Fishes and basal tetrapods

Key points

• Vertebrates are characterized by a skeleton made from bone (apatite).

• The oldest vertebrates are small fish-like creatures from the Early Cambrian of China.

• Armored fishes were abundant in Devonian seas and lakes.

• After the Devonian, the cartilaginous and bony fishes radiated in several phases.

• Conodonts commonly occur as tooth-like elements that are useful in biostratigraphy, as are some other fish teeth and scales (ichthyoliths).

• Tetrapods arose during the Devonian from lobe-finned fish ancestors, and fish-eating amphibians diversified in the Carboniferous.

• The first reptiles were small insect eaters.

• Synapsids dominated ecosystems on land during the Permian and Triassic.

• These groups were heavily hit by the end-Permian mass extinction event, and diapsid reptiles, most notably the dinosaurs, were key forms through the Mesozoic.

Most species do their own evolving, making it up as they go along, which is the way Nature intended. And this is all very natural and organic and in tune with mysterious cycles of the cosmos, which believes that there's nothing like millions of years of really frustrating trial and error to give a species moral fiber and, in some cases, backbone.

Terry Pratchett (1991) Reaper Man

The backbone is the key. Human beings are vertebrates, and so are horses, sparrows, alligators, turtles, frogs and trout. What they all share is their bony internal skeleton, and, in particular, vertebrae - the individual elements of the backbone. The skeleton consists of a backbone, a skull enclosing the brain and sense organs, and bones supporting the fins or limbs. Vertebrates are important today because humans are such a successful species, and also because of the huge diversity and abundance of species of bony fishes, birds and mammals. Other groups, such as insects and microbes, are even more abundant and diverse, but vertebrates include the largest animals on land, in the sea and in the air.

Vertebrates are a subgroup of the Phylum Chordata, a major deuterostome clade. Current views and debates about the nearest relatives of vertebrates are considered in Chapter 14. In this chapter, we look at the origin of vertebrates, the evolution of fishes from the Cambrian to the present day, and the Paleozoic tetrapods. The end-Permian mass extinction reset the clock for vertebrates on land, so we save the dinosaurs and their allies and the mammals for Chapter 17. If the vertebrate skeleton is so significant, what is so special about it?

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