Analysis of over 3,000 Edmontosaurus skeletal fossils excavated from the Lance Formation in Wyoming, USA, has revealed new facts about the ecology of the apex predator, Tyrannosaurus rex.
This study, utilizing non-destructive image analysis, established a strict method to identify "true tooth traces" from various marks left on fossils. As a result, the following behavioral patterns of Tyrannosaurus during that time have been highlighted:
- Extremely low incidence of tooth traces: Among the vast number of fossils, definite tooth traces were found on only about 0.4% (12 specimens).
- Evidence of scavenging: There are no signs of healing, highly likely indicating they scavenged on already dead carcasses.
- Obsession with internal organs: To access highly nutritious organs such as the heart and liver, they intentionally bit and broke ribs with their powerful jaws.
This page details the groundbreaking analytical approach to identifying predators from fossil marks and the Late Cretaceous ecosystem where Tyrannosaurus reigned as a giant scavenger.
Introduction: Challenges in Trace Interpretation in Paleontology and a New Paradigm
In paleontology and paleoecology, accurately identifying the various perforations and surface damages left on the fossil skeletons of extinct vertebrates is a crucial process for reconstructing ancient food webs, predator-prey interactions, and overall ecosystem dynamics.
For many years, unnatural marks on the surfaces of fossil bones have often been intuitively interpreted as "tooth traces" or "bite marks" from carnivores. However, identifying the true origin of marks left in the fossil record is challenging. In reality, physical and chemical changes during the fossilization process, other biological factors (insect borings or erosion by plant roots), or simply normal anatomical structures (such as foramina for nerves and blood vessels) are frequently misidentified as marks left by predators.
What is Taphonomy?
A branch of paleontology that studies the physical, chemical, and biological processes (weathering, transport, decay, predation by other animals, etc.) that organisms undergo from the time they die until they are discovered as fossils in strata.
Such misidentification of traces goes beyond mere morphological classification errors. It risks causing incorrect assumptions about ancient animal behaviors and their roles (niches) in the ecosystem, ultimately distorting the overall picture of paleoecosystems. Based on this serious awareness, a detailed study (Siviero et al., 2026) was published focusing on a bonebed centered on Edmontosaurus annectens, excavated from the Lance Formation of the Late Cretaceous (Maastrichtian) in northeastern Wyoming, USA.
Based on this pioneering study, this report clearly explains the quantitative data obtained from screening a vast sample of 3,013 fossil specimens, the strict redefinition of previously ambiguous tooth trace identification criteria, and the elucidation process using the latest non-destructive image analysis technology.
Geological and Taphonomic Background: Lance Formation and Bonebed
Gemini Generated Image
The fossil group analyzed in this study was excavated over 20 years from a quarry in the Lance Formation near the Hanson Ranch Station (HRS) in eastern Wyoming. Formed during the Late Cretaceous, approximately 72 to 66 million years ago, the Lance Formation meticulously records the rich floodplain and braided river environments of that time.
The bonebed (a stratum densely packed with fossils) at Hanson Ranch Station is overwhelmingly dominated by the remains of the herbivorous dinosaur Edmontosaurus annectens. The skeletons unearthed here are highly disarticulated. Given the lack of evidence that bones were swept in a uniform direction by water currents, it is presumed that these remains were not transported and buried all at once, but were exposed on the surface for a long period, subjected to various biological activities and weathering, before being gently buried in mud and sand.
The most notable fact here is that despite examining a massive sample of 3,013 items, only 13 specimens (about 0.4% of the total) were found to have perforations or lesions suspected to be "tooth traces" in the initial screening. This low frequency suggests that in the ecosystem of that time, direct predation pressure by large theropods or the frequency of thorough carcass utilization destroying deep into the bone was likely much lower than previously expected.
