VERTEBRATE LIFE

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 UCMP Web Lift to Taxa from the fine folks at the Museum of Paleontology, University of California at Berkeley.  Start here for vertebrates

• The American Museum of Natural History provides an excellent overview of the classification and relationships of the vertebrates (including in more detail, the dinosaurs) through clear and to the point cladograms.

• Other sites to try are Jack Conrad's, The Vertebrate Phylogeny Pages or Toby White's Vertebrate Notes
• For an introduction to recent vertebrates, try the North Harris CC Montgomery College library  and learning resource center.

PHYLUM CHORDATA
        Subphylum CRANIATA
                Infraphylum MYXINOIDEA
                Infraphylum VERTEBRATA
                        Order PETROMYZONTIFORMES
                        GNATHOSTOMATA
                                Class CHONDRICHTHYES
                                Class OSTEICHTHYES
                                        Subclass ACTINOPTERYGII
                                        Subclass SARCOPTERYGII
                                                Order ACTINISTIA
                                                Order DIPNOI
                                                Division RHIPIDISTIA
                                                       Eusthenopteron, Panderichthys, etc.
                                                        EOTETRAPODA
                                                                Ichthyostega and other primitive "amphibians"
                                                                TETRAPODA
                                                                        Order AMPHIBIA
                                                                        AMNIOTA
                                                                                SYNAPSIDA
                                                                                        Class MAMMALIA
                                                                                Class REPTILIA
                                                                                         Testudines
                                                                                         Sauria (lizards, snakes, crocodilians, dinosaurs, and birds)
 

Phylum CHORDATA

Taxa

• 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 Life

Chordate characteristics

1. gill slits in the pharynx
2. tubular dorsal nerve chord
3. notochord
4. endostyle or thymus
5. postanal tail
6. tadpole larva

Subphylum CRANIATA

Taxa

• the craniates include the hagfish and the vertebrates.
• Learn  more about craniates from the UCMP or from the Tree of Life

Craniate characteristics

1. distinct head and brain
2. specialized [paired] sense organs
3. one or more semicircular canals
4. gills (for respiration) in pharyngeal slits
5. W-shaped myotomes

Infraphylum MYXINOIDEA

• 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 Tree of Life

Infraphylum VERTEBRATA

Taxa

• 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 a hagfish.
• ostracoderms [extinct Paleozoic (Cambrian to Devonian) jawless fish with an external skeleton of bone], lampreys, and gnathostomes.

Vertebrate Characteristics

1. 2 or 3 pairs of semicircular canals
2. mesonephric kidneys
3. bone
4. neural arches

• Learn  more about vertebrates from the UCMP or from the Tree of Life

Order PETROMYZONTIFORMES (upper Mississippian, middle Pennsylvanian, Recent)

• Learn  more about lampreys from the UCMP or from the Tree of Life.
• although lampreys only have a skeleton of cartilage, they are descended from OSTRACODERMS, jawless fish with an external bony armor, which lampreys have secondarily lost.
• The ostracoderms include the Thelodonti, Anaspida, Galeaspida, Pituriaspida, and the Osteostraci.  The Osteostraci are the sister taxa of the jawed vertebrates (gnathotomes).

GNATHOSTOMATA

Learn  more about gnathostomes (vertebrates with jaws) from the Tree of Life

Characteristics

1. internal jaws (palatoquadrate, Meckel's cartilage) are present.
2. paired appendages (paired pectoral and pelvic fins supported by an internal skeleton
3. three semicircular canals

Taxa

• 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.

Characteristics

1. skeleton is composed of cartilage, which may be prismatically calcified
2. placoid scales
3. claspers present in males for internal fertilization

Taxa

• living chondrichthians include sharks, skates, rays, and the ratfish.

CLASS OSTEICHTHYES (late Silurian to Recent)

Taxa

• living bony fish include the ray-finned (Actinopterygii) and lobe-finned (Sarcopterygii) fishes

Characteristics

1. internal skeleton ossified (i.e., bony)
2. swim bladder or lung present

SUBCLASS ACTINOPTERYGII [middle Devonian to Recent]

Learn  more about ray-finned fish from the UCMP or from the Tree of Life.

Taxa

• the ray-finned fish are the majority of Recent vertebrates (around 23,000 species)

Characteristics

• fins with multiple parallel endochondral supports
• fins controlled by muscles in body wall

SUBCLASS SARCOPTERYGII

Learn  more about lobe-finned fish from the UCMP or from the Tree of Life.

Characteristics

1. paired fins that are fleshy, with muscular lobes at base and with only a single element articulating with girdles)
2. forearm with a single proximal element (humerus), followed by the homologs of the radius and ulna.
3. paired external nostrils

Taxa

• 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)

Taxa

• Coelacanths
  • Only one living species is represented (Latimeria chalumnae).
  • 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..

Characteristics

1. internal nares absent
2. numerous paired snout bones
3. caudal fin tri-lobed

SUBCLASS CHOANATA

Characteristics

1. external nostrils connected by a continuous passage to the internal nares (choana)

Taxa

• lungfish, rhipidistians, and tetrapods

Comment

• 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 ancestral to tetrapods.

ORDER DIPNOI (Early Devonian to Recent)

Taxa

• lungfish, 3 living species:
• Neoceratodus (Australia); Lepidosiren (South America) and Protopterus (Africa)
• Learn more about lungfish from the UCMP

Characteristics

1. large tooth plates on palate, no marginal teeth
2. anterior of skull roof a mosaic of small bones tightly interconnected

DIVISION RHIPIDISTIA (Devonian to Permian)

Characteristics

1. skull roof bones homologous with skull and limbs of tetrapods
2. labyrinthodont teeth typically present

Taxa

• includes two orders of lobe-finned fish close to the ancestry of the tetrapods - the Orders Osteolepidoidea (including Eusthenopteron) and Panderichthyida - and the tetrapods.

Osteolepimorpha
 

DIVISION TETRAPODA

• Learn  more about terrestrial vertebrates from the UCMP  
• or from the Tree of Life
• or read Paul Olsen's web page

Taxa

• AMPHIBIA
• AMNIOTA (mammals, reptiles, and birds)

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