial tympanic crest extends onto the ventrolateral surface of the quadrate, whereas the lateral tympanic crest ends at the confluence of the quadratojugal and squamosal. The squamosal also contributes to the lateral margins of the tympanic emargination in several early neodiapsids and saurians, including the archosauromorph Prolacerta broomi Parrington, 1935 (Modesto and Sues, 2004) and the lepidosauromorph Marmoreta oxoniensis Evans, 1991 (Griffiths et al. 2021).
The internal surface of the squamosal braces the tall, dorsal process of the quadratojugal (Fig. 12C). A posterodorsal process overlaps the dorsal process of the quadrate immediately dorsal to the quadrate contact with the paroccipital process (Fig. 11A). The posterior surface of the squamosal of Millereta does not contribute to the quadratojugal foramen; instead, this foramen is framed entirely by the quadrate and quadratojugal, similar to some early neodiapsids, including saurians (e.g. the tanystropheid archosauromorph Macrocnemus bassanii; Miedema et al. 2020). This feature distinguishes Millereta (and other millerettids) from earlier diverging stem reptiles, including araeoscelidians (e.g. Araeoscelis casei Broom, 1913; Vaughn 1955) and ankyramorphans (e.g. Procolophon trigoniceps Owen, 1876; Carroll and Lindsay 1985), in which the squamosal contributes to the lateral margin of the quadratojugal foramen.
The squamosal of Millereta lacks a pterygoid ramus that contacts the quadrate ramus of the pterygoid, as in all early diverging neodiapsids in which this is region is known, including weigeltisaurids (e.g. Coelurosauravus elivensis Piveteau, 1926; Buffa et al. 2021) and Claudiosaurus germaini Carroll, 1981 (Carroll, 1981). The absence of a pterygoid ramus of the squamosal contrasts with the condition in crownward stem amniotes, such as recumbirostrans (e.g. Pantylus cordatus Cope, 1881), diadectomorphs (e.g. Limnoscelis paludis Williston, 1911; Romer 1946), and protorothyridids (e.g. Protorothyris archeri Price, 1937; Clark and Carroll 1973), and early diverging stem reptiles such as Araeoscelis (Vaughn, 1955) and acleistorhinids (e.g. Feeserpeton oklahomensis MacDougall & Reisz, 2012; Macdougall and Reisz 2012), which all possess squamosal– pterygoid contact.
Quadratojugal: The quadratojugal comprises a subtriangular anterior process, a tall dorsomedial process that is sheathed by the squamosal in lateral view, and a posteromedial process that forms the ventral portion of the tympanic fossa. The quadratojugal contacts the jugal anteriorly, the squamosal dorsally and laterally, and the quadrate posteromedially. The description of the quadratojugal that follows is based primarily on the right quadratojugal of BP/1/3822, which is the most complete quadratojugal of the specimens scanned here.
The quadratojugal of Millereta does not contribute to the lower temporal opening except in immature individuals (SAM-PK-7366), in contrast to other millerettids (e.g. Milleropsis; Watson 1957). Instead, the dorsal surface of the anterior process of the quadratojugal is overlapped by the squamosal, and the anterior end of this process contacts the posteroventral margin of the jugal (Figs 1, 10).
The quadratojugal of Millereta can be differentiated from the quadratojugal of most Permian stem reptiles by the tall dorsomedial process that extends dorsally for the entire height of the temporal region, nearly approaching the postorbital or supratemporal internally (Fig. 12; Gow 1972: fig. 9D–F). A similar dorsomedial process is present in Milleropsis (Jenkins et al. 2025), ‘ Millerosaurus ’ (Gow 1972), and Eunotosaurus (Gow 1997b) among late Permian stem reptiles. This dorsal process of the quadratojugal of Millereta is supported by the lateral surface of the quadrate, which bears a distinct groove to receive this process (Fig. 12).
The quadratojugal forms the ventral margin of the tympanic fossa but does not bear any tympanic crests, which are restricted to the squamosal and quadrate. The tympanic fossa on the quadratojugal takes the form of a posteromedially facing convexity that is directed towards a lower, mediolaterally facing fossa on the dorsal process of the quadrate. The quadratojugal forms the lateral margin of the quadratojugal foramen (Fig. 12). The quadratojugal of BP/1/3822 does not bear dermal sculpting on its lateral surface, in contrast to skeletally more mature individuals of Millereta (e.g. R.C. 14; Broom 1938).
Quadrate: The quadrate consists of a vertically oriented shaft that bears a small tympanic fossa and crest, asymmetrical condyles, an anteroposteriorly short pterygoid wing or ramus, and a raised stapedial boss (Fig. 12). Both quadrates of BP/1/3822 are well preserved and are the basis for the following description. The quadrate is sheathed by the squamosal and quadratojugal laterally and contributes to the jaw joint where it contacts the articular ventrally.
The dorsal shaft of the quadrate is visible in lateral view throughout its height owing to the absence of an occipital shelf of the squamosal and quadratojugal, although slight displacement of the quadratojugal and squamosal in BP/1/3822 have partly hidden the quadrate in anterolateral view (Figs 10, 12). The posterior margin of the dorsal shaft is oriented vertically, contrasting with the posterodorsally oriented shaft of Milleropsis (Jenkins et al. 2025). The dorsal end of the shaft of the quadrate of Millereta lacks a cephalic condyle or head overhanging the squamosal in lateral view, in contrast to early saurian, such as the lepidosauromorph Paliguana whitei Broom, 1903 (Ford et al. 2021). The quadrate shaft contributes to the medial wall of the quadratojugal foramen, although the quadratojugal foramen is not well defined in our segmentations (Fig. 12A).
The posterior margin of the quadrate bears a ventrolateral fossa that is confluent with the tympanic recess of the quadratojugal and squamosal, but lacks the posteriorly facing emargination or conch present in some early neodiapsids (e.g. SAM-PK-K7710, or Thadeosaurus colcanapi Carroll, 1981; Buffa et al. in press) and crown-group reptiles (Carroll 1969a). However, the posteromedial surface of the quadrate of Millereta is concave where it contributed to the medial extent of the tympanic fossa, as in Milleropsis pricei (Jenkins et al. 2025) and Eunotosaurus africanus (Gow, 1997b). A small tympanic crest is present on the medial surface of this tympanic fossa (Fig. 12A).
