Trilobites: A short introduction

Dr Robert Sturm (Austria)

Many people interested in palaeontology and collecting fossils have either found fragments of trilobites in the field or marvelled at fossil examples of these animals displayed in museums around the world. Although they are essential components of palaeontological collections, thereby acting as index fossils for the Palaeozoic epochs (from the Cambrian to the Permian), data regarding their systematic and taxonomic categorisation, biology, and ecology are largely unknown among amateur fossil collectors. In this short contribution, I will provide an overview of the main characteristics of trilobites, which are of great importance if you wish to gain an understanding of these fascinating organisms, which became extinct 220Ma.

The systematics of trilobites

The heyday of the trilobites was during the Cambrian (570 to 505Ma). Due to increased competition for food resources (for example, graptolites and brachiopods), they were subject to a successive displacement from their original habitats. This process was also accelerated by the appearance of predators, such as large cephalopods, eurypterids (which looked like gigantic crayfishes) and fishes. The evolution of better predators and competition for food finally resulted in the extinction of the trilobites during the Permian (220Ma) and evolutionary innovations, such as the ability to roll up and the development of masticatory organs and long spines (see below), could only slow their decline.

From a systematic point of view, trilobites belong to the class of the †Trilobita, which belong to the subphylum of the †Trilobitomorpha. This subphylum is assigned to the Arthropoda phylum and the class of the Eutrilobita can then be subdivided into ten orders (Figs. 1, 2 and 3)

• The Agnostida include small, mostly eyeless trilobite forms, which are characterised by a great similarity between the cephalon and pygidium, and the development of only two to three thorax segments. The most important species include Agnostus, Eodiscus and Pagetia, which occurred in the Middle to Upper Cambrian.
• The Redlichiida represent relatively large trilobites, which commonly possessed a hemispherical cephalon with quite long caudal spines. The glabella has a remarkable structure, such that the eyes are often very large and have a nearly hemispherical shape. The pygidium is greatly reduced in size. Index species of this order (among others) include Holmia, Ellipsocephalus, Redlichia and Paradoxides, which all occurred in the Lower Cambrian.
• The Corynexochida represent a group of trilobites with a generally high degree of homogeneity in their external morphology and share some similarities with the Redlichiida. However, unlike the Redlichiida, they developed a larger pygidium and, in many cases, dorsal spines. The most important species of this order is Ogygopsis, which occurred during the Middle Cambrian of western North America.
• The Ptychopariida include most of the known trilobite species. Their glabella is commonly characterised by deep lateral furrows, which are mainly oriented towards the caudal (tail end) part of the cephalon. The most important species include (among others) Ptychoparia, Euloma, Conocoryphe, Dikelocephalus and Olenus. Unlike the other orders, the Ptychopariida are present in all the main epochs of the Palaeozoicum (from the Cambrian to the Permian).
• The Phacopida have a broad, partly bulb-like glabella. Their thorax normally contains eight to 19 segments and, in some cases, the glabella is significantly swollen and carries a specific dotted pattern. The order shows an evolutionary degeneration in their eyes and the development of an additional segment between glabella and neck segment. The most important species of this order include Phacops, Dalmanitina, Cheirurus, Calymene and Homalonotus.
• The Lichida represent quite large trilobites and the genus, Uralichas, reached a length of 75cm. The glabella of these animals is irregularly shaped and also has a granular structure. The cephalon is characterised by long spines, which are mostly bent backwards. The most important species are Lichas and Ceratarges, which both occur from the Ordovician to the Silurian.
• The Nectaspida represent small trilobites, with a large number of spines on the cephalon, thorax and pygidium. Species like Odontopleura occur from the Middle Cambrian to the Upper Devonian.
• The Asaphida, Harpida, and Proetida have recently been removed from the Ptychopariida, due to specific morphological differences found in their hypostome.

The biology of trilobites

Trilobites, whose name is based on the fact that their body plan crudely consists of three parts consisting of a long central axial lobe, flanked on each side by right and left pleural lobes (and not the head, body and tail as is commonly assumed). Among the 1,300 genera described in the literature, many were characterised by a worldwide distribution and have therefore gained a special significance as index fossils. They were present from the lowest Cambrian (570Ma) to the uppermost Permian (220Ma), which provided ample time for evolution to create a diverse range of morphologies.

On their dorsal (top) side, these animals were protected by a dorsal shield, which overlapped a little bit with the ventral (under)side at the lateral margins of the animal. However, the main part of the ventral region was not protected by any chitinous armour. As well as the three lobed division mentioned above, the dorsal shield of the trilobites is subdivided into three main parts, namely the cephalon (head), the thorax (abdomen) and the pygidium (tail). The thorax and pygidium consist of the lateral pleura, represented by long, arrow-like shield segments and the central rhachis, which shows a slight similarity to the spinal column of vertebrates (Fig. 2).

