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Trilobita

Trilobites belong to the arthropods (Arthropoda), and lived from the Cambrian to the Permian in the Paleozoicum. The group did not survive the mass extinction at the end of the Permian. The class trilobita is subdivided 9 orders, of which the Phacopida are arguably the most well known. These 9 orders are further subdivided into about 150 families holding a total of some 15000 species known today. Regularly, new (sub)species are being discovered. The trilobites can be called a successfull group and show a remarkably high morphological diversity. 

Gerastos catervus from the Belgian Middle Devonian (© F Lerouge)

 Morphology

The word 'trilobite' originates from the feit that the general body plan consists of three parts or lobes, two pleural lobes and a central axial lobe. Longitudinally too, the trilobite has three distinct parts; the head or cephalon, the segmented thorax and the tail or pygidium. The exoskeleton largely consists of calcium carbonate, and is relatively easily preserved. This makes for an extremely hard exoskeleton. Trilobites moulted several times during a life cycle, and this exuviae are also easily preserved. Hence, most of the trilobite fossils that are found, are actually moults. The main morphological parts are shown in the drawing below. 

Dorsal morphology of trilobites: main parts

The shape of trilobites is highly variable, indicative for a broad range of life habits. Some species show spectacular forms, bearing impressive spines. Also other forms of exoskeleton ornamentation ('prosopon') are documented, including spines, tubercles, pores and terraces amongst the most common. The percieved functions of these elements are obviously quite various in nature:

- spines: the most obvious function of spines is for defense.

- tubercles: can have various functions, from defense to fortification, or altering the friction coefficient of the exoskeleton, ...

- pores: very common. Presumably played potential roles in respiration, sensory perception or secretion. Some pores completely penetrate the exoskeleton.

- terrace lines: interesting and common form of prosopon. Terrace lines raise the friction coefficient in one direction, and are mostly found at the underside and mergins of the exoskeleton. It is assumed that this aids the trilobite not to sink in or to oppose against fine sediments.

This specimen - with preserved shell- shown pronounced tubercles on the glabella (coll. FL).

Trilobites have compound eyes. Several types are known:

- Holochroal: eyes are composed of many tightly packed lenses.

- Schizochroaal: eyes with a limited number of sophisticated lenses, separated by sclera.

- Abatochroal: specific type limited to one cambrian group of trilobites.

In some cases, the eyes are exquisitely preserved, showing sophisticated features. This goes in particular for the schizochroal eyes, a remarkable type developed only within the Phacopina. Many eyes have a second, curved lens, dramatically improving the eyesight by corecting various forms of aberrations.

  

Examples of a schizochroal eye (left) and a holochroal eye (right) (coll. FL).

The ventral morphology of trilobites is well-known, thanks to some remarkable finds from conservat-lagerstätten which are showing preservation of the delicate soft tissues, including the legs and gills, sensory appendages and traces of the digestive system. Such finds are known from, a.o., the Burgess shale in Canada, the Hunsrück schiefer in Germany, the Beecher beds in the USA, Chengjiang in China and the Fezouata shales in Morocco.

Each individual segment had two double appendages, a walking leg with attached gill system, respectively. The cephalon, and in some cases also the pygidium, carried a pair of flexible antennas, which obviously had a sensory function. Underneath the cephalon, there was a series of legs, of which the basal parts were placed around the mouth opening, forming part of a jaw apparatus. This was covered by the hypostome, a mouthplate-like part that was calcified like the carapce, and is regularly found as a fossil, either loose or associated with the trilobite.

 

Cross-section of the thorax

 

Living habits

Trilobites were exclusively marine animals. They varied in size between some millimeters to more than half a meter. The largest complete specimen found to this date, an individual Isotelus rex from Newfoundland, measures 72 cm. Through evolutionary history, trilobites inhabited a broad scale of ecological niches, with benthic, demersal and pelagic living habits. Some species were active hunters and had good eyesight. Others lived at great depth and showed secondary loss of eyesight.

Artist impression of Walliserops, carrying a remakable forked spine, the function of which remains to date a subject of speculation (© F Lerouge).

Many trilobites could enroll themselves as a defense mechanism. In doing so, the trilobite could cover all soft parts within the hard shell. Some species even have a lock system, where the cephalon and pygidium show notches and bulges that fit tightly. Once properly locked, it was near impossible for a predator to open these trilobites.

 Voorbeeld van een opgerolde trilobiet (Reedops) (coll. FL)

That some trilobits were prey is obvious from the many specimens showing some form of bite mark. Sometimes these marks show secondary healing, indicating the trilobite survived the attack. What predators caused these marks, is not always clear. The Ordovician and Devonian periods are marked by the emergence of Gnathostomata (jawed animals), including the armored fish. But also other invertebrates, including other trilobites but also sea scorpions, have hunted trilobites.

The combination of a hard exoskeleton that could evolve into many forms with flexible pleurae, making the trilobite mobile, was the key to their succes. The hard shell did not grow, and the trilobite had to shed this skin many times during a lifetime, like modern arthropods. To do this, in most cases the cephalon opened along the sutures, lines separating the cranidium from the librigena, enabling the trilobite to crawl out of the old skin. Losse molts are often found as fossils, and are easily recognised by the (slight) dissociation of the different parts, in particular the librigena. Sometimes these molts or exuviae are so numerous, that they form most of the rock.
 

A mass assemblage of exuviae (coll. FL)

 

 Typical molt: both librigena are missing, and the thorax segments are slightly dissociated, shifted and pulled together as the animal shedded the exoskeleton (coll.FL)

 

Also trace fossils of trilobites have been found, of which the most common is put in the form genus cruziana. These are crawling traces. Also resting trace are known (Rusophycus), and walking traces (Diplichnites).

 

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