Cranial anatomy of a new crocodyliform (Archosauria: Crocodylomorpha) from the Lower Cretaceous of Song-Liao Plain, northeastern China

Abstract

Rugosuchus nonganensis, a new genus and species of crocodyliform, is erected on the basis of its peculiar cranial features. This late Early Cretaceous species represents the second, and most complete, crocodyliform known from northeastern China. Its discovery is significant for our understanding of both the local vertebrate fauna and its stratigraphical correlations, and the paleobiogeography of the Crocodyliformes. The palatine contribution to the anterior border of the choana excludes the new form from the Eusuchia, and evidence from osteoderms (not detailed in the present paper) suggests that the new form appears to be phylogenetically closer to the Eusuchia than to the Goniopholididae. Therefore, comparisons are made primarily with those taxa which have a closer relationship to the former than the latter. The unsculpted, depressed, and lobe-like posterolateral process of the squamosal is further demonstrated to be ontogenetically variable and thus inappropriate for use as a character in phylogenetic analyses.

Morphology of the Late Cretaceous Crocodylomorph Shamosuchus djadochtaensis and a Discussion of Neosuchian Phylogeny as Related to the Origin of Eusuchia

“…The posteriormost teeth are not well preserved, although they seem to be more robust and with shorter crowns. The maxillary alveoli are closely packed to each other, as in other neosuchians (e.g., the Glen Rose form, Bernissartia fagesii, Theriosuchus pusillus, Gonipholis simus), but differ from the well-separated alveoli of Rugosuchus nonganensis, as noted by Wu et al (2001a).…”

“…This resembles the condition of Rugosuchus nonganensis , Isisfordia duncani (Salisbury et al, 2006), and some nonneosuchian crocodyliforms (e.g., Sichuanosuchus shuhanensis IVPP V 10594, Simosuchus clarki UA 8679, Notosuchus terrestris MACN-RN 1041). Lateral to this ridge, the dorsal surface of the frontal is flat, differing from the condition in Rugosuchus nonganensis (as noted by Wu et al, 2001a). The anterior end of the frontal tapers between the prefrontals and is markedly constricted above the orbits (being narrower than the width of the nasals).…”

“…The distal body of the quadrate is slightly wider than anteroposteriorly long, resembling the neosuchian condition. The posterior surface of the quadrate body is slightly convex and seems to lack a well-developed ridge, a condition noted by Wu et al (2001a) to be shared with Shamosuchus tersus, Rugosuchus nonganensis, and Bernissartia fagesii. A similar ridge, however, is present in several basal mesoeucrocodylians (e.g., Libycosuchus brevirostris BSP 1912.VIII.574, Sebecus icaeorhinus AMNH FR 3160, Hsisosuchus chungkingensis CNM V 1090).…”

“…14). Wu et al (1996aWu et al ( , 2001a, however, noted that a smooth depressed lobe of the squamosal may have a wider distribution and that it is ontogenetically variable in Sunosuchus junggarensis. Interestingly, Rugosuchus nonganensis has a morphology similar to Shamosuchus djadochtaensis, having the posterolateral region slightly sculpted, depressed, and separated by a weak ridge .…”

“…This suggests that either this depression is not the palpebral articular facet, or that the anterior palpebral of Shamosuchus djadochtaensis is markedly distinct from that of other Crocodyliformes. Although further material and study is needed to test these options this depression seems to be diagnostic of Shamosuchus djadochtaensis or, alternatively, a putative synapomorphy for the higher taxon Shamosuchus more broadly, given the presence of a transverse prefrontal ridge in other species of Shamosuchus (Efimov, 1983;Wu et al, 2001a).…”