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Paleozoic Paleobiology
Trilobite Paleobiology
Trilobite Evolutionary Arms Race
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Gon: The Redlichiida (particularly the Suborder Olenellina) is considered primitive, appearing in Series/Epoch 2 of the Cambrian, and not persisting into the latest Cambrian (Furongian). The Agnostida also appear early and persist to the end of the Ordovician. The Redlichiina give rise to at least the Corynexochida, and the Ptychopariida in the Cambrian. The Lichida may have arisen from early Corynexochida or Redlichiida, indicated with a "?". The Redlichiida, Corynexochida, and the Ptychopariida, as large primitive groups including the ancestors of other orders, must be paraphyletic, which is indicated by dashed lines between these three orders. In 2002, another order was split out of the Ptychopariida; the Order Harpetida (formerly suborder Harpina of the order Ptychopariida)

The Ptychopariida, Harpetida, Asaphida, and Proetida share (in at least the primitive forms) species with a natant hypostomal condition, leading Fortey to suggest the Subclass Libristoma for these orders combined. The recognition of Asaphida, Proetida, and Harpetida as orders is a relatively recent thing; in the 1959 Treatise they were all included within the very large and paraphyletic Ptychopariida. The Ptychopariida and Harpetida maintain the natant state until their extinction at the end of the Devonian, but both the Asaphida and the Proetida develop conterminant and impendent hypostomes in their advanced forms. The major extinction event at the end of the Ordovician greatly affected trilobites, ending the Olenina, Agnostida and the vast majority of Asaphida. The remainder of the Asaphida (superfamily Trinucleioidea) are lost before the end of the Silurian and all other orders except Proetida are lost by the end of the Devonian (most in the major extinction event between the Frasnian and Famennian ages in the Late Devonian, but the Phacopidae hang on until nearly the Devonian-Carboniferous transition). The Proetida persist until the end of the Permian, the last of the orders of trilobites to go extinct. Only Order and Suborder epithets are provided above. There wasn't room for all of the superfamily figures and labels for the Proetida, Asaphida and Lichida.

The origin of the Phacopida is uncertain. The three suborders (Phacopina, Calymenina, and Cheirurina) share a distinctive protaspis type; this similarity in development suggests phylogenetic closeness. The Calymenina is perhaps the most primitive of the Phacopida, and share some characters with the Ptychopariida (including a few species with natant hypostomes), so although the hypostomal condition of the Phacopida is typically conterminant (and impendent in some advanced Phacopina), they may have had their origins with the natant Ptychopariida (which would make the Phacopida another addition to the Libristoma). Others point out the overwhelmingly conterminant hypostomal condition among Lichida, and similarities in the exoskeleton tuberculation of Phacopida and Lichida, so the ancestral sister group of the Phacopida remains unclear, and its clade in the chart above is placed between Ptychopariida and Lichida.

The status of the Order Lichida is likewise not entirely clear. In the first trilobite Treatise (1959), an Order Odontopleurida was recognized, including the large family Odontopleuridae. Thomas & Holloway (1988) acknowledged that the Lichidae and Odontopleuridae are related, but that their post-Cambrian evolutions have been distinct. When Fortey (1997) added Damesellidae to the order Lichida, it was indicated that they are more similar to Odontopleuridae than to Lichidae. If two orders are recognized, it may be Lichidae+Lichakephalidae=Lichida, and Odontopleuridae+Damesellidae=Odontopleurida. However, the family Lichakephalidae may be paraphyletic, with some of its genera similar to Lichidae and others more similar to Odontopleuridae. In particular, some workers reject the synonymy of Eoacidaspididae with Lichakephalidae, and note that at least some genera in Eoacidaspididae are close to Odontopleuridae.

Important notes on the use of this figure: This diagram was created by Sam Gon III, based on information available in the literature, including the 1997 revision of the Treatise, and Fortey's 2001 synopsis of trilobite systematics. This page should not be used for anything other than its original intent: to share with others interested in trilobites my slowly growing understanding of the relationships between the higher taxonomic units. Any similarity to figures published elsewhere is unintentional. I have not seen any handling of quite this sort in the literature. Please contact me before using this image. I don't want to embarass myself or anyone else with proliferation of inaccuracies. Any such blunders are entirely mine. For example, since 1999, I adjusted the extinction of the Asaphida to the early Silurian, set the origin of the Phacopida to the Cambrian-Ordovician boundary, adjusted all of the Devonian extinctions to reflect the Frasnian-Famennian boundary, and added the small persistent tail on the Phacopida until its last family (Phacopidae) disappeared near the Devonian-Carboniferous boundary. When the Order Harpetida was recognized in late 2002, the entire chart had to be redone! In May of 2006, adjustments were made on the geological time scale to reflect the emerging treatment of the Cambrian, in which the appearence of trilobites is considered the bottom of Series/Epoch 2, and dashed lines indicating paraphyly in the primitive orders were added. I intend to make other revisions to this figure as I receive feedback or learn more. Thank you for your understanding! Look for a version of this figure to appear in a peer-reviewed publication early in 2007.

