Sclerothorax, Huene, 1932

Schoch, Rainer R., Fastnacht, Michael, Fichter, Jürgen & Keller, Thomas, 2007, Anatomy and relationships of the Triassic temnospondyl Sclerothorax, Acta Palaeontologica Polonica 52 (1), pp. 117-136 : 130-133

Relationships of Sclerothorax

Previous hypotheses

In modern cladistic parlance, Sclerothorax was considered a basal stereospondylomorph temnospondyl by Huene (1932), and subsequent authors have followed him in that consideration ( Romer 1947; Schoch and Milner 2000). Huene referred especially to palatal characters, such as the palatine ramus of the pterygoid and the relatively narrow interpterygoid vacuities, but also mentioned the small orbits and the shape of the postorbital as diagnostic. He concluded that, despite a few palatal features shared with metoposaurids and brachyopids, Sclerothorax was most similar to Sclerocephalus (= Onchiodon ) frossardi and Sclerocephalus haeuseri , and therefore placed the taxon within the family Actinodontidae .

True stereospondyls, at that time defined by the structure of their more fully ossified disc−shaped vertebral segment, seemed too different from Sclerothorax to be considered close relatives, although similarities with capitosauroids were explicitly highlighted by Huene (1932). Later, Romer (1947) listed Sclerothorax as a “sterile side branch” of his “neorhachitomes”, a group comprising rhinesuchids and lydekkerinids. In a review of the clade, Schoch and Milner (2000) attempted to place Sclerothorax within the Stereospondyli , albeit as incertae sedis and within its own family Sclerothoracidae as suggested by Huene (1932).

In the following analysis, some of the characters employed by the above listed authors to characterize the taxon turned out to be synapomorphies shared by Sclerothorax and stereospondyls, but their largely diagrammatic reconstruction of the skull was partially erroneous and based on only a fraction of the actual material, which was presumed lost but reappeared due to the active search of one of us (TK).

Phylogenetic analysis

Taxa.—Outgroup: Dendrerpeton acadianum (we exclusively refer to the single find described by Holmes et al. 1998). Dissorophidae —here represented by only two species, Cacops aspidephorus and Dissorophus multicinctus , complementing one another ( Williston 1910; DeMar 1968), the Zatrachydidae represented by Acanthostomatops vorax ( Boy 1989) , the Eryopidae represented by Eryops megacephalus ( Miner 1925; Sawin 1941; Moulton 1974; Pawley and Warren 2006). All other taxa fall into the Stereospondylomorpha of Yates and Warren (2000): Sclerocephalus haeuseri ( Boy 1988) , Peltobatrachus pustulatus ( Panchen 1959) , Australerpeton cosgriffi ( Barberena 1998; Dias and Schultz 2003), Uranocentrodon senekalensis ( Van Hoepen 1915) , Lydekkerina huxleyi ( Pawley and Warren 2005) , Laidleria gracilis ( Kitching 1957; Warren 1998), Siderops kehli ( Warren and Hutchinson 1983) , Gerrothorax pustuloglomeratus ( Hellrung 2003) , Paracyclotosaurus davidi ( Watson 1958) , Mastodonsaurus giganteus ( Schoch 1999a) , Trematolestes hagdorni ( Schoch 2006) , the Metoposauridae ( Dutuit 1976; Hunt 1993). Finally, we included Sclerothorax hypselonotus , based on the findings of the present study.

The choice of taxa followed several general considerations. First, we had to include representatives of all major stereospondyl clades ( Rhinesuchidae , Lydekkerinidae, Brachyopoidea , Plagiosauridae, Capitosauroidea, and Trematosauroidea sensu Schoch and Milner 2000 ). Second, we added two enigmatic temnospondyls, Peltobatrachus and Laidleria , whose phylogenetic whereabouts are far from clear but which both share interesting postcranial features with Sclerothorax such as dermal armour. Finally, as Sclerothorax has some “terrestrial” features in the postcranial skeleton, we included zatracheids, dissorophids, and eryopids, although they all fall well outside the stereospondylomorphs ( Boy 1990; Yates and Warren 2000). We have omitted Lapillopsis , as there are no articulated postcranial skeletons known of that taxon.

Characters.—Seventy characters from all parts of the skeleton were analyzed. All character−states are listed and defined in the Appendix 1 and scored in Table 1. Most of them are taken from the literature, sources are throughout indicated. As most of the considered taxa are not represented by juveniles or larvae, early ontogenetic stages (e.g., in eryopids, zatracheids, or Sclerocephalus ) have been ignored.

Details of analysis.—The programmes PAUP 3.1 ( Swofford 1991) and MacClade 3.0 ( Maddison and Maddison 1992) have been employed to analyze the character distribution, run in the heuristic mode. Throughout, binary charactercodings were preferred, but in five cases (characters 25, 42, 43, 52, and 53) multiple states had to be coded, treated as unordered in PAUP. The analysis (using both outgroups) gave one most parsimonious topology, requiring 163 steps. The consistency index of that tree is 0.485, the retention index 0.690, and the rescaled consistency index is 0.334. We performed analyses of node stability (Bremer Decay Index), their results are listed in the following section. Variation of outgroups (inclusion of both outgroup taxa versus use of either of the two) did not cause any change to the result of the analysis, giving the same topology. Exclusion of the poorly known Peltobatrachus gave the same result, a completely resolved tree requiring 159 steps, a consistency index of 0.497, a retention index of 0.686, and a rescaled consistency index of 0.341. As the states of characters 1 and 58 are known to be subject to ontogeny, we have alternatively omitted these, receiving the same topology as with the full set of characters.

