Palaeos Vertebrates 160.100 Temnospondyli

*Palæos:*		Unit 160: Temnospondyli
*The* *Vertebrates*		100: Temnospondyli

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Temnospondyli

Euskelia

The classical temnospondyl is Eryops: a big, flat slow-looking tetrapod with an enormous mouth. The classical definition of a temnospondyl involved vertebral characters. That is, the principal vertebral component was the intercentrum which (to varying degrees) tended to grow up and around the notochord and provided the basic structural support for the axial skeleton. By contrast, in reptilomorphs (i.e. amniotes and their immediate ancestors), the pleurocentrum grows down over the notochord, fuses with the neural arches, and eventually drives the intercentrum into anatomical oblivion. Actually, the intercentrum of Eryops is not all that well developed, even compared to Ichthyostega. Nevertheless, Eryops is the archetype around which the idea of a temnospondyl revolves.

Like most Carboniferous and Permian tetrapods, Eryops is a cipher. Even on a page devoted to speculation, it is difficult to find a way to fit these parts together. For example, it is all well and good to say that it used an inertial snap of the jaws to capture prey; but it is difficult to snap anything that big underwater, and equally difficult to imagine Eryops as a terrestrial hunter. Nevertheless, the dentition is rather unambiguous. It ate relatively large things that were not happy about the idea. The mouth appears to be well-designed for swallowing animals more or less whole and for keeping them there, in spite of strenuous objections, with such devices as internal fangs.

Perhaps the back forms a sort of an arch, peaking in front of the tall pelvis. This would bring the intercentra into relatively close contact, supporting the arch from below. The neural arches are not smooth curves, as in Ichthyostega. Instead, they are strongly sculpted, working tools for complex attachments of tendon and muscle. One could imagine a style of movement or predation in which substantial energy is stored in arching the back by movement of the hind legs, and then releasing it - perhaps quite suddenly - by stepping forward with the arms and contracting muscles anchored on the dorsal spine. This may be consistent with the odd pattern of early "tetrapod" tracks found in Nova Scotia as well as the structure of the ribs, which appear capable of sliding past each other. ATW000213.

Descriptions

Characters: 20- 300 cm; large heads with akinetic skulls; skull triangular to parabolic, or longirostrine; skull heavily ornamented; sensory line grooves may be present; $ palatal tusks common; $ very large interpterygoid fenestrae; tendency to develop additional cranial bones along midline (e.g. internasal, interfrontal, interparietal); "otic notch" frequently present, but probably did not support tympanum; "vertebrae 'rhachitomous' (with a large, dorsal, crescentic intercentrum and a small, dorsal, paired pleurocentrum) to stereospondylous (without an ossified pleurocentrum, although this element may be retained in a cartilaginous state)" [from Temnospondyli]; $ ornamented scapula; digits 4 (manus), 5(pes); amphibious or aquatic; some forms even with probable external gills.

Discussion:

One of the nice things about temnospondyls is that there are a great many of them. Not only did they manage to diverge into a great many species, but the more successful groups are known from numerous specimens. Thus, it is possible to examine aspects of this taxon which cannot normally be studied in paleontological material -- such as developmental biology. In several cases, there are so many specimens known that it has become possible to piece together the ontogenesis of temnospondyls. Temnospondyls are normally thought of as having three life phases: (a) a larval stage, probably aquatic and gill-breathing, (b) a juvenile transition phase; and (c) an adult form. However, in comparing species, taxonomists have largely restricted themselves to the adult morphs.

Recently, Jean-Sébastien Steyer, a graduate student at the Muséum National d'Histoire Naturelle in Paris, asked himself what would happen if one didn't discard all of the developmental data. Steyer (2000). Rather than (literally!) throwing out the babies with the bath water, Steyer set out to compare six different in-group taxa (plus Dendrerpeton as the out-group) whose developmental course was particularly well charted. The particular taxa and results were not particularly novel in this case, and were fully consistent with Yates & Warren (2000). As intended by Steyer, the point of real interest was not the results, but the method -- a method which has some very exciting possibilities.

