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 “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.

Crusher Teeth, Spines, and the Fall of Trilobites

If you have food in your jaws you have solved all questions for the time being. —Franz Kafka, Investigations of a Dog

Paladin transilis, Upper Carboniferous, Zhirnovsk, Volgograd region, Russia
Paladin transilis, Upper Carboniferous, Zhirnovsk, Volgograd region, Russia. This beautiful trilobite had extremely well developed genal spines for its time. Many late Paleozoic trilobites generally resembled Paladin overall, but had small genal spines or no spines at all. Specimen is about 1.5 cm across genals.

The decline of trilobites accompanies the expansion of gnathostomes (jawed vertebrates). Family-level trilobite diversity nearly held steady with only minor decline throughout the Silurian Period just as gnathostomes began their evolutionary radiation. Trilobite diversity then underwent a series of significant step-wise declines throughout the Devonian Period. This was a time of major expansion for vertebrates, including those with “crusher-type” dentitions, the most likely trilobite hunters. Only four trilobite families survived into the Carboniferous Period.

These crusher-teeth occurred in many common groups of fishes of middle and late Paleozoic age, including bony fishes (Osteichthyes), placoderms, and shark-like fishes (Chrondrichthyes, especially holocephalians). Many fishes with  crusher-teeth likely preyed largely on hard-shelled invertebrates as they do today. It seems plausible, then, that these predators exerted selective pressure on trilobites. It is also therefore reasonable to believe, as some do, that vertebrates played a role in the decline and ultimate extinction of trilobites.

Syntheotodus trisulcatus, Maple Hill Formation, Upper Devonian, Kalona, Iowa
Syntheotodus trisulcatus crusher tooth in occlusal view, Maple Hill Formation, Upper Devonian, Kalona, Iowa. This is an early holocephalian tooth. Holocephalians were likely a much more diverse and widespread group in the middle and late Paleozoic than they are today. Extant holocephalians, the 39 species of chimaeras and ratfishes, are mostly deep water forms that feed on hard-shelled benthic invertebrates. Specimen is 8 mm across.

But correlation, of course, is no proof of causation, especially given the multitude of other changes that occurred during this interval of earth history. Some would even argue that:

A belief in the causal nexus is superstition.—Ludwig Wittgenstein

Lagarodus tooth plate, Harrodsburg Limestone Formation, Washington County, Indiana
Lagarodus sp. (Psammodontidae, Holocephali) tooth plate, Harrodsburg Limestone Formation, Mississippian Period, Washington County, Indiana. Such flat, plate-like crushing teeth would have made short work of a trilobite. Specimen is 1.6 cm across.

Philosophy aside, spines in trilobites are often interpreted to have had a defensive function as they do in many extant marine and aquatic forms. Some predatory fishes of insufficient size, for example, may have difficulty swallowing other fishes because the prey fish can splay out fin spines making passage down the gullet impossible. But spines are no guarantee of safety. Diving birds and waders, fish-eating specialists, can easily manipulate prey into a head-first orientation and eat the spiniest of fishes, even those of large relative size. For every measure, there is a countermeasure. This was likely as true in the Paleozoic as it is today.

The proliferation of dorsal spininess in Devonian trilobites may have been a response to threats from jawed fishes and other predators, ammonites, for example. In the case of soft-bodied predators this makes sense, but I’ve always been skeptical that spines could have been of much protection from vertebrate predators, especially specialized ones. Specialized vertebrate predators are often just too formidable for any invertebrate prey, not matter how thorny. Triggerfish, for example, bite off echinoid spines until the animal’s body is exposed and then eat the soft-tissues. Holocephalians graze on shellfish the way cows graze on grass, groupers grab crabs, and so on.

Further, an evolutionary arms race between spines and teeth would have clearly and immediately favored teeth. Vertebrate teeth are, after all, made of hard, phosphatic tissues and are inherently more than a match for the calcite of the trilobite exoskeleton, no matter how spiny. Trilobite spines, if they were defensive structures at all, were likely only effective against a specific, most likely unknown, soft-bodied threat.

