Trilobites

One of the most popular groups of fossils, and the one that the geopark is famous for! Trilobites are an extinct group of arthropods, distantly related to crustaceans and arachnids. Their similarity to woodlice is only superficial, although it can be striking; many trilobites could roll into a ball as a defence mechanism, as some woodlice can. There are numerous websites dealing specifically with trilobites, including this introduction: https://www.palaeontologyonline.com/articles/2013/fossil-focus-trilobites/

The geopark contains around 60 species of trilobite in the Ordovician rocks of the inlier, and a few others (mostly not yet reported in papers) from the Silurian rocks, so we cover them separately here. The Ordovician trilobites can be divided into several major groups (orders and families), which should make it easier to identify them. Some of these are also not yet published as having lived here, and others are doubtless yet to be found!

For information about the age ranges given in this section, please see here.

Current total number of Ordovician trilobite species in the geopark: 61

Ordovician trilobites

Asaphida

At many levels, these are the most common trilobites in the Ordovician rocks of the geopark.

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• Ogyginus intermedius: distinguished from O. corndensis by having 9-10 ribs in the pygidium (tail) rather than 7-8.
• Basilicus tyrannus: a very large species found very rarely in rocks from the shallowest-water environments; only a few fragments yet found.

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• Ogygiocarella angustissima: can become abundant in the youngest beds, with 13-14 ribs in the pygidium.

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Family Nileidae

Relatively featureless trilobites, with a rounded outline and very few ribs. The number of pygidial ribs (if present) and the shape of the central part of the 'head' are important features for identification.

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• Homalopteon murchisoni

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Family Cyclopygidae

Bizarre pelagic trilobites with enlarged eyes, sometimes even fused at the front.

• Bohemilla sp.

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Family Raphiophoridae

Raphiophorids have a long frontal spine, and a pair of backward-curving spines from the corners of the cephalon (“head”)—although the latter Several species of Cnemidopyge exist in the geopark, which all intergrade with each other – you may find some that are between two species in appearance!

• Cnemidopyge parva: a generally small species with 5-6 pygidial ribs.
• Cnemidopyge nuda: larger, with usually 7-8 pygidial ribs.
• Cnemidopyge bisecta: like C. nuda but with a strong ridge running along the middle of the “head”.

Superfamily Trinucleoidea

A very distinctive group, almost all of which have a strange pitted fringe around the front edge of the cephalon (“head”), which was probably a sensory apparatus. Identifying species can be difficult, as the differences within a genus can be subtle. The arrangements of pits in the fringe is one of the key features.

• Anebolithus simpicior: very thin marginal fringe; rare species known only from one site.
• Bettonia chamberlaini: with a backward-pointing spine from the back of the head; common species in the oldest rocks (artus and lowest murchisoni biozones).
• Bettonia aff. superstes: a single specimen described by Hughes (1972) from the teretiusculus Biozone that seems to be different from B. chamberlaini.
• Dionide cf. turnbulli: strange-looking trinucleoid with expanded but non-pitted fringe; known from one specimen in the teretiusculus Biozone.
• Trinucleus abruptus: marginal fringe sharply truncated; locally common in the oldest rocks.
• Trinucleus fimbriatus: pits in fringe within sharply-defined radial grooves; common in the youngest rocks of the Llanfawr Mudstones Formation (gracilis Biozone).
• Trinucleus sp.: single specimen from teretiusculus Biozone.
• Bergamia prima: known only from two sites in the teretiusculus Biozone.
• Bergamia whittardi: from several sites in the murchisoni to teretiusculus Biozones.
• Cryptolithus instabilis: backward-pointing spine from middle of 'head', with randomised pits in square, backward-pointing extension of the fringe; known from a few sites in the teretiusculus Biozone.
• Cryptolithus? sp.: described by Hughes (1972) from the murchisoni Biozone, but only fragments known.
• Protolloydolithus reticulatus: array of pits in fringe is randomised; teretiusculus BIozone
• Protolloydolithus ramsayi: very rare species in oldest rocks only (artus Biozone).
• Telaeomarrolithus intermedius: similar to Trinucleus fimbriatus but with sharply angled fringe; rare species from gracilis Biozone
• Marrolithus sp.: single specimen described by Hughes (1972) from the gracilis Biozone.

Phacopida

Robust, predatory species with advanced eyes. Very few (except for the family Calymenidae) occur in the geopark, with both being found at a single site.

• Colpocoryphe sp.
• Plaesiacomia sp.
• Platycoryphe vulcani

Family Calymenidae

Chunky trilobites with a thick skeleton, so that they tend to be quite three-dimensional. There are probably multiple unrecognised species from the shallow-water deposits of the murchisoni Biozone, but these remain to be confirmed.

• Gravicalymene aurora
• Platycalymene duplicata
• Platycalymene tasgarensis simulata
• calymenid A
• calymenid B
• calymenid C
• calymenid D
• calymenid E

Odontopleurida

Very spiny trilobites that are normally uncommon, but can appear in abundance in some locations. Only a few species are known from the Ordovician rocks of the geopark, and most of those are from isolated fragments only (hence the incomplete identification!).

• Diacanthaspis sp.
• Meadowtownella serrata: abundant in the type locality, but extremely rare outside it
• Meadowtownella aff. evoluta
• Primaspis? sp.
• Selenopeltis sp.
• Odontopleurid undet. (single librigena recorded by Hughes, 1979)
• Odontopleurid undet. B (single rib fragment, possibly of Acidaspis)

Agnostida

Minute (only a few mm) planktonic trilobites with the front and back being similar, and only two thoracic segments. They only appear in mudstones laid down when the water depth was at its deepest, during the teretiusculus biozone.

• Geragnostus mccoyi: relatively common in the teretiusculus Biozone
• Sphaeragnostus sp.: very rare, with much more rounded outline and sculpture

Lichida

Lichids are large, spectacular trilobites that are often highly ornamented or spined. They are usually very rare, and the geopark is no exception to this rule!

• Placoparia sedgwickii sedgwickii: a very large, highly ornamented species, but very rare in the geopark.
• Lichid undet.: a single partial pygidium represents a distinct species.

Proetida

Small, rather nondescript trilobites with rounded outline and a relatively small glabella (the middle lobe on the 'head'). Very few species.

• Rorringtonia kennedyi: known from a single site in the geopark.
• Rorringtonia sp.: a single pygidium described by Hughes (1979)