Trilobite

The earliest trilobites known from the fossil record are fallotaspids, and redlichiids (both order Redlichiida)

All Olenellina lack facial sutures (see below), and this is thought to represent the original state. The earliest sutured trilobite found so far (Lemdadella), occurs almost at the same time as the earliest Olenellina however, implying that trilobite origin lies before the start of the Atdabanian, but without leaving fossils. Other groups show secondary lost facial sutures, such as all Agnostina and some Phacopina. Another common feature of the Olenellina also suggests this suborder to be the ancestral trilobite stock: early protaspid stages have not been found, supposedly because these were not calcified, and this also is supposed to represent the original state. Earlier trilobites may be found and could shed more light on the origin of trilobites.

Early trilobites show all the features of the trilobite group as a whole; there do not seem to be any transitional or ancestral forms showing or combining the features of trilobites with other groups (e.g. early arthropods).

Morphological similarities between trilobites and early arthropod-like creatures such as Spriggina, Parvancorina, and other “trilobitomorphs” of the Ediacaran period of the Precambrian are ambiguous enough to make detailed analysis of their ancestry far from compelling. Morphological similarities between early trilobites and other Cambrian arthropods (e.g. the Burgess Shale fauna and the Maotianshan shales fauna) make analysis of ancestral relationships difficult.

However, it is still reasonable to assume that the trilobites share a common ancestor with other arthropods before the Ediacaran-Cambrian boundary. Evidence suggests that significant diversification had already occurred before trilobites were preserved in the fossil record, easily allowing for the “sudden” appearance of diverse trilobite groups with complex derived characteristics (e.g. eyes).

For such a long-lasting group of animals, it is no surprise that trilobite evolutionary history is marked by a number of extinction events where some groups perished while surviving groups diversified to fill ecological niches with comparable or novel adaptations. Generally, trilobites maintained high diversity levels throughout the Cambrian and Ordovician periods before entering a drawn-out decline in the Devonian culminating in final extinction of the last few survivors at the end of the Permian period.

Principal evolutionary trends from primitive morphologies (e.g. eoredlichids) include the origin of new types of eyes, improvement of enrollment and articulation mechanisms, increased size of pygidium (micropygy to isopygy) and development of extreme spinosity in certain groups. Changes also included narrowing of the thorax and increasing or decreasing numbers of thoracic segments. Specific changes to the cephalon are also noted; variable glabella size and shape, position of eyes and facial sutures & hypostome specialization. Several morphologies appeared independently within different major taxa (e.g. eye reduction or miniaturization).

Effacement is also a common evolutionary trend. It is the loss of surface detail in the cephalon, pygidium, or the thoracic furrows. Notable examples of this were the orders Agnostida, Asaphida, and the suborder Illaenina of Corynexochida. It is believed that effacement is an indication of either a burrowing lifestyle or a pelagic one. Effacement poses a problem for taxonomists since the loss of details (particularly of the Glabella) can make the determination of phylogenetic relationships difficult.

Phylogenetic biogeographic analysis of Early Cambrian Olenellidae and Redlichidae suggests that a uniform trilobite fauna existed over Laurentia, Gondwana and Siberia before the tectonic breakup of the super-continent Pannotia between 600 million years ago and 550 million years ago.Tectonic breakup of Pannotia then allowed for the diversification and radiation expressed later in the Cambrian as the distinctive olenellid province (Laurentia, Siberia and Baltica) and the separate Redlichid province (Australia, Antarctica and China). Breakup of Pannotia significantly predates the first appearance of trilobites in the fossil record, supporting a long and cryptic development of trilobites extending perhaps as far back as 700 million years ago or possibly further.

Very shortly after trilobite fossils appeared in the lower Cambrian, they rapidly diversified into the major orders that typified the Cambrian—Redlichiida, Ptychopariida, Agnostida and Corynexochida. The first major crisis in the trilobite fossil record occurred in the Middle Cambrian; surviving orders developed isopygus or macropygius bodies and developed thicker cuticles, allowing better defense against predators (see Thorax above). The end Cambrian mass extinction event marked a major change in trilobite fauna; almost all Redlichiida (including the Olenelloidea) and most Late Cambrian stocks went extinct. A continuing decrease in Laurentian continental shelf area is recorded at the same time as the extinctions, suggesting major environmental upheaval.

The Early Ordovician is marked by vigorous radiations of articulate brachiopods, bryozoans, bivalves,

Phacopida and Trinucleioidea are characteristic forms, highly differentiated and diverse, most with uncertain ancestors. The Phacopida and other “new” clades almost certainly had Cambrian forebears, but the fact that they have avoided detection is a strong indication that novel morphologies were developing very rapidly.

