Crusher Teeth, Spines, and the Fall of Trilobites

If you have food in your jaws you have solved all questions for the time being. —Franz Kafka, Investigations of a Dog

Paladin transilis, Upper Carboniferous, Zhirnovsk, Volgograd region, Russia
Paladin transilis, Upper Carboniferous, Zhirnovsk, Volgograd region, Russia. This beautiful trilobite had extremely well developed genal spines for its time. Many late Paleozoic trilobites generally resembled Paladin overall, but had small genal spines or no spines at all. Specimen is about 1.5 cm across genals.

The decline of trilobites accompanies the expansion of gnathostomes (jawed vertebrates). Family-level trilobite diversity nearly held steady with only minor decline throughout the Silurian Period just as gnathostomes began their evolutionary radiation. Trilobite diversity then underwent a series of significant step-wise declines throughout the Devonian Period. This was a time of major expansion for vertebrates, including those with “crusher-type” dentitions, the most likely trilobite hunters. Only four trilobite families survived into the Carboniferous Period.

These crusher-teeth occurred in many common groups of fishes of middle and late Paleozoic age, including bony fishes (Osteichthyes), placoderms, and shark-like fishes (Chrondrichthyes, especially holocephalians). Many fishes with  crusher-teeth likely preyed largely on hard-shelled invertebrates as they do today. It seems plausible, then, that these predators exerted selective pressure on trilobites. It is also therefore reasonable to believe, as some do, that vertebrates played a role in the decline and ultimate extinction of trilobites.

Syntheotodus trisulcatus, Maple Hill Formation, Upper Devonian, Kalona, Iowa
Syntheotodus trisulcatus crusher tooth in occlusal view, Maple Hill Formation, Upper Devonian, Kalona, Iowa. This is an early holocephalian tooth. Holocephalians were likely a much more diverse and widespread group in the middle and late Paleozoic than they are today. Extant holocephalians, the 39 species of chimaeras and ratfishes, are mostly deep water forms that feed on hard-shelled benthic invertebrates. Specimen is 8 mm across.

But correlation, of course, is no proof of causation, especially given the multitude of other changes that occurred during this interval of earth history. Some would even argue that:

A belief in the causal nexus is superstition.—Ludwig Wittgenstein

Lagarodus tooth plate, Harrodsburg Limestone Formation, Washington County, Indiana
Lagarodus sp. (Psammodontidae, Holocephali) tooth plate, Harrodsburg Limestone Formation, Mississippian Period, Washington County, Indiana. Such flat, plate-like crushing teeth would have made short work of a trilobite. Specimen is 1.6 cm across.

Philosophy aside, spines in trilobites are often interpreted to have had a defensive function as they do in many extant marine and aquatic forms. Some predatory fishes of insufficient size, for example, may have difficulty swallowing other fishes because the prey fish can splay out fin spines making passage down the gullet impossible. But spines are no guarantee of safety. Diving birds and waders, fish-eating specialists, can easily manipulate prey into a head-first orientation and eat the spiniest of fishes, even those of large relative size. For every measure, there is a countermeasure. This was likely as true in the Paleozoic as it is today.

The proliferation of dorsal spininess in Devonian trilobites may have been a response to threats from jawed fishes and other predators, ammonites, for example. In the case of soft-bodied predators this makes sense, but I’ve always been skeptical that spines could have been of much protection from vertebrate predators, especially specialized ones. Specialized vertebrate predators are often just too formidable for any invertebrate prey, not matter how thorny. Triggerfish, for example, bite off echinoid spines until the animal’s body is exposed and then eat the soft-tissues. Holocephalians graze on shellfish the way cows graze on grass, groupers grab crabs, and so on.

Further, an evolutionary arms race between spines and teeth would have clearly and immediately favored teeth. Vertebrate teeth are, after all, made of hard, phosphatic tissues and are inherently more than a match for the calcite of the trilobite exoskeleton, no matter how spiny. Trilobite spines, if they were defensive structures at all, were likely only effective against a specific, most likely unknown, soft-bodied threat.

A final observation indicating that spines may have had little to do with defense is that it is the dorsally spiny trilobites (like Comura) that disappear at the end of the Devonian Period. The trilobites that survived into the late Paleozoic, a time when waters teemed with the most menacing piscine predators of the era, were the most conservative forms. Many late Paleozoic trilobites even lacked genal spines.

The reason trilobites retreated into the shadows at the end of the middle Paleozoic and ultimately disappeared near the end of the Permian Period will likely never be completely understood. An analysis of spines versus predators or trilobite predation in general, although an attractive place to look for easy answers, seems unlikely to yield convincing answers about extinction.

Double-crested Cormorant with "Plecostomus, Fiorenza Park, Houston, Texas
Double-crested Cormorant with “Plecostomus” (Pterygoplichthys multiradiatus), Fiorenza Park, Houston, Texas. Loricariid catfish are covered in dermal armor and have formidable fin spines as wicked as any trilobite’s associated with their fins. Cormorants dispatch and eat them with ease.

©2017 Christopher R. Cunningham. All rights reserved. No text or images may be duplicated or distributed without permission.

Leave a Reply

Your email address will not be published. Required fields are marked *