GeoScienceWorld
Volume

Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin

Edited by S. J. Nesbitt, J. B. Desojo and R. B. Irmis

Abstract

Archosaurs, an important reptile group that includes today’s crocodiles and birds, arose during the Triassic in the aftermath of the greatest mass extinction of all time. In the last 20 years, our understanding of the early evolution of the group has improved substantially with the discovery of new fossils and species of early archosaurs and their closest relatives, a better understanding of the relationships of these animals, and new insights into their palaeobiology. In order to synthesize these new data, researchers of early archosaurs from around the world met at the first symposium of early archosaur evolution at the IV Congreso Latinoamericano de Paleontología de Vertebrados (September 2011) in San Juan, Argentina. This symposium facilitated collaboration and strove to paint a better understanding of these extraordinary animals. The resultant body of work is a state-of-the-art examination of early archosaur groups and their close relatives including historical, anatomical, biogeographical, evolutionary and palaeobiological data. This contribution furthers our knowledge of the anatomy, relationships, and palaeobiology of species-level taxa as well as more global patterns of archosaur evolution during the Triassic.

  1. Page 1
  2. Page 9
    Abstract
    Corresponding author (e-mail: martindezcurra@yahoo.com.ar)

    The earliest history of Archosauriformes is mainly represented by members of Proterosuchidae and Erythrosuchidae, which are known worldwide from latest Permian to Middle Triassic beds. These two groups were historically combined within ‘Proterosuchia’, with approximately 30 nominal species. Two morphotypes have been recognized among proterosuchians: proterosuchids with a generally more sprawling gait and elongated and low skulls with an overhanging premaxilla, and the more heavily built erythrosuchids, with a probably less sprawling gait and large, presumably hypercarnivorous, skulls. The systematics of ‘Proterosuchia’ was relatively chaotic throughout most of the twentieth century, but currently there exists consensus regarding the non-monophyly of proterosuchians and their phylogenetic position outside all other archosauriforms. In contrast, the delimitation and taxonomic content of Proterosuchidae and Erythrosuchidae remain unstable. Few studies of proterosuchian palaeobiology have been carried out. Current lines of evidence favour a predominantly terrestrial lifestyle for proterosuchians. Limb bone histology indicates rapid continuous growth rates in Proterosuchus and Erythrosuchus before reaching sexual maturity. A better knowledge of proterosuchian anatomy, systematics, evolution and ecology is important for advancing understanding of the origin and early radiation of Archosauriformes and the patterns of biotic recovery following the Permo-Triassic mass extinction event. There remains much research to be carried out in proterosuchian palaeobiology.

  3. Page 35
    Abstract
    Corresponding author (e-mail: sookias.r.b@gmail.com)

    Euparkeria capensis has long been considered an archetype for the ancestral archosaur morphology, and has been placed just outside of crown Archosauria by nearly all cladistic analyses. Six species are currently considered to be putative members of a clade Euparkeriidae, and have been collected from Olenekian- or Anisian-aged deposits in South Africa (Euparkeria capensis – the only definitive member of the group), China (Halazhaisuchus qiaoensis, Wangisuchus tzeyii, ‘Turfanosuchusshageduensis), Russia (Dorosuchus neoetus) and Poland (Osmolskina czatkowicensis). Four other species (Turfanosuchus dabanensis, Xilousuchus sapingensis, Platyognathus hsui, Dongusia colorata) were historically assigned to Euparkeriidae, but have been removed by recent work. Recent authors deemed Osmolskina czatkowicensis and Dorosuchus neoetus to be the most likely taxa to form a euparkeriid clade with Euparkeria capensis, but Osmolskina czatkowicensis and Euparkeria capensis were not found as sister taxa by the only cladistic analysis to have tested euparkeriid monophyly. Euparkeria capensis was small (<1 m), insectivorous or carnivorous, probably had vision adapted to low-light conditions and a semi-erect crocodile-like stance, and may have been facultatively bipedal. Bone histology demonstrates that Euparkeria capensis had a slow growth rate, which has been suggested to have been an adaptation to relatively stable environmental conditions.

