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GEOLOGY | FOSSILS | KEY FOSSIL GROUPS

Dalmanite Trilobite Fossil Specimen of Dalmanites limulurus from the Silurian of New York.

TRILOBITES

Trilobites rank among the most important of early animals. Our knowledge of them has been gained from the study of their fossils, usually impressions left of their shells after burial in sediment which subsequently hardened into rock.

 

They appeared abruptly in the early part of the Cambrian Period, and came to dominate the Cambrian and early Ordovician seas. They are the earliest organisms which belong to the phylum Arthropoda, which today includes spiders, crabs, centipedes, lobsters and insects.  Trilobites were in decline well before they became extinct at the end of the Permian Period, about 250 mya.

The most distinctive characteristic is that arthropods have a hard exoskeleton, made of a tough natural polymer called chitin.  At points on the interior wall of the exoskeleton apodemes provide insertion points/anchorage for muscles. The trilobite exoskeleton is best described like a suit of armour where various independent plates move in relation to the movement of the animal.  To grow, the animal has to molt, to shed its exoskeleton.

 

Palaeontologists know that trilobites were marine animals because of the rocks in which they are found and the other types of fossils associated with them.

Three is a magic number...

Trilobites get their name from the tripartite arrangement of their body from side to side.  They have three lobes, a central (axial) lobe and two pleural lobes adjoining each side of the axial.  

The head

From anterior to posterior they are divided into the cephalon (head shield), the thorax and the pygidium (tail shield). Side to side across the body are divided into the left pleural lobe, axial lobe and right pleural lobe.

On the ventral surface the cephalon consists of a raised structure called the glabella which is delineated from the adjacent flatter cheeks by a groove known as the axial furrow.  

 

The posterior edges of the cheeks are known as the genal angles, which may in some species be drawn out into genal spines. The cheeks are divided into the inner fixed cheeks (fixigena) and outer free cheeks (librigena), separated by facial sutures.  

 

The glabella and the fixigena together are known as the cranidium.

 

What are facial sutures? 
They are lines on the cephalon along which the parts of the cephalon separate when, during ecdysis,  the trilobite moults its exoskeleton. They typically run from a point on the anterior edge of the cephalon, toward and around the edge of the eye, and continue from there to end at point towards the posterior edge. There are three main categories of facial suture types (proparian, gonatoparian, and opisthoparian). As the configuration of the sutures is species specific, facial sutures are a key tool used by palaeontologists when identifying finds.

The compound eyes are usually prominent and are made of a number discrete calcite lens.  The eyes are located on the inner side of the librigena and rest against a ridge, part of the fixigena, called the palpebral lobe.  The facial suture runs between the eye and the palpebral lobe.

The lateral edge of the cephalon is folded over to continue on the ventral side, forming a narrow rim called the cephalic doublure. A central plate, the hypostome, lies in front of the mouth on the ventral side.  The unbranched antennae originates from the lateral sides of the hypostome.  Three or more pairs of head legs are attached to the cephalon - behind the mouth, which probably had a sensory and feeding function.

The body

The thorax consists of a number (2-40) of mobile, loosely-jointed segments which in some cases allowed the animal to enrol its body into a ball (like a woodlouse). Each segment has a central axial ring, separated by an axial furrow, from a lateral rib-like pleuron.  Appendages were attached to a pair of branched projections (apodemes) on the ventral side of each pleuron.  On the upper branch a feathery gill for gas exchange, would be anchored.  The lower branch would provide the insertion for the muscles of a jointed leg, used for walking or swimming.

The tail

The posterior part of the exoskeleton is formed by a single segment (some Cambrian species) or a series of fused segments.  Paleontologists can determine that the pygidium was a number of fused segments by the smooth form of the rim running along the edge of the tailpiece. The anus opens towards the back of the pygidium.

Trilobite Morphology.001.jpeg

EXOSKELETON

  • Provides protection and support;

  • Provides attachment for muscles;

  • There are three layers: an outer waxy surface, one of chitin and one of protein.

  • The central layer may be augmented with calcium carbonate for strength.

  • May have sensory hairs emerging from the exoskeleton to detect environmental changes eg water pH, temperature.

Cephalon

The semi-circular anterior or head region of a trilobite which consists of at least five fused segments. Some of the key features of the cephalon are:

  • The Glabella (the raised median part of the of the cephalon). The glabella varies dramatically in size and form from species to species.

  • The Compound Eye consisted of many rows of rounded dome-like lenses and fortified by calcite. 

HoW did they feed?

Trilobites are unlikely to have had complex mouthparts. Running along the ventral surface (sternite) was a channel running between the basal structures (coxae) of the appendages. The bristles on the coxae were particularly dense and sharp, forming a gnathobase (jaw-base), which extended into the the channel. The jaw-base effectively occupied the length of the body behind the cephalon.  Food would have been broken down and transported, by a

paddling action of the bristles, forward to the mouth.

Most trilobites were probably not active hunters, but more likely deposit-feeders - scavenging large pieces of organic material if the opportunity presented, or by mining the sediments, ingesting simultaneously organic detritus and mud.

Some species may have been predators, suspension feeders, grazers and some it has been suggested may have evolved complex symbiotic relationships with with sulfur-eating bacteria living on their gills.