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| Dry season at the Mygatt-Moore Quarry showing Ceratosaurus and Allosaurus fighting over the desiccated carcass of another theropod. Types of bite marks observed in the MMQ assemblage with arrows indicating features of note.
in Drumheller, McHugh, Kane, et al., 2020.
Illustration by Brian Engh (dontmesswithdinosaurs.com).
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Abstract
Bite marks provide direct evidence for trophic interactions and competition in the fossil record. However, variations in paleoecological dynamics, such as trophic relationships, feeding behavior, and food availability, govern the frequency of these traces. Theropod bite marks are particularly rare, suggesting that members of this clade might not often focus on bone as a resource, instead preferentially targeting softer tissues. Here, we present an unusually large sample of theropod bite marks from the Upper Jurassic Mygatt-Moore Quarry (MMQ). We surveyed 2,368 vertebrate fossils from MMQ in this analysis, with 684 specimens (28.885% of the sample) preserving at least one theropod bite mark. This is substantially higher than in other dinosaur-dominated assemblages, including contemporaneous localities from the Morrison Formation. Observed bite marks include punctures, scores, furrows, pits, and striations. Striated marks are particularly useful, diagnostic traces generated by the denticles of ziphodont teeth, because the spacing of these features can be used to provide minimum estimates of trace maker size. In the MMQ assemblage, most of the striations are consistent with denticles of the two largest predators known from the site: Allosaurus and Ceratosaurus. One of the bite marks suggests that a substantially larger theropod was possibly present at the site and are consistent with large theropods known from other Morrison Formation assemblages (either an unusually large Allosaurus or a separate, large-bodied taxon such as Saurophaganax or Torvosaurus). The distribution of the bite marks on skeletal elements, particularly those found on other theropods, suggest that they potentially preserve evidence of scavenging, rather than active predation. Given the relative abundances of the MMQ carnivores, partnered with the size-estimates based on the striated bite marks, the feeding trace assemblage likely preserves the first evidence of cannibalism in Allosaurus.
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| Fig 3. Shed lateral tooth of Allosaurus sp. (MWC 5011) found at the Mygatt-Moore Quarry, white arrow indicates the distal denticles. Mesial denticles are present on such teeth, but were not preserved in this specimen. |
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| Fig 2. Types of bite marks observed in the MMQ assemblage with arrows indicating features of note. A, striated marks produced by ziphodont tooth on an Allosaurus sp. pedal claw (MWC 7263); B, a striated score on an Allosaurus sp. vertebral centrum (MWC 8675); C, a score on an Apatosaurus sp. rib fragment (MWC 3853); D, a dense cluster of furrows on a distal Apatosaurus sp. pubis (MWC 861); E, a puncture (white arrow) and a pit (yellow arrow) on an Allosaurus sp. caudal vertebral centrum; F, a dense cluster of striated furrows Apatosaurus sp. ischium (MWC 4011). All scale bars equal 10 mm. |
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| Fig 4. Dry season at the Mygatt-Moore Quarry showing Ceratosaurus and Allosaurus fighting over the desiccated carcass of another theropod.
Illustration by Brian Engh (dontmesswithdinosaurs.com).
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Conclusions:
The Mygatt-Moore Quarry preserves an unusually highly tooth-marked assemblage from the Upper Jurassic Morrison Formation. Bite marks are consistent with a theropod trace maker, and striations place the traces within the range expected for the known large-bodied theropods from the site: Allosaurus and Ceratosaurus. The largest of these traces suggests an individual that is too large to be either taxon based on existing fossils, suggesting they were produced by an even larger taxon such as Saurophaganax or Torvosaurus. While the location of traces on herbivorous dinosaurs are consistent with predation or early access to remains, bite marks found on other theropod material, more specifically Allosaurus, are concentrated on lower-economy bones, suggesting that they represent incidences of scavenging. If the trace maker is Ceratosaurus, this study represents the first incidence of this taxon feeding on another large, contemporaneous theropod. If the trace maker is Allosaurus, this study represents the first time cannibalism has been reported in this taxon and its encompassing clade, Allosauroidea. If the trace maker is a taxon not represented in the fossil assemblage (i.e., Saurophaganax or Torvosaurus), then these bite marks preserve the first indirect evidence of such a taxon in the MMQ, raising the diversity of large carnivores at the site based on bone surface modifications alone in the absence of body fossils. This seems likely for our largest striations, as they are too large to be produced by any taxon of known size in the MMQ.
Together with the high volume of other bone surface modifications, these traces suggest a depositional environment in which remains were exposed at the surface for long stretches of time, allowing more complete utilization of decaying remains than might be expected at other, contemporary sites with more rapid sediment accumulation (e.g., Carnegie Quarry-Dinosaur National Monument). Therefore, the high concentration of bone surface modifications at the MMQ may represent a true sampling of the processes that shaped the fossil site, a signal that seems to have been boosted by a recent shift to bulk collection at the locality. More detailed comparisons of bone surface modification frequencies in samples collected both before and after this change in collection protocol is ongoing, but this case study demonstrates that paleoecological analyses of these taphonomic processes are helped by more complete sampling and are actively biased by targeting of less damaged, more aesthetically-pleasing bones, as is common practice when type and exhibition specimens are preferentially collected.
Stephanie K. Drumheller, Julia B. McHugh, Miriam Kane, Anja Riedel, Domenic C. D’Amore. 2020. High Frequencies of Theropod Bite Marks provide Evidence for Feeding, Scavenging, and possible Cannibalism in A Stressed Late Jurassic Ecosystem. PLoS ONE. 15(5): e0233115. DOI: 10.1371/journal.pone.0233115
In stressed ecosystems Jurassic dinosaurs turned to scavenging, maybe even cannibalism
