Evolution Essay, Research Paper

In Namibia, where the rock record is excellent, Vendian fossils are followed immediately by Cambrian fossils, and they may have overlapped briefly in time. In Siberia, a whole suite of small shelly fossils appears in the rocks as the Cambrian period begins, together with sponges of several different types. Most of the small shells are tiny cones and tubes that we don’t understand properly, but at least some of them are complex animals, including the first molluscs. Soon, archaeocyathid sponges were forming reef patches. These are dramatic changes that reflect rapidly evolving life. New research pegs the base of the Cambrian at about 543 Ma, and the world’s fauna was completely revolutionized in the 20 m.y. that followed.

The same set of small shelly fossils is now known worldwide, and their discovery shows that there were at least two major steps in the evolution of animal skeletons, because the next stage of the Cambrian sees the appearance of more abundant and more complex creatures.

For 10 m.y. or more, there were no animals larger than a few millimeters long except for the archaeocyathid sponges. Then worldwide, in the 2 m.y. between 530 Ma and 528 Ma, we see the appearance of a much larger variety of marine life. Dominant among these animals were trilobites, brachiopods, and echinoderms.

Larger Cambrian Animals

Trilobites are arthropods, complex creatures with thick jointed armor covering them from head to tail. They had antennae and large eyes, they were mobile on the seafloor using long jointed legs, and they were something like crustaceans and horseshoe crabs in structure. They did not have the complex mouth parts of living crustaceans, so their diet may have been restricted to sediment or very small or soft prey. They burrowed actively, leaving traces of their activities in the sediment, and they are by far the most numerous fossils in Cambrian rocks. The number of fossils they left behind was increased by the fact that they molted their armor as they grew, like living crustaceans. Thus, a large adult trilobite could have contributed twenty or more suits of armor to the fossil record before its final death. Even allowing for this bias of the fossil record, it is clear that Cambrian seafloors were dominated by trilobites. Other large arthropods are also known from Early Cambrian rocks, although they are much less common.

Echinoderms familiar to us today are sea urchins and starfish, but peculiar plated echinoderms are found in very early Cambrian rocks. The helicoplacoids looked rather like twisted deflated footballs covered with small plates. They were attached to the seafloor by one pointed end, and three food grooves led to a mouth halfway up on one side. Most echinoderms today are symmetrical, many of them with almost perfect fivefold patterns of arms and plates, but the first known echinoderms were distinctly unlike that, showing that fivefold symmetry is a derived character that evolved later in echinoderm history.

Brachiopods are relatively abundant Cambrian fossils, creatures that had two shells protecting a small body and a large water-filled cavity where food was filtered from seawater pumped in and out of the shell. Brachiopods lived on the sediment surface or burrowed just under it.

These animals are large, and they are easily assigned to living phyla. For the first time, the seafloor would have looked reasonably familiar to a marine ecologist. Trilobites probably ate mud, and echinoderms and brachiopods gathered food from seawater. Yet some ecological puzzles remain. There are no obvious large predators among these earliest skeletonized Cambrian fossils, no obvious grazers unless trilobites ate algae, and no swimmers, only floating plankton.

The Burgess Fauna

I have discussed the “Cambrian explosion” as if it related entirely to the evolution of skeletons. While this is basically true, there is increasing evidence that there was also dramatic evolution at the same time among animal groups with little or no skeleton. The frequency of worm tracks, trails, and burrows increases at the beginning of the Cambrian, and soft-bodied animals appeared with some amazingly sophisticated body plans.

These soft-bodied animals are found in Early and Middle Cambrian rock formation in China, Canada, Greenland, and Poland, and in Utah. I shall call them all the Burgess fauna, after the Burgess Shale in Canada where they were first discovered.

More than half the Burgess animals burrowed in or lived freely on the seafloor, and most of these were deposit feeders. Arthropods and worms dominate the Burgess fauna. Only about 30% of the species were fixed to the seafloor or lived stationary lives on it, and these were probably filter-feeders, mainly sponges and worms. Thus, the dominance of most Cambrian fossil collections by bottom-dwelling, deposit-feeding arthropods is not a bias of the preservation of hard parts: it occurs among soft-bodied communities too. Trilobites are fair representatives of Cambrian animals and Cambrian ecology.

The main delights of the Burgess fauna are the unusual animals which have provided fun and headaches for paleontologists. Aysheaia looks like a caterpillar with thick soft legs. It is called a lobopod because of the strange shape of its limbs.

There are undoubted predators in the Burgess fauna. Priapulid worms today live in shallow burrows and capture soft-bodied prey by plunging a hooked proboscis into them as they crawl by. There were at least seven species of priapulids in the Burgess Shale, including Ottoia. Opabinia is a highly evolved predator. It is long and slim, with a vertical tail fin, so it probably swam about. It has five eyes and one large grasping claw on the front of its head.

Halkieriids are flattened creatures averaging about 5 cm long. They look like flattened worms, with perhaps 2000 spines forming a protective coating embedded into the dorsal surface.

Altogether, the Burgess faunas give us a good idea of the sorts of exciting but extinct soft-bodied creatures that may always have lived alongside the trilobites but were hardly ever preserved.

Solving the Cambrian Explosion

The Cambrian explosion remains a puzzle. The waves of evolutionary novelty that appeared in the seas during the Early Cambrian have few parallels in the history of life. Many groups of fossils appeared quite suddenly in abundance, thanks to their evolution of skeletons, sometimes at comparatively large body size. This was not some ordinary event in the history of life.

Skeletons

A skeleton may support soft tissue, from the inside or from the outside, and simply allow an animal to grow larger. Therefore, sponges could grow larger and higher after they evolved supporting structures of protein or mineral, and they could reach further into the water to take advantage of currents and to gather food. Large size also protects animals from

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