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.

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.

 

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.

Evidence of Great Biocrises in the Field and the Drawer

Life on our planet has been a constant series of cataclysmic events, and we are more suitable for extinction than a trilobite or a reptile. So we will vanish. There’s no doubt in my heart. –Werner Herzog

Griffithidella doris, Lake Valley Formation, New Mexico
Griffithidella doris, Lake Valley Formation, Mississippian Period, New Mexico. A series of biocrises in the Late Devonian Epoch whittled trilobite diversity down to only four families by the opening of the Carboniferous (Mississippian) Period. Smaller trilobite is 1.0 cm long.

Extinction must frequently be on the mind of many trilobite collectors. Every species of once-living thing in their cabinets has been extinct for hundreds of millions of years. What’s more, the diversity of specimens in those cabinets reflects the steady background rate of extinction as well as the mass extinction events sprinkled throughout the Paleozoic Era.

Most noticeable, perhaps, is how the Late Devonian events shape a fossil collection. The Lower Devonian drawers are chock full, the Upper Devonian drawers are sparse, and (unless you’ve made a concerted effort), the late Paleozoic drawers have ample space for additional specimens!

Other transitions are just about as apparent. Ordovician drawers stuffed with asaphids, for example, bear little resemblance to the Silurian drawers—no doubt reflecting the big trilobite die-off at the end of the Ordovician Period.

Megistaspidella triangularis, Kunda level, Ordovician Period, St. Petersburg region, Russia
Megistaspidella triangularis, Kunda level, Ordovician Period, St. Petersburg region, Russia. The Megistaspidae is a group containing large, spectacular shovel-nosed forms. Megistaspids are confined to Lower and Middle Ordovician rocks. Trilobite is about 13 cm long.

Each of the five major mass extinction events of the Phanerozoic, no doubt, looked different to the organisms experiencing them, from cataclysmic bolide impacts to seas draining away or becoming choked with organic detritus . . . .

As a natural history enthusiast who spends a great deal of time in the field (mostly photographing birds), I accept the concept of the Anthropocene, the Age of Man. I also accept that we are experiencing the the sixth great (anthropogenic) mass extinction event of the Phanerozoic Eon. I have no doubt that the fossil record of the future will show evidence of a geologically instantaneous extinction event dating to . . . now.

Older Holocene terrestrial strata will record a diverse vertebrate fauna, and nearshore marine strata will preserve reef facies bristling with invertebrates. Younger Anthropocene strata will show a much decreased biodiversity and a much greater abundance of cow, pig, chicken, human, and dog bones interlaced with scrap metal, broken concrete, and plastic debris!

One more subtle aspect of the unfolding anthropogenic extinction event, I think, is the human importation of often destructive exotic species into many parts of the world.

And now for a bit of shameless self-promotion . . . .

Save the Date (January 18, 2017): A New Two Shutterbirds Presentation at the Houston Audubon Nature Photography Association (HANPA)

Prothonotary Warbler on bottlebrush, Catholic Cemetery, Dauphin Island, Alabama
Looking for a Sweet Treat: Prothonotary Warbler on Bottlebrush, Catholic Cemetery, Dauphin Island, Alabama. Bottlebrushes are Australian plants, but birds everywhere love them because of the copious nectar and pollen they produce. Sweet calorie-rich nectar must be a wonderful treat after a grueling trans-Gulf of Mexico flight! Canon EOS 7DII/600mm f/4L IS (+1.4x TC). High-speed synchronized fill-flash.

Exotics Gone Native!

Synopsis: Human-introduced exotic plants and animals are all around us, and many of them are doing nicely, thank you very much. It’s sometimes hard not to notice them while out photo-birding. The proliferation of these organisms can be troubling to nature lovers, particularly eco-purists. Are these foreign organisms adversely affecting our native plants and wildlife? And if so, how badly? Are some helpful to our native species? Certainly some, like bottlebrush, are helpful to the bird photographer! Whatever your stance on exotics, perhaps the healthiest thing to do is treat them as just another opportunity to experience new species in the wild—even if they are out of place. In this talk, Chris Cunningham will share images of some frequently encountered exotic species and discuss their place in our native landscape. (Note: If this topic is too upsetting, Chris and Elisa will share and some images of native wild birds from their most recent outings to West Texas, the Coastal Bend, and central New Mexico, too!)

Time and Place: 7:00 PM, January 18, 2017 at the Edith L. Moore Nature Sanctuary, 440 Wilchester Blvd., Houston TX 77079. For additional details, please see the Houston Audubon HANPA website.

©2016 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.

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

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.

Book Review: A Sea Without Fish

A Sea Without Fish: Life in the Ordovician Sea of the Cincinnati Region by David L. Meyer and Richard Arnold Davis (2009)

A Sea Without Fish focuses on the famously fossiliferous Upper Ordovician rocks of the Cincinnati region. As someone who lives in Houston, Texas, surrounded by just about the least interesting geology and paleontology imaginable, descriptions of a landscape bristling with early Paleozoic rocks and fossils inspires a bit of jealousy.

Having grown up on marine Ordovician rocks in southern Minnesota, many of the organisms and lithofacies described in this book are familiar, and remind me just how much I miss being able to simply walk or bike to fossiliferous outcrops that record a period of earth history so wildly different from our own.

