Trilobites

TRILOBITES

Morphology

As the name trilobite suggests, it's skeleton is made up of three longitudinal lobes, a axial lobe which protect digestive system and two pleural lobes which cover appendages. Almost in all trilobites, cephalone, thorax and pygidium are developed.

Cephalon

Cephalon has following features

Dorsal side

Glabella : Glabella consists a series of furrows. Furrows were disappeared with the evolution.

Cheeks : Fixigena is fixed cheek which is on inner side while Librigena is free cheek, present on outer side.

Cardinium : It is glabella plus fixigena.

Eyes : Trilobites had most ancient visual system. There were three types of eyes found 

Holochroal eyes 

More than 1500 lenses present.

Lenses were closely packed.

Hexagonal appearance of lenses.

One cornea for all.

Sclera absent.

Schizochroal eyes

Around 700 lenses.
Separated lenses.
One cornea for each lens.
Sclera present.

Present in Phacops.

Blind trilobites

Some early forms like Agnostus, Microdiscus and Conocoryph were blind. Trinucleus had distinct facial sutures, sometimes along the edges of cephalon which made eyes absent in it. 

Facial sutures : These sutures separates fixigena and librigena. Olenellus doesn't have facial sutures. Three main types of facial sutures were

Proparian - Forward of genel angle. Ex - Phacops

Gonatoparian - From genel angle. Ex. Calymene

Opisthoparian - Adaxial to genel angle. Ex. Ptychopariina

Role of facial sutures in Molting or Ecdysis

Trilobites grew by shedding their old, rigid exoskeleton in a process called ecdysis, or molting.  They underwent this process several times during their life time. During this process, the facial sutures separates librigena, the free cheeks, by cardinium. 

Ventral side

Doublure : This is hell continuation under trilobite, present on edges of ventral side of cephalone.

Rostral plate or rostrum : Middle portion of Doublure.

Hypostome : It is a plate of variable size and shape, sited under glabella. On the basis of its attachment and alignment, three types have been classified

Natant - Hypostome free of anterior doublure,

Conterminant - Hypostome attached to anterior doublure and aligned with anterior edge of glabella.

Impedent - Hypostome attached to anterior doublure, but not aligned with the glabella.

Thorax

Thorax is consisted of series of segments which are movable upon each other. List of trilobites with number of thoracic segments is given below

Pygidium

It is tail of trilobites, composed of segments which are immovable. In primitive forms, it was absent like in Nevadia. Then pygidium was developed and varied in size on which trilobites classified as following

Micropygous : Very small than cephalon. Ex. Paradoxide, Phacops, Olenus.

Heteropygous : smaller than cephalon.

Isopygous : Same size as of cephalon. Ex. Agnostus

Macropygous : larger than cephalon. It is rare and found in Lichida. 

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Some important trilobites and their features are

Lower cambrian

Olenellus : 18 - 44 thoracic segments, only one pygidium and no dorsal sutures.

Other trilobites of this age were Limulus, Emuella and Redlichia.

Mid. Cambrian

Paradoxide : Large eyes, 20 - 22 thoracic segment, Micropygous, dorsal sutures present.

Upper Cambrian

Olenus : 15 thoracic segments, sutures present.

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Ordo - Silurian

Calymene : 13 Thoracic segments, 6-8 pygidium and maximum no. of sutures present.

Asaphus - 8 Thoracic segments, Isopygous.

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Ordo - Devonian

Phacops - 11 Thoracic segments, no rostral plate, schizochroal eyes, proparian sutures.

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Devonian - Permian

Phillipsia - 9 Thoracic seg, 12 pygidium segments.

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