Morphological divergence in giant fossil dormice

Abstract

Insular gigantism—evolutionary increases in body size from small-bodied mainland ancestors—is a conceptually significant, but poorly studied, evolutionary phenomenon. Gigantism is widespread on Mediterranean islands, particularly among fossil and extant dormice. These include an extant giant population of Eliomys quercinus on Formentera, the giant Balearic genus † Hypnomys and the exceptionally large † Leithia melitensis of Pleistocene Sicily. We quantified patterns of cranial and mandibular shape and their relationships to head size (allometry) among mainland and insular dormouse populations, asking to what extent the morphology of island giants is explained by allometry. We find that gigantism in dormice is not simply an extrapolation of the allometric trajectory of their mainland relatives. Instead, a large portion of their distinctive cranial and mandibular morphology resulted from the population- or species-specific evolutionary shape changes. Our findings suggest that body size increases in insular giant dormice were accompanied by the evolutionary divergence of feeding adaptations. This complements other evidence of ecological divergence in these taxa, which span predominantly faunivorous to herbivorous diets. Our findings suggest that insular gigantism involves context-dependent phenotypic modifications, underscoring the highly distinctive nature of island faunas.

Island biogeography predicts skull gigantism and shape variation in meadow voles Microtus pennsylvanicus through ecological release and allometry

“…Based on known morphological differences in M. pennsylvanicus , especially for Atlantic region islands (Miller 1896, Chamberlain 1954, Wheeler 1956), we anticipated and detected an association between shape and size (i.e. static allometry), a common pattern across rodents (Marcy et al 2016, Hennekam et al 2020). Furthermore, there is a narrower range of PC scores among populations in the ventral shape data compared to dorsal shape.…”

Comparative morphology of the dormouse skull

“…The maximum within-genus size difference is therefore expected to be larger than displayed in Figure 4. This absence within the dataset is exceptionally notable for the genus Eliomys , for which insular specimens were excluded from this study, but the largest population of which is found on the island of Formentera (Storch, 1978; Hennekam et al, 2020b). The results indicate that relatively large dormouse genera ( Eliomys and Glis ) are more variable in size than smaller genera ( Muscardinus and Glirulus ).…”

“…Considering the large size range within Gliridae, a common allometric trend within the family is to be expected. Strong correlations between size and shape have been identified within dormice species (Hennekam et al, 2020b), resulting in species- or genus-specific allometric trajectories. The size distribution within certain genera (e.g.…”

“…Multiple populations were included for widely dispersed species. Insular populations were avoided, as morphological variation in dormice linked with insularity is described in a previous study (Hennekam et al, 2020b). Even though sexual dimorphism is not considered to be present within dormice (Holden-Musser et al, 2016), whenever possible, an equal representation of both sexes was included within the dataset.…”

Comparative morphology of the dormouse skull and the influence of size and ecology

“…Multiple populations were included for widely dispersed species. Insular populations were avoided, as morphological variation in dormice linked with insularity is described in a previous study (Hennekam, Benson, et al, 2020). Even though sexual dimorphism is not considered to be present within dormice (Holden‐Musser et al, 2016), whenever possible, an equal representation of both sexes was included within the dataset.…”