This is Mazon Monday post #70.  What’s your favorite Mazon Creek fossil?  Tell us at email:esconi.info@gmail.com.

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Arjan Mann gave a presentation at the November 2020 ESCONI General Meeting about Mazon Creek vertebrates.  Now, he’s out with a paper published in the journal Royal Society Open Science that describes a new Mazon Creek tetrapod.  The paper is called “Joemungandr bolti, an exceptionally preserved ‘microsaur’ from the Mazon Creek Lagerstatte reveals pattersn of integumentary evolution in Recumbirostra”.   Joemungandr bolti is named for both the Vikings’ mythic “World Serpent” and John R. Bolt, the late paleontologist and emeritus curator of fossil amphibians and reptiles at Chicago’s Field Museum.

LiveScience has a nice article about the paper.

A long-bodied, sinuous reptile that lived about 310 million years ago has been named for a legendary giant snake in Viking mythology that once battled Thor, the Norse god of thunder.

But while the Vikings‘ mythic “World Serpent,” named Jörmungandr, was large enough to wrap his body around the entire Earth, the ancient reptile Joermungandr bolti (YOR’-mun-gund BOL’-tee) measures just a couple of inches long. 

This creature is a microsaur (“small lizard“), an early group of reptiles that were among the first vertebrates (animals with backbones) to evolve on land. J. bolti had a slender, elongated body with short limbs and a blunt skull, and the fossil was so well preserved that it retained impressions of specialized scales that resemble dirt-repelling scales in modern reptiles. Together, these features suggest that the wee microsaur tunneled underground and slithered like a snake, researchers reported in a new study.

The microsaur fossil was in the collection of Chicago’s Field Museum, and it came from Mazon Creek in Illinois, where deposits have preserved numerous fossils of complete or near-complete organisms dating to the Carboniferous period (about 359 million to 299 million years ago). Microsaurs represent some of the oldest fossils of amniotes, vertebrates that develop embryos in fluid-filled eggs with multiple membrane layers, according to the University of California Museum of Paleontology in Berkeley.

J. bolti (“bolti” is a nod to the late paleontologist John R. Bolt, the emeritus curator of fossil amphibians and reptiles at the Field Museum) is a microsaur from a group called Recumbirostra, which was around for about 40 million to 50 million years, “from the middle of the Carboniferous to the early Permian [299 million to 251 million years ago],” said lead study author Arjan Mann, a postdoctoral fellow of paleobiology at the Smithsonian National Museum of Natural History in Washington, D.C. 

Abstract

The Carboniferous Pennsylvanian-aged (309–307 Ma) Mazon Creek Lagerstätte produces some of the earliest fossils of major Palaeozoic tetrapod lineages. Recently, several new tetrapod specimens collected from Mazon Creek have come to light, including the earliest fossorially adapted recumbirostrans. Here, we describe a new long-bodied recumbirostran, Joermungandr bolti gen. et sp. nov., known from a single part and counterpart concretion bearing a virtually complete skeleton. Uniquely, Joermungandr preserves a full suite of dorsal, flank and ventral dermal scales, together with a series of thinned and reduced gastralia. Investigation of these scales using scanning electron microscopy reveals ultrastructural ridge and pit morphologies, revealing complexities comparable to the scale ultrastructure of extant snakes and fossorial reptiles, which have scales modified for body-based propulsion and shedding substrate. Our new taxon also represents an important early record of an elongate recumbirostran bauplan, wherein several features linked to fossoriality, including a characteristic recumbent snout, are present. We used parsimony phylogenetic methods to conduct phylogenetic analysis using the most recent recumbirostran-focused matrix. The analysis recovers Joermungandr within Recumbirostra with likely affinities to the sister clades Molgophidae and Brachystelechidae. Finally, we review integumentary patterns in Recumbirostra, noting reductions and losses of gastralia and osteoderms associated with body elongation and, thus, probably also associated with increased fossoriality.