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Brief Report

An Isolated Fish Skull from the Nonmarine Eocene of Wyoming, USA: A Case Study in Evaluating a Possible Regurgitalite

by
Spencer G. Lucas
1,*,
Adrian P. Hunt
2,
Edward L. Simpson
3 and
Larry F. Rinehart
1
1
New Mexico Museum of Natural History, 1801 Mountain Road NW, Albuquerque, NM 87104, USA
2
Flying Heritage and Combat Armor Museum, 3407 109th St. SW, Everett, WA 98204, USA
3
Department of Physical Sciences, Kutztown University, Kutztown, PA 19530, USA
*
Author to whom correspondence should be addressed.
Foss. Stud. 2026, 4(2), 11; https://doi.org/10.3390/fossils4020011
Submission received: 3 April 2026 / Revised: 8 May 2026 / Accepted: 8 May 2026 / Published: 11 May 2026
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Abstract

An isolated fish skull from the Early Eocene Green River Lagerstätte from southwestern Wyoming, USA was tentatively identified as a regurgitalite. It consists of a skull of the perciform moronoid Mioplosus labracoides and articulated vertebrae. After mechanical preparation, the specimen was studied in detail and it could represent a regurgitalite, a pabulite (remnant of predation) or a portion of a decayed fish. Fish decay rarely results in an isolated skull, so this specimen probably pertains to a bromalite (pabulite or regurgitalite). We consider that it most likely is a regurgitalite because of the breakage of bones and displacement of elements.

1. Introduction

Regurgitalites are bromalites that are egested through the mouth [1]. They have the least extensive fossil record among bromalites, but there has been increasing interest in them recently (e.g., [2,3,4,5,6,7,8,9,10,11,12,13,14,15]). Regurgitalites can encompass a wide range of morphologies (compare 6, Figures 3, 4 and 12). However, diverse criteria have been utilized to identify regurgitalites (e.g., [6,7,8,13,14,16,17]). The purpose of this paper is to evaluate a possible regurgitalite from the Eocene Green River Formation of Wyoming, USA, as a case study in identifying putative regurgitalites.

2. Materials and Methods

The specimen is contained within a slab of matrix and was obtained by one of us (ELS) from an experienced commercial dealer in 2026. It derives from the Eocene Green River Formation of southwestern Wyoming, USA. The specimen is cataloged in the collection of the New Mexico Museum of Natural History and Science (NMMNH) as NMMNH P-77580. It was mechanically prepared by one of us (LFR) using tungsten carbide hand tools.

3. Geological Setting

Lake systems of the Late Paleocene to Middle Eocene Green River Formation developed in a series of small, elongate intermontane basins associated with the evolving, Sevier-Laramide fold-and-thrust belt [18,19,20,21] (Figure 1). Fossil Lake is one of several extensive lakes linked to this fold and thrust belt (Figure 1) [19,22,23,24]. The strata deposited in Fossil Lake reflect the western extent of Lake Gosiute in what has been estimated to have been the deepest portion of the lake (Figure 1) [22,23,25,26].
Three members of the Green River Formation are recognized at Fossil Lake localities, from oldest to youngest: the fluvial to lacustrine Road Hollow Member, the variable fluvial-lacustrine to profundal Fossil Butte Member, and the evaporitic Angelo Member [20]. These members range from 53 to 51 Ma, making them of Early Eocene, Ypresian age [20]. The NMMNH specimen analyzed here derives from the Fossil Butte Member.
The Fossil Butte Member at Fossil Lake contains two principal mass mortality horizons of fish that are quarried commercially today: the 47 cm (18-inch) layer and the subjacent sandwich layer [21,22]. The 47 cm layer is typically a white-to-bluff limestone with brown kerogenic laminations. In contrast, the sandwich layer is a limestone that is typically lighter in color with faint laminations. Vertebrate fossils are typically darker brown in color in the 47 cm layer than the more orangish brown in the sandwich layer. The putative regurgitalite likely derives from the 47 cm layer. These strata were deposited along the nearshore eastern side of Fossil Lake [21].
The Green River Formation in southwest Wyoming is a significant Lagerstätte that yields abundant fish as well as other terrestrial and volant vertebrates, plants and invertebrates, notably insects [21,22]. The ichnofauna includes tracks, dentalites (bite marks), consumulites (infilling of gastrointestinal tract), coprolites (fossil feces) and pabulites (remnants of predation) [11,21,27,28,29,30,31]. Coprolites are the second most common fossils in the Lagerstätte.
Fossstud 04 00011 g001
Figure 1. Eocene Green River Formation: extent of preserved strata in each basin (tan) and inferred lakes (dark blue) [29]. The specimen described herein is from a quarry located in the Fossil Basin of Wyoming.
Figure 1. Eocene Green River Formation: extent of preserved strata in each basin (tan) and inferred lakes (dark blue) [29]. The specimen described herein is from a quarry located in the Fossil Basin of Wyoming.
Fossstud 04 00011 g001

