Surface Ornamentation in Trilobites

The building’s identity resided in the ornament. –Louis Sullivan

Coronocephalina gaoluensis. Silurian Period, Chadung Province, China
Coronocephalina sp., Silurian Period, China. Fortey (2000) refers to encrinurids such as this (and the trilobite immediately below) with a coarse tuberculate texture on the cephalon as “strawberry-headed.” Trilobite is 4.9 cm long.

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).

Fragiscutem glebalis, Henryhouse Formation, Silurian Period, Oklahoma
Fragiscutum glebalis, Henryhouse Formation, Silurian Period, Oklahoma. Campbell (1967) distinguished F. glebalis from F. rhytium based, in large part, upon minor differences in surface ornamentation. Trilobite is 2.1 cm long.

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.

Flexicalymene granulosa, Cobourg Formation, Bowmanville, Ontario, Canada. Trilobite is about 2.5 cm across genals.
Flexicalymene granulosa, Cobourg Formation, Bowmanville, Ontario, Canada. Trilobite is about 2.5 cm across genals.

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.

Ameura missouriensis, Winterset Limestone, Pennsylvanian Period, Kansas City
Pygidium of Ameura missouriensis, Winterset Limestone Member, Dennis Limestone, Pennsylvanian Period, Kansas City area. Some otherwise smooth trilobites exhibit ornament-like segmentation of the pygidium. Pygidium is about 2.3 cm long.

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?

Paralejurus dormitzeri, Hamar Laghdad Formation, Lower Devonian Series, Morocco
Paralejurus dormitzeri showing terrace ridges, Hamar Laghdad Formation, Lower Devonian Series, Morocco. Levi-Setti (1993) provided several excellent images of whitened specimens showing this surface texture. Trilobite is about 4.5 cm long.

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).

Wanneria surface texture, Cambrian Period, Canada
Free cheek of Wanneria sp. molt showing crinkly surface texture. Eager Formation, Cambrian Period, British Columbia, Canada. Molt is about 12.5 cm long.

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.

References

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.

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

Trilobite Traces

We will go right down to the sea
Bathing in light we will be free to wander . . . . —”Sands of Time” by D. Kirwan (as recorded by Fleetwood Mac)

Cruziana-like trails, Glen Dean Formation, Mississippian Period, Indiana
Cruziana-like trails, probably most correctly termed Diplichnites, Glen Dean Limestone, Chesterian Stage, Mississippian Period, Indiana. Trails are about 2 cm across.

Trace fossils are fossilized behavior. Two types of trace fossils are routinely and confidently assigned to trilobites: Cruziana and Rusophycus. Cruziana traces were produced as trilobites plowed through the substrate, their legs churning through sediment. Cruziana may be described described as “shallow, pocket-like pits, passages or pocket burrows shoveled or scratched by trilobites” (p. W189, Hantzschel, 1962). Rusophycus is a bilobed hyporelief that was produced as trilobites sat in the sediment.

Rusophycus pudicum, Waynesville Formation, Ordovician Period, Ohio
Trilobite Resting Trace: Rusophycus pudicum, Waynesville Formation, Ordovician Period, Ohio. This trace fossil was likely produced by the trilobite Flexicalymene. Trace fossil is 4.4 cm long.

Rare specimens like the one below show a definitive connection between Rusophycus and Cruziana: The maker of one trace is clearly the maker of the other, but simply behaving in a different way.

Cruziana entering Rusophycus, Rose Hill Formation, Silurian Period, Scott County, Virginia
Cruziana entering Rusophycus, Rose Hill Formation, Silurian Period, Scott County, Virginia. A trilobite producing Cruziana moved from left to right, coming to rest and producing a textbook specimen of Rusophycus. Rusophycus is 2.2 cm wide at its maximum.

Diplichnites is a trace made by many arthropods, including trilobites, as they walked across the surface of the substrate. Some trilobite trace fossils, like the one at the top of the post, are difficult to definitively assign to the man-made categories of Diplichnites and Cruziana as trilobites likely moved up and down into sediments of varying consistency. Unless closely associated with Cruziana or Rusophycus, Diplichnites is often difficult to assign to a particular arthropod maker. The Diplichnites below falls into this category. The maker was likely a trilobite, but it is impossible to say for certain.

Arthropod trails, Bisher Formation, Silurian Period, Ohio
Diplichnites: Arthropod Trails, Bisher Formation, Silurian Period, Ohio. Slab is 10 cm across.

An appreciation for trace fossils really brings the hobby of fossil collecting to life—especially when you find a specimen that tells a story. The slab below seems to record an encounter between an arthropod and a soft-bodied invertebrate, possibly an annelid. This is the underside of the slab. The top of the slab exhibits different types of sedimentary structures and traces, including reptile footprints.

It looks like an unknown arthropod, possibly a large trilobite, was walking down the image from around point B toward A, when it encountered an infaunal, soft-bodied invertebrate, a “worm.” A scuffle ensued in the vicinity of C, and the arthropod seized the “worm” and dragged it towards point A. Note the drag mark parallel to the traces made by the arthropod’s walking legs.

Many trilobites were predatory, but so are many other arthropods. Did a large trilobite or other predatory arthropod (maybe even a terrestrial one?) take a stroll in a few millimeters of water at low tide hunting for worms? We’ll never know for sure, but it’s fun to speculate.

Pennsylvanian Worm Hunt, Kansas
Pennsylvanian Worm Hunt, Kansas. This slab seems to record a life and death struggle between two invertebrates that took place on a tidal mud flat during the Pennsylvanian Period in what is now Kansas. I would estimate the width of the arthropod at about 6 cm, which makes it very large for a late Paleozoic trilobite.

Reference

Hantzschel, W. 1962. Trace Fossils and Problematica. in R. C. Moore (ed.), Treatise on Invertebrate Paleontology: Part W, Miscellanea. Geological Society of America and University of Kansas Press, Lawrence, Kansas, W177-W259.

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

Welcome to Trilobite Seas!

Beginnings are always messy. –John Galsworthy

Ditomopyge olsoni pair, Staunton Formation, Coal Run, Indiana.
Ditomopyge olsoni pair, Staunton Formation, Pennsylvanian Period, Coal Run, Indiana. The larger trilobite is about 1.8 cm long.

Hello, and welcome to my new trilobite blog! I have had a life-long interest in paleontology, especially the creatures of the Paleozoic Era. I have also been a long-time collector of trilobites. Over the past years, though, I have drifted away from my fossil friends. It’s time to get back to them!

In this blog, I will focus primarily on the paleobiology of trilobites. There will be some pretty pictures (as trilobites are quite lovely), but most of the time the goal is a scientific understanding of these animals—and getting back to the daily enjoyment of these fascinating animals and their world!

The over-arching purpose of this blog is personal growth, exploration, and enjoyment. In this enterprise, I am taking as my model my own digital bird photography blog, twoshutterbirds.com, that I started about five years ago with my wife, Elisa, as a vehicle to expand our knowledge and capability in the hobbies of birdwatching and nature photography. I hope that trilobite seas.com will get me back to the fossil shows and into the literature, field, shop, and studio!

Without further ado, I launch the website with a new article: “Cryptic Strategies in Trilobites.” Additional articles and posts will follow as time and energy allow.  I invite constructive criticism and comments.

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