Despite their rich fossil record with thousands of genera found throughout the world, the taxonomy and phylogeny of trilobites has 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.
Sensory organs
Many trilobites had eyes; they also had antennae that perhaps were used for taste and smell. Some trilobites were blind, probably living too deep in the sea for light to reach them. Others, such as Phacops rana, had eyes that were quite large.
The eyes of trilobites were made of calcite (calcium carbonate, CaCO3). Pure forms of calcite are transparent, and some trilobites used a single crystallographically oriented, clear calcite crystal to form each lens of each of their eyes. In this, they differ from most other arthropods, which have soft or chitin-supported eyes. The rigid calcite lenses of a trilobite eye would have been unable to accommodate to a change of focus like the soft lens in a human eye would; however, in some trilobites the calcite formed an internal doublet structure, giving superb depth of field and minimal spherical aberration, as rediscovered by Dutch physicist Christiaan Huygens many millions of years later. A living species with similar lenses is the brittle star Ophiocoma wendtii.
The trilobite eyes were typically compound, with each lens being an elongated prism. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in a single eye. In these compound eyes, the lenses were typically arranged hexagonally.
Extinction
The exact reason for the extinction of the trilobites is not clear, although it would seem to be no coincidence that their numbers began to decrease with the arrival of the first sharks and other early fish in the Silurian and Devonian periods with their strong, hinged jaws. Trilobites may have provided a rich source of food for these new animals.
Additionally, their relatively low numbers and diversity at the end of the Permian no doubt contributed to their extinction during that great mass extinction event. Foreshadowing this, the Ordovician mass extinction, though somewhat less substantial than the Permian one, also seems to have significantly narrowed trilobite diversity.
The closest extant relatives of trilobites may be the horseshoe crabs, according to Fortey (2000), or the cephalocarids, according to Lambert (1985).
Source: Wikipedia.Org
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