Trilobites (Class Trilobita) (Cambrian – Permian)

Probably the most popular invertebrate fossils. They are extinct arthropods, living in the sea with calcified dorsal exoskeleton and they grow by molting. Sometimes it’s possible to find specimens in protection position, wound on themselves.

Morphology

The calcite carapace of trilobites (Fig.1) is divided in three longitudinal lobes, one on each side (pleural lobes) and one in the center (axial lobe).

Fig.1: Trilobite morphology

Trilobite carapace may be divided vertically also:

• Cephalon: head shield consists in front segments fused. The dorsal surface typically has a raised central hump called glabella, which is the front end of the axial lobe and it may have different shapes. The corners of the cephalon are known as the genal angle, and they may bear long genal spines (they have an important rule during molting). When a trilobite molted, its cephalon broke apart long a facial suture, that runs from the front of the cephalon and then posteriorly. There are three types of sutures (Fig.2): opisthoparian (the line of the suture ends behind the genal angle, on the posterior border of cephalon), proparian (suture emerges in front of the genal angle) and gonatoparian (suture splits the genal angle). Some specimens have eyes, but there are some trilobites without eyes.

• Thorax: divided in segments with articulating thoracic appendages.

• Pyigidium: tail segments fused. It may have different shapes.

Fig.2: Types of facial suture

Trilobites in thin section

Trilobite carapace is composed of calcite crystals oriented perpendicularly to the wall of the shell. They exhibit undulose extinction patterns. In some cases, they exhibit different layers in their shell. Usually, in thin section there are fragments of thoracic appendages.

Uses in Paleontology

Their small stratigraphic range make them useful tools for biostratigraphy and for paleoecology. They are used for evolution studies, too (in particular their ontogeny).

page created by Dalila Grilli (Department of Earth Sciences, Milan, Italy)

Fossile di Trilobite

Bibliography

• Prothero D.R., Bringing Fossils to Life, 2004 – pp. 255 – 258

Photo
Trilobite fossil, cut perpendicularly to its elongation. PPL image, 2x (Field of view = 7mm)	Trilobite fossil (yellow arrow), cut perpendicularly to its elongation. PPL image, 1,25x (Field of view = 9mm)	Trilobite fossil (yellow arrow), cut perpendicularly to its elongation. PPL image, 2x (Field of view = 7mm)
Trilobite fossil, cut perpendicularly to its elongation. PPL image, 2x (Field of view = 7mm)	Trilobite fossil, cut perpendicularly to its elongation. PPL image, 10x (Field of view = 2mm)	Trilobite fossil, cut perpendicularly to its elongation. XPL image, 10x (Field of view = 2mm)
Trilobite fossil (yellow arrow), cut perpendicularly to its elongation. PPL image, 2x (Field of view = 7mm)	Trilobite fossil, cut perpendicularly to its elongation. PPL image, 10x (Field of view = 2mm)	Trilobite fossil, cut perpendicularly to its elongation. XPL image, 10x (Field of view = 2mm)
Trilobites fossil (yellow arrows). PPL image, 2x (Field of view = 7mm)	Trilobite fossil, cut perpendicularly to its elongation. PPL image, 2x (Field of view = 7mm)	Trilobite fossil, cut perpendicularly to its elongation. PPL image, 10x (Field of view = 2mm)
Trilobites Fragment (yellow arrows). PPL image, 2x (Field of view = 7mm)	Trilobites Fragment (yellow arrows). PPL image, 2x (Field of view = 7mm)	Trilobites Fragment (yellow arrows). PPL image, 2x (Field of view = 7mm)
Trilobites Fragment. PPL image, 2x (Field of view = 7mm)	Trilobites Fragment. PPL image, 2x (Field of view = 7mm)	Trilobites Fragment. PPL image, 2x (Field of view = 7mm)
Trilobites Fragment. PPL image, 10x (Field of view = 2mm)	Trilobites Fragment. PPL image, 10x (Field of view = 2mm)	Trilobites Fragment. PPL image, 10x (Field of view = 2mm)
Trilobites Fragment (yellow arrow). PPL image, 10x (Field of view = 2mm)	Trilobites Fragment (yellow arrow). PPL image, 10x (Field of view = 2mm)	Trilobites Fragment (yellow arrow). PPL image, 10x (Field of view = 2mm)
Trilobites Fragment with Manganese encrustation. PPL image, 2x (Field of view = 7mm)	Trilobites Fragment with Manganese encrustation. PPL image, 10x (Field of view = 2mm)	Trilobites Fragment (yellow arrow) with Manganese encrustation. PPL image, 2x (Field of view = 7mm)
Trilobites Fragment with Manganese encrustation. PPL image, 10x (Field of view = 2mm)	Trilobites Fragment with Manganese encrustation. PPL image, 10x (Field of view = 2mm)