Bottosaurus harlani (Cossette & Brochu, 2018)

Cossette, Adam P., 2021, A new species of Bottosaurus (Alligatoroidea: Caimaninae) from the Black Peaks Formation (Palaeocene) of Texas indicates an early radiation of North American caimanines, Zoological Journal of the Linnean Society 191 : -

SIMILARITIES WITH B. HARLANI

Specimens of B. fustidens bear similarities to B. harlani . Specimens attributable to B. harlani (NJSM 11265 and YPM 56140) share a U-shaped depression of the frontal, at the point of greatest mediolateral constriction between the orbits, with B. fustidens ( Fig. 3). However, the U-shaped depression is much deeper in B. harlani , suggesting a gradient in the expression of this character state.

The teeth are diagnostic for species of Bottosaurus . The B. fustidens holotype (TMM 40148-7) best preserves the dental arcade and shares its dental morphology with the B. harlani lectotype (ANSP 9226) and specimens attributable to it ( Fig. 1). Anterior teeth are relatively low crowned and are circular in cross-section, whereas mid-jaw and posterior teeth are labiolingually compressed, resembling species of Paleosuchus . All teeth bear a mesiodistally oriented carina. There are two regions of enlarged dentary teeth in species of Bottosaurus ; one in the anterior jaw, with the fourth tooth being the largest, and the second in the mid-jaw, with the 12 th tooth being the largest.

The anterior splenial differs between the B. fustidens holotype and the B. harlani lectotype ( Fig. 9). In B. fustidens , the splenial approaches within two alveoli of the mandibular symphysis but does not participate in it. Its anterior process is acute and lies ventral to the Meckelian groove. The splenial is suggested by sutural marks on the dentary of the B. harlani lectotype, where it reaches the mandibular symphysis and might have participated in it.

Both B. fustidens and B. harlani share large jugal foramina ( Fig.2) and postorbital bars that are confluent with the lateral surface of the jugal. Preserved portions suggest that both of these character states are present in B. fustidens , but owing to imperfect preservation these characters are not coded in the matrix.

Dorsal vertebrae belonging to B. fustidens resemble those of B. harlani in all aspects apart from one; the anterior dorsal vertebrae of B. fustidens bear an anteroposteriorly oriented longitudinal furrow on the ventral side of the neural centra ( Fig. 10). The functional significance of this structure is unknown.

Orbital and skull table regions shared by B. fustidens and B. harlani demonstrate similar proportions and morphology. Elements forming the anterior and anterolateral margins of the orbits are gently upturned in both species. The anteromedial corners of the supratemporal fenestrae are preserved in the B. fustidens holotype; their margins are angular and resemble those of B. harlani . Dorsal indentations of the skull table, bordered by the orbits anteriorly and the supratemporal fenestrae posteriorly, are common to both taxa and occur at the three-way sutural junction formed by the frontal, postorbital and parietal.

COMPARISON WITH OTHER CAIMANINES

Bottosaurus fustidens shares features with species of South American caimanines, but may be differentiated from them consistently. The premaxilla bears five teeth in B. fustidens . The teeth and alveoli get larger from the first to the fourth alveolar positions, with the fifth tooth in the series being the smallest. This condition is shared with Gnatusuchus pebasensis Salas-Gismondi et al., 2015 .

Similar to Caiman brevirostis Souza-Filho, 1987 and Gnatusuchus pebasensis , along most of its length the lateral margins of the anterior projection of the frontal are nearly parallel in B. fustidens . This is in opposition to the condition seen in Kuttanacaiman iquitosensis Salas-Gismondi et al., 2015 where the anterior process is wedge shaped.

U-Shaped depressions of the frontal are common among caimanines, such as Melanosuchus niger Gray, 1862 . However, it is deeper and positioned further to the posterior in species of Bottosaurus - the depression is at the point of greatest mediolateral constriction between the orbits.

The prefrontonasal suture trends mediolaterally from posterior to anterior in B. fustidens , as appears to be the case in B. harlani YPM 56140. This morphology is in common with Globidentosuchus brachyrostris Scheyer et al., 2013 . Modern species of Caiman , such as Caiman crocodilus , have sutures that are almost anteroposteriorly aligned. A notable exception is the fossil species Caiman brevirostris Souza-Filho, 1987 , whose prefrontonasal suture trends mediolaterally from posterior to anterior, as found in species of Bottosaurus .

