Diversification of life in the oceans

Assemblages of fossils similar to those in the Burgess Shale have been discovered in Cambrian rocks at a few other places in the world, including China, the United States and South Australia, demonstrating that such faunas were widely distributed in the oceans at this time. Their rarity as fossils is thus due to the unusual conditions required for their preservation. These faunas show the great diversity of marine life at a very early stage in the history of metazoans.

Palaeozoic invertebrate faunas

Archaeocyathids are a group of bottom-dwelling organisms that inhabited warm, shallow seas during the Early Cambrian, becoming extinct in the Middle Cambrian. Their fossil remains, consisting of conical to saucer-shaped skeletons that were attached at the base to the sea floor, are known from all continents except South America. They are particularly abundant in Early Cambrian limestones of the Flinders Ranges in South Australia. The biological affinities are uncertain, but they are believed to have led a sponge-like existence filtering food particles from sea water.

Trilobites are crustacean-like animals that were the most numerous and successful marine creatures of the Early Palaeozoic. The trilobite's body, ranging in size from a few millimetres to about 90 cm, was composed of numerous segments and was encased on the upper surface in a hard shell which is normally the only part to be preserved. The segments of the shell were hinged together, allowing the body to be rolled up into a ball for protection. Each segment of the body had a pair of legs but these are rarely preserved. On the head were a pair of multi-facetted, compound eyes that are the oldest visual organs known. Trilobites were most abundant during the Cambrian Period, but after the Middle Devonian they were in decline and they finally became extinct in the Permian.

Brachiopods are marine shellfish with the soft body enclosed in a pair of shells that can be opened to enable the animal to feed and breathe. In this respect, they resemble the familiar bivalve molluscs that include clams and mussels, but they are not related to these at all. The brachiopod shell may be anchored to the sea floor by variety of means, including a fleshy stalk, spines or cementation, or else the shell may lie free on the sediment surface. There are two major groups of brachiopods: those in which the two shells are hinged together by teeth; and those in which the shells are not hinged but simply held together by muscles. Brachiopods were one of the dominant forms of life in the oceans for much of the Palaeozoic Era, during which time they evolved many different forms. However, at the end of the Palaeozoic they suffered massive extinctions from which they never recovered, and though surviving at the present day they are relatively uncommon.

Corals were abundant during the Palaeozoic Era when they were represented by two groups: the rugose corals, which included forms that constructed colonies as well as solitary forms; and the tabulate corals, which were exclusively colonial. Both groups were involved in the formation of reefs. The rugose and tabulate corals became extinct at the end of the Palaezoic, after which the modern reef-building corals, or scleratinians, arose. Rugose and tabulate corals are believed to have lived in warm, clear shallow seas, as do modern reef-building corals.

Echinoderms include the familar echinoids (sea urchins) and sea stars of the present day, as well as less familar groups such as ophiuroids (brittle stars), crinoids (sea lilies and feather stars) and holothurians (sea cucumbers). These groups were present as long ago as the Palaeozoic, along with a number of other groups that are now extinct. The extinct groups include blastoids (Silurian to Permian) and cystoids (Ordovician to Devonian, both of which were attached to the sea floor by a long stem; the small, discoidal or turret-like edrioasteroids (Middle Cambrian to Carboniferous); spindle-shaped helicoplacoids (Early Cambrian); and the bizarre, asymmetrical carpoids (Middle Cambrian to Late Devonian).

Molluscs are represented in present-day seas by such groups as the bivalves, which include clams, mussels and scallops; the gastropods, or snails; and the cephalopods, which include octopuses, squids and cuttle fish. In the Palaeozoic the same groups were present, but the bivalves and gastropods were not as abundant or diverse as today, and the dominant cephalopods were the nautiloids which are now represented by only a single genus Nautilus with a restricted distribution. Bivalves had a variety of lifestyles in the Palaeozoic as they do today, burrowing into the sediment on the sea floor, attaching themselves to hard objects by strong threads or cementation of the shell, or in a few forms swimming by clapping their shells together. Gastropods also resembled their modern counterparts in the basic shape of their shells and their crawling, burrowing or clinging lifestyles. Nautiloids are squid-like animals that grow a protective shell. This shell is divided internally into a number of chambers that function as buoyancy tanks to assist in flotation and swimming. In the living Nautilus the shell is coiled into a spiral shape, but in most Palaeozoic nautlioids the shell was straight or only slightly curved. Nautiloids were probably carnivores and scavengers in the Palaeozoic, as at the present day. Like their distant modern relatives the squids, they sometimes grew to a very large size, shell lengths of up to 5.5 m having been recorded.

Bryozoans are a group of mostly marine animals living in colonies with a frond-like, net-like or mound-like shape, or forming encrusting mats on shells and rocks. They are very common at the present day but are unfamiliar to many people because they live below low tide, and if their remains are washed up onto the shoreline they may be fragmentary and therefore overlooked, or they may be confused with sea weed. Bryozaons first appear in the fossil record in the Lower Ordovician, and they were common throughout the remainder of the Palaeozoic, forming an important component of coral reefs.

