vertebrates

vertebrates The vertebrates are of interest to us because the human taxonomic group is included within them. Vertebrates are a subphylum of the phylum Chordata and retain the chordate characters of a notochord (a longitudinal stiffening rod), a tubular dorsal nerve cord, and gill slits (lost in the adult stage of land-dwelling vertebrates). However, in addition, vertebrates have developed a brain and an internal bony skeleton that have enabled them to become far more successful and widespread than their chordate relatives the urochordates and cephalochordates. These lower chordates are marine filter-feeders, and it is generally accepted that the cephalochordates, which are rather fish-like in appearance, are the closest relatives of the vertebrates although not their direct ancestors. The presence of the cephalochordate Pikaia in the Middle Cambrian Burgess Shale suggests that vertebrates were already present then, and this has recently been substantiated by the discovery of vertebrates in the Middle Cambrian of Chengjiang, southern China. This seems to indicate that the earliest vertebrates did not possess bone. However, this is present in fossil forms from the Ordovician of North America, Australia, and Bolivia. Sufficient articulated material exists to show that these animals were agnathans (jawless vertebrates), with a bony exoskeleton, and they appear to have inhabited shallow-marine environments.

David K. Elliott