From a biochemical point of view, the dorsal shield is composed of a two-layer cuticula, with a thin, outer layer consisting of prisms and a thick, inner layer including several strata. The armour is moulted periodically (which is referred to as ecdysis), to allow for continued growth by the animal. (A similar phenomenon can be observed in arthropods, such as crayfishes and insects, today.) While the separate segments of the cephalon and pygidium are completely fused, the respective units of the thorax are usually connected by joints, which enabled the animals to partially enrol.

The cephalon includes a central bulge (glabella), which is subdivided by several furrows. Facial sutures separate the different parts of the cephalon (which are not moulted during ecdysis) from those that are moulted, which allows for the growth in size of the head segment to support the growth of the head segment. In most cases, those parts of the cephalon that are moulted also include the compound eyes, which consist of a variable number of CaCO₃-lenses (up to a maximum of 15,000 in some species), with a crystal cone below each lens. The ventral side of the cephalon comprises the rostral plate, the taxonomically important hypostome, as well as the small metastom, which is only known in a very small number of species. The thorax is characterised by a large number of segments, which could move and, therefore, increased the flexibility of the animals.

The pygidium commonly includes a variable number of non-flexible segments. Similar to the cephalon, its connection with the thorax is by means of a joint. It can be either very small or significantly larger than the cephalon, depending on the species.

Due to the extraordinary fossilisation of some trilobites, it has become apparent that these animals developed one pair of pre-oral antennae, as well as a variable number of biramous limbs. The extremities themselves consisted of a so-called telopodite, which was responsible for locomotion and the so-called pre-epipodite, which most probably supported the gills. Some trilobites were marked by sexual dimorphism (that is, males and females with different body size), while others had the ability to roll themselves up. Like other arthropods, trilobites went through a significant number of moults (up to 30) during their lifetime. Therefore, most fossil finds of these animals are the moulted ‘exuvias’, rather than whole organisms.

According to modern palaeontological research, the ontogenetic development of trilobites can be subdivided into three main stages:

• The early protaspid or larval stadium is characterised by a uniform, but slightly structured dorsal shield, which commonly varies between 0.25 and 1.00mm in length.
• The meraspid stage generally marks the first occurrence of a joint between the cephalon and pygidium. During this stage, the animal’s length exceeds that of the protaspid stage by six to twelve times.
• The final holaspid stage is characterised by the attainment of the definitive number of segments.

Further moults after the holaspid stage enabled trilobites to achieve a remarkable increase in size. Most adult trilobites reached a length between 3cm and 8cm. The smallest species measured about 0.5cm in length, whereas the largest known specimen had a body length of 75cm.

Ecology of the trilobites

As we know from a huge number of palaeontological investigations, trilobites were mainly epibenthic residents of shallow marine regions situated near the coast. Species without compound eyes probably burrowed into the mud and therefore adopted an endobenthic life. Due to the absence of differentiating oral extremities (like mouths), trilobites mainly fed on microorganisms, such as protozoa or algae. In some cases, they were simple mud feeders, with the ability to filter nutrients. For some phacopids, masticatory organs have been reported, which may imply a predatory lifestyle.

It is commonly suggested that trilobites produced specific resting, walking, feeding and funeral traces that are known among experts as rusophycus and cruziana and bilobites. In general, the animals were mediocre swimmers and, due to the absence of claws or other defensive mechanisms, they were rather defenceless against all kinds of predators. Therefore, they must have had to hide in rock fissures or in the mud of the sea floor to avoid being eaten. On the one hand, reduced mobility, and on the other, dependence on specific biotopes, resulted in the development of endemism (an endemic species is characterised by a geographically limited occurrence) and specific trilobite provinces. Such provinces are especially recognisable in the Lower and Middle Cambrian (570 to 55Ma).

Further reading

Chatterton, B. D. E. And Speyer, S. E. (1989). Larval ecology, life history strategies, and patterns of extinction and survivorship among Ordovician trilobites. Palaeobiology 15: 118-132.

Fortey, R. A. (2000). Trilobite: Eyewitness to Evolution. New York: Vintage Books.

Fortey, R. A. (2004). The Lifestyles of the Trilobites. American Scientist 92: 446-453.

Fortey, R. A. (2001). Trilobite systematics: The last 75 years. Journal of Palaeontology 75: 1141-1151.

Lehmann, U and Hillmer, G. (1988). Wirbellose Tiere der Vorzeit. Leitfaden der systematischen Paläontologie der Invertebraten. Stuttgart: Enke.

Pete Lawrance and Sinclair Stammers (2014). Trilobites of the World: An atlas of 1000 photographs, Siri Scientific Press, Manchester, 416 pages (softback), ISBN: 9780957453036.

Dr Robert Kennedy and Sinclair Stammers (2018). Trilobites of the British Isles, Siri Scientific Press, Edinburgh, 384 pages (softback), ISBN:9780995749610.