Dinosaurs dominated the terrestrial ecosystems for more than 140 Myr during the Mesozoic era, and among them were sauropodomorphs, the largest land animals recorded in the history of life. Early sauropodomorphs were small bipeds, and it was long believed that acquisition of giant body size in this clade (over 10 tonnes) occurred during the Jurassic and was linked to numerous skeletal modifications present in Eusauropoda. Although the origin of gigantism in sauropodomorphs was a pivotal stage in the history of dinosaurs, an incomplete fossil record obscures details of this crucial evolutionary change. Here, we describe a new sauropodomorph from the Late Triassic of Argentina nested within a clade of other non-eusauropods from southwest Pangaea. Members of this clade attained large body size while maintaining a plesiomorphic cyclical growth pattern, displaying many features of the body plan of basal sauropodomorphs and lacking most anatomical traits previously regarded as adaptations to gigantism. This novel strategy highlights a highly accelerated growth rate, an improved avian-style respiratory system, and modifications of the vertebral epaxial musculature and hindlimbs as critical to the evolution of gigantism. This reveals that the first pulse towards gigantism in dinosaurs occurred over 30 Myr before the appearance of the first eusauropods.
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We thank the Earthwatch Foundation and its volunteers for help during fieldtrips. Support was provided by the Argentinian National Science Agency FONCyT (to C.A. (Pict 2016-236), R.N.M. (Pict 2015-711), I.A.C. (Pict 2015-1021) and D.P. (Pict 2014-1288)), SECITI Gobierno de San Juan (to R.N.M. (2016-2017)) and a PalAss Small Grant (to C.A.). We thank J. Carballido, A. Otero, J. Wilson, S. Nesbitt, T. Rowe and F. Di Fresco for discussions and comments, and D. Abelin for preparation and photographs of the type materials.
The authors declare no competing interests.
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Supplementary Information and References
Phylogenetic matrix of sauropodomorpha
Measurements of the bone elements of Ingentia prima (PVSJ 1086-1087) and new referred specimens of Lessemsaurus sauropoides (CRIALAR PV-302-303, PVL 6580). The numbers of caudal vertebrae and pes phalanges indicate an estimated order but not a specific number of each element. The height of the vertebral centra refers to the posterior face of each centrum. Abbreviations: Ca, caudal vertebra; Cv, cervical vertebra; dc, distal carpal; dt, distal tarsal; H, height; L, length; Lt, lateral; M, medial; mc, metacarpal; mt, metatarsal; ph, phalanx; PR, proximal ulna ratio (lateral margin versus medial margin=radial fossa margin); prox., proximal; TL, transversal length; w/s, without spine (=from neurocentral suture to postzygapophyses level). All measurements are provided in mm and indicate the maximum linear length. Estimated measurements are denoted by asterisk, and absent elements are denoted by double hyphen
Measurements of equivalent bone elements (scapula and ilium) and Body masses estimated for Sauropodomorpha. The measurements of bones are provided in mm and indicate the maximum linear length of each element. The body mass is provided in Kg. Incomplete elements or absent dates are denoted by double hyphen. Abbreviations: Abb., abbreviation used in the figure; BM, body mass; Il, ilium; L, total length; NA, not applicable; Sc, scapula. Simple asterisk indicates the specimen used by Benson et al. 2018 for estimation of BM, double asterisk indicate the new Lessemsaurus specimen reported here; apostrophe indicates length estimated from the literature
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Apaldetti, C., Martínez, R.N., Cerda, I.A. et al. An early trend towards gigantism in Triassic sauropodomorph dinosaurs. Nat Ecol Evol 2, 1227–1232 (2018). https://doi.org/10.1038/s41559-018-0599-y
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