The earliest bird-line archosaurs and the assembly of the dinosaur body plan

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Nature | Letter

The earliest bird-line archosaurs and the assembly of the dinosaur body plan

Nature
doi:10.1038/nature22037
Received
Accepted
Published online
The relationship between dinosaurs and other reptiles is well established1, 2, 3, 4, but the sequence of acquisition of dinosaurian features has been obscured by the scarcity of fossils with transitional morphologies. The closest extinct relatives of dinosaurs either have highly derived morphologies5, 6, 7 or are known from poorly preserved8, 9 or incomplete material10, 11. Here we describe one of the stratigraphically lowest and phylogenetically earliest members of the avian stem lineage (Avemetatarsalia), Teleocrater rhadinus gen. et sp. nov., from the Middle Triassic epoch. The anatomy of T. rhadinus provides key information that unites several enigmatic taxa from across Pangaea into a previously unrecognized clade, Aphanosauria. This clade is the sister taxon of Ornithodira (pterosaurs and birds) and shortens the ghost lineage inferred at the base of Avemetatarsalia. We demonstrate that several anatomical features long thought to characterize Dinosauria and dinosauriforms evolved much earlier, soon after the bird–crocodylian split, and that the earliest avemetatarsalians retained the crocodylian-like ankle morphology and hindlimb proportions of stem archosaurs and early pseudosuchians. Early avemetatarsalians were substantially more species-rich, widely geographically distributed and morphologically diverse than previously recognized. Moreover, several early dinosauromorphs that were previously used as models to understand dinosaur origins may represent specialized forms rather than the ancestral avemetatarsalian morphology.

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Figures

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  1. Geographical and stratigraphical occurrence of Teleocrater rhadinus gen. et sp. nov. from the Ruhuhu Basin, southern Tanzania, Africa.
    Figure 1
  2. Skeletal anatomy of Teleocrater rhadinus gen. et sp. nov.
    Figure 2
  3. Early evolution of avemetatarsalians.
    Figure 3
  4. Skeletal anatomy of the aphanosaurs Dongusuchus efremovi, Yarasuchus deccanensis and Spondylosoma absconditum.
    Extended Data Fig. 1
  5. Histological sections of the limb bones of T. rhadinus gen. et sp. nov.
    Extended Data Fig. 2
  6. The relationships of T. rhadinus gen. et sp. nov. among archosauriforms from the Nesbitt dataset.
    Extended Data Fig. 3
  7. The relationships of Teleocrater rhadinus gen. et sp. nov. among archosauriforms from the Ezcurra dataset.
    Extended Data Fig. 4
  8. Phylogeny of early Avemetatarsalia illustrating the character distributions of the components of the crocodile-normal ankle configuration and showing that this ankle type was plesiomorphic for Archosauria, Avemetatarsalia, and possible less inclusive clades within Avemetatarsalia (for example, Dinosauriformes).
    Extended Data Fig. 5
  9. Ternary diagrams of measurements of the hindlimb elements (femur, tibia and longest metatarsal) of archosauriforms.
    Extended Data Fig. 6
  10. The relationships of S. taylori among archosauriforms from the Nesbitt dataset.
    Extended Data Fig. 7
  11. The relationships of S. taylori among archosauriforms from the Ezcurra dataset.
    Extended Data Fig. 8
  12. Disparity estimates for major archosaur groups and time intervals (weighted mean pairwise dissimilarity (WMPD)).
    Extended Data Fig. 9
  13. New character illustrations for the phylogenetic analysis.
    Extended Data Fig. 10
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