In brief, Steyer scored the same 40 characters for each stage of each species studied, in effect treating a larval Apateon as separate species from the juvenile and adult forms for purposes of scoring. The scores were generally based on Dendrerpeton as the out-group, although there were some missing data and a few other exceptions. Separate cladograms ("ontotrees") were then generated for each morph across all species. Finally, a total evidence cladogram was generated, using all three sets of data. Interestingly, the three ontotrees were quite different. However, the combined data generated a topology which was identical with the larval cladogram and was quite robust (CI = 0.72).

Steyer was kind enough to discuss some of the fine points of the technique with me. In the process, I came to the conclusion that this method is at least as powerful as Steyer hopes it will be. In fact, one cannot easily think of a technique more likely to resolve the intractable deep nodes in the neornithine or mammalian radiations than phylogenetic taxonomy based on developmental biology. Haeckel's Law is not a "law," but it is certainly a frequent observed phenomenon. That is, we have good reason to hope that a systematic analysis of ontogeny will at least help us recapitulate (actually, recalculate) phylogeny. Conveniently, the living embryos of those species are around for all to see, so the work may be much more detailed -- and much easier -- than piecing together the shattered dermal bones of Carboniferous tetrapods.

But, first, there are some surprisingly thorny theoretical questions to be resolved. For example, just what is a synapomorphy in this context? Normally, the textbook answer is: a shared derived characteristic of a clade. Like a family recipe for lamb curry, or a tendency to practice obscurity for its own sake, some things mark a family of humans or other vertebrates and can duly be scored to produce cladograms. But what if say that our family is the one with beards? "So what?", you say, "Lots of people have beards." But no, I explain, in my family everyone has a beard: women, small children, parakeets, the lot. Thus the question of synapomorphy is more complex than presence or absence. It matters when those characters appear. And, if otherwise normal small children have beards in a family, would I score the beards of adult males in that family as a plesiomorphic feature of normal humans (state 0), or as the continuation of a peculiar apomorphy of my family?

Let us take a look at a very concrete example. In Figure 2, it is evident that the Parotosuchus has a large otic notch as a larval form, a notch which is still quite noticeable as a juvenile. This is a derived state. Dendrerpeton, the outgroup for almost all purposes, has little if any otic notch as a larva or juvenile. However, the adult morph of Parotosuchus has a small, narrow otic notch which looks very similar to the small, narrow otic notch of the adult Dendrerpeton. Steyer scores the otic notch of Parotosuchus 1 (larval), 1 (juvenile), and 0 (adult). But, if the otic notch of the adult is developed from the open notch of the juvenile in Parotosuchus, rather than from a notchless skull as in Dendrerpeton, is this really the same state or a developmental homoplasy?

Perhaps the latter is a better choice. That is, "once derived, always derived" is a reasonable rule of thumb. We are essentially dealing with developmental vectors, not the usual scalar quantities of a typical cladistic study. Each stage-specific character not only has a definite description, but also a developmental direction. The adult otic notch looks the same, but it developed from the opposite direction. Once derived, a character does not "underive." In normal cladistics, we may discover, ex post, that a what appeared to be a plesiomorphy was actually a reversal. But the calculation has already been performed. Here, we know ex ante that the notch is developmentally different from the notch of Dendrerpeton, so it should be scored differently from the beginning, and before the tree is calculated.

But Steyer may yet have the better argument. Consider the example of the beards. What if the beards of the women and children (and, of course, the parakeets) are developmental neomorphs brought about by some unique hormonal aberration, while the beards of adult males are the result of the normal pattern of development reasserting itself? How can I justify scoring the adult males as derived just because the children are?

