Synapomorphies of Craniata:

• Head
  • Expanded braincase with cranial nerves
  • Sensory capsules: olfactory, eyes, and otic (organs of balance, and later hearing)
  • Neurocranium (braincase) surrounding cranial nerves and thickened anterior neural tube (i.e., brain)
• Neural Crest
  • New germ cell layer, making craniates effectively "tetrablastic"
  • Appears dorsal and lateral to the neural tube and which contributes to a great variety of adult tissues and structures including: sensory neurons (nerve cells), some skeletal and connective tissues in the skull, and some pigment containing cells and other integumentary tissues. In the skull, the neural crest cells give rise to the brachial arches, jaws and parts of the braincase floor. In the gnathostomes and a number of fossil jawless vertebrates, the neural crest cells are also involved in the formation of the dermal skeleton (scales, teeth, and dermal bones).
• Mouth, with more specialized mouth parts
  • Pharynx with gills, used for both feeding and respiration
  • Branchial apparatus supported by cartiligenous brachial arches (gill bars) external to hemibranchs
  • Pharyngeal muscular pump (hypomeres)
• Larval endostyle transforms into adult thyroid gland
• Gut
  • Liver and pancreas present (formed through endoderm-mesoderm induction)
  • Digestive system invested with smooth muscular lining (rather than cilia)
• Circulatory System
  • Two chambered heart
  • Hemoglobin for oxygen transport
  • Erythrocytes to contain hemoglobin
• Increased size (an order of magnitude)
• Cartilaginous endoskeleton: including fin rays and brachial arches

In terms of metabolic rate and aerobic capacity, only cephalopod mollusks and some arthropods are comparable to craniates.

Living jawless craniates include Myxinoidea(aka Hyperotreti, aka hagfish) and Petromyzontida (aka Hyperoartei, aka lampreys). Together were once called Cyclostomata or Agnatha, but are almost certainly paraphyletic.

Myxinoidea seem to be basal to all other craniates, living or fossil. Myxinoids have tentacles, and (at least the living ones) have large ventrolateral slime glands, esophago-cutaneous duct on the left side, and elongate body shape. Primitive condition is several pairs of small gill openings. Primitive features relative to vertebrates:

• Their body fluid content (more than 10%, whereas it is less than 10% in all other craniates)
• The low oxygen affinity of their blood cells
• Their lack of cardiac innervation
• Their multiple veinous hearts
• Their lack of sensory-line neuromasts (although they have acoustico-lateral nerve fibres)
• Their comparatively simple pituitary gland
• Their lack of muscles in caudal fin web

Only fossil record of hagfish is from Mazon Creek, but some possible Chenjiang forms

Vertebrata, characterized by:

• Metamerically arranged endoskeletal elements flanking the spinal cord (aka vertebrae)
• Extrinsic eye muscles, allowing eye movements
• Radial muscles in fins
• At least two vertical semicircular canals in the labyrinth
• True neuromasts in the sensory-line system

Only living jawless vertebrates are petromyzontids. They are characterized by a large sucker surrounding the mouth, strengthened by an annular cartilage and by pine-shaped processes on gill arches. Also, they unique among extant vertebrates in having a median dorsal "nostril", the nasohypophysial opening, but some other fossil vertebrates also display the same structure. All fossil lampreys are Mazon Creek forms.

A problematic group of basal craniates, probably basal vertebrates: Euconodonta

• Range from Middle Cambrian to the end of the Triassic
• Once lumped with protoconodonts (probably chaetognath jaws) and paraconodonts (uncertain affinity)
• Known primarily from isolated elements composed of calcium phosphate (and may actually be bone, dentine, and enamel!)
  • Elements divided into coniform, ramiform, and platform morphologies
• Animal itself is elongate, has a notochord, chevron-shaped myomeres, a postanal tail, dorsal fin and caudal fin radials, and possible extrinsic eye muscles
• Elements are mouthparts:
  • Coniform and ramiform are graspers
  • Platform are crushers
• Seem to have been pelagic predators a few cm long

Presence of a phosphatic hard parts suggests that they are closer to the "ostracoderms" and gnathostomes than are petromyzontids; however, some dispute claim that conodont elements are true bone, dentine, and enamel

"Ostracoderms": paraphyletic grade of armored jawless vertebrates. "Ostracoderms" share with gnathostomes:

• Mineralized exoskeleton (i.e., a dermal skeleton), including the hard tissues dentine, bone, and enamel
• Sensory-line canals and grooves

An Upper Cambrian form (Anatolepis) is known from bony plates, but the morphology of the whole animal is not known.

Many clades of "ostracoderms" (see here, and websites linked therein, for an extensive overview). "Ostracoderm"-grade vertebrates common in Ordovician through Late Devonian strata.

Major trends among ostracoderms include:

• Evolution of a rigid dermal skeleton head shield
• Evolution of paired fins (and additional midline fins) for control in swimming

However, as long as jawless, gills had to serve "double duty": as organs of feeding and of respiration.

Gnathostomata (jawed vertebrates):

• First appear in Late Silurian; very common ever since
• Development of jaws: hard tissue mobile structures for feeding
  • Liberating gills for respiration only
  • May be specialized versions of brachial arches
• Stomachs (for processing the larger-sized food particles
• Dorsal and anal fins (may be present in more basal forms, and pelvic fins

Placoderms: basalmost gnathostomes, possibly paraphyletic with respect to toothed gnathostomes. Silurian-Devonian range. Jaws lack teeth, and instead have sharp slicing or blunt crushing surfaces of bone.

Remaing gnathostomes have teeth (modified from dermal scales). Basalmost of these are Chondrichthyes, the "cartilagenous fish". Questionable chondrichthyian scales from Late Ordovician and Silurian; unquestioned chondirchthyian fossils from Devonian onward.

Remaining vertebrates have ossified endoskeleton. Include basal forms and Osteichthyes, the "bony fish". Osteichthyes include two major branches:

• Actinopterygii: the "ray-finned fish"
  • Devonian onward
  • Many primitive groups, and the highly specialized Teleostei:
    • Mobile jaws of teleosts allow for many diverse feeding modes
    • First appear in Late Triassic, but become extraordinarily abundant in Late Cretaceous
• Sarcoptergyii: the "lobe-finned fish", including Tetrapoda
  • Also Devonian onward
  • Many diverse aquatic groups
  • One branch became terrestrial
  • Full terrestriality required the development of the amniotic egg

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