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PaleontologyPLoS ONE
Peer-reviewed journal article

A new metoposaurid (Temnospondyli) bonebed from the lower Popo Agie Formation (Carnian, Triassic) and an assessment of skeletal sorting in temnospondyls

Aaron M. KufneriD, Max E. Deckman, Hannah R. MilleriD, Calvin SoiD, Brandon R. Price, David M. LovelaceiD

Abstract

Metoposaurid-dominated bonebeds are relatively commonplace in Upper Triassic continental deposits with at least ten monodominant, densely populated bonebeds globally. The biostratinomy of several classic localities in India, North America, and Poland have been explored in detail, however, variability in methods and resultant conclusions point to the need for a more nuanced approach to understanding both the taphonomic and the ecological origins of metoposaurid-dominated bonebeds. Here we present the first monodominant metoposaurid mass mortality assemblage from the Late Triassic Popo Agie Formation and the stratigraphically lowest known record of several fauna from the Popo Agie Fm including the first occurrence of the Buettnererpeton bakeri in Wyoming. We employ previously used binning methods based on perceived hydrodynamic equivalence ("Voorhies groups") to assess pre-burial skeletal sorting. We suggest a simple counting and normalization method that avoids the inherent bias introduced by the interpretation of hydrodynamic equivalence of skeletal elements in taxa that lack actualistic experimental data. In contrast to other North American metoposaurid bonebeds, the sedimentology and skeletal sorting analyses of the Nobby Knob quarry support an autochthonous origin of this assemblage in a fluvio-lacustrine system with relatively little pre-burial sorting. Despite differences in underlying assumptions regarding the dispersal potential of specific skeletal elements, binning methods tend to follow similar trends regardless of framework used to assess different assemblages.

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Claude Opus 5.5 · xhigh effort · 8 Nov 2026 · from the full text of version 1 (28 Dec 2024) · CC BY 4.0

Nobby Knob, a Carnian bonebed in Wyoming's lower Popo Agie Formation, records an autochthonous, weakly sorted mass mortality of the metoposaurid Buettnererpeton bakeri (MNI 19).

Metoposaurid bonebeds are common in Upper Triassic continental rocks, but studies of their origin differ in methods and conclusions, and temnospondyl bones are binned into Voorhies groups from mammal flume experiments with no actualistic temnospondyl data. This work adds the first monodominant metoposaurid mass mortality assemblage known from the Popo Agie Formation and proposes MNI-normalized element counts to assess pre-burial sorting, even in old collections lacking site maps.

Key results

  1. ShownElongate disarticulated bones in the Nobby Knob quarry show no preferred azimuth, so no current aligned them during deposition, unlike the Lamy amphibian quarry. No bones show weathering or abrasion from transport.

    N=207 elongate elements (length ≥ 4 times width), mostly 4–6 cm long; Hermans-Rasson test T= 1.477, P = 0.910; Pycke test T= −1.772, P = 0.943 (CircMLE 0.3.0); no weathering or abrasion consistent with surface exposure or fluvial transport. · Taphonomy; Spatial distribution of skeletal elements; Bone modifications; Skeletal sorting and transport

  2. ShownAfter normalizing by MNI, much of the skeleton is still present at Nobby Knob, indicating little pre-burial sorting. The Elkins Place bonebed is instead strongly enriched in plate-like skulls, clavicles and interclavicles, consistent with winnowing.

    MNI 19 at NK (19 interclavicles and right clavicles) vs 13 at Elkins Place and 20 for the Site XIII subset; all three overrepresent plate-like bones and underrepresent autopod bones and neural and hemal arches, most strongly at Elkins Place; the Site XIII completeness pattern resembles Elkins Place. · Sorting of skeletal elements; Bone modifications; Skeletal sorting and transport

  3. SuggestedPast Voorhies group assignments for temnospondyls fit observed completeness poorly except in the late-dispersal group. Humerus, fibula and ischium behave more like intermediate elements, yet the alternative grouping schemes still produce similar overall trends.

    Normalized counts at NK, Elkins Place and Site XIII (Fig 4A-C) show highly variable intermediate-group abundances and uniformly poor representation of vertebral arches, metapodials and phalanges; ternary plots under unmodified, metoposaurid-modified and capitosaurid-modified schemes give comparable patterns (Fig 4D). · Skeletal sorting and transport; Implications for the taphonomy of metoposaurids…

  4. ShownThe site preserves partial articulation and very small bones, including articulated denticulate palatal plates not previously found in articulation in a metoposaurid palate. This implies soft tissue persisted at burial in at least some individuals, consistent with an in-place mass mortality.

    Every B. bakeri element except the epipterygoid is present (S1 Table); skulls with articulated mandibles, two partial articulated hindlimbs (UWGM 7044), a partial skeleton with over one dozen nearly articulated intercentra; no tooth or claw scores on any metoposaurid bone. · Spatial distribution of skeletal elements; Sorting of skeletal elements; Incertae sedis; Skeletal sorting and transport

  5. SuggestedThe bonebed yields the stratigraphically lowest diagnostic metoposaurid and the first B. bakeri in the Popo Agie Formation, across a wide size range. A parasuchid jaw moves the base of the Otischalkian teilzone lower in the local section.

