Palæos:		Unit 160: Temnospondyli
The Vertebrates		050: Temnospondyli

Temnospondyli

TETRAPODA
|--+--LEPOSPONDYLI
|  `--REPTILOMORPHA
|
Temnospondyli    
|--Eucritta  
`--+--Edopoidea
   |  |--Edops    
   |  `--Cochleosauridae
   `--+--Dendrerpeton
      `--+--Euskelia
         |  |--Dissorophoidea   
         |  |  `--LISSAMPHIBIA
         |  `--Eryopoidea
         `--Limnarchia
               |--Dvinosauria
               `--Stereospondyli
                  |--Rhinesuchidae
                  `--+--Capitosauria
                     `-Trematosauria
                        |--Trematosauroidea
                        `--+--Metoposauroidea
                           `--+--Plagiosauroidea
                              `--+--Rhytidosteidae
                                 `--Brachyopoidea

1. Eucritta X
2. Temnospondyli  

Temnospondyl Systematics

Traditionally, there were only two temnospondyl suborders, the Ratchitomi and the Stereospondyli, which were determined according to vertebral type, and these are mentioned in the older books.  Romer provided a third suborder, the fish-eating marine-going trematosaurs.  Other paleo-tetrapod authorities such as Nilsson and Panchen removed the short-headed Peltobatrachidae and Plagiosauridae from the Brachyopoids and placed them in a seperate order.  Later writers like Carroll and Benton did away with the ratchitome - sterospondyl classification altogether, arguing that the sterospondyls were an artificial (polyphyletic) group, the stereospondylous condition having evolved seperately a number of times.

However, the relationships between the different temnospondyl families, and sometimes even the question of which family goes in which superfamily, remained very controversial.  Many of these problems in temnospondyl taxonomy were addressed by Yates & Warren (2000) in comprehensive cladistic analysis from which the following cladogram is taken.

In this diagram, the geological period is shown on the top.  The known occurance of taxa (genera and families) in the fossil record is indicated by solid bars.  Thin lines indicate hypothetical ancestry (cladistic techniques forbid deriving one known monophyletic genus or family from another, rather, both are derived from a hypothetical "most recent common ancestor", which gives the branching pattern as shown above).

Several things need to be said about this diagram.  First, not every temnospondyl taxon is included.  Many known types are not shown here.  However, most of the important forms are shown.  Secondly Yates and Warren revive the monophyletic Trematosauria  and Stereospondyli, but replaced the Ratchitomi with the clades Euskelia and Limnarchia. And third, not everyone agrees with these particular phylogeny. A common alternative for example is to have the Dvinosauria as a very early and quite distinct clade, prior to the Euskelia and the higher "Limnarchia" (several of the cladograms displayed here show this option)

Descriptions

Temnospondyli: Dendrerpeton. 

Range: from the Early Carboniferous  

Phylogeny: Tetrapoda*: (Lepspondyli + Reptilomorpha) + *: Eucritta + (Edopoidea + (Dendrerpeton + (Euskelia + Limnarchia))). 

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. 

Links: Dendrerpeton and Joggins, Nova Scotia; Temnospondyli; Basal Temnospondyli; Historický vývoj (Czech); Phylogeny and Apomorphies of Temnospondyls. 010806.

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.

References: Steyer (2000); Yates & Warren (2000).

Eucritta:  E. melanolimnetes Clack 1998

Range: Early Carboniferous (Brigantian, latest Viséan) of Europe (Britain). 

Phylogeny: Temnospondyli: (Edopoidea + (Dendrerpeton + (Euskelia + Limnarchia))) + *.

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.  

Links: AE TREE2000.pdf; relics: The creature from the black lagoon.  

References: Clack (1998) [C98], Clack (2001) [C01]. ATW020820.  

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