The ventral extents of the quadrate condyles of Millereta are asymmetrical, with the medial condyle extending further ventrally than the lateral condyle (Fig. 12A). The short pterygoid ramus, which extends anteriorly from the condylar region, is dorsally elevated, as in Neodiapsida (Pritchard and Nesbitt 2017), but also present in Milleropsis (Jenkins et al. 2025) and some other Permian stem reptiles (e.g. Sauropareion anoplus Modesto et al., 2001; Macdougall and Modesto 2011). The pterygoid ramus of the quadrate of Millereta sheaths the quadrate ramus of the pterygoid laterally in a loose, overlapping suture. At the junction of the quadrate condyles and pterygoid ramus, the medial surface of the quadrate bears a distinct stapedial boss that cartilaganously or ligamentously supported the ventral surface of the stapedial shaft (Fig. 12A), like other neoreptiles, including early neodiapsids (e.g. Acerosodontosaurus piveteaui; Currie 1980) and some parareptiles (e.g. Feeserpeton oklahomensis; Macdougall and Reisz 2012). This differs from early diverging or stem amniotes, such as captorhinids (e.g. Labidosaurus hamatus Cope, 1896; Modesto et al. 2007), protorothyridids (e.g. Paleothyris Carroll, 1969; Carroll 1969b), and araeoscelidians (Vaughn 1955) that bear a distinct fossa (the stapedial recess) for the reception of the stapedial shaft.
Scleral ossicles: Approximately 12 overlapping scleral ossicles are present in the right orbit of BP/1/3822 (Gow 1972); only seven of these were partly reconstructed owing to poor contrast in this region, which impeded segmentation on the scleral ossicles that were partly exposed by preparation. The sclerotic ring of Millereta is large, ~ 4.5 mm in external diameter in an orbit that is 5 mm in total diameter. The internal diameter of the sclerotic ring is ~ 1.5–2 mm, which is proportionally small and suggests that the visual system of Millereta was adapted to well-lit conditions (Schmitz 2009, Schmitz and Motani 2011).
Palate
The palate is particularly well preserved in BP/1/3822, with these elements remaining in tight articulation with the skull laterally (Fig. 3A). Dorsoventral crushing of BP/1/3818 has disarticulated many of the palatal elements, such that the vomer, palatines, and cultriform process of parabasisphenoid nearly contact the ventral surface of the frontal and nasal (Fig. 1D). Overall, the palate of Millereta is similar to that of Milleropsis (Jenkins et al. 2025), although it is proportionally wider. The palate of Millereta bears slender vomers anteriorly, an extremely long interpterygoid vacuity that separates the pterygoids for most of their length, and a small suborbital foramen located between the ectopterygoid, palatine, and maxilla, which was reported as absent by Gow (1972) but figured in their reconstruction of R.C. 14 (Gow 1972: fig. 4; Fig. 4B).
Vomer: The vomer is a mediolaterally slender and rectangular bone that contacts the pterygoid medially and the palatine posterolaterally immediately anterior to the anterior margin of the orbit (Fig. 13). The vomer extends anteriorly to the anterior margin of the internal naris, where it presumably contacted the palatal process of the premaxilla, although this contact is not preserved in BP/1/3818 and BP/1/3822 because the premaxillae are damaged in these specimens (Fig. 4A).
The vomer is approximately five times longer anteroposteriorly than it is mediolaterally wide, similar to the proportions of Milleropsis pricei (Gow, 1972), although it is shorter relative to the total skull length in Millereta. The medial suture between the vomers of Millereta are separated by the palatal process of the pterygoid posteriorly, but they contact each other anteriorly at the level of the sixth or seventh maxillary tooth position (Figs 4, 8).
The lateral margin of the vomer forms the medial margin of the elongate, but mediolaterally narrow, internal naris (Fig. 4). The vomer of Millereta and other millerettids lacks the ‘alar process’ of pareiasaurs (Tsuji 2006) that partly extends into the internal nares. Instead, the lateral margin of the vomer is straight (Fig. 13).
The vomer of Millereta lacks the asymmetric vomerine process sensu Ford and Benson (2019), and instead the anterior end of this element attenuates to a single point. Immediately posterior to the premaxilla articulation is an anterior vomerine buttress that supports three enlarged vomerine teeth, although the right vomer is missing a single tooth and the left vomer is missing two (Fig. 13B, C). These enlarged vomerine teeth are separated from the midline vomerine tooth row by a diastema or gap in the vomerine dentition, a feature diagnostic of the Millerettidae (Gow 1972). A large foramen, the vomerine aperture of Gow (1972), pierces the anterior surface of the vomerine buttress. Enlarged vomerine teeth are also present in Eunotosaurus africanus (Bever et al. 2015) and Milleropsis pricei (Jenkins et al. 2025), although they are relatively smaller in Milleropsis. There is only a single, medial tooth row on the ventral surface of the vomer, which lacks the lateral or choanal tooth row or rows seen in early neodiapsids (e.g. Youngina; Hunt et al. 2023) or the anterolaterally directed tooth row that extends onto the vomer from the palatine in procolophonians (e.g. Saurodektes kitchingorum; Reisz and Scott 2002). A single vomerine tooth row is described in Eunotosaurus aficanus (X.A.J. pers. obv. CM-777) and in Milleropsis pricei (Jenkins et al. 2025).
The dorsal surface of the vomer of Millereta is extremely flat, lacking the ascending process (‘alar projection’ of Heaton 1979) or midline flange that frames the choana in crownward stem amniotes, such as captorhinids (Fox and Bowman 1966, Heaton 1979) and recumbirostrans (e.g. Euryodus dalyae Carroll & Gaskill, 1978; Carroll and Gaskill 1978, Gee et al. 2021). However, low ridges are present on the dorsal surface, similar to the early diverging neodiapsid Youngina capensis (Hunt et al., 2023) and the millerettid Milleropsis pricei (Jenkins et al. 2025; Fig. 13A).
Palatine: The palatine of Millereta is a complex bone. The palatines, present in both BP/1/3818 and BP/1/3822, are bordered by the palatal process of the pterygoid medially, the ectopterygoid posteriorly, the maxilla laterally, and the vomers anteriorly (Figs 4, 8).
The palatine of Millereta forms the anterior margin of the suborbital foramen together with the maxilla and ectopterygoid, and its anterior margin is incised by the choana or internal naris (Fig. 4). A row of teeth (‘T2’ of Welman 1998) extends onto the palatine from the pterygoid on a slight ventral ridge, ending immediately posterior to the incision for the choana (Fig. 4). A distinct depression or pocket is present lateral to the ridge supporting T2 and medial to the maxillary ramus of the palatine. This pocket was also reported in other millerettids (e.g. Milleropsis pricei; Gow 1972). The maxillary ramus of the palatine of Millereta is extremely robust, unlike the dorsoventrally thin and anteroposteriorly constricted maxillary ramus present in taxa with an elongate suborbital fenestra, such as Orovenator mayorum Reisz et al., 2011 (Ford and Benson, 2019) and neodiapsids, such as the early diverging Youngina capensis (Hunt et al. 2023).
