Dissertation - A student’s guide to the geology of the Maltese Islands

The citation and abstract to my B. Ed. (Hons) dissertation have been added to the links on the right-hand side of this blog. The page can also be accessed through here.

More on Pleistocene rodents

Dorothea Bate (1878-1951) was a British explorer and palaeontologist whose main interests were Mediterranean Pleistocene mammals. The first woman scientist to hold a permanent post at the Natural History Museum in London, she discovered and named a number of new species from Crete, Cyprus, Malta and the Balearic Islands.

Two Pleistocene vole species bearing her authorship are known from deposits in Għar Dalam, Malta. Voles, from the rodent family Cricetidae, are very similar to mice, but have a stouter body and a hairy tail. They are nowadays extinct from the Maltese Islands - Gulia (1858; 1890) and some subsequent authors list the vole Arvicola amphibius (L., 1758) as occurring in Malta, but this statement is not supported in any way, either by more recent authors on mammals (Lanfranco, 1969) or by the present writer's observations.

The fossil species named by Dorothea Bate are Terricola melitensis (Bate, 1920) and Terricola pauli (Bate, 1935), both originally placed in the Pitymys genus before it was known that this actually indigenous to North America (Chaline et al., 1999, Kotsakis, 2004). T. melitensis is closely related to Terricola savii (de Sélys-Longchamps, 1838), shown above, which nowadays lives mainly along the Italian peninsula (Brunet-Lecomte & Chaline, 1992; Chaline et al., 1999; Wilson & Reeder, 2005). T. pauli is a large enigmatic species with less obvious affinities, and indeed with a name not encountered much in the literature. The figure of the holotype from Bate's original description is reproduced below.


References:

Bate, D. M. A., 1920. Note on a new vole and other remains from the Ghar Dalam Cavern, Malta. Geological Magazine, 57: 208-211.

Bate, D. M. A., 1935. Two new mammals from the Pleistocene of Malta, with notes on the associated fauna. Proceedings of the Zoological Society of London, 247-264.

Brunet-Lecomte, P. & Chaline, J., 1992. Morphological convergences versus biochemical divergences in the holarctic ground voles: Terricola and Pitymys (Arvicolidae, Rodentia). Neues Jahrbuch für Geologie und Paläontologie, 12: 721-734.

Chaline, J., Brunet-Lecomte, P., Montuire, S., Viriot, L., & Courant, F., 1999. Anatomy of the arvicoline radiation (Rodentia): palaeogeographical, palaeoecological history and evolutionary data. Annales Zoologici Fennici, 36: 239–267.

Gulia, G., 1858. Repertorio di Storia Naturale. Malta, 68 pp.

Gulia, G., 1890. Elenco dei Mammiferi Maltesi. Il Naturalista Maltese, 1 (1): 2-3.

Kotsakis, T., 2004. The Plio-Pleistocene rodents of the Mediterranean islands: origin and evolution. 18th Senckenberg Conference 2004 in Weimar.

Lanfranco, G., 1969. Maltese mammals (Central Mediterranean). Malta: Progress Press, 36 pp.

Wilson, D. E. & Reeder, D. M. [eds], 2005. Mammal Species of the World - A Taxonomic and Geographic Reference [3rd edition]. Johns Hopkins University Press, 142 pp.

Picture of Terricola savii (de Sélys-Longchamps, 1838) from http://www.cogecstre.com/Vertebrati/images/ArvicolaDiSavi.jpg

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.

References:

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.

Picture of Eliomys quercinus L., 1766 from http://upload.wikimedia.org/wikipedia/commons/d/d5/Eliomys_quercinus02.jpg

Introduction Part 1: The stratigraphy of the Maltese Islands

The exposed sedimentary rocks of the Maltese Islands date back from the late Oligocene, in the Chattian period, to the late Miocene, in the Messinian period. Strata as old as the Cretaceous are present beneath the oldest exposed layers, evidence for these was obtained through examination of fossilized spores (Pedley et al., 1976).

The first Oligocene stratum exposed in Malta is the Lower Coralline Limestone (LCL). This rock is composed mainly of rhodophytes with occasional coral horizons. The predominant rhodophyte genera are Lithothamnion and Archaeolithothamnion, both of which still exist. Since these require light for autotrophic processes, the environment was probably largely shallow water with calm conditions. In some areas where this layer is exposed, gigantic foraminifers such as Heterostegina and Lepidocyclina are preserved. The top of the stratum consists of the so-called Scutella bed (Spratt, 1843), which is composed of tests of the burrowing echinoid Scutella subrotunda. A phosphorite layer (C0) is also present in some areas (Gatt, 2005).

