The building’s identity resided in the ornament. –Louis Sullivan
From a collector’s viewpoint, the variation in “ornamentation” (granules, pustules, tubercles, ridges) is the raw material of building a collection.
However, some have objected to the commonly used term ornamentation:
“Such surface sculpture is frequently referred to as ornament, but as Gill (1949) argued in proposing to call it prosopon, ornament is a general word that gives an erroneous impression of mere decoration, whereas surface sculpture has biological significance.” (Whittington and Wilmot, 1997, p.77).
Point taken, but as a birder I know that “mere” decoration can and often does have biological significance. Bright colors, plumes, and iridescence in male bird feathers are meant to appeal to the females—they are decorations! Such flamboyant structures are used in species recognition and dominance and courtship rituals (sexual selection) in many other groups of organisms, too. Think of shaggy manes, antlers, even oversized pincers in fiddler crabs.
A danger, however, lies in the over-interpretation of the functional significance of morphological features, especially minor superficial ones (see Mayr, 1983). Genetic mutation, the raw material of evolution, is a random process. The phenotypic expression of these mutations will be preserved in populations if the changes they represent are adaptive, or at least not too deleterious.
But it’s fun to speculate on the possible functional significance of the sculpted texture like that found in Flexicalymene granulosa (above), as contrasted with the more typical smooth skin found in F. meeki, for example. It seems to me that such rough or pebbly textures may have better blended into a sandy bottom than smooth textures.
The Treatise references Chatterton (1980) who suggested that bumpy exoskeletal surface textures could foil the attacks of predators with sucker disks. While interesting, extant cephalopods grab rough-skinned crustaceans with little problem, and I would think that a rough surface texture would, in general, be easier to grab. Think about a soccer ball versus an American football.
Why do some mostly smooth trilobites preserve external segmentation of the pygidium, like Ameura, whereas many trilobites are smooth over their entire exoskeletons (e.g. Asaphus)? Is this functional, or simply a superficial expression of some deeper developmental difference? Is this difference ornamental?
Among the more lovely forms of surface sculpture are the terrace ridges. Although several studies have attempted to establish a functional explanation for this type of texture through relating it to surface and deeper features of the exoskeleton, its purpose is still unknown. See discussion in Whittington and Wilmot (1997).
Rarely, the surface texture of a fossil specimen can provide a window into the possible physiological paleobiology of trilobites. Does the thin, crinkly-looking texture of the Wanneria molt above indicate that valuable minerals from the exoskeleton were reabsorbed by the animal prior to molting—as some extant arthropods do?
Finally, even if trilobite ornamentation is difficult to interpret, the similarities and differences among species are the visible evidence of evolution. And gaining a further appreciation for the evolutionary history of our favorite group is always fascinating and worthwhile.
Campbell, K. S. W. 1967. Trilobites of the Henryhouse Formation (Silurian) in Oklahoma. Oklahoma Geological Survey Bulletin 115.
Chatterton, B. D. E. 1980. Ontogenetic studies of Middle Ordovician trilobites from the Esbataottine Formation, Mackenzie Mountains, Canada. Palaeontographica (Abt. A) 137: 1-74.
Fortey, R. 2000. Trilobite! Eyewitness to Evolution. Alfred A Knopf, New York, 284p.
Gill, E. D. 1949. Prosopon, a term proposed to replace the biologically erroneous term ornament. Journal of Paleontology 23: 572.
Levi-Setti, R. 1993. Trilobites. The University of Chicago Press. 342p.
Mayr, E. 1983. How to carry out the adaptationist program? The American Naturalist 121 (3): 324-334.
Whittington, H. B. and Wilmot, N. V. 1997. Microstructure and sculpture of the exoskeletal cuticle. 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, 74-84.
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