  4. Page 49
    Abstract
    Corresponding author (e-mail: suesh@si.edu)

    Doswelliidae is a clade of armoured non-archosaurian archosauriform reptiles more closely related to Archosauria than are Proterosuchidae, Erythrosuchidae and possibly Euparkeria capensis. It is currently known from the late Middle Triassic (Ladinian) of Germany, the late Middle to early Late Triassic (Ladinian–Carnian) of Argentina and Brazil, and the Late Triassic (Carnian–Norian) of the USA. To date, two unambiguous synapomorphies diagnose Doswelliidae: (i) osteoderm ornamentation coarse, incised, and composed of central regular pits of subequal size and shape, and (ii) osteoderms with anterior articular lamina. Five taxa are currently recognized: Archeopelta arborensis, Doswellia kaltenbachi, Doswellia sixmilensis, Tarjadia ruthae and a new taxon from Germany. Based on skeletal features and occurrence, doswelliid archosauriforms may have had a semi-aquatic mode of life.

  5. Page 59
    Abstract
    Corresponding author (e-mail: jtrotteyn@unsj.edu.ar)

    Proterochampsia is a monophyletic group of crocodile-like archosauriforms currently endemic to the late Middle and early Late Triassic of South America considered as one of the potential successive sister-taxa of the crown group Archosauria. The proterochampsians come from the Ischigualasto-Villa Unión Basin in the west of Argentina and the Parana Basin in the south of Brazil. The traditional composition of the group includes the genera Cerritosaurus Price 1946, Proterochampsa Reig 1959 (with two species: P. barrionuevoi in Argentina and P. nodosa Barberena 1982 in Brazil), Chanaresuchus Romer (with two species from Argentina: C. bonapartei Romer and C. ischigualastensis Trotteyn et al. 2012), Gualosuchus reigi Romer 1971 and Tropidosuchus romeri Arcucci 1990. After a precladistic history of confusion about their relationships with crocodilians, in the last 20 years new discoveries of taxa, and more systematic and phylogenetic studies, have clarified their position as non-archosaurian archosauriforms and their relationships with other Triassic archosaurs.

  6. Page 91
    Abstract
    Corresponding author (e-mail: mstocker@utexas.edu)

    Phytosauria is a nearly cosmopolitan clade of large, quadrupedal, carnivorous archosauriforms. They are known unambiguously from Late Triassic deposits, although the clade’s ghost lineage extends at least to the late Early Triassic. Their nares are uniquely located close to the orbits rather than anteriorly in the rostrum as in modern crocodylians, and the rostrum is formed by elongated premaxillae bearing many teeth. Phytosaurs have roughly triangular, ornamented paramedian osteoderms, rounder appendicular osteoderms, and a unique ‘gular shield’ assembled from multiple, irregular osteoderms under the throat. Phytosaurs are reconstructed as semi-aquatic because of their general similarity to modern crocodylians and common preservation in fluvial and shallow-marine deposits. Currently, over thirty species are recognized. New specimens continue to be collected, some representing new taxa. The taxonomic status of other named taxa is uncertain and requires re-investigation. Since their discovery, phytosaurs have been used as biostratigraphic and biochronological index taxa for correlating Late Triassic sediments worldwide. Recent systematic and taxonomic revisions cast doubt on some of those correlations. Our understanding of the evolution of Phytosauria is far from complete. With reevaluation of well-known specimens, rigorous and comparative morphological descriptions, and robust phylogenetic hypotheses of ingroup relationships, studies of phytosaurs can address larger palaeobiological questions.

