Platyognathus
Introduction '''''Platyognathus''''' is an extinct genus of protosuchian crocodyliform. Fossils are known from the Early Jurassic Lower Lufeng Formation in Yunnan, China and belong to the type and only species, ''P. hsui''.
Description The skull of ''Platyognathus'' is small, being about  in length. The snout is narrow and shorter than the remainder of the skull, as indicated by the anterior position of the antorbital fenestra. Two teeth, dentaries 5 and 6, are enlarged into caniniforms in the lower jaw. Paired caniniform teeth are not seen in any other described crocodyliform taxa, but they have been recorded from an unidentified crocodyliform from the Lower Jurassic Kayenta Formation and from a protosuchid from the Lower Jurassic McCoy Brook Formation in Nova Scotia.
History and classification ===Holotype===''Platyognathus'' was first named by Chung-Chien Young in 1944 on the basis of a partial lower jaw found from the Dark Red Beds of the Lower Lufeng Formation in 1939.The material was first mentioned briefly four years earlier in a paper on Lufeng fossil vertebrates. The holotype jaw consisted of 11 alveoli (tooth sockets) on the left side and 12 alveoli on the right side, in addition to one broken caniniform tooth. Several diagnostic features were mentioned, including an expansion at the tip of the jaw, a tooth cross section that is octangular in outline, and a lateral constriction of the jaw behind the caniniform tooth, which is followed by an expansion further back. The anterior teeth are small and angled forward, while there is evidence in the jaw of at least two caniniform teeth that were larger and oriented obliquely and anteriorly. Further back in the jaw, the posterior teeth are about the same size and project vertically. The entire jaw is well ossified, and sutures between the bones could not be identified in the holotype. The symphysis, the point where the two sides of the jaw come together, is represented by a broad ventral groove.Based on these features, Young found no close similarities between the jaw of ''Platyognathus'' and any other known early crocodylomorph such as ''Notochampsa'', ''Sphenosuchus'', ''Erythrochampsa'', and ''Pedeticosaurus'' (at the time, these crocodylomorphs were collectively known as the "Pseudosuchia"). The jaw of ''Protosuchus richardsoni'', described by Barnum Brown in 1933 from Arizona, is similar in size to that of ''Platyognathus'', although it lacks many of the distinguishing features of the Asian form as described by Young. Because ''Platyognathus'' differed from any other known pseudosuchian, Young suggested that it may belong to its own family.During World War II, the holotype was either lost or destroyed. The classification of the genus continued to be debated, as descriptions of the fragmentary jaw were all that was available to study. ''Platyognathus'' continued to be referred to Pseudosuchia in the following years, and was suggested to have a close relationship with ''Sphenosuchus''. In 1955, it was suggested to have affinities with aetosaurs, and in 1956 Alfred Romer tentatively assigned it to the Notochampsidae (later synonymized with Protosuchidae).===New material===CUP 2083 in the Field Museum of Natural History.In 1965, additional specimens from the Lower Lufeng were referred to ''Platyognathus''. It was placed in a new pseudosuchian family, the Platyognathidae, and was considered to be intermediate between Pseudosuchia and Protosuchia. Most later studies of ''Platyognathus'' were based on one specimen of the new material known as CUP 2083 which preserved the lower jaw. Some of these studies concluded that ''Platyognathus'' was a pseudosuchian related to ''Sphenosuchus'' and ''Pedeticosaurus'', but most considered it to be a protosuchian. One 1986 study also considered the material to be from a protosuchian, but did not consider CUP 2083 to be from ''Platyognathus'' because the mandibular symphysis was elongate and unfused, unlike the holotype described by Young which was ossified. Later that year, a new paper also concluded that the newer material did not represent ''Platyognathus'', but considered the specimens and ''Platyognathus'' to belong to different basal crocodyliform taxa of uncertain affinities. In that paper, the validity of ''P. hsui'' was questioned on the basis of Young's description of the holotype===Neotype===In 1996, a newly described partial skull known as IVPP V8266 that was referable to ''Platyognathus'' was designated as a neotype. It was collected in 1984 from the Lower Lufeng in Yunnan. The authors of the paper that described the neotype agreed with the previous studies that the material described in 1965 does not belong to ''Platyognathus'' because CUP 2083 lacks many of the features of the holotype. In CUP 2083, the mandibular symphysis is unfused, there is a single caniniform tooth in the dentary, and there is no groove along the symphysis. This contrasts with the holotype, which has a fused symphysis, at least two caniniform teeth, and a conspicuous trough along the symphysis. The authors also mention that the holotype and CUP 2083 come from two different strata within the "Dark Red Beds" of the Lower Lufeng; the holotype is from stratum 6  of the Dahuangtian locality while CUP 2083 is from stratum 5 of the Dadi locality, about 400m southeast of the Dahuangtian locality. This shows that the two specimens are from different horizons and are likely to belong to different taxa.The new skull revealed an additional diagnostic feature of ''Platyognathus'' that distinguishes it from other protosuchians: the distinctive curvature of the jugal. Additional diagnostic characters were proposed by the authors, but their character-states cannot be determined in other protosuchian taxa. These characters include a width of the mandibular symphysis across the swellings of the caniniform teeth that is almost equal to the anteroposterior length of the symphysis and an anterolateral process of the ectopterygoid with an inverted V-shaped ridge on the dorsal surface. The shortness of the snout relative to the rest of the skull and the presence of a laterally and ventrally open notch between the premaxilla and maxilla indicate that ''Platyognathus'' is a member of Protosuchia as defined by Wu ''et al.'' (1994). Because the infratemporal fenestra of ''Platyognathus'' is very small and the mandibular symphysis extends posteriorly to the level of the seventh to ninth tooth, the genus is thought to be more derived than ''Orthosuchus''. The authors of the description of the neotype suggested that ''Platyognathus'' may be closely related to the clade containing Protosuchidae and ''Shantungosuchus'' because it possesses a dentary tooth that fits into a notch between the premaxilla and maxilla. Because ''Platyognathus'' is known from such incomplete material, there is yet to be an adequate cladistic analyses that can determine a phylogenetic position for ''Platyognathus'' within Protosuchia.An unnamed mesosuchian from the Salt Wash Member of the Late Jurassic Morrison Formation of western Colorado in the United States possesses several features that are also seen in ''Platyognathus''. The dentition of the mesosuchian is similar to that of ''Platyognathus'' and like ''Platyognathus'', the dentary is laterally expanded. However, the dentary of the unnamed mesosuchian is much broader and spatulate.
References
External links *  ''Platyognathus'' in the Paleobiology Database