Silurian Period

 Silurian - depiction - artwork

Silurian Marinescape

© 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

Richard Paselk

Middle Silurian - 430


Early Silurian

These maps of major tectonic elements (plates, oceans, ridges, subduction zones, mountain belts) are used with permission from Dr. Ron Blakey at Northern Arizona University. The positions of mid-ocean ridges before 200 Ma are speculative. Explanation of map symbols

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:

Eurypterus_Smithsonian_txt.jpg

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.

Silurian Animal (Metazoan) Fossils

Trilobites

(ToL: Trilobites<Arthropoda<Ecdysozoa<Bilateria<Metazoa<Eukaryota)

sketch drawing of a trillobite

Trilobites

Trilobites were on the decline in the Silurian, but still abundant locally. Species included in this display are:

fossil image icon Cyphaspis christyi

fossil image icon Diacalymene clavicula

fossil image icon Calymene, enrolled


Euryptides

ToL: Euryptida<Chelicerata<Arthropoda<Ecdysozoa<Bilateria<Metazoa<Eukaryota)

Euryptides engraving thumbnail

Sea Scorpions

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.

Eurypterus remipes  Eurypterus remipes


Echinoderms

(ToL: Echinodermata<Deuterostomia<Bilateria<Metazoa<Eukaryota)

Three extant groups of echinoderms are represented on a single specimen:

fossil image icon starfish, brittlestars, and sea-lilies. (Can you find them all?)

Crinoid engraving thumbnail

Crinoids (Crinoidea)

A number of sea-lilies (stalked crinoids) are displayed:

fossil image icon Eucalyptocrinites crassus theca note the plates and attached snail

fossil image icon stems and fragments a cystoid (see below) is also present.

fossil image icon 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.

fossil image icon crinoid stem embedded in rock.

Blastoid  engraving thumbnail

Cystoids (Blastoidea?)

Three specimens of two cystoids (primitive blastoids?), generally stemmed organisms with globular or pear-shaped theca with round or slit piercings, are represented:

fossil image icon Caryocrinus sp.

fossil image icon Caryocrinus sp. cystoid on the upper right

fossil image icon Holocystites scutellatus

fossil image icon Holocystites scutellatus


Mollusks

(ToL: Mollusca<Lophotrochozoa<Bilateria<Metazoa<Eukaryota)

Gastropod engraving thumbnail

Gastropods

Three gastropod snails are represented:

fossil image icon Tropidodiscus sp. embedded in rock

fossil image icon Platyceras (note the brachiopod and crinoid fragments also present)

fossil image icon unidentified snail on crinoid theca

Cephalopod  engraving thumbnail

Cephalopods

A nautilous, cone-shaped cephalopod is represented.

fossil image icon cephalopod


Brachiopods

(ToL: Brachiopoda<Lophotrochozoa<Bilateria<Metazoa<Eukaryota)

brachiopod  engraving thumbnail

Brachiopods

Specimens of two species of brachiopod are displayed:

fossil image icon Eosirifer sp.

fossil image icon Pentamerus sp.


Corals

(ToL: Cnidera<Metazoa<Eukaryota) 

Coral engraving thumbnail

Cnidarians (corals)

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:

fossil image icon Horn coral

fossil image icon Syringopora, tabulate colonial coral

fossil image icon unidentified colonial coral


Sponges

(ToL: Porifera<Metazoa<Eukaryota)

sponge engraving thumbnail

Sponges

Three different species of sponge are displayed. First is the cup-shaped Astraeospongia meniscus. Note the star-shaped spicules apparent in this specimen.

 Caryospongia and Astrospongia. Astraeospongia meniscus

There are two, small specimens of globular sponges: Caryospongia and Astrospongia:

fossil image icon Caryospongia

fossil image icon Astrospongia

stromatoporidea engraving thumbnail

Stromatoporidea

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:

fossil image icon fragment

fossil image icon encrusting


Silurian Plant Fossils

Green Algae (ToL: Green Plants<Eukaryota)

Recepticulites

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

fossil image icon globular specimen

fossil image icon pore structure

fossil image icon rectangular plate structure


Vascular Plants

(ToL: Embryophytes [land plants] <Green Plants<Eukaryota)

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.

fossil image icon Bryophyte-grade thaloid 

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

Associated Exhibit : 
Life Through Time