Trilobita sandbox

From Palaeos.org

Jump to: navigation, search
(sandbox for trilobita)
Line 1: Line 1:
{{Trilobita}}
{{Trilobita}}
 +
==Introduction==
The '''trilobites'' are an extinct class of marine arthropods. By the time true trilobites appear in the fossil record, during the Cambrian, they are already highly diverse and possess two characteristics that likely served them well to survive in the Paleozoic seas, a well- mineralized exoskeleton, and a highly advanced visual system. They particularly flourished during the Cambrian when the peak number of families existed.
The '''trilobites'' are an extinct class of marine arthropods. By the time true trilobites appear in the fossil record, during the Cambrian, they are already highly diverse and possess two characteristics that likely served them well to survive in the Paleozoic seas, a well- mineralized exoskeleton, and a highly advanced visual system. They particularly flourished during the Cambrian when the peak number of families existed.
 +
==Fossil Record==
Class Trilobita left an extensive fossil record spanning the Paleozoic Era, beginning in the Lower Cambrian and extending to the Upper Permian. As the earth’s continental configuration changed markedly due to continental drift, the trilobites that had a mineralized dorsal exoskeleton left exquisitely preserved fossils. As the earth changed and cycled, continents broke apart and drifted, and sea levels rose and fell, the trilobites underwent repeated declines and radiations. The associated adaptations among the trilobites from progenitors to descendents are expressed in their complex exoskeletons and well recorded in the rock record. This record has proved valuable in the study of plate tectonics, and their huge diversity has enabled the trilobite to become an exemplar for modern cladistic analysis.
Class Trilobita left an extensive fossil record spanning the Paleozoic Era, beginning in the Lower Cambrian and extending to the Upper Permian. As the earth’s continental configuration changed markedly due to continental drift, the trilobites that had a mineralized dorsal exoskeleton left exquisitely preserved fossils. As the earth changed and cycled, continents broke apart and drifted, and sea levels rose and fell, the trilobites underwent repeated declines and radiations. The associated adaptations among the trilobites from progenitors to descendents are expressed in their complex exoskeletons and well recorded in the rock record. This record has proved valuable in the study of plate tectonics, and their huge diversity has enabled the trilobite to become an exemplar for modern cladistic analysis.
 +
==Taxonomy and Phylogeny==
Despite a rich fossil record dispersed both stratographically and across thousands of genera, taxonomy and phylogeny of the Class Trilobita remains problematic. A huge literature describes the systematic division of trilobites into nine distinct orders. Earlier work was largely based on stratigraphical approaches. Recent decades has benefited by cladistics approaches and parsimony analysis. Something close to a consensus has emerged with respect to placement of genera within families and superfamilies. Additionally, with the possible exception of Phacopida, all trilobite orders appeared prior to the end of the Cambrian. Higher level systematics among the orders and suborders is more problematic, that is, when and how did the seven orders, excluding Agnostida, descent from the Redlichiids or their progeny, mostly before the end of the Cambrian period?
Despite a rich fossil record dispersed both stratographically and across thousands of genera, taxonomy and phylogeny of the Class Trilobita remains problematic. A huge literature describes the systematic division of trilobites into nine distinct orders. Earlier work was largely based on stratigraphical approaches. Recent decades has benefited by cladistics approaches and parsimony analysis. Something close to a consensus has emerged with respect to placement of genera within families and superfamilies. Additionally, with the possible exception of Phacopida, all trilobite orders appeared prior to the end of the Cambrian. Higher level systematics among the orders and suborders is more problematic, that is, when and how did the seven orders, excluding Agnostida, descent from the Redlichiids or their progeny, mostly before the end of the Cambrian period?
 +
==Gallery of Images==
<gallery>
<gallery>
Image:Peronopsis-interstricta.jpg|Peronopsis interstricta, a [[Agnostid]] trilobite from the middle Cambrian Wheeler Formation of Utah, USA.
Image:Peronopsis-interstricta.jpg|Peronopsis interstricta, a [[Agnostid]] trilobite from the middle Cambrian Wheeler Formation of Utah, USA.
Line 18: Line 22:
