Perryella and convergence in basal tetrapod clades

A recent paper by Schoch 2018,
once again stirs confusion into the phylogeny of basal tetrapods due to taxon exclusion. Schoch reports, “the enigmatic taxon Perryella (Fig. 1) is found to nest just outside Dissorophoidea (phylogenetic defintion), but shares a range of synapomorphies with this clade.” Schoch derives tiny Perryella from the much larger taxa, Trimerorhachis and Sclerocephalus apparently without testing a wide gamut of taxa (as in the LRT), but relying on a wide consensus of tradition, omitting several key taxa.

Figure 1. Perryella is not a transitional taxon in the LRT, but a terminal taxon nesting with Dendrerpeton.

Figure 1. Perryella is not a transitional taxon in the LRT, but a terminal taxon nesting with Dendrerpeton.

A little Perryella history:
In Carlson 1987 the classification of Perryella was uncertain because it shared features with two groups, Trimerorhachidae and Dissorophoidea, which were thought to be distantly related. In the LRT those taxa are indeed distantly related.

In Ruta and Bolt 2006, a phylogenetic analysis placed Perryella between trimerorhachids and other dvinosaurs (basal tetrapod in Ruta and Bolt that includes lepospondyls like Cacops, Fedexia, Dissorophus, frogs, salamanders and caecilians, all taxa nesting in the lepospondyls in the LRT). Here all similarities with trimerorhachids are convergent.

Figure 1. Trimerorhachis and kin to scale. Here are Panderichthys, Tiktaalik, Ossinodus, Dvinosaurus, Acanthostega, Batrachosuchus and Gerrothorax. Maybe those tabular horns on Acanthostega are really supratemporal horns, based on comparisons to related taxa.

Figure 2. Trimerorhachis and kin to scale. Here are the players in today’s blogpost: Panderichthys, Tiktaalik, Ossinodus, Dvinosaurus, Acanthostega, Batrachosuchus and Gerrothorax. Maybe those tabular horns on Acanthostega are really supratemporal horns, based on comparisons to related taxa. These taxa are not related to lepospondyls (including frogs) despite the convergent appearance.

As you’ll note in the cladogram below
Trimerorhachis and Dvinosaurus nest together in the first large (15 tested taxa, Fig. 3) clade of basal tetrapods in the LRT, the trimerorhachids. Meanwhile Perryella nests several nodes away with Dendrerpeton in the lepospondyl clade of the LRT.

Figure 1. Subset of the LRT focusing on basal tetrapods, colorized according to chronology. Note the wide dispersal of Early Carboniferous taxa, suggesting a Late Devonian radiation as yet largely undiscovered.

Figure 3. Subset of the LRT focusing on basal tetrapods, colorized according to chronology. Note the wide dispersal of Early Carboniferous taxa, suggesting a Late Devonian radiation as yet largely undiscovered. Perryella is not listed here, but nests with Dendrerpeton.

By convergence
Trimerorhachis and Perryella both have wide circular interpterygoid vacuities (Figs. 1, 2) and a largely similar set of skull bones. The difference is in the details and you don’t find convergence (if present) unless you test a wide gamut of candidate taxa.

Figure 1. Trimerorhachis was considered a dvinosaurian temnospondyl. Here both Trimerorhachis and Dvinosaurus nest low on the basal tetrapod tree, close to the fin/finger transition.

Figure 4. Trimerorhachis was considered a dvinosaurian temnospondyl. Here both Trimerorhachis and Dvinosaurus nest low on the basal tetrapod tree, close to the fin/finger transition, far from Perryella and the lepospondyls.

A bad traditional paradigm
is at the bottom of this problem. According to Ruta and Bolt (2006), “Whereas the three extant clades (frogs, salamanders, and caecilians) are generally small creatures with feeble skeletons, most of which feed on small invertebrates, their likely stem group, Paleozoic temnospondyls, encompasses 1–2m long, heavily ossified predators. The evolutionary transition between the Paleozoic giants and the dwarfed modern forms has long been sought among the Dissorophoidea, a speciose clade of mainly terrestrial and presumably insect-eating Carboniferous–Triassic temnospondyls that were usually smaller and had less massive skeletons than their fish-eating fellows, but alternative scenarios are still debated.”

Figure 2. Utegenia nests as a sister to Diplovertebron.

Figure 5. Utegenia nests at the base of the Lepospondyli and the Lissamphibia in the LRT. It is also the proximal sister to the Reptilia in the LRT. Do not exclude this taxon from your basal tetrapod studies!

In the LRT
(Fig. 3) there are no huge, heavily ossified ancestors to the lepospondyls and lissamphibians. Rather Ossinodus is the largest ancestor in the lissamphibian line. Ruta and Bolt report, “Watson (1940) wrote a review paper on the origin of anurans in which he sought the ancestry of frogs among amphibamids, notably the tiny, lightly built Amphibamus grandiceps.” This is supported and confirmed by the LRT. They continue, “The discovery of Eocaecilia and Gerobatrachus brought an end to the long-practiced separate treatment of temnospondyls and lissamphibians, because both Eocaecilia and Gerobatrachus retained dermal bones that are not present in any extant lissamphibian, but are well known from temnospondyls.” In the LRT basal lepospondyls, like Utegenia (Fig. 5) which is not a seymouriamorph, but close!) retain dermal bones not present in extant lissamphibians. A keyword search of Ruta and Bolt failed to bring up the taxon, ‘Utegenia.’

Figure 7. Subset of the LRT including Perryella.

Figure 6. Subset of the subset in figure 3 of the LRT that now includes Perryella.

Once again,
that’s taxon exclusion crimping otherwise well-considered and serious studies, like Ruta and Bolt 2006.

References
Carlson KJ 1987. Perryella, a new temnospondylous amphibian from the Lower Permian of Oklahoma. Journal of Paleontology. 61 (1): 135–147.
Ruta M and Bolt JR 2006. A reassessment of the temnospondyl amphibian Perryella olsonifrom the Lower Permian of Oklahoma. Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 97 (2): 113–165.

wiki/Dendrerpeton
wiki/Tersomius
wiki/Perryella

 

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