Convergence in Trilobites

Name any name and then remember everybody you ever knew who bore that name. Are they all alike? I think so. –Gertrude Stein

Broggerolithus broggerei, Harnage Shales, Ordovician Period, Welshpoole, Wales, United Kingdom
Broggerolithus broggerei (Family Trinucleidae), Harnage Shales, Ordovician Period, Welshpoole, Wales, United Kingdom. Trilobite is 2.5 cm across the genals.

When unrelated or only distantly related organisms evolve a similar form as an adaptation to a common way of life, you have convergence. And convergence is one of the great patterns in the history of life–and one of the clearest lines of evidence that evolution by means of natural selection is real.

Distinguishing features that are identical by descent (blood relationship) from those that are convergent is the central challenge in reconstructing the evolutionary histories of living things.

Aristoharpes sp., Devonian Period, Morocco
Aristoharpes sp. (Family Harpidae), Devonian Period, Morocco. Trilobite is 4.5 cm long.

Evidence of convergence is to be seen throughout the Trilobita. An easy place to recognize it is among the filter feeders. All the trilobites in this post were likely filter feeders, their large cephalons used as filtration chambers. Aristoharpes and Broggerolithus are not closely-related to each other, and Cordania is only distantly related to the others (they all belong to the Ptychopariida). Their superficial resemblance is likely due to a common way of life.

How many instances of convergence can you recognize in your collection?

Cordania wessmani, Bois d'Arc Formation, Devonian Period, Coal County, Oklahoma.
Cordania wessmani (Family Brachymetopidae), Bois d’Arc Formation, Devonian Period, Coal County, Oklahoma. This trilobite is more closely related to proetids than it is to any of the other trilobites in this post. Trilobite is 2.5 cm long.

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

Homalonotids: Large Weird Trilobites

Fear has many eyes and can see things underground. –Miguel de Cervantes

Trimerus, Rochester Shale, Silurian Period, New York
Trimerus delphinocephalus, Rochester Shale, Silurian Period, Middleport, New York. This homalonotid trilobite occurs alongside such familiar forms as Dalmanites and Calymene. At this scale of observation the exoskeleton looks smooth, but under a hand lens it is covered in tiny pustules. Specimen is 17.3 cm long.

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.

Dipleura dekayi, Devonian Period, New York
Dipleura dekayi, Skaneateles Formation, Devonian Period, Hamilton County, New York. Here trilobation is nearly absent. Specimen is 16 cm long.

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.

Dipleura detail, Devonian Period, New York
Orange-peel skin: Dipleura dekayi detail showing pitted surface texture. Was this trilobite covered in hair-like filaments?

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.

"Homalonotus," Devonian Period, Morocco
“Homalonotus,” Devonian Period, Morocco. This specimen has a smooth exoskeleton, unlike the Dipleura above–there is no reason to think that this animal was hairy. Axial length of pygidium is 19mm.

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

The Uniquely Spiny Thysanopeltis

Always remember that you are absolutely unique. Just like everyone else. –Margaret Mead

Thysanopeltis, Devonian Period, Morocco
Thysanopeltis sp., Hamar Laghdad Formation, Devonian Period, Morocco. Trilobite is about 80 mm long.

Spines are a persistent preoccupation of the trilobite enthusiast. Scutellids, by and large, are not known for significant spininess, although the group is among the most ornamented. Members bear every conceivable form of prosopon including pustules, terrace lines, and pygidial ribs. There are spiny exceptions, however, like Weberopeltis from the Silurian of Russia, Kolihapeltis from the Devonian of Morocco—and of course, Thysanopeltis.

Thysanopeltis: detail of margin of pygidium, Devonian Period, Morocco
Thysanopeltis sp.: detail of spiny margin of pygidium, Hamar Laghdad Formation, Devonian Period, Morocco.

In the case of each spiny scutellid, though, the arrangement of spines is very different. Weberopeltis has long marginal spines projecting backwards from the pygidium as extensions of pygidial ribs, as well as spike-like spines projecting backwards from the glabella and occipital ring. Kolihapeltis has large spines projecting backwards from the tops of the eyes and the occipital ring of the cephalon—but no marginal spines around the pygidium. Thysanopeltis is unique in the scutellid group and unusual among all trilobites in having numerous small spines fringing the pygidium.

Pygidium of Platyscutellum, Devonian Period, Morocco
Pygidium of Platyscutellum, AM Limestone Formation, Devonian Period, Morocco. Platyscutellum is not a common trilobite and has a row of small spines down the axial lobe, but like most other scutellids no marginal pygidial spines at all. Pygidium is 45 mm across at its widest.