  7. Page 119
    Abstract
    Grup de Recerca del Mesozoic, Institut Català de Paleontologia ‘Miquel Crusafont’ (ICP), C. Escola Industrial 23, E-08201 Sabadell, Spain (e-mail: fabio.dallavecchia@icp.cat)

    Pterosaurs are a clade of highly specialized, volant archosauromorphs recorded from the Upper Triassic to the uppermost Cretaceous. Problematic remains referred to the Pterosauria are reported from the Triassic of Europe and both North and South America, but unequivocal pterosaur specimens are only known from the Alps (Italy, Austria and Switzerland: Preondactylus buffarinii, Austriadactylus cristatus, Peteinosaurus zambellii, Eudimorphodon ranzii, Carniadactylus rosenfeldi, Caviramus schesaplanensis and Raeticodactylus filisurensis) and Greenland (‘Eudimorphodon’ cromptonellus). Pterosaurs are diagnosed mostly by features associated with the advent of powered flight. They are generally considered to be archosaurians more closely related to dinosaurs than to crocodilians, but non-archosaurian positions have also been proposed. There is a lack of general agreement about ingroup relationships, particularly among the basal pterosaurs. Triassic pterosaurs differ from other non-pterodactyloid pterosaurs in features of the dentition and caudal vertebral column. A ‘Big Bang’ model for their early history fits better with the fossil record: the earliest unequivocal pterosaurs show a sudden and geographically limited appearance in the fossil record, as well as a relatively high burst of diversity and considerable morphologic disparity. Absence of pterosaur remains from deposits where they are expected to be found suggests that they had not yet evolved in pre-Norian times.

  8. Page 157
    Abstract
    Corresponding author (e-mail: mclanger@ffclrp.usp.br)

    Ichnological evidence suggests that dinosauromorphs originated by the Early Triassic, and skeletal remains of non-dinosaur representatives of the clade occur from the Anisian to the end of the Triassic. These taxa are small- to medium-sized, vary in feeding and locomotor features, and occurred over most of western Pangaea. They include the small lagerpetids from the Mid–Late Triassic of Argentina and the United States, and the larger, quadrupedal Silesauridae, with records in the Middle Triassic of Africa and Argentina, and in the Late Triassic of Europe, the Americas and northern Africa. The former group represents the earliest diverging dinosauromorphs, whereas silesaurids are more closely related to Dinosauria. Other dinosauromorphs include the archetypal early dinosauriform Marasuchus lilloensis (Middle Triassic of Argentina) and poorly known/controversial taxa such as Lewisuchus admixtus and Saltopus elginensis. The earliest diverging dinosauromorphs may have preyed on small animals (including insects), but cranio-dental remains are rare; by contrast, most silesaurids probably included plant material in their diet, as indicated by their modified jaw apparatus and teeth. Our knowledge of the anatomy and thus relationships of non-dinosaurian Dinosauromorpha is still deficient, and we suspect that future discoveries will continue to reveal novel patterns and hypotheses of palaeobiology and biogeography.

  9. Page 187
    Abstract
    Corresponding author (e-mail: belen_vb13@yahoo.com.ar)

    The ornithosuchids were a group of archosaurs with body lengths ranging from 2 to 4 m recorded from Upper Triassic beds in Argentina and Scotland. The group was defined as a node-based clade including Ornithosuchus longidens, Riojasuchus tenuisceps, Venaticosuchus rusconii and all descendants of their most recent common ancestor. The ornithosuchids are diagnosed by the following apomorphies observed in the three known species of the clade: downturned premaxilla; premaxilla–maxilla contact with a diastema in the alveolar margin equal in length to two teeth; palatine–pterygoid fenestra; and orbit with a distinct ventral point surrounded by ‘V’-shaped dorsal processes of the jugal. The most remarkable postcranial apomorphy of the group is the presence of the so-called crocodile reversed ankle joint, a condition that seems to be unique for the ornithosuchids among amniotans. The systematic history of Ornithosuchidae is complex and Ornithosuchus was allied with dinosaurs or phytosaurs prior to the implementation of numerical phylogenetic analyses. Currently, there is consensus that Ornithosuchidae is positioned within Pseudosuchia, but their phylogenetic position within the group remains strongly debated. Nevertheless, all hypotheses agree in inferring an extremely long ghost lineage at the base of the clade. The presence of derived pseudosuchians in the late Olenekian produces a ghost lineage of c. 16–18 millions of years for Ornithosuchidae, indicating that only the late evolutionary history of the clade is currently sampled in the fossil record.

