title: Fossorial Origin of the Turtle Shell.
abstract: The turtle shell is a complex structure that currently serves a largely protective function in this iconically slow-moving group [1]. Developmental [2, 3] and fossil [4-7] data indicate that one of the first steps toward the shelled body plan was broadening of the ribs (approximately 50 my before the completed shell [5]). Broadened ribs alone provide little protection [8] and confer significant locomotory [9, 10] and respiratory [9, 11] costs. They increase thoracic rigidity [8], which decreases speed of locomotion due to shortened stride length [10], and they inhibit effective costal ventilation [9, 11]. New fossil material of the oldest hypothesized stem turtle, Eunotosaurus africanus [12] (260 mya) [13, 14] from the Karoo Basin of South Africa, indicates the initiation of rib broadening was an adaptive response to fossoriality. Similar to extant fossorial taxa [8], the broad ribs of Eunotosaurus provide an intrinsically stable base on which to operate a powerful forelimb digging mechanism. Numerous fossorial correlates [15-17] are expressed throughout Eunotosaurus' skeleton. Most of these features are widely distributed along the turtle stem and into the crown clade, indicating the common ancestor of Eunotosaurus and modern turtles possessed a body plan significantly influenced by digging. The adaptations related to fossoriality likely facilitated movement of stem turtles into aquatic environments early in the groups' evolutionary history, and this ecology may have played an important role in stem turtles surviving the Permian/Triassic extinction event.
authors.affiliation: Department of Earth Sciences, Denver Museum of Nature and Science, Denver, CO 80205, USA; Evolutionary Studies Institute, University of the Witwatersrand, PO Box Wits, 2050 Johannesburg, South Africa. Electronic address: tyler.lyson@dmns.org.
authors.forename: Tyler R
authors.initials: TR
authors.lastname: Lyson
authors.affiliation: Evolutionary Studies Institute, University of the Witwatersrand, PO Box Wits, 2050 Johannesburg, South Africa.
authors.forename: Bruce S
authors.initials: BS
authors.lastname: Rubidge
authors.affiliation: Paläontologisches Institut und Museum, Universität Zürich, Karl Schmid-Strasse 4, 8006 Zürich, Switzerland.
authors.forename: Torsten M
authors.initials: TM
authors.lastname: Scheyer
authors.affiliation: Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA.
authors.forename: Kevin
authors.initials: K
authors.lastname: de Queiroz
authors.affiliation: Department of Cell Biology & Anatomy, School of Medicine, Louisiana State University Health Sciences Center, New Orleans, LA 70112, USA.
authors.forename: Emma R
authors.initials: ER
authors.lastname: Schachner
authors.affiliation: Evolutionary Studies Institute, University of the Witwatersrand, PO Box Wits, 2050 Johannesburg, South Africa; Karoo Palaeontology, Iziko South African Museum, Cape Town 8000, South Africa.
authors.forename: Roger M H
authors.initials: RM
authors.lastname: Smith
authors.affiliation: Karoo Palaeontology, National Museum, PO Box 266, Bloemfontein 9300, South Africa; Department of Zoology and Entomology, University of the Free State, Bloemfontein 9300, South Africa.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Botha-Brink
authors.affiliation: Evolutionary Studies Institute, University of the Witwatersrand, PO Box Wits, 2050 Johannesburg, South Africa; Department of Anatomy, College of Osteopathic Medicine, New York Institute of Technology, PO Box 8000, Old Westbury, NY 11568, USA; Center for Functional Anatomy and Evolution, Johns Hopkins University School of Medicine, 1830 East Monument Street, Baltimore, MD 21205 USA.
authors.forename: G S
authors.initials: GS
authors.lastname: Bever
mesh_terms: D060105:Animal Shells / Q000033:anatomy & histology*; D000818:Animals; D005075:Biological Evolution; D005580:Fossils / Q000033:anatomy & histology*; D000071421:Life History Traits; D008124:Locomotion; D010802:Phylogeny; D013019:South Africa; D014426:Turtles / Q000033:anatomy & histology*