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PALAEONTOLOGY | The key fossil groups

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


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.  The interior wall of the exoskeleton acts as an 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.  


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 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.


  • 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.


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 


Structures of the thorax:


Structures of the Pygidium:

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