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Originally Posted by Vindex Urvogel
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.
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You have already started with assumptions. 1) “Shallow and incipient” are your words hinting at an infant state of the furculum. How will you definitively show me and others that the Allosaurus fragilis furculum was really a “robust-furculum-in-progress?” 2) Was or was not the furculum of Allosaurus fragilis a development of Neotetanurae and how will you prove your answer by the fossil record? Also, you will recall I originally asked for “the non-existence of, then the gradual development of, and finally, the full arrival of a furculum.” What then is the ancestor of Allosaurus fragilis wherein the furculum is not present, and how does the fossil record connect the two?
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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,
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Exactly what line do you intend on developing the avian furculum through? Have we now shifted to Coelurosauria and Scipionyx samniticus? If so, we are left with a major gap or are you counting on the “incipient” furculum found in a “cousin” Allosaurus fragilis to address this problem? I am not sure you should be allowed to do this. What if we were dealing with kiwis and emus, both of the order Struthioniformes, where, say, kiwis were progenitors of true flight but emus were a bifurcation? Should I not insist that you stay with kiwis and demonstrate the gradual development of avian features through its lineage alone? I see your change in direction as a problem.
Aside from that however, you are still faced with a gap. How will you move Allosaurus fragilis along in the fossil record to Scipionyx samniticus and convincingly present the gradual development of the furculum from “incipient” to “rather more robust?” And, in a temporal sense, will this be a lateral movement or a forward movement?
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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.
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Gaps, parallel evolution, and reverse evolution, suggesting temporal discontinuity, all plague your paragraph. Why has our train of thought jumped tracks over to Tyrannosauroidea, unless it is to include a few instances of a near robust furculum in antecedents of Sinornithosaurus millenni and Archaeopteryx? Again, I do not know that you should be allowed to do this. Why are you not able to trace a single line of fossil data from Coelurosauria to Avialae without introducing offshoots and parallel branches within clades and families? And please offer a full explanation of why the “gracile” furculum in Daspletosaurus torosus apparently throws the evolutionary movement into regression, yet you do not view it as problematic. If you are counting on Daspletosaurus to fill a slot in your continuum, then you are presented with a temporal inconsistency.
But parallel and reverse evolution aside, you are still faced with gaps. Here, your argument has taken us from Scipionyx samniticus and thrown us into a “sister clade of Maniraptora” where we must more or less come to some vague agreement with you that the fossil record bears out a smooth, gradualistic development of the furculum from “rather more robust” in Scipionyx samniticus to exactly who knows what in Tyrannosauroidea - Gracile? Robust? Remember my clear initial request: can you present “the non-existence of, then the gradual development of, and finally, the full arrival of a furculum [for flight]” in these creatures? Here, I must insist that you gradually move us from Scipionyx samniticus all the way through Tyrannosauroidea and use the fossil record to do so, otherwise, one might assume you do not have a clear roadmap to follow.
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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.
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After trudging through a related clade, we suddenly arrive at Maniraptora where “robust” furcula exist in number. Are we to think that the avian lineage came through the related clade or do we need to insist that you lay out clear fossils from Scipionyx samniticus to Sinornithosaurus millenni? If passing through the related clade is acceptable procedure, are we then to believe that you arrived at Sinornithosaurus millenni, who has a furculum “as robust as that of Archaeopteryx,” after the “gracile” furculum of Daspletosaurus torosus?
Here is the question: Can you or can you not provide a fossil record which plainly and unmistakably establishes the non-existence of, then the gradual development of, and finally, the full arrival of an avian furculum in dinosaurs? If you are unable to do this as you claimed you could, and instead, you begin arguing from a gradualistic clade-to-clade or family-to-family approach, we will have no choice but to suspect that, whether intentionally or by default, you espouse punctuated equilibrium. To be sure, your argument here smacks of punctuated equilibrium already.
A final note. Creationism and punctuated equilibrium are not bedfellows. Somehow or other, I feel, you have been led to believe that creationists adopted that theory after having applauded Gould’s admission of serious gaps in the fossil record. Nothing could be further from the truth. Creationists and Gould remained antagonists until his demise. Though some splinter group of creationists may hold to the theory of punctuated equilibrium, I do not nor does any creationist I personally know.
I do not intend to address the sternum or any more anatomical features until I am satisfied with answers to my questions or until we both agree that the fossil record is too scanty at present to prove your assertions.