In imagining the purpose of the marginal spines of Thysanopeltis it’s logical to consider the case of enrollment. Clearly an enrolled Thysanopeltis would have a well projected “zone of weakness” between the cephalon and pygidium, a “picket fence” if you will. Why this trilobite needed such a feature and other scutellids did not is, of course, completely unknown. Absent a breakthrough in our understanding in the functional morphology of the trilobite exoskeleton, all we can do is enjoy the fantastic diversity of our favorite arthropods.

Scabriscutellum furciferum, Devonian Period, Morocco
Scabriscutellum furciferum, Hamar Laghdad Formation, Devonian Period, Morocco. Some specimens of Scabriscutellum have small stout spines projecting from the tops of the eyes and occipital ring, but this one does not. S. furciferum is a common species with no marginal spines. Specimen is 40 mm long.

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

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.

Agnostida: Tiny, Blind (Mostly) Trilobites

I generally wade in blind and trust to fate and instinct to see me through. –Peter Straub

Goniagnostus nathorsti, Maya Formation, Cambrian Period, Lena River region, Siberia, Russia
Goniagnostus nathorsti, Maya Formation, Cambrian Period, Lena River region, Siberia, Russia. Trilobite is 10 mm long.

Agnostids are quite familiar to collectors of North American trilobites from the Cambrian of Utah, especially the Wheeler and Marjum Formations. How many trilobite collectors (or geologists for that matter) got their start when a parent or grandparent bought them an agnostid from Utah at a museum gift shop for a buck or two?

Baltagnostus eurypx, Wheeler Shale, Millard County, Utah
Baltagnostus eurypx, Wheeler Shale, Cambrian Period, Millard County, Utah. Trilobite is 4 mm long.

A quick perusal of the Treatise, however, reveals a bewildering variety of similar forms from the Cambrian and Ordovician of the world. Something about this small, blind, isopygous morphotype allowed for great success in the oceans of the early Paleozoic Era.

Peronopsis interstricta, Wheeler Shale, Cambrian Period, Millard County, Utah
A Typical Introduction to the World of Fossil Collecting: Peronopsis interstricta, Wheeler Shale, Cambrian Period, Millard County, Utah. Trilobite is 7 mm long.

The Order Agnostida contains two suborders, the Agnostina and Eodiscina. Agnostina are the more common and familiar to most collectors: These are all blind and have two thoracic segments. Some Eodiscina have eyes and possess two or three thoracic segments. The relationship between these groups has been controversial, some even arguing that the two suborders share no close relationship, their affinities resting with other trilobites.

Ptychagnostus michaeli, Marjum Formation, Millard Coounty, Utah
Ptychagnostus michaeli, Marjum Formation, Millard County, Utah. A spiny agnostid? Sure enough. Trilobite is 7 mm long (exclusive of spines).

As is the case with most trilobite groups, the mode of life of these little creatures is a matter for speculation. Some believe these trilobites occupied a planktonic niche. Whatever the case, agnostids (except for the rare ones from exotic locales like the Goniagnostus above) provide an easy entrée into the fascinating world of fossil collecting for children and adults alike.

Cephalopyge notibilis, Jbel Wawrmast Formation, upper Lower Cambrian, Taroudant, Morocco
Cephalopyge notibilis, a blind eodiscoid (Family Weymouthiidae), Jbel Wawrmast Formation, upper Lower Cambrian Epoch, Taroudant, Morocco. Trilobite is 9 mm long.

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

Giant Trilobites

Very few species have survived unchanged. There’s one called lingula, which is a little shellfish, a little brachiopod about the size of my fingernail, that has survived for 500 million years, but it’s survived by being unobtrusive and doing nothing, and you can’t accuse human beings of that.–David Attenborough

Acadoparadoxides briareus, Cambrian Period, Morocco
Acadoparadoxides briareus, Cambrian Period, Morocco. Commercial specimens of these giant Moroccan trilobites that have not been extensively tinkered with are hard to find. This is a nice specimen that is mostly real. This specimen would benefit from a cosmetic re-preparation. Trilobite is 33 cm long.

Some may tend to think of trilobites as small animals, but in the context of their times a few species were large animals. This is because the largest animals of the Paleozoic generally were not giants by Recent standards. Some orthoconic cephalopods (e.g. Cameroceras) grew to perhaps 5 meters in length, and some fishes (Dunkleosteus, Titanichthyes) grew to similar sizes. But these were outliers, the vast majority of Paleozoic animals were very much smaller.

