Trilobites:
Among the most familiar of all fossils, trilobites belong to the most diverse group of animals alive today: the arthropods. This phylum includes insects, crustaceans, myriapods (centipedes and millipedes), arachnids, and a host of less familiar creatures. They have, between them, adapted to almost every conceivable environment, marine, freshwater and terrestrial. A quite ridiculous proportion of the total known diversity of species on the planet are arthropods, and a lot of those are beetles. They are also, with the exception of nematodes, probably the most abundant group of animals alive today.
It might come as a surprise, then, that their fossil record is generally quite poor. In some relatively recent rocks, insects and crustaceans can be quite common, but the further back we go, the scarcer they become. This is because the skeleton of most arthropods, although crunchy, is not mineralised. It decays, given half a chance. The only exceptions to this are the barnacles (rarely found as fossils because they tend to live in highly destructive environments, the ostracodes, which are abundant from the Ordovician onwards, but nearly microscope, a small group of crustaceans called conchostracans, and the trilobites. Some large crustaceans such as crabs and lobsters have a significant proportion of calcite in their shell, and these can also preserve well. But that’s the lot.
Trilobites are extraordinarily abundant in the Cambrian, Ordovician and parts of the Silurian, unusual in the Devonian, rare in the Carboniferous, and almost invisible in the Permian, during which time they finally went extinct. Their decline was long, gradual, and enhanced by large-scale extinction events, particularly at the end of the Ordovician and Silurian. They recovered slowly after extinctions, and appear to have been simply unable to adapt sufficiently to the changing ecosystems, although the reasons for this are a mystery. In their prime, they were extraordinarily successful, with many thousands of species having now been described. The diversified into a huge range of forms, from monsters nearly a metre long to minute planktonic species that barely reached a few millimetres.
Biology
The trilobite skeleton is made of a series of calcite plates: the cephalon (‘head’), thorax (made of individual segments), and the pygidium (‘tail’). It is misleading to call the cephalon the head, because the bulbous bit in the middle is actually its stomach; the mouth is underneath, and points backwards. The pygidium and each thoracic segment are single pieces, but the cephalon is usually made of a central region, the cranidium, flanked by the ‘free cheek,’ and with another plate, the hypostome, covering the mouth on the underside.
The free cheeks are important during moulting, when the skeleton is shed to allow for growth. The trilobite basically separates the free cheeks, pushing them aside, and crawls out the front. Almost all trilobite fossils are in fact moults, and missing free cheeks are a giveaway. On the other hand, an absolutely complete specimen, with the cheeks still in place, probably represents a whole animal. There are exceptions, though; trinucleids, for example, do not have free cheeks, but split the skeleton along the edge of the cephalon, leaving the lower part (with spines attached) behind.
One of the most remarkable features of trilobites is their advanced sensory apparatus, and most famously, their eyes. Those that were not blind (as the trinucleids were) possessed compound eyes similar to those of flies, but which were made of crystals of calcite. Calcite has some strange optical properties, including the production of a double image when you look through it in any direction except one. In trilobite eyes, the crystals are in the right orientation to avoid it (c-axis perpendicular to the skeleton). An advantage of compound eyes is that they are easily enlarged by evolution. A variety of planktonic trilobites, most notably the cyclopygids, made use of this to produce eyes that ran over half their total length, and in some cases actually fused at the front to produce an enormous array, giving vision in almost all directions, above, below, and through 360 degrees. These are remarkable enough but one group, called the phacopids, were still more advanced. They are not found in the Builth Inlier, but do occur in the Silurian rocks of the Welsh Borders, and can be recognised by eyes in which the lenses are separated, rather than forming a dense hexagonal array. The lenses are also round, and if we cut through them and study them with a microscope, we find that they are made of two parts, with an undulating division between them. This design corrects for spherical aberration, a small but significant effect that means the edges of a field of view are not quite ‘true.’ Each lens in the eye of one of these phacopids (called a ‘schizochroal eye’) is about as perfect as there is any need to be. They’re remarkable things.
Many trilobites (e.g. Ogygiocarella) have a series of fine ridges running over the edge of the skeleton. There are reports that in very well preserved specimens, there are sometimes lines of pits along the bases of the ridges, passing entirely through the skeleton. These initially baffling structures may also be a sensory apparatus, with the canals probably housing hairs. Using the movement of the hairs to indicate currents, the array could have become an extremely sophisticated motion sensor, detecting changes in the water around them that indicated the presence of food or a predator. Potentially, such an array could be as effective as, for example, a bat’s echolocation. The peculiar fringe around the cephalon of trinucleids may have had an equivalent function. We’re not sure, by any means, but a motion/chemical sensor array is perhaps the most likely interpretation.