Ultimately, it may be a judgment call. Or one might separately score the direction and value, so that the 1 -> 0 transition itself is given a score different from the 0 -> 0 development of Dendrerpeton. The problem is that this practice clearly violates the rule of independence. That is, the scoring of transitions depends in a simple way on the underlying states, with the result that the states may effectively be double-counted. This may distort the results significantly. In Steyer's 3-stage study, for example, scoring transitions would essentially require us to count the middle, juvenile state twice as often as the larval and adult forms (i.e. la->ju and ju->ad). This is a form of weighting which has no obvious theoretical justification.

This is a powerful technique with a great deal of promise, but working out exactly how to use it is not an easy proposition.

Characters: $ snout short with nasals square or hexagonal; full compliment of dermal roofing bones [C98]; orbit anteroventrally embayed (perhaps only in larger individuals) [C01]; frontals long & narrow, without participation in orbit [C01]; prefrontal only weakly sutured to lacrimal & nasals [C01]; $ postorbital broadly crescentic without ventral process into orbit margin [C98]; $ skull table approximately square [C98]; parietals short, forming hexagonal plate [C01]; pineal foramen just posterior to orbits [C01]; postparietals relatively long [C01]; intertemporal present [C01]; $ supratemporal broadly crescentic [C98]; supratemporal contacts postparietal [C98]; supratemporal surrounds most of otic notch (not squamosal) [C98]; $ distance from apex of otic notch to orbit less than diameter of orbit [C98]; tabulars square, without button or horn (C01 notes that horn may have been lost) [C01]; paraquadrate foramen present [C01]; quadrate with

broad dorsal plate and ventral articular surface [C01]; parasphenoid with broad triangular body thickened at edges in anterior portion [C98] [C01]; parasphenoid body with median smooth depression [C01]; parasphenoid with narrow cultriform process [C01]; basicranial articulation not fused [C98]; palate closed [C98]; broad vomerine plate [C98]; pterygoids meeting on midline [C98]; lower jaw of "standard" tetrapod pattern [C01]; coronoid-type crest & retroarticular process both absent [C01]; $ maxillary tooth count 38-40 with peak at positions 7-14 [C98]; fang pairs on vomers and palatines [C98]; palatines, vomers maybe all palatal bones, denticulated & striated [C01]; possible ectopterygoid tooth row [C01]; pterygoids and parasphenoid denticulated [C98]; cultriform process not denticulated [C01]; denticles and striations even on distal portion of quadrate ramus (!?) [C01]; pterygoids "striated" [C01]; dentary teeth unknown [C01]; axial skeleton poorly ossified circle [C98]; cervical ribs long, straight & somewhat expanded distally [C01]; trunk ribs only slightly curved & not expanded [C98] [C01]; single pair of stout sacral ribs [C01]; cleithrum present, long, straight, well-ossified, and expanded distally to wedge-shaped terminus [C98] [C01]; clavicles do not meet on mid-line [C01]; clavicle with dorsal blade having posterior face concave (cleithral articulation) [C01]; interclavicle diamond-shaped [C98]; interclavicle anterior edge crenellated (as some temnospondyls -- ATW) [C01]; scapulocoracoid single ossification, poorly ossified [C01] [C98]; humerus L-shaped [C98]; humerus entepicondyle quarter-circle [C98] [3]; humerus entepicondylar foramen present [C01]; ectepicondyle low [C01]; ulna more slender and slightly longer than radius [C01]; ulna with moderately developed olecranon process [C01]; manual unguals slender & tapered [C01]; ilium with both dorsal and posterior processes [C98]; femur ~ 18 mm (vs. 12-14 mm humerus) [C01]; tibia & fibula 11 mm, very similar, with interepipodial space [C01]; pes with five digits [C98]; pes phalangeal formula 2345? [C01]; ventral armor of narrow gastralia [C98]; dermal bone with pattern of radiating ridges [C01].

Notes: [1] as Clack (1998) notes, Eucritta is very close to basal temnospondyls in characters of the skull, except for the closed palate. [2] Clack (2001) makes the interesting point that the posterior stem on the interclavicle is a developmental artifact. It gradually grows out into a full diamond-shape during ontogeny. [3] in Clack (2001) the entepicondyle is described as "triangular." From the figure in the later paper (at right), the "quarter-circle" may refer to the ectepicondyle.

