Peltobatrachus: Panchen's 'shield frog'

Been on a bit of a hiatus lately (busy month academically), but we're back for #TemnospondylTuesday! I'm still musing whether to do another themed series (probably starting in two weeks in November), so between now and then, I'll cover one oddball each week. Up this week is Peltobatrachus, a peculiar late Permian temnospondyl from Tanzania! As a bonus, it has a toy version that you can buy for not an exorbitant amount of money!

Skeletal reconstruction of Peltobatrachus (figure from Panchen, 1959). Detailed areas are those represented by known fossil material.

Etymology
Peltobatrachus is a pretty good name. It derives from two Greek roots that collectively mean 'shield frog,' in reference to its labyrinthodont affinities and the extensive dermal armour. It's kind of a funny name because it reminds me of schildkröte, which is the German word for turtle but which literally translates to 'shield toad.' 

A brief history of study
The original material of Peltobatrachus was collected by the famous British palaeontologist F.R. Parrington, who's usually better known for his work on amniotes, in 1933 in the Ruhuhu Valley of Tanzania. Work in this region was subsequently expanded in the 1930s, the 1960s, and most recently in the 21st century, which is half of a 2017 JVP memoir. Evidently future work never recovered additional material of Peltobatrachus, or at least none that was immediately recognizable, as all of the material that was described decades later in 1959 by British palaeontologist A.L. Panchen (for his dissertation) was from Parrington's 1933 collection, and none has ever been described since then. Panchen's description is very detailed (more so in his dissertation) - even many of the comparisons still hold up - and although it's been re-examined by numerous authors over the years, it's never merited the re-description needed for plenty of other old temnospondyls.

Illustrations of the holotype skull of Peltobatrachus (figure from Panchen, 1959).

The phylogenetic position of Peltobatrachus
Unsurprisingly, figuring out where something like Peltobatrachus fits is very difficult. Panchen (1959) originally hypothesized it to be a close relative of the plagiosaurids, which would have in turn suggested a separation of plagiosaurids from virtually all other Mesozoic temnos. In fact, he spent 16 pages making compelling arguments (some perhaps a little less compelling than others) in favour of this. In my opinion, it seems that Panchen was particularly taken by the similarity of the pustulated ornamentation of Peltobatrachus, which was and still remains, otherwise found only in some plagiosaurids, and then from there, his comparisons are increasingly restricted (selective?). There definitely remain some pretty pronounced differences from plagiosaurids that can't really be accounted for in any way. For example, the remarkably terrestrial morphology compares far more favourably with virtually any other terrestrial temnospondyl (Panchen compared to Eryops for example), which doesn't imply a relationship with those taxa, but also doesn't suggest much of any affinities with plagiosaurids. The particular flavour of stereospondyl vertebrae found in Peltobatrachus is truly similar to plagiosaurids in having a single large centrum with the rib articulations formed between adjacent centra, rather than being fully within a single centrum, but there are plenty of cautionary tales about relying too much on vertebral type to infer relationships (see lepospondyls, for example). Histological sampling of the osteoderms (Witzmann & Soler-Gijón, 2010), which were compared to a few other taxa, showed that those of Peltobatrachus are most comparable to that observed in plagiosaurids, although that may reflect a similar purpose rather than a close phylogenetic relationship.

Of course, Panchen's work was in the pre-quantitative phylogeny era. When Peltobatrachus is put into phylogenies, it is typically agreed upon to be a basal stereospondyl whose phylogenetic relationships are probably complicated by a poor record of Paleozoic stereospondyls; it has rarely been placed in a formalized family, although 'Peltobatrachidae' shows up from time to time (e.g., Ruta et al., 2007).

• Yates & Warren (2000): this study (all major temno clades) recovered Peltobatrachus​ at the base of Stereospondyli.
• Dias-da-Silva & Marsicano (2010): this study (rhytidosteid-focused) recovered Peltobatrachus near the base of Stereospondyli, closely related to Lapillopsis and Lydekkerina.
• Schoch (2013): this study (all major temno clades) identified Peltobatrachus as a wild-card taxon in a full analysis of all major temnospondyl groups. Parsimony analysis did recover it as being closely related to eryopids, but Schoch suggested that this reflected a long period of evolutionary isolation (at least with existing gaps in the fossil record) and similarities in postcranial anatomy associated with shared terrestriality (how terrestrial eryopids were is, of course, still debated). 
• Eltink & Langer (2014): this study (focused on placement of Australerpeton within all major temno clades) recovered Peltobatrachus at the base of Stereospondyli. Subsequent analysis of additional archegosauroids in the same matrix by Pacheco et al. (2017) maintained its position. 
• Eltink et al. (2016): this study (rhinesuchid-focused) recovered Peltobatrachus at the base of Stereospondyli next to Lapillopsis.
• Eltink et al. (2018): this study (stereospondylomorphs only) recovered Peltobatrachus at the base of Stereospondyli. The results from this analysis are shown below, with major nodes and clades marked in bold. 

