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Cambrian

Ordovician 488.3 to 443.7 Million years ago

Silurian

Plate Tectonic Reconstructions

The Ordovician* lasted about 45 million years and saw the transition from very primitive to relatively modern life-forms in the seas. The “Ordovician radiation” which followed the late Cambrian extinctions, lead to the highest diversity levels seen during the Paleozoic Era. As a result, all of the common invertebrate fossil groups and a few vertebrates were present by the end of this period. Starfish, brittle stars, crinoids, and echinoids, all of which have living representatives, first appeared in the Ordovician. Pandemic species of planktonic graptolites and conodotes appear in the fossil record during this Period. Their world-wide distribution and evolution during the Ordovician make them key species for correlating fossil deposits. There were significant fluctuations in climate with prolonged periods of warm “hothouse” conditions with periodic cold, “icehouse” intervals and turnovers of the oceans. The Ordovician ended with a mass extinction event associated with sea level changes and glacier formation.

Tectonically, the Ordovician saw a rapid reorganization of plates around the Iapetus Ocean. Plate movement resulted in a migration of the South Pole from North Africa to central Africa. The period ended with a strong lowering of sea levels (a strong negative eustatic event) as high latitude glaciation occurred.

* The term Ordovician was proposed by Lapworth in 1879 for strata which had been included in Cambrian or Silurian systems. It is named for the Ordovices, a tribe of northern Wales.

Ordovician Animal (Metazoan) Fossils
Trilobites (ToL: Trilobites<Arthropoda<Ecdysozoa<Bilateria<Metazoa<Eukaryota)
	A rich new fauna of trilobites appeared in the Ordovician, well represented in this case by 9 different specimens: Calymene callicephala, Cryptolithus tessalatus, Isotelus sp., Flexicalymene meeki, Homotelus bromidensis, Pseudogygites canadensis, Flexicalymene meeki,Flexicalymene enrolled, and a second enrolled Flexicalymene.
Echinoderms (ToL: Echinodermata<Deuterostomia<Bilateria<Metazoa<Eukaryota)
	Sea stars (Asteroidea) such as our specimen, Auluroidea sp.) as well as their relatives, the Brittle stars (Ophiuroidea) first appear during the Ordovician.
	Crinoids (Crinoidea), that also first appear in this Period, are represented in our collection by Iocrinus subcrassus.
	Edrioasteroids (Edrioastroidea),  were an extinct , round, sessile form of echinoderm .  Note the short thick, plate-covered stalk on our specimen of Isorophus cincinnatiensis  (attached to a brachiopod shell). The five rayed feeding grooves, which often make these organisms look like starfish, are damaged but discernable.
Graptolites (ToL: Hemichordata <Deuterostomia<Bilateria<Metazoa<Eukaryota)
	Graptolites were strange colonial animals with a plant-like appearance in their fossils. They are thought to have been members of the Hemichordata. Two specimens are displayed: The first has three species on the same slab. All three share a saw toothed appearance but they differ in branching pattern. The second specimen shows blade shaped graptolites, probably as a result of compression into a 2-dimensional plane.
Vertebrates (ToL: Vertebrata<Chordata<Deuterostomia<Bilateria<Metazoa<Eukaryota)
	Ostracoderms (Ostracodermi), represented in our collections by a small skull, were primitive jawless fish without paired fins. They were armored with a coat of large bony plates or scales. They were the first fish to use their gills for breathing only, feeding with pharyngeal gill pouches. Originating in the Ordovician, they began a decline with the advent of jawed fish, dying out with the end Devonian extinction event.
Mollusks (ToL: Mollusca<Lophotrochozoa<Bilateria<Metazoa<Eukaryota)
	Gastropods (Gastropoda) were small and rare during the Cambrian, but they became a diverse and sometimes large bodied group in the Ordovician. A medium sized snail, Salpingostoma richmondense, represents this group in our collection.
	Cephalopods (Cephalopoda), like their relatives the gastropods, diversified and grew in size in the Ordovician. These relatives of modern squids were one of only two families of animals (starfish are the other) that appear to have been predators. All other animals at this time seem to have been detritus feeders, filter feeders, or grazers. Displayed are fragments of straight cone-shaped nautiloids (a & b) and a partially coiled cephalopod, Liluites lituus. The mural specimen of Liluites lituus is found on the touch shelf.
Brachiopods (ToL: Brachiopoda<Lophotrochozoa<Bilateria<Metazoa<Eukaryota)
	Brachiopods are the dominant fossils in Ordovician deposits. An example is the brachiopod assemblage, which also includes bryozoan, coral, annelid, and gastropod fossils. (Can you find them?) Brachiopods and their fragments dominate a second assemblage , while a third consists of a single species, Plaesiomys subquadrata. Other specimens include: Hebertella sinuata, Platystriophia acutilirata, Rhynchotrema sp. (note the solitary coral attached to one of the shells), and Strophomena neglecta (a & b).
Corals (ToL: Cnidera<Metazoa<Eukaryota)
	The corals appear for the first time early in the Ordovician: the tabulate colonial corals, such as Foerstiphyllum vacua (a & b), are seen first in the fossil record. The rugose corals appear by the Middle Ordovician. On display are specimens of the solitary rugose coral Streptelasma sp. (a & b). Another example of a solitary coral may be seen growing on a brachiopod shell, Rhynchotrema sp.

The engravings are from Dana, James D. (1870) Manual of Geology, or Encyclopedia Britannica, 11th ed. (1911).

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Last modified 29 January 2007 | ©1998, HSU NHM