Tree of Chordates

Using a combination of anatomical, molecular, and fossil evidence, biologists have developed hypotheses for the evolution for chordate groups. The first transition was the development of a head that consists of a brain at the anterior end of the dorsal nerve cord, eyes and other sensory organs, and a skull. These innovations opened up a completely new way of feeding for chordates: active predation. All chordates with a head are called craniates. The origin of a backbone came next. The vertebrates are distinguished by a more extensive skull and a backbone, or a vertebral column, composed of a serious of bones called vertebrae. These skeletal elements enclose the main parts of the nervous system. The skull forms a case for the brain, and the vertebrae enclose the nerve cord. The vertebrae skeleton is an endoskeleton, made of either flexible cartilage or a combination of hard bone and cartilage. Bone and cartilage are mostly nonliving material. But because there are living cells that secrete the nonliving material, the endoskeleton can grow with the animal. The next major transition was the origin of jaws, which opened up new feeding opportunities. The evolution of lungs or lung derivatives, followed by muscular lobed fins with skeletal support, opened the possibility of life on land. Tetrapods, jawed vertebrates with two pairs of limbs, were the first vertebrates on land. The evolution of amniotes, tetrapods with a terrestrially adapted egg, completed the transition to land. -Biology book page 390

 

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