09-14-2003, 09:47 PM
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Registered User
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Join Date: Aug 2003
Posts: 57
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Quote:
This aside, let us address your challenge: provide evidence from the fossil record of the gradual development of the furcula and carinate sternum in dinosaurs. Excellent, and not at all hard. Let us look at the phylogenetic map of Theropoda, and isolate the taxa where these characters appear, their initial morphology, and their subsequent ontogenetic variation over time, leading first to the par-avian and then the bona fide avian condition.
Furculae first appear in the Allosauroidea, being present in Allosaurus fragilis itself. In Allosauroidea the furcula is shallow and incipient. The presence of furcula in Allosauroidea indicates that it may be a synapomorphy of Neotetanurae itself, which pushes the origin of the structure back to the terminal Triassic at earliest and early Jurassic at latest.
The earliest appearance of a furcula in Coelurosauria--the containing clade for Maniraptora and thus of more phylogenetic relevance than Allosauroidea--is in Scipionyx samniticus, from the Aptian. The furcula is rather more robust in this basal coelurosaur than in its larger allosauroid cousins, although it is still not as hypertrophied as later maniraptoran furculae. Further derivation of furculae in the coelurosaurian lineage includes their presence in Tyrannosauroidea, the sister clade of Maniraptora. Tyrannosauroids displaying furculae include Gorgosaurus libratus in which the furcula is nearly as robust as that of velociraptorines, and Daspletosaurus torosus, in which the furcula is more gracile and may indicate a character reversal to the earlier furcula morphology observed in basal coelurosaurs.
The most par-avian of furculae, appear within Maniraptora. They are present in Sinornithosaurus millenni, in which the furcula is as robust as that of Archaeopteryx, in Ingenia yanshini where indeed the furcula exhibits a 'greater' degree of hypertrophy than that seen in the urvogel, in Oviraptor spp., in which the furcula is equally robust as that seen in Ingenia, and finally, robust furculae are known from multiple velociraptorine specimens.
Thus traced, we have gradualistic variation following the derivation of the furcula, in which it progresses from the basal state, to the par-avian, to the avian. You must find a way to explain away the clearly preserved furculae of multiple theropod lineages, to refute these data.
Now let us turn to sterna. Ossified sterna lacking carinas first appear amongst the basal Neotetanurae and Allosauroidea, particularly in Xuanhanosaurus qilixiaensis and Sinraptor spp. In these taxa the sterna are fairly small, and lack derived sternocostal articulations. Nevertheless, as early as Sinraptor spp. we see an incipient carina of the sternum, hinting at the hypertrophy of this element in Coelurosauria.
Sterna are persistent elements of the pectoral girdle throughout Coelurosauria, and are seen in both Tyrannosauroidea (namely Gorgosaurus libratus), in basal form, and in Maniraptoriformes and Maniraptora in increasingly par-avian morphology. The sterna of oviraptors are largely ossified and no longer cartilaginous, and display an incipient carina. Indeed, the oviraptorosaur sterna are more robust than those of the urvogel and are no smaller than those seen in archaic avian lineages, such as the confuciusornithids. The same holds true for the sterna of velociraptorines, which are indeed larger than those of oviraptorosaurs (and they dwarf those of Archaeopteryx). Perhaps most daming is the fact that Rahonavis and the Alvarezsaurs, possess sterna which are more strongly carinate than the earliest Avialae.
And here we are again, a nice, gradualistic change in character state for the sternum. To refute these data one must somehow account for the presence of sterna in these dinosaurs, and moreover, why they should appear to grow increasingly avian in form, over time. The same holds true with the case of the furculae.
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I have not even begun to research your claims here and am readily identifying gaps and reverse evolution and temporal discontinuity. Why do you solidly claim a "nice gradualistic change in character state" for these features? Here is just one contradiction from your post several days ago (after some research, other conflicts with regard to gradualism will be shown):
The lack of a robust, carinate sternum, ossified uncinate processes, a basal sternocostal morphology, the incipient nature of the acrocoracoid and acromion process, and lack of a triosseal canal system indicating that the supracoracoideus and pectoralis major were still fairly generalized comparative to Pygostylia, suggests that Archaeopteryx was not a skilled aerialist
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