Phylum Arthropoda / Class Trilobita

Overview

Trilobites are easily the most recognized fossil invertebrates. Their name derives from Greek meaning 'three-lobed'. While trilobite morphology is divided into three parts: the cephalon (head), thorax (abdomen) and pygidium (tail), the three lobes referred to in the name are actually the axial lobes, divided by grooves more or less running the length of the body.

Trilobites are plentiful in Cambrian rocks; they are likely to be found in greater abundance than any other invertebrate. Although their range stretches to the Permian, their diversity sharply decreased after the Ordovician and then gradually declined until their extinction 225 million years ago.

Skeleton

The image below is Paradoxides from the Museum's collections:

The cephalon, or head region, of trilobites underwent rapid evolution. Because of this, many features can be of use in taxonomy. At the center of the cephalon is the glabella. Behind it is the occipital ring which adjoins to the first of the axial rings of the thorax. Between the occipital ring and the glabella is the occipital furrow. The glabella is often subdivided by additional furrows in many species; these furrows are the external representations of internal segmentation and muscle attachment points.

The eyes are placed on either side of the glabella. Some early Cambrian species sported simple eyes while others had compound eyes. Because of this rapid appearance of compound eyes, their evolution had been difficult to trace. Eyes are usually crescent-shaped and may be placed on ridges or stalks.

Facial sutures allowed the cephalon to be opened. This was essential to the molting process, which was the only way trilobites (and all arthropods with a hard external skeleton) could grow. A suture running from the front of the cephalon to the side or back edge opened during molting, allowing the animal to crawl out of its hard case. At one point in their classification, the position of this suture was of great importance in figuring out their family tree, but it has been shown since that the facial suture is much less critical than previously thought. After the suture is opened, portions of the cephalon on either side of the glabella region can come free. These separated regions are called the free cheeks. The areas aside of the glabella (usually including the eyes) are the fixed cheeks.

Around the front edge of the cephalon is the doublure. It gives structural strength to the margin. Moving laterally around the margin from the doublure is the genal region (including the free cheeks in species with facial sutures). Often a genal spine is present at the posterior corner.

The number of segments of the thorax varied from two or three to a few dozen depending on the species. Each segment articulated freely, allowing the trilobite to roll up--presumably as protection. Segments were composed of a central axial ring with two lateral extensions of the skeleton called pleurae. Axial furrows divided segments.

The pygidium, or tail region, shows great diversity among species in shape, size, segmentation and ornamentation. The number of segments range from one to 30 or more. Unlike the thorax, these segments are fused into a single rigid shield.

Soft-Part Anatomy

Some trilobites have been preserved in black shale, their sulfur-containing soft parts replaced by fine crystals of pyrite. X-ray photos taken of these specimens have revealed many features of their internal anatomy that generally are not preserved otherwise. The digestive system has been detailed. Their musculature resembles primitive crustaceans. Most, if not all, segments supported at least one pair of appendages. A pair of antenna in front of the mouth was used for sensing. Several pairs of appendages behind the mouth were probably used in feeding. Each thoracic segment had two pairs of appendages. Like crustaceans, one pair was used for walking and the other in respiration, but may also have aided in swimming or gathering of food.

Classification

Classifying trilobites is a daunting task and one over which much study continues to take place. 140 families and over 10,000 species are recognized. Many classification schemes for trilobites still exist. One scheme is gainly popularity (though it was proposed almost 90 years ago) in which two orders exist: one containing trilobites with two or three thoracic segments and another for those with more than three segments. While economical on this level of the classification hierarchy, the latter order contains 95% of trilobite genera. For simplicity's sake, a complete trilobite classification is not presented here. In fact, none is presented here. (However, if I find a lot of spare time on my hands, I'll add the families and the periods they're found in. Until then, check out the trilobite book in the bibliography.)


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