Silurian "quick glimpses" No. 1 - What was it like in the Silurian?
Silurian "quick glimpses" No. 1 - What was it like in the Silurian?
This is a model ~ for man did not co-exist with either the
eurypterids or the dinosaur, although a survey of
Australians in 2010 suggests about one in three
may not know this.
Someone once wrote "The Ludlow was a moderately warm era. The Ice Age which ended the Ordovician had long been over, although a small polar ice cap was probably still present in Africa. Oxygen had reached 50-100% of modern levels in the atmosphere, favoring the evolution of larger animals in suitable, shallow-water environments. There were a good many of these, since the Ludlow experienced the highest sea levels of the Paleozoic, and shallow seas covered many continental areas. Carbon dioxide levels were still quite high, perhaps 10 times today's concentrations. Indeed, carbon dioxide may actually have been increasing due to the extensive volcanic activity which accompanied the formation of Laurasia. The enormous Gondwanan peninsula formed by Australia and Antarctica prevented the formation of a continuous current around the South Pole, and the shores of the Gondwanan mainland were warmed by tropical waters flowing southeast through the Rheic Ocean" - What is the 'Ludlow', and how did they work out what is was like ('moderately warm')? It is beyond scope of a short webpage to enter into all that but a few glimpses are given here of the Silurian, how it was defined and subdivided, etc. are given here. Discerning 'global' events, like climatic sealevel changes, from event caused by more regional tectonics is always difficult and work towards that end continues.
Tectonics versus Silurian global "events":
Although often difficult to discern tectonic-causes from possible global-eustatic ones, here is a case where 'tectonics clearly wins'. Here the Ordovician-Silurian boundary, more generally known as an example of marine transgression is seen exposed inverted on the Southern tip of the Hovedøya Island in Norway. It has been overturned due to the Caledonian orogeny. The Ordovician grey limestone (upper left) is here seen on top of the younger Silurian strata. (Photo: Petter Bøckman )
The Silurian is famous as the time period when life, both plants and animals, first moved from the sea to land. From Silurian strata come our first good evidence of life on land. The moving onto land was done by relatives of the spiders and centipedes; and also the earliest fossils of vascular plants occur in the Silurian.
The story of how ‘animals conquered the land’ is perhaps the most potent and iconic of all the stories composing the larger Darwinian narrative about evolution.
Evolution 'has' to take steps onto land if life is to make it to the complexity of the present day. So in the BBC programme "Walking with Monsters" the Cambrian fish Haikouichthys smoothly transform itself into a land-walking amphibian as it sprouted fleshy fins and its fins turned into limbs, as if its evolutionary history were like an embryo developing in the womb. Was the transformation based on fact, or was it the most inexcusable propaganda (as certain creationists complained about the show)? Creationists may claim that all evolution is just propaganda of evil Marksists or devil workers, but geologists believe in it so have to only mildly chide the BBC (not entirely slap down the much good work it does do) if it did perhaps take excesses or liberties in the way it depicted the known facts for popular presentation.
The jawless fish invade brackish and fresh water, as do eurypterids, xiphosurids, and scorpions (which may have been semi-aquatic?). The plants - rhyniophytes, primitive lycophytes, and the myriapods, became the first proper land organisms.
The Silurian is the 'smallest' (shortest) geological period in the Palaeozoic.
This is the result of some strong bickering between two geologists who were erecting the Silurian and Cambrian systems of stratigraphic subdivision and found that they were both 'claiming' a quantity of formations/strata in common, such that a compromise was to put the contested beds into a period of their own - the Ordovician. That cut down the time span that was originally being contemplated for the Silurian.
The Silurian system was erected by British geologist Roderick Murchison.
Sir Rodereick Murchison
In 1831 Murchison began working in the border region of England and Wales to subdivide the beds underlying the Old Red Sandstone. This research was embodied in The Silurian System (1839). He named the sequences for a Celtic tribe of Wales, the Silures, following the practice of his friend Adam Sedgwick who had established the Cambrian system in Wales. In 1835 the two men presented a joint paper, under the title On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales. However, as it was further traced, the "Silurian" series soon came to overlap Sedgwick's "Cambrian" sequence. This provoked some provoking furious disagreements about 'whose' system various strata belonged to, and the arguments ended the friendship between the two men. Charles Lapworth resolved the conflict by defining a new Ordovician system to accomodate the contested beds. The establishment of the Silurian system was followed by that of the Devonian system, an investigation in which Murchison assisted, both in England and the Rhineland. Murchison continued with work in Russia, and with others produced a work on Russia and the Ural Mountains in 1845. This completed the first and most active half of Murchison’s scientific career, and in 1846 he was knighted.