4. Description

The original specimen was largely covered by thin laminae of silt (Figure 2A). It comprised a lenticular mass approximately 72 mm in length and 36 mm in width. Mechanical preparation revealed an abundance of cranial material of a medium-sized fish (Figure 2B).
L. Grande (pers. commun., 2026) identified the specimen as a poorly preserved skull of the perciform moronoid Mioplosus labracoides (Figure 2B and Figure 3). Cope [32] described five species of Mioplosus from the Green River Formation, but most recent authors only recognize M. labracoides as valid [22,33,34,35]. The holotype of M. sauvagenus is currently unavailable, and no illustration has been published, so the name is retained only as a nomen dubium [22,35].
The right side of the skull is embedded in the matrix. Overall, the skull is slightly flattened dorsoventrally and partially disarticulated (Figure 2B). The articulated jaws at the left of the specimen are displaced anterior to the remainder of the skull (Figure 3A). Two-thirds of the anterior part of the left dentary is missing. The left premaxilla, containing numerous mm-sized teeth, and its associated maxilla, are preserved, and the anterior part of the right premaxilla emerges from under the anterodorsal skull. A short nasal is located between the left premaxilla and the frontal. Most of the skull below the frontal, except for the parasphenoid, is missing. The anterior and dorsal orbit borders are observable, but the orbit is mostly filled with scales and bone fragments. Small fragments of palatal bones with very small (<0.5 mm) teeth are located posterior to the maxilla and the orbit. The ventro-posterior section of the skull is preserved. Three articulated vertebrae, and an incomplete fourth, are visible at the posterior end of the specimen (Figure 3B). The posteriormost vertebra is broken close to the articular surface perpendicular to its long axis. Elements in the dorso-posterior region of the skull are disarticulated and broken.

5. Interpretation

5.1. Introduction

The concentration of organic material in a discrete mass with a limited matrix suggested that it represents a bromalite [3,8,16]. The large volume of articulated skeletal elements indicates that the specimen does not represent a coprolite. Thus, it could represent a regurgitalite, a pabulite or a fragment of a decayed carcass.

