@article{0d26211bd44843908986661e4038fc36,
title = "The evolution of the manus of early theropod dinosaurs is characterized by high inter- and intraspecific variation",
abstract = "The origin of the avian hand, with its reduced and fused carpals and digits, from the five-fingered hands and complex wrists of early dinosaurs represents one of the major transformations of manus morphology among tetrapods. Much attention has been directed to the later part of this transition, from four- to three-fingered taxa. However, earlier anatomical changes may have influenced these later modifications, possibly paving the way for a later frameshift in digit identities. We investigate the five- to four-fingered transition among early dinosaurs, along with changes in carpus morphology. New three-dimensional reconstructions from computed tomography data of the manus of the Triassic and Early Jurassic theropod dinosaurs Coelophysis bauri and Megapnosaurus rhodesiensis are described and compared intra- and interspecifically. Several novel findings emerge from these reconstructions and comparisons, including the first evidence of an ossified centrale and a free intermedium in some C. bauri specimens, as well as confirmation of the presence of a vestigial fifth metacarpal in this taxon. Additionally, a specimen of C. bauri and an unnamed coelophysoid from the Upper Triassic Hayden Quarry, New Mexico, are to our knowledge the only theropods (other than alvarezsaurs and birds) in which all of the distal carpals are completely fused together into a single unit. Several differences between the manus of C. bauri and M. rhodesiensis are also identified. We review the evolution of the archosauromorph manus more broadly in light of these new data, and caution against incorporating carpal characters in phylogenetic analyses of fine-scale relationships of Archosauromorpha, in light of the high degree of observed polymorphism in taxa for which large sample sizes are available, such as the theropod Coelophysis and the sauropodomorph Plateosaurus. We also find that the reduction of the carpus and ultimate loss of the fourth and fifth digits among early dinosaurs did not proceed in a neat, stepwise fashion, but was characterized by multiple losses and possible gains of carpals, metacarpals and phalanges. Taken together, the high degree of intra- and interspecific variability in the number and identities of carpals, and the state of reduction of the fourth and fifth digits suggest the presence of a {\textquoteleft}zone of developmental variability{\textquoteright} in early dinosaur manus evolution, from which novel avian-like morphologies eventually emerged and became channelized among later theropod clades.",
keywords = "archosaur, birds, carpus, Coelophysis bauri, dinosaur, manus, Megapnosaurus rhodesiensis, theropod",
author = "Barta, {Daniel E.} and Nesbitt, {Sterling J.} and Norell, {Mark A.}",
note = "Funding Information: We thank M. Fitzpatrick (Natural History Museum of Zimbabwe, Bulawayo) for loan of Megapnosaurus specimens to S.J.N. We thank C. Mehling (AMNH), M. Lamanna, A. Henrici, and D. Berman (CM), S. Pierce and J. Cundiff (MCZ), Rainer Schoch (SMNS), and S. Lucas (NMMNH) for access to collections. CT scan data for MCZ 4329 were provided by T. Rowe and M. Colbert (UT Austin). CT scanning in the AMNH Microscopy and Imaging Facility was facilitated by M. Hill and H. Towbin. Illustrations and photography were provided by N. Wong and M. Ellison. We thank S. Bell and C. Mehling for help with archival information about Plateosaurus at AMNH. E.-T. Lamm (Museum of the Rockies, Bozeman, MT, USA) prepared AMNH FARB 27435 and made interpretative maps of the jacket, which proved crucial to reconstructing the manus of this specimen. Invaluable discussion about this project was provided by A. Bronson, J. Flynn, C. Griffin, A. Heers, M. Hopkins, J. Meng, A. Watanabe, and the students and instructors of the Richard Gilder Graduate School Vertebrate Paleobiology course. A Richard Gilder Graduate School Fellowship to D.E.B. and the AMNH Division of Paleontology supported this research. Funding for travel to MCZ was provided by NSF Doctoral Dissertation Improvement Grant DEB-1601315 to M.A.N. and D.E.B. The Macaulay Family Endowment and the Newt and Callista Gingrich Endowment provide funding to M.A.N. We thank two anonymous reviewers for their constructive comments on the manuscript. Funding Information: We thank M. Fitzpatrick (Natural History Museum of Zimbabwe, Bulawayo) for loan of Megapnosaurus specimens to S.J.N. We thank C. Mehling (AMNH), M. Lamanna, A. Henrici, and D. Berman (CM), S. Pierce and J. Cundiff (MCZ), Rainer Schoch (SMNS), and S. Lucas (NMMNH) for access to collections. CT scan data for MCZ 4329 were provided by T. Rowe and M. Colbert (UT Austin). CT scanning in the AMNH Microscopy and Imaging Facility was facilitated by M. Hill and H. Towbin. Illustrations and photography were provided by N. Wong and M. Ellison. We thank S. Bell and C. Mehling for help with archival information about Plateosaurus at AMNH. E.-T. Lamm (Museum of the Rockies, Bozeman, MT, USA) prepared AMNH FARB 27435 and made interpretative maps of the jacket, which proved crucial to reconstructing the manus of this specimen. Invaluable discussion about this project was provided by A. Bronson, J. Flynn, C. Griffin, A. Heers, M. Hopkins, J. Meng, A. Watanabe, and the students and instructors of the Richard Gilder Graduate School Vertebrate Paleobiology course. A Richard Gilder Graduate School Fellowship to D.E.B. and the AMNH Division of Paleontology supported this research. Funding for travel to MCZ was provided by NSF Doctoral Dissertation Improvement Grant DEB-1601315 to M.A.N. and D.E.B. The Macaulay Family Endowment and the Newt and Callista Gingrich Endowment provide funding to M.A.N. We thank two anonymous reviewers for their constructive comments on the manuscript. Publisher Copyright: {\textcopyright} 2017 The Authors. Journal of Anatomy published by John Wiley & Sons Ltd on behalf of Anatomical Society.",
year = "2018",
month = jan,
day = "1",
doi = "10.1111/joa.12719",
language = "English",
volume = "232",
pages = "80--104",
journal = "Journal of Anatomy",
issn = "0021-8782",
publisher = "Wiley-Blackwell Publishing Ltd",
number = "1",
}