Results

• Our results ( Fig. 7 ) principally corroborate Yates and Warren’s (2000) Stereospondylomorpha as a monophylum, and differ from the concept of Schoch and Milner (2000) by the inclusion of Peltobatrachus and the plagiosaurids. The Stereospondylomorpha as represented here is supported by one synapomorphy (25) and three−steps Bremer Support.

• The relationships of the three Euskelian clades ( Dissorophidae , Zatracheidae , Eryopidae ) are resolved, but the support of these successive clades is weak. We found the Zatrachydidae and Dissorophoidea to form a clade, with the Eryopidae nesting as their shared sister group, each supported by one step Bremer Support. While the clade as a whole is supported by two synapomorphies (16, 58), zatracheids and dissorophoids are not supported by synapomorphies but two homoplasies (23, 24) and one step Bremer Support.

• Peltobatrachus is found to nest more basally than even Australerpeton , which is not necessarily in contradiction to Yates and Warren (2000) and the more recent analysis of Pawley and Warren (2005) who suggested it be nested between Sclerocephalus (plus the archegosaurids which we did not consider) and the rhinesuchids; these authors did not code Australerpeton which was then still poorly known. Peltobatrachus itself is poorly known for many parts of the skull (margins, snout, anterior palate) and unknown for others (interclavicle), so that its relationships may remain obscure to some extent. In our analysis, the position of Peltobatrachus is supported by three synapomorphies (17, 38, 49) and three−steps Bremer Support. Omitting Peltobatrachus did not critically change the topology of the cladogram.

• Australerpeton , the rhinesuchids ( Uranocentrodon ), and lydekkerinids ( Lydekkerina ) are found to be successive sister taxa, together forming the basalmost grade of the Stereospondyli sensu stricto ( Yates and Warren 2000; Schoch and Milner 2000). These successive nodes are supported as follows: Australerpeton crownwards (6, 59, one−step Bremer Support), Uranocentrodon crownwards (27, 50, 68, four−steps Bremer Support), and Lydekkerina crownwards (19, 26, one−step Bremer Support). This hypothesis is consistent with those of Yates and Warren (2000) and Pawley and Warren (2005) in the general order of succession, but it differs from Yates and Warren in that Benthosuchus and Mastodonsaurus are not nested with Lydekkerina , a group also not found by Damiani (2001) or Pawley and Warren (2005).

• Laidleria , Siderops , and Gerrothorax as a monophylum. This is interesting, as it shares the view put forward by Yates and Warren (2000) that the short−faced stereospondyls may form a clade. It certainly differs from Schoch and Milner’s (2000) concept, in which the former two were nested with lydekkerinids, and the plagiosaurids were no stereospondyls at all. On the other hand, there are two major differences between our result and that of Yates and Warren: (1) our short−faced clade is nested well below the capitosauroid−trematosauroid dichotomy, whereas it nests within the “trematosaur” clade of Yates and Warren (2000); (2) Laidleria and Siderops are here found to be monophyletic with the plagiosaurids forming their sister group, while Yates and Warren found Laidleria and plagiosaurids to be sister taxa with Siderops (and all other brachyopoids) their sister taxon. In our analysis, the monophyly of Laidleria , Siderops , and Gerrothorax is supported by two synapomorphies (42−2, 57), and four−steps Bremer Support, while the relationship between Laidleria and Siderops is indicated by one character (3) and one−step Bremer Support. The ( Laidleria + Siderops ) + Gerrothorax clade nested above Lydekkerina and below the capitosauroid−trematosauroid dichotomy is supported by two synapomorphies (35, 61) and one−step Bremer Support.

• The basal capitosauroid−trematosauroid dichotomy suggested by all more recent authors ( Yates and Warren 2000; Schoch and Milner 2000; Damiani 2001; Pawley and Warren 2005) is also found here. Most interestingly, Sclerothorax is here supported as the sister taxon of capitosauroids ( Mastodonsaurus plus Paracyclotosaurus ), while Benthosuchus forms the sister group to Trematolestes and the Metoposauridae . Sclerothorax plus the capitosauroids is supported by three synapomorphies (20−2, 48, 51) and two−steps Bremer Support; the two capitosauroids are indicated as a monophylum by three characters (21, 33, 41−2) and a two−step Bremer Support. The trematosauroid clade plus Benthosuchus is supported by two synapomorphies (15, 36) and one−step Bremer Support, while the monophyly of Trematolestes and the Metoposauridae is indicated by two characters (4) and one−step Bremer Support.

Figures

Fig. 7. Results of phylogenetic analysis. Sclerothorax is deeply nested within the higher Stereospondyli, forming the sister taxon of the two best−known capitosauroids. Interestingly, plagiosaurids (Gerrothorax), Laidleria, and brachyopoids (Siderops) are found to be monophyletic, which is in accordance with Yates and Warren (2000), while in contrast to the findings of these authors, the latter clade is only remotely related to trematosaurids and metoposaurids. Peltobatrachus, another enigmatic terrestrial temnospondyl, is here found to be a primitive stereospondylomorph. Numbers below each node refer to synapomorphies supporting them.