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

Abstract

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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Figure 1: Geographical and stratigraphical occurrence of Teleocrater rhadinus gen. et sp. nov. from the Ruhuhu Basin, southern Tanzania, Africa.
Figure 2: Skeletal anatomy of Teleocrater rhadinus gen. et sp. nov.
Figure 3: Early evolution of avemetatarsalians.

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Acknowledgements

We acknowledge A. Tibaijuka for help with fieldwork logistics in Tanzania. Supported by National Geographic Society Research & Exploration grant (9606-14, S.J.N.), National Science Foundation EAR-1337569 (C.A.S.) and EAR-1337291 (K.D.A., S.J.N.), a Marie Curie Career Integration Grant (630123, R.J.B.), a National Geographic Society Young Explorers grant (9467-14 M.D.E.), and the Russian Government Program of Competitive Growth of Kazan Federal University and RFBR 14-04-00185, 17-04-00410 (A.G.S.). We thank S. Chapman, A. C. Milner, M. Lowe and S. Bandyopadhyay for access to specimens, S. Werning, G. Lloyd, R. Close and K. Padian for discussions, and H. Taylor for photographs of the holotype.

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Contributions

S.J.N., R.J.B., M.D.E. and P.M.B. designed the research project; C.A.S. and K.D.A. designed the field project; S.J.N., C.A.S., K.D.A., R.M.H.S. and M.R.S. conducted fieldwork; S.J.N., R.J.B., M.D.E., P.M.B., M.R.S. and A.J.C. described the material; S.J.N., M.D.E. and M.R.S. conducted the phylogenetic analyses; R.J.B. conducted disparity analyses; and S.J.N., R.J.B., M.D.E., P.M.B., M.R.S., K.D.A., R.M.H.S., C.A.S., G.N. and A.G.S. wrote the manuscript.

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Correspondence to Sterling J. Nesbitt.

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Reviewer Information Nature thanks K. Padian and H.-D. Sues for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Skeletal anatomy of the aphanosaurs Dongusuchus efremovi, Yarasuchus deccanensis and Spondylosoma absconditum.

a, b, D. efremovi. ct, Y. deccanensis. ucc, S. absconditum. a, b, Left holotype femur of D. efremovi (Borissiak Paleontological Institute of the Russian Academy of Sciences, Moscow, Russia (PIN) 952/15-1) in posteromedial (a) and anterolateral (b) views. ce, Right partial femur of Y. deccanensis (Indian Statistical Institute, Kolkata, India (ISIR) unnumbered) in posterolateral (c), proximal (d) and anterolateral (e) views. f, g, Left tibia of Y. deccanensis (ISIR 334) in posterior (f) and distal (g) views. h, i, Left calcaneum of Y. deccanensis (ISIR unnumbered) in proximal (h) and lateral (i) views. j, Second sacral vertebra of Y. deccanensis (ISIR BIA 45/43) in ventral view. k, Right ischium of Y. deccanensis (ISIR 334) in ventrolateral view. l, m, Posterior cervical vertebrae of Y. deccanensis (ISIR BIA 45/43) in posterior (l) and right lateral (m) views. n, o, Right humerus of Y. deccanensis (ISIR 334 53) in anterior (n) and posterior (o) views. p, Right ulna of Y. deccanensis (ISIR 334) in anterior view. q, Trunk vertebra of Y. deccanensis (ISIR BIA 45/43) in left lateral view. r, s, Posterior cervical vertebrae of Y. deccanensis (ISIR BIA 45/43) in posterior (r) and right lateral (s) views. t, Triple-headed rib of Y. deccanensis (ISIR BIA 45) in anterior view. u, x, Original condition (u) of a cervical vertebra (from Huene 1942) of S. absconditum (Paläontologische Sammlung der Universität Tübingen, Tübingen, Germany (GPIT) 479/30) compared to that of the current condition (x). v, w, y, Original condition of a more posterior cervical vertebra of S. absconditum (from Huene 1942) in left lateral (v) and anterior (w) views compared to that of the current condition of the same vertebra in left lateral (y) view. z, Trunk vertebra of S. absconditum in posterior view. aa, Second sacral vertebra of S. absconditum in dorsal view. bb, cc, Right scapula of S. absconditum in lateral (bb) and posterior (cc) views. a., articulates with; ain, anteriorly inclined anterior margin of the neural spine; as, astragalus; ct, calcaneal tuber; dp, deltopectoral crest; fi, fibula; hy, hyposphene; mic, M. iliotrochantericus caudalis scar; mie, M. iliofemoralis externus scar; pr, posterolateral; r, ridge. Scale bars, 1 cm. Outline of Africa and Tanzania obtained from Google Maps.

Extended Data Figure 2 Histological sections of the limb bones of T. rhadinus gen. et sp. nov.

a, Right fibula (NMT RB 488) in lateral (left) and medial (right) views. b, Photo of the histological section of the fibula (NMT RB 488) in regular transmitted light (bright field) (1 plane polarizer). c, Photo of the same section as in b using a full wave retarder (l = 530 nm). d, Left humerus (NMT RB476) in posterior (left) and anterior (right) views. e, Photo of a partial histological section of the humerus (NMT RB476) in regular transmitted light (bright field) (1 plane polarizer). f, Photo of the same section as in e using a full wave retarder (l = 530 nm). Scale bars, 1 mm. Arrows in a, d indicate where each element was sampled. Arrows in b, c, e, f indicate growth marks in the outer cortex.

