Ordovician Period, 465 million years old, Tazzarine, Morocco, private collection from Netherlands.
Trilobites are extinct marine arthropods. They constitute an extinct class of arthropods, the Trilobita, made up of ten orders, over 150 families, about 5,000 genera, and over 20,000 described species. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (521 million years ago) and they flourished throughout the lower Paleozoic before slipping into a long decline, when, during the Devonian, all trilobite orders except the Proetida died out. The last extant trilobites finally disappeared in the mass extinction at the end of the Permian about 252 million years ago. Trilobites were among the most successful of all early animals, existing in oceans for almost 270 million years. This makes trilobites the single most diverse class of extinct organisms, and within the generalized body plan of trilobites there was a great deal of diversity of size and form. The smallest known trilobite species is under a millimeter long, while the largest include species from 30 to over 70 cm in length.
Trilobites had three body lobes, two of which lay on each side of a longitudinal axial lobe. The trilobite body was segmented and divided into three regions from head to tail: the cephalon, or head region, separated from the thorax, which was followed in turn by the pygidium, or tail region. Trilobites, like other arthropods, had an external skeleton, called exoskeleton, composed of chitinous material. For the animal to grow, the exoskeleton had to be shed, and shed trilobite exoskeletons, or portions of them, are fossils that are relatively common. Each trilobite body segment bore a pair of jointed appendages. The forwardmost appendages were modified into sense and feeding organs. Trilobites had compound eyes, consisting of a number of separate lenses. The number of lenses and the complexity of the eye structure varied enormously. Some trilobites had large, convex compound eyes (like a fly’s) with a large number of lenses, giving them a wide field of view forwards, backwards, sideways, upwards and even downwards, depending on the actual curvature of the eye. Other trilobites had much smaller eyes, with fewer lenses, giving them a more restricted view. Some trilobites were active predators, whereas others were scavengers, and still others probably ate plankton.
Flexicalymene is a genus of trilobites belonging to the order Phacopida, suborder Calymenina and Family Calymenidae. Flexicalymene specimens can be mistaken for Calymene, Gravicalymene, Diacalymene and a few other Calymenina genera. They are used as an index fossil in the Ordovician.
Trilobite (Flexicalymene) fossils are in good condition. Age-related wear, chip and fractures. Size approx. 8,5cm x 4,8cm and 8,6cm x 5,5cm. Sell as a set.
References, sources and citations:
A Guide to the Orders of Trilobites, A website devoted to understanding trilobites, Dr. Sam Gon III. (https://www.trilobites.info)
Britannica, The Editors of Encyclopaedia. "trilobite". Encyclopedia Britannica, 12 Oct. 2018. (https://www.britannica.com/animal/trilobite)
Trilobites, Discovering Geology - Fossils and geological time, British Geological Survey. (https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/trilobites)