The publication in 1839 of Murchison's Silurian System incited Barrande to carry on systematic researches on the equivalent strata in Bohemia. For ten years (1840-1850) he made a detailed study of these rocks, engaging workmen specially to collect fossils, and in this way he obtained upwards of 3500 fossil species. The first volume of his great work, Système silurien du centre de la Bohême appeared in 1852; and from that date until 1881, he issued twenty-one quarto volumes of text and plates. His great work stands almost unrivalled in the palæontological literature. The 22 volumes contained 1160 plates. Two other volumes were issued after his death in 1887 and 1894. It is estimated that he spent nearly £10,000 on these works. In addition he published a large number of separate papers. His stratigraphic interpretation was modified in 1854 by Edward Forbes (who re-assigned Barrande's later stages F, G and H to the Devonian) but despite such trimming of Barrande's 'Silurian', his established Bohemia as a classic ground for the study of the Silurian.
DEFINITION OF THE SILURIAN SYSTEM (and its possible global events?)
The Silurian extends from the end of the Ordovician period, about 443.7 ± 1.5 M to the beginning of the Devonian period, about 416.0 ± 2.8 Ma (ICS, 2004).
eriod/System Epoch/Series Age/Stage When began Devonian Early Devonian Lochkovian 416.0 mya Silurian Pridoli (not subdivided) 418.7 Ludlow Ludfordian 421.3 Gorstian 422.9 Wenlock Homerian 426.2 Sheinwoodian 428.2 Llandovery Telychian 436.0 Aeronian 439.0 Rhuddanian 443.7 Ordovician Late Ordovician Hirnantian 445.6
Some major extinction events which are widely recognised. There is a strong extinction event that 'defines' the
base of the Silurian in life terms, whereas life forms continue on relatively uninterrupted into the Devonian. The
best known extinction event, at the Cretaceous-Tertiary boundary, was probably caused by an asteroid or
comet impact. However, the causes of older extinction events remain very uncertain.
Each major extinction event shown in the above chart eliminated reefs from the oceans, and millions of years may have passed before reefs re-evolved. As the chart shows, a major die-off event occurred at the base of the Silurian and life conditions for such biota continued smoothly from Silurian through till the late Devonian. Often the same families persist through, increasing in diversity into the Devonian.
Of interest for possibly being global are a number of Ordovician-Silurian extinction events
PALAEOGEOGRAPHY
The Silurian world. Australia at this time was part of the "Gon" (Gondwanaland) supercontinent shown here.
( From "Paleogeographic Globes" of Prof. Ron Blakey of Northern Arizona University )
Another of Prof. Blakey' s images for the Silurian ( http://www2.nau.edu/rcb7/430_Silurian_2globes.jpg )
During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late Ordovician glaciation. The melting of icecaps and glaciers contributed to a rise in sea level. Other cratons and continent fragments drifted together near the equator, starting the formation of a second supercontinent known as Euramerica.
GLOBAL CONDITIONS
Climate - The Silurian was a time when the Earth underwent considerable changes that had important repercussions for the environment and life within it. The Silurian witnessed a relative stabilization of the earth's general climate, ending the previous pattern of erratic climatic fluctuations. One result of these changes was the melting of large glacial formations. This contributed to a substantial rise in the levels of the major seas. The Earth entered a long warm greenhouse phase. However latitudinal variations in climate were rather similar to today, with glaciers occurring in the higher latitudes (over 65o). Regions of marked aridity occurred within 40o of the Silurian equator. Warm shallow seas covered much of the equatorial land masses. Early in the Silurian, glaciers retreated back into the South Pole until they almost disappeared in the middle of Silurian. The period witnessed a relative stabilization of the Earth's general climate, ending the previous pattern of erratic climatic fluctuations. Layers of broken shells provide evidence of a climate dominated by violent storms generated then as now by warm sea surfaces. Later in the Silurian, the climate cooled slightly, but in the Silurian-Devonian boundary, the climate became warmer again.