What makes it so weird?
Given what we know now, the weirdest part of Peltobatrachus is its combination of features that's otherwise unparalleled. For example, the pustulated ornamentation is undoubtedly reminiscent of that seen in the Triassic plagiosaurids, a fully aquatic clade. However, the postcranial skeleton reflects a terrestrial lifestyle and looks a lot more like what you would see in a large dissorophoid or eryopid, with stout, well-ossified limbs and robust girdles. To tack onto that, the armour of Peltobatrachus is also quite unusual. As you can see on the right, it forms a distinct carapace with multiple plates per band, and the armour extends over the pelvis onto the anterior part of the tail. Compared to a lot of other temnos with armour, the covering is far more extensive; it's not like the osteoderms of dissorophids that are only associated with the vertebral spines. Undoubtedly these complex coverings were primarily involved with protection, probably from synapsids in the case of Peltobatrachus.

Results of the phylogenetic analysis of non-stereospondyl stereospondylomorphs, with Peltobatrachus nested at the base (and marked by the semi-opaque red box); figure from Eltink et al. (2018).

Peltobatrachus in context
Peltobatrachus occupies an interesting time in temnospondyl evolution. For one, it's very terrestrial and relatively large-bodied (15 cm skull length; >50 cm reconstructed body length). From the Carboniferous and into the middle Permian, many temnospondyls are similarly terrestrial, like the dissorophoids that I work on for my dissertation, and of a similar size. But by the late Permian, when Peltobatrachus was stomping around Africa, most temnospondyls were aquatic stereospondylomorphs that passed through the Permo-Triassic extinction and that radiated during the Mesozoic. Essentially all terrestrial Mesozoic temnospondyls are lissamphibians (if you buy that temnospondyls give rise to lissamphibians) and/or very small-bodied forms like Lydekkerina. Additionally, the African record of temnospondyls has been relatively poor outside of the well-known Karoo Basin in South Africa, which spans the late Permian to the Early Triassic - other temnospondyls recovered from the interior of the continent have often proven to be similarly difficult to place phylogenetically (or been recovered in unexpected positions) and exhibit marked anatomical differences from the proposed closest relatives (e.g., Nigerpeton and Saharastega from the late Permian of Niger). Further exploration in these regions will hopefully produce more material that can better contextualize regional patterns of temnospondyl evolution.

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Refs

• Dias-da-Silva, S. and Marsicano, C., 2011. Phylogenetic reappraisal of Rhytidosteidae (Stereospondyli: Trematosauria), temnospondyl amphibians from the Permian and Triassic. Journal of Systematic Palaeontology, 9(2), pp.305-325. doi: ​10.1080/14772019.2010.492664
• Eltink, E. and Langer, M.C., 2014. A new specimen of the temnospondyl Australerpeton cosgriffi from the Late Permian of Brazil (Rio do Rasto Formation, Paraná Basin): comparative anatomy and phylogenetic relationships. Journal of Vertebrate Paleontology, 34(3), pp.524-538. doi: 10.1080/02724634.2013.826667
• Eltink, E., Dias, E.V., Dias-da-Silva, S., Schultz, C.L. and Langer, M.C., 2016. The cranial morphology of the temnospondyl Australerpeton cosgriffi (Tetrapoda: Stereospondyli) from the Middle-Late Permian of Paraná Basin and the phylogenetic relationships of Rhinesuchidae. Zoological Journal of the Linnean Society, 176(4), pp.835-860. doi: 10.1111/zoj.12339
• Eltink, E., Schoch, R.R. and Langer, M.C., 2019. Interrelationships, palaeobiogeography and early evolution of Stereospondylomorpha (Tetrapoda: Temnospondyli). Journal of Iberian Geology, 45(2), pp.251-267. doi: 10.1007/s41513-019-00105-z
• Pacheco, C.P., Eltink, E., Müller, R.T. and Dias-da-Silva, S., 2017. A new Permian temnospondyl with Russian affinities from South America, the new family Konzhukoviidae, and the phylogenetic status of Archegosauroidea. Journal of Systematic Palaeontology, 15(3), pp.241-256. doi: 10.1080/14772019.2016.1164763
• Panchen, A.L., 1959. A new armoured amphibian from the Upper Permian of East Africa. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 242(691), pp.207-281. doi: 10.1098/rstb.1959.0005
• Schoch, R.R., 2013. The evolution of major temnospondyl clades: an inclusive phylogenetic analysis. Journal of Systematic Palaeontology, 11(6), pp.673-705. doi: 10.1080/14772019.2012.699006
• Witzmann, F. and Soler‐Gijón, R., 2010. The bone histology of osteoderms in temnospondyl amphibians and in the chroniosuchian Bystrowiella. Acta Zoologica, 91(1), pp.96-114. doi: 10.1111/j.1463-6395.2008.00385.x
• Yates, A.M. and Warren, A.A., 2000. The phylogeny of the ‘higher’ temnospondyls (Vertebrata: Choanata) and its implications for the monophyly and origins of the Stereospondyli. Zoological Journal of the Linnean Society, 128(1), pp.77-121. doi: 10.1111/j.1096-3642.2000.tb00650.x