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SARCOPTERYGII | TETRAPODA |--Acanthostega `--+--Ichthyostega `--+--Crassigyrinidae `--+--Colosteidae | `--Greererpeton `--+--Spathicephalus `--+--Baphetidae `--Tetrapoda* |--TEMNOSPONDYLI `--+--LEPOSPONDYLI `--REPTILIOMORPHA
Superficially, the colosteids appear quite different from their eryopid cousins. However, the differences are largely matters of shape or degree, rather than fundamental structure. The basic form of the head is very similar, surprisingly unchanged from the common panderichthyid ancestor. Possibly because of its reduced width, the jaw apparatus is more strongly braced in the center. That is, the outer elements serve more as the arched beams of a roof, supported by continuous long bones, the frontals and prefrontals, along the mid-line. By contrast, the skull of Eryops is too wide to support in this manner and the dermal bones of the jaw area are more evenly sized and generally oblong.
Unlike the eryopoids, the colosteids have a shoulder girdle which may be somewhat integrated with the skull, although the extent of the integration is difficult to reconstruct. In a fish, the cleithrum would integrate the girdle closely with the opercular bones forming the cover of the gill apparatus. colosteids do not possess an operculum, but retain the cleithrum. The cleithrum in this tetrapod group is a long thin bone with no obvious static connection to any particular structure. It may well have served a function analogous to the scapula of later vertebrates, and stabilized the shoulder girdle on the trunk, generally, rather than on the skull. Of course, the colosteids also possessed a scapulocoracoid with a similar, though smaller, dorsal extension. However, the coracoid element predominates. That is, the scapulocoracoid was largely a ventrolateral plate. It is believed that the colosteids may have also possessed a cartilaginous suprascapular, but its size, properties and function are unknown.
These elements were overlaid with a layer of three dermal bones, in the form of two large, generally diamond-shaped lateral clavicles and a ventral interclavicle. Together, they formed a broad thoracic shield. In addition, the central interclavicle had a long tongue-like process which extended forward to cover the throat and perhaps articulated, at least loosely, with elements of the lower jaw. Finally, the dermal bone layer was, in turn, overlaid by skin bearing heavy scales.
There is something to be said for the proposition that tetrapod evolution is a matter of adding neurons and subtracting bones. But, aphorisms aside, why all this complexity? Defense does not seem a viable answer. Anything big enough to take on a scale-covered meter-long carnivorous salamander with fangs would not be much deterred by these relatively thin bones and, in any case, would be unlikely to try a frontal attack.
Perhaps a more likely answer lies in the way the trunk is put together, and this requires a digression into vertebral structure. Primitively, the vertebrae consisted of discrete bony elements having no fixed relation to each other. The neural arches developed around the dorsal nerve cord, above the notochord. Pleurocentra rode generally on top or sides of the notochord, on either side and/or below the neural arches. Intercentra supported the notochord from below. (Although terms like "rode" and "supported" cannot be taken literally in this context). In some lineages, one or the other of the centra came to dominate the centrum, with the virtual elimination of the other central bone, as well as the notochord. Thus, for example, the vertebral body of amniotes is composed almost entirely of the pleurocentral element.
In the early temnospondyls and their sister group, the colosteids, the various elements were more or less evenly matched. The notochord remained an important structural element. The intercentra formed a continuous crescent around the ventral half of the notochord, and the paired pleurocentra flanked a large neural arch bearing a substantial dorsal spine. All of the bony elements articulated with each other, but they did not form a solid block as in later terrestrial vertebrates. This arrangement had the advantage of flexibility, but the disadvantage of central weakness. For an eel-like organism, this would make little difference. However, the colosteids, to judge by their small but reasonably serviceable limbs, used a mixture of central, eel-like, undulation and paraxial (off-center) limb-powered motion. To accomplish this complex mixture of motions, the colosteids needed some method of transferring force from the limbs to the axis of the body without, as it were, getting all bent out of shape and without losing central flexibility.
Their solution to this mechanical quandary seems to have been to have several levels of substantial tendonous attachments spread out across the dorsal surface. In addition to the notochord, Colosteids had strong neural spines which presumably bore tendons linking the arches. More tellingly, the colosteids also retained strong, generally horizontal ribs which attached both to the neural arches and to the intercentrum (that is, they were bicipital and could not bend dorsoventrally). The ribs bore uncinate processes -- flanges about halfway out -- and had a twisted, spatulate (broad & flat) distal end. Again, the most likely interpretation is that longitudinal tendons linked the ribs at both of these levels. Thus, instead of relying on a single, strong vertebral column, the Colosteids used as many as six dorsal cables to achieve strength without loss of flexion.