  10. Page 203
    Abstract
    Corresponding author (e-mail: julideso@macn.gov.ar)

    Aetosauria is a clade of obligately quadrupedal, heavily armoured pseudosuchians known from Upper Triassic (late Carnian–Rhaetian) strata on every modern continent except Australia and Antarctica. As many as 22 genera and 26 species ranging from 1 to 6 m in length, and with a body mass ranging from less than 10 to more than 500 kg, are known. Aetosauroides scagliai was recently recovered as the most basal aetosaur, placed outside of Stagonolepididae (the last common ancestor of Desmatosuchus and Aetosaurus). Interrelationships among the basal aetosaurs are not well understood but two clades with relatively apomorphic armour – the spinose Desmatosuchinae and the generally wide-bodied Typothoracisinae – are consistently recognized. Paramedian and lateral osteoderms are often distinctive at the generic level but variation within the carapace is not well understood in many taxa, warranting caution in assigning isolated osteoderms to specific taxa. The aetosaur skull and dentition varies across taxa, and there is increasing evidence that at least some aetosaurs relied on invertebrates and/or small vertebrates as a food source. Histological evidence indicates that, after an initial period of rapid growth, lines of arrested growth (LAGs) are common and later growth was relatively slow. The common and widespread Late Triassic ichnogenus Brachychirotherium probably represents the track of an aetosaur.

  11. Page 241
    Abstract
    Corresponding author (e-mail: sjn2104@gmail.com)

    ‘Rauisuchia’ comprises Triassic pseudosuchians that ranged greatly in body size, locomotor styles and feeding ecologies. Our concept of what constitutes a rauisuchian is changing as a result of discoveries over the last 15 years. New evidence has shown that rauisuchians are probably not a natural (monophyletic) group, but instead are a number of smaller clades (e.g. Rauisuchidae, Ctenosauriscidae, Shuvosauridae) that may not be each other’s closest relatives within Pseudosuchia. Here, we acknowledge that there are still large gaps in the basic understanding in the alpha-level taxonomy and relationships of these groups, but good progress is being made. As a result of renewed interest in rauisuchians, an expanding number of recent studies have focused on the growth, locomotor habits, and biomechanics of these animals, and we review these studies here. We are clearly in the midst of a renaissance in our understanding of rauisuchian evolution and the continuation of detailed descriptions, the development of explicit phylogenetic hypotheses, and explicit palaeobiological studies are essential in advancing our knowledge of these extinct animals.

  12. Page 275
    Abstract
    Corresponding author (e-mail: irmis@umnh.utah.edu)

    Non-crocodyliform crocodylomorphs, often called ‘sphenosuchians’, were the earliest-diverging lineages of Crocodylomorpha, and document the stepwise acquisition of many of the features that characterize extant crocodylians. The first crocodylomorph fossils are approximately 230 million years old (upper Carnian, Late Triassic), and at least one of these early lineages persisted until at least 150 million years ago (Late Jurassic). These taxa occupied a wide variety of terrestrial environments from equatorial regions to high-paleolatitudes during the early Mesozoic. Despite a quarter-century of quantitative phylogenetic work, the interrelationships of early crocodylomorphs remain in a state of flux, though recent studies suggest that these lineages are paraphyletic with respect to Crocodyliformes, rather than forming a monophyletic early offshoot of Crocodylomorpha as some previously hypothesized. Nearly all early crocodylomorphs were upright quadrupedal small-bodied taxa, but lumping them all together as small cursorial faunivores masks ecological and morphological disparity in diet and limb functional morphology. With the accelerated pace of recent discovery of new specimens and taxa, future consensus on early crocodylomorph phylogeny will provide a solid framework for understanding their change in diversity and disparity through time, potential biogeographic patterns, and the morphological transformation leading to Crocodyliformes.

