There is no ghost so difficult to lay as the ghost of an injury. –Alexander Smith
A few types of injuries to trilobite exoskeletons consistent with predation are known. Johnson (2003), for example, figured a number of (healed) injured specimens of Cambrian, Ordovician, and Devonian age. Apart from the interesting details of specific cases, the significance of these injuries lies in the revelation that even from the earliest beginnings of the Paleozoic Era organisms existed within paleocommunities with trophic levels. As in extant communities, there were predators and there were prey. This basic aspect of the biological structure of the world has, then, existed for at least 500 million years.
V- or W-shaped divets or “bite marks” from the pleura of Cambrian trilobites are fairly commonly encountered trace fossils. These injuries are typically attributed to Anomalocaris and relatives, the “terrors of Cambrian seas.” Foster (2014) summarized criticisms of the interpretation that Anomalocaris caused these injuries, though. Most significantly, how did a predator with soft mouth parts inflict injuries on hard-shelled prey? Further, similar looking injuries persist into the Ordovician Period (and beyond) even though anomalocarids are thought to be extinct by this time. Also giving pause to the anomalocarid hypothesis, are stomach contents of the arthropod Utahcaris orion from the Spence Shale (middle Cambrian) of Utah containing broken trilobite sclerites (Kelley et al, 2013), but to my knowledge there are no known anomalocarid stomach contents containing trilobite remains.
The V-shaped injuries of Ordovician age are reasonably attributed to cephalopods and their triangular mouth parts (rhyncholites). Certainly nautiloid cephalopods were abundant predators in many paleoenvironments at this time–and are routinely depicted as preying on trilobites in natural history exhibits! Like anomalocarids, cephalopods were likely the top of the food chain. Some grew to almost monstrous sizes (e.g., Cameroceras) and injuries to a variety of fossil organisms (trilobites, gastropods, brachiopods, and crinoids) have been attributed to them in the literature. It is important to note, though, that there is no known direct evidence (nautiloid coprolites or stomach contents) to render this interpretation more solid. Of course, even such evidence might not prove predation rather than scavenging.
The Dalmanites limulurus specimen shown below is missing about half the pleural lobe on the left side of the body over a length of five segments. Inspection of the injury reveals that the edge of the wound is not smooth, but rather appears ragged and to have been inflicted by a serrated or “toothed” edge.
The predator responsible for this injury is not known, but educated guesses about the identity of the maker are possible. Being Silurian in age makes the specimen quite early for gnathostomes, although the earliest history of jawed vertebrates is murky. A cephalopod is a definite possibility as nautiloids are known from the Rochester Shale (e.g., Dawsonoceras), but injuries likely associated with cephalopods are usually triangular given the shape of their mouth parts. The most likely identity of the attacker is, I think, a clawed arthropod with serrated chelicera (pincers), perhaps a eurypterid.
By the middle Paleozoic Era, the seas were full of predators capable of devouring trilobites. Cephalopods, straight and coiled nautiloids (plus flattened, nearly spheroidal and other exotic forms) as well as early ammonites, and gnathostomes were present in some paleoenvironments.
Most trilobites from their earliest history were spiny: genal, pleural, and pygidial spines are everywhere from the Cambrian Period onwards. But the dorsal spininess of some groups of trilobites increased dramatically during the Devonian Period. This increase in spines is often interpreted as a defense against predators.
Anyone who has seen a triggerfish eating a sea urchin would doubt, however, that the delicate, finely pointed spines of forms such as Comura, Dicranurus, or Cyphaspis could provide much direct protection against fishes armed with so-called “crusher dentitions.”
I think any of crusher-toothed chondrichthyans, hybodontoids, holocephalians, and others, that were so inclined could easily pulverize any trilobite they could get their mouths around: if you can crush a bivalve, then you can crush a trilobite! Significantly, the crusher-toothed chondrichthyans first appeared during the Late Devonian Epoch. See Long (1995) for many figured examples of Paleozoic crusher dentitions.
To my mind, the dorsal spininess may have provided direct defense against some unknown (but plausibly inferred) soft-bodied predators. I suspect, however, that the spines may in some cases have functioned as sensory organs, probing the extent of the environs as the animals crawled though the nooks and crannies of complex three-dimensional reef paleoenvironments, any one of which could contain a predatory worm or cephalopod. See Harrington (1959) for a summary of trilobite integumentary sensory organs.
Late Paleozoic (Carboniferous and Permian) trilobites possess genal spines and at most modest pleural spines only, and show a consistent, conservative body plan —none of the exuberance of the Devonian Period.
Whatever function the elaborate dorsal spines had, they were no longer needed by the Carboniferous Period and beyond. Perhaps trilobites had simply disappeared from paleoenvironments in which they served a purpose. Perhaps dorsal spines were simply a hopeless defense against the formidable predators of the Late Paleozoic Era.
Foster, John. 2014. Cambrian Ocean World: Ancient Sea Life of North America. Indiana University Press. 416 p.
Harrington, H. J. 1959. General Description of Trilobita in Raymond C. Moore (ed.), Treatise on Invertebrate Paleontology Part O, Arthropoda 1. O38-O117.
Johnson, Thomas T. 2003. Discovering the Mysterious Trilobites. Geofin s.r.l. Publishing House, Italy. 175 p.
Kelley, Patricia, Michal Kolawewski, and Thor A. Hansen (eds.) 2013. Predator-prey Interactions in the Fossil Record. Springer Science+Business Media, New York. 464 p.
Long, John A. 1995. The Rise of Fishes. The Johns Hopkins University Press. 223 p.
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