Vertebrates have been of immense importance in aquatic and terrestrial ecosystems from the Ordovician onwards. They are commonly present as predators and scavengers, becoming almost ubiquitous in modern faunas. However, their characteristic, multipart, internal skeleton fossilizes poorly, and they are under-represented in fossil assemblages.
The vertebrates are the most important group of the phylum Chordata. The characteristic element of a chordate is its notochord, the strengthened rod running down the back of the animal. This is usually mineralized in vertebrates to form a backbone, which surrounds a long nerve sheaf. In all skeletonized vertebrates the material used is calcium phosphate mixed with an organic material used as a template. This unusual and costly choice has physiological implications that may have contributed to vertebrate success. When organisms use oxygen faster than they can acquire it they begin to function anaerobically. This builds up high acidity in the body, which would quickly begin to corrode calcium carbonate secretions. However, calcium phosphate is resistant to such dissolution and allows vertebrates to overexert themselves for short periods, providing a useful energy boost.
The first vertebrates are Cambrian in age and include con-odonts and rare fish. A close ancestor of vertebrates is the Burgess Shale animal, Pikaia. Successive radiations of different vertebrate groups have taken place, with fish becoming common and diverse in the Silurian, amphibians in the Devonian, different descendents of reptiles in the Permian, Triassic, and Jurassic, and birds and mammals in the Cenozoic (Fig. 11.1). Ecological niches have frequently been filled by successive groups of vertebrates, one after another, either competitively or by passive replacement.
Fig. 11.1 The evolutionary relationships of major groups of vertebrates.
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