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What are trilobites anyway ?

Trilobite Cheirurus Surely, this would be an excellent question to be asked in any popular quiz show, as most people are not acquainted with this subject at all (not even quiz masters! <g>). It is quite common for trilobite collectors to be confronted with irritated glances when talking about their field of study. But we are determined to do something about that and revive these little bugs in the hearts and minds of our fellow human beings. Therefore, please be presented with the minimum requirements in knowledge about trilobites for the average person:

Trilobites are dorsally armored animals with a segmented body plan which existed until approximately 250 million years before the present day and which played a major part in the ecology of early life in the ancient oceans. They became extinct way before the much more popular (especially with kids) dinosaurs conquered the planet, and their fossils – alongside other marine life forms - have been serving as stratigraphic indicators in sedimentary rocks of the Palaeozoic ever since. The start of the Palaeozoic marks the first era in which we can find a radiation of highly developed life forms – life forms which – according to most leading scientists – represent but a screenshot of a fast and furious evolution that seems to have started long before we can find the first trilobite fossils embedded in rocks. Life seems to be much older than could be imagined from the first appearance of trilobites.

Trilobite Scabriscutellum Trilobites belonged to the first hard-shelled arthropods, a stem of animals with a segmented body plan and in many cases multiple pairs of legs that worked in a coordinated way to propell the body in whatever direction. Unfortunately those legs (and antennae) are scarcely preserved as fossils because of their plain organic consistence in the living animal. (Under exceptional circumstances they were fossilized in one way or another and certain quarries yield such fossils – e. g. the famous Burgess Shale in British Columbia, Chenjiang in China and the Hunsruck Slates in the Southwest of Germany, where extremities are preserved as pyrite impressions.) They form the extinct class of the Trilobita, consisting of nine accepted orders, more than 150 families, over 5,000 genera and well exceeding 15,000 described species (as of 2002). Since there are no direct progenitors, the frequently used description in today’s dictionaries, decrying trilobites as an “ancient type of crab” is but completely false. Today we regard the recent Limulidae or horseshoe crabs as the most closely related existing life form.

First notes on trilobites: In 1698 a British clergyman, the reverend Dr. Edward Lhwyd, described a fossil which he believed to be the remains of a type of flatfish or flounder, and he mentioned this idea in a letter to his friend Martin Lister. Lhwyd had found a good number of these fossils in limestones near the town of Llandeilo in South Wales and had drawn sketches of them (see. ill.). But it soon became clear that his fossils did not represent a common and disdainful ancient flounder but a new and as yet unknown type of marine life form. His “flatfish”, which can be found in abundance in Ordovician rock formations in Wales, turned out to be remains of a creature which would soon become known as “trilobite”, and in 1822 it received the melodious name Ogygiocarella debuchii (for the scholars of Greek mythology: This trilobite genus is named after Ogygia, the seventh daughter of Amphion and Niobe and not after the island on which the brave Ulysses continued to prove his manhood to the sensible nymph Calypso for seven sultry years).

"Flounder" Ogygiocarella In the following years more and more of these fossils were excavated – collectors now knew what they had to look out for – and in the following century, once it had been realised that these aninmals had to be assigned to a a class of its own (WALCH, 1771), they quickly advanced to drawing the attention of many interested natural scientists and collectors. What was even more important: these fossils could be used to determine the age of the rocks they were found in.

More than 300 years after their first appearance in literature we still find new and describe as yet unknown species every single year, most of them from the northern parts of Africa. This makes trilobites one of the most diverse groups among all extinct life forms. And within their basic body plan there is an enormous variety in size and morphology.

