Malpaisomys, WITH

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 : 567-568

RELATIONSHIPS OF MALPAISOMYS WITH MAINLAND

TAXA

Within this morphological pattern among modern and fossil taxa, Malpaisomys clearly falls within the intermediate cluster of herbivorous diet. This supports a palaeoecological interpretation of the taxon as herbivorous, but poses the problem of identifying its mainland ancestor. In particular, herbivory could either be inherited from this ancestor or the result of a parallel evolution on the islands.

Island populations of rodents are known to frequently display an increase in size compared to mainland relatives ( Foster, 1964). A decrease in size associated with insular conditions therefore seems unlikely, and is in any event rare in evolutionary lineages ( Fig. 7C ). Such an interpretation excludes large taxa as being potential ancestors of Malpaisomys , especially Aethomys , Arvicanthis , Thallomys, Pleistocene Paraethomys , and Late Pliocene Stephanomys . Shape, by contrast, reveals a different pattern. Among modern taxa, Aethomys (and to a lesser extent, Arvicanthis ) are the most similar regarding first upper molar shape ( Figs 3 , 4B ) while Malacomys and Praomys are the most different ( Fig. 7D ). Some discrepancies exist between the shape of the first upper or lower molars. Based on the upper molars, the extreme stephanodonts Oenomys, Pleistocene Paraethomys and Pliocene Stephanomys appear to be more differentiated than Malpaisomys , which would be closest to Occitanomys (e.g. O-MTH) or Late Pliocene Paraethomys ( Fig. 3B ). Based on the first lower molar shape, however, Malpaisomys appears to be closer to the more derived, Pleistocene Paraethomys , while remaining associated with Late Pliocene Occitanomys ( Fig. 3C ). In agreement with our results, the morphological relatedness of the stephanodont genera Malpaisomys , Oenomys and Stephanomys had previously been shown by phenetic distances of dental characters ( López-Martínez et al., 1998). Skull characters provide a different picture, suggesting a similarity between Stephanomys and Malpaisomys , Oenomys having been found to be more similar to non-stephanodont genera like Rattus ( López-Martínez et al., 1998) . Mosaic evolution likely occurred due to diverse selective pressures acting on different characters. The skull is more influenced by habitat whereas dental patterns reflect diet. The similarity of the skulls of Malpaisomys and Stephanomys may be a consequence of a similar mode of life within a rocky landscape (López- Martínez et al., 1998).

Combining these various results shows that morphologically the taxa closest to Malpaisomys are to be found among Pliocene fossil lineages related to either Paraethomys or Occitanomys . A hypothesized colonization of the eastern Canary Islands by members of these genera minimizes the evolution in both size and shape. A third alternative is colonization by an ancestor of the arvicanthine group, smaller in size than the modern representatives and already displaying a morphology characteristic of the intermediate, herbivorous group. Gene flow between mainland and canarian populations has been promoted by low sea level during glacial periods in some birds ( Idaghdour et al., 2004). However, any hypothesis considering such a recent colonization event for Malpaisomys would imply extremely high evolutionary rates and reversion in size and/or shape. Morphometric methods favour hypotheses which minimize morphological distance and evolutionary rates. Nonetheless, observations of the fossil lineages ( Fig. 7 ) consistently reveal that such high rates and reversion seldom occur. We therefore favour the hypothesis of an ancient colonization event by a mainland taxon already exhibiting a trend toward herbivory.

Biogeographical considerations should also be considered when evaluating the different hypotheses of colonization. The sea-surface current system around the Canary Islands ( Stramma & Siedler, 1988), as well as the dominant trade winds, favour a colonization event from North Africa or the Iberian Peninsula ( Fig. 1), providing additional support for a relatively ancient colonization event involving Paraethomys or Occitanomys . A Late Miocene or Pliocene colonization leading to Malpaisomys has been previously suggested ( Hutterer et al., 1988; Michaux et al., 1991) and is supported by our results. This hypothesis is not contradicted by dating of the insular volcanic complex, indicating that subaerial areas in the eastern Canary Islands are as old as 15 Myr ( Carracedo & Soler, 1995; Ancochea et al., 1996).

Figures

Figure 3. Morphological differentiation of the dental outline among murine rodents. A, upper molars. Group means on CA1 and 2. Dotted lines show the fossil lineages leading from Progonomys hispanicus to Occitanomys and to Stephanomys, and from Pr. cathalai to Paraethomys. Outlines correspond to some group means. B, upper molars, CA1 and 3. C, lower molars, CA1 and 2. For locality and sample abbreviations see Table 1.

Figure 4. Cluster analysis of the shape of the upper molar. Euclidean distances between group means were calculated based on Fourier coefficients up to the ninth harmonic (CF9). Clustering method was UPGMA (unweighted pair-group method, arithmetic average). Symbols as per Fig. 3. For locality and sample abbreviations see Table 1.

Figure 7. Relationship between time interval and morphological evolution regarding size (A, C) and shape (B, D) of the first upper molars. Size distance is estimated as differences in A0, shape distance as the Euclidean distance calculated based on the Fourier coefficients of the first nine harmonics. (A, B) Log-log relationship between evolutionary rates and time interval. (C, D) Relationship between time interval and morphological distance. Lines correspond to a linear regression on the Occitanomys/Stephanomys lineage (black line), the Paraethomys lineage (light grey line), and modern taxa (dark grey line). Dotted lines P <0.05; solid line P <0.001. (A) pair including Malpaisomys; (Δ) corresponds to the distance between a pair of Malpaisomys samples, Costa Calma and CVA4. Locality or sample abbreviations per Table 1.