Palaeos Vertebrates 150.400 Tetrapoda: Carboniferous Forms

*Palæos:*		Unit 150: Tetrapoda
*The* *Vertebrates*		200: Carboniferous Forms

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Tetrapoda: Carboniferous Forms

Greerpeton & Spathicephalus

Taxa on This Page

Colosteidae

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????)

Spathicephalus: another aquatic frisbee

Like some of the much earlier Silurian galeaspids and the much later Triassic plagiosaurs Spathicephalus somehow made a living with an almost two-dimensional head. Exactly how it managed to do so is -- as with galeaspids and plagiosaurs -- a matter of considerable uncertainty. The enormous jaw and flat head making closing the mouth against the resistance of water a major concern. But the size of the antorbital fossa and the adductor chamber suggest that the jaw adductors were actually weaker than comparable organisms of the same, more or less baphetid, design.

Of course the peculiar teeth of Spathicephalus tell us that it did not feed in the same way. Unlike the baphetids, Spathicephalus had numerous small, thin teeth, no fang pairs, and very few denticle-bearing areas on the palate. The teeth seem to have present all at the same time, as opposed to the usual tetrapod arrangement involving unsynchronized replacement of a smaller number of large teeth. The theory seems to be that Spathicephalus strained water or substrate through these teeth.

Descriptions

Characters: Limited ossification of vertebral centra, no occipital condyle. No anocleithrum. Cheek (very!) moveable on skull roof, like Reptilomorpha, intercentra & pleurocentra roughly equal, orbits close to midline, tabular does not contact parietal.

Note: The current (2002) belief is that Crassigyrinus is a rather weird anthracosaur. Eventually, we will get around to moving it there. Meanwhile, the current belief is also that Crassigyrinus is one of the major causes of cladogram instability in this area. Accordingly, until things settle down a bit, it will stay in the old neighborhood.

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).

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)


Spathicephalus: skull in dorsal and lateral views. From Beaumont & Smithson (1998).	Spathicephalus: skull in palatal and occipital views. From Beaumont & Smithson (1998).

Characters: skull length to 22 cm [BS98]; skull very flat, broad & blunt [C02] [BS98]; skull width almost equal to length [BS98]; snout bones laterally expanded [BS98]; internasals absent [BS98]; antorbital large & vacuity smoothly continuous with orbit (no "key-hole") [BS98]; orbits closely spaced & strongly excavated [C02] [BS98]; orbits set 75% skull length back [BS98]; orbits face dorsally [BS98]; frontals enter orbit margin with thickened interorbital bar [BS98]; skull table very short [BS98]; intertemporal absent [BS98]; supratemporal forms anterior border of otic notch [BS98]; small pineal opening [BS98]; tabular horns and tabular posterior boss present [BS98]; quadratojugal strongly sutured to quadrate [BS98]; single occipital element [BS98]; occipital bone(s) makes no contact with skull table [BS98]; posttemporal fenestra absent [2] [BS98]; supraoccipital absent [BS98]; braincase: otic capsules broad & well ossified [BS98]; otic capsules strongly attached to skull roof [BS98]; stapes robust with large plate-like foot on fenestra ovalis and large distal region resting against quadrate ramus [BS98]; parasphenoid broad, forming floor of fenestra ovalis [BS98]; widely separated basipterygoid processes [BS98]; parasphenoid sutured to basipterygoid processes [BS98]; cultriform process short & narrow [BS98]; palate: premaxilla without midline buttress [BS98]; pterygoids sutured along midline and probably fused with basipterygoid processes [BS98]; pterygoids, vomers & parasphenoid ornamented with pits & ridges (as pterygoid quadrate ramus of baphetids) [BS98]; pterygoid quadrate ramus forms medial wall of adductor fossa and meets squamosal dorsally [BS98]; lower jaw: very shallow (22 mm in known specimen) [BS98]; dentaries & presplenials strongly sutured and immobile [BS98]; parasymphysial plate present (apparently with row of small teeth) [BS98]; parasymphysial teeth point medially, toward opposite ramus [BS98]; coronoids without denticles, but with fine ornamentation [BS98]; dentition: all teeth small, chisel-shaped and numerous [C02] [BS98]; vomer with row of smaller teeth [BS98]; teeth pectinate (filter feeder?) [C02]; teeth all present at same time [C02]; dentary tooth count probably 110-120 [BS98]; palatal bones & coronoids without denticles [BS98]; postcranial: interclavicle rhomboidal [B77].

Note: [1] Spathicephalus is shown as outside the Baphetidae because that's the way it came out when scored and run using a slightly modified version of the database of Clack (2001). No one has much confidence in any of the cladistic results in this area of phylospace at the moment. However, Spathicephalus scores so consistently primitive in many different runs, that this placement may be correct. This makes the Loxommatoidea (Baphetidae + Spathicephalus) paraphyletic. This is perhaps unlikely, but not really any stranger than most other possibilities. [2] This may be a matter of definition. The geometry of the occiput is odd, because the whole structure is so vertically compressed, laterally extended, and strongly fused. In addition, the extent of vertical compression has been exaggerated by crushing during preservation. It may be impossible to say exactly what the exact state of the occiput might have been at this point.

Tetrapoda: Carboniferous Forms

Greerpeton & Spathicephalus

Abbreviated Cladogram

Contents

Taxa on This Page

Colosteidae

Spathicephalus: another aquatic frisbee

Descriptions