Triassic reptiles from the elgin area: Stagonolepis, Dasygnathus and their allies

Abstract

The maxilla named by T. H. Huxley Dasygnathus longidens , from the Trias of Findrassie near Elgin, is re-described. A pterygoid from the same locality is referred to this species and described for the first time. These two bones indicate a large, carnivorous pseudosuchian apparently allied to Erythrosuchus . A detailed description is given of the osteology of the pseudosuchian Stagonolepis , amplified by a large number of hitherto undescribed specimens. The material (considered to represent at least twenty-one individuals) shows an almost complete segregation into two size-groups, distinguished by only a few minor morphological differences of the postcranial skeleton. The larger and small individuals are considered to be males and females respectively, of the one species S. robertsoni Agassiz. Study of Stagonolepis has demonstrated its close relationship to Aetosaurus from the Stubensandstein (Keuper) of Stuttgart. The latter genus, however, was in urgent need of revision. Accordingly a brief account is included of the principal respects in which previous descriptions of this form require modification. Specimens referred by von Huene (1921) to A. crassicauda are shown to have been misidentified in many cases; this material is a composite of a small coelurosaur and a true aetosaurid. Some modifications are also suggested to previous accounts of Typothorax and Desmatosuchus from the Trias of North America, and new restorations are given of the skulls of these forms. The extremely close relationship between Stagonolepis and Aëtosaurus , perhaps even indicating generic identity, makes unavoidable the merging of the two families previously founded on these genera. On grounds of priority the name Aetosauridae is retained for the taxonomic unit which includes Aëtosaurus, Stagonolepis, Typothorax and Desmatosuchus as principal members. A restricted diagnosis of the family Aëtosauridae is given, based on the above four forms, and this is followed by a review of the genera which have from time to time been included in the former families Stagonolepidae and Aëtosauridae. The great majority of these genera are excluded from the group as now defined. Possible evolutionary trends within the family are briefly outlined; the sequence of increase of specialization appears to be Aëtosaurus, Stagonolepis, Typothorax, Desmatosuchus . The conclusions of Dollo (1884), Adams (1919) and others concerning the function of the preorbital fossa in archosaurian reptiles are endorsed, and it is suggested that a trend towards the reduction of the anterior pterygoid muscle took place in aetosaurids, in parallel with a similar trend in ornithischian dinosaurs. Many features of the skeleton of aetosaurids recall the Ornithischia. These include the elongate naris, reduced dentition, vertical or forwardly inclined quadrate, slipper-shaped jaw, small skull and well-developed dermal armour. However, a direct ancestor-descendant relationship appears to be ruled out by the position of the supratemporal fossa, reduction of the infratemporal opening, probable loss of the coronoid and typically pseudosuchian pubis, although the aëtosaurids may well lie close to the root-stock of the Ornithischia. The mode of life of the aetosaurids is considered, and it is concluded that these animals were herbivorous, or possibly feeders upon invertebrates obtained, in the case of Stagonolepis at least, by digging with the peculiar expanded snout-tip and dentary rostrum. A simple muscular mechanism is postulated whereby this could be effected. The stratigraphical implications of these studies are briefly examined and the suggestion, based primarily on the close relationship between Stagonolepis and Aëtosaurus , is put forward that the Triassic sandstone of Elgin occupies a higher horizon than has previously been considered.

The Late Triassic pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs

“…coracoid and ilium). Interestingly, the squamosal of Revueltosaurus is most similar to aetosaurs (Walker 1961), although it is not rotated laterally as in that group. It is premature to conduct a phylogenetic analysis of the new material of R. callenderi given the confused state of basal archosaur relationships, notably 'Rauisuchia', and poor understanding of the polarity of basal archosaurian characters (Gower & Wilkinson 1996).…”

The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades

“…aetosaurs largely as the result of the work of Huxley (1877) and especially Walker (1961). Although casts produced from sandstone molds represent nearly all the specimens, Walker (1961) laboriously worked to produce a rather complete anatomy of Stagonolepis. Details of the pes and other bones are missing because of the preservation of the material.…”

“…8. Skull reconstructions of pseudosuchian archosaur terminal taxa: A, Riojasuchus tenuisceps in lateral and B, dorsal views; redrawn from Sereno (1991a); C, Gracilisuchus stipanicicorum in lateral and D, dorsal views; redrawn from Romer (1971b); E, Arizonasaurus babbitti in lateral view; redrawn from Nesbitt (2005); F, Stagonolepis robertsoni in lateral and G, dorsal views; redrawn from Walker (1961); H, Revueltosaurus callenderi in lateral view; based on PEFO 34561. I, Effigia okeeffeae in lateral view; redrawn from ; J, Xilousuchus sapingensis in lateral view; based on IVPP V 6026.…”

“…Even though specimens from outside the Lower Stubensandstein have been referred to Aetosaurus (Jenkins et al, 1994;Heckert and Lucas, 1998;Small, 1998) only the individuals numbered SMNS 5770 are scored here. KEY REFERENCES: Fraas, 1877;Huene, 1920;Walker, 1961;Wild, 1989;Schoch, 2007.…”

Supplemental Material for 'Postcranial osteology of Azendohsaurus madagaskarensis (?Middle to Upper Triassic, Isalo Group, Madagascar) and its systematic position among stem archosaur reptiles. (Bulletin of the American Museum of Natural History, no. 398)'

“…Our recovery of a clade of bizarre herbivores, Allokotosauria, demonstrates that herbivory among archosauromorphs was far more prevalent early in the Triassic than previously realized. Beyond the herbivorous groups that have been known for over a century (e.g., rhynchosaurs Huxley, 1869;Benton, 1983;aetosaurs, Agassiz, 1844, Walker, 1961andsauropodomorphs von Meyer, 1861, Weishampel andNorman, 1989), a wealth of presumably herbivorous archosauromorphs have come to light from the Triassic in recent years (e.g., Revueltosaurus callenderi, Parker et al, 2005;silesaurids, Langer et al, 2010and possibly shuvosaurids, Nesbitt and Norell, 2006), as have other postTriassic lineages (crocodylomorphs, Buckley et al, 2000). Additionally, taxa that have a typically "carnivorous" dentition such as Protorosaurus speneri have been found with plant fragments in their digestive track (Munk and Sues, 1993).…”