Malpaisomys, AND ITS

Renaud, Sabrina & Michaux, Jacques, 2004, Parallel evolution in molar outline of murine rodents: the case of the extinct Malpaisomys insularis (Eastern Canary Islands), Zoological Journal of the Linnean Society 142 (4), pp. 555-572 : 556-557

BACKGROUND ON MALPAISOMYS AND ITS MAINLAND

RELATIVES

Malpaisomys insularis is known from Holocene and Pleistocene deposits ( Hutterer et al., 1988; Michaux, Hutterer & López-Martínez, 1991; Castillo, Martín- González & Coello, 2001) in the eastern Canary Islands, including Fuerteventura, Lanzarote and nearby islets ( Fig. 1). It became extinct during historical times, probably because of the arrival of man ( Boye et al., 1992). Malpaisomys owes its name to the Spanish word, malpaís, meaning lava fields where its fossil remains are sometimes found in cavities. A study of its skeletal characteristics suggested that Malpaisomys lived in fissures opened in the lava fields ( Boye et al., 1992).

Its phylogeny, based on dental or skeletal characters, can at present only be hypothesized; these characters can be of poor value for phylogenetic reconstruction, especially because of cases of convergent evolution. Malpaisomys was first interpreted as related to Acomyinae ( Hutterer et al., 1988). Acomyinae now constitutes a genetically well-identified group showing morphological convergence in dental pattern with the Murinae, in particular in the first two upper cheek teeth ( Chevret, Michaux & Catzeflis, 1993). Immunogenetic comparisons, however, cast some doubt on the relationships of Malpaisomys and Acomys , suggesting that Malpaisomys was more closely related to Murinae than to Acomyinae ( Montgelard, 1992). This interpretation has been reinforced by the reconsideration of some dental features typical of murine rodents, such as the structure of the third upper molar ( Denys & Michaux, 1992). Consequently, Malpaisomys is now considered to be a true murine. In addition, Malpaisomys displays a dental specialization termed stephanodonty ( Schaub, 1938; Misonne, 1969), which is characterized by the swelling of the cusps and the development of longitudinal crests that extend the cusps and join them in a garland-like pattern on the upper molars. Such a trend has been interpreted as an adaptation to a diet rich in green vegetation or grass by analogy with the extant African stephanodont murines Oenomys ( Dieterlen, 1967) and Aethomys ( Denys, 1994) , and functional morphological arguments ( Michaux, 1978; van Dam, 1997). The similarity of the dental patterns is stressed by the grouping of Malpaisomys together with the stephanodont murines Oenomys and Stephanomys ( López-Martínez, Michaux & Hutterer, 1998) .

Stephanodonty in Malpaisomys could be the result either of evolution from a stephanodont ancestor or of convergent evolution. In order to investigate both hypotheses, we compared Malpaisomys with murine rodents showing a stephanodont trend and with taxa exhibiting a basic murine dental pattern ( Fig. 2 ). Stephanodont rodents include several fossil taxa and some tropical extant species. An evolutionary lineage leading to the evolution of stephanodonty starts in south-western Europe with Progonomys hispanicus evolving into Occitanomys . A cladogenesis around 7 Mya leads to Occitanomys alcalai on the one hand and to Stephanomys on the other ( Michaux, 1971; van de Weerd, 1976; van Dam, 1997). The evolution along this lineage leads from small and primitive molars in P. hispanicus to large and specialized, highly stephanodont molars in Stephanomys , with Occitanomys having intermediate features ( van de Weerd, 1976; Cordy, 1978; van Dam, 1997). Stephanodonty evolved independently along the lineage of Paraethomys ( Jaeger, 1977; Coiffait, 1991; Benammi et al., 1995, 1996), a genus characteristic of North African rodent fauna from the Late Miocene until its extinction late in the Pleistocene ( Jaeger, 1977; Jaeger, Michaux & Thaler, 1975). Progonomys cathalai was present earlier than Paraethomys in North Africa, and compared to the latter it is characterized by a more primitive evolutionary stage in dental morphology. Consequently, it is considered in the present study as the morphological reference for an ancestor of the Paraethomys lineage. Within extant species, Oenomys displays complete stephanodonty ( Misonne, 1969) while Aethomys shows some stephanodont characteristics ( Denys, 1994).

Malacomys , Praomys and Mastomys were chosen as examples of rodents characterized by a basic dental pattern associated with an omnivorous diet. Mastomys peregrinus was considered because its geographical range, including western North Africa, makes a colonization of the Canary Islands by this species more likely than by a tropical taxon ( Fig. 1). The arvicanthine group ( Ducroz, Volobouev & Granjon, 2001) includes murine rodents with a herbivorous diet. Thallomys and Arvicanthis are part of this group but do not display stephanodont trends, unlike Aethomys and Oenomys . However Thallomys and Arvicanthis were included in order to evaluate the role of diet on the shape of molars.

Figures

Figure 2. Schematic representation of the modern and fossil murine rodents compared to Malpaisomys. Top, molecular phylogeny after Chevret (1994) and Chevret et al. (2001). A calibration of the time of divergence between taxa is given by reference to the divergence of the Rattus rattus lineage, estimated as having occurred 12 Mya (Chevret et al., 2001). The grey box includes the arvicanthine rodents, characterized by a herbivorous diet. Bottom, fossil lineages with investigated localities. Thick grey lines indicate taxa with stephanodont characteristics, thick black lines, complete stephanodonty. Crosses indicate extinct taxa.