Tuesday, May 11, 2010

Bryzoans, Past and Present

Four colonies of a marine bryzoan on Laminaria

Sea mats, ectoprocts or bryzoans are to be found all along the Irish coast, most commonly seen as skeletal remains attached to Laminaria washed up on the midshore line. This mineralized skeleton consists of an extracellular secretion of calcium carbonate crystals set in an organic matrix. There are clear similarities between bryozoan skeletons with the shells of brachiopods and molluscs in the calcareous composition of the skeleton, the morphology of the crystallites, and the mode of secretion (Taylor, 2005 Encyclopedia of Geology pp. 310-321).

The earliest known bryozoans are reported in the Paltodus deltifer Conodont Zone of late Tremadocian age (487 – 478 million years ago) (Zhang et al., 2009 Palaeoworld 18 pp. 67–73). All three classes of bryzoans are represented in Ireland; the marine classes Stenolaemata, with 27 species, and Gymnolaemata, with 171 species, and the freshwater Phylactolaemata, with 8 species (Ferriss et al. (ed.), 2009 Irish Biodiversity: a Taxonomic Inventory of Fauna).

The specimens pictured were both found on the same seashore but are separated by millennia.

A Fenestrate fossil bryozoan in limestone

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