Introduction[1]
P. davidis is a species of the extinct class trilobita that lived during the Cambrian period. It was among the largest of all trilobites, upwards of half a meter long[2]. Trilobites in general are readily recognized even in popular culture due to their exoskeletons being so readily preserved as fossils (due to their high mineral content, including calcium carbonate).
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Kingdom: Animalia
Superphylum: Ecdysozoa Phylum: Arthropoda Class: †Trilobita Order: †Redlichiida Suborder: †Redlichiina Family: †Paradoxididae Genus: †Paradoxides Species: †P. davidis |
Morphology
Trilobite bodies of all species are divided into three tagmata (Cephalon, Thorax, Pygidium) and three lobes (second image, hence the name, three + lobe). A further segmented thorax (14-21 spines in P. davidis) allowed trilobites to roll up into a defensive position, protecting the antennae, mouth, limbs, and soft ventral surface, yet still allowing the trilobite to see. Many trilobites had pygidiums that interlock neatly with features on the celphalon that aided in enrollment.
The majority of trilobites had compound eyes, which gave them extreme sensitivity to motion, likely helping in detecting predators. Some trilobites evolved and subsequently lost eyes, and at least one species is thought to have been completely eyeless. Recreations of trilobite internal organ systems is fairly poorly understood, relying on knowledge of living arthropod anatomy and presuming that trilobites had similar systems. |
Left: tagmata; Right: lobes
(diagrams do not picture P. davidis) |
Feeding
Trolibites have a ventral mouth (as opposed to anterior), which probably let early trilobites such as P. davidis feed on smaller invertebrates on the sea floor (benthic zone). However, a large number of other trilobites are hypothesized to be benthic particle feeders, filter feeders in the open ocean, or benthic filter feeders.
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Reproduction
As other arthropods do, trilobites are believe to have reproduced sexually and laid eggs. However, only one instance of fossilized eggs has been found. One hypothesis is that trilobites may have held eggs in the celphalon (as horseshoe crabs do), and in some species there are swellings that support the idea. Details of embryo development are unknown. As hatched trilobites mature, they develop more and more segments in the thorax and pygidium.
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Molting
As ecdysozoans, trilobites molt several times during their lives, and they have a number of adaptations helpful to the process. Sutures cover the face of the skeleton that split open to create an exit for the creature. The highly segmented thorax allows the body to arch, as the trilobite plants its posterior spines, along with the rostral plate (chin) into the mud floor to anchor the exoskeleton.
Left: diagram and fossil evidence of molting, showing the facial sutures unlocked; Below: P. davidis molting
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Evolution & Extinction
Trilobites as a whole lived from the early Cambrian to the permian extinction, about 525-250 Mya or basically all of the Paleozoic era, but trilobite diversity dwindled severely up until the end of the Permian, leaving only 5 species for the extinction event. P. davidis was extinct before making it out of the Cambrian. Early arthropods like trilobites are suspected to have evolved from the lobopods[3, p684] or from Parvancorina[1].
References
[1]http://www.trilobites.info (vast majority of information on this page)
[2]http://fieldmuseum.org/explore/trilobite-paradoxides
[3]Campbell & Reece, Biology, 8th Edition
[2]http://fieldmuseum.org/explore/trilobite-paradoxides
[3]Campbell & Reece, Biology, 8th Edition