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Fabbri M, Navalón G, Mongiardino Koch N, Hanson M, Petermann H, Bhullar BA. A shift in ontogenetic timing produced the unique sauropod skull. Evolution 2021; 75:819-831. [PMID: 33578446 DOI: 10.1111/evo.14190] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/10/2020] [Accepted: 01/30/2021] [Indexed: 11/29/2022]
Abstract
Sauropod dinosaurs include the largest terrestrial vertebrates that have ever lived. Virtually every part of the sauropod body is heavily modified in association with gigantic size and associated physiological alterations. Sauropod skulls are no exception: they feature elongated, telescoped facial regions connected to tilted neurocrania and reoriented jaw adductor muscles. Several of these cranial features have been suggested to be adaptations for feeding on the one hand and the result of paedomorphic transformation near the base of Sauropoda on the other. However, the scarcity of sauropodomorph ontogenetic series has impeded further investigation of these hypotheses. We re-evaluated the cranial material attributed to the early sauropodomorph Anchisaurus, which our phylogenetic analyses confirm to be closely related to sauropods. Digital assembly of μCT-scanned skulls of the two known specimens, a juvenile and an adult, permitted us to examine the detailed ontogeny of cranial elements. The skull anatomy of Anchisaurus is distinguished by a mosaic of ancestral saurischian and sauropod-like characters. Sauropod-like characters of the braincase and adductor chamber appear late in ontogeny, suggesting that these features first evolved by the developmental mechanism of terminal addition. Shape analyses and investigation of allometric evolution demonstrate that cranial characters that appear late in the ontogeny of sauropodomorphs closely related to sauropods are already present in the embryos and juveniles of sauropods, suggesting a predisplacement-type shift in developmental timing from the ancestral anchisaurian condition. We propose that this developmental shift relaxed prior constraints on skull morphology, allowing sauropods to explore a novel range of phenotypes and enabling specializations of the feeding apparatus. The shift in timing occurred in concert with the evolution of gigantism and physiological and locomotory innovations.
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Affiliation(s)
- Matteo Fabbri
- Department of Earth and Planetary Sciences, Yale University, New Haven, Connecticut, 06511.,Peabody Museum of Natural History, Yale University, New Haven, Connecticut, 06511
| | - Guillermo Navalón
- Department of Earth Sciences, University of Oxford, Oxford, OX1 3AN, United Kingdom.,Unidad de Paleontología, Departamento de Biología, Universidad Autónoma de Madrid, Madrid, 28049, Spain
| | - Nicolás Mongiardino Koch
- Department of Earth and Planetary Sciences, Yale University, New Haven, Connecticut, 06511.,Peabody Museum of Natural History, Yale University, New Haven, Connecticut, 06511
| | - Michael Hanson
- Department of Earth and Planetary Sciences, Yale University, New Haven, Connecticut, 06511.,Peabody Museum of Natural History, Yale University, New Haven, Connecticut, 06511
| | - Holger Petermann
- Department of Earth and Planetary Sciences, Yale University, New Haven, Connecticut, 06511.,Peabody Museum of Natural History, Yale University, New Haven, Connecticut, 06511.,Denver Museum of Nature and Science, Denver, Colorado, 80205
| | - Bhart-Anjan Bhullar
- Department of Earth and Planetary Sciences, Yale University, New Haven, Connecticut, 06511.,Peabody Museum of Natural History, Yale University, New Haven, Connecticut, 06511
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