Trilobites / Trilobita: Morphology, Geological History and Evolutionary Trends, Stratigraphic utility of Trilobita

The class trilobita includes extinct forms which were highly evolved invertebrates though their life span was restricted to Paleozoic era.
They were benthic animals which crept or swimmed along the bottom and fed on small organisms and debris in the bottom mud.

Morphology

Body is divided into three parts from posterior to anterior and hence the name.
The transverse division of the body divided animal into three regions: Cephalon (head), Thorax (body) , Pygidium (tail segments).

Cephalon is made up of six fused segments. The median part of the cephalon is inflated and is termed as glabella and the lateral are the Cheeks.
The cheeks are triangular in shape and divided by Facial suture. The shape of facial suture vary in different forms. They are of following five types:
- Protoparian – suture is marginal . free and fixed cheeks . e.g – Agnostus
- Hypoparian – similar to above but eyes are absent e.g – Microdiscus
- Opiathoparian – suture commences on the posterior margin inside the genal angle and is present on top
- Proparian – The suture starts from posterior or lateral margin and end at anterior lateral margin . eg – Phacops
- Gonatoparian – facial suture is intermediate between opisthoparia and proparia

Trilobites appeared at the very beginning of Cambrian, and this causes difficulty for palaeontologist to make any suggestions about their origin and ancestry.
Arising from some annelid ancestry , trilobite appeared for the first time in lower Cambrian marine beds and very soon become most dominant animal of that period.
Cambrian- Ordovician is considered to be the climax of this animals.They become totally extinct at the end of Permian , possibly failed to compete with the incoming fishes and large cephalopods.

Structural changes in Glabella – All the furrows except the last glabella furrow disappeared and finally the last furrow also disappeared giving rise to smooth and furrowless glabella for easy burrowing . The general outline of the cephalon changes with the change in the shape of the glabella .
Changes in Eyes – Modification in eyes were required for adaptation to differing requirements of sight which may be low light intensity in deep water condition , burrowing mode of life and intense light in shallow water conditions . The first two conditions lead to the loss of eyes by disintegration .
Changes in facial suture – in early forms there were no facial suture , free cheeks and eyes . Gradually protoparian and hypoparian facial suture developed .
Changes in Thorax –Trilobita have evolved from annelid group . thus the primitive trilobites have large number of thoracic segments
Spine – Except in Agnostida generally all other forms including swimming and free floating forms possess the spines.

Trilobites are of considerable importance as regards the zonation of Cambrian-Ordovician rocks and are successfully used as a good zone fossils in many countries.
Almost everywhere in marine sequence their first appearance usually marks the beginning of Lower Cambrian.