trilobites and related fossils

trilobites and related fossils The trilobites are among the best-known fossil examples of the phylum Arthropoda, an extensive group of organisms in which the body is segmented and has a chitinous exoskeleton and numerous jointed legs. The arthropods are the most successful living organisms, comprising almost 75 per cent of all described animals, and trilobites are the earliest known examples, occurring in rocks of Early Cambrian age just above the earliest Cambrian faunas. This entirely marine group was widespread and diverse throughout the Palaeozoic and became extinct in the Permian. They are important geologically, for they are widespread and numerous in Palaeozoic rocks and their rapid evolution is easy to detect in their distinctive morphological features, making them valuable for biostratigraphic correlation. A number of other arthropod groups are present in the Palaeozoic, although their relationship to the trilobites is unclear. Some of these, insects and crustaceans, are dealt with in separate entries. One major group, the eurypterids, will, however, be considered here.

Morphology

Trilobites take their name from the division of the exoskeleton into three longitudinal portions which is clearly seen in most species (Fig. 1). The axis, or axial lobe, extends from near the anterior margin to the posterior extremity of the body and is flanked on either side by the pleural regions. The most anterior section of the dorsal exoskeleton is termed the head or cephalon, and this bears a central hump or glabella that is flanked laterally by the paired compound eyes. A thin lineation, the facial suture, runs from the postero-lateral margin of the cephalon forward and around the eyes before terminating at the anterior margin. This follows different courses in different trilobite species and is a weak area of the skeleton that apparently split during ecdysis to facilitate moulting of the exoskeleton. Ventrally, the mouth of the trilobite was protected by a flat plate, the hypostome. Behind the head are a series of thoracic segments, which vary in number in different taxa. Each of these is articulated with its anterior and posterior neighbour, allowing the animal a considerable degree of flexibility, in some instances even allowing it to roll up completely. The most posterior section of the animal is termed the pygidium, a flat plate of fused segments similar to the thorax. Ventrally, the appendages are very rarely preserved since they were apparently not well mineralized. However, unusually preserved specimens show the presence of paired antennae anteriorly, followed by paired two-branched appendages under the cephalon and one pair for each segment through the thorax and the pygidium. The appendages are divided into a gill branch and a walking leg, thus combining the functions of locomotion and respiration.

Taxonomy

Arthropods are extremely numerous and show an amazing diversity of morphology. Because of this a number of different taxonomic schemes have been raised for them and there is still much controversy over whether they were derived from single or multiple ancestors. In a series of papers in the 1970s S. M. Manton championed the view that ‘arthropodization’ occurred several times. She therefore divided arthropods into four phyla: Uniramia, which includes the millipedes, centipedes, and insects; Crustacea, which includes lobsters and crabs, and also ostracods and barnacles; the Chelicerata, composed of spiders and scorpions as well as the extinct eurypterids; and finally the Trilobita. In other schemes the phylum Arthropoda is retained and the major taxa are given subphylum or class status.

The trilobites are divided into nine orders according to the morphology of the dorsal exoskeletons, and in particular details of the cephalon. The important forms are the Agnostida, very small blind trilobites with only two thoracic segments, and the Redlichiida, large forms with many lateral spines that are characteristic of Lower Cambrian rocks. The Corynexochida were an extensive and rather heterogeneous group that lasted until the Middle Devonian. The Phacopida were Ordovician to Devonian trilobites characterized by large well-developed eyes. The Ptychopariida are a diverse group characterized generally by a forward-tapering glabella. The Proetida were the last of the trilobites, holding on until final extinction in the Permian.

Ecology

Evidence for the life habits of trilobites comes both from their morphology and from the trace fossils that they formed in the sediment. The traces are of several distinct types and appear to record a variety of activities. Some show a series of V-shaped markings indicating movement on the surface; others consist of elongated bilobed trails, convex down and with oblique striations formed by the legs, which are probably related to burrowing activity. Short ovoid markings of a similar structure seem to indicate ‘resting’ traces excavated by the animal near the surface, although some reported from the Lower Cambrian of Sweden are related to the burrows of priapulid worms and might record hunting activity by the trilobites.