The dorsal surface of the palatine of Millereta is relatively flat posteriorly. A distinct ascending process rises dorsally to form the anteroventral margin of the orbit, medial to the maxilla and lacrimal. This ascending process is visible in posterolateral or medial views (Fig. 5C) and forms most of the contact with the ventral process of the prefrontal. A distinct foramen, the foramen orbitonasale, is present on the lateral surface of the palatine immediately lateral to the ascending process. The foramen orbitonasale is widespread in those early amniotes that have a broad prefrontal–palatal contact, including synapsids (e.g. Edaphosaurus boanerges; Modesto 1995), procolophonids (e.g. Libognathus sheddi Small, 1997; Mueller et al. 2024), the early diverging neodiapsid Youngina capensis (X.A.J., pers. obv. BP/1/2871), early turtles (e.g. Palaeochersis talampayensis, Rougier et al., 1995; Sterli et al. 2007), and rhynchosaurian archosauromorphs (e.g. Hyperodapedon sanjuanensis Sill, 1970; Gentill and Ezcurra 2022). The palatine of Millereta lacks a transversely oriented orbitonasal ridge that contributes to nasal septum, similar to Orovenator (Ford and Benson, 2019) and neodiapsids (e.g. Youngina capensis; Hunt et al. 2023) but in contrast to crownward stem amniotes, such as recumbirostrans (e.g. Euryodus; Gee et al. 2021) and captorhinids (Heaton 1979). Likewise, the dorsal surface of the palatine lacks a choanal ridge that would contribute to the dorsolateral portion of the internal nares, contrasting with the condition in Captorhinus laticeps (Heaton, 1979).
Ectopterygoid: The ectopterygoid of Millereta, best preserved in BP/1/3822, is the shortest bone in the palate, approximately one-quarter of the anteroposterior length of the palatine (Fig. 4). The main body of the palatine is subrectangular, but an elongate posterolateral process (twice the anteroposterior length of the main body) is emarginated by the adductor fossa and acts as a brace between the palate and the jugal and maxilla (Figs 4, 13).
The main body of the ectopterygoid of Millereta is located anterior to the transverse flange of the pterygoid and contributes broadly to the adductor fossa. The ectopterygoid bears a posterolateral process that contacts the jugal and maxilla laterally, where it bears a slightly dorsoventrally expanded articular facet for these bones. A posterolateral process of the ectopterygoid is present in the early reptile Orovenator mayorum (Ford and Benson 2019), millerettids (X.A. Jenkins, pers. obv. BP/1/720), and early neodiapsids (e.g. Youngina capensis; Hunt et al. 2023). This contrasts with the condition in traditional eureptiles, such as araeoscelidians (e.g. Petrolacosaurus; Reisz 1981), in which the ectopterygoid lacks a posterior expansion and is relatively rectilinear in ventral view.
The anterior margin of the ectopterygoid of Millereta overlaps the palatine and forms the posterior margin of the suborbital foramen at this contact, together with a lateral contribution from the maxilla in BP/1/3822, but not BP/1/3818 (Fig. 4). In BP/1/3818, the suborbital foramen is small and located between the ectopterygoid and palatine, but this is attributable to an anterior displacement of the ectopterygoid (Fig. 4C, D). The suborbital foramen is considered absent (Gow 1972) or small (Lee 1995) in millerettids, which was hypothesized as the plesiomorphic condition among stem reptiles by studies such as those by Gauthier et al. (1988) and Laurin and Reisz (1995). However, the suborbital foramen is, in fact, present in all millerettids: Broomia (Thomassen and Carroll, 1981: fig. 1), Millereta (this study), and Milleropsis pricei (Jenkins et al. 2025).
Pterygoid: Complete pterygoids from both sides are evident in the tomography data of BP/1/3818 and BP/1/3822, although both pterygoids of BP/1/3818 are displaced dorsally such that the anterior ends of the palatal processes nearly contact the ventral surface of the frontals (Fig. 1D). The pterygoids of Millereta are the longest elements of the palate. They contact all other bones of the palate and connect the palate to the braincase and quadrate posteriorly (Figs 4, 8), as in most other amniotes. The palatal ramus of the pterygoid extends anteriorly, contacting the vomer anteromedially, the palatine medially, and the ectopterygoid posteromedially (Figs 4, 8).
The long interpterygoid vacuity of Millereta separates the palatal rami of the contralateral pterygoids over nearly their entire length, a feature diagnostic of millerettids, including Broomia perplexa (Thomassen and Caroll, 1981) and Milleropsis pricei (Gow, 1972), but that is also present Eunotosaurus aficanus (Bever et al. 2015: fig. 1C). Two tooth rows are present on the palatal ramus of the pterygoid of Millereta; an anterolaterally oriented tooth row that extends onto the palatine (T2) and a much longer tooth row (‘T3’ of Welman 1998) along the margin of the interpterygoid vacuity that extends anteroposteriorly from the basipterygoid region of the pterygoid onto the vomer, anteriorly (Fig. 4). This contrasts with the middle Permian millerettid Broomia perplexa and some neodiapsids (e.g. Youngina capensis, Hunt et al. 2023), which bear two tooth rows in the region of T2 that extend onto the palatine, but is similar to Milleropsis.
The transverse flange of the pterygoid of Millereta extends posteroventrally into the subtemporal fossa, projecting far ventral to the maxillary tooth row as visible in lateral view (Fig. 4). The transverse flange of Millereta is directed laterally in ventral view, differing from Milleropsis, in which the transverse flange is directed weakly anterolaterally. It is bordered by the ectopterygoid anteriorly but not laterally, unlike some procolophonians (e.g. Saurodektes kitchingorum; Reisz and Scott 2002), in which the ectopterygoid descends posteroventrally along the lateral margin of the transverse flange of the pterygoid. The transverse flange of the pterygoid of Millereta bears a single large row of teeth on its posterior margin and several rows of smaller teeth anteriorly (Fig. 4), although denticles in the form of a shagreen are absent. A distinct sulcus is present on the ventral surface of the pterygoid, located between the anterior margin of the transverse flange and T2 of the palatal ramus in ventral view. This sulcus extends onto the posterior margin of the ectopterygoid (Figs 3, 9C). The basicranial recess of the pterygoid is positioned immediately medial to the transverse flange and takes the form of a posteriorly facing, subcircular facet (Fig. 14A) that receives the basipterygoid processes of the parasphenoid, excluding the epipterygoid from contributing to the basicranial recess, as in other millerettids (Jenkins et al. 2025) and neodiapsids including crown reptiles (Gow 1974), but unlike the condition in other early stem reptiles, including acleistorhinids (e.g. Delorhynchus cifelli; Reisz et al. 2014) and araeoscelidians (e.g. Petrolacosaurus kansensis; Reisz 1981), in which the epipterygoid contributes to the basicranial recess.