    Nearly complete skulls of <20 to ∼40 cm midline length vs mostly ∼30 cm at the Elkins Place type locality; parasuchid fragments (UWGM 1995, 7578) lie within 5 m above the bonebed, placing the teilzone base at ∼5 m above the purple unit base; ocher-unit zircon ages imply late Carnian (ca. 230 Ma). · Systematic paleontology; Biochronological implications

Methods, data and reproducibility

The team excavated and mapped the Nobby Knob quarry, measured stratigraphic sections and described facies, and tested long-bone azimuths for preferred orientation. Element counts were normalized to MNI-based expected counts (skeletal template from Dutuitosaurus and Metoposaurus krasiejowensis) and compared with Elkins Place, Site XIII and Voorhies group binning schemes.

  • Nobby Knob bonebed metoposaurid remains, purple unit of lower Popo Agie Fm, Fremont County, WY; excavated 2014, 2016, 2018 and 2019 (UWGM; S1 Table) · Buettnererpeton bakeri (metoposaurid temnospondyl) · n = MNI 19; 207 elongate bones measured for azimuth
  • Elkins Place bonebed specimens (Camp Springs Conglomerate, basal Dockum Group, Texas), counted firsthand at University of Michigan Museum of Natural History (S2 Table) · Buettnererpeton bakeri (type locality) · n = MNI 13
  • Site XIII blocks (Timezgadiouine Formation, Morocco) counted from published photographic plates; Muséum National d'Histoire Naturelle (S3 Table) · Dutuitosaurus ouazzoui (type locality) · n = MNI 20 (subset of site)
  • Unionoid bivalve external molds cf. Antediplodon sp. (UWGM 7567, 7571–7573, 7584) plus dozens of uncatalogued impressions; SEM-EDS of one mold and matrix · Antediplodon sp. (unionoid bivalve)
  • Actinopterygian scales, teeth, possible fin and supracleithrum mold (UWGM 7575, 7576, 7579, 7586) · Redfieldiidae indet. (ray-finned fish)
  • Shed archosauromorph teeth (UWGM 7569, 7570, 7585) and ex situ phytosaur mandible fragments (UWGM 1995, 7578) · Archosauromorpha indet.; Parasuchidae indet. · n = three shed teeth
  • Three overlapping trenched stratigraphic sections measured in 2018, one passing through the Nobby Knob quarry
  • Plant megafossils, root traces and coprolites from the bonebed interval (UWGM 7577, 7580–7583, 7587–7589) · plants and tetrapods (indet.)
Methods & tools
CircMLE R package v0.3.0 (HR_test, pycke_test), Hermans-Rasson test, Pycke test, Voorhies dispersal groups (unmodified, metoposaurid- and capitosaurid-modified), Minimum number of individuals (MNI) normalization of element counts, Ternary diagrams of dispersal-group proportions, SEM with energy dispersive spectroscopy (EDS), Modified Jacob's staff and 'Jake for stratigraphy' iOS app, Pneumatic air scribes and sodium bicarbonate air abrasion, Paraloid B-72 and Butvar B-98 consolidants, Dino-Lite Edge 3.0 USB Digital Microscope, Adobe Lightroom, Photoshop and Illustrator
Reagents & environment
R package CircMLE version 0.3.0 (HR_test, pycke_test); R script for the azimuth analyses and the sorting and Voorhies group plots is supplied as S2 File.
Reproducibility gaps
No code or data repository URLs; specimen lists are in S1–S3 Tables and the R script in S2 File. Site XIII was counted only from published plates, and preparation of the NK material is still under way.
Limitations (6)
  • The normalization assumes equal bone density across all elements, which the authors note seldom holds for air-breathing aquatic tetrapods, and no actualistic flume experiments exist for temnospondyl skeletons. · stated by the authors
  • Small elements (∼1 cm) may be underrepresented at Nobby Knob because several quarry blocks are not fully prepared, and the field crew tended to focus on jacketing fragile plate-like skull and pectoral girdle elements. · stated by the authors
  • The Site XIII counts come only from published photographic plates of blocks that may have been chosen for their diagnostic skulls and girdles, so the subset may not represent the whole locality. · stated by the authors
  • Elkins Place has no quarry map, which rules out spatial or orientation comparisons. Its reinterpretation as a debris-flow deposit relies on published images, not the authors' own field observations, and remains inconclusive. · stated by the authors
  • The lateral limits of the NK bonebed have not been found, so Site XIII 'drying pond' criteria that require complete excavation cannot be compared. Other North American bonebeds lack raw element counts, which prevents direct comparison. · stated by the authors
  • Sorting is judged by eye from normalized proportions and ternary plots, with no formal statistical test or uncertainty intervals on the MNI-based ratios, so 'little sorting' is a qualitative conclusion. · noted in this summary

Who should read:Read this if you study temnospondyl or Triassic bonebed taphonomy or Carnian biostratigraphy of western North America, or want an MNI-normalized alternative to Voorhies-group binning for assessing skeletal sorting.

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