The largest known complete trilobite specimen, Isotelus rex from the Ordovician of the Canadian Arctic, is about 72 cm in length and dwarfs most Ordovician invertebrate species. Known only from fragmentary remains, Terataspis grandis from the Devonian of New York achieved similar, but likely slightly smaller, sizes. It’s important to note that because of plate tectonic processes what we know of the life of Paleozoic Era is confined to species that inhabited the epicontinental seas, not the open oceans. The sizes achieved by the denizens of those vast open waters remains completely unknown. Likely some creatures were large, perhaps very large. The largest animals of today, the baleen whales, are creatures of the ocean basins.

Dikelokephalina sp., "relict capstone," Ordovician Period, Dra Valley, Morocco
Dikelokephalina sp., “relict capstone,” Ordovician Period, Dra Valley, Morocco. Trilobite is 23 cm long.

It’s notable that the relative size of trilobites compared with the largest creatures of the time also changed throughout the Paleozoic Era. During the Cambrian Period, for example, the largest trilobites were a significant fraction of the size of the largest known animals. The largest trilobites of that time approached half a meter in length, and the largest known mobile animals, like Anomalocaris, reached about a meter. Some sponges likely grew to well over a meter in height.

By the middle Paleozoic, the very largest known trilobites were over half a meter in length (Tetrataspis, Uralichas), and the largest predatory fishes were about ten times that long. But by the late Paleozoic the largest trilobites were very much smaller than the largest animals and probably tried to go about their business as unobtrusively as possible. By the time trilobites became extinct at the end of the Permian Period, the land and water teemed with monsters, and a really large trilobite was about 10 cm long . . . .

And why, since these be changed enow,
Should I change less than thou.–Elizabeth Barrett Browning, Change Upon Change

Isotelus maximus (cast). Waynesville Formation, Ordovician Period, Ohio
Isotelus maximus (cast). Waynesville Formation, Ordovician Period, Ohio. This is a high quality cast of a specimen collected and prepared by Thomas T. Johnson. Trilobite is 39 cm long—about half the size of the largest known trilobite.

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

The Bizarre Unicorn Trilobites

The Unicorn in association with heraldry is usually drawn as a horse with a single long twisted horn, lion’s tail and the legs of a stag. The Unicorn symbolizes extreme courage, strength and virtue.—clancunninghamglobal.com

Ampyx linleyensis, Hope Shales, Shelve Formation, Ordovician Period, Unite Kingdom
A Blind Unicorn: Ampyx linleyensis (molt), Hope Shales, Shelve Formation, Ordovician Period, United Kingdom. Ampyx and other members of the Family Raphiophoridae lack eyes, but not all raphiophorids have a rostral spine. Note: genal spines are missing due to this specimen being a molt. Trilobite is 2.5 cm across genals.

Rostral protuberances are common in trilobites, but a handful of families (Raphiophoridae, Alsataspididae; Hapalopleuridae) of generally similar morphology contain members with a single, needle-like, forward-projecting glabellar spine. Many trilobites with this spine are blind or have greatly reduced eyes (the atheloptic condition), and are usually considered to have inhabited an offshore, deep water, low light, benthic paleoenvironment. Often, they occur in siliciclastic rocks.

Cnemidopyge nuda, Ordovician Period, Wales, United Kingdom
Cnemidopyge nuda (molt), Ordovician Period, Wales, United Kingdom. Another blind raphiophorid unicorn. As in the Ampyx specimen above, genal spines are missing. Trilobite is 1.9 cm across the genals.

Most trilobite spines are interpreted to have had some sort of defensive function. In the case of the unicorns, however, many think that the glabellar spine, in conjunction with the long genal spines, acted to spread the trilobite’s weight over a larger area thus allowing them to live at the surface of soft, soupy sediments, perhaps as filter feeders in a fashion similar to the trinucleids that were discussed in the last post.

In any case, unlike “real unicorns,” the trilobitic ones are quite common, and the trilobite enthusiast can easily assemble a nice little collection of them!

Seleneceme acuticaudata, Hope Shales, Ordovician Period, Leigh, Shropshire, United Kingdom
Seleneceme acuticaudata, Hope Shales, Ordovician Period, Leigh, Shropshire, United Kingdom. This species is one of the most extreme examples of the unicorn morphology and belongs to the Family Alsataspididae. Note that this specimen is not a molt and possesses a least one long, sweeping genal spine. Trilobite is 1.2 cm long, exclusive of spines.