Changes within the trilobite fauna during the Ordovician foreshadowed the mass extinction at the end of the Ordovician allowing many families to continue into the Silurian with little disturbance. Ordovician trilobites were successful at exploiting new environments, notably reefs. However, the end Ordovician mass extinction did not leave the trilobites unscathed; some distinctive and previously successful forms such as the Trinucleioidea and Agnostida became extinct.

The Ordovician marks the last great diversification period amongst the trilobites, very few entirely new patterns of organisation arose post-Ordovician; later evolution in trilobites was largely a matter of variations upon the Ordovician themes. By the Ordovician mass extinction vigorous trilobite radiation has stopped and gradual decline beckons.

Most Early Silurian families constitute a subgroup of the Late Ordovocian fauna. Few, if any, of the dominant Early Ordovician fauna survived to the end of the Ordovician, yet 74% of the dominant Late Ordovician trilobite fauna survived the Ordovician. Late Ordovician survivors account for all post-Ordovician trilobite groups except the Harpetida.

Silurian and Devonian trilobite assemblages are superficially similar to Ordovician assemblages, dominated by Lichida and Phacopida (including the well-known Calymenina). However, a number of characteristic forms do not extend far into the Devonian and almost all the remainder were wiped out by a series of drastic Middle and Late Devonian extinctions. Three orders and all but five families were exterminated by the combination of sea level changes and a break in the redox equilibrium (a meteorite impact has also been suggested as a cause). Only a single order, the Proetida, survived into the Carboniferous.

The Proetida survived for millions of years, continued through the Carboniferous period and lasted until the end of the Permian (when the vast majority of species on Earth were wiped out). It is unknown why order Proetida alone survived the Devonian. The Proetida maintained relatively diverse faunas in deep water and shallow water, shelf environments throughout the Carboniferous. For many millions of years the Proetida existed untroubled in their ecological niche. An analogy would be today’s crinoids, which mostly exist as deep water species; in the Paleozoic era, vast ‘forests’ of crinoids lived in shallow near-shore environments.

Exactly why the trilobites became extinct is not clear; with repeated extinction events (often followed by apparent recovery) throughout the trilobite fossil record, a combination of causes is likely. After the extinction event at the end of the Devonian period, what trilobite diversity remained was bottlenecked into the order Proetida. Decreasing diversity of genera limited to shallow water, shelf habitats coupled with a drastic lowering of sea level (regression) meant that the final decline of trilobites happened shortly before the end of the Permian mass extinction event. With so many marine species involved in the Permian extinction, the end of nearly 300 million successful years for the trilobite is hardly surprising.

The closest extant relatives of trilobites may be the horseshoe crabs, or the cephalocarids.

Trilobites appear to have been exclusively marine organisms, since the fossilized remains of trilobites are always found in rocks containing fossils of other salt-water animals such as brachiopods, crinoids, and corals. Within the marine paleoenvironment, trilobites were found in a broad range from extremely shallow water to very deep water. Trilobites, like brachiopods, crinoids, and corals, are found on all modern continents, and occupied every ancient ocean from which Paleozoic fossils have been collected. The remnants of trilobites can range from the preserved body to pieces of the exoskeleton, which it sheds in the process known as ecdysis. In addition, the tracks left behind by trilobites living on the sea floor are often preserved as trace fossils.

There are three main forms of trace fossils associated with trilobites: Rusophycus; Cruziana & Diplichnites – such trace fossils represent the preserved life activity of trilobites active upon the sea floor. Rusophycus, the resting trace, are trilobite excavations involving little or no forward movement and ethological interpretations suggest resting, protection and hunting.Cruziana, the feeding trace, are furrows through the sediment, which are believed to represent the movement of trilobites while deposit feeding. Many of the Diplichnites fossils are believed to be traces made by trilobites walking on the sediment surface. However, care must be taken as similar trace fossils are recorded in freshwater and post Paleozoic deposits, representing non-trilobite origins.

Trilobite fossils are found worldwide, with many thousands of known species. Because they appeared quickly in geological time, and moulted like other arthropods, trilobites serve as excellent index fossils, enabling geologists to date the age of the rocks in which they are found. They were among the first fossils to attract widespread attention, and new species are being discovered every year.

A famous location for trilobite fossils in the United Kingdom is Wren’s Nest, Dudley in the West Midlands, where Calymene blumenbachi is found in the Silurian Wenlock Group. This trilobite is featured on the town’s coat of arms and was named the Dudley Bug or Dudley Locust by quarrymen who once worked the now abandoned limestone quarries. Llandrindod Wells, Powys, Wales, is another famous trilobite location. The well-known Elrathia kingi trilobite is found in abundance in the Cambrian age Wheeler Shale of Utah.

