20 extant species of deep-sea scavengers (hagfishes)
have most of the basic characteristics of vertebrates ...except vertebrae!
in addition, hagfishes have several 'derived' (specialized) traits:
- mucous glands and thread cells
- degenerate eyes
- "teeth" on tongue
- unique feeding
Infraphylum VertebrataClass: Petromyzontida
includes >40 species of parasitic fishes (lampreys)
demonstrate the basic vertebrate design as larvae:
But as adults, they have many specializations, e.g.:
digestive system specialized for parasitism
tidal ventilationSeveral species are pests of commercial and sport fisheries.
The Origin of Jaws
Most vertebrates have jaws and paired appendanges: when did these evolve?
probably evolved in the mid Ordovician (~480 mya)
underwent a second duplication of the Hox genes;
possess numerous derived characteristics (specializations) compared to agnathans:
jaws, derived from the first two pharyngeal arches- teeth
two pairs of appendages (pectoral, pelvic), probably derived from skin folds
- gills with counter-currentexchange
gametes excreted via thearchinephric duct (sperm)and oviduct (eggs)
lateral line (canal containing groups of hair cells) for sensing motion in water
may also detect electrical fields
Do any living species exhibit just the basic gnathostome design?
No. Gnathostomes diversified during the Ordovician (~450mya), and include two derived lineages:
Class Chondrichthyes("cartilageous fishes")
"bony fishes" and tetrapods.
What’s special about the Chondrichthyes?
Living chondrichthyans have several distinctive characteristics:
skeleton calcified, but notossified; fins supported by keratinized rods
ventral mouth; intestine with spiral valve; liver often oil-filled for buoyancy
osmoregulation via urea
eggs generally develop in a protein sac(oviparous), but some species are ovoviviparous[female retain eggs during development] or viviparous [developing young are supported by a placenta]; internal fertilization via pelvic claspers
well-developed electroreception; reflective layer in retina enables vision in dim light; strong olfactory sensitivity
Is there much variation within Chondrichthyes?
Chondrichthyes underwent three major radiations; two major groups remain, including ~900 species:
feed on crustaceans
upper jaw fused to cranium; teeth modified into grinding plates; one gill slit
modern sharks, rays, skates
active predators and planktivores
jaw-opening mechanismenables jaw protrusion andsuction; stiff fins and heterocercal tail provide lift; multiple gill slits