Trilobita sandbox
From Palaeos.org
| TRILOBITA | |
| Taxonomy | Phylogeny |
o Arthropoda `--o TRILOBITA |?-Agnostina `--+--o Redlichiida | |--Olenellina | `--Redlichiina |--Corynexochida |--Phacopida |--Lichida `--o Librostoma |--Proetida |--Harpetida [Harpoidea] |--Ptychopariida |?-Eodiscina `--Asaphida |
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Contents |
[edit] Introduction
The trilobites are already highly diverse when they appear in the fossil record in the lower Cambrian, providing strong evidence that they branched from their soft-bodied arthropod ancestors well before Cambrian time. Two prominent morphological characteristics assured their survival and further diversity in the early Paleozoic seas, a well-mineralized exoskeleton, and a highly advanced visual system. They particularly expanded their presense during the Cambrian when the peak number of families existed.
[edit] Fossil Record
An extensive fossil record of Class Trilobita spans the Paleozoic Era from the Lower Cambrian and to the Upper Permian. As the earth changed and cycled, continents broke apart and drifted, and sea levels rose and fell, the trilobites underwent repeated declines and radiations, all the while leaving a detailed record in the form of exquisitely-preserved, mineralized dorsal exoskeletons. The record provides important data in geology and paleontology for the study of arthropod evolution, paleoenvironments, and plate tectonics. The huge and diverse fossil record has allowed the trilobites to become an exemplar for modern cladistic analysis.
[edit] 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?
[edit] Gallery of Images
 Peronopsis interstricta, an Agnostid trilobite from the middle Cambrian Wheeler Formation of Utah, USA. |
 Olenellus fowleri, a Redlichid trilobite from the lower Cambrian Pioche Shale Formation of Nevada, USA. |
 Trilobite of Order Lichida from "Systeme Silurian du Centre De La Boheme par Joachim Barrande" (circa 1852) |
 Homotelus florencevillensis, an Ordovocian Asaphid Trilobite from Iowa, USA (a good example of trilobite effacement)[[1]] |
 Illaenus schmidti var cardinalis, an Ordovician Corynexochid Trilobite the Wolchow River region near Saint Petersburg, Russia [[2]] |
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.
[edit] Trilobite Ancestors and Origins
The currently most accepted phylogeny has Trilobita as a Class within the Superclass Arachnomorpha, one of two Superclasses within the Subphylum Schizoramia of Phylum Arthropoda. By this phylogeny, trilobites are more closely related to the Chelicerates, than to Myriapods, Crustacea or Hexapods. Subphylum Schizomoria also contains Crustaceomorpha among whose members are primitive arthropods common to the Burgess shale and Chengjiang.
[edit] 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.
- Fortey RA 1990. Ontogeny, hypostome attachment and trilobite classification. Palaeontology 33:529-576.
- Fortey RA. 2000. Trilobite! Eyewitness to Evolution. HarperCollins, London.
- Fortey RA A. 2001. Trilobite systematics: The last 75 years. Journal of Paleontology 75:1141–1151.
- Kaesler RL, ed. 1997. Treatise on Invertebrate Paleontology, Part O, Volume 1, revised, Trilobita. Geological Society of America and University of Kansas Press, Lawrence, Kansas.
- Levi-Setti R 1993. Trilobites. University of Chicago Press, Chicago.
- A Guide to the Orders of Trilobites - website by Dr. Sam Gon III
- The Virtual Fossil Museum - Class Trilobita
- Zhang Xiguang & Pratt, B.R. 1999. Early Cambrian trilobite larvae and ontogeny of Ichangia ichangensis Chang, 1957 (Protolenidae) from Henan, China. J. Paleontol. 73: 117-128.
