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Elrathia kingii damage explained


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I just prepped this Elrathia kingii I found last year. It’s more 3-dimensional than most of them. It also has a slight reverse C-shape curve to it. It’s left side was covered with matrix when found, but after removing the matrix, almost nothing was actually found under it. Looks like something took a big bite out of it. Can anyone hypothesize what the damage actually is caused by?

Link to images: https://imgur.com/gallery/wzDoG




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Can't help but nice specimen. Love all the agnostids. But it may just have happened after it died, just started to come apart in the current like modern crustaceans do after they die.

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I don't know a lot about the Cambrian but I've seen photos of trilobites like that explained as narrow escapes from something like Anomalocaris.



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Anomalocaris is the most likely candidate that we know about. 

Trilobite bits have been found in coprolites where Anomalocaris is the only predator to be capable of leaving coprolites of this size. 

See :



But I'm not sure they could actually 'bite', it seems they couldn't close the mouth entirely and crunched up their prey in the mouth and gullet. 

Lets see if @piranha has any more information about this. 

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Elrathia kingii Trilobite With Preserved Bite Mark
Anomalocaris Food

Elrathia kingii

Trilobite Order Ptychopariida, Family Ptychoparioidea

Geological Time: Middle Cambrian

Size (25.4 mm = 1 inch): Trilobite 20 mm long by 14 mm wide

Fossil Site: House Range, Wheeler Formation, Millard County, Utah

Fossil Code: TR149


Elrathia-kingii-t.jpgComing from the Cambrian Wheeler Formation deposits of Millard County Utah this is a detailed example of the trilobite Elrathia kingii. These trilobites are often found with disarticulated or missing free cheeks. This fine example is well-articulated, with a dark black exoskeleton which contrasts well with the beige shale matrix which derives its color from its high organic content. This is a small example, rarely seen in such a fine state. What makes this even more rare is the fact a section of the left pleural lobe is missing, the result of the trilobite’s being bitten, presumably by an Anomalocaris. Interestingly, these bites are typically found on the right side of the trilobite, indicating some preferred attack mode by the terror of the Cambrian. This one was preyed upon by a directionally-challenged one, however, as it is the left lobe that is damaged, only the third such example I have seen.

click fossil pictures to enlarge

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I certainly wouldn't know, but very interesting!



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"Worms were newly abundant and large, and the mouth of Anomalocaris was flexible and circular, so it is much more likely that worms were their preferred prey."


Oxman, K. (2014)

Comparative analysis of a unique specimen of a new species of Anomalocaris from the Kinzers Formation of Lancaster County yields a reassessment of the feeding habits of the genus. Franklin and Marshall College Archives, Undergraduate Honors Thesis, 58 pp.  PDF LINK



"Even if they weren’t living up to their reputation as trilobite crunchers, there is no doubt that Anomalocaris and kin ruled the oceans over half a billion years ago."


Daley, A.C., & Paterson, J.R. (2012)

The Earth's first Super-Predators.

Australasian Science, 33:16-19  PDF LINK



Hagadorn, J.W. (2010)

Putting Anomalocaris on a soft-food diet?

GSA Denver Annual Meeting - Paper No. 125-1

Anomalocaridids have a circular mouth consisting of 32 inwardly-facing pointed plates that cap a plate-studded esophageal area. How these structures functioned or what they were used to eat is not understood. To address these knowledge gaps, we constructed CAD models of an anomalocaridid mouth and twelve possible trilobite ‘prey’, and characterized their response to different biting kinematics and stresses.


In life, anomalocaridid mouth plates were connected along their long edges by flexible tissue, and mouth plates, esophageal plates and preoral appendages were composed of unmineralized cuticle. The mouth could close like a sphincter, and its plates could have moved synchronously or moved asynchronously with opening initiated by four large cardinal plates. Whether moving in an inverted, everted, or in-plane position, a rapidly closing anomalocaridid mouth could generate sufficient external pressure change to allow suctorial feeding - yet no anomalocaridid mouth could close more than half-way.


