The Jawless Vertebrates

•      Vertebrate Origins

•      Diversity

•      Origin of jaws

Vertebrate Origins

•      Vertebrates first evolved in marine habitats

•      Appeared by the early Cambrian period (500 mya)

•      Were jawless (Agnatha)

•      Were more active than the cephalochordates

–   Cephalochordate à agnathan

–   Filter feeder à more active predator

–   Cilia à muscular pharyngeal pump

Bone

•      Feature of early vertebrates

–   Absent in early Cambrian forms and in living jawless vertebrates

–   First appeared as teeth in the Conodonts

–   First appeared in external layers in dermal armor of the ostracoderms

Conodonts

•      Based on tooth-like microfossils  from the Cambrian to the end of the Triassic

•      New fossils have recently been found of laterally-compressed soft-bodied organisms with a complete set of teeth in the pharynx

–    v-shaped myotomes

–    Notochord and dorsal nerve cord

–    Caudal fin rays = postanal tail?

–    Teeth similar to other vertebrate teeth

Ostracoderms

•      Several lineages of extinct armored fishes

–    Sheets or small elements of bone with external layers of dentine and enamel-like tissue

•      Widely known form the Silurian and early Devonian periods

–    Wiped out during the late Devonian extinctions

Superclass Agnatha

•      Jawless Fishes

•      Diverse fossil record

–    Great radiation during Silurian and early Devonian

–    Few living species remain

•      Special senses

–    Complex eyes controlled by muscles

–    Inner ear with semicircular canals

–    Lateral line system

•      Paired appendages

•      Cellular bone in outer exoskeleton

Class Myxini

•       Hagfish

•       Entirely marine

•       Scavengers = Feed on dying or dead fishes, annelids, mollusks or crustaceans

•       Considered most primitive of living agnathans

Class Pteraspidomorphi

•      Appeared in late Cambrian through the late Devonian

•      Most had a head shield formed by fusion of several large bony plates and exoskeleton formed of small plates and scales

•      Paired nasal openings

Class Cephalospidomorphi

•      Long fossil record from early Silurian to Devonian and to present

•      Fossil forms heavily armored with a head shield of bony plates and smaller scales covering rest of the body

•      Some fossils had anterior lobes projecting from sides of head

•      Includes lampreys

Origin of Paired Fins

•      Early fishes - lacked paired fins

•      Pitch, Yaw and Roll

•      Fins as stabilizers

Fin-fold Theory

•      F.M. Balfour and J.K. Thacher (1850’s) and Erik Jarvik (1980’s)

•      Arose from a series of ventrolateral folds that condensed into pectoral and pelvic fins

Superclass Gnathostomata

•      Development of jaws

–    Derived from anterior pharyngeal arches

–    Allowed capture of larger prey

•      Early forms possessed two sets of paired fins

–    Pectoral and pelvic fins

–    Supported on cartilagenous or bony girdles

Origin of Jaws

•      Early evolution took place in marine habitats

•      Characterized by an increasingly active lifestyle

•      Three Steps:

–    Prevertebrate  = ciliated filter-feeding pharynx

–    Agnathan = muscular pharynx

–    Gnathostome = jaws

Prevertebrate

Agnathan

Gnathostome

Origin of Jaws

•      Agnathan

–    Mouth not supported by jaws

–    Splanchnocranium supported the roof of the pharynx and the pharyngeal slits

•      Acanthodian and Placoderms

–    First appearance of jaws

–    Arose from one of the anterior pair of gill arches

Class Placodermi

•      Early Silurian but flourished during the Devonian

•      Possessed jaws

•      Notochord

–    Ossified neural and hemal arches

–    Synarcual

•      Marine and freshwater

Class Acanthodii

•      Extinct, early Silurian to Permian

•      Most were small (20 cm).  Couple of large (2m) species

•      Dermal armor reduced to small plates

•      Row of spines along top and sides of body

•      Notochord prominent with ossified neural and hemal arches