The quadrate ramus of the pterygoid of Millereta extends posterolaterally from the basicranial recess to contact the pterygoid ramus of the quadrate (Fig. 14). The quadrate ramus of Millereta does not contact the squamosal, which lacks a pterygoid ramus. The quadrate ramus is entirely edentulous and is divided into two processes, best visible in posterior view: the dorsal (or quadrate) flange and a well-developed arcuate flange that extends medially from the ventral margin of the quadrate flange, forming an angle of 90°. An edentulous quadrate ramus of the pterygoid is widespread among stem reptiles (e.g. the araeoscelidian Halgaitosaurus gregarius Henrici et al., 2023; Henrici et al. 2023), although a quadrate ramus bearing denticles is known in acleistorhinids (e.g. Delorhynchus cifelli; Reisz et al. 2014) and in the possible bolosaurian Erpetonyx arsenaultorum Modesto et al., 2015 (Modesto et al., 2015). The dorsal flange of the quadrate ramus of Millereta bears a distinct groove on the ventrolateral surface for the footplate of the epipterygoid (Fig. 14B). The ventral margin of the dorsal flange of the quadrate ramus bears a low crest marking the origination of the m. pterygoideus lateralis (Fig. 14B).
The dorsal surface of the pterygoid bears a longitudinal sulcus for the medial palatine ramus of the facial nerve (CN VII) and the inferior nasal artery (Fig. 4B), located immediately anterior to the groove for the epipterygoid. The anterolateral pathway of this sulcus is more well defined in Millereta than in Milleropsis (Jenkins et al. 2025) and can be seen to extend towards the articulation between the palatine and ectopterygoid in dorsal view (Fig. 4B). The orbitotemporal ridge (sensu Heaton 1979) marking the orbitotemporal membrane is visible but poorly developed and is located on the dorsal surface of the transverse flange of the pterygoid (Fig. 4B).
Braincase
The braincase of the Millereta specimens described here is largely disarticulated owing to dorsoventral crushing in both specimens. In BP/1/3822, the occiput is dorsoventrally crushed, causing the basioccipital to drift posteriorly, the exoccipitals laterally, and the opisthotics medially, although the supraoccipital remains in its original position relative to the skull roof dorsally. In BP/1/3818, the occiput is relatively intact, although the otic capsules are splayed ventrolaterally, no longer in articulation with the parabasisphenoid ventrally.
Epipterygoid: The epipterygoid of Millereta consists of an anteroposteriorly broad and subtriangular footplate that rests on the lateral surface of the quadrate ramus of the pterygoid, and a dorsal columella that does not form a bony contact with the skull roof (Fig. 14). The epipterygoid is present in both scanned specimens of Millereta, although the right epipterygoid of BP/1/3822 is missing and the left epipterygoid has been rotated nearly 180°, such that the dorsal columella faces ventrally. The epipterygoids remain articulated in BP/1/3818 (Fig. 14).
The epipterygoid of Millereta bears a medial process that overhangs but does not contribute to the basicranial articulation, as described by Gow (1972), and which is also present in other millerettids. Gow (1974) also noted that a similar process was present in the neodiapsids Youngina capensis and Prolacerta broomi. The lack of a contribution of the epipterygoid to the basal articulation contrasts with the condition in other early amniotes, including synapsids (Romer and Price 1940), captorhinids (Heaton 1979), and bolosaurids (e.g. Belebey; Reisz et al. 2007). A small foramen pierces the ventral portion of the dorsal columella in the right epipterygoid of Millereta BP/1/3818, although this is not visible in the other epipterygoids and is of an uncertain identity (Fig. 14A).
Parabasisphenoid: The parasphenoid and basisphenoid are indistinguishably fused in Millereta and are described here as the parabasisphenoid. The parabasisphenoid of Millereta is remarkablycomplex,extendingfromtheocciputposteriorlytonearlythe posterior margin of the choana (Fig. 4). The parabasisphenoids of both BP/1/3818 and BP/3822 are present, although both are weakly displaced (Fig. 1). The parabasisphenoid of Millereta contacts the pterygoids anteriorly, the prootics dorsally, and the basioccipital posteriorly.
The elongate cultriform process of Millereta extends from the rostral process posteriorly to the posterior margin of the choana in ventral view and remains in the same dorsoventral plane for its entire length (Fig. 4). This differs from the condition in captorhinids (e.g. Captorhinus aguti Cope, 1882; Fox and Bowman 1966, Heaton 1979) or synapsids (e.g. Dimetrodon milleri Romer, 1937; Romer and Price 1940), in which the cultriform process rises anterodorsally above the palate. The cultriform process of Millereta forms a strong ‘V’-shape in cross-section, bearing a ventral keel on the ventral surface where two anteroposteriorly oriented laminae converge, probably supporting a trough for a cartilaginous interorbital septum. In Millereta, a row of teeth is present on the ventral keel of the cultriform process throughout its length (Fig. 4), as in many early amniotes but unlike members of the reptile crown group, Sauria, in which teeth on the cultriform process are either absent or restricted to the region adjacent to the basipterygoid processes (e.g. in the stem lepidosaur Fraxinisaura rozynekae Schoch & Sues, 2018; Schoch and Sues 2018). Oval-shaped crista trabeculae are present at the junction of the cultriform process and anterior surface of the rostral process of the parabasisphenoid and are visible in anterior view (Fig. 15C).
The basipterygoid processes of Millereta extend anteroventrolaterally as short processes that bear a single articular facet on the anterior surface for the basicranial recess of the pterygoid (Fig. 15C). The ventral surface of the parabasiphenoid of Millereta bears a groove for the exit of the palatal branch of the carotid artery immediately medial to the basipterygoid processes (Fig. 4D). The posterior margin of the ventral surface of the parabasiphenoid of Millereta is weakly concave between the crista ventrolaterales, except for two anteromedially oriented ridges that bear a row of approximately six teeth (Fig. 4A), similar to the condition in the millerettids Milleropsis pricei (Gow, 1972) and Broomia perplexa (Cisneros et al. 2008), and also present in mesenosaurine varanopids (e.g. Mesenosaurus romeri; Reisz and Berman 2001). The ventral surface, or plate, of the parabasisphenoid widens posteriorly, such that the crista ventrolaterales diverge, similar to Broomia (Cisneros et al. 2008) but unlike Milleropsis, in which the crista ventrolaterales are parallel throughout their length (Jenkins et al. 2025). The ventral plate of the parabasiphenoid bears two tooth rows that merge with dentition of the cultriform process (Fig. 4A, E), as in other millerettids (e.g. Milleropsis, Gow 1972; Broomia, Cisneros et al. 2008).
The lateral surface of the parabasisphenoid of Millereta is deep anterior to the dorsum sellae, but quickly decreases in dorsoventral height posteriorly, becoming extremely thin (Fig. 15B). The entry foramen for the vidian canal is visible in lateral view, immediately anteroventral to the clinoid processes (Fig. 15B). The vidian sulcus, which is entirely closed within the lateral wall of the parabasisphenoid, continues anteriorly, where it divides into separate pathways for the cerebral and palatal branches of the internal carotid arteries, similar to the condition in Milleropsis pricei (Jenkins et al. 2025). The course of the cerebral branch rises anterodorsally, exiting in paired foramina in the posterior wall of the pituitary fossa (Fig. 15D), whereas the palatal branch exits via a foramen medial to the basipterygoid processes (Fig. 15C). The carotid artery is also within the lateral wall of the braincase in procolophonian reptiles and saurians, such as early lepidosauromorphs and testudines (Shishkin 1968; Müller et al. 2011). In contrast, the pathway of the carotid arteries extends along the ventral plate of the parabasisphenoid in most other early amniotes, including many stem reptiles (Ford and Benson 2019).