If this interpretation is correct, the complex pectoral girdle can be explained as a method of transferring paraxial forces flexibly across a number of layers of dorsal support in a manner that did not tax the strength of any one. (99????)
Range: Early Carboniferous to Late Carboniferous.
Phylogeny: Tetrapoda :::: (Spathicephalus + (Baphetidae + Tetrapoda*)) + * : Greererpeton.
Introduction: The Colosteids were a small group of medium-sized to large secondarily aquatic fish-eaters, with elongated, eel-like bodies with up to 40 presacral (trunk and neck) vertebrae and well-developed lateral line canals in the skull. The legs were small and many species probably spent their whole lives in water. The skull and lower jaw were low and flat.
Colosteids are usually considered very primitive members of the temnospondyl order. Their ancestry is by no means certain. Computer-assisted phylogenetic analyses places them close to the Baphetidae. (see: Michel Laurin's on-line essay Phylogeny of Terrestrial Vertebrates.). But the pattern of the braincase and skull roof are very primitive and resemble that of the Acanthostega (Carroll (1988: 170)), so it is possible they may have even evolved separately to the baphetid-crassigyrinid line. In any case, it is clear that the Colosteids represent one of a number of short-lived early Carboniferous tetrapod radiations. Certain specialized features make it unlikely that the Colosteids were directly ancestral to the temnospondyls. (MAK 010305)
Characters: Secondarily aquatic salamander-like forms with elongate, flattened bodies and small limbs, up to 1.5m, with tail 30-50% of length. Labyrinthodont dentition; one pair of premaxillary tusks which are large and fit into notch on dentary; dentary teeth markedly larger than maxillary teeth; elongate prefrontal extending to nares which (a) contacts the maxilla and premaxilla and (b) excludes lacrimal and nasal from nares; intertemporal minute or absent; broad contact between postorbital and parietal; orbits dorsal in adults; large stapes supports braincase on pterygoids (no impedance matching ear); large interpterygoid vacuities; braincase very primitive (like Ichthyostega); no embayment of squamosal (i.e., no otic notch); otic capsule incompletely ossified; gills(?); well-developed, about 40 presacral vertebrae; rachitomous vertebrae with ossified centra and approximately equal mass of pleurocentra and intercentra; ribs conspicuously bicipital, bearing uncinate processes, flattened and twisted distally; in G, probable lateral flattening and fin like surface over proximal 1/3 of tail; supraglenoid foramen absent; pectoral girdle forms broad thoracic shield; median process of interclavicle extends far anteriorly; cleithrum present, long but thin; humerus short; 4 digits on manus; one sacral rib; sacral rib unspecialized and not fused or firmly attached to ilium; no fin rays; lateral line grooves present; extensive scales dorsal & ventral; ventral scales rhomboidal in V-shaped pattern (dorsal scales vary).
Links: Batrachomorpha [Amphibia]; Dinosaurios, Tutorial interactivo (Spanish); Geol 437 amphibia, Fall, 1995; Phylogeny of stegocephalians; p7; Filogenias (Portuguese -- very strange phylogeny here, but this is an excellent and usually well-informed site);
References: Carroll (1988); Godfrey (1989). ATW030522.
Range: Early Carboniferous of North America
Phylogeny: Colosteidae : *.
Greererpeton buckemorani Romer
Adult Length: upto 1.5 metres long
Duration: Early Carboniferous (late Visean)
Region: equatorial Euramerica
Fossil remains: from Bickett Shale, Bluefield Formation, West Virginia
Comments: Greererpeton was one of a number of eel-like tetrapods that frequented early Carboniferous rivers and swamps. A member of the colosteid group of very primitive tetrapods, it had a low flat head, about 18 cm in length, a short neck, and an elongated body and long tail. The back contained about 40 vertebrae, about twice the usual labyrinthodont number. The legs were too small to support the weight of the creature on land. There is no otic notch for the ear-drum, but instead the skull has open grooves which in life were marked by lateral line sensory canals which could detect water-borne vibrations. (MAK 010305)
Links: Dinosaurios, Tutorial interactivo (Spanish); WVGES Mini-Museum, Photographs of Selected Fossils. ATW021030.
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