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Micro-CT Study of Rhynchonkos stovalli (Lepospondyli, Recumbirostra), with Description of Two New Genera.

Szostakiwskyj M, Pardo JD, Anderson JS - PLoS ONE (2015)

Bottom Line: We revisit the homology of the supraoccipital, median anterior bone, and temporal bone of recumbirostrans, and discuss implications of alternate interpretations of the homology of these elements.Finally, we evaluate the characteristics previously used to unite Rhynchonkos stovalli with caecilians in light of these new data.These proposed similarities are more ambiguous than previous descriptions suggest, and result from the composite nature of previous descriptions, ambiguities in external morphology, and functional convergence between recumbirostrans and caecilians for head-first burrowing.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada.

ABSTRACT
The Early Permian recumbirostran lepospondyl Rhynchonkos stovalli has been identified as a possible close relative of caecilians due to general similarities in skull shape as well as similar robustness of the braincase, a hypothesis that implies the polyphyly of extant lissamphibians. In order to better assess this phylogenetic hypothesis, we studied the morphology of the holotype and three specimens previously attributed to R. stovalli. With the use of micro-computed x-ray tomography (μCT) we are able to completely describe the external and internal cranial morphology of these specimens, dramatically revising our knowledge of R. stovalli and recognizing two new taxa, Aletrimyti gaskillae gen et sp. n. and Dvellacanus carrolli gen et sp. n. The braincases of R. stovalli, A. gaskillae, and D. carrolli are described in detail, demonstrating detailed braincase morphology and new information on the recumbirostran supraoccipital bone. All three taxa show fossorial adaptations in the braincase, sutural articulations of skull roof bones, and in the lower jaw, but variation in cranial morphology between these three taxa may reflect different modes of head-first burrowing behaviors and capabilities. We revisit the homology of the supraoccipital, median anterior bone, and temporal bone of recumbirostrans, and discuss implications of alternate interpretations of the homology of these elements. Finally, we evaluate the characteristics previously used to unite Rhynchonkos stovalli with caecilians in light of these new data. These proposed similarities are more ambiguous than previous descriptions suggest, and result from the composite nature of previous descriptions, ambiguities in external morphology, and functional convergence between recumbirostrans and caecilians for head-first burrowing.

No MeSH data available.


Reconstructions of the sphenethmoid and the descending flange of the frontal and parietal.Specimens previously attributed to Rhynchonkos (A, C, E) and select microsaurs (B, D, F) in right lateral view. A, Aletrimyti gaskillae, gen. et sp. nov. (FM-UR 1040); B, Nannaroter mckinziei (OMNH 73107); C, Rhynchonkos stovalli (FM-UR 1039); D, Micraroter erythrogeios (FM-UR 2311); E, Dvellecanus carrolli, gen. et sp. nov. (UCMP 202940); F, Huskerpeton englehorni (UNSM 32144). Dashed line indicates the bony margins of the orbit. Scale bars equal 1mm. Abbreviations: fro, frontal; par, parietal; ps, parasphenoid; se, sphenethmoid; II, optic nerve foramen.
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pone.0127307.g014: Reconstructions of the sphenethmoid and the descending flange of the frontal and parietal.Specimens previously attributed to Rhynchonkos (A, C, E) and select microsaurs (B, D, F) in right lateral view. A, Aletrimyti gaskillae, gen. et sp. nov. (FM-UR 1040); B, Nannaroter mckinziei (OMNH 73107); C, Rhynchonkos stovalli (FM-UR 1039); D, Micraroter erythrogeios (FM-UR 2311); E, Dvellecanus carrolli, gen. et sp. nov. (UCMP 202940); F, Huskerpeton englehorni (UNSM 32144). Dashed line indicates the bony margins of the orbit. Scale bars equal 1mm. Abbreviations: fro, frontal; par, parietal; ps, parasphenoid; se, sphenethmoid; II, optic nerve foramen.

Mentions: It is only when considering the internal morphology that the largest differences between specimens are observed. Possibly the greatest difference in internal morphology is seen in Aletrimyti, where the elements of the braincase and palate articulate with those of the dermatocranium. It is a common feature amongst recumbirostrans for the sphenethmoid to reach the skull roof (Fig 14). In Aletrimyti, not only does the sphenethmoid brace against the frontals, but the pleurosphenoids articulate with the parietals, the vomers with the nasals, and the ‘median anterior braincase bone’ with the frontals (Fig 8A). Rhynchonkos shows a lesser degree of dorsoventral bracing, with only the sphenethmoid and epipterygoids reaching the skull roof, while in Dvellecanus only the frontals contact the sphenethmoids. Likewise, Dvellecanus shows an enlarged dorsal sinus, similar to the condition seen in Pelodosotis, beginning between the sphenethmoids and the parietals and spanning to the supraoccipital (Fig 12A). Aletrimyti shows a much smaller sinus that occupies the area dorsal to the supraoccipital and otic capsules (Fig 8A), while the only evidence of a sinus in Rhynchonkos is a narrow gap between the pleurosphenoids and parietals (Fig 4A). The shape of the occiput of Rhynchonkos differs from both Aletrimyti and Dvellecanus. Whereas Rhynchonkos has a somewhat flattened set of occipital condyles, Aletrimyti and Dvellecanus have a much more convex occiput with more pronounced exoccipitals that form the knob-like condyles that flank the cotyle of the basioccipital.


