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( Trilobite)
Subclass Librostoma Trilobites are very well-known, and possibly the second-most famous fossil group, after the dinosaurs. When trilobites appear in the fossil record of the Lower Cambrian they are already highly diverse and geographically dispersed. Because of their diversity and an easily fossilized exoskeleton, they left an extensive fossil record with some 17,000 known species spanning Paleozoic time. Trilobites have been important in biostratigraphy, paleontology, and plate tectonics research. For example, trilobites have been important in estimating the rate of speciation during the period known as the Cambrian Explosion because they are the most diverse group of metazoans known from the fossil record of the early Cambrian (Lieberman, 1999), and are readily distinguishable because of complex and well preserved morphologies. The trilobites are often placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha (equivalent to the Arachnata) (e.g., Cotton & Braddy 2004), although several alternative taxonomies are found in the literature. Different trilobites made their living in different ways. Some led a benthic life as predators, scavengers or filter feeders. Some swam (a pelagic lifestyle) and fed on plankton. Still others (particularly the family Olenidae) are thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food. Despite their rich fossil record with thousands of genera found throughout the world, the taxonomy and phylogeny of trilobites have many uncertainties. Nonetheless, the systematic division of trilobites into nine distinct orders is represented by a widely held view that will inevitably change as new data emerge. Except possibly for the members of order Phacopida, all trilobite orders appeared prior to the end of the Cambrian. Most scientists believe that order Redlichiida, and more specifically its suborder Redlichiina, contains a common ancestor of all other orders, with the possible exception of the Agnostina. While many potential phylogenies are found in the literature, most have suborder Redlichiina giving rise to orders Corynexochida and Ptychopariida during the Lower Cambrian, and the Lichida descending from either the Redlichiida or Corynexochida in the Middle Cambrian. Order Ptychopariida is the most problematic order for trilobite classification. In the 1959 Treatise on Invertebrate Paleontology, what are now members of orders Ptychopariida, Asaphida, Proetida, and Harpetida were grouped together as order Ptychopariida; subclass Librostoma was erected in 1990 by Fortey (1990) to encompass all of these orders, based on their shared ancestral character of a natant (unattached) hypostome. The most recently recognized of the nine trilobite orders, Harpetida, was erected in 2002. The progenitor of order Phacopida is unclear.
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