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.

Erpetosuchus, a small archosaurian reptile from the Late Triassic of Scotland and North America, hasoften been implicated in the ancestry of crocodilians. A restudy of the type specimen, using new high‐fidelity casts, as well as examinationof new, hitherto undescribed material, allows a detailed description and restoration of Erpetosuchus granti from the LossiemouthSandstone Formation (late Carnian, Late Triassic). This small reptile is known only from the front end of its body; a complete skull,cervical vertebral column, anterior dorsals and ribs, shoulder girdle, and forelimb. The skull shows a number of unusual features: a reducedrow of only 4–5 teeth on the anterior part of the maxilla, a large antorbital fenestra set in a deep fossa whose margins aremarked by distinct sharply angled ridges, a jugal that is divided into a lateral and a ventral portion by a sharp ridge, a deeplyrecessed tympanic area, the angular and surangular marked by a strong ridge running back from the ventral margin of the mandibular fenestra,and teeth oval in cross‐section and lacking anterior and posterior carinae and marginal serrations. The remains suggest that Erpetosuchus wasa light, cursorial animal that may have fed on insects. A cladistic analysis of crurotarsan archosaurs indicates that Erpetosuchus isthe closest sister group of Crocodylomorpha among known basal archosaurs. It shares with them a deep recess in the cheek region framed bythe quadrate and quadratojugal which slope forward side‐by‐side at an angle of 45° above horizontal, and reach the uppermargin of the lower temporal fenestra. In Erpetosuchus the recess is entirely lateral, while in crocodylomorphs, the recesspenetrates medially as well, since the quadrate/quadratojugal bar meets the side wall of the braincase. © 2002The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 25–47.

New data on the braincase of the aetosaurian archosaur Stagonolepis robertsoni Agassiz are presented, based on new preparation, synthetic casting, and interpretation of fossil material from the Triassic Elgin Sandstones, Scotland. The metotic fissure is not divided by bone. The perilymphatic foramen is completely bound by bone, and faces away from the otic capsule in a posterolateral direction. A prominent subvertical ridge on the anterolateral edge of the exoccipital and upper part of the basioccipital cannot be directly associated with the subcapsular process of the chondrocranium of extant crocodilians. This ridge projects laterally beyond the ventral ramus of the opisthotic, and lies anterior to the external foramina for the hypoglossal nerve. The overall structure of the braincases (especially the otic region) of S. robertsoni and other aetosaurians, where known, is more similar (in terms of derived archosaurian characters) to those of crocodylomorphs than are the braincases of other major suchian groups. This provides evidence for the currently unorthodox hypothesis that, among major suchian clades, Aetosauria and Crocodylomorpha are each others’ closest relatives. Support for this hypothesis is found in features of the palatine and prefrontal that have not been considered in recent studies of suchian phylogeny. This alternative phylogenetic hypothesis demands further investigation but, combined with the new morphological data that it explains, it provides a framework for the understanding of the evolution of the derived and distinctive braincase structure of extant crocodilians. © 2002 The Linnean Society of London, Zoological Journal of the Linnean Society, 2002, 136, 7−23.

No abstract

The unique holotype of Hallopus victor (Marsh), from the Upper Jurassic of Garden Park, Colorado, is redescribed. The bones previously identified as pubes (Marsh 1890) or ischia (von Huene 1914) are regarded here as the left radius and ulna, and the ‘ ulna5 and ‘radius5 of previous workers are considered to be the left radiale and ulnare. Marsh’s identification (1890) of the ischium and his orientation of the scapula and femur (1896) are upheld. The presence of a humerus on the larger slab is confirmed. Hallopus is interpreted as a highly specialized, cursorial crocodilian, with slender, hollow bones, a greatly elongated radiale and ulnare, and a roller-like joint between these and the metacarpals. The manus is pentadactyl with a symmetrical distribution of lengths about the central axis and some proximal wedging-out of the metacarpals. The iliac blade is elongated and resembles that of Orthosuchus , the ischium is reminiscent of that of Protosuchus . The femur has a lesser trochanter, a fourth trochanter and a ‘pseudointernal' trochanter, but no greater trochanter. The tibia is longer than the femur. The tarsus is basically crocodilian in pattern, but greatly compressed and specialized. The first metatarsal is reduced to an elongated splint, permanently recessed into metatarsal II. Metatarsals II to IV are symmetrical in length with III longest, metatarsal V is reduced, pointed, and lacks phalanges. The interpretation put forward provides a consistent explanation of the peculiarities of the skeleton of Hallopus as a variant on the basic crocodilian plan. The details of the articulation of the carpal and tarsal joints are described as far as preservation permits, and possible movements are considered. The carpometacarpal and tarsal joints are simple hinges, but the proximal carpal joint appears to have been relatively immobile and the elongation of the radiale and ulnare is viewed as a device to compensate for the increase in length of the tibia. The femur has an off-set, ball-like head and evidently moved essentially in a parasagittal plane. The pes is functionally tridactyl, with the metatarsals locked together proximally. It is concluded that both fore- and hind-feet were digitigrade during movement, although in a stationary pose the metatarsus may have been in contact with the ground. Some aspects of the pelvic and hind-limb musculature are briefly discussed. Functional analogies from the locomotory point of view are limited by the lack of cursorial quadrupedal archosaurs for comparison. It is concluded that a hare-like bounding gallop was the most probable type of fast locomotion in Hallopus . Although no skull bones have been identified, evidence from the postcranial skeleton is adduced to show that Hallopus is of pedeticosaurid descent. The relationships of early crocodilomorphs are discussed, and it is deduced that two basic stocks diverged from a common ancestry during the middle part of the Trias. These two groups are included in an expanded Order Crocodylomorpha. The Suborder Crocodylia has the Triassic Stegomosuchidae as its radicle and contains ‘ normal ’ crocodiles (including the Sebecosuchia but not the Baurusuchidae). The suborder Paracrocodylia is proposed for mainly cursorial forms, to include the infraorders Pedeticosauria, Baurusuchia and Hallopoda. Diagnoses for these groupings are presented. An origin for both stocks from a form close to Cerritosaurus is postulated. Erpetosuchus and Dyoplax are not now regarded as crocodilomorphs. The possibility of an early cursorial phase in crocodilian evolution is briefly discussed, and it is tentatively suggested that the gallop occasionally observed in young crocodiles (Cott 1961) may be a relic of a primitive type of locomotion in the group. The significance of this to the emergence of the crocodilian type of shoulder-girdle is considered.