Mean atmospheric O2 content over period duration = ca. 14 Vol %[1] (70 % of modern level)
Mean atmospheric CO2 content over period duration = ca. 4500 ppm[2] (16 times pre-industrial level) Mean surface temperature over period duration = ca. 17 °C [3] (3 °C above modern level) Sea level (above present day) = around 180m, with short-term negative excursions[4]
The change in environmental conditions appears to have been rapid at the beginning of the Silurian but conditions were rather stable continuing into the Devonian.
During the Silurian , the Earth seems to have witnessed many changes in the way in which landmasses were distributed around the globe. Although there were no major volcanic events, a deglaciation and rise in sea levels occurring at that time produced varying periods of continent coverage and exposure. The variation of ocean levels occurred alongside the process of continental fragmentation and grouping that occurred from the Cambrian to the present.
At that time, the continents were distributed very differently than they are today. The Silurian world consisted of a vast north polar ocean and a south polar supercontinent (Gondwana) with a ring of approximately six continents. By the Silurian period, a large portion of the Rodinian landmass had become fragmented, and those fragments migrated toward the equatorial region. Most of these fragments were eventually assembled by a series of plate collisions into the super-continents of Laurussia and Laurasia. The modern Philippine islands were most likely inside the Arctic Circle, while Australia and Scandinavia resided in the tropics; South America and Africa were probably over the South Pole.
There was no major volcanic activity during the Silurian; however, the period is marked by major orogenic (mountain-building) events in eastern North America and in northwestern Europe, resulting in the formation of the mountain chains there. This was called the Caledonian Orogeny. In other areas, large igneous rock formations of the Middle Silurian arose, such as those in Central Europe, as well as light sedimentation throughout the Baltic region. While not characterized by dramatic tectonic activity, the Silurian world experienced gradual continental changes that would be the basis for greater global consequences in the future, such as those that created terrestrial ecosystems.
The Silurian oceans are also of particular interest for activity between the regions known as Laurentia, Baltica and Avalonia. The ocean basins between these areas substantially closed together, continuing a geologic trend that had begun much earlier. The new marine habitats produced by these profound changes in the Silurian seas provided the framework for significant biological events in the evolution of life.
Coral reefs made their first appearances in the fossil record during the Silurian:
A Silurian reef at Thornton Quarry, Illinois (reef core at right, flanking beds at left and above).
The Silurian period was a time when the earth underwent considerable changes that had important repercussions for the environment and the life within it. The Silurian witnessed a relative stabilization of the world's general climate, ending the previous pattern of erratic climatic fluctuations. One significant feature of these changes was the melting of large glacial formations. This contributed to a substantial and significant rise in the levels of the major seas, creating many new marine habitats.
The Silurian period's condition of low continental elevations with a high global stand in sea level can be strongly distinguished from the present-day environment. This is a result of the flood of 65% of the shallow seas in North America during the Llandovery and Wenlock times. The shallow seas ranged from tropical to subtropical in climate. Commonly present in the shallow seas were coral mound reefs with associated carbonate sediments. Due to reduced circulation during the Ludlow and Pridoli times, the process of deposition of evaporites (salts) was set in motion. Some of these deposits are still found in northern Europe, Siberia, South China and Australia.
SEARCH FOR SILURIAN "CYCLES" AND BIG EVENTS
"Silurian Cycles Linkages of Dynamic Stratigraphy With Atmospheric, Oceanic, and Tectonic Changes".
A New York State Museum Bulletin. Ed Landing (Editor). Includes a global sea-level curve
constructed for the Silurian from many localities around the world.
Key events of the Silurian
As shown below, the Silurian is the shortest period of the Palaeozoic (in part following the friction between Murchison and Sedgwick and splitting the Ordovician off from Murchison's original Silurian period.