Extended Data Figure 3 The relationships of T. rhadinus gen. et sp. nov. among archosauriforms from the Nesbitt dataset.

The dataset used has been described in ref. 19. Strict consensus of 36 most parsimonious trees (tree length = 1,374; consistency index = 0.3559; retention index = 0.7807). Bremer support values (first), absolute (second), and GC (third) bootstrap frequencies presented at each branch.

Extended Data Figure 4 The relationships of Teleocrater rhadinus gen. et sp. nov. among archosauriforms from the Ezcurra dataset.

The dataset used has been described in ref. 20. Strict consensus of four most parsimonious trees (tree length = 2,684; consistency index = 0.2955; retention index = 0.6284). Bremer support values (first), absolute (second), and GC (third) bootstrap frequencies presented at each branch.

Extended Data Figure 5 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).

a, Left calcaneum of the pseudosuchian Nundasuchus songeaensis (NMT RB48). b, Right calcaneum of the aphanosaur T. rhadinus gen. et sp. nov. (reversed) (NMT RB490). c, Left calcaneum of the dinosauriform silesaurid A. kongwe (NMT RB159). Proximal view (left), distal view (middle) and lateral view (right). Scale bars, 1 cm. red, character state present; blue, character state absent; red and blue, basal condition could be either; ?, unknown condition. See Fig. 3 for silhouette sources. 4th, fourth tarsal; a., articulates with; as, astragalus; ct, calcaneal tuber; fi, fibula.

Extended Data Figure 6 Ternary diagrams of measurements of the hindlimb elements (femur, tibia and longest metatarsal) of archosauriforms.

Colour of data points relates to femoral length.

Extended Data Figure 7 The relationships of S. taylori among archosauriforms from the Nesbitt dataset.

The dataset used has been described in ref. 19. Strict consensus of 792 most parsimonious trees (tree length = 1,378; consistency index = 0.3549; retention index = 0.7803) (see Supplementary Information). Bremer support values (first), absolute (second), and GC (third) bootstrap frequencies presented at each branch.

Extended Data Figure 8 The relationships of S. taylori among archosauriforms from the Ezcurra dataset.

The dataset used has been described in ref. 20. Strict consensus of four most parsimonious trees (tree length = 2,693; consistency index = 0.2945; retention index = 0.6280) (see Supplementary Information). Bremer support values (first), absolute (second), and GC (third) bootstrap frequencies presented at each branch.

Extended Data Figure 9 Disparity estimates for major archosaur groups and time intervals (weighted mean pairwise dissimilarity (WMPD)).

Ani, Anisian; C, Changhsingian; Car, Carnian; H, Hettangian; I, Induan; J, Jurassic; Lad, Ladinian; Lo, Lopingian; Nor, Norian; Olen, Olenekian; P, Permian; Rha, Rhaetian.

Extended Data Figure 10 New character illustrations for the phylogenetic analysis.

See Supplementary Information. ac, Archosaurian iliac comparisons for character 414 in the modified dataset of ref. 19. a, Left ilium of Teleocrater rhadinus (NHMUK PV R6795) in lateral view. b, Right ilium of Asilisaurus kongwe (NMT RB159) in lateral view. c, Left ilium of Batrachotomus kupferzellensis (Staatliches Museum für Naturkunde Stuttgart (SMNS) 80273) in lateral view. dg, Avemetatarsalian fibula comparisons for character 415 in the modified dataset of ref. 19. d, e, Left fibula of T. rhadinus (NHMUK PV R6795) in lateral (d) and posterior (e) views. f, g, Left fibula of A. kongwe (NMT RB159) in lateral (f) and posterior (g) views. Arrows highlight the posterior ridge, character 415 state 1. h, i, Archosauriform femoral comparisons for character 417 in the modified dataset of ref. 19. h, Right femur of Erythrosuchus africanus (NHMUK PV R3592) in ventral view. i, Right femur of T. rhadinus (NHMUK PV R6795) in posteromedial view. White dotted regions highlight character 417, state 1. jm, Avemetatarsalian second primordial sacral comparisons for character 416 in the modified dataset of ref. 19. j, k, Second primordial sacral vertebra of T. rhadinus (NMT RB519) in ventral (j) and posterior (k) views. l, m, The second primordial sacral vertebra of A. kongwe (NMT RB159) in ventral (l) and dorsal (m) views. Arrows highlight the posterior process of the sacral rib, character 416, state 1. Scale bars,. 1 cm (ag, im) and 5 cm (h).

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Nesbitt, S., Butler, R., Ezcurra, M. et al. The earliest bird-line archosaurs and the assembly of the dinosaur body plan. Nature 544, 484–487 (2017). https://doi.org/10.1038/nature22037

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