Trilobite Rostral Protuberances

You are like a chestnut burr, prickly outside, but silky-soft within, and a sweet kernel, if one can only get at it . . . ― Louisa May Alcott, Little Women

Coronocephalina jastrowi, Silurian Period, China
Coronocephalina jastrowi, Silurian Period, East Hubei Sheng Province, central China. This specimen would benefit from further preparation, including excavation of the right genal spine. Trilobite is 3.9 cm long.

On an aesthetic level, nothing excites trilobite collectors more than spines, horns, and the other projections found emanating from the exoskeletons of our favorite arthropods. Understanding the function and significance of these structures, however, is not an easy task. Further, I would suggest that many of these features will never be fully comprehended and will forever remain a matter for imaginative speculation. Case in point: rostral processes or protuberances of the cephalon.

Coronocephalina jastrowi, Silurian Period, China
Portrait: Coronocephalina jastrowi, Silurian Period, East Hubei Sheng Province, central China. The saw-tooth spines of the anterior margin of the cephalon do not match those of the pygidium.

As a simple classification, I would offer two speculative categories for these structures: 1) protuberances related to enrollment, and 2) those unrelated to enrollment. Within category 1, it seems one could make the case for two sub-types, “tweezers” and ” picket fences.”

The Coronocephalina above is an example of the “picket fence” type morphology. Upon enrollment, the spines on the pygidium and cephalon would form a prickly barrier protecting the zone of  weakness between the leading and trailing edges of the animal. In general, there seems to be no one-to-one correspondence between the size, shape, an number of spines on the cephalon and pygidium in a picket fence.

Apparently only two of the three possible permutations of the picket fence structure exist. Some trilobites adopting this strategy have spines on the pygidium only (e.g., Comura, Greenops, and many others), and some have spines on both the pygidium and cephalon, like Coronocephalina. I know of no examples of spines on the cephalon only with no pygidial spines at all, but would not be surprised to discover that this permutation exists rarely, also.

Huntonia oklahomae, Devonian Period, Oklahoma
Huntonia oklahomae, Haragan Formation, Devonian Period, Coal County, Oklahoma. Trilobite is 7.8 cm long.
Huntonia oklahomae, Haragan Formation, Devonian Period, Coal County, Oklahoma.
Huntonia oklahomae, Haragan Formation, Devonian Period, Coal County, Oklahoma. Trilobite is 7.8 cm long.

On to the second sub-type, the “tweezers.” I consider the Huntonia above to be an example of the “tweezers” type enrollment-related morphology. The rostral projection (“anterior cephalic process” of Campbell, 1967) fits tightly against the single pygidial spine, like the opposing tines of a tweezers, thus making the enrolled animal a tough nut to crack for any potential predator. In some well-preserved and prepared specimens, the tip of the rostral projection articulates with the pygidial spine.

Zlichovaspis rugosa, Hamar Laghdad Formation, Devonian Period, Morocco
Zlichovaspis rugosa, Hamar Laghdad Formation, Devonian Period, Morocco. Although this species possesses a much more modest rostrum than Huntonia, it, too, likely operated like a “tweezers” upon enrollment.

I would note that there are many forms similar to Huntonia (with a anterior cephalic process) within dalmanitacean trilobites, including species that show variations in the number, length, and shape of spines and the addition of other embellishments. Sometimes these animals have picket fences and sometimes tweezers. In Comura bultyncki, for example, a blade-like anterior cephalic process is present along with many pygidial spines. I would consider this a picket fence arrangement.

Finally, Clarkson and Whittington (1997) noted that some dalmanitids formed tightly closed capsules upon enrollment whereas others (e.g. Glyptambon verrucosus) left gaps between the trailing and leading edges of the animal. The precise functional significance of these variations, if any, are, of course, unknown.

Moroccanites, Devonian Period, Morocco
Arthropod Unicorn: Moroccanites malladoides, Devonian Period, Morocco. Trilobite is 6.8 cm long, exclusive of “horn.”

The most spectacular rostral protuberances would seem to be unrelated to enrollment. For example, I find it hard to believe that the spectacular tridents of the trident comurids, the anterior prongs of Philonyx, or the rhinoceros-like horn of Moroccanites had anything to do with enrollment. Rather, one can imagine these projections perhaps being used in jousting or pushing and shoving matches over patches of seafloor used for spawning or for mating rights. Likewise, I can well imagine Moroccanites sitting in burrow guarding a brood of eggs with its fearsome horn pointing outwards—a nasty surprise for any foraging soft-bodied predator.

In any case, contemplations such as these remind me again and again how little we know, perhaps can know, about the remote past. All we can do is keep working . . . and imagining.

Cornuproetus, Devonian Period, Morocco
Cornuproetus cornutus, Devonian Period, near Zguid, Morocco. Subtle protuberances of the cephalon, like the showy ones, will likely forever remain, to some degree or other, a mystery. Trilobite is 2.8 cm across the genals.

Reference

Campbell, K. S. W. 1967.  Trilobites of the Henryhouse Formation (Silurian) in Oklahoma. Oklahoma Geological Survey Bulletin 115.

Clarkson, E. N. K., and Whittington, H. 1997. Enrollment and Coaptative Structures, in Roger L. Kaesler (ed.), Treatise on Invertebrate Paleontology: Part O, Arthropoda 1, Trilobita, Revised. Geological Society of America and University of Kansas Press, Lawrence, Kansas, 67-74.

©2016 Christopher R. Cunningham. All rights reserved. No text or images may be duplicated or distributed without permission.

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