Scaptochelys: Generic Revision and Evolution of Gopher Tortoises

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Abstract
Comparative morphologic studies of the modern and fossil gopher tortoises of North America have led to the documentation of 2 spp. complexes. These species groups are judged sufficiently distinct so as to warrant formal generic separation. Accordingly, the genus Gopherus is restricted to an assemblage of highly specialized tortoises ranging from the early Middle Miocene (Hemingfordian) to Recent times. G. polyphemus (= type species) and G. flavomarginatus are the living representatives. A new genus, SCAPTOCHELYS [type-species Xerobates agassizii], is proposed for the more generalized gopher tortoises ranging in age from Middle Oligocene (Orellan) to Recent. S. agassizii comb. nov. and S. berlandieri comb. nov. are the surviving members of this genus. [The fossil members are S. laticunea comb. nov., S. mohavense comb. nov. and an undescribed species.] Gopherus is characterized by a suite of morphological features which distinguish it from Scaptochelys and all other known testudinids. These include: a hypertrophied inner ear containing a massive saccular otolith; short, robust cervical vertebrae with enlarged, closely joined pre- and postzygapophyses; a specialized, interlocking joint between cervical 8 and the 1st dorsal vertebra; attachment of the 1st dorsal vertebra to a distinct, bony strut on the nuchal plate; and a modified manus typified by enlarged, spatulate ungual phalanges, 3 to 4 subradial carpal bones, a restricted mesocarpal joint and true unguligrade stance. Scaptochelys exhibits none of these derived characters. In the development of its otolithic ear, Gopherus appears to be unique among tetrapods. Structural specialization of the cranium and neck in Gopherus are functionally related to burrowing. Members of this genus appear to use the head and neck to brace or stabilize the trunk while digging with the forelimbs. Burrowing in Scaptochelys presumably does not involve comparable head-bracing behavior. Unlike Gopherus, the skull and neck of Scaptochelys show to modification for withstanding large mechanical stresses. The unique otolithic ears of Gopherus most probably function as high-gain seismometers for the detection of weak ground vibrations. The homologies of the chelonian carpus and those of land tortoises in particular are reevaluated using newly acquired data from Eocene and Oligocene testudinids. The primitive carpal arrangement of turtles is similar to that of certain stem reptiles (i.e., Permian captorhinids). Homologies established by previous workers for the carpus of gopher tortoises are incorrect. The carpus of Gopherus is structurally adapted for digging in friable soils; that of Scaptochelys is more suited to overland travel and burrow excavation in resistant soils. Palenotologic evidence points to an Early Miocene (Arikareean) origin of Gopherus in the Great Plains region of North America. At least 3 environmental factors may have contributed to the origin of the genus by favoring increased fossorial ability. These factors are: climatic cooling; increased aridity and seasonality of rainfall; and the widespread development of sandy soils.