Scaptochelys: Generic Revision and Evolution of Gopher Tortoises

Abstract

Comparative morphologic studies of the modern and fossil gopher tortoises of North America have led to the documentation of two species 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. Gopherus polyphemus (=type species) and G. flavomarginatus are the living representatives. A new genus, Scaptochelys, is proposed for the more generalized gopher tortoises ranging in age from Middle Oligocene (Orellan) to Recent. Scaptochelys agassizii (=type species) and S. berlandieri are the surviving members of this genus. Gopherus is characterized by a suite of morphological features which distinguish it from Scaptochelys and all other known testudinids. These include: 1) a hypertrophied inner ear containing a massive saccular otolith; 2) short, robust cervical vertebrae with enlarged, closely joined pre- and postzygapophyses; 3) a specialized, interlocking joint between cervical 8 and the first dorsal vertebra; 4) attachment of the first dorsal vertebra to a distinct, bony strut on the nuchal plate; 5) 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 utilizing 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 shown to be incorrect. The carpus of Gopherus is structurally adapted for digging in friable soils whereas that of Scaptochelys is more suited to overland travel and burrow excavation in resistant soils.