Ordovician

The Ordovician period (500 to 440 million years ago) comes after the Cambrian in the early Paleozoic era. The period is named for a Celtic tribe named the Ordovices who once lived in the area of Wales (in Britain) where the rocks were first studied. Ordovician limestones are over 6.4 kilometers (4 miles) thick in places and are found on all continents except Antarctica. The uniformity and thickness of the bed indicates a long period of warm and stable climate that allows them to develop.

In fact, the Ordovician period was as remarkable for the diversity of its species as the Cambrian period was for the appearance of most major phyla. A burst of evolutionary creativity in shape, size, and function tripled the number of marine species that appeared. Specialization became the dominant theme of life, with new forms filling every possible niche.

The appearance of highly efficient predators such as the nautiloids and the lobster-size sea scorpions forced the marine community to evolve protective strategies or disappear. Various species responded by developing larger size, thicker shells, or more elaborate defenses. A proliferation in the shapes of the shells of bivalve mollusks allowed them to burrow deeply into sand or mud. Other mollusks learned to swim freely by rapidly clapping their valves together. And still others developed intricate teeth-and-socket arrangements that allowed them to close so tightly that they were almost impossible to open.

Exploring the oceans of the Ordovician world would have been quite similar to exploring the oceans of today. Sea urchins, starfish, and sea lilies lived in profusion among the rocks. The first great coral reefs appeared and gave shelter to crustaceans of all kinds. Sea mats, sea snails, and sea cucumbers abounded in the tide pools. A huge diversity of bivalve mollusks made their slow way across the muddy ocean floor, leaving their tracks and burrows in the fossil record.

The very first primitive fishes appeared, slow and heavily armored, without fins or heads with brains. These agnathans (jawless fishes) were the first animals to have a notochord (flexible rod spine), a precursor of a true spinal chord. These chordates were the ancestor of all animals with backbones.

While almost all animals of the Ordovician were marine, another remarkable occurrence is recorded in the rocks of northwest England. There, arthropods (animals with jointed legs) that lived in shallow, freshwater pools left the first tracks in fossilized mud. Scientists speculate that evaporation of their pools forced these centipede-like creatures to adapt to terrestrial conditions. From this point on, the arthropods, a group that includes insects, spiders, and crabs, ruled the land for 40 million years.

The massive Ordivician limestone ends abruptly with a jumble of glacial till, indicating an ice age that so disrupted Earth's climate that more than half of all species became extinct. This first great extinction wiped out huge numbers of trilobites, with their precise and sensitive eyes, brachiopods, crinoids, and other marine invertebrates. The life-forms that survived the cataclysmic end of the Ordovician contributed to the genetic makeup of the animal kingdom to the present.