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Trilobites:
Trilobites were arthropods that mainly lived on or in the bottoms of the oceans, although
some species probably swam around in the middle water layers. They evolved in the Cambrian
and became extinct in the Permian. More than 15,000 different species are known, varying
in length from 1 mm (0.04 in) to more than 60 cm (2 ft). They are called trilobites
because their body, lengthwise, consists of three lobes. Widthwise we can also clearly
distinguish three parts: the head (cephalon), the middle part (thorax), and the tail
(pygidium). Just like modern arthropods such as crabs and lobsters, trilobites threw off
their external chitin carapace (the so-called exoskeleton) when it became too small, after
which they grew a newer, bigger one. A lot of trilobite fossils that are found are not the
remains of the actual animal, but are the exoskeletons that, because of their hardness,
fossilized relatively easily. Trilobites had compound eyes (just like modern insects), and
were able to roll themselves up to protect their soft ventral side.
Bibliography:
Système silurien du centre de la Bohême. Vol. I, Recherches paléontologiques I:
Trilobites, J. Barrande, 1852, Czech National Museum (Prague, Czech Rep.)
Trilobites, R. Levi-Setti, 1984 (2nd ed.), University of Chicago Press
(Chicago, US)
Bohemian trilobites, M. Snajdr, 1990, Czech National Museum (Prague, Czech Rep.)
Trilobites: Fossils Illustrated, vol. 2, H.B. Whittington, 1992, Boydell Press (Ipswich,
UK)
Trilobite! Eyewitness to evolution, R. Fortey, 2001, Flamingo Publ. (London, UK)
Trilobites of New York, T.E. Whiteley, 2002, Cornell University Press (Ithaca, US)