  13. Page 303
    Abstract
    Corresponding author (e-mail: tiagoraugust@hotmail.com)

    Proterochampsians are basal archosauriforms whose record is restricted to the Middle and Upper Triassic in Argentina and Brazil. They are quadruped forms that present characteristics consistent with a semi-aquatic lifestyle, such as an anteroposteriorly elongated skull that is flattened dorsoventrally with dorsally located orbits. In 2003, specimen UFRGS-PV-0877-T was discovered at the Schoenstadt site, in the city of Santa Cruz do Sul (Santacruzodon Assemblage Zone, Santa Maria Formation). This specimen, consisting of disarticulated cranial elements (such as nasals, frontals, parietals, postorbitals, a left squamosal, a left pterygoid and a fragment of a right mandibular ramus that bears teeth) and postcranial elements (such as femora, the left tibia, one vertebral centrum and two rib fragments), is assigned to the ‘proterochampsian’ Chanaresuchus bonapartei Romer (1971). This assignment is based on the shared V-shaped frontal-parietal suture of the new specimen and Chanaresuchus bonapartei, which differs from the transversely aligned and zigzagged pattern of C. ischigualastensis.

  14. Page 319
    Abstract
    Corresponding author (e-mail: gniedzwiedzki@biol.uw.edu.pl)

    We present the first comprehensive description of Prorotodactylus and Rotodactylus dinosauromorph tracks from the Early and Middle Triassic of the Holy Cross Mountains, Poland. We describe and comprehensively figure tracks that have been mentioned briefly in previous accounts as well as new, recently discovered material, and analyse the variation and stratigraphic distribution of these specimens. Tracks have been recorded from four sites – Koszary, Stryczowice, Wióry and Baranów – which span the early Olenekian–early Anisian (c. 250–246 Ma). These tracks therefore represent an ichnological record of the evolutionary succession of early dinosauromorphs during the earliest part of their evolutionary history. Recognized track types include cf. Prorotodactylus isp., Prorotodactylus isp., Prorotodactylus mirus, Rotodactylus cursorius, Rotodactylus isp. and cf. Rotodactylus isp. At least three distinct Early and early Middle Triassic early dinosauromorph ichnofaunas can be recognized. The oldest, which is early Olenekian in age, is characterized by the presence of Prorotodactylus isp., cf. Prorotodactylus isp. and non-archosaurian archosauromorph or archosaur tracks (e.g. Synaptichnium isp., Protochirotherium isp.), recorded at the Stryczowice and Koszary sites. The following assemblage, recorded at the late Olenekian Wióry site, displays the highest ichnodiversity of dinosauromorphs, with four track types present (Prorotodactylus isp., Prorotodactylus mirus, Rotodactylus cursorius and cf. Rotodactylus isp.). The youngest site, Baranów, includes Rotodactylus isp., as well as other larger dinosauromorph tracks. The first body fossil evidence of dinosauromorphs is a few million years younger than the youngest Polish tracks, so Prorotodactylus and Rotodactylus tracks currently provide the oldest record of dinosauromorph morphology, biology and evolution.

  15. Page 353
    Abstract
    Corresponding author (e-mail: mclanger@ffclrp.usp.br)

    Silesauridae is an exclusively Triassic group of dinosauromorphs, knowledge on the diversity of which has increased dramatically in the last few years. Silesaurid relationships are still contentious, as a result in part of different homology statements, particularly regarding the typical edentulous mandible tip of these animals. One of the most complete silesaurids yet discovered is Sacisaurus agudoensis from the Caturrita Formation (Late Triassic: Norian) of Rio Grande do Sul, Brazil, represented by numerous isolated bones recovered from a single site. The anatomy of S. agudoensis is fully described for the first time here, and comparisons are provided to other basal dinosauromorphs. S. agudoensis is a small-bodied animal (less than 1 m in length) that possesses a dentition consisting of leaf-shaped crowns with large denticles in the carinae, a plesiomorphic propubic pelvis with an almost fully closed acetabulum, elongate distal hindlimbs suggesting well-developed cursorial ability, and a laterally projected outer malleolus in the tibia. All previous numerical phylogenies supported a non-dinosaur dinosauromorph affinity for Silesauridae, but the reanalysis of one of those studies suggests that a position within Dinosauria is not unlikely, with silesaurids forming the basal branch of the ornithischian lineage.