Silurian time scale from Geology Museum, University of Oslo, http://norges.uio.no
The Silurian lasted from about 443 to 417 million years ago. Its stratigraphy is subdivided into four epochs (from oldest to youngest): the Llandovery, the Wenlock, the Ludlow, and the Pridoli. Each epoch is distinguished from the others by the appearance of new species of graptolites. Graptolites are a group of extinct colonial, aquatic animals that put in their first appearance in the Cambrian period (543 -490 million years ago) and persisted into the Early Carboniferous (354-290 million years ago). The beginning of the Silurian (and the Llandovery) is marked by the appearance of Parakidograptus acuminatus, a species of graptolite.
The Llandovery (443-428 million years ago) preserves its fossils in shale, sandstone, and gray mudstone sediment. Its base (beginning) is marked by the appearance of the graptolites Parakidograptus acuminatus and Akidograptus ascensus. The Llandoverian epoch is subdivided into the Rhuddanian, Aeronian, and Telychian stages.
At the close of the Telychian stage, the appearance of Cyrtograptus centrifugus marks the start of the Wenlockian epoch (428-423 million years ago). The fossils are found in siltstone and mudstone under limestone. Missing from the fossil record of the Wenlock was the conodont Pterospathodus amorphognathoides, present in earlier strata. This is an epoch with excellent preservations of brachiopod, coral, trilobite, clam, bryozoan, and crinoid fossils. The Wenlock is subdivided into the Sheinwoodian and Homerian stages.
The Ludlow (423-419 million years ago) consists siltstone and limestone strata, marked by the appearance of Neodiversograptus nilssoni. There is an abundance of shelly animal fossils. The Gorstian and Ludfordian stages make up the Ludlow epoch.
Platy limestone strata rich in cephalopods and bivalves characterize the Pridolian (419-417 million years ago), the final epoch of the Silurian. It is marked by the appearance of the index fossil Monograptus parultimus, and also by two new species of chitinozoans (marine plankton), Urnochitina urna and Fungochitina kosovensis, which appear at the base or just above the base of the Pridoli.
The classic Silurian subdivisions/places in England
Wenlock Edge, marked by dashed line. Here viewed looking north east from above Craven Arms. Wenlock Edge
is lower and immediately in front of Callow Hill, as shown by the green line. (Photo: Peter Toghill)
Much Wenlock Limestone - The Much Wenlock Limestone of Wales and the Welsh Borderland reveals one of the most diverse, and well-preserved fossil assemblages known, with well over 600 species of invertebrates recorded. The fine preservation allows investigations as to the interactions of the animals involved. For example, high quality of preservation of crinoidal remains revealed a range of small-scale morphological features among the aboral surfaces of the arms and calyces of this group. Small, circular to sub-circular depressions and parabolic traces are randomly situated in the calcite plates of a number of the crinoid taxa. A small number of these features indicate in vivo formation through the presence of rims and gall-like features surrounding the trace. These structures are interpreted as a response by the crinoid to the presence of another organism through either a mechanical or a chemical stimulant. The lack of penetration into the body cavity and the extent of the reaction structures suggest a symbiotic relationship existed between host crinoid and trace maker. For work to determine the taxonomic identity of the pit-producing organisms see Widdison ( 1999).
Crinoid, Wenlock Limestone. Periechocrinus moniliformis, Miller. (Sedgwick Museum)
Abundantly fossiliferous thin bedded marly/argillaceous limestone sequence of Wrens Nest (Wenlock Limestone) at Dudley, England. (Photo: Geoff Broughton - Geoff.Broughton@aeat.co.uk
Excellent preservation of bryozoans, brachiopods and corals seen in the Wenlock Limestone (Sedgwick Museum)
The trilobite Calymene, Wenlock Limestone. (Sedgwick Museum)
Although many Silurian forms are distinct some are still of familiar shapes - at rigght a striped dog whelk,
Nassarius glans, scavenging on shallow seafloor of the Great Barrier Reef. (Photp: GBRMPA)
SOME MORE ABOUT SILURIAN FOSSILS
The Silurian is a time when many biologically significant events occurred.
Following the Ordovician extinction event there was a rapid recovery of invertebrate faunas during the Silurian. The high sea levels and warm shallow continental seas provided a hospitable environment for marine life of all kinds. The biota and ecological dynamics were basically still similar to that of the Ordovician, but may have been more diverse
In the oceans, there was a widespread radiation of crinoids, a continued proliferation and expansion of the brachiopods, and the oldest known fossils of coral reefs. The time period also marks the wide and rapid spread of jawless fish, along with the important appearances of both the first known freshwater fish and the appearance of jawed fish. Other marine fossils commonly found throughout the Silurian record include trilobites, graptolites, conodonts, corals, stromatoporoids, and molluscs.
