It is not of the essence of the vertebrate type to be tetrapodal.

Either the hordes of chordates of the vertebrate subgate (or vertebrate subgate) characterized by the skeleton. Vertebrates are bilaterally symmetric and have an internal skeleton of bone or cartilage, divided into the nervous system of the brain and the spinal cord. Vertebrates have well-developed body cavities (called body cavities), including hollowed heart, large digestive organs, liver, pancreas, kidney, which blood contains red and white bodies. Vertebrates include fish, amphibians, reptiles, birds and mammals

Quadrupeds are limb vertebrates, now represented by amphibians, reptiles, birds and mammals. The tetrapod in the early Devonian era (419-390 to 590 million years ago) was very interested in paleontologists. They are the first vertebrate entering the land and opened the way to the life of the vertebrate in the future on land. Migration from water to land necessarily affects all aspects of these animal biology, but so far no serious attempt has been made to investigate their life history. Although well-preserved cockroaches are rarely seen, they are only supposed to represent adults

Vertebrate During the Devonian era, a typical evolutionary event occurred in life history: Reef Finfish lived in dry land. The two best candidates of the earliest quadruped (4 legged vertebrate) are Acanthostega and Ichthyostega, which evolved from early exclusive marine vertebrates such as Tiktaalik and Panderichthys. Surprisingly, many of these early tetrapods have seven or eight numbers on each leg. This means that they represent a "dead end" of evolution Animals use 5 fingers, 5 toes are body plans