Agnostida AGNOSTIDA - Among the early trilobites, with a basic, clamshell-like appearance.
Suborders Agnostina and Eodiscina.
Representative species pictured here: Ptychagnostus akanthodes (Agnostina)
Redlichiida REDLICHIIDA - Including the most primitive orders of trilobites in the lower Cambrian. Suborders Olenellina and Redlichiina.
Representative species pictured here: Redlichia sp. (Redlichiina)
Corynexochida CORYNEXOCHIDA - An often spiny group united by a shared hypostomal attachment.
Suborders Corynexochina, Illaenina, and Leiostegiina.
Representative species pictured here: Kootenia sp. (Corynexochina)
Lichida LICHIDA - Some of the most ornately sculptured species fall into this group.
No suborders, but three Superfamilies.
Representative species pictured here: Arctinurus boltoni (Lichioidea)
Phacopida PHACOPIDA- The well-known Phacops, with its beautiful compound eyes belongs here.
Suborders Calymenina, Phacopina, and Cheirurina.
Representative species pictured here: Phacops sp.(Phacopina)
Proetida PROETIDA - Includes some of the last trilobite species before the Permian Extinction.
No suborders, but three Superfamilies.
Representative species pictured here: Proetus granulosus (Proetoidea)
Asaphida ASAPHIDA - All share a ventral median suture, and most a similar development.
No suborders, but six Superfamilies comprising ~20% of all trilobites.
Representative species pictured here: Homotelus sp. (Asaphoidea)
Ptychopariida PTYCHOPARIIDA - Bearing the "generic trilobite" body plan, but many weird variations!
Suborders Ptychopariina and Olenina (Harpina has recently been elevated to order Harpetida; see below)
Representative species pictured here: Modocia sp. (Ptychopariina)
Harpetida
HARPETIDA - Bearing the distinctive, broad, often intricately pitted, cephalic fringe.
Very recently (2002) split out of the Ptychopariida and elevated from suborder to full order.
Representative species pictured here: Eoharpes sp.
Nektaspida
NEKTASPIDA - The so called "soft-shelled trilobites" such as Naraoia have been classified as an order of trilobites by some. Click on the image or link to learn more

Literature Cited:

Chen, J. & G. Zhou. 1997. Biology of the Chengjiang Fauna. in The Cambrian Explosion and the Fossil Record. Bulletin of the National Museum of Natural Science 10:11-106.

Cotton, T.J., and S.J. Braddy. 2004. The phylogeny of arachnomorph arthropods and the origins of the Chelicerata. Transactions of the Royal Society of Edinburgh: Earth Sciences, 94: 169–193,

Ebach, M.C. & K.J. McNamara. 2002. A systematic revision of the family Harpetidae (Trilobita). Records of the Western Australian Museum 21: 235-67.

Edgecombe, G. & L. Ramskøld. 1999. Relationships of Cambrian Arachnata and the systematic position of Trilobita. J. Paleontology. 73(2):263-87.

Fortey, R.A. 1997. Classification. In Kaesler, R. L., ed. Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida. xxiv + 530 pp., 309 figs. The Geological Society of America, Inc. & The University of Kansas. Boulder, Colorado & Lawrence, Kansas.

Fortey, R.A. 2001. Trilobite systematics: The last 75 years. J. Paleontology. 75(6) 1141-51.

Lauterbach, K.-E. 1983. Synapomorphien swischen Trilobiten- und Cheliceraten-sweig der Arachnata. Zoologischer Anzweiger 210:213-38.

Størmer, L. 1944. On the relationships and phylogeny of fossil and recent Arachnomorpha. A comparative study on Arachnida, Xiphosurida, Eurypterida, Trilobita, and other fossil Arthropoda. Skrifter Utgitt av Det Norske Videnskaps-Academi I Oslo. I. Matematisk-Naturvidenskapelig Klasse 5:1-158.

Wills, M.A., D.E.G. Briggs, R.A. Fortey, M. Wilkinson & P.H.A. Sneath. 1998. An arthropod phylogeny based on fossil and Recent taxa. In: G.D. Edgecomb, ed. Arthropod Fossils and Phylogeny. Columbia University Press, N.Y.

References:
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