Although dinosaurs may be the more famous of fossils, trilobites do belong to the most cherished of objects in palaeontology (the science of prehistoric life). The fascination they create has never been described more properly than by the author of one of the first popular books on trilobites:

"Trilobites tell me of ancient marine shores teeming with budding life, when silence was only broken by the wind, the breaking of the waves, or by the thunder of storms and volcanoes. The struggle of survival already had its toll in the seas, but only natural laws and events determined the fate of evolving life forms. No footprints were to be found on those shores, as life had not yet conquered land. Genocide had not been invented as yet, and the threat to life on Earth resided only with the comets and asteroids." - Riccardo Levi-Setti

Trilobite Destombesina Enormous diversity: The smallest known trilobite is just 1mm in length (Acanthopleurella sp.) while the largest reached more than 70 cms (Isotelus rex). In view of such eminent diversity in form and size we can only speculate on the different ecological roles different trilobites played, some of which lived as floating plancton while others lead a benthic or semi-benthic life, filtering sediment for food particles, scavenging or preying on other animals in the Palaeozoic seas.

Most trilobites reached only a few centimeters in length and it is obvious that their attractivity to a large extend has to be attributed to their handiness, making it possible to hold a complete specimen in the palm of your hand. Try that trick with your average carnivorous dino skeleton!

© Image of Scabriscutellum courtesy of PaleoDirect

The general trilobite body plan

Trilobite Independent of size and outer shape – many trilobites display a completely different and sometimes much more spectacular appearance than the example in our line drawing (many examples can be admired in our various galleries) – all fossilized trilobite remains follow a similar if not identical general body plan.

The calcified (hardened by mineralization) exoskeleton consists of three major parts: The cephalon or head shield, a segmented thorax (upper body or chest) and the pygidium (tail shield) as shown in our sketch.

Size and shape of the cephalon, the number of thoracic segments as well as the form and size of the tail shield vary considerably from species to species, but the general layout is predominant throughout the whole class.


All line drawings on this page ©1999, 2000 by S. M. Gon III

three lobes

Popular misconception: The name "trilobite" (three lobes) does, however, not refer to the three main parts that we described above, but reflects on the fact that all trilobites feature a long, median axial lobe (rhachis) which is usually separated by clearly visible furrows from the side or pleural lobes as shown on the right. The one median and the two side lobes, which extend over the whole length of the trilobite body are the characteristics that gave the trilobite its name and they are common in all species, regardless of their other morphological peculiarities and their different sizes. In fact there are trilobites in which this lateral segmentation seems to have all but vanished in the course of evolution (e. g. Homanolotus which looks like a fat worm rather than a trilobite). But this is just an effacement, a reduction in and flattening of morphological detail in the course of adapting to their specific habitats and their feeding habits.

In the thorax, the pleural lobes are formed by very agile and movable segments, connected in the living animal by what we believe was strong but flexible organic tissue (perhaps chitinous), which enabled most trilobites to enroll for protection when being attacked by a predator or when facing any other perilous situation. Many trilobites had tipped posterior appendages to their head shield, the so-called genal spines. Quite a few owned spiny extensions to their pleural segments, therefore called pleural spines.

Birth of a name: The name "trilobitae" (later amended to "trilobita") was first introduced in 1771 by the German professor Johann Ernst Immanuel Walch (* August 29, 1725 in Jena; † December 1,1778 in Jena; German theologian and geologist), but it took until the beginning of the 19th century for this name to prevail in the nomenclature of palaeontology (WALCH , J.E.I. 1771. Die Naturgeschichte der Versteinerungen zur Erläuterung der Knorrischen Sammlung von Merkwürdigkeiten der Natur. Nürnberg). The Swedish scientist G. Wahlenberg proposed the name "Entomostracites" in 1821 to denote the class and five years later his fellow countryman J. W. Dalman still preferred another name - "Palaeades". However it was Walch’s creation that finally made it through due to its obvious and direct relation to the distinct segmentation of the trilobite body.

More detailed descriptions of the trilobite body plan can be found on the following sub-page: Morphology.

All line drawings on this page ©1999, 2000 by S. M. Gon III

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Last Update : 12/12/2010 11:46 AM

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