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New Data Towards the Development of a Comprehensive Taphonomic Framework for the Late Jurassic Cleveland-Lloyd Dinosaur Quarry, Central Utah

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New Data Towards the Development of a Comprehensive Taphonomic Framework for the Late Jurassic Cleveland-Lloyd Dinosaur Quarry, Central Utah

Joseph E Peterson et al. PeerJ.

Abstract

The Cleveland-Lloyd Dinosaur Quarry (CLDQ) is the densest deposit of Jurassic theropod dinosaurs discovered to date. Unlike typical Jurassic bone deposits, it is dominated by the presence of Allosaurus fragilis. Since excavation began in the 1920s, numerous hypotheses have been put forward to explain the taphonomy of CLDQ, including a predator trap, a drought assemblage, and a poison spring. In an effort to reconcile the various interpretations of the quarry and reach a consensus on the depositional history of CLDQ, new data is required to develop a robust taphonomic framework congruent with all available data. Here we present two new data sets that aid in the development of such a robust taphonomic framework for CLDQ. First, x-ray fluorescence of CLDQ sediments indicate elevated barite and sulfide minerals relative to other sediments from the Morrison Formation in the region, suggesting an ephemeral environment dominated by periods of hypereutrophic conditions during bone accumulation. Second, the degree of abrasion and hydraulic equivalency of small bone fragments dispersed throughout the matrix were analyzed from CLDQ. Results of these analyses suggest that bone fragments are autochthonous or parautochthonous and are derived from bones deposited in the assemblage rather than transported. The variability in abrasion exhibited by the fragments is most parsimoniously explained by local periodic re-working and re-deposition during seasonal fluctuations throughout the duration of the quarry assemblage. Collectively, these data support previous interpretations that the CLDQ represents an attritional assemblage in a poorly-drained overbank deposit where vertebrate remains were introduced post-mortem to an ephemeral pond during flood conditions. Furthermore, while the elevated heavy metals detected at the Cleveland-Lloyd Dinosaur Quarry are not likely the primary driver for the accumulation of carcasses, they are likely the result of multiple sources; some metals may be derived from post-depositional and diagenetic processes, and others are potentially produced from an abundance of decomposing vertebrate carcasses. These new data help to support the inferred depositional environment of the quarry as an ephemeral pond, and represent a significant step in understanding the taphonomy of the bonebed and Late Jurassic paleoecology in this region.

Keywords: Bonebed; Geochemistry; Morrison; Paleoecology.

Conflict of interest statement

Christopher R Noto is an Academic Editor for PeerJ.

Figures

Figure 1
Figure 1. Regional stratigraphy of the CLDQ vicinity.
(A) Stratigraphic column of the Morrison Formation in the area around the Cleveland-Lloyd Dinosaur Quarry (CLDQ) and the Johnsonville (JONS) sites, shown in meters above the basal contact of the Salt Wash Member of the Morrison Formation with the upper Summerville Formation. Standard USGS symbols of rock units are used in the diagram. (B) Map showing sites, stratigraphic section line, and regional stratigraphy in context of the San Rafael Swell.
Figure 2
Figure 2. Vertebrate fauna of the Cleveland-Lloyd Dinosaur Quarry.
Vertebrate fauna of the Cleveland-Lloyd Dinosaur Quarry, illustrating the 3:1 ratio of predators to prey and minimum number of individuals for each taxon, based on left femoral count. Modified from Gates, 2005.
Figure 3
Figure 3. Fossils and characteristics of the Cleveland-Lloyd Dinosaur Quarry and the Johnsonville site.
(A) A photogrammetric reconstruction of the North Butler building of the Cleveland-Lloyd Dinosaur Quarry (CLDQ), illustrating the locations from which sediment samples were taken for IBF and geochemical analyses. Scale bar equals 1 m; (B) Arrow annotating the location where approximately 30 kg of sediment was collected for analyses. Sediment was collected following the excavation of a series of theropod thoracic ribs. Scale bar equals 10 cm; (C) Arrow annotating the location where approximately 30 kg of sediment was collected for analyses. Sediment was collected following the excavation of a theropod femur and tibia. Scale bar equals 10 cm; (D) Allosaurus manual ungual (left) and Allosaurus pedal phalange (right) as examples of bone preservation from the CLDQ, Scale bar equals 5 cm; (E) Photograph of the Johnsonville (JONS) site, with arrows annotating the locations from which sediment samples were taken for IBF and geochemical analyses. Scale bar equals 1 m; (F) Sauropod caudal vertebra collected from JONS. Scale bar equals 10 cm; (B) Shed theropod teeth (left), crocodilian vertebra (center), and turtle shell (right) as examples of fossils commonly collected from JONS. Scale bar equals 5 cm.
Figure 4
Figure 4. Schematic diagram of the sediment processing tanks.
Sediment was placed into the meshed boxes and submerged with gentle air agitation for approximately 48–72 h.
Figure 5
Figure 5. Examples of fragment abrasion stages, based on Peterson, Scherer & Huffman (2011).
Stage 0—angular fragments; Stage 1—subangular fragments; Stage 2—subrounded fragments; Stage 3—rounded fragments. Scale bar equals 5 mm. All fragments imaged were collected from CLDQ.
Figure 6
Figure 6. X-ray fluorescence results.
The concentrations of selected metals detected in sediment (gray) and bone (green) collected from the CLDQ are compared with those of sediments from local Morrison Formation strata, labeled “Strat Samples” (blue) and sediment and bone from MMQ (red and yellow, respectively). CLDQ sediments and bone stand out in contrast to most other elements, yet are similar to those found at MMQ. All values are given in ppm.
Figure 7
Figure 7. Diagenetic alteration to bones and bone fragments at CLDQ.
(A) Intramatrix bone fragment from CLDQ in petrographic thin section. Arrows annotating the location of bone tissue (BT) and pyrite crystals (Py) in porous cavities within the fragment at 5× magnification (scale bar equals 10 µm), and (B) at 20× magnification (scale bar equals 2.5 µm). (C) Allosaurus caudal vertebra (UMNH.A.2012.26.020) collection from CLDQ possessing barite growth across articular processes. Arrows annotating the presence of diagenetic nodules adhered to the surface of the bone. Scale bar equals 5 cm.
Figure 8
Figure 8. Distribution of abrasion stages of intramatrix bone fragments at the CLDQ (gray) and JONS (white) localities.
Error bars represent standard error.
Figure 9
Figure 9. Conceptual model of the CLDQ depositional system.
(A) Dinosaur carcasses are washed into the CLDQ deposit during a flood stage. High rates of organic matter decay leads to hypereutrophy, calcite and barite precipitation, and discourages biostratinomic influences (e.g., scavenging). (B) As water levels recede during drier conditions, bones that were not buried during the flood stage remain at the surface. (C) During arid conditions, bones present at the surface undergo weathering and abrasion from subaerial exposure, generating the intramatrix bone fragments present at the CLDQ. (D) Floodstage returns, incorporating new carcasses and re-worked bones and fragments to the deposit. The cycle repeats until the deposit maintains a higher water table, producing the limestone above the bone-bearing siltstone.

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Grant support

This work was supported by the University of Wisconsin-Oshkosh Faculty Development Fund (Grant No. FDM250) and the Bureau of Land Management Youth Field Program. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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