Theoretical Framework for Tooth Trace Identification: Ichnotaxa and Morphological Criteria
To scientifically and objectively prove the involvement of predators and scavengers from the complex damage patterns left on bone surfaces, this study reevaluated past literature and theoretically systematized strict criteria for identifying true tooth traces. Based on mechanical vectors and tooth penetration depth, tooth traces are classified into the following four basic types:
| Tooth Trace Type | Definition and Mechanical Mechanism | Morphological Characteristics and Depth |
|---|---|---|
| Pit | A shallow depression caused by direct vertical or oblique pressure applied to the bone surface. | Does not fully penetrate the cortical bone (the hard outer surface) and often has a bowl-shaped cross-section. |
| Puncture | Caused by stronger pressure than that forming a pit, where the tooth deeply penetrates the bone tissue. | A deep hole that completely penetrates all layers of cortical bone, reaching the inner cancellous bone. |
| Score | A scratch mark caused by a tooth dragging laterally or obliquely while in contact with the bone surface. | A shallow linear mark that does not penetrate cortical bone. It has a U-shaped or V-shaped cross-section. |
| Furrow | Similar to a score caused by dragging, but dug in much deeper with a stronger force. | Severe and deep linear damage that completely penetrates or extensively destroys the cortical bone layers. |
What are Ichnotaxa?
Rather than the fossils themselves, these are taxonomic names assigned to the "traces" (footprints, burrows, bite marks, etc.) left by biological activity. For example, regular "striations" carved into bones by the serrated teeth of carnivorous dinosaurs are finely classified under names like Knethichnus and Linichnus, serving as evidence to identify the attacker.
The Complex Spectrum of Pseudopathologies and Image Analysis Approach
During the fossilization process, bones are exposed to various factors, leaving "pseudopathologies" on the surface that can easily be mistaken for tooth traces. Examples include root traces formed by plant roots secreting acid, bone borings by insects, tool marks created during excavation cleaning, pathological holes from bacterial infections, and normal anatomical structures for blood vessels and nerves (neurovascular foramina).
To accurately distinguish these diverse pseudopathologies from true tooth traces, the research team employed high-resolution microscopic observation, high-contrast photography using focus stacking technology, and medical-grade non-destructive CT scanning to thoroughly visualize the three-dimensional internal structures.
A Surprising Paradigm Shift in Specimen HRS05499
The most symbolic discovery in this study was the reevaluation process of specimen HRS05499 (a fossil surangular bone), which was initially screened as having "crocodile tooth traces." While linear holes aligning with a crocodilian dental arcade were present on the surface, a CT scan of the interior revealed that each hole merged deep within the bone to form a single large space (channel). This is a typical characteristic of a "neurovascular foramen."
Furthermore, comparative anatomical investigation revealed that this bone did not belong to the abundant Edmontosaurus, but surprisingly to a ceratopsian, Torosaurus latus. This dramatic overturning of identification served as a warning to the paleontology community about how dangerous intuitive conclusions relying solely on partial visual similarities can be.
Identifying the Apex Predator through Quantitative Analysis
Twelve fossil specimens cleared the strict criteria and were excluded as pseudopathologies, leaving true tooth traces (pits, punctures, scores, and furrows).
To identify the attacker who left the specific tooth traces, the research team adopted an approach of precisely measuring the spacing of microscopic striations and serration marks left on the fossil bones and mathematically comparing them with the "denticle (serration) density" of various theropod teeth discovered from the same formation.
| Theropod Taxon | Measured Denticle Density (per 2mm) | Compatibility with Fossil Bone Tooth Traces |
|---|---|---|
| Nanotyrannus lancensis | More than 4 density (>4 / 2mm) | Physically incompatible |
| Pectinodon bakkeri | More than 4 density (>4 / 2mm) | Physically incompatible |
| Acheroraptor temertyorum | More than 4 density (>4 / 2mm) | Physically incompatible |
| Tyrannosaurus rex | Exactly 2 (2 / 2mm) | Perfect match (Confirmed) |
As a result of the measurements, the density of the notches left in the grooves of the fossil tooth traces all indicated "exactly 2 per 2mm." As the table shows, small tyrannosaurs and troodontids other than Tyrannosaurus rex all have finely packed serrations of 4 or more per 2mm, making it physically impossible for them to have formed these traces.
