Wednesday, April 7, 2010

The giant Maltese dormice

Insular gigantism is an evolutionary process that leads to individuals in a population becoming progressively larger in size than their ancestors.

This process occurs when a population is isolated - for example, on a part of land which detaches from the mainland, beoming an island - and therefore divorced from its previous ecosystem, with all the biotic and abiotic factors which this entails.

In the new environment, the successful species may become free from a considerable number of predators (which may not have enough resources in the new ecosystem, therefore decreasing or becoming extinct) and also competitors (which may not be so successful in the new ecosystem).

In Malta, during the Pleistocene, this process can be seen to have occurred with several species of reptiles and mammals. The extinct giant lizard Lacerta siculimelitensis Böhme & Zammit-Maempel, 1982, still has congeners of smaller size living around the Mediterranean.

Two very interesting 'giants' from Maltese Pleistocene deposits are Leithia melitensis (Adams, 1863) and Leithia cartei (Adams, 1867). Fossils of these two animals were found from Middle Pleistocene deposits in Mnajdra, Mqabba and Wied Inċita, and a jaw from the National Museum of Natural History is pictured above. The genus is from the rodent family Gliridae (the dormice), which does not exist anymore on the islands of Malta. The size of the former was about twice that of the recent species Eliomys quercinus L., 1766, a specimen of which can be seen below. L. cartei was only slightly smaller than L. melitensis - indeed, differentiation based solely on size may give errors (Petruso, 2004).

There is also another genus of Maltese Pleistocene dormice, which consisted of animals smaller than Leithia but still large by 'modern' standards. This genus, Maltamys, consists of two species - Maltamys wiedincitensis Zammit Maempel & de Bruijn, 1982 and Maltamys gollcheri de Bruijn, 1966.

Zammit Maempel & de Bruijn (1982) are of the opinion that both Maltamys and Leithia are derived from Eliomys, due to similarity in anatomical features. However, more recent publications (Petruso, 2002; Petruso, 2004) state that convergent evolution is more probable, meaning that Maltamys and Leithia are more distantly related than previously thought.

Some material of the mentioned four fossil species has since been found in other Mediterranean countries, but all type material comes from Malta.

In a future post we shall look at an evolutionary pattern which is the diametrical opposite of insular gigantism - insular dwarfism.


Böhme, W. & Zammit Maempel, G.,
1982. Lacerta siculimelitensis sp. n. (Sauria: Lacertidae), a giant lizard from the Late Pleistocene of Malta. Amphibia-Reptilia, 3 (2-3): 257-268.
Bruijn, H. de, 1966. On the Pleistocene Gliridae (Mammalia, Rodentia) from Malta and Mallorca. Proceedings Koninklijke Nederlandse Akademie van Wetenschappen, B 69 (1): 480-496.
Petruso D., 2002. Il contributo dei micromammiferi alla Stratigrafia e Paleogeografia del Quaternario continentale siciliano. Ph. D. thesis, University of Napoli.
Petruso, D., 2004. New data on Pleistocene endemic Sicilian-Maltese dormice (Gliridae, Mammalia). 18th Senckenberg Conference 2004 in Weimar.
Zammit Maempel, G. & Bruijn, H. de, 1982. The Plio/ Pleistocene Gliridae from the Mediterranean Islands reconsidered. Palaeontology, B 85 (1): 113-128.

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