The question ot the origin of birds can be equated with the origin of Archueopte,:vx. thc oldest known bird. Analysis of the five presently known skeletal specimens of Archaeopteryx. and comparison with the skeletal anaromv of the several reptilian groups that have heen proposed as possible ancestors of birds (Ornithopoda. Thcropoda. Pscudosuchia and Sphenosuchidac). confirm the conclusions (long rejected by most suhscqucnt workers) of tleilmann (1926). I.owe (1935, 1944) and Holmgren (1955). namely. that the skeletal anatomy of Archaeopteryx is extraordinarily similar to that of contemporancous and succccJing coelurosaurian dinosaurs. Rejection of these similarities as adaptive structures only (parallel or convergent similarities), and therefore of no phylogenetic importance, is hcre considlered invalid. I4eilmann was thc 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 thcropods, 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 he false.With the exception of fused claviclcs 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 unrelateQ 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.Ornithopod-A rchaeopteryx ancestral-descendant affinities may be dismissed becausc of the false "avian" organization of the pelvis in the Berlin specimen of Archaeopteryx and the merely superficially bird-like construction of the ornithischian 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 Archueopteryx and subsequent birds.All available evidence indicates unequivocally that Archaeop...
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