Herridge, V.L.; (2010) Dwarf elephants on Mediterranean islands: a natural experiment in parallel evolution. Doctoral thesis , UCL (University College London).

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Abstract

Mediterranean dwarf elephants represent some of the most striking examples of phyletic bodysize change observed in mammals and are emblematic of the ‘island rule’, where small mammals become larger and large mammals dwarf on islands. The repeated dwarfing of mainland elephant taxa (Palaeoloxodon antiquus and Mammuthus meridionalis) on Mediterranean islands provide a ‘natural experiment’ in parallel evolution, and a unique opportunity to investigate the causes, correlates and mechanisms of island evolution and body-size change. This thesis provides the first pan-Mediterranean study that incorporates taxonomic and allometric approaches to the evolution of dwarf elephants, establishing a framework for the investigation of parallel evolution and key morphological correlates of insular dwarfism. I show that insular dwarfism has evolved independently in Mediterranean elephants at least six times, resulting in at least seven dwarf species. These species group into three, broad size-classes: ‘smallsized’ (P. falconeri, P. cypriotes and M. creticus), ‘medium-sized’ (P. mnaidriensis and P. tiliensis) and ‘large-sized’ (Palaeoloxodon sp. nov. and ‘P. antiquus’ from Crete). Size-shape similarities between independent lineages from the east and central Mediterranean indicate that homoplasy is likely among similar-sized taxa, with implications for the existence of meta-taxa. These homoplasies appear to result from the exploitation of ontogenetic trajectories common to the Elephantidae, underpinning the evolution of small size. Interspecific allometry between dwarf and full-sized species can be seen to result from these common, but grade-shifted ontogenetic trajectories, and this may also be true of broader macroevolutionary trends in the Proboscidea. These size-related grade-shifts suggest that similar, but increasingly extreme, modifications of pre-natal development underpin the evolution of insular dwarfism in elephants. By incorporating research into the morphology and ontogeny of teeth and post-crania in fullsized extant and extinct elephants, this thesis provides new insights into insular dwarfism, elephant systematics and elephant functional morphology and adaptation.

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