摘要: Choristoderes were an important clade of semi-aquatic predators that occupied
Laurasian freshwater ecosystems from the Middle Jurassic to the Miocene. During the Early
Cretaceous, the neochoristodere lineage evolved large size and long snouts, converging on
the body plan of modern crocodilians. Here, we describe a new longirostrine choristodere,
Mengshanosaurus minimus gen. et sp. nov. from the Lower Cretaceous Mengyin Formation of
Shandong Province, China. The holotype is the smallest reported neochoristodere individual, with
a skull length of only 35 mm. The poorly ossified braincase, along with retention of a fontanel
at the frontal-parietal suture, indicates this individual was a juvenile. Phylogenetic analyses
recovered Mengshanosaurus as a neochoristodere, a placement supported by the presence of
a single narial opening, fusion of the nasals, and expansion of the temporal fenestrae. In the
Neochoristodera, Mengshanosaurus is sister to a clade consisting of Ikechosaurus, Tchoiria,
Simoedosaurus, and Champsosaurus. It differs from other neochoristoderes in having the
lacrimal foramen between the prefrontal and lacrimal, in addition to having large vomerine teeth
(exceeding one-third the width of corresponding maxillary teeth). The closely arranged marginal
teeth and large vomerine teeth suggest juvenile choristoderes may have fed on invertebrates and
insects, similar to juveniles of modern crocodilians. However, the observation that very young
neochoristoderes had similar skull proportions and marginal tooth shapes to adults, along with
features suggesting a more fully aquatic ecology, suggest that neochoristoderes exhibited less
pronounced ontogenetic niche shifts than modern crocodilians.