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Early Ordovician Epoch |
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Paleozoic Era
Cambrian Period Terreneuvian Epoch Epoch 2 Epoch 3 (Middle Cambrian) Furongian Epoch Ordovician Period Early Ordovician Epoch Tremadoc Age Floian Age (Early Arenig) Middle Ordovician Epoch Late Ordovician Epoch Silurian Period Devonian Period Carboniferous Period Permian Period |
The Early Ordovician Geography References Notes |
From
the point of view of a hypothetical Late Ordovician oberver, the Early
Ordovician was the "good old days." Things were simpler
then. It was really just an extension of the
Cambrian, but with a new
generation of trilobites -- sleeker, probably a bit stronger, but still
trilobites. Lots of phylogenetic change was in the air, or actually in the
water, but it was mostly happening in the same lush, warm epicontinental
seas. Besides, organisms stuck to their roots back then. There were
crinoids from Baltica and other crinoids from Laurentia, and yet others from
East Gondwana, and you could really tell the difference. None of this
messy business with everyone running around everywhere in a totally disorganized
fashion. It all would have been just fine if it hadn't been for that
worthless Taconic Orogeny. Everything was going along just great, but then
Baltica and Laurentia got too close, and suddenly we've got mountains twelve
thousand meters high sprouting like mushrooms overnight. Things went to
hell pretty fast after that, I'd say.
As in the ramblings of
many old-timers (such as ourselves), hypothetical or otherwise, there is both
more and less here than meets the eye. Things really were simpler in the
Early Ordovician. Metazoan diversity was on the edge of another leap
almost comparable to the Cambrian
explosion. However, this was a slower matter, and the long run results
were mixed. The echinoderms
are a good example. Crinoids
had evolved well before the Early Ordovician. But during this epoch, they
became dominant in many ecosystems on the continental shelves, a dominance they
were destined to maintain into the Mid-Jurassic in some regions. Asteroids
("star fishes") and ophiuroids
("brittle stars") both evolved in the Early Ordovician. Asteroids didn't become important until the Cretaceous.
Ophiuroids have never amounted to much. The Early Ordovician also saw the
evolution of two new high-level taxa of blastoids
-- but the entire
clade was extinct by the end of the Ordovician. Other novelties of the
Early Ordovician met similarly mixed fates, e.g., several new types of
articulate brachiopods (Strophomenida &
Rhynchonellida)
and planktonic graptolites.
The world of the Early Ordovician was also much like the Cambrian. Stromatolites were still common. In fact, there had been a moderate resurgance of stromatolites in the Furongian. However, the Early Ordovician was the last epoch in which these massive bacterial colonies would be common. The renewed decline of stromatolites was simply more opportunity for sponges, bryozoans, and corals, who flourished by replacing stromatolites in the Early Ordovician. Similarly, bioturbation of near-shore sediments by annelid worms, bivalves, trilobites and other burrowing forms was not yet universal, and sea bottoms stiffened by algal mats were still relatively common. Again, the Early Ordovician was also probably the last epoch for which this was true.
The
fauna of the Early Ordovician was also strongly endemic. In fact, sharp
differences between local faunas continued well into the Middle Paleozoic.
However, this feature of Paleozoic life was exaggerated in the Early Ordovician
due to the wide dispersal of continental cratons in the temperate and equatorial
zones. The exact position of many of the continents remains controversial,
probably because of this dispersal. Its hard to tell, for example,
precisely where Avalonia and Baltica were in the Early Ordovician, since they
weren't in contact with any other land masses. Avalonia and Baltica lay to
the south and east of Laurentia. Siberia was to the northeast. The
are between them was the broad and loosly confined Iapetus Sea. Further
east, poorly known microcontinents were assembling other bits and pieces of what
would one day be Asia. Finally, yet further east, Australia and the
Chinese terranes formed the tail end of East Gondwana. The rest of
Gondwana lay far to the south, as a huge south polar continent. Since no
land plants had yet evolved, and Ordovician climates were well stratified from
north to south, this was a broad, lifeless barrier to dispersal, not the
east-west faunal highway it would become in the Devonian and Late
Paleozoic.
Image credits: Isograptus from Geological Society of Australia, Victoria Division. Colcocoryphe from the Paleontological Museum, University of Oslo. Alocorthis from Paterson & Brock (2003).
ATW041125. Text
public domain. No rights reserved.
This map is derived from several sources, including two different time slices from Jan Galonka, Dr. Ron Blakey's Paleogeographic Globes, the Paleomap project, and Mac Niocaill et al. (1997). We'd probably put Baltica a bit further east, if we were to do it over again. Note that Laurentia and Baltica are rotated about 90° clockwise from their present orientations.
ATW041124. Map public domain. No rights reserved. An enormous, 2400 x 1200 pixel, unlabelled version of this map is available (free) in all the usual formats, including a Photoshop® .psd file with each topographical type on a different layer. That one is 8 MB, so you'd best have a fast connection. CD also available for the cost of mailing with various large graphics. Email augwhite@sbcglobal.net.
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