There are a wide range of other modifications, including spines and tubercles whose functions are largely unknown. The enormous frontal and genal (from the side of the cephalon) spines in Cnemidopyge may well have been for balance while swimming, increasing its manoeuvrability. In contrast, the bafflingly spiny odontopleurids defy any reasonable explanation. The rare examples from the Builth Inlier are spectacular, but positively reserved compared with, for example, the staggeringly ornate Devonian trilobites of Morocco. Some of the more subtle ornament may have been related to camouflage, encouraging the growth of algae or encrusting organisms, as a variety of crustaceans do today.
The fossils that we find are only the skeletons, whether it is a moult or represents an entire animal. The remainder of the trilobite, the soft tissue, legs and gills, all decay quickly and are almost never preserved. Salter, the great trilobite authority of the middle nineteenth century, believed that they had no legs, but instead glided around like snails! Shortly afterwards, the first fossils of trilobite soft tissue were discovered, and since then there have been several more. The most informative examples are the Burgess Shale, Beecher’s Trilobite Bed, and the Hünsruck Slate. From these examples, we now have a good idea of the structure of the legs and antennae, the form of the digestive system, and the arrangement of the gills.
All the information on soft tissue allows us to place the trilobites more precisely within the arthropods as a whole, and it turns out that they are most closely related (among living forms) to the ‘horseshoe crabs’. These are not actually crabs, but remnants of an ancient group of arthropods called the xiphosurans, whose occasional remains go back to the Silurian. Among more familiar arthropods, they fall within the chelicerates, the group that also contains arachnids, the spiders and scorpions. However, they are not very closely related to any living forms, and represent an early branch of the arthropod family.
Ecology
The diversity of shapes in trilobites almost certainly reflects an equally great diversity in lifestyles. The tiny, blind agnostids were planktonic, probably filtering plankton from the water. In contrast, we know from wonderful trace fossils (the preserved burrowing activity) that some Cambrian trilobites were active predators, specialising in catching large worms between the spiny bases of their legs. The shape of the hypostome can also be used as an indicator: in predatory forms it tends to be large, forked, and sharp – useful for holding and slicing up a struggling worm. It is likely that at least some of the Builth Inlier trilobites were also predators, but there is as yet no direct evidence, and their hypostomes are generally ambiguous.
The majority were probably detritus feeders, scavenging organic matter from wherever they could find it. Possibly some of them were deposit feeders, ingesting sediment in bulk and extracting the nutrients. Still others, particularly Ogygiocarella, may have had little helpers. It has been suggested that large, flat trilobites with wide pleurae, and small hypostomes and glabellas (representing the stomach), may have housed symbiotic bacteria on their gills. This would have allowed them to make a living in environments that were low in oxygen and poor in digestible food – in other words, the black shales that are found through much of the upper part of the succession at Builth.
The details of the life history of trilobites are uncertain. There are areas where young trilobites and larvae are common, but in general they are much rarer than expected. It is possible that the few places where they are abundant represent ‘nurseries.’ There is also some evidence that trilobites may have spent some time looking after their eggs and juveniles, rather than simply ignoring them. Although not common in the Builth Inlier, it is common in some places (notably Bohemia) to find a pair of adult trilobites, of the same species and age, sheltering inside an empty nautiloid shell. It’s a peculiarly revealing insight into the habits of a long-extinct creature. I find it surprisingly powerful in making trilobites come alive, and forcing us to see the fossils as the remains of long-dead creatures, rather than shapes in the rock. Details like this remind us of how little we are really seeing, and how much of their lives will forever remain hidden from us. Long live the trilobites.
[4,5]Barrandia cordai M'Coy 1849. Up to ~ 60 mm long.
[4]Barrandia expansa Hughes 1979. Pygidium only. The rest of the skeleton is very similar to B. cordai. Up to ~60 mm.
[4]Barrandia ultima Hughes 1979. Cranidium only; the rest of the trilobite is very similar to B. cordai. Up to ~60 mm.
[4]Bergamia prima (Elles 1940). Up to 25 mm long.
[4]Bergamia whittardi Hughes 1971. Up to 30 mm long.
[1,2]Bettonia chamberlaini Elles 1940. Up to ~20 mm long.
[2]Calymenid indet. A. Width ~ 20 mm.
[2]Calymenid indet. B. Width ~ 15 mm.
[2]Calymenid indet. C. Width up to ~35 mm.
[4]Calymenid indet. D. Width of only specimen 15 mm.
[1]Calymenid indet. E. Width of only specimen (cranidium) 10 mm.
[4,5]Cnemidopyge bisecta Elles 1940. Body length up to 40 mm.
[4,5]Cnemidopyge nuda (Murchison 1839). Body length up to 40 mm
.[4,5]Cnemidopyge parva Hughes 1969. Body length up to ~ 30 mm.
