542-488 million years ago

jawless fish

Although the earliest fish lacked vertebrae (and are thus classified as invertebrates), they developed cartilaginous rays in their tails. In the head, cartilage formed the splanchnocranium and parts of the chondrocranium.

Are hagfish and lampreys vertebrates? Most people assume that all fish are vertebrates and possess a vertebral column. However, in primitive fish, the notochord was not merely an embryonic structure, it formed the major axial support structure in adults as well. The notochord remained a continuous support rod through various lineages of fish and even into early amphibians. In hagfish and lampreys, the notochord is still the major support in the adult fish. The solid structures we refer to as vertebrae evolved gradually over hundreds of millions of years. Vertebrae began as small pieces of cartilage around the notochord which would gradually fuse and eventually replace the notochord. Hagfish do have cartilaginous rays in their tails (evident in the following photo), but since they are restricted to the tail they do not qualify as primitive vertebrae. Hence, hagfish are fish but they are not vertebrates. Since their brain is much more developed than that of more primitive chordates, the term "craniate" is often used to designate the group composed of hagfish and true vertebrates.

tail fin
The vertebrate skull is composed of three components: the splanchnocranium, chondrocranium, and dermatocranium. The splanchnocranium originated as the gill supports of the primitive chordates which were composed of cartilage in the earliest fish. The second portion of the skull to evolve was the chondrocranium and all craniates possess a chondrocranium under their brain (Kardong, 2002, p. 233, 239). The chondrochranium and splanchnocranium compose the head skeleton of jawless fish and cartilaginous fish. The head of lampreys consists of a number of cartilaginous structures and includes studs on the dorsal cranium similar to those observed in some fossil sharks and a pair of plates proceeding caudally from the occipital region (Dean, 1900). In higher vertebrates, the chondrocranium and splanchnocranium serve as embryonic scaffolding for the development of the structures of the head and throat. Some parts ossify and contribute to the bony adult skull. In the shark skull below, the splanchnocranium forms the upper jaw (palatoquadrate), lower jaw (Meckel's cartilage), and a series of branchial arches (including the hyomandibula of the second arch). Obviously the gill arches are lost in adult amniotes, but the embryonic splanchnocranium contributes to the malleus (Meckel's cartilage), incus (palatoquadrate), alisphenoid (palatoquadrate), stapes (hyomandibula of arch II), the hyoid bone (ceratohyal and basihyal components of arch II and the epibranchial, ceratobranchial, and hypobranchial portions of arch III), and larynx (derivatives of arches IV and V). The embryology of the skeletal and muscular structures of the jaws suggest that jaws developed evolved from a modified pair of gill arches (arch I) (Kardong, 2002, p. 237)