Characters: $ marginally elongate premaxilla; retain intertemporal and moveable articulation between base of braincase and pterygoid; squamosal embayment supported ventrally by the quadratojugal and a quadrate process.

Characters: marginal dentition with pseudocanine peaking with maxillary swellings above the peaks.

Introduction: This group is best known from Cochleosaurus bohemicus (Fritsch 1885), from the Late Carboniferous of Eastern Europe (equatorial Pangea). A reconstruction is shown at right from Milner (1980). Cochleosaurus was a medium-sized (120-160 cm) temnospondyl with a flattened skull of up to 16 cm. It lived as a fresh water aquatic predator of the "East Edaphosaurid-Nectridean Empire." Numerous specimens of various growth stages are known, and it is believed that Cochleosaurus was a common predator in its size range.

Characters: $ depressed areas with subdued sculpture between parallel sculpture ridges on the skull table; $ relatively elongate prechoanal region of the vomer; $ ectopterygoid separating from the subtemporal fossa.

Characters: Up to 100 cm; large laterally facing orbits; $ jugal narrowing to a point, making a point contact with the lacrimal; large, rounded "otic notch" in squamosal; stapes massive [suggesting support structure, not related to hearing -- but the "massive stapes" may be controversial, see Biology 356]; possible lanceolate expansion on anterior tip of cultriform process; lateral line sulci absent; short presacral column of 24 vertebrae, less than twice skull length; large stout limbs.

Note: according to one source, these characteristics "suggest a terrestrial lifestyle distinct from the aquatic and semiaquatic adaptations of most contemporary Carboniferous amphibians"

Characters: Some large, wide forms up to 2 m. Description is largely of Eryops. Large, wide skull; skull shape generally U-shaped; lower jaw triangular in lateral view; long, sharp labyrinthodont teeth; some accessory fangs on ectopterygoids, palatine, vomers, etc. (probably used inertial snap of jaws); nares almost terminal; depth of skull increases posteriorly, with fairly pronounced upward curvature ant to orbits; orbits face antero-laterally (variable); dermal skull ornamented with pits; $ intertemporal absent; $ parasphenoid firmly attached (sutured) to pterygoid, with no moving articulation between braincase and pterygoid; no separate cervical vertebral series (no neck!); dorsal vertebrae with moderately tall, thick neural arch; both pleurocentrum and intercentrum rather small; ribs broad (pronounced in Eryops); ribs shorten post, and may be absent at level of sacrum; 1 sacral, not (or not completely?) fused to ilia; caudal ribs variable in number and morphology, but tail is not longer than dorsal series; pectoral girdle not attached to skull; massive scapula; coracoid expanding to plate ventro-medial to glenoid; ilia oddly tall; humerus and femur short and massive; humerus & tibia oriented horizontally; radius & ulna short and well-separated, as are tibia & fibula; 3 carpals; 5 (4?) digits on manus, 5 on pes; Branchiosaurs may be embryonic and/or neotenous euskelians with external gills.

Characters: prefrontal not contacting postfrontal; otic notch extremely large & semi-circular; otic notch occupies entire back of squamosal; $ palatine dorsolateral margin exposed in orbital margin; $ narial, prefrontal and supratympanic flanges (?); $ orbits and pineal foramen large; interclavicle short & square; clavicles with narrow blades; long, slender limb elements; humerus slightly elongate & lacks supinator process.

Notes: Those who favor a temnospondyl origin for extant amphibians generally identify the dissorophids as the ancestral stock. ATW021002.

Note: The Eryopoidea were for a long time a sort of waste-basket taxon for mostly late Carboniferous and Early Permian temnospondyls that could not be slotted anywhere else. Yates & Warren (2000) reduce this to two families, the Eryopidae and Zatracheidae, although other families may also belong here.

Temnospondyli

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Euskelia

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