[4]Cryptolithus instabilis Hughes 1971. Up to ~ 25 mm long.
[4]Degamella wattisoni (Hughes 1979). Up to ~ 30 mm.
[1]Diacanthaspis (Hughes 1979)sp. Approximately 15 mm.
[4]Emmrichops planicephalus Marek 1961. Up to 40 mm.
[3,4]Geragnostus mccoyi (Salter in Murchison 1869). Up to ~ 4 mm long.
[2]Gravicalymene aurora (Hughes 1969). Up to 80 mm long.
[4]Homalopteon radians (M'Coy 1849). Up to 70 mm.
[4]Microparia lusca Marek 1961. Up to 30 mm.
[4]Microparia major Salter 1863. Up to ~30 mm.
[4]Microparia (Heterocyclopyge) nigra Höbinger & Vanek 1985. Up to ~30 mm.
[4]Microparia (Heterocyclopyge) shelvensis Whittard 1961. Up to ~30 mm.
[?4,5]Nobiliasaphus powysensis Hughes 1979. Hughes (1979) described two fragmentary specimens as indeterminate Opsimasaphus? species, but these are not certainly distinct from N. powysensis, and are therefore not treated separately here. Up to ~120 mm.
[4]Odontopleurid indet (Hughes 1979). ~ 10 mm.
[4]Odontopleurid indet b, possibly Acidaspis sp. Single thoracic rib pleural fragment, ~ 8 mm wide.
[3,4,5]Ogygiocarella debuchii (Brongniart 1822). Up to 150 mm. O. angustissima Salter 1865 [4,5] is identified by having 13-14 pygidial (tail) ribs, rather than 10-12.
[1,2,3,4]Ogyginus corndensis (Murchison 1839). Up to 180 mm, more commonly ~ 50 mm. O. intermedius [1,2?,3?] has 9-10 pygidial (tail) ribs, rather than 7-8, and the eyes are slightly further back.
[4]Placoparina sedgwickii sedgwickii (M'Coy 1849). Up to 100 mm.
[1]Plaesiacomia sp. (Hughes 1969). Estimated up to ~ 40 mm.
[4,5]Platycalymene duplicata (Murchison 1839). Up to 100 mm.
[4]Platycalymene tasgarensis simulata Hughes 1969. Estimated up to 60 mm.
[1]Platycorpyphe vulcani (Murchison 1839). Estimate up to 40 mm.
[4]Protolloydolithus reticulatus (Elles 1940). Up to 25 mm.
[4]’Rorringtonia’ kennedyi Owens 1981. Probably should not be assigned to Rorringtonia. At least 10 mm, probably larger.
[4]Rorringtonia sp. (Hughes 1979). Approx. 30 mm.
[4]Sagavia cf. novakelliformis Koroleva 1982. Up to ~30 mm.
[4]Sphaeragnostus sp. (Hughes 1969). Up to 4 mm.
[5]Telaeomarrolithus intermedius Hughes 1971. Up to 30 mm.
[4]Trilobite indet. sp. A. This is a novelty - a trilobite that I have not the faintest idea what to call. However, it's known from a single, very poorly-preserved internal mould of a cephalon at the moment, and the reconstruction, although the best I can do from the material, may be subject to major revision... The flat genal fields are ornamented by radaiting grooves (effectively elongated reticulation), and although there is some similarity to the cranidium of Cryptolithus instabilis I'm pretty certain there was never any pitted fringe in this beastie. The glabella is bulbous, and slightly buckled, so that any real scultpure is difficult to detect. The specimen is 4 mm across.
Update! - almost certainly a new species of Dionide or related genus... there are a few specimens known from SW Wales, according to Kennedy (1989), but they're all a bit different.
[1,2?]Trinucleus abruptus Hughes 1971. Up to 20 mm.
[5]Trinucleus fimbriatus Murchison 1839. Up to 35 mm.
To be drawn:
Asaphus (Basilicus) peltastes Salter 1866
Bettonia aff. superstes Whittard 1961
Cnemidopyge nuda granulata Hughes 1969
Homalopteon murchisoni Hughes 1979
[4]Trinucleus? sp. (aff. T. fimbriatus)
[1]Protolloydolithus ramsayi? Recorded by Pete Lawrence, from one specimen.
[3]Meadowtownella sp. a
[1]Selenopeltis sp. Recorded by Pete Lawrence, from a single specimen.
[1]Primaspis? sp. Recorded by Pete Lawrence.
[1]Meadowtownella aff. evolutaRecorded by Pete Lawrence.
[1]Anebolithus simplicior sp. Recorded by Pete Lawrence.
Telaeomarrolithus radiatus (Murchison 1839).