Spectacularly preserved trilobite fossils, often showing soft body parts (legs, gills, antennae, etc.) have been found in British Columbia, Canada (the Cambrian Burgess Shale and similar localities); New York State, U.S.A. (Ordovician Walcott-Rust quarry, near Russia, and Beecher’s Trilobite Bed, near Rome); China (Lower Cambrian Maotianshan Shales near Chengjiang); Germany (the Devonian Hunsrück Slates near Bundenbach) and, much more rarely, in trilobite-bearing strata in Utah (Wheeler Shale and other formations), Ontario, and Manuels River, Newfoundland and Labrador.The French palaeontologist Joachim Barrande (1799–1883) carried out his landmark study of trilobites in the Cambrian, Ordovician and Silurian of Bohemia, publishing the first volume of Système silurien du centre de la Bohême in 1852.

When trilobites are found, only the exoskeleton is preserved (often in an incomplete state) in all but a handful of locations. A few locations (Lagerstätten) preserve identifiable soft body parts (legs, gills, musculature & digestive tract) and enigmatic traces of other structures (e.g. fine details of eye structure) as well as the exoskeleton.

Trilobites range in length from 1 millimetre (0.04 in) to 72 centimetres (28 in), with a typical size range of 3–10 cm (1.2–3.9 in). The world’s largest trilobite, Isotelus rex, was found in 1998 by Canadian scientists in Ordovician rocks on the shores of Hudson Bay.

The exoskeleton is composed of calcite and calcium phosphate minerals in a protein lattice of chitin that covers the upper surface (dorsal) of the trilobite and curled round the lower edge to produce a small fringe called the “doublure”. Three distinctive tagmata (sections) are present: cephalon (head); thorax (body) and pygidium (tail).

As might be expected for a group of animals comprising c. 5,000 genera, the morphology and description of trilobites can be complex. However, despite morphological complexity and an unclear position within higher classifications, there are a number of characteristics that distinguish the trilobites from other arthropods: a generally sub-elliptical, dorsal, chitinous exoskeleton divided longitudinally into three distinct lobes (from which the group gets its name); having a distinct, relatively large head shield (cephalon) articulating axially with a thorax comprising articulated transverse segments, the hindmost of which are almost invariably fused to form a tail shield (pygidium). When describing differences between trilobite taxa, the presence, size, and shape of the cephalic features are often mentioned.

During moulting, the exoskeleton generally split between the head and thorax, which is why so many trilobite fossils are missing one or the other. In most groups facial sutures on the cephalon helped facilitate moulting. Similar to lobsters and crabs, trilobites would have physically “grown” between the moult stage and the hardening of the new exoskeleton.

A trilobite’s cephalon, or head section, is highly variable with a lot of

morphological complexity. The glabella forms a dome underneath which sat the “crop” or “stomach”. Generally the exoskeleton has few distinguishing ventral features, but the cephalon often preserves muscle attachment scars and occasionally the hypostome, a small rigid plate comparable to the ventral plate in other arthropods. A toothless mouth and stomach sat upon the hypostome with the mouth facing backwards at the rear edge of the hypostome.

Hypostome morphology is highly variable; sometimes supported by an un-mineralised membrane (natant), sometimes fused onto the anterior doublure with an outline very similar to the glabella above (conterminant) or fused to the anterior doublure with an outline significantly different from the glabella (impendent). Many variations in shape and placement of the hypostome have been described. The size of the glabella and the lateral fringe of the cephalon, together with hypostome variation, have been linked to different lifestyles, diets and specific ecological niches.

The anterior and lateral fringe of the cephalon is greatly enlarged in the Harpetida, in other species a bulge in the pre-glabellar area is preserved that suggests a brood pouch. Highly complex compound eyes are another obvious feature of the cephalon.

Facial or Cephalic sutures are the natural fracture lines in the cephalon of trilobites. Their function was to assist the trilobite in shedding its old exoskeleton during ecdysis (or molting).

All species assigned to the suborder Olenellina, that went extinct at the very end of the Early Cambrian (like Fallotaspis, Nevadia, Judomia, and Olenellus) lacked facial sutures. They are believed to have never developed facial sutures, having pre-dated their evolution. Because of this (along with other primitive characteristics), they are thought to be the earliest ancestors of later trilobites.

Some other later trilobites also lost facial sutures secondarily. The type of sutures found in different species are used extensively in the taxonomy and phylogeny of trilobites.

The dorsal surface of the trilobite cephalon (the frontmost tagma, or the ‘head’) can be divided into two regions – the cranidium and the librigena (“free cheeks”). The cranidium can be further divided into the glabella (the central lobe in the cephalon) and the fixigena (“fixed cheeks”). The facial sutures lie along the anterior edge, at the division between the cranidium and the librigena.

Trilobite facial sutures on the dorsal side can be roughly divided into five main types according to where the sutures end relative to the genal angle (the edges where the side and rear margins of the cephalon converge).