GEOL 102 Historical Geology

Spring Semester 2017
The Middle Paleozoic: The Silurian & Devonian


Global and Regional Geology of the Silurian and Devonian

Phanerozoic Eon: 541.0 - 0 Ma

Paleogeography and Geology of the Silurian:
Huge reef complexes in many parts of the world. In fact, the Silurian and Devonian see the largest volume of metazoan-generated carbonates in Earth's history.

Global oceanic highstand after the big terminal Ordovician regression

In Appalachian Orogen of Laurentia, late flysch and molasse from the Taconic Orogeny, Appalachians erode down to base, resumption of stable margin (carbonate and sandstone) deposits.

In the foreland basin and mid-craton of Laurentia, reefs so big that they allowed evaporation of water in the middle of the basin.

Paleogeography and Geology of the Devonian:
Extremely warm conditions worldwide: evaporites common North and South of 30°.

Continued widespread tabulate-stromatoporoid reef complexes around the world.

In eastern Laurentia, the Taconic Appalachians had worn all the way down by the beginning of the Devonian. No longer molasse deposits, but instead once again stable platform deposits (Helderberg Group, Oriskany Sandstone, etc.).

Collision of Laurentia, eastern Avalonia, and Baltica to form Laurussia (sometimes called Euramerica):

In northern Laurentia, Ellesmere Orogeny (possible collision with one of the Asian blocks)

In western Laurentia, the first of the Cordilleran orogenies:

Towards end of Devonian, CO2 levels begin to drop:

Major mass extinction event in Late Devonian


Evolution of Life in the Silurian and Devonian

Colonization of the land:

Why colonize the land?

Land dwellers need to combat:

Different groups of organisms combat these features in different ways.

Plants (Plantae):

Presence of plants modified the surface of the Earth (at least around lakes and streams), because ground cover would retard erosion. Retention of sediment on land, and incorporation of decaying plant matter in that sediment: development of first biological soils. Represents a vast new carbon sink: carbon dioxide levels begin to drop.

Devonian flora:
Plants continue the "conquest" of land started in the Ordovician.

Rhynia:

Other Devonian plant developments:

New groups of plants in Devonian:

Arthropods (Arthropoda):

Other early colonists (with no fossil record) probably included fungi and various "worms" (nematodes, earthworms, etc.).

Very simple early terrestrial community in the Silurian with simple plant producers, millipede herbivores, centipede and arachnid carnivores, worm and myriapod detritivores, and fungi decomposers.

Devonian hexapods still wingless. Scorpions common, many still partly aquatic. Spider-like arachnids (but not true spiders) also present.

Marine life of the Devonian:
Tabulate-stromatoporoid reefs continue to flourish, supporting large and diverse ecosystem

Devonian Nekton Revolution: During the Devonian, huge increase of the diversity of swimming forms, especially ones that could swim well within the water column (and not just above the substrate).

Eurypterids begin to decline, but still present

Ammonoids:

MAJOR fish radiation: Devonian sometimes called "Age of Fish"
Advanced fish begin with bony exoskeleton and partly bony endoskeleton.
Some major groups:

During Devonian, one branch of the sarcopterygians (lobe-fins) develop first wrists, then digits (fingers and toes). These represent the first stegocephalians ("terrestrial" vertebrates):

Relative to their kin, stegocephalians (tetrapods and their extinct relatives) are distinguished by:

Footprints showing tetrapods (stegocephalians with toes) present in the Early-Middle Devonian boundary, about 25 million years older than the oldest body fossil of such animals.

Throughout the Devonian, stegocephalians remain predominantly aquatic.

So by the later part of the Devonian Period there were vertebrates which had a bony skeleton to support their bodies; bony limbs with wrists, ankles, and digits to push along on land or on the lakebed; lungs to breath air (but still had gills to breath in the water).

Many of these Late Devonian stegocephalians still lived their lives essentially only in the water (such as Acanthostega), and thus were essentially fish-with-fingers. Others (such as Ichthyostega may have gotten most of their food from land. It was from these latter sort that the more fully terrestrial vertebrates--the Tetrapoda ("four footed ones"), would evolve.

With expansion of swimming predators (eurypterids in Silurian, ammonoids and jawed fish in Devonian), major shift in prey species: trilobites and jawless fish decline in diversity.

Late Devonian mass extinction:


To Lecture Notes.

Last modified: 19 January 2017