The Conquest of the Aquatic Environment
The vertebrates discovered in the upper Cambrian and lower Ordovician are animals without jaws (Agnatha), the first vertebrates with jaws (Gnathostomes) not being identified until the lower Silurian, that is at least 50 Ma later. These two groups have anatomically different structures (Fig. 3.1) corresponding to two modes of nutrition: microphagous for the Agnatha, macrophagous for the Gnathostomes. The first Gnathostomes were the fishes. It should be specified here that the popular sense of the term “fish” applies to free aquatic vertebrates with “cold blood” (the tunnies are warm-blooded as an exception in the group, since only the mammals, birds, and possibly the dinosaurs have acquired this characteristic), gills, and fins, a description applying to both the Agnatha and the fishes. The term “fish” has some validity in systematics, but it designates a group which cannot be characterized by the presence of a specific character, a paraphyletic group. The presence of a rigid exoskeleton led to the first vertrebrates being qualified as armored Cyclostomes or Ostracoderms. This term groups together the Heterostraci, the Osteostraci, and even the Anaspidae.
KeywordsPermian Silurian Sandstone Cretaceous Jurassic
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