Biology : The Time of Ancient Life : Part Four
The Cambrian Extinctions
The Cambrian period is marked by as many as four mass extinctions. The first of these mass extinctions resulted in the disappearance of the archaeocyathids and a major group of trilobites. The later extinctions limited the diversity of conodonts, brachiopods, and other trilobite groups. The Cambrian is bracketed by large-scale glaciation at the close of the Proterozoic and by a similar occurrence early in the Ordovician. Development of continental glaciers would have several consequences: cooling of the planet and a drop in sea level. Both of these might have happened quickly enough for a mass extinction to have resulted.
Brachiopods were present (although not major organisms in the environments) during the Cambrian, but after the Cambrian extinction, they underwent an adaptive radiation in the Ordovician. Brachiopods have bilaterally symmetrical shells, which make them among the most common marine fossils in Paleozoic rocks. Traditionally brachiopods were divided into articulate and inarticulate groups, depending on the presence or absence of a hinge between the halves of the shell. The earliest brachiopods were inarticulate, and lacked that hinge. Lingula is an example of this type, with fossils very similar to that genus being found in Cambrian rocks, and persisting even today.
Extant Lingula from the Philippines (left) and an Ordovician fossil assignable to the genus (right).
Conodonts are another group of "odd fossils". At one time considered to be mouthparts of annelid worms, conodonts are now known to have been part of the "teeth" of an early vertebrate fish.
The phylum Mollusca increased in significance in the Ordovician faunas. Where the Cambrian seas had Anomalocaris as the largest free-swimming predator, the Ordovician had swimming molluscs, a type known as a cephalopod. Nautiloids resemble, somewhat, their living distant relatives the chambered nautilus and squids.
Gastropods, another class of the phylum Mollusca, also become more prevalent in the Ordovician seas. Ordovician deposits yield snails, as well as large, sedentary gastropods such as Maclurites.
Perhaps the most profound advance during the Ordovician was the development of plants from an ancestral group of green algae. Fossil and biochemical evidence indicates plants are descended from multicellular green algae. Algae dominated the oceans of the precambrian time over 700 million years ago. Between 500 and 400 million years ago, some algae made the transition to land, becoming plants by developing a series of adaptations to help them survive out of the water.
The Ordovician plant record consists of spores with a trilete mark, a Y-shaped feature formed when the spores were formed by (presumably) meiosis. One possible interpretation of the presence of spores is that the land plant (trilete) spore have evolved prior to the move of plant ancestral types onto land.
The vertebrates also evolved during the Cambrian period, possibly from a creature similar to the Burgess Shale animal Pikaia or an organism similar to the "conodont" animal. The oldest group of fish, the ostracoderms, first appeared during the Cambrian, before their extinction at the close of the Devonian. The exact appearance of the Ordovician fish in unclear. What we do have is fragments of hard coverings of soft tissue.
Reconstruction of the presumed appearance of an early ostracoderm fish.
The end of the Ordovician period saw another mass extinction, accounting for the second most severe loss of marine species during the Phanerozoic. This mass extinction saw the loss of one third of all brachiopod and bryozoans, and the loss of groups of conodonts, trilobites, and graptolites. The falling sea level caused by the growth of the continental glaciers caused much of the reef-building fauna to become either locally or globally extinct. Over one hundred families of marine invertebrates disappeared. However, this loss paved the way for an adaptive radiation during the next period, the Silurian.
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