  16. Page 393
    Abstract
    Corresponding author (e-mail: bjsmall2008@gmail.com)

    A small aetosaur skull and skeleton and referred material from the Chinle Formation, Eagle Basin of Colorado, USA, is described as a new taxon, Stenomyti huangae gen. et sp. nov, distinguished from other aetosaurs by the following autapomorphies: three premaxillary teeth; four palpebrals; pronounced midline ridge on frontals and parietals; paired ridges flanking midline ridge on parietal and frontal; exclusion of quadratojugal from ventral margin of skull by contact between jugal and quadrate; exclusion of postorbital from infratemporal fenestra; infratemporal fenestra a horizontally oriented oval that embays the posterior edge of the jugal; retroarticular process longer than distance between articular glenoid and posterior edge of external mandibular fenestra; oval to irregularly shaped ventral osteoderms that do not contact each other. Paramedian and lateral osteoderms of S. huangae are nearly identical to those of Aetosaurus ferratus, and other shared cranial characters suggest that these taxa are closely related and lie outside the clade Typothoracisinae + Desmatosuchinae. This discovery indicates that other reports of Aetosaurus across Laurasia based on osteoderms should be reassessed. Similar confusion with the osteoderms of other non-typothoracisine/desmatosuchine aetosaurs such as Aetosauroides, Stagonolepis and Calyptosuchus suggests that osteoderms are not always reliable taxonomic indicators.

  17. Page 413
    Abstract
    Corresponding author (e-mail: jtaborda@macn.gov.ar)

    Recent palaeohistological studies on paramedian osteoderms of aetosaurs revealed the presence of growth lines (lines of arrested growth or LAGs) and a minimal or nonexistent secondary remodelling in the bone matrix of these elements. This feature allows the age of individuals to be estimated through growth line count. In the present contribution we study the growth curve of the South American aetosaur Aetosauroides scagliai. We estimated the age (obtained from LAG counting) and body size (body length and body mass were used as proxies) of different aetosaur specimens in order to reconstruct the growth curve of the South American species. The data obtained for Aetosauroides scagliai were compared with that of other aetosaurs, such as Neoaetosauroides engaeus, Aetosaurus ferratus, Aetobarbakinoides brasiliensis, Typothorax coccinarum and Paratypothorax sp. Our results indicate that, if body length is considered as proxy, all studied aetosaur specimens have a similar or almost identical growth rate. However, important variations arose among aetosaur taxa if body mass is considered as proxy, which would be related to a body morphology ranging from slender (e.g. Aetobarbakinoides brasiliensis) to very wide (Typothorax coccinarum) morphotypes. In comparison with extant pseudosuchians (i.e. crocodylians), Aetosauroides scagliai possesses a relatively lower growth rate.

  18. Page 425
    Abstract
    Corresponding author (e-mail: bmmastrantonio@yahoo.com.br)

    The osteology of an almost complete braincase of the rauisuchian archosaur Prestosuchus chiniquensis from the Middle Triassic of Brazil is described for first time, based on two specimens (UFRGS-PV-0629-T and UFRGS-PV-0156-T). A comparative description with other taxa of rauisuchians is presented that forms the basis of a phylogenetic analysis. To perform the phylogenetic analysis, we describe and discuss each character codification for a modified version of the recent matrices of Gower (2002), Gower & Nesbitt (2006) and Brusatte et al. (2010). The analysis resulted in two most parsimonious trees that differ from the topologies recovered by Gower (2002) in a few aspects within Rauisuchia, and Prestosuchus chiniquensis was unequivocally depicted as deeply nested within Pseudosuchia, as the sister taxon of Batrachotomus kuperferzellensis in both topologies, supported by a single synapomorphy: the reduced to small fissure of the post-temporal fenestra between parietal, supraoccipital and exoccipital-opisthotic.

