CT analysis shows new relationships of Euryodus, an extinct amphibian

Computed tomographic analysis of the cranium of the early Permian recumbirostran ‘microsaur’ Euryodus dalyae reveals new details of the braincase and mandible

Bryan M. Gee, Joseph J. Bevitt and Robert R. Reisz

Summarized by Danielle Miller, who is a geology major at the University of South Florida, working towards becoming a paleontologist. She loves watching hockey and listening to music. She has two dogs. She also loves spending time out on the boat, fishing, and hanging out in the Gulf of Mexico.

What data were used? The researchers used two specimens that were identified as Euryodus dalyae from the Richards Spur locality, a fossil site in Oklahoma. Researchers compared the specimens of Euryodus dalyae to the holotype fossils of this species; a holotype is a specimen that is used by paleontologists to define a species. These two specimens are gymnarthrids, which belong to an extinct family of amphibians called microsaurs. These new specimens were compared using data that was collected through a type of CT that is called neutron tomography and x-ray tomography. The researchers also performed a phylogenetic analysis to understand how the newly discovered specimens were related to other microsaurs. The researchers created a number of characters based on the cranial (skull) shapes across the taxa used, as well as on the vertebrate of the taxa. The collected morphological data was put into the matrix (i.e., all of the morphogical characters in total) to be analyzed to determine the evolutionary relationships between the specimens.

Methods: The researchers did tomographic and phylogenetic analyses of the specimens. The phylogenetic analyses were done to see the evolutionary relationship between the specimens and the holotype of Euryodus. During the phylogenetic analysis, the researchers coded the two new specimens into a phylogenetic matrix that was created in a previous study done in 2017. Coding of several other microsaurs were used in the analyses. One of the other microsaurs used in the analyses was from the same genus as the new specimens, while the others were from different genera. The analyses were done using PAUP*, which is a computer program that is used to infer evolutionary trees. For the tomographic analysis of the specimens, the researchers did neutron tomography and X- ray tomography. Tomographic analyses are techniques that are used to represent a cross section of solid objects through X- rays or ultrasound. Neutron tomography is performed by rotating the specimens 180°, then taking neutron radiographs, which are photos produced on film by x-rays, at defined angular positions. This can produce a 3D image of a specimen’s composition. 1200 radiographs of one of the new specimens were produced in this analysis. The other new specimen was analyzed using X- ray tomography. The photos from both tomographic analyses were then analyzed in ImageJ, which is an image processing program that can be used to measure and analyze images. 

There are three pictures of the skull of Euryodus sp. Picture A is a triangular shaped, brownish skull shown from the bottom. Picture B is a colorful computer-generated model of the skull in the same orientation of Picture A. Picture C is also a colorful computer-generated model of the skull, but this time it is looking from the back of the skull. The sky-blue colored bone in Picture B is the upper jaw of the specimen discussed in the article. The dark pink bone in Picture B is the mandible also discussed in the article. Pictures B and C have abbreviations pointing out the different bones of the skull.
Figure 1. The skull of a specimen of Euryodus sp. Picture A is a picture of Euryodus sp.’s skull, viewed from the bottom. Picture B is a rendering of Euryodus sp.’s skull in the same profile. This picture has different colors representing the different parts of the specimen. Picture C is a rendering of Euryodus sp.’s skull from the back of the skull angled from the bottom to the top. The colors in the pictures B and C represent different parts of the skull. The colors in C represent the same parts as they do in Picture B. There are abbreviations in pictures B and C that represent the different anatomical parts of Euryodus sp. The abbreviation ‘m’ stands for the maxilla which is the upper jaw and while it is not abbreviated in this photo, the darker pink section is the part of the jaw that was discussed in the article. Scale bar= 1 cm.

Results: The result of this study shows that there is an especially close relationship between one of the recently discovered specimens and the holotype of Euryodus dalyae. The majority of differences between the skeletons are probably due to damage to the specimen over millions of years in the rock record, and not due to biological differences; however, there were some differences between the specimens on the internal portion of the skeleton These two specimens are also very similar to other species of extinct amphibians. The two specimen that were identified as Euryodus dalyae are now described as Euryodus sp. because of this study. Euryodus dalyae and Euryodus sp. look almost the same on the outside, but they are different internally. One such internal difference is the presence or absence of a presphenoid, which is the front part of a bone that is found at the base of the skull of the specimen. The researchers are unsure if these differences are due to ontogeny, the growth of the specimens, or if this is a signal that the two specimens represent different species. This difference is one reason that the researchers encourage more exploration of recumbirostrans, which are the amphibian group that include the family that Euryodus is part of. There was also the presence of an offset partial tooth row in the new specimens. This feature has been seen in a group called the captorhinids, which are lizard-like reptiles. This helps us better identify the broader groups that Euryodus is related to, because the offset teeth are only in certain species. This means that scientists can more confidently say that these groups belong in the same group. However, the offset teeth weren’t identical across the specimens studied here, so further tests need to be done.

Why is this study important? This is important because it provides a new understanding of the Euryodus clade. It can also be used to help determine if microsaurs are really a sister clade (meaning, the most closely related clade) to captorhinids, lizard-like reptiles that ranged from small to large and lived during the Permian, as they have been hypothesized to be before. This study also provides a better understanding of the anatomy of gymnarthrids and other microsaurs. Understanding the anatomy of gymnarthrids and other microsaurs is useful since future researchers can use that data to put other specimen into the group, if they fit that description.

The big picture: If we know the evolutionary relationships between the specimens in this study, then we could start to ask other questions about the group. We could investigate how the group changed across different extinction events or we could better understand the anatomy of these amphibians and see how this anatomy developed in amphibians of today.  

Citation: Gee, B. M., Bevitt, J. J., & Reisz, R. R. (2020). Computed tomographic analysis of the cranium of the early permian recumbirostran ‘microsaur’ Euryodus Dalyae reveals new details of the braincase and mandible. Papers in Palaeontology, 7(2), 721–749. https://doi.org/10.1002/spp2.1304

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