In general, trilobites appear to have been benthonic organisms feeding on organic detritus on the sea floor, but some may have captured small soft-bodied organisms which would have been passed forward to the mouth by the limbs. Forms that carried out this type of benthonic feeding were oriented parallel to the surface and may have been able to form a seal with it, below which the legs would churn up the sediment and remove organic particles. Some trilobite lineages show adaptations that are consistent with a pelagic or swimming lifestyle. These include large cephalons with extremely large eyes, and an overall morphology that seems maladapted to movement on the sea floor. By analogy with modern amphipods it is thought that these animals may have swum on their backs and fed on suspended particles. It has also been suggested that agnostid trilobites might have developed a pelagic mode of life, for some of them are very widely distributed, the same species occurring, for instance, in the Middle Cambrian of Scandinavia and western North America.

Trilobite eyes

Trilobites characteristically possess well-developed compound eyes. These represent the first visual systems known and are thus of particular interest. Most trilobite eyes are holochroal, having many round or polygonal lenses in close contact covered by a single cornea. These are the oldest eye-types known and persist until the extinction of the trilobites in the Permian. The lenses are commonly long prisms formed of calcite with the crystallographic axis parallel to the light path to reduce birefringence (splitting of light). A second type of eye, termed schizocroal, is found only in phacopids; it consists of an array of large lenses separated from each other. Work by Euan Clarkson of the University of Edinburgh has shown that the strongly biconvex lenses were constructed in two parts separated by a complex surface, a structure that would tend to improve focusing ability. Such an array may have given the animals sharp stereoscopic vision through 360° and could also have provided them with improved vision in the dark.

Geological history

Trilobites appeared in the Early Cambrian with no obvious ancestral forms, although it has been suggested that arthropod-like forms in the Proterozoic Ediacara Fauna may be on the ancestral line. Their mobility allowed them to take advantage of available habitats and they developed rapidly during the Cambrian; all the orders except the proetids appeared during this period. This trend was reversed, however, during the Ordovician, probably through increased competition from other benthonic forms and the rise of predatory cephalopods. The redlichiids became extinct before the end of the Cambrian; several other groups, including the agnostids, did not survive the Ordovician. A gradual decline then set in until the extinctions of the Middle and Late Devonian disposed of the majority of the trilobite groups. Only the proetids continued until the end of the Permian, when they died out during the major extinction event that affected all the shallow-water marine invertebrate groups. Because of this history, trilobites are most useful in the biostratigraphy of the Cambrian on a worldwide scale, and in the Ordovician and Devonian locally.

Eurypterids

Eurypterids are arthropods of the phylum Chelicerata that have a fossil record ranging from the Ordovician to the Permian, reaching their acme during the Silurian and Devonian. They occur in marine, brackish, and freshwater rocks and appear to have been active benthonic predators that were also capable of swimming. The animal had a large head region or prosoma (Fig. 2) with prominent compound eyes dorsally, and six pairs of appendages ventrally. The first pair were adapted for feeding, the next four were walking legs, and the most posterior pair formed large swimming paddles. Behind the head was a long body or opisthosoma of 12 segments terminating in a stout telson. Analysis of the sixth appendage has shown that the blade moved from a vertical orientation during the propulsive stroke to a horizontal orientation during the recovery stroke, an arrangement necessary for efficient swimming. There is, however, evidence from fossil trackways (a type of trace fossil) that suggests that some euryptids may have been able to walk on land. Most eurypterids were probably predators on other invertebrates and possibly fish. Although most were quite small, the largest, at almost 2 metres long, were the largest arthropods that have ever existed.

David K. Elliott

Bibliography

Boardman, R. S., Cheetham, A. H., and Rowell, A. J. (eds). (1987) Fossil invertebrates. Blackwell Scientific Publications, Oxford.
Clarkson, E. N. K. (1979). The visual system of trilobites. Palaeontology, 22, 1–22.
Manton, S. M. (1973). Arthropod phylogeny—a modern synthesis. Journal of the Zoological Society of London 171, 111–30.