Brachiopods, bryozoa, molluscs, hederelloids and trilobites were abundant and diverse. The brachiopods are the most common hard-shelled organisms known from the Silurian, making up 80% of the total species. Among these, pentamerids first appear and are abundant, rhynchonellids, and the spire-bearing athyridids and atrypidids are also common, as are other groups that continue from the Ordovician.
The Silurian was also a remarkable time in the evolution of fishes. Not only does this time period mark the wide and rapid spread of jawless fish, but also the highly significant appearances of both the first known freshwater fish as well as the first fish with jaws. The first bony fish, the Osteichthyes, appeared, represented by the Acanthodians covered with bony scales; fishes reached considerable diversity and developed movable jaws, adapted from the supports of the front two or three gill arches.
A diverse fauna of Eurypterids (Sea Scorpions) - some of them several meters in length - rowled the shallow Silurian seas. Leeches also made their appearance during the Silurian Period.
Perhaps most striking of all biological events in the Silurian was the evolution of vascular plants, which have been the basis of terrestrial ecology since their appearance. Most Silurian plant fossils have been assigned to the genus Cooksonia, a collection of branching-stemmed plants which produced sporangia at their tips. None of these plants had leaves, and some appear to have lacked vascular tissue. Also from the Silurian of Australia comes a controversial fossil of Baragwanathia, a lycophyte. If such a complex plant with leaves and a fully-developed vascular system was present by this time, then surely plants must have been around already by the Ordovician. In any event, the Silurian was a time for important events in the history of evolution, including many "firsts," that would prove highly consequential for the future of life on earth.
The early plants : Cooksonia, on the left, has usually been considered the oldest known land plant. Fossils assigned to several species are known from North America, Europe, Asia, and Africa, and from both the Late Silurian and Early Devonian. The lycophyte Baragwanathia, on the right, is structurally more complex than Cooksonia, but Silurian fossils of this plant have been found in Australia, significantly earlier than in the Northern Hemisphere.
Fish were evolving in the Silurian and this is a ' typical' Silurian (Wenlock-Ludlow) vertebrate
fauna as known from Saaremaa Island in Estonia. Towards the end of the period the jawed
fish appeared for the first time, but remain relatively insignificant until the Devonian..
Plate showing trilobites from Barrande's Système silurien du centre de la Bohême
Appreciating the Silurian in Door County, Wisconsin. (Sea poster created by Sura Njaa with drawings by Flora M. Langlois)
Wisconsin Silurian marine life
Coral reefs made their first appearance during this time, built by tabulate and rugose corals.
http://www.mpm.edu/collections/learn/reef
Eurypterids - a 'favourite' Silurian fossil group
http://www.g-3d.com/images/Eurypterid_No_G2.jpg
The first creatures known to be capable of existing both on land and in water were the eurypterids. They seem to have lived in shallow-water environments, such as the zone between low and high tide. Although they resembled scorpions in some ways, they were not related. Some species reached lengths of over 2 metres and have the distinction of being the largest arthropods ever to have lived; others were much smaller. The earliest fossil eurypterids go back to the Early Cambrian (from the Paseky Shale, Czech Republic) and are thus part of the Cambrian Explosion. They are not preceded by any simpler organism evolving into a eurypterid, and they became extinct in the Permian ( http://www.earthhistory.org.uk/recolonisation/first-steps-on-land ).
l
Left: Upper Silurian (Onondaga) Middle: Upper Devonian (Old Red Sandstone) Right: Jaekelopterus rhenaniae
A claw of the giant eurypterid, Jaekelopterus rhenaniae was found in 2007 by Markus Poschmann of the Mainz museum, in a quarry near the town of Prum in western Germany. The species, first recognised from other German finds last Century but reached lengths in excess of 2.5 m.
One of thousands of spectacularly preserved eurypterid specimens from the Ciurca collection.
