Fossil identification resources

Identifying fossils

The rocks of the Heart of Wales Geopark have produced hundreds of species of fossils, many of which have not yet been described or named. Many are rare, difficult to see, or very difficult to identify, but nonetheless, new species are discovered here every year. For most people, the fossils you are likely to see belong to certain major groups. This page aims to give you a starting point, by helping you identify what broad type of fossil you have. Each group contains many species, and in some cases you can work down through the pages to try to identify them.

Some of the animals that have been preserved are relatively familiar from their living relatives, but others are not. Remember also that fossils often preserve only a small part of an animal, and that they can be preserved in different ways, depending on the conditions; they can look very different depending on the process of fossilisation. We have a section on how fossils are formed here. As general rule, the more fossils you see, the easier it becomes to recognise them!

Trilobites

These are the classic fossils of the Builth–Llandrindod Inlier, for which it has long been famous. They are extinct arthropods with a segmented exoskeleton over the upper surface that is commonly fossilised because it was made of the mineral calcite.

A complete skeleton is unmistakable, but in most cases the skeletons fell apart into separate plates after the animal died or moulted. The most obvious plates are the pygidium (“tail”), and the cranidium (the central part of the “head”), but there are also the ribs of the thorax, the “free cheeks”, and a strange plate on the underside, called the hypostome. The various parts do not look much like a complete trilobite, and many people mistake a pygidium for a butterfly, for example!

There are well over 50 species of trilobite known from the geopark, including a few Silurian ones in the outlying areas, and they are divided into several major groups... but most of them are relatively easily identifiable.

Graptolites

Among the most common fossils in both the Ordovician and Silurian rocks of the geopark are planktonic colonies of animals known as graptolites. The colonies have a saw-toothed form with one or more branches (or many branches in bushy forms known as dendroids), and consisted of a series of tiny tubes that the individual animals lived inside. The colonies were made of tough organic material, so they are usually fossilised as black or silver films.

Graptolites are very diverse, and are widely used to precisely work out the age of rocks that they are found in, so they are important to get to know. Identifying the species is possible for beginners, but it needs care and sometimes accurate measurements need to be taken under a microscope.

Brachiopods

Extremely common fossils in Ordovician and Silurian rocks, but most species are found in shallower-water sediments like sandstones and limestones. There are two main groups: the rhynchonelliforms (mainly in sandstones), with calcite shells and looking like familiar molluscs such as cockles, and the lingulids (abundant in mudstones), with much flatter, simpler phosphatic shells. Brachiopods can be separated from bivalved molluscs by their symmetry: there is a mirror plane through the middle of each shell.

Many species are identifiable, but you have to know whether you're looking at the inside or the outside, and whether it is the upper or lower shell, since each view can be different.

Molluscs

Molluscs are familiar animals of seashore and garden, including snails, clams and squid. In the geopark, several groups are also reasonably common in a range of rocks.

Nautiloids are ancestors of squid and octopus, but have a shell. Normally this is a straight, conical shell (but some species are curved or coiled), with the narrower end divided into chambers. They can be very large, even up to a metre or more, but most are up to 10 cm long. The outside can be ornamented with a range of ornamental ridges and patterns, and they can be found in both Ordovician and Silurian rocks.

Bivalves were relatively common in shallow water in the Ordovician rocks of the geopark, but very rare in the deeper-water mudstones. In the Silurian rocks, there are a few species found in the shales that had adapted to these quieter conditions.

Gastropods (sea snails) are uncommon, but have helically coiled shells that are very distinctive when found. There are also some non-helical shells known as tergomyans, which are either a primitive groups of gastropods, or a related group of extinct molluscs.

Echinoderms

This diverse group includes the familiar starfish, the less-familiar sea lilies (crinoids), and various bizarre and extinct critters. They all share an internal skeleton of calcite plates, usually with at least one arm-like structure for feeding with. Their skeletons are very delicate, and fall apart rapidly after death, so complete specimens are rare, and indicate at least rapid burial.

Crinoids are stalked animals, with a small conical body, and five (often branched) arms. They were common in shallow-water environments of the time, and the stalk breaks up into hundreds of little 'polo mints' that are very distinctive. The shapes of these, and the patterns on the surfaces, suggest that there were many species in the Silurian rocks as well, although only a few have been found complete.

Starfish are very rare in Ordovician rocks, but there were several species, only a few of which have been described. They all have five arms, and are very distinctive fossils, if generally small!

Cystoids are another stalked group of echinoderms, but are very much rarer, with very few specimens recorded. These resemble crinoids, but have larger, globular bodies and spindly little arm-like structures known as brachioles.

Stylophorans are the weirdest extinct group, being either roughly bilateral, or asymmetric, and with a single feeding arm. Some of them have a U-shaped body, and others are more oval. Isolated plates of them are quite common in Ordovician mudstones, but there are only a few species.