The dorsal surface of the parabasiphenoid of Millereta is extremely complex in comparison to its mostly flat ventral surface (Fig. 15D). A depression for the pituitary or hypophyseal fossa is located posterior to the crista trabeculae, which is framed by a low parabolic ridge posteriorly (Fig. 15D). The pituitary fossa bears two foramina that mark the exit of the cerebral branch of the carotid arteries (Fig. 15D). A fossa for the retractor bulbi muscle is present posterior to the ridge for the pituitary fossa, which in turn is framed by the low dorsum sellae of the parabasisphenoid. The clinoid processes are located laterally to the dorsum sellae and are approximately four times taller than the maximum height of the dorsum sellae (Fig. 15C). The dorsomedial surface of the parabasisphenoid is bifurcated for the pathway of the abducens nerve, medial to the prootic articulation of the clinoid process (CN VI; Fig. 15C). This is similar to the condition in Milleropsis pricei (Jenkins et al. 2025), the early diverging neodiapsid Youngina capensis (Gardner et al. 2010), and early saurians, such as Prolacerta broomi (Evans, 1986), but unlike the condition of most other early amniotes, including captorhinids (e.g., Captorhinus laticeps; Heaton, 1979) and araeoscelidians (e.g. Petrolacosaurus kansensis; Peabody, 1952), in which CN VI travels through the wall of the dorsum sellae (Romer and Price 1940, Heaton 1979).
Basioccipital: The basioccipital of Millereta is a plate-like bone in dorsal and ventral view, consisting of a thin, anterior lamina that overlaps the parabasiphenoid, paired dorsal facets for the exoccipitals, and a posteriorly facing occipital condyle that bears a notochordal pit on its posterior surface (Fig. 15). Both basioccipitals are preserved in the scanned specimens of Millereta; however, the basioccipital of BP/1/3822 is poorly ossified and weakly disarticulated relative to the rest of the braincase. The basioccipital of BP/1/3818, however, is more ossified and bears a well-defined occipital condyle.
A sagittal ridge extends across the dorsal surface of the basioccipital, possibly for the bifid ligament of the medulla. In the holotype specimen, R.C. 14, this sagittal ridge is flanked by two depressions (Gow 1972: fig. 9), although these are difficult to resolve in our segmentations. There is no fissure between the basioccipital and parabasisphenoid, contra Laurin and Reisz (1995), who considered a basal cranial fissure an autapomorphy of millerettids that was acquired convergently in procolophonians.
The ventral surface of the basioccipital is flat, lacking the development of ventral basal tubera present in the holotype of Millereta rubidgei, R.C. 14, and that of Milleropsis pricei (Jenkins et al. 2025), although these two specimens belong to mature individuals, in contrast to the material described here (Fig. 15A). The basioccipital does not contribute to the fenestra ovalis in BP/1/3822, a feature that is widespread in early stem reptiles, such as the araeoscelidian Araeoscelis casei (Vaughn 1955), but absent in the millerettid Milleropsis and early neodiapsids (e.g. Youngina capensis; Gardner et al. 2010).
Exoccipital: The exoccipital of Millereta is falciform in posterior view, flaring ventrally where it abuts the dorsal surface of the basioccipital, nearly contacting the contralateral exoccipital (Fig. 15A). The exoccipitals of Millereta are distinct early in ontogeny (Fig. 15A), but fuse to the basioccipital in mature individuals, such as the holotype, R.C. 14 (Gow 1972). The exoccipital of Millereta contacts the opisthotic laterally, basioccipital ventrally, and the supraoccipital dorsally (Figs 11, 16).
The exoccipitals of Millereta form the lateral margins of the foramen magnum but do not exclude the basioccipital from participating in its ventral margin. At the suture between the exoccipital and opisthotic lies a large and undivided metotic canal for nerves IX–XI (Gow 1972). The pathways of cranial nerves IX–XI are visible as a groove framed by a mediodorsal ridge, best visible in anterior view of BP/1/3818. Paired foramina for the hypoglossal nerve (CN XII) are present on the posterior surface of the exoccipital (Fig. 15A), a condition that is widespread in early amniotes and contrasts with the presence of a single hypoglossal foramen present in early tetrapods and some stem amniotes, such as recumbirostran ‘microsaurs’ (e.g. Euryodus dalyae; Gee et al. 2019). No facets for the proatlas are visible on the posterior surface, and the exoccipitals do not contribute to the occipital condyle.
Prootic: The prootic of Millereta is well ossified but disarticulated relative to other braincase bones in both specimens. The prootic consists of a main body housing the vestibular apparatus that contacts the opisthotic posteriorly (forming the fenestra ovalis) and the supraoccipital dorsally, and a ventral process that contacts the parabasiphenoid ventrally (Fig. 15B). The prootic of Millereta contributes to the anterior margin of the paroccipital process for a short distance posterodorsal to the fenestra ovalis, best seen in anterior view (Fig. 15C).
The anterior surface of the prootic of Millereta bears a marked swelling for the anterior semicircular canal (Fig. 17A). This is often referred to as the ‘alar process’ in descriptions of early reptiles (e.g. Heaton 1979, Hamley et al. 2021), although it differs from the ‘alar process’ of squamates, which is a distinct, wing-like anterodorsal process (Evans 2008). This ‘alar process’ overhangs the notch for the trigeminal nerve (CN V), which is exposed in lateral view, contrasting with early synapsids, in which CN V is located posterodorsal to an infratrigeminal process (Romer and Price 1940; Fig. 15B). In Millereta, the supratrigeminal process is expressed as a medially projecting flange that frames the dorsomedial margin of CN V and indicates the pathway of the middle cerebral vein (Fig. 17A). The floccular fossa is present along the medial surface of the prootic immediately posterior to the supratrigeminal process (Figs 15C, 17B).
The ventral process of the prootic articulates with the clinoid process of the parabasisphenoid (Fig. 15B).
The foramen for the facial nerve (CN VII) is present as a notch on the ventral surface of the ventral process (Figs 15B, 17), similar to Milleropsis pricei and unlike most other early reptiles, in which it is expressed as a distinct foramen (e.g. the procolophonid Eomurruna yurrgensis Hamley et al., 2021; Hamley et al. 2021). A low crest trends anteroventrally from the opisthotic onto the ventral process of the prootic, somewhat similar to the crista prootica described in crown reptiles (e.g. Prolacerta; Gow 1974), although it is much more poorly developed in Millereta (Fig. 16). A similar crest is present in Milleropsis (Jenkins et al. 2025), but in Milleropsis, the crest does not extend onto the opisthotic. The fenestra ovalis of Millereta is well defined and formed primarily by the prootic and opisthotic, with a weak ventral contribution of the parabasisphenoid (Fig. 15B). The large fenestra ovalis of Millereta is relatively well defined across all ontogenetic stages and matches the size of the enlarged stapedial footplate.
