This is Mazon Monday post #283.  What’s your favorite Mazon Creek fossil?  Tell us at email:esconi.info@gmail.com.

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Another week, another new Mazon Creek paper,.. “Mazon Creek bromalites evidence a specialized, xiphosurid-rich diet for Pennsylvanian predators” was published in the journal Palaios. It was authored by Russell Bicknell, Julien Kimming, Andew Young, Bruce Lauer, Rene’ Lauer, and Victoria McCoy.  Those are some familiar names for anyone that follows Mazon Creek.  

The paper takes a close look at some unusual Mazon Creek fossils, called bromalites.  If you’re not familiar with the term, bromalites are fossils of stomach or intestinal contents. These can be coprolites (fossilized feces) or regurgitalites (fossilized vomit). While they may not sound glamorous, they offer some of the clearest evidence of what ancient animals were eating. These specimens, now part of the Lauer Foundation collection, are packed with broken pieces of the horseshoe crab Euproops danae, one of the most common animals in the Braidwood assemblage.

The study shows that these bromalites are best interpreted as regurgitalites (vomited stomach contents) rather than coprolites (fossilized feces). The fragments represent a very specific size range of Euproops, suggesting that predators either targeted medium-sized individuals or that the largest ones managed to escape.

So who was eating all these horseshoe crabs? The authors rule out Arthropleura, eurypterids, and tetrapods. Instead, they point to large lungfish such as Ctenodus cristatus as the most likely culprits. With broad tooth plates well-suited for crushing hard shells, these fish could easily feed on Euproops and then spit out the indigestible bits.

This discovery highlights the important role of lungfish in Pennsylvanian food webs and shows that horseshoe crabs were a regular menu item in the Carboniferous—very different from today, where only a few animals prey on them. It’s another reminder that even the “leftovers” of ancient meals can tell us a lot about life at Mazon Creek.

Abstract: The Mazon Creek Konservat-Lagerstätte is a globally significant Pennsylvanian (late Carboniferous) fossil assemblage characterized by exceptional soft-tissue preservation in siderite concretions. Coprolites documented from this site have provided crucial insights into trophic interactions within a Pennsylvanian deltaic environment but have not been widely studied. To expand the limited record in the literature of Mazon Creek bromalites, we investigate four shelly specimens, considered regurgitalites, with high concentrations of fragmented xiphosurid (Euproops danae) exoskeletal elements, indicating a specialized, durophagous diet. The sizes of prosomal and thoracetronic fragments within regurgitalites reveal a constrained prey size range, suggesting either prey size selection by the predator or differential survival of larger individuals. These findings highlight successful predation on xiphosurids within the late Carboniferous and provide direct evidence of their position within benthic food webs. This study underscores the utility of bromalites in informing paleoecological reconstructions and contributes to a broader understanding of trophic structures within late Carboniferous deltaic ecosystems.

Introduction: The Mazon Creek fossil assemblage is a world-class Konservat-Lagerstätte (sensu Kimmig and Schiffbauer 2024) that presents insight into a Pennsylvanian (late Carboniferous) deltaic environment (Johnson and Richardson 1968; Schram 1979; Shabica and Hay 1997; Clements et al. 2019; Burke et al. 2024). Mazon Creek siderite concretions exceptionally preserve invertebrates, vertebrates, and plants from this paleoecosystem (see Baird et al. 1986; Shabica and Hay 1997; Clements et al. 2019; Burke et al. 2024 for reviews). Soft-bodied preservation common in the Mazon Creek fossil assemblage has uncovered data on the ontogeny, morphology, and taxonomy of . 465 plant and . 350 animal species (Clements et al. 2019; Burke et al. 2024), as well as rare anatomical information (e.g., neural systems and muscles) for vertebrate (Bardack 1979; Mann and Gee 2019; McCoy et al. 2023) and invertebrate (Tetlie and Dunlop 2008; Bicknell et al. 2021; Plotnick et al. 2023) groups. This combination has resulted in the Mazon Creek Lagerstätte being subject to detailed taphonomic, geological, and paleontological examination.

Aggregations containing fragmented animal parts record predation or scavenging in the fossil record and are useful for reconstructing trophic interactions in deep time (Knaust 2020).  Shell-rich aggregations, commonly considered bromalites, often reflect shell-crushing (durophagous) activity and can have a higher preservation potential than their producers (Vannier and Chen 2005). Shelly bromalites rich in arthropod fragments illustrate the position of arthropods as prey in the fossil record (Habgood et al. 2003; Northwood 2005;  Qvarnström et al. 2016). Shelly, arthropod-rich bromalites from Paleozoic deposits require exceptional preservation, and are, therefore, rare. The majority of Paleozoic shelly bromalites are Cambrian-aged (Mikuláš 1995; Nedin 1999; Vannier and Chen 2005; English and Babcock 2010; Daley et al. 2013; Kulkarni and Panchang 2015; Kimmig and Strotz 2017; Bicknell and Paterson 2018; Kimmig and Pratt 2018; Bicknell et al. 2022a), with rare Ordovician (Briggs et al. 2015; Bicknell et al. 2024), Silurian (Caster and Kjellesvig-Waering 1964; Bicknell et al. 2023), and Devonian (Habgood et al. 2003; Zatoń and Rakociński 2014) examples. Limited evidence for Carboniferous, arthropodrich bromalites has also been presented (Fisher 1979). These bromalites originate from the Mazon Creek Konservat-Lagerstätte and contain horseshoe crab (Euproops danae) and millipede fragments (Fisher 1979). Despite the paleoecological implications of this material, such specimens have not been considered in detail. To address this, we present four new records of Euproops-rich bromalites from the Mazon Creek Lagerstätte, explore the origins of this material, its possible producers, and outline indications for limitations on prey size.

Acknowledgments: This research was funded by a MAT Program Postdoctoral Fellowship (to R.D.C.B). We highlight LF 7282, LF 7283, and LF 7284 were originally housed within the David and Sandra Douglass Collection, who made them available to us for preliminary investigation. The specimens were then donated to the Lauer Foundation for Paleontology, Science and Education in Wheaton, Illinois, to place them into a publishable repository (see https://www.lauerfoundationpse.org/). We thank Michele Micetich for sharing with us her vast knowledge of the mining history of the Mazon Creek area. Finally, we thank Adrian Hunt and an anonymous reviewer for their insightful comments.