New publication: Lissamphibian-like toepads in an exceptionally preserved amphibamiform from Mazon Creek (Mann & Gee; Journal of Vertebrate Paleontology)

Title: Lissamphibian-like toepads in an exceptionally preserved amphibamiform from Mazon Creek
​Authors: A. Mann; B.M. Gee
Journal: ​Journal of Vertebrate Paleontology e1727490
DOI to paper: 10.1080/02724634.2019.1727490

Note: this paper came out on Wednesday, but it took a little while to get around to the blog post. As you can imagine, it's a little hard to write a blog in the middle of a pandemic. I actually plan to try to rev things back up in a week or two after I finish a bunch of problem sets for my stats classes, so stay tuned...

General summary: Late Carboniferous Lagerstätte (fossil sites with remarkable preservation) are incredibly important for our understanding of the early evolution of many tetrapod clades. Mazon Creek in Illinois is one of the most famous Lagerstätte in North America and has produced a slew of both vertebrates and invertebrate material of great interest to paleontologists across the board (this is also where the famed Tully Monster comes from). The tetrapods have been studied by some of the most preeminent Paleozoic workers, including Moodie, Gregory, Watson, Westoll, Olson, and Baird, as well as by living paleontologists such as Jason Anderson and Andrew Milner. Preservation at Mazon Creek occurs in nodules that can be cracked in half to hopefully reveal a marvelous fossil within (or just a boring clay ball). Not only do these nodules often preserve complete, articulated skeletons, but they often preserve soft tissue structures (which is how we get a lot of records of squishy invertebrates). Here we describe a nice amphibamiform specimen (recalling that amphibamiforms are widely hypothesized to be on the lissamphibian stem) that preserves the oldest (quite possibly the only) record of toepad ('toe pad' with a space?) structures in a temnospondyl that superficially resemble those of lissamphibians. Arjan's doctoral dissertation is on Mazon Creek (the amniotes mostly); talk to him (arjanmann[at]cmail[dot]carleton[dot]ca) if you wanna know more about the exciting stuff going on! 

Amphibamiforms revisited: Amphibamiforms, a group of dissorophoid temnospondyls that includes the groups historically termed Branchiosauridae and Amphibamidae, has long been of interest for the temnospondyl hypothesis of lissamphibian origins (taxonomic reconfiguration by Schoch, 2019). These are all small-bodied dudes and occur almost exclusively in Laurasian Permo-Carboniferous deposits. Soft tissue impressions are common in the aquatic branchiosaurids (as well as the other aquatic dissorophoids, micromelerpetids), but they don't always have the fingers or toes, and what is known appears to show a relatively thin and tapering soft tissue morphology (see part A on the right). The preservation of soft tissue in general, which can also include tail flukes and torso impressions, is facilitated by deposition in a fine-grained aquatic sedimentary environment that the animals probably lived in. Terrestrial temnos conversely have to serendipitously washed in and then rapidly buried. 

Autopodia of Micromelerpeton credneri (figure from Fröbisch et al., 2014).

Remarkable soft tissue preservation isn't the only way to get information about the soft tissues of course. It has long been known that temnospondyls have chunky digits based on trackways that preserve some fat impressions, and there are a lot of temnospondyl ichnotaxa (identified usually by the presence of only four fingers on the hand) that have been reported in teh literature. Limnopus (figures from Baird, 1952) is one of the best-known ones and can be seen below. However, not surprisingly, we rarely get the soft tissue preservation except in aquatic taxa, which are not the ones making these types of tracks; preservation in terrestrial taxa is even less likely. 

This paper is about direct soft tissue though. There's a remarkable amount of detail preserved, including the contour of the body, a partially digested bolus of food mush in the stomach, and the toe impressions that we're focusing on here. We also have good osteological preservation, including a lot of dermal ossifications and part of the scleral ring that supports the eye. You can see a close-up of the toes from our paper here on the left, compared to a modern salamander. There's no evidence for adhesive cups at the ends of the fingers like what you would see in many treefrogs (enabling them to climb smooth surfaces), but the toes are definitely expanded throughout their length and at the ends, contrary to the more slender and tapering digits that we see in aquatic temnospondyls. 

What's it all mean? This is a short paper by intention. We did want to point out that this doesn't look like Amphibamus, the most notable of the Mazon Creek amphibians and a derived non-branchiosaurid amphibamiform, but further resolution is part of some other work reassessing more temno material from the site. The more significant point is that it suggests a deep origin of soft tissue features of modern amphibians (over 300 mya!)

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Refs

• Baird D. 1952. Revision of the Pennsylvanian and Permian footprints Limnopus, Allopus and Baropus. Journal of Paleontology 26(5): 832-840. [link]
• Fröbisch NB, Bickelmann C, Witzmann F. 2014. Early evolution of limb regeneration in tetrapods: evidence from a 300-million-year-old amphibian. Proceedings of the Royal Society B: Biological Sciences 281(1794): 20141550. DOI: 10.1098/rspb.2014.1550
  
• Schoch RR. 2019. The putative lissamphibian stem-group: phylogeny and evolution of the dissorophoid temnospondyls. Journal of Paleontology 93(1): 137-156. DOI: 10.1017/jpa.2018.67