Other Arthropods:
The arthropods are today the most diverse animals on the planet, and probably also the most abundant, with the exception of the nematodes. (It’s said that if the entire planet became invisible, leaving only the nematodes, you’d still be able to make out everything down to the beermats.) As Haldane noted, the Creator must have been inordinately fond of beetles, for there are perhaps as many species of them as there are of everything else combined.
We have no reason to think that it was different, even back to the Cambrian. Alright, so the insects appeared only after the colonisation of land, but there is still a huge diversity of arthropods in the sea. Most of them, however, do not have a mineral skeleton, and so, unlike the trilobites and ostracodes, are rarely fossilized. Trilobites in themselves account for a large part of Cambrian diversity, and they and ostracodes are certainly significant in the Ordovician, but that is all we normally see. Occasionally, in sites like the Burgess Shale where almost everything is preserved, we can obtain a truer impression. In the Burgess itself, the bulk of the diversity is made up of arthropods: crustaceans, chelicerates (or at least chelicerate-like things that are probably quite close to the ancestors of real chelicerates), the nightmarish anomalocaridids, and a whole host of more ambiguous creatures. One beautiful little creature, Marrella, is known from tens of thousands of specimens, but several others are rare.
A similar pattern occurs later in the Palaeozoic; the Silurian Herefordshire Lagerstätte, for example, also has a high diversity of arthropods, as does the Devonian Hunsrück Slate. Other, lesser deposits of soft-bodied fossils also include a high proportion of arthropods, and their diversity and abundance in general are well described. Some of this may be an artefact, though, a bias of preservation. Although early arthropods in general were not mineralised, their cuticle was still relatively resistant to decay (compare with, say, a jellyfish), and because they moulted, an individual produced numerous copies through its lifetime. Some groups, such as the ceratiocarid crustaceans, are relatively common as fossils in a few areas, and can turn up anywhere.
In the Builth Inlier, unmineralized arthropods are rare. However, there are some examples known from several sites, including bivalved ‘crustaceans’ from several sites in the Llanfawr Mudstone. In all but two cases, it is just the elliptical carapace that has been found. In one of the others, there is also the trunk and tail protruding from it, and in the last there are even (debatably) faint traces of the appendages as well. There appears to be some diversity within the group, and the carapace diameter ranges from a millimetre to several centimetres. Pieces of what appear to be arthropods of some sort, including bivalved carapaces and perhaps parts of a large telson (tail spine), have also been found in ashy shales in the Builth Volcanic Formation, as well as a very peculiar, and very poorly preserved, trilobite-like creature. Reports of still more unusual creatures are awaiting confirmation…
Elsewhere, the pattern is similar: bivalved carapaces of ceratiocarids, dactylocarids and phyllocarids are occasional interesting finds, but there is rarely anything more. Something vaguely trilobite-like, but not, is cause for celebration; there are a large number of groups of this type, including aglaspidids, cheloniellids, and nektaspids, but any of these is a significant discovery. Anything more unusual than that, such as marrellomorphs, hexapods, isopods or arthropleuridans, really is something to write home about.
Even a fragment of such a thing is important, but what we really want are complete specimens with appendages preserved. A lot of arthropod classification (which nowadays includes studies of their evolutionary history) requires a detailed knowledge of the appendages in the head region – how many antennae, mandibles, and so on. Unfortunately, the appendages are the parts of the skeleton most prone to decay, and to preserve them requires very unusual conditions indeed. It’s not impossible, but most palaeontologists go through life without ever finding an unmineralized arthropod with appendages. The presence of such things in a deposit is often used informally as a marker for ‘exceptional preservation.’
Overall, the unmineralised arthropods are very interesting creatures, and they are generally poorly studied because of their extreme rarity. They can, however, give a glimpse of the real diversity of the original ecosystem. It’s like ballooning over a silent rain forest, and watching a parrot break through the canopy.
[4]Bivalved arthropod indet. Carapace has irregular wrinkling from top left to bottom right, but this may be a diagenetic feature. 5 mm long.
[5]Canadaspid (possibly). ~ 10 mm.
[5]Ceratiocarid A. ~ 4 mm.
[3]Ceratiocarid B. ~ 5-6 mm.
[3]Ceratiocarid C. ~ 8 mm.
[3]Ceratiocarid D. 3-4 mm.
[5]Phyllocarid. Carapace approx. 10 mm.
To be drawn:
[3]Ceratiocarid sp. E
[2]Trilobitomorph? indet.
[4]large bivalved arthropod carapace (~ 4 cm)
[1]Large ceratiocarid? Reported by Brian Beveridge/Pete Lawrance.
[1]Eurypterid? fragments. Reported by Pete Lawrance and Brian Beveridge.
[5]Peracarid crustacean-like form.
note that we're getting confused by the range of forms in bivalved arthropod carapaces now... some are probably ceratiocarids, the phyllocarid we're pretty sure of, but the others are getting horribly confusing!