An important continuing debate is whether or not Order Redlichiida is paraphyletic. Redlichiida has two suborders, Olenellina and Redlichiina that have an unresolved relationship. The Olenellids are differentiated by the lack of facial sutures, a distinction that in the past has led to arguments to exclude them from Class Trilobita. Stratigraphical data and cladistic analysis both support Fallotaspidoidea within Suborder Olenellina as the earliest trilobites, and that the many trilobite orders have a lineage tracing back to the Suborder Redlichiina, which must then be considered paraphyletic. Most phylogenies have Suborder Redlichiina giving rise to Orders Corynexochida and Ptychopariida during the Lower Cambrian. The Lichida are variously shown as having arisen from either the Redlichiida or Corynexochida in the Middle Cambrian. Order Ptychopariida remains, as it has always been, the most problematic order for trilobite classification. In the 1959 Treatise on Invertebrate Paleontology, what is now Orders Ptychopariida, Asaphida, Proetida, and Harpetida were grouped together as Order Ptychopariida, a huge and paraphyletic group; subclass Librostoma was erected in 1990 by Fortey (1990) to encompass these orders that are united at least in earlier forms by a natant hypostomal condition. The final reorganization of trilobita occurred when Ebach & McNamara (2002) raised Harpetida to order status because all members lack a rostral plate and have a marginal facial suture, and therefore can not be defined as Ptychopariida. Consequently, they raised Harpetida to ordinal status within the trilobite subclass Librostoma. Asaphida, Proetida, and Harpetida arose from Ptychopariida in the Cambrian. The origin of Order Phacopida is unclear. The three Phacopid suborders, Phacopina, Calymenina, and Cheirurina, are united by a unique protaspis characteristic. The Calymenina are likely the earliest Phacopids with characteristics that would ally them with the Ptychopariida, whereas other characteristics would ally Phacopida with Order Lichida.
An important continuing debate is whether or not Order Redlichiida is paraphyletic. Redlichiida has two suborders, Olenellina and Redlichiina that have an unresolved relationship. The Olenellids are differentiated by the lack of facial sutures, a distinction that in the past has led to arguments to exclude them from Class Trilobita. Stratigraphical data and cladistic analysis both support Fallotaspidoidea within Suborder Olenellina as the earliest trilobites, and that the many trilobite orders have a lineage tracing back to the Suborder Redlichiina, which must then be considered paraphyletic. Most phylogenies have Suborder Redlichiina giving rise to Orders Corynexochida and Ptychopariida during the Lower Cambrian. The Lichida are variously shown as having arisen from either the Redlichiida or Corynexochida in the Middle Cambrian. Order Ptychopariida remains, as it has always been, the most problematic order for trilobite classification. In the 1959 Treatise on Invertebrate Paleontology, what is now Orders Ptychopariida, Asaphida, Proetida, and Harpetida were grouped together as Order Ptychopariida, a huge and paraphyletic group; subclass Librostoma was erected in 1990 by Fortey (1990) to encompass these orders that are united at least in earlier forms by a natant hypostomal condition. The final reorganization of trilobita occurred when Ebach & McNamara (2002) raised Harpetida to order status because all members lack a rostral plate and have a marginal facial suture, and therefore can not be defined as Ptychopariida. Consequently, they raised Harpetida to ordinal status within the trilobite subclass Librostoma. Asaphida, Proetida, and Harpetida arose from Ptychopariida in the Cambrian. The origin of Order Phacopida is unclear. The three Phacopid suborders, Phacopina, Calymenina, and Cheirurina, are united by a unique protaspis characteristic. The Calymenina are likely the earliest Phacopids with characteristics that would ally them with the Ptychopariida, whereas other characteristics would ally Phacopida with Order Lichida.
 +
==Trilobite Ancestors and Origins==
The currently most accepted theory is that the Trilobita is a Class within the Superclass Arachnomorpha, one of two Superclasses within the Subphylum Schizoramia of the Phylum Arthropoda, and as such are more closely related to Chelicerata, than to Myriapoda, Crustacea or Hexapoda, including insects (see chart above). Subphylum Schizomoria also contains Crustaceomorpha among whose members are primitive arthropods common to the Burgess shale and Chengjiang.
The currently most accepted theory is that the Trilobita is a Class within the Superclass Arachnomorpha, one of two Superclasses within the Subphylum Schizoramia of the Phylum Arthropoda, and as such are more closely related to Chelicerata, than to Myriapoda, Crustacea or Hexapoda, including insects (see chart above). Subphylum Schizomoria also contains Crustaceomorpha among whose members are primitive arthropods common to the Burgess shale and Chengjiang.
-
References:
+
==References==
*Ebach, M.C. & K.J. McNamara. 2002. A systematic revision of the family Harpetidae (Trilobita). Records of the Western Australian Museum 21:135-67.
*Ebach, M.C. & K.J. McNamara. 2002. A systematic revision of the family Harpetidae (Trilobita). Records of the Western Australian Museum 21:135-67.

Revision as of 16:46, 4 March 2007

Personal tools