The “Horns” of Dicranurus

From the exterior face of the wall towers must be projected, from which an approaching enemy may be annoyed by weapons, from the embrasures of those towers, right and left. –Vitruvius

Dicranurus hamatus elegantus, Devonian Period, Oklahoma
Dicranurus hamatus elegantus, Haragan Formation, Early Devonian Epoch, Oklahoma. Specimen approximately 7 cm long. Specimen prepared by Maestro Bob Carroll.

Among the spiny trilobite monsters of the Devonian Period, Dicranurus stands out as one of the most spectacular “horned” forms. Emlen (2005) blithely considered the horns of this trilobite (as well as a variety of spines and exoskeletal projections in other trilobite taxa) as “weapons,” likely used by males in infraspecific combat. A more cautious discussion of the evidence and reasoning used to draw this type of conclusion (but in the case of raphiorids) can be found in Knell and Fortey (2005).

I find the interpretation of the horns of Dicranurus as analogous to the horns of ungulates or even horned beetles to be unconvincing. The notion that animals covered in fine, delicate, and easily breakable spines would purposely engage in pushing, shoving, or wrestling matches seems unlikely. Further, the horns of Dicranurus are simply an extreme example within odontopleurids. Ceratonurus and Miraspis, for example, both have similar, although more gracile horns.

These other horned odontopleurids, however, also have stalked eyes anterior to the horns. This would seem to inevitably lead to losing an eye or two if the horns were used to attack each other! Use of horns as weapons in stalk-eyed forms would seem even less likely than in Dicranurus, and the idea that the horns in Dicranurus had a function different from that in other horned trilobites stretches credulity further.

I tend to be of the opinion that the spines in the spiniest Devonian trilobites played a role in gathering sensory information about the environment. As they crawled through their reefy habitats the spines would have mapped out a corridor of clear navigation. If they encountered a soft-bodied predator, it would  be delivered an unpleasant poke. The curling around of the rams-horns of Dicranurus may simply be an adaption to crawling around in patches of habitat with lots of overhangs, such as branching bryozoans or corals.

For those of us willing to entertain non-adaptationist interpretations, the possibility exists that the extreme horns of Dicranurus and others served no particular function in and of themselves. The gene(s) responsible for horn development may have been linked to other genes that did have adaptive significance, perhaps spininess in general.

Until sexual dimorphism is clearly demonstrated in these trilobites, and evidence is found of battles (one trilobite’s spine lodged in another or two specimens entangled in each other’s horns), I remain a skeptic of the spines and horns as weapons concept.

Dicranurus mostrosus, Devonian Period, Morocco
Dicranurus monstrosus, Devonian Period, Morocco. Did the horns curl under simply to avoid entanglements from above? Trilobite is approximately 5.5 cm across genals.

References

Emlen, Douglas J. 2008. The Evolution of Animal Weapons. The Annual Review of Ecology, Evolution, and systematics 39: 387-413.

Knell, Robert J., and Fortey, Richard A. 2005. Trilobite spines and beetle horns: sexual selection in the Palaeozoic? Biology Letters 1 (2): 196-199

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

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