The Cover features stunning reconstruction "The Cincinnatian" by John Agnew
The book’s cover features a stunning reconstruction of a marine scene from the Late Ordovician Epoch, “The Cincinnatian,” by John Agnew (2007).

To begin, the book provides the customary general discussion of geological and biological terminology. A somewhat lengthy summary of the history of paleontological exploration of the area is also included. The bulk of the volume surveys the major groups of fossil organisms found in Cincinnatian rocks, from algae to hemichordates and conodonts. Interestingly, the authors also devote significant space to trace fossils, biofacies, depositional paleoenvironments and stratigraphy. It is, therefore, a well-rounded treatment of this fascinating stratigraphic interval and geographic area.

The Ordovician Period included a time when North America was essentially submerged, by some estimates (e.g., Hallam) the high water mark of the Phanerozoic, and the Midwest teemed with marine organisms. Of course, the vast majority of these organisms are now extinct. Ohio and surrounding areas resembled the Caribbean or Persian Gulf more than the Midwest of today. By and large, though, the rocks of the Cincinnatian were likely deposited in water less than 35 m deep. Further, the Late Ordovician Epoch was a time of hurricanes, high atmospheric carbon dioxide, and low oxygen levels. The Midwest was also in the Southern Hemisphere.

Primaspis crossota on bryozoan fragment, Kope Formation,Late Ordovician Epoch, Hamilton County, Ohio. Trilobite is 9 mm long.
A Rare Cincinnatian: The Diminutive Odontopleurid Trilobite Primaspis crossota on a bryozoan fragment, Kope Formation, Late Ordovician Epoch, Hamilton County, Ohio. P. crossota is often associated with bryozoans. Figure 11.6E; F in A Sea Without Fish also shows this trilobite preserved on bryozoans. I suspect that a symbiotic relationship existed between these organisms. Trilobite is 9 mm long.

Chapter 11, the arthropod chapter, along with Chapter 16: “Life in the Cincinnatian Sea,” which contains paleoecological information on facies and units and figured examples of trilobites associated with other organisms (nautiloid living chambers), will be of most interest to trilobite enthusiasts. The broad stratigraphic relationships of the Cincinnatian summarized in Figure 15.1 is also a useful touch.

Flexicalymene retrorsa, Sunset Member, Arnheim Formation, Mt. Orab, Ohio. Trilobite is 3.8 cm long.
A Common Cincinnatian: Flexicalymene retrorsa, Sunset Member, Arnheim Formation, Late Ordovician Epoch, Mt. Orab, Ohio. The only Cincinnatian trilobite the casual collector is ever likely to find in the field. Trilobite is 3.8 cm long.

On to quibbles. I consider the title to be a little odd, focusing on something that is absent (unless you consider conodont animals to be vertebrates or “fish”), rather than what is present. Fishes do not become a major part of the fossil record until the Devonian Period. Ordovician agnathans (“jawless fishes”) do appear in a few places around the world, mostly on Gondwanaland (South America and Australia) and North America (Harding Sandstone of Colorado), but open marine Ordovician rocks are typically free of the remains of anything normally called a “fish” (vertebrates minus tetrapods). I think Cincinnatian rocks are interesting enough, and filled with enough organic remains, to warrant a positive descriptive title based upon what is there, rather than what is not.

One last quibble involves collecting localities. The reader can consult Appendix 1 under field guides for collecting localities where the statement “Localities listed in older guidebooks may no longer be accessible.” Because this book seems to have as its audience serious amateur geologists and fossil collectors, a detailed up-to-date list of localities where enthusiasts can safely and legally collect Cincinnatian fossils would, I think, be appreciated.

One of the things that has soured me on fossil collecting in the past is trying to chase down localities from antiquated references—that and fruitless run-ins with constabulary (who object to fossil collecting even though there is no strictly legal basis for their attentions) and other locals who raise biblical or other silly objections to “outsiders” poking around in their rocks. Once, for example, I was collecting on a road cut in Kansas and was approached by a cop because I was creating a “distraction” that might cause motorists to loose control and wreck their vehicles. But as I was violating no law or ordinance, he had to leave me to my diggings.  Ah yes! Nothing rounds out a hot and dusty day in the field better than a scolding by a creationist or policeman!

All in all, I found A Sea Without Fish to be an interesting and worthy addition to my trilobite library. This volume occupies a place of honor on a shelf next to other excellent recent titles about Paleozoic geology and paleontology such as Foster’s Cambrian Ocean World (2014) and Erwin and Valentine’s The Cambrian Explosion: The Construction of Animal Biodiversity (2013). I highly recommend A Sea Without Fishes for all trilobite lovers, no matter where they live.

Diplichnites, Kope Formation, Late Ordovician Epoch, Mason County, Kentucky. Trace fossil is about 1.5 cm across.
Diplichnites, Kope Formation, Late Ordovician Epoch, Mason County, Kentucky. This book figures and discusses many common ichnotaxa of the Upper Ordovician. It also briefly discusses Seilacher’s ichnofacies concept, in which this specimen would be considered repichnial, a locomotory trace. Trace fossil is about 1.5 cm across.

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