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

Filter Feeding Trilobites

Congratulations, you have a sense of humor. And to those who didn’t: Go stick your head in the mud. –Jesse Ventura

Nankingolithus, Ordovician Period, Morocco
“Nankingolithus sp.”, “Trilobite Beds,”Ordovician Period, near Mecissi, Morocco. The broad, pitted fringe of the cephalon may have served as a strainer for particles kicked up by flailing legs—like in a trinucleid, which this animal strongly resembles. Trinucleids, however, have six thoracic segments whereas this look-alike Moroccan form has only four. Trilobite is 35 mm across the widest part of the cephalon.

Trilobites are thought to have pursued a variety of feeding strategies. Some may  have been burrowing predators, and others are thought to have been scavengers or detritus feeders, perhaps wandering the bottom in search of whatever they could find. On the other hand, species with large cephalic chambers may have been filter feeders. A large number of specimens in the collection fall into this common general morphology, and just a few examples are shown here to illustrate.

Lloydolithus lloydi, Middleton Formation Ordovician Period, near Betton, Shropshire, United Kingdom
Lloydolithus lloydi, Middleton Formation Ordovician Period, near Betton, Shropshire, United Kingdom. This trinucleid trilobite is 17 mm across the widest part of the cephalon.

In general, these likely filter feeders have large, broad cephalons, presumably to house a filtration apparatus. Also, they tend to have long genal spines, which in  forms like some brachymetopids (e.g. Cordania) and harpetids (e.g. Aristoharpes) are deep and blade-like.

Filter feeding trilobites may have plowed head-first into the sediment, their massive cephalons balanced on genal spines. Beating legs may have either churned through sediments or generated currents that pushed stirred up detritus or small organisms into the filtration apparatus.

Cryptolithoides, Ordovician Period, Oklahoma
A North American Trinucleid: Cryptolithoides sp. (molt), Ordovician Period, Oklahoma. Trilobite is 15 mm across the widest part of the cephalon.

Restricted to Ordovician rocks, the trinucleids are perhaps the most specialized of the filter feeders and had pitted, bilaminar cephalic margins that acted like strainers. Pits may have allowed water to flow through the margin leaving food particles stranded behind. Specially adapted limbs may have swept these particles into the mouth, but this is speculative. This biomechanical interpretation is figured nicely in Gon (2003).

It’s fun to think of trilobites as wandering boldly around the Paleozoic sea-floor looking for prey, or perhaps carving out territories for mating or egg-laying purposes. In many cases, however, trilobites probably lived far less exciting lives than we imagine. Head-first into the mud, the filter feeders probably picked through the sediment as quietly and unobtrusively as they could.

Cordania falcata, Haragan Formation, coal County, Oklahoma
Enrolled Cordania falcata, Haragan Formation, Devonian Period, Coal County, Oklahoma. This brachymetopid is unrelated to the above species but shares a similar cephalic structure, and perhaps a similar filter feeding lifestyle. Specimen is 15 mm across the genals.

Reference

Gon, Samuel M., III, 2003. A Pictorial Guide to the Orders of Trilobites. 88p.

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

The Hypostome

The Ancients understood the omnipotence of the underside of things. ― Louis Pasteur

Ceraurus(?) in ventral aspect, Gull River Formation, Ordovician Period, Belleville, Ontario, Canada
Ceraurus(?) in ventral aspect, Gull River Formation, Ordovician Period, Belleville, Ontario, Canada. The shield-shaped element in the center of the cephalon is the hypostome. In this case, it has been displaced from its conterminant position—attached to the anterior doublure, the shelf-like ventral edge of the cephalon. Trilobite is 2.7 cm across the tips of the genal spines.

Perhaps the most interesting calcitic ventral structure in trilobites is the hypostome (or hypostoma). Although not completely understood, this exoskeletal element is usually interpreted as a mouthpart.

Flexicalymene with hypostome, Ordovician Period, Ohio
Flexicalymene sp. with hypostome, Ordovician Period, Ohio. Trilobite is 2.0 cm across the genals.

In the majority of trilobite species, specimens with the hypostome in life position are not known. There are several reasons for this. In some trilobites, the hypostome was attached to the animal by the ventral membrane only (the natant condition).

Isotelus hypostome, Ordovician Period, Ohio
Isotelus sp. hypostome, Ordovician Period, Ohio. Isotelus hypostomes are common finds in Ordovician rocks of the Midcontinent. Specimen is 2.5 cm across at the widest.

In some trilobites, the hypostome was fused to the rostral plate, a separate anterior element that functioned as part of the doublure, or the doublure itself. Sometimes a flexible(?) suture existed between the hypostome and the doublure. Sometimes a stalk existed between the hypostome and the rest of the exoskeleton. In these two latter cases, given the vagaries of preservation, it’s easy to understand why the hypostome is not often found in association with the rest of the exoskeleton.