The medial wall of the prootic of Millereta is well ossified in comparison to crownward stem amniotes, such as recumbirostrans (Pardo and Anderson 2016), with the pathway of the anterior semicircular canal being entirely enclosed by bone along its length (Figs 16, 18). In contrast, the lateral semicircular canal of Millerta remains open medially owing to increasingly incomplete ossification of the medial wall of the prootic as it approaches the paroccipital process of the opisthotic. However, this is likely to be ontogenetic, because the medial surface of the prootic is much more ossified in adult specimens of Milleropsis pricei (Jenkins et al. 2025).
Opisthotic: The opisthotic of Millereta consists of a rod-like paroccipital process, a dorsal expansion that contacts the supraoccipital, and a ventral ramus that contacts the exoccipital posteriorly and forms the posterior border of the fenestra ovalis anteriorly (Fig. 15). The most complete opisthotic is the right opisthotic of BP/1/3822; other opisthotics are present in the tomography data but are less complete. The right opisthotic of BP/1/3822 is rotated inwards ~90°, such that the paroccipital process is oriented anteriorly.
The anteroventral surface of the ventral ramus of the opisthotic bears a groove along its anteroventral surface that approaches the morphology described for the ‘otic trough’ of diadectomorphs and synapsids, although this groove is limited to the ventralmost surface and does not lead into the fenestra ovalis (Berman et al. 1992, Cisneros et al. 2020). In Millereta, the anterior surface of the ventral ramus of the opisthotic bears a well-developed crista interfenestralis (Fig. 16D; sensu Pardo et al. 2017), which separates the metotic fossa from the vestibule, a feature widespread in Amniota (Romer and Price 1940, Heaton 1979) but absent in early tetrapods (e.g. Seymouria Broili, 1904; Pardo et al. 2017). A lagenar crest separates the lagena from the structure identified as the ‘recessus scalae tympani’ by Heaton (1979), although this structure in captorhinids almost certainly does not correspond to the recessus scalae tympani of crown reptiles (Fig. 16D). The medial wall for the lateral semicircular canal is well developed, although less so than in the holotype of Milleropsis, BP /1/720 (a mature individual), in which the medial wall nearly canalizes the lateral semicircular canal (Fig. 17D). It is probable that this feature would be more strongly developed in more ontogenetically mature individuals of Millereta, given the ontogenetic variation of this trait in Milleropsis (Jenkins et al. 2025). There is no indication of a perilymphatic foramen.
The opening for the metotic foramen is located between the opisthotic and exoccipital, approximately at the confluence of the paroccipital process and ventral ramus (Fig. 11). The paroccipital process of the opisthotic forms the ventral margin of the posttemporal fenestra in Millereta and is circular in cross-section throughout its length (Fig. 11). The paroccipital process of Millereta is laterally directed and contacts the medial surface of the dorsal shaft of the quadrate, and is overall mediolaterally shorter in comparison to Milleropsis pricei (Jenkins et al., unpublished research). A ventromedially sloping ridge is present at the junction of the paroccipital process and the ventral ramus of the opisthotic, whereas in Milleropsis this ridge is restricted to the paroccipital process (Jenkins et al. 2025). Internally, the lateral semicircular canal travels through the paroccipital process (best visible in anterior view) where it joins the posterior semicircular canal within the rounded lagenar recess (Fig. 17).
Supraoccipital: The supraoccipital of Millereta is a broad, plate-like bone that forms the dorsal margin of the foramen magnum and the medial margins of the posttemporal fenestrae (Fig. 11). The supraoccipital of Millereta contacts the exoccipitals and opisthotics laterally and the postparietals dorsally (Fig. 15). The supraoccipital is preserved in both BP/1/3818 and BP/1/3822, but it is best observed in BP/1/3822 owing to slight posteroventral displacement.
A low, sagittal ridge is present along the midline of the dorsal surface of the supraoccipital (Fig. 15A, D). Paired fossae are present along the anterior margin of the dorsal surface of the supraoccipital and are likely to represent facets for the paired postparietals (Fig. 2B). The supraoccipital of Millereta lacks the median and lateral ascending processes described in recumbirostran ‘microsaurs’ and captorhinid eureptiles that broadly underlie the skull roof (Pardo et al. 2017). The absence of such processes in Milleropsis is most similar to other neoreptiles, such as the owenettid Saurodektes kitchingorum (Reisz and Scott 2002) and the early diverging neodiapsid, Youngina (Gow, 1974).
The pathways of the posterior and anterior semicircular canals are visible as a boomerang-shaped impression on the ventral surface of the supraoccipital in BP/1/3822. The pathways of these canals through the supraoccipital are poorly ossified, in contrast to the more ossified ventral wall of Milleropsis, which encloses the semicircular canals. This is possibly related to ontogeny; the holotype of Milleropsis is a mature individual, whereas BP/1/3822 is a juvenile or subadult. The common crus between the anterior and posterior semicircular canals is partly housed by the supraoccipital (Fig. 17). The ventral surface of the supraoccipital of Millereta lacks paired endolymphatic fossae.
Stapes: The stapes of Millereta is a robust bone, bearing a stapedial footplate that is approximately the size of the ventral ramus of the opisthotic and a lateral expansion that is dorsoventrally taller than the footplate (Fig. 12). The left stapes of BP/1/3822 is preserved, as are both stapes in BP/1/3818. The stapes of Millereta contacts the prootic and opisthotic medially at their contribution to the fenestra ovalis, and the shaft is directed towards tympanic emargination of the quadrate, quadratojugal, and squamosal (Gow 1972). It is likely that a cartilaginous extension, the extracolumella, would have contacted the tympanic membrane, as in extant reptiles (Livens et al. 2019).
The large stapes of Millereta is extremely similar in overall morphology and proportions to that of Eunotosaurus aficanus, which also has a lateral expansion of stapes (Gow 1997b). The millerettid Milleropsis pricei also bears a lateral expansion of the stapes, but the stapes remains a much smaller element of the occiput. However, the stapes of Millereta is still remarkably reduced relative to the massive, blade-like stapes in captorhinids (Heaton 1979, Abel and Werneburg 2021) and early amniotes (e.g. Dimetrodon; Romer and Price 1940), being smaller than the ventral ramus of the opisthotic.
The stapes of Millereta unequivocally lacks a dorsal process (as noted by Watson 1957 and Gow 1972; Fig. 12). Therefore, the stapes of Millereta does not contact the ventral surface of the paroccipital process of the opisthotic (Fig. 15). A large stapedial foramen pierces the quadrate process at approximately its midlength, marking the pathway of the stapedial artery (Fig. 12).