A final observation indicating that spines may have had little to do with defense is that it is the dorsally spiny trilobites (like Comura) that disappear at the end of the Devonian Period. The trilobites that survived into the late Paleozoic, a time when waters teemed with the most menacing piscine predators of the era, were the most conservative forms. Many late Paleozoic trilobites even lacked genal spines.

The reason trilobites retreated into the shadows at the end of the middle Paleozoic and ultimately disappeared near the end of the Permian Period will likely never be completely understood. An analysis of spines versus predators or trilobite predation in general, although an attractive place to look for easy answers, seems unlikely to yield convincing answers about extinction.

Double-crested Cormorant with "Plecostomus, Fiorenza Park, Houston, Texas
Double-crested Cormorant with “Plecostomus” (Pterygoplichthys multiradiatus), Fiorenza Park, Houston, Texas. Loricariid catfish are covered in dermal armor and have formidable fin spines as wicked as any trilobite’s associated with their fins. Cormorants dispatch and eat them with ease.

©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.

Trilobite Multiples

The first law of ecology is that everything is related to everything else. –Barry Commoner

Ampyxina bellatula, Marquoketa Formation, Ordovician Period, Missouri
Ampyxina bellatula molts, Maquoketa Formation, Ordovician Period, Missouri. These trilobites lack free cheeks (note absence of long genal spines) and are therefore molts. Did these animals gather to molt communally? Largest molt is 1.0 cm long.

Associations of large numbers of monospecific trilobite molts on a single bedding surface occur worldwide throughout marine rocks of Paleozoic age. Often, it looks as though trilobites gathered to molt at a specific place and time. Sometimes it’s not easy to tell if the assemblage reflects paleobiology and not simply a hydraulic accumulation of molted exoskeletal sclerites, though.

Elrathia kingii, Wheeler Shale Formation, Cambrian Period, Utah
Elrathia kingii (multiple), Wheeler Shale Formation, Cambrian Period, Utah. Most of these trilobites have free cheeks and are probably not molts. These animals likely died at the same time, in the same place. Largest trilobite is 3.2 cm long.

Sometimes a single bedding surface may contain a monospecific (or nearly) assemblage of complete trilobite specimens. More rarely, one finds several species of complete specimens on the same bedding surface (as below).

Raymondites plate, Ordovician Period
Ceraurus globulobatus (multiple), Raymondites spiniger (center right), and Bumastoides milleri (upper left), Bobcaygeon Formation, Ordovician Period, near Brechin, Ontario. This slab contains three species of trilobites, one of which (Ceraurus) is in a variety of preservational states ranging from complete, outstretched and articulated to scattered and disarticulated. Largest Ceraurus is 3.4 cm long.

Although a complete understanding of these associations will likely forever elude us, these multi-species plates are of great interest to the collector. This is especially true if it is certain that the slab reflects a completely natural assemblage of rare or unusual species.

Raymondites plate detail, Ordovician Period
Raymondites (upper right) plate detail, Ordovician Period.

Many multiple commercial specimens from Russia and Morocco, on the other hand, are likely the product of manipulation. Large slabs may have had a pit or pits excavated into it, and trilobites or other fossils added and epoxied into place. A texture added to the surface can conceal the additions. This being the case, a collector should pay no more than he/she would for the specimens in isolation, the association being neither paleoecological nor sedimentological (i.e., scientifically meaningless).

Russian double, Ordovician Period
Asaphus cornutus (left) and Pseudoasaphus globifrons (right), Ordovician Period, St. Petersburg region, Russia. Real trilo-buddies or a composite? Most likely the latter. Larger trilobite is 8.1 cm long.

©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 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.

My What Big Eyestalks You Have!

Keep your eyes on the stars, and your feet on the ground. –Theodore Roosevelt

Cybele panderi, Ordovician Period, Russia
Cybele panderi, Ordovician Period, St. Petersburg region, Russia. Small holochroal eyes sit atop spectacularly tall eyestalks. Other encrinurids from the Ordovician of Russia (e.g., Cybele bellatula; Cybellela rex) have eyestalks, too, but not as tall. Trilobite is about 5.0 cm long.