  19. Page 441
    Abstract
    Corresponding author (e-mail: alexandreliparini@yahoo.com.br)

    Prestosuchus chiniquensis is an extinct species of terrestrial archosaur from the Middle Triassic Epoch restricted to southern Brazil. In this paper the thigh musculature of P. chiniquensis is reconstructed based on a well-preserved specimen and on myological descriptions of extant birds and crocodylians. Among the 16 analysed muscular groups, 13 were recognized as present and homologous to both extant groups of archosaurs, and two only to the crocodylian line of archosaurs, so that 15 muscular groups were reconstructed in the fossil specimen. Morphological particularities of the pelvic girdle and the hindlimbs of P. chiniquensis gave a distinct arrangement for the muscular origin and insertion sites, leading to different lines of action and functions when compared with extant archosaurs. The comparison between extinct and extant archosaurs showed a basal condition sustained in some aspects, such as the morphology of the femur and the flexion of the knee, although other aspects were considered as derived, such as the morphology of the pubis and ischium, and their associated muscle origin locations.

  20. Page 469
    Abstract
    Corresponding author (e-mail: marquinhobio@yahoo.com.br)

    Unlike most rauisuchians, which are known based on partially preserved specimens, fossils attributed to Decuriasuchus quartacolonia include a monotaxonomic assemblage composed of nine associated individuals (MCN-PV10.105a–i), three with almost complete skulls (MCN-PV10.105a,c,d), and a partial disarticulated skull (MCN-PV10.004) collected in the Middle Triassic (Ladinian, Dinodontosaurus Biozone) beds of the Santa Maria Formation, in south Brazil. Because of its completeness and possible phylogenetic position, as one of the most basal loricatans, D. quartacolonia is a key taxon for anatomic, evolutionary and biomechanical studies of rauisuchians. The comparative description of its osteology reveals that the skull and mandible of D. quartacolonia are very similar to those of cf. Prestosuchus chiniquensis and Saurosuchus galilei, sharing a drop-shaped subnarial fenestra, a subtriangular antorbital fenestra with an elongated and narrow anterior point, a ‘roman nosed’ nasal, and a posteroventrally oriented ridge on the lateral surface of the ventral ramus of the squamosal. Among the differences are the autapomorphies of D. quartacolonia: numerous maxillary teeth (17), lateral expansion of the nasal/lacrimal covering the antorbital fenestra dorsally, and squamosal and quadratojugal forming a subtriangular projection that invades the lower temporal fenestra.

  21. Page 503
    Abstract
    Corresponding author (e-mail: William_Parker@nps.gov)

    The partial postcrania of Poposaurus gracilis, a bipedal poposauroid convergent with theropod dinosaurs, has been known for nearly a century, but the skull of P. gracilis has proven elusive. P. gracilis is part of a clade of morphologically divergent pseudosuchians (poposauroids) whose members are sometimes bipedal, lack dentition (i.e. beaks) and some have elongated neural spines (i.e. sails). However, the timing and acquisition of these character states is unknown given the uncertainty of the skull morphology of the ‘mid-grade’ poposauroid P. gracilis. Here, we present the first confirmed skull remains of P. gracilis directly associated with diagnostic pelvic elements that overlap with the holotype. The incomplete skeleton (PEFO 34865) from the Chinle Formation of Petrified Forest National Park (Arizona, USA) includes a left maxilla with a large, mediolaterally compressed tooth, left dentary, right prearticular and a partial postcranium. The character states of P. gracilis (bipedal, ‘sail-less’ and toothed) demonstrate that the evolution of bipedalism, the origin/loss of a dorsal ‘sail’ and the shift to an edentulous beak are complex in poposauroids. P. gracilis is widespread in the Upper Triassic formations in the western USA and is restricted temporally prior to the Adamanian–Revueltian faunal turnover during the Norian.