Sphenethmoid: All known specimens of Millereta rubidgei lack an ossified sphenethmoid, a condition similar to other known millerettids (e.g. Milleropsis; Jenkins et al. 2025) and early neodiapsids (e.g. Youngina; Gow 1974). This contrasts with recumbirostrans, which often bear paired ossification of the interorbital cartilages referred to as ‘orbitosphenoids’ (Szostakiwskyj et al. 2015). In most crownward stem amniotes (e.g. X.A.J. pers. obv. Captorhinus, OMNH 44816), synapsids (e.g. Oedaleops campi Langston, 1965), araeoscelidians (X. A. J. pers. obv. AMNH FARB 4686), and ankyramorphans (Macdougall et al. 2019), a ‘y-shaped’ sphenethmoid is present and forms an interorbital septum.
Endosseous labyrinth: The endosseous labyrinth of Millereta was segmented using the well-ossified otic capsule of BP/1/3822 (Fig. 17). The endosseous labyrinth, preserved within the prootic, opisthotic, and supraoccipital, of Millereta is dorsoventrally low, except for the dorsal expansion of the anterior semicircular canal (ASC) and the ventral development of the vestibule and lagenar recess (Fig. 17).
The ASC is located in the anterior margin of the prootic and occupied an anterolaterally oriented plane, approximately orthogonal to the planes of the lateral (LSC) and posterior (PSC) semicircular canals. The ASC curves anterodorsally from its ampulla anterior to the vestibule through the alar process of the prootic,thencurvesposterodorsallytowardsthecommoncruslocated within the supraoccipital. The PSC curves posteroventrally from the common crus, exiting the supraoccipital and continuing into the opisthotic. The PSC then curves ventrolaterally towards its ampulla immediately lateral to the lagenar recess along the medial portion of the paroccipital process of the opisthotic. The LSC is located within the paroccipital process of the opisthotic and the lateral portion of the internal surface of the prootic. The LSC curves anterolaterally from the ampulla for the PSC and exits the opisthotic onto the prootic, then curves anteromedially towards the ampulla for the ASC. The LSC is only weakly curved in dorsal view and is separated from the vestibule owing to the ossification of the medial wall of the prootic, similar to other neoreptiles but unlike the condition in taxa such as captorhinids (Bazzana 2022b) and ‘microsaurs’ (Pardo and Anderson 2016), in which the LSC remains open to the vestibule.
The vestibule of Millereta has a subtriangular outline in lateral view (Fig. 17), a feature that distinguishes amniotes from earlier tetrapods, such as diadectomorphs, in which the vestibule is subrectangular (Klembara et al. 2021). A subtriangular lagenar recess extends well ventral to the vestibule, contrasting with the inner ear of taxa close to the amniote stem, such as captorhinids (Bazzana 2022b) and varanopid synapsids (Bazzana et al. 2022a), in which a distinct lagena differentiated from the vestibule is absent or is poorly developed. However, the lagenar recess of Millereta is still comparatively large relative to neodiapsids, such as Youngina (Gardner et al. 2010), in which the lagenar recess is extremely narrow in lateral view. Overall, the inner ear of Millereta is similar to that of Milleropsis (Fig. 17).
Lower jaw
Both lower jaws are known in BP/1/3822 and are mostly complete (Fig. 1B). Unfortunately, the single right lower jaw of BP/1/3818 is poorly preserved (Fig. 1D). The lower jaw of Millereta is relatively slender dorsoventrally, but it is markedly reduced in relative anteroposterior length in comparison to early diverging millerettids, such as Broomia perplexa (Thomassen and Caroll, 1981; Cisneros et al., 2008) or Milleropsis pricei (Jenkins et al. 2025). The dentary of Millereta is comparatively deeper and bears fewer, but larger, teeth (Fig. 18). In Millereta, the posterior elements of the mandible, the surangular and angular, are reduced in anteroposterior length relative to Milleropsis. There is a well-developed retroarticular process, formed primarily by the articular but with some contribution by the angular and surangular.
Dentary: The dentary of Millereta forms the anterior half of the mandible and prevents the exposure of the splenial in lateral view (Fig. 18A). On the lateral surface, the dentary contacts the angular posteroventrally and the surangular posterodorsally, forming approximately equal contacts with both elements (Fig. 18A).
The alveolar shelf of the dentary of Millereta is moderately broad, and its anterior end overhangs the Meckelian canal. It bears 15 alveoli for large, conical teeth; two more than the 13 positions described by Gow (1972), but eight fewer than 23 alveoli of Milleropsis (Jenkins et al. 2025). As in the maxilla, plicidentine grooves are present at the base of each tooth (Fig. 6). The tooth implantation of the dentary can be categorized as subthecodont, with both the labial and lingual walls extending dorsally to approximately the same height as each other. This contrasts with the maxilla, in which the labial wall extends slightly further ventrally than the lingual wall (Fig. 6). Tooth replacement is occurring in the 5th and 13th dentary tooth positions, with replacement teeth located basal to the erupted teeth near the centre of the alveoli.
The anterior end of the dentary forms the mandibular symphysis (pers. obv. X.A.J. R.C. 14) in medial view, although this region is missing in both specimens scanned here. The anterior end of the dentary is pierced by the Meckelian canal, a feature present in many stem reptiles, including procolophonians (Carroll and Lindsay 1985) and non-saurian neodiapsids (e.g. Youngina; Hunt et al. 2023). The symphyseal facet of Millereta is located dorsal to the Meckelian canal. The ventromedial surface of the dentary of Millereta is overlapped by the splenial along its entire length, covering the medial contact of the dentary and prearticular, which takes place at approximately the level of the 15th dentary alveolus. The medial surface of the dentary bears a flat facet for the anterior process of the coronoid, but the dentary contact with the coronoid is reduced by an anterodorsal process of the surangular (Fig. 18C). A row of nutrient foramina is present along the lateral surface of the dentary in SAM-PK-K-10581 (Fig. 3C) but is not visible in the specimens scanned here, probably owing to damage during preparation.
The posterior end of the dentary of Millereta attenuates, unlike the early diverging neodiapsid Youngina capensis, in which the posterior end of the dentary bifurcates in dorsal view (Hunt et al. 2023). The dentary does not contribute to the low coronoid eminence (Fig. 18A).
Angular: The angular of Millereta is a low element in lateral view, with a weakly convex ventral margin. The angular extends from the midlength of the retroarticular process to the level of the 13th dentary alveolus (Fig. 18A). The angular contacts the articular posteromedially, the surangular posterolaterally, the prearticular posteromedially, the dentary anterolaterally, and the splenial anteromedially (Fig. 18B). In medial view, the angular of Millereta contributes to the posterior margin of the posterior inframeckelian foramen. The ventral surface of the angular of Millereta is sharply keeled, unlike the more rounded condition in pareiasauromorphs (e.g. the pareiasaur Nochelesaurus alexander Haughton & Boonstra, 1929; Van den Brandt et al. 2021).
