This section gives a brief overview of the classification of the vertebrates.
For more information the following sites are highly recommended:
- Tree of Life: "A
collaborative Internet project containing information about phylogeny and
biodiversity" run by David Maddison and Wayne
Maddison of the University of Arizona. The vertebrates begin here.
- The American
Museum of Natural History provides an excellent overview of the classification
relationships of the vertebrates (including in more detail, the dinosaurs) through clear
and to the point cladograms.
Eusthenopteron, Panderichthys, etc.
Ichthyostega and other primitive "amphibians"
Sauria (lizards, snakes, crocodilians, dinosaurs, and birds)
- besides the vertebrates, the chordates include tunicates (Urochordata) and lancelets
(Cephalochordata), chordates that, among other characteristics, differ from
the vertebrates in lacking a backbone
- Learn more about chordates from the UCMP or from the Tree of
- gill slits in the pharynx
- tubular dorsal nerve chord
- endostyle or thymus
- postanal tail
- tadpole larva
- the craniates include the hagfish and the vertebrates.
Learn more about craniates from the UCMP or from the
- distinct head and brain
- specialized [paired] sense organs
- one or more semicircular canals
- gills (for respiration) in pharyngeal slits
- W-shaped myotomes
- the most primitive group of craniates, includes the living and fossil hagfish.
- in traditional classifications, hagfish, lampreys, and the extinct armored jawless fish
are grouped together in the paraphyletic Class Agnatha.
- Learn more about hagfish from the UCMP or from the
- Vertebrates first
appear in the early Cambrian. Two genera, Myllokunmingia
and Haikouichthys are known
from China. They lack bone, but have skeletons of cartilage.
Haikouichthys looks like a lamprey, Myllokunmingia looks like
- ostracoderms [extinct Paleozoic (Cambrian to Devonian) jawless fish with an external
skeleton of bone], lampreys, and gnathostomes.
- 2 or 3 pairs of semicircular canals
- mesonephric kidneys
- neural arches
Order PETROMYZONTIFORMES (upper Mississippian, middle Pennsylvanian, Recent)
- Learn more about lampreys from the UCMP or from the
- although lampreys only have a skeleton of cartilage, they are descended from
jawless fish with an external bony armor, which lampreys have secondarily lost.
- The ostracoderms include the Thelodonti,
and the Osteostraci.
The Osteostraci are the sister taxa of the jawed vertebrates (gnathotomes).
Learn more about gnathostomes (vertebrates with jaws) from the Tree
- internal jaws (palatoquadrate, Meckel's cartilage) are present.
- paired appendages (paired pectoral and pelvic fins supported by an internal skeleton
- three semicircular canals
- the jawed vertebrates include the extinct (upper Silurian to early Carboniferous),
armored, jawed-fish (Class Placodermi), the cartilaginous fish, and the bony fish
(including the tetrapods).
CLASS CHONDRICHTHYES (?early Silurian, early Devonian to Recent) Learn
more about cartilaginous fish from the UCMP.
- skeleton is composed of cartilage, which may be prismatically calcified
- placoid scales
- claspers present in males for internal fertilization
- living chondrichthians include sharks, skates, rays, and the ratfish.
CLASS OSTEICHTHYES (late Silurian to Recent)
- living bony fish include the ray-finned (Actinopterygii) and lobe-finned (Sarcopterygii)
- internal skeleton ossified (i.e., bony)
- swim bladder or lung present
SUBCLASS ACTINOPTERYGII [middle Devonian to Recent]
Learn more about ray-finned fish from the UCMP or
from the Tree
- the ray-finned fish are the majority of Recent vertebrates (around 23,000 species)
- fins with multiple parallel endochondral supports
- fins controlled by muscles in body wall
Learn more about lobe-finned fish from the UCMP or from the
- paired fins that are fleshy, with muscular lobes at base and with only a single element
articulating with girdles)
- forearm with a single proximal element (humerus), followed by the homologs of the radius
- paired external nostrils
- the lobe-finned fish include the Dipnoi (lungfish), coelacanths (Actinistia), the
extinct rhipidistian fish, and their descendants, the tetrapods
ORDER ACTINISTIA (Middle Devonian to Recent)
- Only one living species is represented (Latimeria
- This group was thought to have gone extinct in the Cretaceous until a living coelacanth
from the depths of the Indian Ocean was recognized in 1939..
- internal nares absent
- numerous paired snout bones
- caudal fin tri-lobed
- external nostrils connected by a continuous passage to the internal nares (choana)
- lungfish, rhipidistians, and tetrapods
- Dingus and Rowe state that the lungfish are most closely related to the tetrapods.
However, Maisey (and others) interpret that lungfish are too specialized to be
ORDER DIPNOI (Early Devonian to Recent)
- lungfish, 3 living species:
- Neoceratodus (Australia); Lepidosiren (South America) and Protopterus
- Learn more about lungfish from the UCMP
- large tooth plates on palate, no marginal teeth
- anterior of skull roof a mosaic of small bones tightly interconnected
DIVISION RHIPIDISTIA (Devonian to Permian)
- skull roof bones homologous with skull and limbs of tetrapods
- labyrinthodont teeth typically present
- AMNIOTA (mammals, reptiles, and birds)