To test the hypothesis that anomalocaridids ate trilobites, we conducted finite element analyses of how different biting stresses would deform and cause failure of twelve commonly malformed Cambrian trilobites. A spectrum of trilobite sizes, shapes, thicknesses, and ornamentation were subjected to both horizontal and vertical bite-impact angles, using the known range of anomalocaridid plate tip sizes. Young’s modulus, Poisson’s ratio, and ultimate tensile strength were derived from strain measurements on modern marine arthropod cuticle ranging from freshly molted Callinectes sapidus to well sclerotized claws of Homarus americanus. These cuticle material properties were applied to each possible combination of anomalocaridid biting geometry and trilobite type. In the absence of significant bending stresses, cuticle failure always occured at the locus of bite impact in all trilobites modeled; where bending stresses predominated, failure occurred near the axial furrow. FEA suggests that although tiny or protaspid trilobites could be eaten whole by most anomalocaridids, and some freshly molted trilobites could possibly be deformed by interacting with anomalocaridid mouth plates or preoral appendages, anomalocaridid mouth plates would break before most trilobite thoracic cuticle would fail.



Hagadorn, J.W. (2009)

Taking a bite out of Anomalocaris.

In: Walcott 2009 - International Conference on the Cambrian Explosion


Anomalocaridids are hypothesized to have consumed trilobites. Other than their large size, their midgut glands, and malformed trilobites, there is little direct evidence that they did so. New taphonomic, compositional, and modelling evidence suggests that anomalocaridid mouths were soft, could not close completely or chew, had biting kinematics incompatible with many trilobite malformations, and were well suited to manipulate or suck soft prey. Anomalocaridid mouth plates and their tips are never broken, nor are tips worn. If plates were hard, and were used to manipulate, puncture, crush, or masticate biomineralized prey, they would be expected to show evidence of abrasion or breakage. Absence of this evidence is striking given the frequency (0.01-1%) of healed malformations in extant marine arthropods, most of which are due to prey manipulation or feeding. Moreover, anomalocaridid plates and their biting tips are commonly wrinkled, exhibit preburial shearing and tearing, and mantle or are deformed by biomineralized fossils such as brachiopods, trilobites, and Scenella. Plates are preserved as organic carbon and exhibit fracture patterns typical of desiccating arthropod cuticle. Thus anomalocaridid plates, including their tips, were unmineralized and pliable in life.


Computer aided design modelling of the kinematics of mouth opening and closure, together with comparison with muscle movements used by modern circular-mouthed organisms, suggests several plausible models for anomalocaridid mouth movement. These include sphincter-like constricting closure of the circlet of plates, and full- or half-eversion or inversion of the circlet; the latter two movements generate sufficient subambient pressure for suction feeding. In all closure modes, laterally-adjacent opposing plates intersect one another when the mouth closes, which prevents the circlet from closing more than half-way. Orientations of plate tips are consistent with a partial mouth closure model; if full closure was possible, opposing plate tips would not articulate or interlock with one another, as is expected from teeth optimized to masticate or puncture, or teeth which intersect at tooth tips to crush, puncture, or break.


Although bilaterally-oriented trilobite malformations can plausibly be explained by a closure of a circular mouth, most trilobite malformations are arc- or U-shaped. Suction-, eversion-, and sphincter-movement of anomalocaridid jaws cannot produce U-shaped bite marks; these are better explained by predators who had opposable jaws or claws. Finite Element Analysis, and modelling of anomalocaridid plates using cuticle yield strengths from modern shrimp (Pandalus) and lobster (Homarus), illustrate that plates could withstand maximum forces of up to 6.2 and 13.0 N, should they have bitten into the thoracic segments of a trilobite. The most commonly malformed Cambrian trilobites had maximum skeletal yield strengths ranging from 3.7–37.1 N, suggesting that only weakly mineralized taxa, such as Elrathia kingii, could have been broken by an anomalocaridid bite.


Anomalocaridids may have bitten some soft trilobites, but it is more likely that they were suctorial feeders, perhaps using their preoral appendages to comb soft-bodied invertebrates from the benthos. This feeding strategy makes sense given the recent discovery of multiple rows of inwardly pointing serrated plates inside some anomalocaridids’ oral cavity; these may have prevented prey from exiting the mouth, or may have been part of a buccal cavity or eversible grasping organ.

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