Furthermore, it was found that the parallel score marks left on the forelimb bones were exactly "10mm" apart. This completely matches the distance between the teeth (interdental distance) in the jaw of a Tyrannosaurus rex. This fact vividly illustrates the action of the giant apex predator powerfully clamping the forelimb of the Edmontosaurus with its jaws and biting down intensely as if to scrape off the meat.
T. rex Feeding Strategy and the Fate of Carcasses
A critical feature common to all 12 identified bones with tooth traces is the complete absence of bone remodeling or healing signs. If Tyrannosaurus frequently attacked living prey, there should be a certain probability of finding bones with traces of injuries that had healed after the prey escaped. However, the complete lack of healing marks strongly suggests that Tyrannosaurus rex actively engaged in "scavenging" behavior, consuming carcasses already dead and lying on the surface.
Also, the extremely low tooth trace incidence rate of only about 0.4% out of 3,013 items indicates that in the ecosystem of that time, a massive amount of carrion far exceeding the consumption capacity of predators was supplied due to the mass deaths of Edmontosaurus. Scavengers likely did not need to crush hard bones, instead eating only the easily accessible soft meat before moving on to another carcass.
Moreover, the smooth fracture patterns concentrated on the ribs highlight a dramatic behavioral pattern where a giant carnivore like Tyrannosaurus used its powerful jaws to intentionally bite and break the ribs of the rib cage, forcibly prying them open to directly access extremely nutritious internal organs like the heart and liver first.
Conclusion
The greatest academic contribution of this study lies in demonstrating the danger of judging fossil damage merely by superficial shapes, and in establishing strict evaluation criteria for accurately identifying pseudopathologies through integrated methods like CT scanning and quantitative microstructural analysis.
For the 12 authentic tooth trace specimens, based on undeniable mathematical data such as microscopic serration density and interdental distance, it was deduced that the attacker was a specific species: Tyrannosaurus rex. The extremely low incidence of tooth traces, the lack of healing marks, and the destructive marks on the ribs vividly depict a scene in the Late Cretaceous ecosystem where Tyrannosaurus rex fed as a scavenger.
References
- Identification of tooth traces from a Cretaceous (Maastrichtian) Edmontosaurus annectens bonebed in the Lance Formation, Wyoming, U.S.A. | PLOS One - Research journals, https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0351939
- T. rex was likely responsible for some tooth marks on fossil bones from Cretaceous-era Wyoming | EurekAlert!, https://www.eurekalert.org/news-releases/1135306
- A T. rex bit a duck-billed dino and we can still see the teeth marks | Popular Science, https://www.popsci.com/science/t-rex-teeth-marks-dinosour-bones/
- Tyrannosaurus rex Scavenged Duck-Billed Dinosaurs in Ancient Wyoming, Bite Marks Reveal | Sci.News, https://www.sci.news/paleontology/tyrannosaurus-rex-edmontosaurus-annectens-scavenging-bite-marks-14922.html
- Full article: Analysis of pseudopathologies in Edmontosaurus annectens bones: taphonomic implications from biogenetic and diagenetic bone alterations from a Cretaceous bonebed in the Lance Formation, Wyoming - Taylor & Francis, https://www.tandfonline.com/doi/full/10.1080/02724634.2025.2600392
- (PDF) Paleoecological Information in Predator Tooth Marks - ResearchGate, https://www.researchgate.net/publication/283459834_Paleoecological_Information_in_Predator_Tooth_Marks
- REFINEMENT OF TOOTH TRACE CRITERIA THROUGH EXPERIMENTATION AND LITERATURE REVIEW | Request PDF - ResearchGate, https://www.researchgate.net/publication/321407071_REFINEMENT_OF_TOOTH_TRACE_CRITERIA_THROUGH_EXPERIMENTATION_AND_LITERATURE_REVIEW