Origin of Jawed Vertebrates

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

Origin of Jaws

•      The Evidence:

–    Jaws in  series with branchial arches in Shark

–    Both jaws and branchial arches derived from neural crest cells

–    Cranial nerves and blood vessels in series with arches

–    Jaw musculature derived and modified  from branchial arch musculature

•      Problem: where did jaws actually come from?

–    1^st, 2^nd, 3^rd or 4^th arch?

Serial Theory

•      One arch one mandible theory

•      Jaws arose from 1st or 2nd branchial arch

•      Hyoid came from next arch in line

Composite theory

•      Erik Jarvik

•      Based on anatomy of fossil fishes and fish embryology

•      Started with 10 branchial arches

•      Terminal, premandibular, mandibular, hyoid and 6 branchial arches

•      Series of losses or fusions of various elements

Ancentral Jawless vertebrate

•      Two sets of gill supports (External and internal branchial arches)

•      Afferent and efferent arteries found internally

Lamprey

•      Internal arches are lost = move to tidal ventilation

Gnathostome Ancestor

•      Internal branchial arches enlarged

–   Support strengthened breathing muscles

First Gnathostome

•      First branchial arch modified into a jaw

•      Mouth opening enlarges

Jon Mallot

•      Initial enlargement of the mandibular arch into proto-jaws

–   Improved gill ventilation rather than feeding

•      Progressive enlargement of the internal support in the gills

–   Related to strengthening muscles that move the arches back and forth during ventilation

Gnathostome