Fear has many eyes and can see things underground. –Miguel de Cervantes
Homalonotids are well-known fossils of Silurian and Devonian age from around the world. Despite occurring in deposits alongside other more typical-looking trilobites, they have a number of unusual features.
Many specimens show “indistinct trilobation,” giving them a streamlined torpedo-like appearance. They generally lack spines, although some “Burmeisteria armatus” (aka “Elvis”), widely faked and composited specimens from Morocco, apparently have short, stout spines. Such streamlining (in all but Elvis) could be used to make a case for a burrowing lifestyle.
What gives pause to the notion of burrowing, however, is the pitted orange-peel texture exhibited by some species. There are a variety of types of pores and canals, often associated with bumps or pustules, that perforate the exoskeletons of trilobites. Interpretations of the functions of these structures vary and include openings for the diffusion of oxygen, a chemosensory function, secretion, and most often setae (hair-like filaments or bristles) that could have had a protective or sensory function.
Presence of bristles over the surface of the body would seem to be at crossed purposes with a burrowing lifestyle where smoothness would most helpful. Perhaps the pores of such animals as Dipleura just allowed easier diffusion of oxygen through the shell to the gills below and are unrelated to setae. Maybe a secreted slime layer flowed through the pores and allowed easy movement through a gritty substrate. Or perhaps they were for setae–but allowed a buried, often immobile, animal to sense prey or predators in the surrounding sediment. We will likely never know.
In the seascape of my imagination, though, homalonotid trilobites like Dipleura were covered in hairs like giant asp caterpillars wandering the seabed. Perhaps, like asps, these trilobites, too, were venomous–offering up the most unpleasant possible mouthful for any passing monster cephalopod or placoderm.
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 Bulletin115.
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 Paleontology23: 572.
Levi-Setti, R. 1993. Trilobites. The University of Chicago Press. 342p.
Mayr, E. 1983. How to carry out the adaptationist program? The American Naturalist121 (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.
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
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.
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.
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.
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.
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.
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.
If there were no mystery left to explore, life would get rather dull, wouldn’t it?–Sidney Buchman
Illaenids are common trilobite fossils, especially in Ordovician rocks. The illaenid lifestyle—if there was a commonality in their way of life–was likely a very successful one. Based on logic and inference, many illaenids are thought to have been at least partly infaunal, their cephalons sitting at the sediment-water interface and their bodies inclined steeply into burrows.
As noted by Whittington (1997), however, no illaenids have been identified in burrows, so there is significant doubt that any members of the group were infaunal.
In general, most illaenids may be described as smooth, or sometimes covered in an ornamentation of fine parallel ridges. See the Moroccan Ectillaenus above. Such surface texture has been interpreted as a method for strengthening the shell. Smoothness and general lack of spininess could be consistent with a burrowing or infaunal lifestyle. A few illaenids, like Illaenus tauricornis, famously sport impressive genal spines, though.
Many illaenids were also capable of complete spheroidal enrollment, typically without an “overbite” or “underbite” of the cephalon. Enrollment is usually interpreted as a defense mechanism against predators and dangers of the physical or chemical environment. Perhaps such perfection of enrollment was necessary given the general lack of spininess.
One fact that leads me to question the notion that illaenids necessarily shared a common lifestyle can be seen in the eyes. There is a tremendous range in the relative size of the holochroal eyes, from very large in Bumastus (below) to a complete absence in some species of Ectillaenus (above). Eye reduction or absence in trilobites has been associated with great water depth (>700 m) or turbidity where light levels are low (Clarkson, 1967), or an infaunal lifestyle (Berkowski, 1991).
Such a range of eye size in illaenids is certainly fuel for paleoecological conjecture. In my collection, blind or nearly blind species of trilobites (across many taxa) tend to occur in siliciclastic rocks whereas species with well-developed eyes tend to occur in carbonates. This makes sense in that I would generally expect marine carbonates to reflect clear, brightly illuminated paleoenvironments given the typical importance of photosynthetic calcareous algae here. It’s easy to imagine murky, silty or muddy paleoenvironments, perhaps near estuaries, where eyes would be of little utility to a benthic marine invertebrate.
On the other hand, perhaps the blind or nearly blind illaenids lived a completely infaunal existence, whereas those with eyes may have emerged from their hiding places often. If an illaenid is ever found in situ within a burrow, or perhaps inside a larger invertebrate shell or another hiding place, we would have more basis for informed speculation.
Given the intensity of search for trilobites in illaenid-bearing strata, however, it may be that these arthropods have not been found in burrows simply because they never occupied them, and the burrowing lifestyle . . . another “just-so story.”
Berkowski, B. 1991. A blind phacopid trilobite from the Femennian of the Holy Cross Mountains. Acta Palaeontologica Polonica36 (3): 255-264.
Clarkson, Euan N. K. 1967. Environmental significance of eye reduction in trilobites and Recent arthropods. Marine Geology5 (5-6): 367-375.
Whittington, H. B. 1997. Mode of life, habits, and occurrence. 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, 137-169.
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)
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.
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.
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.
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.
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.