Hypodiacranotus striatulus hypostome, Verulam Formation, Ordovician Period, Colbourne, Ontario, Canada
Hypodicranotus striatulus hypostome, Verulam Formation, Ordovician Period, Colbourne, Ontario, Canada. The hypostome of this trilobite exhibited long, spine-like posteriorly directed processes. Specimen is 8 mm long.

Further, the hypostome was typically shed during molting along with the rest of the exoskeleton when it became just another particle in the sedimentary rock record.

Given the position of the hypsotome, it’s logical to suppose that it functioned in feeding. If this is the case, the wide variety of sizes, shapes, and manner of attachment to the dorsal exoskeleton likely means that trilobites exhibited a wide variety of specific feeding strategies. The details of these, of course, will likely never be known.

Huntonia sp. hypostome, Bois d'Arc Formation, Devonian Period, Coal County, Oklahoma
Huntonia sp. hypostome, Bois d’Arc Formation, Devonian Period, Coal County, Oklahoma. Specimen is 1.9 cm long.

The not-infrequent discovery of a trilobite hypostome in the field is usually a happy moment. For even if articulated specimens remain elusive, the presence of these strange and mysterious little elements means that trilobites were around, and hope can remain for the discovery of the rest of the animal!

Phacops (Drotops) megalomanicus cephalon in ventral aspect, Devonian Period, Morocco
Phacops (Drotops) megalomanicus cephalon in ventral aspect with hypostome, Devonian Period, Morocco. Specimen is 8 cm across the genals.

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

The Phacopid World View

We do not see things as they are, we see things as we are.—Vikas Runwal

Drotops megalomanicus, Devonian Period, Issoumour region, Morocco
Phacops (Drotops) megalomanicus, Devonian Period, Issoumour region, Morocco. Trilobite is about 14 cm long.

One of the most persistent things you notice when examining a large collection of different species of phacopids is that the schizochroal eyes are usually tilted downward by 30-40 degrees. This is true for phacopids of every description. Large species covered in pustules like Drotops (above) or much smaller smooth forms like Crythops (below)–and every morphology in between–show this feature.

Such an orientation would seem to waste part of the field of view given that it means the anterior third or so of the lenses looked down into the mud if the animal was walking level across the bottom. Likewise, the posterior one-third of lenses would be looking up into the water column behind the animal as it moved across the sea floor.

Eocrythops, Devonian Period, Morocco
Crythops cryptopthalmus (aka “Rotops”), Devonian Period, Atchana, Morocco. Trilobite is about 4.0 cm long.

It’s possible to imagine a functional significance for such an orientation of the eyes with the animal extended on the sea floor, though. Perhaps these trilobites were keeping an eye out for nasty piscine or cephalopod predators that came swimming down from behind and above. Perhaps they were simultaneously inspecting the sea floor at the ten- and two-o’clock positions for prey or detritus.

Paciphacops birdsonensis, Devonian Period, Tennessee
Paciphacops birdsongensis, Birdsong Shale, Devonian Period, Benton County, Tennessee. Built for looking out of a burrow? Trilobite is about 2.0 cm across the genals.

Much more likely, in my view, is that these were infaunal or semi-infaunal animals, and perhaps spent a significant amount of time with their heads poking out of the sediment. Phacopids in just the position one would expect for an animal peering out of a burrow are known from the Devonian of Oklahoma. In this posture, the orientation of the eyes makes sense: The sea floor could be surveyed with maximum efficiency, the lenses probing the widest possible scene.

Phacopid, Devonian Period, Morocco
Thar she blows! Indeterminate phacopid, Devonian Period, Morocco. Perhaps trilobites plowing through sediments looking for food occasionally breached the surface and surveyed the sea floor for menaces or opportunities. Trilobite is about 3.5 cm across the genals.

Trilobites are known burrowers, and they are also known to have exhibited cryptic behaviors. Cruziana, the trace fossil associated with trilobitic burrowing, is a common fossil. In some deposits, such as the those at the famous Ichnological Park at Penha Garcia (Lower to Middle Ordovician), Portugal, Cruziana specimens compose the rock.

Definitive examples of trilobites preserved in burrows are rare, however, and limited to a hand-full of examples. The taphonomic requirements for the preservation of trace fossils and body fossils are different. Although it has happened, finding an example of a trilobite that died in its tracks in a burrow is highly unlikely. All we can do is keep looking!

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