5.2. Identifying Regurgitalites

Regurgitalites have no single unique characteristic, but they can be recognized on the basis of a suite of features [6,7,8,9,10,11,12,14,15,16,17,36,37]:
  • The geometry of putative regurgitalites ranges from three-dimensional cylindrical or amorphous bodies to discrete concentrations of associated organic matter on a bedding plane (e.g., [7,8,15,38,39]).
  • The non-biotic content of regurgitalites includes a relative (to coprolites) lack of phosphatic matrix, and some specimens have none [6,8,10,15,40].
  • The biotic content of regurgitalites can include the following features:
    • The principal inclusions are often elements that are not easy to digest (e.g., vertebrate skeletal elements, invertebrate valves and cuticles, and dermal elements—fur and feathers) [7,16,40]. Some hard parts, such as invertebrate shells, can pass through the digestive tracts of durophagous fish and some birds [41,42]. Conversely, regurgitalites can also be identified if they preserve soft tissue that would not survive complete digestion [8].
    • Skeletal elements are usually relatively larger and often more articulated in regurgitalites than in coprolites [7].
    • Regurgitalites often preserve evidence of digestion, but less than in coprolites, including physical damage such as broken elements or evidence of chemical digestion, including etching or pitting [7,8,37,43,44,45].
    • Dentalites are less likely to be preserved in elements in regurgitalites that have shorter residence time in the digestive tract than in coprolites [37].
    • Recent and fossil avian pellets (ornithoregurgitalites) can be characterized by a prevalence of skulls, articulated elements and the association of one or more discrete skeletons [7,10,16]. Rare ejectalites include entire articulated carcasses [12].
    • Elements are often aligned about their long axes and closely packed [16,45].

5.3. Identifying Pabulites

Pabulites have only recently been explicitly named, and no single, unique characteristic distinguishes them. This is a new term but an old concept. They can be recognized and distinguished from other bromalites based on the possession of two or more of the following features [11,14,46,47,48]:
  • Lack of even diffuse bromalitic matrix around preserved elements [47].
  • The carcass is often incomplete but with obvious evidence of biological causation of dismemberment (e.g., dentalites present or associated coprolites, regurgitalites, or tracks).
  • The representation and preservation of remaining elements are consistent with partial predation (e.g., [48,49]).
  • There is often articulation of some elements of the body [7,11,13,14,47].
  • The elements are restricted to a limited area [47].
  • Dentalites (bite marks) are often present [50].
  • A high degree of element breakage is common [50].
  • Carnivore teeth are often associated with the specimen (e.g., [51]).

5.4. Identifying Decayed Fish

There has been extensive study of the natural decay of fish carcasses and their recognition in fossil assemblages (e.g., [36,52,53,54,55,56,57,58,59,60]). The sequence of decay in fish is markedly different from that of terrestrial tetrapods. Notably, the tetrapod skull is a bony structural box, whereas an actinopterygian fish skull is a collection of many delicate elements that are less strongly articulated. Thus, work on tetrapod taphonomy, going back to the classic study of Voorhies [61], has demonstrated that isolated fossil tetrapod skulls are commonly preserved intact. In contrast, in actinopterygians, disarticulation begins at the head–trunk boundary, then continues gradually anteriorly into the skull region and eventually posteriorly into the posterior abdominal region, and, as disarticulation progresses, only the caudal region remains articulated (e.g., [58]). Saurichthys, a Mesozoic genus with an elongate and robust skull, is an exception, and complete skulls can survive decay processes ([62], Figure 10). In conclusion, relatively complete and isolated fish skulls of Paleogene actinopterygians are rare.

6. Discussion

Published taphonomic studies [54,57,58] indicate that the isolated skull with a few articulated vertebrae evaluated here is inconsistent with the usual manner of natural decay of actinopterygians in an Eocene Green River lake. The large volume of bone and the absence of a phosphatic matrix preclude this specimen from being a coprolite [11,63]. Thus, the specimen presumably represents a pabulite or a regurgitalite.
A pabulite is a remnant left after partial predation or scavenging, whereas a regurgitalite represents food that is ingested and then regurgitated [11,13,47]. The sharp termination of the vertebral column and the lack of the anterior dentary of the specimen evaluated here suggest predation and mechanical separation of the skull from the trunk. The disruptions and distortion of the skull indicate taphonomic processes after the amputation.
Characteristics suggesting the specimen is a pabulite include an incomplete carcass, possibly resulting from biting and disarticulation of skeletal elements. A regurgitalite is suggested by the fusiform shape of the specimen in lateral view, possible evidence of digestion (displacement of elements), breakage of bones and preservation of the semi-articulated skull (common in some regurgitaites). Additionally, most of the preserved elements are aligned along their long axes and are closely packed [4,8]. Overall, we think it most likely that this represents a regurgitalite of a large carnivore. The breakage of bones and movement of elements is unlikely to have resulted from partial dismemberment and more likely to have occurred during ingestion.

