Ostracodes

Ostracodes:

These tiny crustaceans are perhaps the most deceptive of fossils found in the Builth Inlier, excepting the spectacular palaeoscolecid worms. On the surface, they appear as tiny bean-shaped objects, up to a couple of millimetres long, if you’re lucky with a few small ridges to make them interesting. They are very easy to overlook, and not, at first glance, inspiring – even if you recognise them as fossils. They are locally abundant – meaning that in most rocks you won’t find any, but in several places there are thousands of the things – and almost invariably preserved as moulds in relatively fine sediments, particularly fine sandstones. But what are they?

What we see as ostracodes are in fact the small bivalved shells of the creatures, which in life resembled a tiny shrimp, all squished up inside the carapace. They are ubiquitous today in practically every aquatic environment, from the deep ocean to puddles on leaves in the canopy of rain forests. Many ecologists are now beginning to recognise their uses in recording environmental changes, since the group as a whole is very sensitive to things like temperature, sediment, and chemical conditions. They also appear to be useful as fossils; their small size and obscure details actually hide a high diversity of species, all with different environmental preferences. At the moment, though, we just don’t know enough about them to really make use of this.

The best way to study fossil ostracodes is to dissolve limestones, and hope that silicified casts will fall out, so you can put them in a scanning electron microscope (SEM). However, in the Builth area, not only are there few limestones, but the ostracodes aren’t silicified. The originally calcitic shells are normally preserved simply as internal and external moulds, the shell itself long dissolved out and not replaced. Occasionally there is some quartz precipitated into the mould, but it doesn’t help unless it’s in a limestone, because the acid needed to dissolve a mudstone will also dissolve quartz. Instead, we must use very fine latex casts, which can reproduce details down to a micrometer, and then put these in the SEM instead.

Once this is done, the exquisite ornament of the fossils leaps out. There are spines and ridges, pores and knobbles, flanges and reticulations. Some have peculiar hand-like projections waving from the top, others have rows of spines all around the edges, and over the ridges. The diversity is enormous. Broadly, they can be divided into two main groups: the binodicopes and palaeocopes. There are several others such as the myodicopes, but these are generally very rare creatures. Palaeocopes are generally rather conservative in basic sculpture, with a ridge running parallel to the curved edge of the shell, and from which two other ridges also run upwards across the middle of the shell. Beyond that, though, the ornament and sculpture, and the outline, are highly variable. The binodicopes are far more diverse still. Most have an assortment of bulbous bits with a deep sulcus running down from the middle of the flat upper edge, but their number and arrangement are quite diverse. Likewise the ornament, which is a riot of all sorts of possible patterns. Some have lost all the bulbous bits, and become effectively smooth. Others have exaggerated the ornament, so produce a delicate latticework, or a fingerprint-like pattern.

Some ostracodes, the palaeocopes in particular, are known to exhibit dimorphism – that is, the female is a different shape to the male. The giveaway tendency is for a broad flange, called a velum, to appear in females around the curved margin. This must, of course, be distinguished from a pseudovelum (a broad flange around the curved margin!) by the fact that there are other specimens from the same site that don’t have it. These will be the tecnomorphs – either male or juvenile. It all gets a bit confusing after that, and ostracode terminology is a quagmire. Formal taxonomic descriptions are impenetrable to the uninitiated.

The main interest in the ostracodes lies in their ecological and evolutionary stories. Because there are so many species, with such fine detail, they are extremely useful for looking at patterns of distribution, through time and space. At Builth, the most diverse faunas are from immediately below and above the main volcanic episode, and may be related to the environmental disturbances of the time. In contrast, the youngest sediments yield only a few species, but whether this is a genuine indication of diversity, or merely a trick of the fossil record, we cannot yet be certain. Only more faunas will help, so let them be overlooked no more! It’s worth noting that the Builth Inlier faunas are the most diverse early faunas from Britain, and of great interest in themselves for the evolution of the major ostracode groups.

[3]Bullaeferum sp. A. Up to ~ 1.5 mm.