BIOL 1004 Lecture Notes - Lecture 20: Paraphyly, Chondrichthyes, Yolk Sac
Document Summary
Diversity iii chapter 34: most chordates are vertebrates but not all, hagfish are the only animals that have a skull but are not vertebrates, gnathostomes: vertebrates with jaws, tetrapods: 4-limbed vertebrates. Shared derived characteristics include: notochord: forms what we call the axial skeleton which provides anchoring for muscles in the body. Humans only have this as embryos (becomes the intervertebral discs in adult humans: dorsal hollow nerve cord: the brain attaches to the top end. In humans, the nerve cord is our spinal cord: post anal tail: the axial skeleton extends beyond the anus: in fish it is used as a tail to swim. Lancelets are marine suspension feeders that retain characteristics of chordate body plan as adults. Vertebrates developed in the early cambrian period, about 530 million years ago with a huge variety of marine vertebrates. One type of animal gave rise to vertebrates. The first tetrapods appeared 365 million years ago.