Basal Archosaurs: Phylogenetic Relationships and Functional Implications

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. ABSTRACT-Archosaurs first appeared in the Middle Triassic and within a short interval of time came to dominate all faunas of large vertebrates for the remainder of the Mesozoic. It is widely held that shortly after archosaurs first appeared the group split into two clades, now termed "Pseudosuchia" and "Ornithosuchia." Each clade independently evolved a rotary-style ankle design ("crocodile-normal" and "crocodile-reversed," respectively) and each independently evolved from semi-erect to erect limb posture. Erect posture, in turn, has been identified as the key adaptation (possibly associated with locomotor stamina) that resulted in the archosaurian radiation.These hypotheses are examined in light of a numerical cladistic analysis of basal archosaurs. Contrary to previous schemes, Archosauria is divided into Crurotarsi and Ornithodira. Crurotarsi (Suchia + Ornithosuchidae + Parasuchia) is characterized by the rotary crurotarsal ankle joint and other postcranial synapomorphies; this style of ankle joint thus appears to have evolved once rather than twice. Ornithodira is divided into Pterosauria and Dinosauromorpha. Pterosaurs, therefore, occupy a basal position within Ornithodira and do not constitute the sister-group to Dinosauria as has been suggested. The supposed pterosaur precursor, Scleromochlus, is reexamined and its proximity to Pterosauria is questioned.The analysis underscores the perils of employing functional constructs, such as ankle "types," rather than character data in phylogeny reconstruction. Current scenarios for the evolution of upright posture in archosaurs-either as an "improvement" in design or as a correlate of locomotor stamina-are not supported by the cladistic pattern. Erect archosaurs coexisted for millions of years alongside more abundant sprawling or semi-erect rhynchosaurs and synapsids and radiated only after their demise. Osteological features associated with locomotor stamina in living tetrapods do not appear to be strongly correlated with erect posture among archosaurs. Once erect posture had been achieved in archosaurs, however, it may have promoted the evolution of bipedal locomotion, which appears at about the same time. Bipedal locomotion, in turn, may have permitted more extensive modification of the forelimbs, which in a short interval of time were fashioned as wing supports for powered flight in pterosaurs.Archosaurs include familiar forms, such as crocodiles, pterosaurs, and dinosaurs, as well as a variety of less familiar forms that went extinct before the end of the Triassic, including the gavial-like phytosaurs, armored aetosaurs, and a variety of quadrupedal and bipedal rauisuchians and dinosaur precursors. Although several monophyleti...

Histology and postural change during the growth of the ceratopsian dinosaur Psittacosaurus lujiatunensis

“…The possibility that basal members of all three major dinosaurian clades retained a quadrupedal hatchling stage is intriguing and cannot presently be excluded, but awaits further testing based on future discoveries. Similarly, the quadrupedal-to-bipedal shifts that occur in at least some taxa may recapitulate the evolutionary transition from quadrupedal basal archosaurs to bipedal avemetatarsalians that took place during the Early and Middle Triassic 29,30 , but this scenario will remain speculative until more information is available regarding the ontogeny of the early avemetatarsalian bipeds themselves.…”

The origin and early radiation of dinosaurs

“…The advent of numerical cladistic analyses in the mid 1980s crystallized support for both dinosaur monophyly and the hypothesis that birds evolved from theropod dinosaurs (e.g., Gauthier, 1986;Benton and Clark, 1988;Sereno, 1991a). Today, higher-level phylogenetic analyses continue to find robust support for dinosaur monophyly (e.g., Juul, 1994;Benton, 1999Benton, , 2004Sereno, 1999;Ezcurra, 2006;Langer and Benton, 2006;Irmis et al, 2007a;Brusatte et al, 2008a;Nesbitt et al, 2009b;Brusatte et al, 2010b;Nesbitt et al, 2010), although the exact characters diagnosing the dinosaur group continue to change as new fossils are found and old ideas are reinterpreted.…”

“…Over 50 characters have been cited as dinosaur synapomorphies in both pre-cladistic and cladistic studies (Bakker and Galton, 1974;Benton, 1984;Gauthier, 1986;Benton and Clark, 1988;Novas, 1989;Sereno, 1991a;Novas, 1992;Sereno and Novas, 1994;Novas, 1996;Benton, 1999;Sereno, 1999;Fraser et al, 2002;Langer and Benton, 2006;Irmis et al, 2007a;Nesbitt et al, 2009bNesbitt et al, , 2010Brusatte et al, 2010b). Potential dinosaur characteristics are distributed throughout the body.…”

New Interpretation of the Palate of Pterosaurs

“…For the phylogenetic relationships of pterosaurs, it has been suggested that Pterosauria (1) was the sister group of the Dinosauria (Sereno, 1991;Benton, 1999), (2) were basal archosauromorphs (Bennett, 1996), (3) or were members of the group Prolacertiformes (Peters, 2000). Recently, Benton (2007, 2008) provided new evidence to support the origin of the Pterosauria within Archosauria and we follow their definition here.…”

“…The isolated and three-dimensionally preserved cranial elements of this specimen provide a unique insight into the finer morphology and structure of some palatal bones of Dorygnathus. On the basis of the archosaurian affinities of the group (Sereno, 1991, Benton, 1999Hone and Benton, 2007), we adopted an extant phylogenetic bracket (''EPB,' ' Witmer, 1995a) approach to identify some of the isolated cranial elements and to reconstruct the palate of Dorygnathus. Comparison of the newly reconstructed palate of Dorygnathus with those of other pterosaur taxa helped to clarify some hitherto undescribed or misinterpreted bony elements and fenestrae in these taxa, and opened the way to propose possible evolutionary changes in the construction of the pterosaur palate.…”