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Archaeopteryx and the origin of birds

1976; Oxford University Press; Volume: 8; Issue: 2 Linguagem: Inglês

10.1111/j.1095-8312.1976.tb00244.x

1095-8312

John H. Ostrom,

Ichthyology and Marine Biology

The question of the origin of birds can be equated with the origin of Archaeopteryx, the oldest known bird. Analysis of the five presently known skeletal specimens of Archaeopteryx. and comparison with the skeletal anatomy of the several reptilian groups that have been proposed as possible ancestors of birds (Ornithopoda, Theropoda, Hseudosuchla and Sphenosuchidac), confirm the conclusions (long rejected by most subsequent workers) of Heilmann (1926), Lowe (1935, 1944) and Holmgren (1955), namely, that the skeletal anatomy of Archaeopteryx is extraordinarily similar to that of contemporaneous and succeeding coelurosaurian dinosaurs. Rejection of these similarities as adaptive structures only (parallel or convergent similarities), and therefore of no phylogenetic importance, is here considered invalid. Heilmann was the first to identify the only evidence that has been cited so far for dismissing coelurosaurian-avian ancestral–descendant relationships, the supposed absence of clavicles in all theropods, and on that basis suggested a common Archaeopteryx–dinosaur ancestry among pseudosuchian reptiles. That evidence is negative and thus inconclusive, and is now known to be false. With the exception of fused clavicles and unique ischial morphology, virtually every skeletal feature of Archaeopteryx is known in several contemporaneous or near-contemporary coelurosaurian dinosaurs and many of these conditions are unrelated, specialized features (the detailed morphology of the manus, metacarpus, carpus, humerus, scapulocoracoid, pes, metatarsus, tarsus, femur, pubis, ilium, skull and mandibles). The presence of so many derived characters in common clearly establishes that the closest ancestral affinities ot Archaeopteryx are with coelurosaurian theropods. There is no contrary evidence and any other explanation is illogical. All available evidence indicates unequivocally that Archaeopteryx evolved from a small coelurosaurian dinosaur and that modern birds are surviving dinosaurian descendants. Stated simply, avian phylogeny was: Pseudosuchia Coelurosauria Archaeopteryx higher birds. The question of the origin of birds can be equated with the question of the origin of Archaeopteryx. This last question evokes two possible answers, depending upon how one views the importance of "primitive versus derived characters" in assessing phylogenetic relationships. One possible answer is: Archaeopteryx is a direct descendant of some unknown, but presumably Euparkeria-like pseudosuchian. This answer is predicated on the belief that Archaeopteryx only parallels or converges with various coelurosaurs in certain skeletal similarities. This is the view now held by the majority of biologists– a view that I find unacceptable. The second possible answer is: Archaeopteryx is directly descendant from a small unknown Ornitholestes-like coelurosaurian dinosaur. This answer assumes that skeletal similarities between coelurosaurs and Archaeopteryx are derived from a common ancestor, itself a coelurosaur. This is the view advocated here. There is no evidence to support an ornithischian ancestry of birds. The pubis of Archaeopteryx apparently was not reflected backward as in ornithischians and modern birds, and in any case, the ornithischian pubis is only superficially like that of living birds. Nor is the so-called ornithopod foot like that of birds. Evidence of close theropod–Archaeopteryx relationships, however, is abundant: the presence of the same, multiple, specialized adaptations in both Archaeopteryx and various coelurosaurs (tridactyl manus, metacarpus and carpus morphology, forelimb and pectoral girdle structure, four-toed pes, reversed hallux, metatarsal morphology, mesotarsal joint, hindlimb construction, pelvic form, plus elongated forelimbs, bipedal posture, vertebral structure and formula, and basic cranial morphology). The presence in Archaeopteryx, coelurosaurs and pseudosuchians of several primitive characters in common (thecodont dentition, sclerotic ring, possibly amphicoelous vertebrae, long caudal series, gastralia, pubic symphysis, short coracoids) indicates only a probable common ancestry. It does not establish that the Coelurosauria could not have given rise to Archaeopteryx–and higher birds. There is no evidence (outside of Lagosuchus and Lagerpeton) of shared derived characters to suggest a close evolutionary relationship between classic pseudosuchians and Archaeopteryx. Similarly, there is no clear-cut evidence in the form of shared derived characters to link Archaeopteryx with Sphenosuchus. The absence of clavicles in theropods (now known to be false), once considered as conclusive evidence against a coelurosaurian ancestry of birds, is no more significant than is the absence of a sternum in all known pseudosuchians as evidence against a pseudosuchian ancestry of all other archosaurs. The absence of any known "ideal" coelurosaurian pre-Archaeopteryx is only negative and inconclusive evidence, especially in view of our meagre and exceedingly deficient knowledge about Early and Middle Jurassic terrestrial vertebrates. All available evidence indicates that the immediate ancestor of Archaeopteryx was a small coelurosaurian dinosaur and that the phylogeny of avian ancestry was: Pseudosuchia–Coelurosauria–Archaeopteryx:– higher birds. Ornithopod-Archaeopteryx ancestral-descendant affinities may be dismissed because of the false "avian" organization of the pelvis in the Berlin specimen of Archaeopteryx and the merely superficially bird-like construction of the ornithisehian pelvis. The suite of specialized characters unique to ornithischians (e.g., predentary, tooth morphology), that occur even in Triassic representatives, is further evidence for dismissing close affinity between ornithopods and Archaeopteryx. The supposed close relationship between birds and pseudosuchians is judged to be remote at best, due to the completely primitive nature of the few anatomical features which pseudosuchians have in common with Archaeopteryx. Sphenosuchus, a primitive and early archosaur, is also a potential avian ancestor, but existing evidence consists of primitive archosaurian features plus a few similarities with certain modern birds. These similarities, which are present in two groups that are separated from each other by more than 200 million years, and which cannot be demonstrated in Archaeopteryx, are considered irrelevant to the origins of Archaeopteryx and subsequent birds.