Much has been written about the remarkable and complex physics of trilobite eyes. Each lens in the holochroal or compound eye of a trilobite, for example, is a single calcite crystal. Perhaps even more remarkable is that each of these numerous, tightly packed crystals is oriented such that its optic axis is perpendicular to the visual surface. This allowed glass-like (isotropic) behavior, rather than birefringent behavior (variable index of refraction) of the lens (see Levi-Setti, 1993). Anyone who has observed calcite sliced in arbitrary crystallographic directions through a petrographic microscope has seen this strong birefringence. Of course, the added benefits of trilobite optics include the fits they produce in creationists!

Extant arthropods with compound eyes have poor, short-range vision by vertebrate standards. Their eyes are, however, good at detecting movement . . . .

Pseudocybele nasuta, Pogonip Group, Nevada
Pseudocybele nasuta, Pogonip Group, Ordovician Period, Nevada. Although superficially similar to Cybele, this trilobite belongs to a different family (Pliomeridae). The prominent, tall anterior eyes show that vision was important to this animal. It’s easy to imagine how natural selection could have operated on a visually-oriented animal with tall conical eyes like this and produced stalk-eyed forms. Whatever this trilobite was on the look-out for, though, will likely forever remain a mystery. Trilobite is 2.3 cm long.

Russian asaphids, like encrinurids, show a great range in eyestalk height. Some, like Asaphus ornatus or A. expansus, have typical low holochroal eyes similar to what one might see in Isotelus. A. punctatus possesses modest eyestalks, and slightly taller ones exist in A. intermedius. The most spectacular asaphid eyestalks famously occur in A. kowalewskii.

Asaphus punctatus, Ordovician Period, Russia
Asaphus punctatus (anterior view), Ordovician Period, St. Petersburg region, Russia. This trilobite clearly had a good view of the sea-floor around it. Trilobite is 7.0 cm long.
Asaphus punctatus, Ordovician Period, Russia
Asaphus punctatus (lateral view), Ordovician Period, St. Petersburg region, Russia. Trilobite is 7.0 cm long.

Some dalmanitacean trilobites seem to have taken a different approach to getting their eyes elevated above the bottom: These trilobites evolved tall, cone-shaped schizochroal eyes. The most spectacular of these, perhaps, can be seen in Erbenochile erbeni, a Devonian form from Morocco that even seems to have a “brim” at the top of the eye to shade to lenses from the glare of the sun! Surely this animal operated in shallow water. A similar but far more common tall-eyed dalmanitacean trilobite available to collectors is Coltraenia.

Treveropyge prorotundifrons, Devonian Period, Morocco
Coltraenia sp. (aka “Treveropyge prorotundifrons”), Devonian Period, Morocco.

We have only logic and reasoning by modern analogy to ascertain what the possible significance of eyestalks in trilobites was. It makes sense that stalked eyes were used to detect the presence of predators or prey as well as obstructions that might be encountered while strolling around on the bottom. They might reasonably be assumed to be helpful if the sea bottom was soupy, or perhaps stirred up such that a thin cloudy layer developed. Being able to peer from a burrow to see if the coast was clear before divulging one’s presence would also seem to be adaptive.

On the other hand, more exciting interpretations of eyestalks are possible. Eyestalks in male stalk-eyed flies (Diopsidae), for example, are known to be implicated in sexual selection. Male flies engage in competetive confrontations in which they compare eyestalk height. Females flies prefer males with taller eyestalks. The strongly overlapping fields of view in these stalked fly eyes also creates stereoscopic vision.

Perhaps trilobites wandered their ocean worlds with a good perception of the three-dimensional world around them. Perhaps they also engaged in ritual confrontations over mates or spawning grounds. The truth of the matter, of course, has been lost in the mists of time . . . .

Reference

Levi-Setti, R. 1993. Trilobites. The University of Chicago Press. 342p.

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