Alligatoroidea Gray, 1944

Stout, Jeremy B., Massonne, Tobias, Samuels, Joshua X. & Schubert, Blaine W., 2025, Small, enigmatic alligatoroid from the Middle Eocene Clarno Formation, John Day Fossil Beds, Oregon, Fossil Record 28 (2), pp. 309-319 : 309-319

Alligatoroidea Gray, 1944

Type species.

Alligator mississippiensis [ Daudin, 1802] from “les bords du Mississipi ”.

Referred specimen.

UCMP 164865 , anterior portion of left dentary.

Description.

UCMP 164865 (Fig. 3 ) is the anterior-most portion of a reconstructed left dentary that measures 6.3 cm in length. The mandibular symphysis measures 2.4 cm long and extends posteriorly to the fourth dentary alveolus. UCMP 164865 does not preserve the alveoli immediately posterior to the first, though they are assumed to be alveoli two and three, as the next preserved tooth is the largest of the preserved dentary and spatially consistent with other known alligatoroids ( Brochu 2004; Cossette and Tarailo 2024) and most members of Crocodylia as a whole ( Brochu 1999). Furthermore, the assumed fourth alveolus is preserved along its anterior extent and is not confluent with another. Pursuant to the aforementioned assumption, counting anterior to posterior, the first and fourth through twelfth alveoli are preserved. Similar to other small-bodied alligatoroids of the time ( Cossette and Tarailo 2024), the dentary curvature between the fourth and twelfth alveoli is deep, with a noticeable dorsal rise between the eleventh and twelfth alveoli. The height of the dentary at the first and fourth alveoli is approximately equal to the dorsal extent at the twelfth alveolus.

The lingual side of UCMP 164865 is better preserved than the lateral. Though the splenial is not preserved, its presence can be inferred as a scar ( Stout 2021) along the mandibular ramus and is well preserved. The splenial had a robust presence constituting the medial wall along the entirety of the anterior mandibular ramus. When present, the splenial appears to have reached and participated in the mandibular symphysis. The angle that the lingual margin of the mandibular ramus reaches the symphysis is exceptionally shallow. The Meckelian groove is preserved across most of the mandibular ramus and attenuates as it approaches the symphysis. In life, it may have been completely enclosed along its mesial margin by the splenial. With the exception of the mandibular symphysis and the inferred presence of the anterior portion of the splenial, no articulation with other bones of the mandible is preserved. Several openings of the mandible preserve important phylogenetic information in alligatoroids ( Brochu 1999), but are not preserved in UCMP 164865 : the mandibular fenestra and posterior foramen intermandibularis oralis existed posterior to the preserved portion and the anterior foramen intermandibularis oralis, if present, would be located on the splenial.

Seven teeth are partially or wholly preserved in UCMP 164865 : the first, fourth through sixth, seventh and ninth (partial) and twelfth. The largest tooth and its associated alveolus is the fourth, which is separated from the (not preserved) third. All teeth project dorsally or anterodorsally and each are pointed, except for the distinctly globidontan twelfth. All are unserrated, but possess anterior and posterior carinae. All possess some degree of labiolingual compression. A portion of the thirteenth alveolus is preserved at the posterior-most extent of UCMP 164865 and may have been much larger than the preceding eight. The teeth possess smooth carinae and robust basiapical ridges on their lingual margins.

Comparison.

In comparison with contemporaneous North American alligatoroids, UCMP 164865 differs from late Eocene Alligator sp. ( Whiting and Hastings 2015) and from Allognathosuchus Mook, 1921 spp. in possessing a mandibular symphysis reaching only the fourth or fifth alveolus ( Mook 1921; Brochu 2004). It differs from Procaimanoidea Gilmore, 1946 species in possessing a splenial that participates in the mandibular symphysis ( Wassersug and Hecht 1967). From Tsoabichi greenriverensis Brochu, 2010 , it differs in that its largest alveolus posterior to the fourth is probably not the eleventh or twelfth ( Brochu 2010). The preserved portion of its mandibular ramus exhibits deeper curvature than that observed in Ceratosuchus burdoshi Schmidt, 1938 ( Bartels 1984). It is unlike Ahdeskatanka russlanddeutsche Cossette & Tarailo, 2024 in not possessing a long and robust symphysis reaching the ninth alveolus. It differs from YPM VPPU. 017369 in its deep dentary curvature, globidontan tooth at the twelfth alveolus (which also marks the steep dorsal rise) and the other “ Golden Valley crocodylian ” taxa that all possess more robust mandibular symphyses ( Cossette and Tarailo 2024). From Chrysochampsa mlynarskii Estep, 1988 , it differs in the small alveolus located as position twelve ( Cossette and Tarailo 2024). The labiolingual compression of the teeth is similar to Paleosuchus and Bottosaurus , but unlike other alligatoroids ( Cossette and Brochu 2018). Preserved teeth and alveoli immediately posterior to the inferred fourth are unlike most contemporaneous alligatoroids in their relatively large size and wide spacing (pers. obs. e. g. Allognathosuchus, Brochu (2004) : Eocene Alligator, Whiting and Hastings (2015) ), but similar to Ahdeskatanka ( Cossette and Tarailo 2024). The middle Eocene caimanine Chinatichampsus wilsonorum Stocker et al., 2021 from southern Texas does not preserve a dentary and, thus, cannot be compared.

The following phylogenetic characters were observed or inferred on UCMP 164865 (character numbers and states from Walter et al. (2025)) and could prove useful in future analyses: 46 (1) alveoli size of dentary tooth four is larger than the third and separated from it, 47 (1) anterior dentary teeth project anterodorsally, 48 (0) dentary symphysis extends to fourth or fifth alveolus, 49 (1) deep dentary curvature between fourth and tenth alveoli, 53 (0) splenial participates in mandibular symphysis, 79 (0) maxillary and dentary teeth with smooth carinae, 195 (0) anterior maxilla and dentary teeth without or weak dorsoventral ridges on the lateral surface, 219 (1) the level of the first and fourth dentary alveoli are equal to or higher than the level of the eleventh and twelfth dentary alveoli. None of these characters alone is apomorphic for Alligatoroidea ( Brochu 1999; 2004; Walter et al. 2025), but the suite of character states taken together and the distinctly globidontan posterior tooth ( Brochu 1999), supports inclusion into the clade.

Figures

Figure 3. UCMP 164865, anterior left dentary of an alligatoroid from the Eocene age Hancock Mammal Quarry (Clarno Formation, John Day Fossil Beds National Monument) in lingual (top), occlusal (middle) and lateral (bottom) views. Abbreviations: dfs (deeply furrowed striations), Mc (Meckelian canal), ps (posterior extent of the symphysis), sps (splenial scar), sym (mandibular symphysis). Numbers indicate dental alveoli (as counted from anterior to posterior). White arrows denote dorsal margin of splenial scar. Dotted line follows the angle that the mandibular ramus contacts the symphysis. Anterior is to the right (top and middle) and left (bottom). Scale bar: 1 cm.