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Thread: A New Hypothesis of Dinosaur Relationships and Early Dinosaur Evolution

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    A New Hypothesis of Dinosaur Relationships and Early Dinosaur Evolution

    It might explain the lack of feather-ish integument in Sauropodomorphs if this is true but Pterosaurs have them too. Notice either way proto-dinosaurs are omnivores. Did Ornithischians lose a bird-like airsac system? One is present in Pterosaurs, Sauropodomorphs and Theropods. Orithischians are well nested within a clade of hot blooded airsac creatures.

    Herrerasaurs share the bizarre pattern of digit reduction seen in theropods (reducing distal digits first, and inwardly) rather than that of true theropod ceratosaurs and birds (normal tetrapod pattern of digit reduction at both sides). It is not a dogma that meat eating dinosaurs could not have evolved twice because herrerasaur relationships were problematic from their discovery and their dinosaurian status was outright doubted in the past. The Saurischian-Ornithischian split wasn't concrete either: some people like Bakker favoured a Phytodinosauria (it was never supported by cladistics just wierd and therefore strong characters like a double sternum) and in the days of taxonomic pessimism the Sauropodomorpha were often treated as a third, completely unrelated archosaur order perhaps closest to crocodiles. A well tested hypothesis is not a dogma as journos are claiming, even if its wrong.

    Eoraptor is the most primitive theropod and silesaurs are still not Ornithischians. As I said there are no pterosaurs included nor is Scleromochlus. Nor is Chilesaurus. I notice there are no Marginocephalians which might have affected where heterodontosaurs live in the Ornithischian part of the tree.



    For 130 years, dinosaurs have been divided into two distinct clades—Ornithischia and Saurischia. Here we present a hypothesis for the phylogenetic relationships of the major dinosaurian groups that challenges the current consensus concerning early dinosaur evolution and highlights problematic aspects of current cladistic definitions. Our study has found a sister-group relationship between Ornithischia and Theropoda (united in the new clade Ornithoscelida), with Sauropodomorpha and Herrerasauridae (as the redefined Saurischia) forming its monophyletic outgroup. This new tree topology requires redefinition and rediagnosis of Dinosauria and the subsidiary dinosaurian clades. In addition, it forces re-evaluations of early dinosaur cladogenesis and character evolution, suggests that hypercarnivory was acquired independently in herrerasaurids and theropods, and offers an explanation for many of the anatomical features previously regarded as notable convergences between theropods and early ornithischians.
    http://www.nature.com/nature/journal...ture21700.html

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    Now there are aphanosaurs at the stem with somewhat croc-like ankles and a bird-like metabolism. Aphanosauria includes the Anisian archosaur Teleocrater, and three less well known genera of Anisian age that have been problematic - Yonghesuchus, Dongusuchus, and Spondylosoma. Teleocrater was more mysterious until very recently.

    https://www.nature.com/nature/journa...ture22037.html


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    By the way this is the definitions involved excluding the herrerasaurs. I would prefer stem definitions myself as it gets less confusing, especially when talking to the public.



    The Ornithoscelida paper proposes an improper definition of Saurischia. Correctly herrerasaurs are now sauropodomorphs (accepting the ornithoscelida phylogeny). Though the deeper ornithischians nest in the tree, the harder it is to make sense of their absence of bird-like airsacs.

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