Solutans are even rarer, being known from only two sites in the geopark, both of them protected. They are like stylophorans, but with a circular or heart-shaped body, a feeding arm from one end, and a similar-looking appendage from the other.

The rarest group recorded so far are sea urchins (echinoids), known from only a few partial, crushed specimens at one site. Any new example would be an important discovery.

Bryozoans

Colonial creatures known as 'moss animals', bryozoans were common fossils mainly in shallow-water during this time. They formed sheet-like, or branching stick-like colonies made of calcite, rather like apartment blocks, the animals living inside tubes that opened as tiny (less than 1 mm) pores on the outer surface.

In the Ordovician rocks, they are mostly found in sandstones, where they can be diverse and quite abundant, but occasionally they lived as sheets encrusting onto the shells of nautiloids. In Silurian rocks of the geopark, they are normally only seen in the shell beds, washed together with brachiopods and other remains. The bryozoans of this area have not yet been described, although some of the species are bound to be known from work in other areas.

Ostracodes

Also known as 'seed-shrimps', ostracodes are ubiquitous but mostly tiny crustaceans, with their soft tissues entirely hidden inside a pair of millimetre-sized shells made of calcite. The shells can fossilise, and are often quite beautiful when magnified. They have no growth lines, but many are strongly sculptured and ornamented.

At least some of the Ordovician species in the inlier have already been described in a monograph from 1984-5, although others remain to be studied. The Silurian ones have also been described, including giant (up to 1 cm) myodocope ostracodes that are found at several sites surrounding the inlier.

Conulariids

An unusual group of fossils consisting of thin, pyramid-shaped shells that housed something similar to a jellyfish, attached to the sea floor. They were originally phosphatic, and are normally preserved black. Usually each face has a line down the centre, and very fine transverse lines run across each side. These fossils have not been described for over a century, but are not uncommon at some localities.

Sponges

The most diverse group of fossils in the geopark, but among the most difficult to recognise. Sponges are animals, most of them with a skeleton of microscopic opal needles called spicules (straight, or cross-shaped) forming the skeleton. The animal is normally vase-shaped to spherical, with an opening at the top, and the structure of the skeleton can be extremely variable. The spicules are often replaced by iron minerals, and at many sites there is a rusty stain marking the skeleton and the outline of the body; any strange black, brown or orange blobs in shales have a chance of being sponges... if they are not just weathering patterns!

There are well over 100 species of fossil sponges in the geopark, but only 30 or so have yet been described. Many more doubtless await discovery.

Palaeoscolecids

Worms do not normally fossilize, except at the most exceptional sites like Castle Bank, but there is an exception. The palaeoscolecids had a cuticle studded with minute (very microscopic!) phosphate plates, and this can be preserved reasonably commonly at some sites. They normally appear on the rocks as thin (down to less than a millimetre wide) orange or brown wiggles on the surface of shales, sometimes (if you're lucky) with a trace of extremely fine transverse lines... but an electron microscope is needed to see the fine detail.

About ten species have been identified in the geopark so far, which is more than any other Ordovician area in the world... but many others are yet to be described. These need to be studied by a specialist with access to an electron microscope in order to identify them, and are one of the best reasons to work with the professionals if you find one!

Conodonts

Minute, millimetre-sized teeth, often with multiple spines, are occasionally found in the Ordovician rocks, preserved black or white on the shale surfaces. These belonged to the conodont animals, which are now known to be early relatives of fish from before the origin of jaws. The teeth lined a sucker-like mouth, and were used to catch and draw in prey in a similar way to hagfish and lampreys.

These fossils are well known in general, and like graptolites, are used extensively to date the rock sequence (especially where they are found in limestones, from which one can dissolve them). The ones from the geopark have not yet been studied, however, although work is currently starting on some of them.

Phyllocarids

Very rarely, the elongate oval shells of bivalved arthropods are found, in either the Ordovician or Silurian rocks. At first glance they can be difficult to separate from bivalved molluscs, although they do not have growth lines, and usually have a rim around the edge. The shells were organic, thin, and delicate, and usually show some feature such as a short spine projecting from one or both ends that can help to identify them.

These fossils belong to a range of groups of early crustacean-like animals, including ceratiocarids and caryocarids, but most cannot be identified without extra detail of their segmented body or tail structures. Extremely rarely, complete animals are found, but these have not yet been described from the area. It is likely that any of these that turn up are new species.

Algae

Seaweeds can also fossilize, under particular conditions, but are not common. Within the geopark, they are sometimes found as wispy, amorphous structures with no distinctive shape; none have yet been formally described.

In addition to these groups, there are a range of 'exceptional' fossils that have been found in the geopark, but these are generally at protected, unique sites like Castle Bank.