© G. Paselk
Artist’s depiction of Silurian animals, including, from left: bryozoans, crinoids, clams, cephalopod, jelly, sea scorpion (Pterygotus), brachiopod, jawless fish (Birkenia), gastropod shell, brittle star, trilobite, bivalve mollusk, sponges, sea star.
443.7 to 416.0 Million years ago
The Silurian* lasted about 28 million years. There was a rapid recovery of biodiversity after the great extinction event at the end of the Ordovician. A warm climate and high sea level gave rise tolarge reefs in shallow equatorial seas. Tabulate corals and stromatoporid sponges were the main builders of these first coral based reefs, but rugose corals and recepticulite algae also contributed. Invertebrates remained dominant because vertebrates were relatively rare. Sea scorpions (euryptids) reached their maximum diversity. These predators were commonly 5–9 inches long, (see the Eurypterus remipes in case) but could be as large as 4.3 feet. Alife- model is shown below:
Mollusks, bryozoans, and especially brachiopods flourished, but trilobites and graptolites were on the decline. Invertebrates remained dominant, vertebrate fossils are rare. Fish with moveable jaws appear, and the first bony fish (osteichthyans) evolved. Fishes and some invertebrate groups, such as eurypterids, invaded freshwater habitats during the Silurian period.
Simple vascular plants emerged on land with moss forests growing along streambeds and lakeshores. The Silurian is the first period with fossils of extensive non-microscopic life on land. Land fauna included spider-like and millipede-like predators, in addition to herbivorous (live plant) and/or detritus feeders.
During the Silurian period Earth's continents joined together, closing the Iapetus Ocean and forming two supercontinents: Laurasia in the north, and Gondwanaland to the south. The South American and southern African Gondwana plates moved slowly toward and then over the South Pole. At the same time the northern continents moved together and began forming Laurasia. Glaciers retreated and nearly disappeared as continental warming began. Much of the equatorial land mass was covered by warm shallow seas. There were dramatic worldwide sea-level changes and oceanic turnovers (exchanges of bottom waters and surface waters) resulting in a moderate level of extinctions during the Period. The Silurian ended with a series of relatively minor extinction events linked to climate change.
* The Silurian was named by Murchison in 1839 for the Silures, a tribe of the Welch borderland.
Trilobites were on the decline in the Silurian, but still abundant locally. Species included in this display are:
Eurypterides (Sea Scorpions) were predatory arthropods closely related to horse-shoe crabs. They appear during most of the Paleozoic (Ordovician - Permian). On display is a specimen from New York.
Three extant groups of echinoderms are represented on a single specimen:
starfish, brittlestars, and sea-lilies. (Can you find them all?)
A number of sea-lilies (stalked crinoids) are displayed:
Eucalyptocrinites crassus theca note the plates and attached snail
stems and fragments a cystoid (see below) is also present.
unidentified species showing the flower-like crown on a stem. Note the second stem showing a few of the less often preserved arms coming off of it.
crinoid stem embedded in rock.
Three specimens of two cystoids (primitive blastoids?), generally stemmed organisms with globular or pear-shaped theca with round or slit piercings, are represented:
Caryocrinus sp. cystoid on the upper right
Three gastropod snails are represented:
Tropidodiscus sp. embedded in rock
Platyceras (note the brachiopod and crinoid fragments also present)
unidentified snail on crinoid theca
A nautilous, cone-shaped cephalopod is represented.
Specimens of two species of brachiopod are displayed:
Specimens from two extinct coral groups, the large, generally solitary Horn coral (subclass Rugosa), and the colonial tabulate pipe coral Syringopora (subclass Tabulata) are shown:
Syringopora, tabulate colonial coral
unidentified colonial coral
Three different species of sponge are displayed. First is the cup-shaped Astraeospongia meniscus. Note the star-shaped spicules apparent in this specimen.
There are two, small specimens of globular sponges: Caryospongia and Astrospongia:
These important Silurian and Devonian period reef-building organisms are thought to be sponges, though they were previously classified as corals. The fossils are encrusting, cylindrical, or massive. The museum has two Silurian specimens on display:
Until recently classified as sponges, recepticulites are now thought to be fossils of marine algae of the division Chlorophyta. They lived from the Ordovician to the Permian. Whole specimens are globular to platter shaped. On display are a near complete globular example, and two fragments showing the pore and rectangular plate structures characteristic of these organisms
rectangular plate structure
The black blotches are the remains of thaloids from a bryophyte-grade land plant from the early Silurian, ca. 440 Ma, making them some of the earliest remains of land plants.
The engravings of Silurian fossils are from Dana, James D. (1870) Manual of
Geology; Le Conte, Joseph (1898) A Compend of Geology; Louis Pirson and Charles Schuchert, A Text-Book of Geology.
(1920), or H. Alleyne Nicholson (1876) The Ancient Life-History of the Earth.
Last modified 10 August 2012 | ©1998, HSU NHM