METAPHIOMYS (RODENTIA: PHIOMYIDAE) FROM THE PALEOGENE OF SOUTHWESTERN TANZANIA

Journal of Paleontology, Mar 2006 by Stevens, Nancy J, O'Connor, Patrick M, Gottfried, Michael D, Roberts, Eric M, Et al

INTRODUCTION

THE EVOLUTIONARY history of the living African rodent families is a topic of considerable debate, yet it is generally agreed that the modern cane rats (Thryonomys Fitzinger, 1867) and dassie rats (Petromus Smith, 1831) have an evolutionary history within the infraorder Phiomorpha (e.g., Wood, 1968). Phiomorphs possess hystricognathous mandibular morphology, multiserial incisor enamel, and hystricomorphous attachment of the masseteric musculature (e.g., Lavocat, 1978; Holroyd, 1994). In his initial work on the group, Wood (1968) placed all phiomorph taxa into a single family, and named a handful of morphologically diverse species based mainly on size. Lavocat (1978) later revised the taxonomy of the group, raising many of the differences among species to the family level. More recently, Holroyd (1994) observed that these contrasting views likely stemmed from the fact that Wood's phiomorph work emphasized the overall similarity of Paleogene specimens from the Fayum of Egypt, whereas Lavocat endeavored to explain the diverse Miocene rodent faunas from East Africa, envisioning that each of the Miocene forms had an ancestor among the Paleogene taxa. In this paper we adopt HoIroyd's (1994) revised version of family-level relationships among the phiomorphs.

A variety of well-preserved specimens pertaining to phiomorph evolutionary history have been described from the Paleogene of northern Africa and Oman (Osborn, 1908; Wood, 1968; Jaeger et al., 1985; Fejfar, 1987; Holroyd, 1994) and Asia (e.g., Flynn et al., 1986; Jaeger, 1988; Marivaux et al., 2002; Marivaux and WeIcomme, 2003). However, Paleogene terrestrial deposits in subSaharan Africa are extremely rare, hence this constitutes the earliest phiomorph record from East Africa. Here we announce a new microsite in the Mbeya Region of southwestern Tanzania, preserving Metaphiomys Osborn, 1908, a rodent taxon previously described exclusively from the early Oligocene of Egypt, Libya, and Oman (Wood, 1968; Fejfar, 1987; Thomas et al., 1989). Not only does this represent a significant geographic extension of the taxon, it also constitutes the first rodent material described from the Paleogene of East Africa.

Location.-The specimen described herein derives from a laterally extensive, richly fossiliferous, muddy sandstone unit (TZ01) located in the Songwe Valley portion of the Rukwa Rift Basin, Mbeya Region, southwestern Tanzania (Fig. 1). TZ-01 is located at approximately 8�56''S, 33�12''E (precise locality coordinates are on file at Ohio University). These fossil-bearing strata pertain to Unit 2 of the Red Sandstone Group, a series of proximal fluvial and overbank deposits (Roberts et al., 2004). A range of isolated to articulated microvertebrate fossils have been recovered from this locality, primarily from a tabular, 2 m thick sandstone bed, interpreted as either channel lag or flood-stage fluvial deposits. The TZ-01 fauna is characterized by small (

Although age estimates for red sandstones in the study area have ranged from the Jurassic to the late Cenozoic (e.g., Tiercelin, 1988; Ebinger et al., 1989; Morley et al., 1992; Damblon et al., 1998), recent work has refined the geological and paleontological context of deposits in the region, clearly documenting the presence of both Cretaceous and Paleogene sequences in the primary study area (O'Connor et al, 2003; Roberts et al., 2004; Stevens et al., 2004). These age assignments are consistent with thermal history reconstructions of the Rukwa and Malawi rift flanks by Van der Beek et al. (1998). Based on apatite fission-track data, they document a phase of rapid cooling and denudation of rift flanks between 40 and 50 Ma, associated with an early Cenozoic tectonic event and subsequent sedimentation. The TZ-01 deposits are most likely associated with reactivation of preexisting basement structures and basin development during a Paleogene tectonic event. This age estimate is also supported by Tiercelin et al. (1988), who reported a middle Eocene to early Oligocene age for nepheline basalts capping sedimentary deposits in the southern part of the field area.

Abbreviations.-AMNH, American Museum of Natural History, New York; CM, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania; TNM, National Museums of Tanzania, Dar es Salaam; YPM, Yale Peabody Museum, New Haven, Connecticut.

MATERIALS AND METHODS

The specimen (TNM 03111) was collected by standard handquarrying methods and mechanically prepared at the Stony Brook Vertebrate Preparation Facility. Length and width measurements were recorded using a Nikon SMZ 1500 stereomicroscope bundled with SPOT Advanced (version 3.5) software. The accuracy of measurements is on average /- 0.01 mm. Reference specimens and casts were borrowed from collections at the American Museum of Natural History, the Carnegie Museum of Natural History, the Duke University Primate Center, and Southern Methodist University, and included holotypes and/or representatives of numerous genera including Phiomys Osborn, 1908, Metaphiomys, Diamantomys Stromer, 1922, Phiocricetomys Wood, 1968, Paraphiomys Andrews, 1914, Gaudeamus Wood, 1968, Paraulacodus Hinton, 1933, Myophiomys Lavocat, 1973, and Elmerimys Lavocat, 1973.