HIERARCHICAL TAXONOMY OF THE STEGOCEPHALI

SYSTEMATIC BIOLOGY
CLASS STEGOCEPHALI+

Stegocephali (ste-go-SE-fa-le) is derived from two Greek roots that mean roofed or covered head [roof -stege (στέγη); and head -kephale (κεφαλή)]. This is a reference to the heavy, bony, flattened skulls of these animals.

This is a very problematic collection of taxa. They are the oldest tetrapods and can be identified by the presence of toes, a character shared with all other tetrapods. In addition, they retain a suite of sarcopterygian characters like the retention of a lateral line, broad fish-like tails (in some), the retention of gills in adult forms (in most), and the retention of fish-like teeth on the palate (a trait seen also in other tetrapods). Thus, there is no single synapomorphy that defines the group. Clearly, this is a paraphyletic group of taxa, and adding other large taxonomic entities to them would not solve the problem short of including all tetrapods in one large, rambling taxon. Systematists have similar problems with basal groups that show rapid radiation and give rise to other successful groups.

HIERARCHICAL TAXONOMY OF THE CLASS STEGOCEPHALI+. The following descriptions come from Benton (2005) and Nelson (2006). The structure of the following system is also based, in part, on the phylogenetic treatment by Mikko's Phylogeny Web (from the Finnish Museum of Natural History). Taxa in red are extinct.

Drawing of Acanthostega and Ichthyostega from: http://www.bertsgeschiedenissite.nl/geschiedenis%20aarde/devoon2.htm

CLASS STEGOCEPHALI[i] +

A class with a single order and 8 paraphyletic families.

ORDER UNNAMED

FAMILY ELGINERPETONTIDAE

Upper Devonian

Elginopteron

FAMILY ICHTHYOSTEGIDAE

Freshwater. Fishlike in form with functional gills as adults. Therefore, primarily aquatic. Particularly massive ribs. Broad tail with a swimming fin and lateral line system. The bones of the ankles and wrists were weak. Skull very much like that of Euthenopteron and Panderichthyes (sarcopterygians). Seven toes front and back; and the foot was paddle-like.

Upper Devonian

Ichthyostega

FAMILY ACANTHOSTEGIDAE

Freshwater. The animal was very similar to Ichthyostega except that it had a shorter skull and shorter gill coverings. Also, its ribs were quite small. Eight toes front and back, foot paddle-like. Clearly adapted primarily for an aquatic lifestyle.

Upper Devonian

Acanthostega

FAMILY TULERPETONTIDAE

Similar to Acanthostega and Ichthyostega but found in marine sediments.

Upper Devonian

Tuleropteron

FAMILY COLOSTEIDAE

Aquatic with lateral line system. Very long body with 40 trunk-neck vertebrae. Broad tail. Eyes placed forward. Along with the smaller teeth, they had some large, fang-like teeth.

Lower Carboniferous (Mississippian)

Greererpeton

FAMILY CRASSIGYRINIDAE

Members of this family remained aquatic as evidenced by a diminutive forelimb and reduced hind limb. It had a long body and likely had a large flattened tail. The mouth had large teeth and the skull retained a notch that is interpreted as a spiracle opening.

Lower Carboniferous (Mississippian)

Crassigyrinus

FAMILY WHATCHEERIIDAE

They had a leg and foot adapted to walking rather than swimming. The feet were pentadactyl.. In their compliment of teeth, they had at least one pair of massive teeth on the upper jaw. They retained the fish-like teeth on the upper palate. They had a somewhat high rather than flattened skull.

Lower Carboniferous

Whatcheeria, Pederpes

FAMILY BAPHETIDAE

Also called the loxommatids, these had very low skulls (the lower jaw was thicker than the cranium-upper jaw complex) with an unusual opening in front of the orbit (for a gland?). They had sharp pointed teeth with 6 or more fangs attached to the palate. They had traces of a lateral line. They seem to have developed a muscle for opening the mouth that ran from the back of the articulation of the joint to the side of the skull, a structure unlike fish and much like later tetrapods. Aspects of the postcranial skeleton are not known.

Lower Carboniferous (Mississippian)

Megalocephalus, Baphetes, Loxomma, Spathicephalus.

[i] I use this term in the old, more narrow sense to include all of the basal tetrapod families. In a strict phylogenetic interpretation, Stegocephali would include all of the animals with digits.