major fossil groups: their origins

major fossil groups: their origins The major fossil groups are those that have resistant hard parts and are thus preservable. They are not representative of the entire range of organisms that have lived, and many important taxa have left no fossil record. The major groups represented are invertebrates, an informal division of the kingdom Animalia that ranges from some organisms almost as simple as protists to highly complex animals such as insects. The number of invertebrate phyla is probably close to fifty, of which only eight have a significant fossil record. In addition the phylum Chordata (which includes the vertebrates) also has a significant fossil record.

A major problem in palaeontology is the origin of invertebrate phyla from a presumed eukaryotic (unicellular) ancestor at some time in the Precambrian. Evidence for a solution to this problem comes from the fossil record and also from the comparative anatomy and embryology of living representatives. Comparative anatomy shows that the invertebrates can be divided into groups by the way in which the coelomic cavities (cavities between the endoderm and ectoderm) formed. If this scheme is taken as a basis, the most primitive group consists of Protista, Porifera (sponges), and Cnidaria (corals). From these developed three other groups: Chordata and Echinodermata (starfish, etc.); Bryozoa and Brachiopoda; and Arthropoda, Mollusca, and Annelida (worms). The sequence of development of the mouth and anus proceeds in opposite directions in the Chordata–echinoderm group (deuterostomes) and the arthropod–annelid–mollusc group (protostomes), while the bryozoan–brachiopod group (lophophorates) shows affinities with both.

The fossil record of the earliest metazoans consists of trace fossils that can be recognized at the base of the Ediacaran interval about 680 million years ago. These show a change through the interval from simple shallow horizontal burrows to more complex horizontal and vertical burrows, and finally to arthropod traces. A fauna of complex metazoans found in the Ediacaran fauna consists of soft-bodied organisms of unusual design (the Vendozoa) that may not be directly related to the development of any later taxa. At the base of the Cambrian, a new fauna of animals with hard parts appears. This Tommotian fauna contains sponges, brachiopods, primitive molluscs, the calcareous archaeocyathids, and other organisms of unknown affinity. Shortly after this, the ‘Cambrian Explosion’ marked the rapid development and diversification of invertebrate taxa.

On the basis of this information it has been proposed that the earliest metazoans were probably wormlike organisms, similar to the larvae of modern cnidarians and developed from flagellate protists. From this form the cnidarians and poriferans developed; and after the development of a coelom gave more rigidity to the body, the wormlike ancestral form was able to live infaunally and leave burrowing traces. A necessary response to the requirements of locomotion is segmentation. Once this had appeared, the protostomes could develop. Deuterostomes and lophophorates did not develop full segmentation, and the lophophorates might have developed directly from filter-feeding worms. Deuterostomes were a later development, for echinoderms are not present until Early Cambrian times, and chordates, which probably developed from a swimming ancestor, are not present until the Middle Cambrian.

David K. Elliott