Animals with a notochord (a cylindrical sheath forming a flexible support for the back) or a backbone.
subphylum CEPHALOCHORDATA (Present)
Fish-like animals without bones or fins, but with a notochord (e.g. Amphioxus).
subphylum UROCHORDATA (Permian to Present)
The sea squirts or tunicates. Notochord only present in the larvae, which are active swimmers. Adults marine; benthic or pelagic; some colonial.
subphylum VERTEBRATA (Ordovician to Present)
Chordates with a skeleton of cartilage or bone.
class AGNATHA (Ordovician to Permian; also Present)
Vertebrates without jaws; they include Palaeozoic ostracoderms, with external plates, and the modern soft-bodied cyclostomes (hagfishes and lampreys). Most ostracoderms appear to have lived in rivers and freshwater lakes, but some were marine.
class PLACODERMI (Devonian and Carboniferous)
Jawed fishes with heavy external plates. Marine and fresh water.
class CHONDRICHTHYES (Devonian to Present)
Sharks with a cartilaginous skeleton. Mostly marine.
class OSTEICHTHYES (Silurian to Present)
This large class is split into several classes by some authors. It includes the Palaeozoic spiny sharks (Acanthodii), the lungfish and the Crossopterygians (some of which were ancestral to the first amphibians) and all the other bony fishes. The actinopterygian bony fishes include the Chondrostei, the Holostei and the Teleostei. The chondrosteans were common in the Upper Palaeozoic and Triassic. The holosteans were the dominant bony fishes in the Mesozoic. During the Cretaceous the teleosts became more common, and were the dominant fish of the Cenozoic. The ear-bones of bony fishes are known as otoliths. Mostly marine, but many freshwater.
class AMPHIBIA (Devonian to Present)
The adults are land animals, but the eggs are typically laid in water and the early life stages are usually aquatic.
class REPTILIA (Carboniferous to Present)
Mainly land animals, which develop from an egg that can be laid on land, though same are marine. They reached their maximum development in the Mesozoic. Reptilian teeth are usually simple cones.
t subclass ANAPSIDA (Carboniferous to Present)
Includes the earliest reptiles and also the turtles and the mesosaurs. The mesosaurs were stream-lined swimmers, in contrast to the turtles, which nevertheless were (and are) also mobile aquatic animals.
subclass LEPIDOSAURIA (Permian to Present)
Includes the lizards, the snakes and the lizard-like rhynchosaurs.
subclass ARCHOSAURIA (Permian to Present)
The ancestral thecodonts gave rise to the dinosaurs, which included many varied types which were dominant on the land during the Mesozoic. This subclass also includes the crocodiles and the winged pterosaurs.
subclass EURYAPSIDA (Permian to Cretaceous)
Mostly aquatic or amphibious, this subclass includes the early and relatively unspecialized nothosaurs, the long-necked plesiosaurs with paddle-like legs, the short-necked pliosaurs, and the fish-shaped ichthyosaurs.
subclass SYNAPSIDA (Permian to Jurassic)
These terrestrial mammal-like reptiles (paramammals) include the pelycosaurs, which had large dorsal spines.
class AVES (Jurassic to Present)
Birds typically have feathers, and show several skeletal differences from the reptiles (e.g. hollow bones). Except for some early forms, the birds have no teeth. They do not have a very complete fossil record.
class MAMMALIA (Triassic to Present)
Fossil mammals can be distinguished from the reptiles by many details of their skeleton, including the teeth which are among the more durable parts preserved fossil. Mammalian cheek teeth typically have several cusps. Fossil mammals are rare until the Palaeocene, that is, until after the extinction of the dinosaurs at the end of the Cretaceous.
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