One of the lesser-known geologic periods in the earth's history, the Ordovician didn't witness the same burst of evolutionary activity that characterized the preceding Cambrian period; rather, this was the time when the earliest arthropods and vertebrates expanded their presence in the world's oceans. The Ordovician is the second period of the Paleozoic Era (542-250 million years ago), preceded by the Cambrian and succeeded by the Silurian, Devonian, Carboniferous and Permian periods.
Climate and geography. For most of the Ordovician period, global conditions were as stifling as during the preceding Cambrian; air temperatures averaged about 120 degrees Fahrenheit worldwide, and sea temperatures may have reached as high as 110 degrees. By the end of the Ordovician, however, the climate was much cooler, as an ice cap formed on the south pole and glaciers covered adjacent landmasses. Plate tectonics carried the earth's continents to some strange places; for example, much of what would later become Australia and Antarctica protruded into the northern hemisphere! Biologically, these early continents were only important insofar as their coastlines provided habitats for shallow-water marine organisms; no life of any kind had yet conquered land.
Marine Life During the Ordovician Period
Invertebrates. Few non-experts have heard of it, but the Great Ordovician Biodiversity Event (also known as the Ordovician Radiation) was second only to the Cambrian Explosion in its importance to the early history of life on earth. Over the course of 25 or so million years, the number of marine genera around the world guadrupled, including new varieties of sponges, trilobites, arthropods, brachiopods, and echinoderms (early starfish). One theory is that the formation and migration of new continents encouraged biodiversity along their shallow coastlines, although climatic conditions also likely came into play.
On the other side of the evolutionary coin, the end of the Ordovician period saw the first great extinction in the history of life on earth (or, one should say, the first for which we have ample fossil evidence; there may have been periodic extinctions during the preceding Proterozoic Era). Plunging global temperatures, accompanied by drastically lowered sea levels, wiped out a huge number of genera, although marine life as a whole recovered fairly rapidly by the start of the ensuing Silurian period.
Vertebrates. Practically all you need to know about vertebrate life during the Ordovician period is contained in the two "aspises": Arandaspis and Astraspis. These were two of the first jawless, lightly armored prehistoric fish, measuring anywhere from six to 12 inches long and vaguely reminiscent of giant tadpoles. The bony plates of Arandaspis and its ilk would evolve in later periods into true skeletons, further reinforcing the basic invertebrate body plan. Some paleontologists also believe that the numerous, tiny, worm-like "conodonts" found in Ordovician sediments counted as true vertebrates; if so, these may have been the first vertebrates on earth to evolve teeth.
Plant Life During the Ordovician Period
As with the preceding Cambrian, evidence for plant life during the Ordovocian period is maddeningly elusive. If plants did exist, they consisted of microscopic green algae floating on or just underneath the surface of the water, along with equally microscopic early fungi. It wasn't until the ensuing Silurian period that the first terrestrial plants appeared for which we have solid fossil evidence.
Next: the Silurian Period