Trilobita sandbox

From Palaeos.org

Jump to: navigation, search
(a sandbox for trilobita - all are welcome to add and kibitz)
Line 8: Line 8:
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.
 +
 +
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:

Revision as of 16:38, 4 March 2007

Personal tools