The research collection of Samuel J. Ciurca Jr. of Rochester, now at the Yale Peabody Museum is undoubtedly the largest collection of eurypterids and fossil scorpions in the world, accumulated over 40 years. It includes thousands of eurypterids, fossil scorpions, xiphosurans and their associated fauna from North America. For a project addressins the paleoecology, taphonomy and phylogeny of eurypterids and scorpions (mainly from the classic lithologies in New York State and Ontario) see:
http://earth.geology.yale.edu/~deb47/index.cgi?page-selection=2The eurypterids were undoubtedly affected by environmental constraints imposed by their physiology and gross morphology, as are modern aquatic organisms. Kjellesvig-Waering (1961) proposed a series of distinct ecological phases for eurypterids, defined by the environment in which they lived, and based on the Upper Silurian Welsh Borderland fauna. However, evidence from the eurypterid fauna of the Upper Silurian Bertie Waterlime Formation, New York, suggests that two distinct transitional assemblages existed, perhaps caused by a difference in the environmental preferences of juvenile and adult eurypterids (‘ontogenetic segregation’) (Manning 1993) (Manning 1999).
http://geology.stlawu.edu/facilities/paleontology-collection
Some corals
Halysites catenularius Wenlock Edge. (per Ludlow Library and Museum)
Halysites catenularia (Linnaeus), Middle Silurian, Louisville, Kentucky, USA. (Photo: K. Carlson, photo )
A large corallite Halysites at Wrens Nest, collected by Geoff Broughton. Geoff.Broughton@aeat.co.uk
What looks like a very similar form to this occurs in New South Wales.
Halysites sp. Middle Silurian, Louisville Limestone, Louisville, Jefferson Co., KY
(Large colony collected in the 1880s, Falls of the Ohio Park collection)
Les coraux Silurien - Les coraux paléozoïques sont très différents des coraux actuels (scléractiniens hermatypiques). On ne sait pas s'ils étaient hermatypiques ou non. On compte deux grands groupes: les Tabulata et les Rugosa. Bien que localement ils ont construits de modestes récifs, ils n'ont pas été de grands constructeurs comme les stromatoporoïdés.
Grande colonie du corail tabulé Favosites à la surface d'un lit. Formation de Chicotte, Silurien de l'île d'Anticosti, Québec. Pièce de 2 dollars (2,8 cm) comme échelle.
Corail Favosites vu en coupe montrant bien la structure de la colonie. Les éléments verticaux sont les corallites dans lesquelles se retrouvent les tabulae horizontales. Formation de Chicotte, Silurien de l'île d'Anticosti, Québec.
Les Favosites en "galettes" (c), tout comme les stromatopores (s), peuvent contribuer à stabiliser les sédiments, comme ici dans ce dépôt (bank) à crinoïdes (les particules arrondies) vu en coupe verticale. À remarquer que les stromatopores encroûtent les coraux par endroits. Formation de Chicotte, Silurien de l'île d'Anticosti, Québec.
Grande colonie du corail tabulé Halysites, un corail dont la caractéristique est d'être "en chapelet". Formation de Chicotte, Silurien de l'île d'Anticosti, Québec. Pièce de 2 dollars comme échelle.
Vue rapprochée d'Halysites montrant bien son architecture. Pièce de 1 cent (2 cm) comme échelle. Formation de Chicotte, Silurien de l'île d'Anticosti, Québec.
Structure interne d'Halysites vue en coupe verticale. Les éléments verticaux sont les corallites dans lesquelles se retrouvent les tabulae horizontales. Formation de Chicotte, Silurien de l'île d'Anticosti, Québec.
Petit récif dont la construction est assurée par le corail Halysites (H) et des stromatopores (s). Formation de Chicotte, Silurien de l'île d'Anticosti, Québec.
Corail Rugosa (R) solitaire dont la base s'évase pour qu'il puisse bien s'ancrer dans le sédiment. À noter la présence des septes verticaux dans la corallite (calice). Les Rugosa solitaires ne possèdent qu'une seule corallite, plus grande que chez les tabulés. Présence aussi de tiges de crinoïdes (Cr) et du corail Favosites sur la même surface de litage. Formation de Chicotte, Silurien de l'île d'Anticosti, Québec. [Or might this be an overturned broad-caliced coral like Mucopyllum ? - JGB]