The squamosal prong is moderately long and trends mediolaterally from anterior to posterior in species of Bottosaurus . This morphology is similar to that of K. iquitosensis and Caiman brevirostris , although the former has a more acute posterior projection and the latter a slightly shorter prong.

The positioning of the foramina for the passage of neurovasculature on the posterior exoccipital differs among caimanines. Unfortunately, specimens referable to B. fustidens preserve little morphology in this region, and comparisons are limited. Relative to extant caimanines, the carotid foramen is more proximal to the foramen for CN XII in B. fustidens .

The sutural contact between the pterygoid and ectopterygoid differs between species of caimanines. Anteroventrally, preservation of the element is imperfect. The interpretation here is that the ventralmost tip of the element along its anterior face is blunt. This differs from most caimanines, including B. harlani , whose tip tapers to a point. Posteriorly, the sutural contact between the elements is variable among caimanines. Along the posterodorsal face of the ectopterygoid, the element sends a medial lamina along the wing of the pterygoid. The medial lamina is proportionally larger in B. fustidens relative to extant caimans and B. harlani . The lamina ends in a blunt tip in B. fustidens , similar to Caiman yacare (Daudin, 1802) and Caiman crocodilus . This character state is in opposition to that of Caiman latirostris Daudin, 1801 and Paleosuchus palpebrosus (Cuvier, 1807) , which have pointed tips.

The mandibular symphysis of B. fustidens is robust and extends to the level of the fourth dentary alveolus. This morphology is shared with B. harlani but is in opposition to the proportionally longer mandibular symphyses found in Gnatusuchus pebasensis , K. iquitosensis , Caiman latirostris and Caiman wannlangstoni Salas-Gismondi et al., 2015 , which extend to or beyond the sixth alveolus, respectively.

The dentary of B. fustidens is gently curved, forming a convexity between the first and fourth alveoli. Although incomplete, NJSM 11265 (a specimen referable to B. harlani ) suggests a similar morphology. This is in opposition to species of Paleosuchus , in which the dentary is relatively linear in this region. Tsoabichi greenriverensis , Caiman crocodilus , Caiman yacare and M. niger demonstrate a curvature of the dentary between alveoli 1 and 4 that is intermediate between species of Bottosaurus and Paleosuchus . The dentary of Caiman latirostris bears a curvature of the dentary between the first and sixth alveoli, and Gnatusuchus pebasensis demonstrates a gentle curvature of the dentary between the first and seventh alveoli.

COMPARISON WITH CRETACEOUS GLOBIDONTANS

The Cretaceous globidontans Albertochampsa langstoni , Br. montana , Br. sealeyi and S. mccabei are recovered here in a close relationship with species of Bottosaurus . Stratigraphic overlap exists between B. harlani and some Cretaceous globidontans, but not for B. fustidens . Additionally, the species are found in different formations and geographical locations. Bottosaurus harlani is from the latest Cretaceous– earliest Palaeogene Hornerstown Formation of New Jersey, whereas Br. montana is from the Maastrichtian Hell Creek Formation, Br.sealeyi is from the Campanian Menefee Formation of New Mexico, S. mccabei is from the late Campanian–Maastrichtian Horseshoe Canyon Formation of Alberta, Albertochampsa langstoni is from the Campanian age Dinosaur Park Formation of Alberta and B. fustidens is from the Tiffanian (Palaeocene) North American Land Mammal Age Black Peaks Formation of Texas.

Cretaceous globidontans and species of Bottosaurus share anteroposteriorly short, blunt snouts and derived dentition. Although similar, fragmentary remains of species of Bottosaurus are not likely to be confused with Cretaceous globidontans. The largest specimens of Bottosaurus are considerably larger than the largest specimens of Cretaceous globidontans, skull element proportions vary, and sutural contacts differ between the species. However, isolated postcrania might be confused, as is the case for most species of alligatoroids.

Ornamentation of skull and mandibular elements differs between the species. Ornamentation is slight in species of Bottosaurus . Ornamentation is moderate on most surfaces in Albertochampsa langstoni and more pronounced in S. mccabei and species of Brachychampsa .

The orientation of the external narial aperture differs between species of Cretaceous globidontans and B. harlani . The condition for B. fustidens is difficult to determine owing to deformation, but is probably similar to that of B. harlani , in which the aperture opens anterodorsally. All Cretaceous globidontans preserve external narial apertures that project dorsally.

The curvature of the premaxillary tooth row differs between the species. In species of Brachychampsa , especially Br. montana , the tooth row is relatively more linear when compared with S. mccabei or species of Bottosaurus . Comparison with Albertochampsa langstoni is difficult, because the anteromedial premaxilla and mesialmost premaxillary alveoli are not preserved.

The frontal of species of Bottosaurus differs from that of all Cretaceous globidontans. No species of alligatoroid from this geological period is known to have a depression at the point of greatest mediolateral constriction between the orbits. Moreover, no Cretaceous globidontan preserves a depression anterior to the orbits.

The frontoparietal suture differs between the species. In Cretaceous globidontans, the frontoparietal suture makes modest entry into the supratemporal fenestrae at maturity. This differs from species of Bottosaurus , in which the frontoparietal suture is entirely on the skull table. Additionally, the frontoparietal suture differs between Cretaceous globidontans and species of Bottosaurus ; the suture is concavoconvex for the former and linear between the supratemporal fenestrae in the latter.

Skull tables may be compared for all species. The supratemporal fenestrae of Albertochampsa langstoni , Br. montana and S. mccabei are proportionally larger than those of B. harlani . Comparison with B. fustidens and Br. sealeyi is not possible owing to poor preservation of this region. Lateral margins of the skull table differ. They are nearly parallel in S. mccabei and B. harlani but trend laterally from anterior to posterior in Br. montana and Albertochampsa langstoni . Comparison with Br. sealeyi and B. fustidens is impossible owing to lack of preservation.

The posterior skull table differs between the species, especially the posterior prongs of the squamosals. The posterior prongs of the squamosals are long in species of Bottosaurus and Br. montana but are short in Albertochampsa langstoni and S. mccabei . Comparison with Br. sealeyi cannot be made owing to lack of preservation.

Exposure of the supraoccipital is small in S. mccabei and Albertochampsa langstoni but large in B. harlani and species of Brachychampsa . Although B. harlani and species of Brachychampsa have similar codings for the exposure of the supraoccipital, the shape of the element differs between the species. In B. harlani , the length and width of the element are similar, and the anterior margin is linear. Brachychampsa sealeyi demonstrates a supraoccipital with similar lengths and widths, but whose anterior margin culminates in a point, whereas Br. montana preserves an element whose width greatly exceeds its length and whose anterior margin is rounded.

Species of Bottosaurus have mesiodistal carinae and labiolingually compressed teeth. Species of Brachychampsa have teeth that bear mesiodistal carinae but no labiolingual compression. Preserved teeth in Albertochampsa langstoni and S. mccabei do not have pronounced carinae or labiolingual compression. Posterior teeth are much lower crowned, forming a nearly flat occlusal surface, in species of Brachychampsa , Albertochampsa langstoni and S. mccabei . This is in opposition to species of Bottosaurus , in which posterior teeth maintain a slight apex.

Values differ for the character describing the largest alveolus in the maxillary tooth row. The largest maxillary alveolus in S. mccabei , Albertochampsa langstoni and B. fustidens is the fourth maxillary tooth, whereas in species of Brachychampsa it is the fifth. This character cannot be coded for B. harlani owing to lack of preservation.

Mandibular morphology differs between the species. The mandible is robust and nearly circular in crosssection in species of Bottosaurus , with B. fustidens preserving a relatively more robust mandible than B. harlani . The mandibular symphysis extends to the fourth mandibular alveolus in B. fustidens and is reconstructed as having reached the fourth mandibular alveolus in B.harlani . However, the symphysis extends to the sixth alveolus or beyond in Cretaceous globidontans.

The angular–surangular suture contacts the external mandibular fenestra at the posterior angle in Br. montana and S. mccabei but passes broadly along the ventral margin of the external mandibular fenestra in B. harlani . This region is not coded in B. fustidens , Br. sealeyi or Albertochampsa langstoni owing to lack of preservation.

The retroarticular process is incompletely known for species of Bottosaurus , but is reconstructed as relatively long and trending dorsally from anterior to posterior. In Br. montana , the process is long and trends somewhat posteriorly relative to species of Bottosaurus , whereas in Br. sealeyi and S. mccabei the process is short and trends dorsally from anterior to posterior. A mandible is not preserved for Albertochampsa langstoni .