Rhizomyinae Winge 1887

Wilson, Don E. & Reeder, DeeAnn, 2005, Order Rodentia - Family Spalacidae, Mammal Species of the World: a Taxonomic and Geographic Reference (3 rd Edition), Volume 2, Baltimore: The Johns Hopkins University Press, pp. 907-926 : 907

Rhizomyinae Winge 1887

Rhizomyinae Winge 1887 , E Museo Lundii, Vol. 1: 109.

Synonyms: Rhizomyini Winge 1887 ; Rhizomyidae Miller and Gidley 1918 .

Genera: 2 genera with 4 species:

Genus Cannomys Thomas 1915 (1 species)

Genus Rhizomys Gray 1831 (3 species)

Discussion: Diagnosed and reviewed by Ellerman (1961), Carleton and Musser (1984), and also by Flynn (1990), who summarized cladistic relationships among extant and extinct genera. Differences in dentition, infraorbital canal, and zygomatic plate have been used to separate African mole rats as a subfamily from the Asian Rhizomyinae ( Chaline et al., 1977; Miller and Gidley, 1918). Ellerman (1940, 1941) interpreted these distinctions as demonstrating Tachyoryctes to be phyletically remote from Asian rhizomyines, instead emphasized its dental similarities to the nesomyine Brachyuromys , a link recognized earlier by Major (1897), and combined the two genera in Tachyoryctinae of an inclusively defined Muridae . The distant relationship of Tachyoryctes to Asian rhizomyines was also supported by Lavocat (1978), who listed Tachyoryctinae as one of six subfamiles of Nesomyidae , all occurring primarily in Africa and presumably descended from afrocricetodontines rather than some ancient Asian stock. Recent analysis of masticatory muscles and skeletal architecture in Rhizomys , Cannomys , and Tachyoryctes demonstrated different masticatory systems, implying functional convergence between African Tachyoryctes and Asian Cannomys and Rhizomys ( Endo et al., 2001) . Their findings are consistent with the dual subfamily classification favored by Flynn (1990), who recognizes Tachyoryctinae for Tachyoryctes and related extinct genera and Rhizomyinae for Rhizomys and Cannomys and their extinct relatives, both within Rhizomyidae .

Rhizomyines and tachyoryctines were more geographically widespread and taxonomically diverse during the Miocene and Pliocene, a radiation succinctly summarized by Flynn (1990) and reflected in the eleven fossil genera described from Africa ( Flynn and Sabatier, 1984; Mein et al., 2000 a; Tong and Jaeger, 1993), Thailand ( Mein and Ginsburg, 1997), and Pakistan, India, and China ( Black, 1972; de Bruijn et al., 1981; Flynn, 1982 a, b; Patnaik, 2001). Two lineages have been identified in the Siwalik localities of Pakistan and India, one leading to extant African Tachyoryctes , the other to living Asian Rhizomys and Cannomys ( Black, 1972; Flynn, 1982 a). Prokanisamys arifi , from 19 million-year-old (early Miocene) sediments in Pakistan ( de Bruijn et al., 1981), is the earliest Asian "rhizomyid," and the middle Miocene Pronakalimys ( Tong and Jaeger, 1993) and late Miocene Nakalimys and Harasibomys ( Flynn and Sabatier, 1984; Mein et al., 2000 a) represent the earliest African "rhizomyid" examples. Whether these forms can be classified as rhizomyine or tachyoryctine is unclear, although they do resemble middle Miocene Asian taxa ( Flynn, 1990) or even early spalacines ( Mein et al., 2000 a). Flynn (1990) surmised that the group originated in Asia, with an earlier migration to Africa represented by Nakalimys (and presumably Pronakalimys and Harasibomys ), possibly before 14 million years ago ( Tong and Jaeger, 1993), and a second resulting in extant Tachyoryctes . The oldest record of the latter is T. pliocaenicus , represented by fragments from late Pliocene sediments in Ethiopia ( Sabatier, 1978); Tachyoryctes shares more derived features with late Miocene Siwalik taxa than with the older African forms ( Flynn, 1990). Although all extant and some extinct rhizomyines and tachyoryctines are fossorial and subterranean, osteological fragments from the early fossil record of both groups indicate that early forms were not subterranean. Specialized fossorial adaptations appear in rhizomyines around 8.5 million years ago and after 7 million years in tachyoryctines (Flynn, 1980).

The subfamilial separation of African mole rats from Asian bamboo rats is a reasonable hypothesis supported to date by morphological data from extant and extinct species and one testable by cladistic analyses that incorporate a wider range of anatomical systems and molecular sources.