The stratum immediately above the LCL may date from the late Oligocene (Janssen, 2004). This layer is the Lower Globigerina Limestone (LGL), which marks a deepening of the sea-level since it is composed of mainly of planktonic foraminifera, which require a deep water column for such large populations to arise. Other deep-sea species such as the endemic echinoid Coelopleurus melitensis also occur (Zammit-Maempel, 1969).The LGL is succeeded by the similar but paler Middle Globigerina Limestone (MGL), which does not contain many fossils where exposed. Finally, the Upper Globigerina Limestone (UGL) caps the deep-sea limestone unit. The C1 phosphorite layer divides the LGL and MGL while the C2 phosphorite layer divides the MGL and the UGL. Phosphorite horizons represent a slowing-down in the deposition rate of sediments and a welling of nutrients from deeper areas of the Mediterranean. Several fossils, mostly of holoplanktonic molluscs and echinoderms, as well as teeth of Chondrichthyes, are frequent.

The Blue Clay Formation (BCF) succeeds the UGL. This still shows a deep-sea deposition environment, but instead of carbonate particles, there is a shift towards clayey minerals probably derived from volcanic detritus in a nearby area, creating a muddy area. The BCF contains limonite (iron compound) nodules, and fossils found in this layer are frequently composed of the same mineral.

The Greensand Formation (GF) lies directly above the BCF. This thin layer is almost entirely absent in Malta, though quite frequent in Gozo, especially in the Gelmus area where it reaches a thickness of about 11m (Pedley et al., 1976). It is lithologically a soft sandstone containing high proportions of the mineral glauconite (a complex silicate). The foraminifer Heterostegina appears again, but as another species several times smaller than that found in the LCL. Echinoids, mostly Clypeaster spp., are extremely common, if not always intact. Some authors regard the GF as part of the Upper Coralline Limestone (UCL). This marks a return to shallow water conditions and is composed of organisms similar to those in the LCL, though this time the species Mesophyllum commune is the most important and abundant coralline alga (Bosence, 1983). Some brachiopods and molluscs with algal habitats can also be found together with relatively shallow-water species such as the bivalve Lima lima. Fossilized wave impressions in some areas of the UCL exposure shows that currents were extremely strong, confirming the shallow water hypothesis.

Further sedimentary rocks of Quaternary origin, formed after the Maltese Islands had emerged from the water, are also present at some areas. The Maghlaq coast is the location of an alluvial fan formed by the delta of a large river, while Fiddien Valley in Rabat has a considerable area of lacustrine (lake-formed) tufa with pulmonate gastropod fossils (Pedley, 1980) and imprints of tracheophytes such as Laurus nobilis (Zammit-Maempel, 1977). However, the most important Quaternary deposit is that at Ghar Dalam, which shows the successive remains from the early Pleistocene fauna to the arrival of Neolithic Man in Malta (Zammit-Maempel, 1989).

Figures:

Fig. 1. Heterostegina cf. depressa from the Lower Coralline Limestone, Xghajra, Zabbar, Malta

Fig. 2. Globigerinoides ruber from the Upper Globigerina Limestone at San Lawrenz, Gozo, Malta

Fig. 3. Heterostegina costata from the Greensand Formation at Gelmus Hill, Gozo, Malta

References:

Boehme, 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), pp. 257-268.
Bosence, D. W. J. (1983), Coralline algae from the Miocene of Malta. Palaeontology, 26, pp. 147-173.
Gatt, P.A. (2005), Syntectonic deposition of an Oligo-Miocene phosphorite conglomerate bed in Malta. The Central Mediterranean Naturalist, 4 (2), pp. 109-119.
Janssen, A. W. (2004), Fossils from the Lower Globigerina Limestone Formation at Wardija, Gozo (Miocene, Aquitanian), with a description of some new pteropod species (Mollusca, Gastropoda). The Central Mediterranean Naturalist, 4 (1), pp. 1-33, 4 pl.
Pedley, H. M. (1980), The occurrence and sedimentology of a Pleistocene travertine in the Fiddien valley, Malta. Proceedings of the Geologists’ Association, 91, pp. 195-202.
Pedley, H. M., House, M. R., & Waugh, B. (1976), The Geology of Malta and Gozo. Proceedings of the Geologists’ Association, 87, pp. 325-341.
Spratt, T. A. B. (1843), On the Geology of the Maltese Islands. Proceedings of the Geological Society, 4 (2:97) pp. 225-230.
Zammit Maempel, G. (1969), A New Species of Coelopleurus (Echinoidea) from the Miocene of Malta. Palaeontology, 12 (1), pp. 42-47, 6 pl.
Zammit Maempel, G. (1977), An Outline of Maltese Geology. pp. 1-44, Progress Press, Malta.
Zammit Maempel, G. (1989), Ghar Dalam – Cave and Deposits. pp. 1-74, PEG, Malta.