Name any name and then remember everybody you ever knew who bore that name. Are they all alike? I think so. –Gertrude Stein
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.
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?
The first law of ecology is that everything is related to everything else. –Barry Commoner
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.
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).
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.
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).
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
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.
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
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
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.
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!
Congratulations, you have a sense of humor. And to those who didn’t: Go stick your head in the mud. –Jesse Ventura
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.
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.
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.
Gon, Samuel M., III, 2003. A Pictorial Guide to the Orders of Trilobites. 88p.
The Ancients understood the omnipotence of the underside of things. ― Louis Pasteur
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.
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).
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.
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.
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!
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
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.
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)
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.
Keep your eyes on the stars, and your feet on the ground. –Theodore Roosevelt
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 . . . .
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.
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.
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 . . . .
Levi-Setti, R. 1993. Trilobites. The University of Chicago Press. 342p.
The building’s identity resided in the ornament. –Louis Sullivan
From a collector’s viewpoint, the variation in “ornamentation” (granules, pustules, tubercles, ridges) is the raw material of building a collection.
However, some have objected to the commonly used term ornamentation:
“Such surface sculpture is frequently referred to as ornament, but as Gill (1949) argued in proposing to call it prosopon, ornament is a general word that gives an erroneous impression of mere decoration, whereas surface sculpture has biological significance.” (Whittington and Wilmot, 1997, p.77).
Point taken, but as a birder I know that “mere” decoration can and often does have biological significance. Bright colors, plumes, and iridescence in male bird feathers are meant to appeal to the females—they are decorations! Such flamboyant structures are used in species recognition and dominance and courtship rituals (sexual selection) in many other groups of organisms, too. Think of shaggy manes, antlers, even oversized pincers in fiddler crabs.
A danger, however, lies in the over-interpretation of the functional significance of morphological features, especially minor superficial ones (see Mayr, 1983). Genetic mutation, the raw material of evolution, is a random process. The phenotypic expression of these mutations will be preserved in populations if the changes they represent are adaptive, or at least not too deleterious.
But it’s fun to speculate on the possible functional significance of the sculpted texture like that found in Flexicalymene granulosa (above), as contrasted with the more typical smooth skin found in F. meeki, for example. It seems to me that such rough or pebbly textures may have better blended into a sandy bottom than smooth textures.
The Treatise references Chatterton (1980) who suggested that bumpy exoskeletal surface textures could foil the attacks of predators with sucker disks. While interesting, extant cephalopods grab rough-skinned crustaceans with little problem, and I would think that a rough surface texture would, in general, be easier to grab. Think about a soccer ball versus an American football.
Why do some mostly smooth trilobites preserve external segmentation of the pygidium, like Ameura, whereas many trilobites are smooth over their entire exoskeletons (e.g. Asaphus)? Is this functional, or simply a superficial expression of some deeper developmental difference? Is this difference ornamental?
Among the more lovely forms of surface sculpture are the terrace ridges. Although several studies have attempted to establish a functional explanation for this type of texture through relating it to surface and deeper features of the exoskeleton, its purpose is still unknown. See discussion in Whittington and Wilmot (1997).
Rarely, the surface texture of a fossil specimen can provide a window into the possible physiological paleobiology of trilobites. Does the thin, crinkly-looking texture of the Wanneria molt above indicate that valuable minerals from the exoskeleton were reabsorbed by the animal prior to molting—as some extant arthropods do?
Finally, even if trilobite ornamentation is difficult to interpret, the similarities and differences among species are the visible evidence of evolution. And gaining a further appreciation for the evolutionary history of our favorite group is always fascinating and worthwhile.
Campbell, K. S. W. 1967. Trilobites of the Henryhouse Formation (Silurian) in Oklahoma. Oklahoma Geological Survey Bulletin115.
Chatterton, B. D. E. 1980. Ontogenetic studies of Middle Ordovician trilobites from the Esbataottine Formation, Mackenzie Mountains, Canada. Palaeontographica (Abt. A) 137: 1-74.
Fortey, R. 2000. Trilobite! Eyewitness to Evolution. Alfred A Knopf, New York, 284p.
Gill, E. D. 1949. Prosopon, a term proposed to replace the biologically erroneous term ornament.Journal of Paleontology23: 572.
Levi-Setti, R. 1993. Trilobites. The University of Chicago Press. 342p.
Mayr, E. 1983. How to carry out the adaptationist program? The American Naturalist121 (3): 324-334.
Whittington, H. B. and Wilmot, N. V. 1997. Microstructure and sculpture of the exoskeletal cuticle. in Roger L. Kaesler (ed.), Treatise on Invertebrate Paleontology: Part O, Arthropoda 1, Trilobita, Revised. Geological Society of America and University of Kansas Press, Lawrence, Kansas, 74-84.
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.
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.
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.
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.