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Basal Archosaurs: Phylogenetic Relationships and Functional Implications

Paul C. Sereno
Memoir (Society of Vertebrate Paleontology)
Vol. 2, Basal Archosaurs: Phylogenetic Relationships and Functional Implications (Dec. 31, 1991), pp. 1-53
DOI: 10.2307/3889336
Stable URL: http://www.jstor.org/stable/3889336
Page Count: 53
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Basal Archosaurs: Phylogenetic Relationships and Functional Implications
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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.

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