7. Conclusions

The Green River Lagerstätte in southwestern Wyoming yields a significant ichnofauna in addition to the body fossil fauna. A newly collected specimen was suspected to be a putative regurgitalite and was prepared and studied in detail. It consists of a skull of the perciform moronoid Mioplosus labracoides and articulated vertebrae. Fish decay rarely results in an isolated skull, so this specimen probably pertains to a bromalite—pabulite or regurgitalite. We consider that it most likely is a regurgitalite because of breakage of bones and displacement of elements.

Author Contributions

Conceptualization, A.P.H. and S.G.L.; methodology; formal analysis, S.G.L., A.P.H., E.L.S. and L.F.R.; investigation, S.G.L., A.P.H. and E.L.S.; writing—original draft preparation, S.G.L., A.P.H. and E.L.S.; writing—review and editing, S.G.L., A.P.H. and E.L.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

No data were collected during the research and writing of this article.

Acknowledgments

Lance Grande identified the specimen described herein. We thank two anonymous reviewers for their helpful comments.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
NMMNHNew Mexico Museum of Natural History, Albuquerque, New Mexico, USA

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Figure 2. Overview of a putative regurgitalite containing Mioplosus labracoides skull. (A) Unprepared; (B) prepared specimen.
Figure 2. Overview of a putative regurgitalite containing Mioplosus labracoides skull. (A) Unprepared; (B) prepared specimen.
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Figure 3. Close-ups of putative regurgitalite containing Mioplosus labracoides skull. (A) Anterior skull; (B) posterior skull. Abbreviations: cl, cleithrum; d, dentary; f, frontal; mx, maxilla; n, nasal; o, orbit; pm, premaxilla; ps, parasphenoid; tb, tooth-bearing palatal fragments; v, vertebra.
Figure 3. Close-ups of putative regurgitalite containing Mioplosus labracoides skull. (A) Anterior skull; (B) posterior skull. Abbreviations: cl, cleithrum; d, dentary; f, frontal; mx, maxilla; n, nasal; o, orbit; pm, premaxilla; ps, parasphenoid; tb, tooth-bearing palatal fragments; v, vertebra.
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MDPI and ACS Style

Lucas, S.G.; Hunt, A.P.; Simpson, E.L.; Rinehart, L.F. An Isolated Fish Skull from the Nonmarine Eocene of Wyoming, USA: A Case Study in Evaluating a Possible Regurgitalite. Foss. Stud. 2026, 4, 11. https://doi.org/10.3390/fossils4020011

AMA Style

Lucas SG, Hunt AP, Simpson EL, Rinehart LF. An Isolated Fish Skull from the Nonmarine Eocene of Wyoming, USA: A Case Study in Evaluating a Possible Regurgitalite. Fossil Studies. 2026; 4(2):11. https://doi.org/10.3390/fossils4020011

Chicago/Turabian Style

Lucas, Spencer G., Adrian P. Hunt, Edward L. Simpson, and Larry F. Rinehart. 2026. "An Isolated Fish Skull from the Nonmarine Eocene of Wyoming, USA: A Case Study in Evaluating a Possible Regurgitalite" Fossil Studies 4, no. 2: 11. https://doi.org/10.3390/fossils4020011

APA Style

Lucas, S. G., Hunt, A. P., Simpson, E. L., & Rinehart, L. F. (2026). An Isolated Fish Skull from the Nonmarine Eocene of Wyoming, USA: A Case Study in Evaluating a Possible Regurgitalite. Fossil Studies, 4(2), 11. https://doi.org/10.3390/fossils4020011

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