Sunday, May 9, 2010

On some fossil brachiopods from the Maltese Islands

Brachiopods are filter-feeders usually indicating seabeds densely populated with algae. The calcium carbonate shell of these animals is fragile and therefore sufficient shelter from waves is usually needed. In fact, they are mostly found in the Upper and Lower Coralline limestones.

The Il-Mâra Lower Coralline Limestone in the north-east areas of Malta occasionally contains populations of a dwarf form of Terebratula vitrea (von Born, 1778). These fragile brachiopods are usually found in association with cidaroid sea-urchin fragments and large species of foraminifers, including Heterostegina.

Terebratula vitrea (von Born, 1778)

In the westernmost parts of Malta, conspicuous 10 to 20cm thick layers of brachiopod shells are often present in cliff faces composed of the Mtarfa Upper Coralline Limestone, amongst rhodoliths and bryozoan fragments. These fossils are often crushed and identification is difficult unless the shell is broken open. Material from these brachiopod layers was used by Cooper (1983) to describe a new genus and species both named for the type locality, namely Maltaia maltensis Cooper, 1983, considered by other writers (Gaetani & Saccà, 1983) to be a synonym of Terebratula sinuosa (Brocchi, 1814). The latter is also found in considerable numbers in the same strata. However, the former species is markedly different in form and is also found in areas where ‘normal’ T. sinuosa is absent .

Terebratula sinuosa (Brocchi, 1814) (lower picture - detail)


Maltaia maltensis (Cooper, 1983) (detail)

The small (6mm) Megathiris detruncata (Gmelin, 1790) and other small species like Argyrotheca sp. are also very common throughout some parts of the Upper Coralline Limestone Formation.

Megathiris detruncata (Gmelin, 1790)

Deep sea such as that in which the Globigerina Limestone formed also offers protection from strong currents that may damage brachiopods. Despite this, brachiopods in this layer are few and far in between. Disarticulated phosphatised valves of the still-extant Megerlia truncata (Linné, 1767) are infrequently found in the C2 Phosphorite Conglomerate Bed.

Megerlia truncata (Linné, 1767)


References:


Cooper, G. A., 1983. The Terebratulacea (Brachiopoda), Triassic to Recent: a study of the brachidia (loops). Smithsonian Contributions to Paleobiology, 50: 1-445.
Emig, C. C., 2006. Brachiopoda world database. [www document, url: http://www.marinespecies.org/brachiopoda, last accessed: 10.II.2010]
Gaetani, M. & Saccà, D., 1983. Brachiopodi neogenici e pleistocenici della provincia di Messina e della Calabria meridionale. Geologica Romana, 22: 1-42.

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