This is Mazon Monday post #289.  What’s your favorite Mazon Creek fossil?  Tell us at email:esconi.info@gmail.com.

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More exciting Mazon Creek research is out. Featured here is a short paper in the journal Vertebrate Anatomy Morphology Palaeontology (VAMP), which details the first occurrence of the urocordylid Ctenerpeton remex in the Mazon Creek fossil deposit!

Arjan Mann, a curator of early fossil vertebrates at the Field Museum of Natural History, instigated the research when he found detailed notes and a partial description by John Bolt, his predecessor. Blue M. Byrnes, one of his summer 2025 interns, collaborated with Arjan to finish the research and publish the results.

The lead author, Blue M. Byrnes, is a junior at Pitzer college studying biology and studio art. Blue was an intern in the Mann Lab during the summer of 2025. It’s rare for an intern to be a lead author on an important find like this. Congratulations, Blue!

Urocordylids are a family of interesting animals belonging to the order Nectridea. Diplocaulus is a well-known member of their clade. Urocordylids are known from the slightly younger Carboniferous locality near Linton, Ohio. This is the first occurrence of this order in Mazon Creek. The Linton locality is a focus of some of the research in the Mann Lab at the Field Museum. Ctenerpeton remex had striking tail vertebrae that helped it swim efficiently.

Ctenorhynchus remex was described by Edward Drinker Cope (1840-1897) in 1868. Cope was an American zoologist, paleontologist, comparative anatomist, herpetologist, and ichthyologist. He was a prolific writer (1400+ papers) who described and named more than 1000 vertebrate species, including hundreds of fish, dozens of dinosaurs, and many early tetrapods. He is probably best remembered as a participant in the so-called Bone Wars in the late 19th century.

First occurrence of the urocordylid Ctenerpeton remex from the Mazon Creek Lagerstätte, Illinois, USA

Abstract: Despite being one of the most complete records of a Paleozoic ecosystem, tetrapod fossils at the Mazon Creek Lagerstätte, in Illinois, USA, remain relatively rare compared to other contemporary late Carboniferous localities, making new additions to this fauna particularly important. Here we report a new concretion from Mazon Creek that preserves an impression of a series of articulated caudal vertebrae of the early tetrapod Ctenerpeton remex. It can be differentiated from Ptyonius and other nectrideans by the characteristics of its neural and haemal spines and arches, most notably the anterior and posterior projections on the haemal arches, the accessory zygapophyses on the neural arch, and the size and shape of the spines compared to known specimens of other urocordylids and nectrideans. The addition of Ctenerpeton remex to the Mazon Creek tetrapod assemblage increases the known diversity of nectrideans in this fauna and aids in our understanding of the different environments inhabited by urocordylids in the late Carboniferous.

Introduction: The Carboniferous, late Pennsylvanian, Moscovian-age (309−307 Ma), Mazon Creek Lagerstätte of northern Illinois contains one of the best-known records of a late Paleozoic ecosystem (Shabica and Hay 1997; Clements et al., 2018). The tetrapod fauna from Mazon Creek has been recently revisited in a series of papers detailing new discoveries, most of which belong to the clade of highly fossorial recumbirostran ‘microsaurs’ (Mann and Maddin 2019; Mann et al. 2019a, 2019b; Mann and Gee 2019; Mann et al. 2021; Mann et al. 2022). This recent research has challenged long standing beliefs that the Mazon Creek Lagerstätte is lower in tetrapod diversity than contemporary sites such as Linton, Ohio (Moodie, 1912; Gregory, 1998; Shabica and Hay 1997). Despite these new works, there are still many aspects of the tetrapod fauna that require further revision, including the diversity of temnospondyls, aistopods, and nectrideans from the site. Tetrapods from Mazon Creek are uniquely found in siderite concretions, broken apart though freeze-thaw cycles (natural or artificial), into part and counterpart, usually down the midline of the animal, revealing natural molds of tetrapod remains inside (Shabica and Hay 1997).

Here we report a new tetrapod fossil from Mazon Creek that was collected from an unknown spoil pit locality, likely Pit 11 (Clements et al. 2019), by Helen and Ted Piecko.

The specimen, FMNH PR 1799, preserves a partial articulated tail of a large nectridean that most closely resembles the caudal vertebrae of those found in Ctenerpeton remex, a nectridean tetrapod known previously only from Linton and Five Points, Ohio. At Linton, Ctenerpeton is a relatively rare member of the vertebrate fauna, known from only a handful of specimens, with more complete fossils only coming to light in recent years (Hook and Baird 1986). The first fully articulated remains of Ctenerpeton, including the skull, are from Five Points, Ohio, but these have yet to be described in detail (Hook and Baird 1993). Previously, the only record of a nectridean from Mazon Creek was Ptyonius marshii, known from two relatively small individuals (FMNH PR 1353, USNM PAL 18125). FMNH PR 1799, although limited in the preserved remains, adds an important rare tetrapod to the assemblage at Mazon Creek.

Description: Comparative Osteology of FMNH PR 1799: This specimen, preserved in a siderite nodule, consists of a natural mold of 21 complete and two partial caudal vertebrae in part and counterpart. The anatomy preserved in FMNH PR 1799 has notable overlap with the holotype of Ctenerpeton remex, AMNH 6907, which consists of 23 caudal vertebrae. The following description is based on epoxy casts (Fig. 2) that were made from latex peels of FMNH PR 1799.

The neural and haemal spines are strikingly symmetrical, both being slender and elongate fans, and are characteristic of caudals belonging to nectrideans (Bossy 1979). The distal ends of adjacent spines never touch. Both neural and haemal spines are grooved distally with the grooves extending into fine serrations at the margin. In lateral aspect, the haemal spines have two parallel emarginations on either side of a median ridge extending distally, whereas the neural spine frequently has a single median groove. This condition is also observed in the holotype of Ctenerpeton remex, AMNH 6907.

The neural arch possesses well developed pre- and post-zygapophyses which are oriented horizontally. Three pairs of horizontal accessory apophyses between neural arches, characteristic of Urocordylinae (Carroll et al. 1998), are present (Fig. 2).

The articular regions of the haemal arch consist of thin, semi circular laminae, that occasionally bear small finger-like projections on both anterior and posterior ends (Figs. 2, 3A). The articular regions occasionally abut and do not seem to consistently possess the three finger-like projections that typically are described as articulating with the adjacent haemal arches (Bossy 1976).

Acknowledgements: We thank the Field Museum’s Prince Internship program for generously providing project funding for Blue M. Byrnes. We thank Bill Simpson and Adreinne Stroup for providing us with collections assistance at the Field Museum of Natural History. We are further grateful to Rich Holm and the Earth Science Club of Northern Illinois for providing access to Mazon Creek localities around Chicagoland and helping with fieldwork. We are grateful to Paul Demkovich and the Lauer Foundation for Science and Education for providing continued support for the Mann Lab research program through funding and access to new fossil material. We thank Stephen Godfrey and Jason Anderson for providing helpful reviews. Finally, this project was inspired by the personal research notes of the late Dr. John R. Bolt, former Curator of Fossil Amphibians and Reptiles at the Field Museum of Natural History, who first recognized the specimen as belonging to Ctenerpeton. John R. Bolt is included here as an author to honor his memory and many contributions to the field of early tetrapod paleontology

Specimen in the study

Field Museum PR 1799. This fossil was originally accessioned in paleobotany. In 1992, Mike Henderson reaccessioned it into the vertebrate collection. The specimen was likely collected from Pit 11 by Helen Piecko or her son Ted and later donated to the Field Museum.