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SPECIES Rhagamys orthodon

Author:Hensel, 1856.
Citation:Z. Dtsch. Geol. Ges.: 281.
Common Name:Tyrrhenian Field Rat
Type Locality:Unknown (see Alcover et al., 1998), presumably from somewhere on either Corsica or Sardinia.
Distribution:Recorded only from Corsica and Sardinia (Kotsakis, 1980; Caloi et al., 1986; Martín Suárez and Mein, 1998; Alcover et al., 1998; Pereira et al., 2001).

An extinct insular species that was large in body size (up to 50 grams; Libois et al., 1993) compared with its continental ancestors, and with bulky, high-crowned molars. Represented only by fossils recovered from late Pleistocene to Holocene sediments (Sondaar et al., 1984; Sondaar, 2000; Amori, 1993; Vigne, 1988; Pereira et al., 2001). A smaller species, minor, known by fossils from early and middle Pleistocene deposits on both islands, was described as a species of Rhagamys (Brandy, 1978; Pereira et al., 2001; Sondaar, 2000) but later transferred to Rhagapodemus. This extinct genus dates back to late Miocene, and all the other species are represented by fossils from the Mediterranean continental mainland (Martín Suárez and Mein, 1998); Rhagapodemus minor has never been found in late Pleistocene horizons on Corsica or Sardinia, the time zone during which Rhagamys orthodon is first recorded. Rhagamys orthodon coexisted with the introduced Apodemus sylvaticus at the beginning of the third millennium but vanished at the end of the first millennium BC (Libois et al., 1993). Alcover et al. (1998) assumed that all mammalian species living on islands during late Pleistocene and Holocene survived until the arrival of humans and, except for a small cervid on Corsica, the other "Late Pleistocene mammals co-existed for several centuries with the exotic species introduced by man, from the beginning of his colonization of the Tyrrhenian islands" (Masseti, 1993:211; see also Vigne, 1992). The more than 25 species of extant mammals now found on Corsica and Sardinia represent gradual introductions by humans. However, before humans colonized the islands, about 9000 years ago (Mesolithic), the endemic fauna was unbalanced as is typical of insular assemblages and consisted of large soricids (Episoriculus similis on Sardinia and E. corsicanus on Corsica), a talpid (Talpa tyrrhenica), a fox-sized canid (Cynotherium sardous), an otter (Cyrnaonyx majori), a small cervid (Megaloceros cazioti), a pika (Prolagus sardus), a vole (Microtus henseli), and Rhagamys orthodon. Megaloceros was exterminated soon after human arrival, but the shrew, vole, pika and Tyrrhenian field rat coexisted with humans until between 2000 and 1000 years ago (between Roman time and Middle Ages) and disappeared sometime between the arrival of Rattus rattus (during the Roman period) and the present (Vigne, 1990; Vigne and Marinval-Vigne, 1991; Vigne, 1992). The extinction of the four small mammal species "may be correlated with drastic deforestation" and "a major increase in agriculture, the effects of which were probably aggravated by the appearance of Rattus rattus, an aggressive competitor" (see Vigne, 1992:93, for comprehensive review and pertinent literature references).

Martín Suárez and Mein (1998) described molars of Rhagamys orthodon as large and highly derived compared with its relatives in Rhagapodemus, a consequence of insular evolution. They speculated R. orthodon possibly to have been derived from Rhagapodemus or even Apodemus, but lack of pertinent fossils prevents resolution of its evolutionary origin. Except for the hypsodonty exhibited by R. orthodon, Barrett-Hamilton (1900:422) thought its molar occlusal patterns were similar to A. sylvaticus and A. agrarius, "and may have been a direct offshoot from a common stock."




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