Surangular: Both surangulars are preserved in BP/1/3822. The surangular contributes broadly to the lateral surface of the mandible, with a dorsoventral exposure nearly twice that of the angular (Fig. 18A). The surangular contacts the dentary anteriorly, the coronoid anterodorsally, the angular ventrally, and the articular ventromedially.
The dorsal surface of the surangular bears a medial expansion, forming a dorsally facing shelf that overhangs the Meckelian fossa, similar to the condition in Milleropsis pricei (Jenkins et al. 2025), the early diverging neodiapsid Youngina capensis (Hunt et al. 2023), and some varanopid synapsids (e.g. Heleosaurus scholtzi Broom, 1907; Ford and Benson, 2019). This contrasts with the thin dorsal exposure of the surangular seen in stem or early amniotes, such as the captorhinid Captorhinus laticeps (Heaton, 1979) and the araeoscelidian Petrolacosaurus kansensis (Reisz, 1981), in which the surangular bears no medial expansion dorsally. An anterodorsal process of the surangular limits the posterior contact between the coronoid and dentary, a feature otherwise noted as being present in varanopids (Ford and Benson 2019), although this feature might be more widespread across Permian reptiles. A low adductor crest is present on the dorsolateral surface of the surangular of Millereta. The posterior end of the surangular bears a C-shaped flange bracing the articular (Fig. 18C). A posterior surangular foramen, possibly for the chorda tympani nerve of CN VII, is present laterally at the articulation with the articular (Gow 1972) (Fig. 18A).
Splenial: The single splenial of Millereta is a mediolaterally thin bone that forms the medial wall for the Meckelian canal (Fig. 18B). It contacts the dentary ventromedially, the angular and prearticular medially, and the coronoid posterodorsally.
The splenial of Millereta does not contribute to the mandibular symphysis, terminating anterior slightly short of this region, as in most Permian stem reptiles, including the early diverging millerettid Broomia perplexa (Thomassen and Carroll, 1981). This contrasts with the condition in some early or stem amniotes, such as the recumbirostran Pantylus cordatus (Romer, 1969) or the diadectomorph Limnoscelis paludis (Fracasso, 1983), in addition to pareiasaurian reptiles (Van den Brant et al. 2021), in which the splenial forms the ventral portion of the mandibular symphysis. The splenial of Millereta contributes to the anterior margin of the posterior inframeckelian foramen (Fig. 18B). There is no anterior inframeckelian foramen visible in the tomography data, contrasting with some early reptiles, including Orovenator mayorum (Ford and Benson, 2019), the millerettid Milleropsis pricei (Jenkins et al. 2025), and the early diverging neodiapsid Youngina capensis (Hunt et al. 2023). A postsplenial is absent in Millereta.
Coronoid: The single coronoid of Millereta, present in BP/1/3822, extends anteriorly from the low coronoid eminence to the level of the 12th alveolus, where it becomes acuminate (Fig. 18B). The coronoid of Millereta contacts the dentary anterolaterally, the surangular posterolaterally, and the splenial ventrally.
The coronoid of Millereta is completely edentulous, unlike early tetrapods and some acleistorhinids among stem reptiles (Haridy et al. 2018). The anterior process of the coronoid is reduced, attenuating at the level of the 12th dentary tooth position. The contact between the coronoid and dentary is reduced by an anterodorsal process of the surangular (Fig. 18C). The coronoid of Millereta is not exposed in lateral view, and therefore this taxon lacks a distinct coronoid eminence. The posterior end of the coronoid forms the anterior margin of the adductor fossa and bears a short posteroventromedial process, contrary to Milleropsis pricei, which lacks this process (Jenkins et al. 2025).
Prearticular: Only a single, left prearticular is present in BP/1/3822. It is anteroposteriorly elongated, extending from the retroarticular process medially to approximately the anterior termination of the coronoid (Fig. 18B).
The medial surface of the prearticular of Millereta is relatively complex, twisting medially where it overlaps the articular. Two ridges on the prearticular mark the contact between the angular ventrally and the coronoid dorsally (Fig. 18B). An additional ridge is present laterally for the lateral contact of the angular, although this is not visible in lateral view. The prearticular contributes to the dorsal margin of the posterior inframeckelian foramen, although the slight posterior disarticulation of the splenial obscures this in medial view (Fig. 18B).
Articular: The articular of Millereta, best preserved in BP/1/3822, is the shortest element of the mandible, consisting of two dorsally facing cotyles for reception of the quadrate condyles and a well-developed retroarticular process posteriorly. The articular is overlapped by the surangular anterolaterally, the angular ventrolaterally, and the prearticular medially (Fig. 18). The medial cotyle of the articular is located ventral to the lateral cotyle, mirroring the ventral extent of the quadrate condyles. The posterior end of the retroarticular process is weakly upturned as in other millerettids, including Milleropsis (Jenkins et al. 2025).
Hyoid apparatus: A single left first ceratobranchial or ceratohyal is present in BP/1/3818. The tube-like first ceratobranchial is anteroposteriorly reduced, although, as preserved, it is visible in lateral view, extending posterior to the occiput. A ceratohyal extending posterior to the jaw articulation was considered to be synapomorphic of mesenosaurine varanopids, although this feature is widespread in Reptilia (e.g. Archosauromorpha; Ezcurra 2016) and is easily misinterpreted because the ceratohyals are often dislodged posteriorly. The posterior face of the ceratobranchial is cup-like and probably would have contacted a cartilaginous second ceratobranchial. No median copula or basihyal is known in any specimen of Millereta nor other millerettids, similar to the condition in the non-saurian neodiapsids Claudiosaurus, Carroll 1981 (Carroll 1981), Hovasaurus Piveteau, 1926 (Currie 1981), Thadeosaurus (Currie and Carroll 1984), and early saurians (Prolacerta; Modesto and Sues 2004). It is unlikely that a basihyal was ossified, unlike the condition in ankyramorphan reptiles (e.g. Delorhynchus cifelli; Reisz et al. 2014) and the mesosaurid Mesosaurus tenuidens Gervais, 1865 (Modesto, 2006), in which the basihyal is ossified.
description: Published as part of Jenkins, Xavier A., Benson Maya Elliot, Roger B. J., Jeppson, Gabriel, Dollman, Kathleen, Fernandez, Vincent, Browning, Claire, Ford, David P., Choiniere, Jonah & Peecook, Brandon R., 2025, New information on the anatomically derived milleretid Millereta rubidgei from the latest Permian based on µCT data, pp. 109 in Zoological Journal of the Linnean Society (Statistics in Society) (Statistics in Society) 203 (3) on page 109, DOI: 10.1093/zoolinnean/zlaf004, http://zenodo.org/record/14972230