Graptolites are extinct marine animals that formed twig-like colonies composed of one or more branches. The colonies were originally three dimensional but usually became completely flattened during fossilisation, though they are still easily recognisable. Some graptolite colonies may have been attached to the sea floor, but most floated freely in the sea. Graptolites appeared in the Cambrian Period and reached their greatest diversity in the Ordovician. They suffered a major decline in the Silurian, and finally became extinct in the Carboniferous. They are one of the most important group of fossils for dating rocks of Ordovician to Early Devonian age.

Fishes: The first vertebrates

The earliest traces of fish are bony scales found in rocks of the Late Cambrian period, and the first complete fish have been found in Australian and Bolivian rocks of Middle and Early Ordovician age. The Australian form, Arandapis at 15 cm in length, is about half the size of its South American cousin but both are marked by the absence of jaws, merely an opening to the mouth lined with bony plates, and a simple tail with no other fins present. Within the body there are no bones at all, they are confined to superficial bony scales. A rigid notochord served to support the body.

By the end of the Ordovician, many types of more advanced jawless fishes (agnathans) had evolved. They had developed primitive fins to propel and stabilise themselves in the water. Their bodies were encased in great shields of bone set in the skin of the head and upper back, or by thick overlapping scales. These were ostracoderms, or 'shell-skinned' fishes. We can gain a much better understanding of these earliest vertebrates by studying the living hagfish and lampreys. They are similar to these ancient fish in many ways except that they lack bony scales.

The first fishes with jaws and teeth evolved in the Early Silurian, some 80 million years after the jawless fishes had appeared. They were the spiny-skinned sharks or acanthodians. They had bony fin spines together with dermal bones in the form of a mosaic of tiny plates embedded in their skins. In addition, within their bodies there were bones forming the vertebrae and braincase. Unlike the more superficial or dermal bones, these formed deeper in the body. These endochondral bones were first laid down as cartilage which was then subsequently replaced by bone during the growth of the animal, a major innovation in the evolution of vertebrates.

With the evolution of biting jaws and endochondral bones, the lives of fishes changed. No longer were they restricted to grovelling through the bottom mud for minute food particles. Rather they could now actively seek prey, grasping it in their jaws and, if need be, manipulate it as it struggled. This made it possible for fishes to radiate into a wide variety of quite different ways of life. As a consequence, what followed in the Devonian was an explosive increase in the number and varieties of these animals that has been called the Age of Fishes. By the end of that period, all major groups of fishes had appeared.

Major Devonian groups such as the armoured fishes or placoderms became extinct shortly after the close of that period. Other major groups persisted into the late Palaeozoic, only to disappear then. Two groups continued to the present and dominate the seas: the chondrosteans or cartilaginous fishes (sharks and rays) and the osteicthyians or bony fishes (bony fishes; e.g. trout, tunneys, and tuna). They continued to evolve after the Devonian, adapting to the changing conditions of their environment.

At the end of the Palaeozoic Era sharks and rays almost became extinct. The few survivors diversified in the early Mesozoic, giving rise to a variety of new groups. Subsequently, there was another near extinction that again nearly brought about the total disappearance of the group. But once again the survivors radiated widely to become the sharks, rays and ratfishes in the seas today.

The bony fishes may be divided into the ray-finned and lobe-finned forms. The lobe-finned fishes, which include the living lungfish, have a central axis of large bones running the length of the paired fins, and the tiny bones or rays supporting the fleshy part of the fin radiate from this axis. The bones of the central axis in one early group of lobe-finned fishes, the rhipidistians, evolved into the limb bones of amphibians, reptiles, birds and mammals. In the ray-finned fishes, on the other hand, there is no central bony axis in the fins; rather, the rays supporting the fins originate within the body of the fish.

From a fish's perspective, it is the ray-finned which are clearly the success story. Crossing a significant adaptive threshold by modifying their skeletons, the most advanced group of ray-finned fishes, the teleosts, beginning in the Jurassic, underwent an extensive evolutionary radiation resulting in their forming more than 90% of all species of fish today.

However, from the perspective of a human or other land dweller, it was the lobe-finned fishes that provided the path to the future. For it was the structure of that type of fin with its strong central axis which was capable of being modified by evolution into the limbs of land dwelling vertebrates. Although the lobe-finned fishes today are only the few lungfish of South America, Africa, and Australia together with the coelocanth of Africa, first found alive in 1938, it was out of this major group, in particular the rhipidistians, that the terrestrial vertebrates arose.

References:

Carrol, R. L. 1997. Patterns and processes of vertebrate evolution. Cambridge University Press, Cambridge.

Conway Morris, S. & Whittington, H. B. 1979. The animals of the Burgess Shale. Scientific American 241 (1): 122-133.

Doyle, P. 1996. Understanding fossils. An introduction to invertebrate palaeontology. Wiley, Chichester.

Fairbridge, R. W. & Jablonski, D. (eds) 1979. Encyclopedia of paleontology, Dowden, Hutchinson & Ross, Inc., Stroudberg, Pennsylvania.

Gould, S. J. 1989. Wonderful Life. The Burgess Shale and the nature of history. Penguin, London.

Long, J. A. (ed.) 1993. Palaeozoic vertebrate biostratigraphy and biogeography. Belhaven Press, London.

Moy-Thomas, J. A. & Miles, R. S. 1971. Palaeozoic fishes. 2nd edn, Chapman & Hall, London.

Stanley. S. M. 1986. Earth and life through time. W. H. Freeman & Co., New York.