Wednesday, March 24, 2010

Getting Along With Geologic Time #1 - The Silurian

Graptolite Monograptus tomasii and early plant Baragwanathia longifolia
in Silurian silicics of Australia. Image from Geological Society of Australia.

Welcome to what hopes to be a long list of "Getting Along with Geologic Time" series. The introductory period will, of course, be the Silurian. Why the Silurian first? Why not?

This series of posts will not be (geo)chronological. If it were, it would be no fun. Plus, I know a good quantity of Oneonta Staters that wouldn't log onto this blog for months if they knew the evil, evil Devonian was coming along.

So here we go, Silurian Stylee.

The Beginning

As with all geologic periods, the beginning of the Silurian is characterized by new life after a substantial loss of biotas worldwide. In the case of the Ordovician/Silurian (OS) boundary, there was a mass extinction of 60% of earth's biotas at this time, mainly devastating marine shelf-life. The mass extinction(s) were second in magnitude of deceased marine species, only to pale in comparisson to the horrible events circa 249 MYA. Interestingly, the OS mass extinction is classified by two events, one at approximately 442 MYA another a short 1 MY later. During this time, Gondwanda was drifting into the higher southern latitudes, eventually passing near (possibly completely over) the Southern Pole. This process led to a cooling of the land masses, and likely a global cooling to boot. This cooling, of course, led to extensive glaciation of the Ordivician world. Thus, the world's waters became trapped within extensive ice masses, and sea level dropped. With the corresponding drop in sea level, shallow marine life suffered due to extensive habitat loss. Bryozoans, brachiopods, bivalves and echinoderms suffered the greatest.

The Origins

Silurian refers to a Welsh tribe, the Silures. In 1835, the term "Silurian" was coined by none other than British geologist Roderick Murchison, in a series of events which led to the initial geologic time scales. In a joint paper with fellow Adam Sedgwick, On the Silurian and Cambrian Systems, Exhibiting the order in which the Older Sedimentary Strata Succeed each other in England and Wales; the birth of the geologic time scale, as we know it. Sort of. The partnership of Sedgwick and Murchison would later dissolve over disagreements between the end and beginning of said periods. However, Charles Lapworth later defined a new period in between, the Ordovician, resolving the conflict.

The Epochs

(Europe, North American Equivalent(s))

Llandovery, Alexandrian (443-428 MYA)
Wenlock, Lockportian/Tonawandian (428-422 MYA)
  • Oldest known Tracheophytes appear
Ludlow, Cayugan (422-418 MYA)
Pridoli (418-416 MYA)
  • Named for the town of Pridoli in the Czech Republic, where excellent exposures of the SD (Silurian/Devonian) occur
Paleogeography of the Silurian

Image courtesy Dr. Ron Blakey, Northern Arizona University

As stated earlier, the Silurian geography can be characterized by a slow, southernly drift of supercontinent Gondwanda. Aside from Gondwanda, there were a number of other land masses and cratons drifting towards the equator. These multiple land masses were in the business of colliding around the equator, leading to the Caledonian Orogeny. During this time, land masses of the current New York State, Greendland and Norway were colliding in order to form a second supercontinent, Euramerica.

If you take a look at the image above, generously shared via Wikipedia and UNA's website by Dr. Ron Blakey; you'll notice a massive ocean dominating the northern hemisphere. This ocean, known as Panthalassa, was accompanied by the southern oceans of Proto-Tethy and Paleo-Tethys, which later evolved into, you guessed it; the Tethys Ocean. Also present was the Rheic Ocean, a seaway of the Iapetus.

Paleoclimate of the Silurian

After the Ordivician-Silurian cooling event, the Silurian period experienced a prolonged greenhouse phase. The glaciers which formed with Gondwanda's passing over the South Pole almost disappeared during the Silurian. With the rebound in Ordovician sea levels, the Silurian saw a return to general climate normalcy, in contrast to the erratic conditions of the Cambrian and Ordovician.

Multiple stratas of coquinas (sedimentary rock composed of broken shell fragments) characterize many units of Silurian formations. These conquinas are evident that violent storms were common in the Silurian. Plentiful, violent storms are typical of time periods in which shallow, easily warmed sea surfaces.

End Silurian Mass Extinction Event(s)

The end Silurian extinction, in comparison to others, has been rather neglected in terms of research and understanding. Rumblings has produced trite reasoning such as "probably caused by climate change..." and other vague scenarios.

The widely accepted Lau Event at 420 MYA, the final of three extinction events ending the Silurian; is viewed as a peak in sigma 13 C (blogger does not allow for greek lettering and sub/post scripts) levels. However, the Lau Event, characterized as a massive conodont extinction events, lags behind the disappearance of index fossils by at least one million years. Building upon this event, some paleoclimatologists have theorized another glaciation event as a possible climatic situation contributing to the extinctions.

So what's so great about the Silurian?

So we've reviewed the main concepts of the Silurian. No much ice (lame), a good amount of warm-weather storms (very awesome) and relatively quiet orogenies (Caledonian Range is all we really have to show for it).

What really sticks out about the Silurian is the organisms. The Silurian period is home to the first tracheophytes and land-dwelling vascular plants.
Cooksonia, above, and Baragwanathia, shown in the first image on this page; are some early examples of these early vascular plants. Vascular plants are those which are able to carry food through lignified tissue, and are exclusive to land. Modern examples of vascular plants include ferns, clubmosses, flowering plants, conifers and gymnosperms.

We have the Silurian to thank for these precious lifeforms today.

While the Silurian was an integral time for the plant life of planet Earth, life in this time was dominated by the oceans.

Fishes underwent important changes. The first fish with jaws evolved, and those without jaws diversified.

Some of the first animals to take to land, various centipedes and scorpions, did so during the Silurian.

And there's these:

Mixopterus multispinosus

Eurypterids, arthropods of the Chilicerata subphylum; first diversified and became prominent in the Ordivician, but they're just way too cool to not be mentioned. They absolutely ruled Silurian waters (sea water, that is. Eurypterids are expected to have taken to fresh water in the Carboniferous.)

One particular Silurian eurypterid is known as one of the largest arthropods ever know (and by known, of course, we mean fossilized), Pterygotus. Pterygotus was capable of reaching an enourmous 2.3 m (that's about 7', you imperialists.) However, as is so common in the fossil record, biggest is certainly not the best. Pterygotus died out by the middle Devonian. Four species of Pterygotus existed, with P. anglicus being the type species. The genus was first described by none other than the legendary Louis Agassiz in 1839. (Go Switzerland!)

And, for a little joke:
Did you know elephants diversified in the Silurian? Neither did I!

Stay tuned for more Getting Along with Geologic Time.


Don't be completely discouraged by Wikipedia. Most of the time, the articles are written by legitimate scientists with proper sources. I always check the sources before using the information.

The Paleontology Portal

Wikipedia - The Silurian

Wikipedia - Vascular Plants

No comments: