Thursday, 13 May 2010

Today's Critter: Dielasma

Today my tutor lent me the book Zechstein Reef Fossils and Their Palaeoecology  by Hollingworth and Pettigrew, so expect more post about the Zechstein in future. Sadly it only covers an area way up near Middlesbrough, but it does give a lot of insight into my own locality (Ashfield brick-clay pit). I'm going to see if I can order it online as it is a useful and potentially cheap resource. It contains some decent information on the different fossils, so I thought I would start with the easiest I identified from Conisbrough and add the info from the book. Here is Dielasma elongatum, a Permian brachiopod which was abundant in the Zechstein:

The brachiopods pictured are bigger than most of the ones I found, which can be seen below:
Many which I found appear to be juveniles or smaller individuals, perhaps suggesting a high infant mortality rate. It may also suggest that the substrate was not firm or that there was crowding and therefore increased competition. The Dielasma specimen in the book is five times bigger than my smallest and around twice the size of my largest. Although I haven't compared yet, this seems to be true of the bivalves I have as well. This would seem to support the idea that there was crowding, though perhaps this did also affect infant mortality.

I won't list the taxonomic details from the book, as those change often and the book was published in the late 80s. For any interested, the description given is:

Shell is oval in outline, pedicle and brachial valves convex, pedicle valve with broad shallow sulcus and brachial valve with corresponding broad fold. Ornament of fine growth lamellae.

Dielasma  was an epifaunal suspension feeder, meaning that it lived on the sea floor (as opposed to within it) and sifted out its food from the water (using an appendage known as a lophophore). They are the ultimate couch potatoes of the sea. They had a thick, fully functional pedicle, which is the bit which attaches to the substrate so that the brachiopod stays put (also indicating that it is epifaunal; without it would suggest it was part of the infauna). The pedicle may have divided into rootlets to give an even stronger hold. Dielasma are often found as nests and with a wide range in size. 

It would be interesting to find Dielasma attached to other organisms, such as a bryozoan reef, as this sort of thing often happened. Many fossils have little brachiopods attached to them, particularly sponges, even way back in the Cambrian. 

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