An Introduction to the Fossil Group Trilobita
A Basic Overview of the Earliest Known Arthropod Group
Dec 23, 2008
Appearing in the early Cambrian Period (542 million years ago), reaching highest diversity and distribution throughout the Cambrian and Ordovician Periods (488-444 million years ago), and finally becoming extinct in the great Permian extinction event (251 million years ago)
The Trilobite Body Plan
Over this 350 (approximately) million year range the body plan of the trilobite remained relatively unchanged, consisting of three regions; the head plate known as the Cephalon; a thorax; and a tail end known as the Pygidium. The Cephalon was a large shelly plate made up of several fused elements, located anteriorally and housed the sensory organs (antennae and eyes in some species), feeding apparatus (on ventral side) and is also thought to have contained the stomach in a lobed section of the head called the Glabella.
The thorax was comprised of a central column of articulated axial rings, along which a series of segments known as pluerons (number of which varied between species) articulated. Each pluerons housed a matching series of bifurcated structures that split into a jointed limb and a gill branch. It is also thought that the central column contained an intestinal canal, terminating in the tail section.
The final, tail segment or Pygidium was, like the Cephalon, a composite plate of fused shelly elements. Sizes of the Pygidium vary between species, but generally are smaller than (Heteropygous) or equal to (Isopygous) the size of the Cephalon. The highly articulated nature of the trilobite body meant that many could curl into a defensive position, and indeed many fossils have been found with the organism curled into a ball.
Trilobite species have a vast size range, with the largest species, Isotelus rex measuring approximately 72 cm in length and 40 cm in width; and the smallest being Bynumina lirae at just 1mm in length. The tests (shells) of some trilobites are adorned with spines and various ornamentation, of which can be exceedingly ornate, with sweeping and curling spines protruding from various points along the organisms bodies. It is likely that this spinosity was to discourage predators or serve as sexual attractors.
Trilobite Life Styles and Habitat
Trilobite fossils are exclusively found in marine deposits and as such are considered marine organisms. A variety of life styles can be inferred by their morphological differences and associated trace fossils. Generally, most trilobites were bottom dwellers (benthic), crawling along the sea bed in search for food (actively predating upon smaller invertebrates, scavenging particulate matter or grazing on algal mats). This mode of life is supported by the existence of trace fossils (literal traces of an organism..E.g. faeces, footprints, burrows..), preserved as crawling tracks (Cruziana) across fossilised sea bed. However some species of trilobite are thought to have been pelagic, that is being actively able to swim within the water column, suggested by their more streamlined, smaller bodies.
The Trilobite Eye, An Innovation
The trilobites where the first organisms to evolve visual sensory organs. Trilobite eyes are compound lenses and, like modern arthropods, are composed of radially arranged elements pointed in a variety of directions to maximise field of view. There are two types of eye format within the Trilobita: Holochroal and Schizochroal. Holochroal eyes are by far the most common and are comprised of many, small rounded or polygonal lenses which all touch at the edges and are covered by a continuous transparent cuticle. Schizochroal eyes are much rarer and comprise of a variety of large lenses separated from each other, set in an element called the sclera and covered by their own cuticle.
Source: E.N.K. Clarkson (1998) Invertebrate Palaeontology and Evolution. Blackwell Science Publishing.
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