  22. Page 525
    Abstract
    Biology Department, Southern Connecticut State University, New Haven, CT 06515, USA (e-mail: weinbaumj1@southernct.edu)

    Postosuchus kirkpatricki is a Late Triassic (Norian) ‘rauisuchid’ archosaur from North America. The initial description of the Postosuchus type material included elements from two poposaurids. This confusion has prevented adequate description of the material. Recent examination of the type material and other specimens of Postosuchus, and of related taxa, has helped clarify the osteology of Postosuchus. The type specimens represent c. 75% of the skeleton. Together with other referred material, Postosuchus remains one of the most completely known rauisuchids. The paratype skeleton, which is relatively complete, would have been c. 3.5–4 m in length, and the holotype would have been closer to 5–6 m.

    Analysis of the postcranial skeleton of Postosuchus suggests that it may have been an obligate biped (based in part on limb proportions, which are similar to some theropod dinosaurs, the size of the manus (30% of the size of the pes) and the highly reduced nature of the digits and vertebral measurements). Possible postcranial autapomorphies of Postosuchus include a large, rugose triangular supra-acetabular buttress confluent with the dorsal margin of the iliac blade, and a symmetrical pes with digits two and three being roughly equal in length.

  23. Page 555
    Abstract
    Corresponding author (e-mail: hollidayca@missouri.edu)

    Archosauromorphs radiated into numerous trophic niches during the Mesozoic, many of which were accommodated by particular suites of cranial adaptations and feeding behaviours. The mandibular symphysis, the joint linking the mandibles, is a poorly understood craniomandibular joint that may offer significant insight into skull function and feeding ecology. Using comparative data from extant amniotes, we investigated the skeletal anatomy and osteological correlates of relevant soft tissues in a survey of archosauromorph mandibular symphyses. Characters were identified and their evolution mapped using a current phylogeny of archosauriforms with the addition of non-archosauriform archosauromorphs. Extinct taxa with the simple Class I condition (e.g. proterochampsids, ‘rauisuchians’), rugose Class II (aetosaurs, protosuchians, silesaurids) and interdigitating Class III symphyses (e.g. phytosaurs, crocodyliforms) and finally fused Class IV (avians) build the joints in expected ways, although they differ in the contributions of bony elements and Meckel’s cartilage. Optimization of the different classes of symphyses across archosauromorph clades indicates that major iterative transitions from plesiomorphic Class I to derived, rigid Class II–IV symphyses occurred along the lines to phytosaurs, aetosaurs, a subset of poposauroids, crocodyliformes, pterosaurs and birds. These transitions in symphyseal morphology also appear to track changes in dentition and potentially diet.

  24. Page 573
    Abstract
    Corresponding author (e-mail: alan.turner@stonybrook.edu)

    The first large (>1 m) diapsids appeared near the Permian–Triassic extinction and a subset of diapsids, the archosauriforms, expanded their body size range soon after in the Early–Middle Triassic. Here, we examine body size at key evolutionary events within Archosauriformes during the Triassic and through the end-Triassic extinction. Using femoral length as a body size proxy and a temporally calibrated phylogeny of Archosauriformes, we estimate ancestral body sizes using a maximum likelihood approach and test for the presence of an adapative radiation by comparing the fit of competing evolutionary models. Archosauriform body size is characterized by punctuated change with more change occurring early in the Triassic. Archosaurs crossing the Triassic–Jurassic boundary show a wide range in ancestral size, and dinosaurs (sauropodomorphs and theropods) are considerably larger in the Jurassic. Crocodylomorph origins are characterized by a drop in body size; however, both the relative amount of change and the rate of change are matched among other archosaur clades. Archosauriforms increase in absolute body size through the Triassic and evidence suggests that a directional trend in size increase occurred in the early Mesozoic. The morphological signature of adaptive radiation is rare in comparative data from extant animals but is present at the origination of Archosauriformes.

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