Micro-CT Study of Rhynchonkos stovalli (Lepospondyli, Recumbirostra), with Description of Two New Genera.

Szostakiwskyj M, Pardo JD, Anderson JS - PLoS ONE (2015)

Reconstructions of the sphenethmoid and the descending flange of the frontal and parietal.Specimens previously attributed to Rhynchonkos (A, C, E) and select microsaurs (B, D, F) in right lateral view. A, Aletrimyti gaskillae, gen. et sp. nov. (FM-UR 1040); B, Nannaroter mckinziei (OMNH 73107); C, Rhynchonkos stovalli (FM-UR 1039); D, Micraroter erythrogeios (FM-UR 2311); E, Dvellecanus carrolli, gen. et sp. nov. (UCMP 202940); F, Huskerpeton englehorni (UNSM 32144). Dashed line indicates the bony margins of the orbit. Scale bars equal 1mm. Abbreviations: fro, frontal; par, parietal; ps, parasphenoid; se, sphenethmoid; II, optic nerve foramen.
© Copyright Policy
Related In: Results  -  Collection

License
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getmorefigures.php?uid=PMC4465623&req=5

pone.0127307.g014: Reconstructions of the sphenethmoid and the descending flange of the frontal and parietal.Specimens previously attributed to Rhynchonkos (A, C, E) and select microsaurs (B, D, F) in right lateral view. A, Aletrimyti gaskillae, gen. et sp. nov. (FM-UR 1040); B, Nannaroter mckinziei (OMNH 73107); C, Rhynchonkos stovalli (FM-UR 1039); D, Micraroter erythrogeios (FM-UR 2311); E, Dvellecanus carrolli, gen. et sp. nov. (UCMP 202940); F, Huskerpeton englehorni (UNSM 32144). Dashed line indicates the bony margins of the orbit. Scale bars equal 1mm. Abbreviations: fro, frontal; par, parietal; ps, parasphenoid; se, sphenethmoid; II, optic nerve foramen.
Mentions: It is only when considering the internal morphology that the largest differences between specimens are observed. Possibly the greatest difference in internal morphology is seen in Aletrimyti, where the elements of the braincase and palate articulate with those of the dermatocranium. It is a common feature amongst recumbirostrans for the sphenethmoid to reach the skull roof (Fig 14). In Aletrimyti, not only does the sphenethmoid brace against the frontals, but the pleurosphenoids articulate with the parietals, the vomers with the nasals, and the ‘median anterior braincase bone’ with the frontals (Fig 8A). Rhynchonkos shows a lesser degree of dorsoventral bracing, with only the sphenethmoid and epipterygoids reaching the skull roof, while in Dvellecanus only the frontals contact the sphenethmoids. Likewise, Dvellecanus shows an enlarged dorsal sinus, similar to the condition seen in Pelodosotis, beginning between the sphenethmoids and the parietals and spanning to the supraoccipital (Fig 12A). Aletrimyti shows a much smaller sinus that occupies the area dorsal to the supraoccipital and otic capsules (Fig 8A), while the only evidence of a sinus in Rhynchonkos is a narrow gap between the pleurosphenoids and parietals (Fig 4A). The shape of the occiput of Rhynchonkos differs from both Aletrimyti and Dvellecanus. Whereas Rhynchonkos has a somewhat flattened set of occipital condyles, Aletrimyti and Dvellecanus have a much more convex occiput with more pronounced exoccipitals that form the knob-like condyles that flank the cotyle of the basioccipital.

Bottom Line: We revisit the homology of the supraoccipital, median anterior bone, and temporal bone of recumbirostrans, and discuss implications of alternate interpretations of the homology of these elements.Finally, we evaluate the characteristics previously used to unite Rhynchonkos stovalli with caecilians in light of these new data.These proposed similarities are more ambiguous than previous descriptions suggest, and result from the composite nature of previous descriptions, ambiguities in external morphology, and functional convergence between recumbirostrans and caecilians for head-first burrowing.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada.

ABSTRACT
The Early Permian recumbirostran lepospondyl Rhynchonkos stovalli has been identified as a possible close relative of caecilians due to general similarities in skull shape as well as similar robustness of the braincase, a hypothesis that implies the polyphyly of extant lissamphibians. In order to better assess this phylogenetic hypothesis, we studied the morphology of the holotype and three specimens previously attributed to R. stovalli. With the use of micro-computed x-ray tomography (μCT) we are able to completely describe the external and internal cranial morphology of these specimens, dramatically revising our knowledge of R. stovalli and recognizing two new taxa, Aletrimyti gaskillae gen et sp. n. and Dvellacanus carrolli gen et sp. n. The braincases of R. stovalli, A. gaskillae, and D. carrolli are described in detail, demonstrating detailed braincase morphology and new information on the recumbirostran supraoccipital bone. All three taxa show fossorial adaptations in the braincase, sutural articulations of skull roof bones, and in the lower jaw, but variation in cranial morphology between these three taxa may reflect different modes of head-first burrowing behaviors and capabilities. We revisit the homology of the supraoccipital, median anterior bone, and temporal bone of recumbirostrans, and discuss implications of alternate interpretations of the homology of these elements. Finally, we evaluate the characteristics previously used to unite Rhynchonkos stovalli with caecilians in light of these new data. These proposed similarities are more ambiguous than previous descriptions suggest, and result from the composite nature of previous descriptions, ambiguities in external morphology, and functional convergence between recumbirostrans and caecilians for head-first burrowing.

No MeSH data available.