Book Review: Cambrian Ocean World

Cambrian Ocean World: Ancient Sea Life of North America by John Foster (2014)

Cambrian Ocean World: Ancient Sea Life of North America by John Foster
The Cover of Cambrian Ocean World features a lovely painting by John Agnew that reconstructs Middle Cambrian time during deposition of the Spence Shale.

Cambrian Ocean World is a treasure trove of information, general and specific, for the trilobite enthusiast. Although the first fifty or so pages contain the obligatory general discussions/explanations of paleontological, geological, and biological concepts and terminology (and can likely be skipped by those with significant background in the earth and life sciences), the bulk of the book is a detailed march through the stratigraphy, depositional settings, and paleontology of the Cambrian section of North America–with minor digressions to places like Sirius Passet of Greenland.

In general, the book has a density of information similar to that of an undergraduate survey course textbook, and so it would be difficult to summarize its contents in detail. Below find a few snippets of commentary relating to some of the major features/interesting or unusual highlights contained within.

Modocia typicalis, Marjum Formation, Cambrian Period, Millard County, Utah
The elegant Modocia typicalis, Marjum Formation, late Middle Cambrian Epoch, Millard County, Utah. This book describes in some detail the depositional setting and fossils of the Marjum Formation, including the cyclic nature of its deposition. Sediments of the Marjum formation, along with those of many famous trilobite-bearing formations, were deposited along the northern margin of tropical Laurentia as shown in Figure 1.4B of the book. Specimen is 2.0 cm long.

Early in the book, a summary of the Precambrian history of the world includes a lengthy discussion the Ediacaran Period, its unique life forms, and the unresolved question of whether or not these organisms were true animals or represent an unrelated radiation of multicellular life before the dawn of the Cambrian Period. Another early point of interest in the book is a discussion of recently discovered moss spores in the Bright Angel Shale (of Grand Canyon section fame) and the notion that Cambrian terrestrial ecosystems may have also included algae, slime molds, and lichens–very different from the traditional view of the early Paleozoic landscape as being essentially barren.

The heart of the book begins with the appearance of Treptichnus pedum, a circular (in part) trace fossil of world-wide distribution and its preservation of the first complex burrowing/feeding behavior that marks the official beginning of the Cambrian Period. At this point in earth history, though, trilobites are still twenty million years in the future. The sudden appearance, fully formed, of the the oldest trilobites in North America, those of the Fritzaspis Zone of the Montezuma Range in Esmeralda County, Nevada, has led to speculation about soft-bodied trilobites or trilobite-ancestors extending back into the Proterozoic Era and other hypotheses of trilobite origins–ideas discussed by Foster.

Bolaspidella housensis, Wheeler Shale, Cambrian Period, Millard county, Utah
Cluster of Bolaspidella housensis, Wheeler Formation, Middle Cambrian Epoch, Millard county, Utah. The Wheeler Formation has been interpreted is a deep water, outer detrital belt unit deposited in a series of cycles reflecting sea-level rises and falls. Slab is 5.0 cm long.

Chapter 4 begins with an interesting discussion of Early Cambrian reefs. These reefs, composed primarily of archaeocyathids and other exotic organisms likely unrelated to extant reef-forming organisms, will be unfamiliar to most readers. The placement of trilobites in this exotic paleoenvironment is attention-getting and unexpected. Figure 4.3, for example, shows a Cruziana trace within this, what will be for most, weird paleoecological setting.

This middle part of the book also contains a somewhat lengthy discussion of trilobite morphology, taxonomy, stratigraphic distribution, and paleoecology. In addition to the typical discussion of the dorsal exoskeleton, the book explains trilobite nervous, digestive, circulatory, reproductive, and respiratory systems. Interestingly, the book references research suggesting that the outer branch of the trilobite limb (the filamentous branch), which traditionally has been considered a gill, rather was used to push water across the ventral surface for the purpose of respiration.

An important feature of the last half of the book is a series of descriptions of significant Cambrian fossil localities. Among many examples are localities in the Marble Mountains of California, Ruin Wash in the Pioche Shale of Nevada, and the House Range in Utah.

One chapter is devoted entirely to the Middle Cambrian Burgess Shale of British Columbia, a topic familiar to most trilobite enthusiasts. A highlight of the Burgess Shale chapter is a discussion of the functional morphology of the seven species of anomalocarids that occur in the Mount Stephen Trilobite Beds–and the lack of evidence that anomalocarids were the makers of “bite marks” in the dorsal exoskeletons of trilobites. Foster also touches on the potential role of submarine brine seeps in the paleoecology and preservation of fossils in the Burgess deposits. This is a topic not covered in most works for a general audience. Lovely stipple drawings of Burgess animals by Matt Celesky (reminiscent of those by Marianne Collins in Gould’s Wonderful Life) grace this part of the work.

Glyphaspis capella(?), Wolsey Shale, Bear Tooth Lake, Montana
An early asaphid: Glyphaspis capella(?), Wolsey Shale, Bear Tooth Lake, Montana. Like many groups of invertebrates, asaphid trilobites begin their adaptive radiation in the Middle Cambrian. Asaphids are important by the Late Cambrian and a major component of many Ordovician communities. They disappear near the end of the Silurian Period. Specimen is 2.2 cm long.

The concluding sections of the book focus on some technical aspects of paleobiology such as taphonomy, paleoecology, the nature of the Cambrian explosion itself, and the biological legacy of the Cambrian Period. This part of the book will likely challenge readers without significant formal background in paleontology, but are worth slugging though for the committed.

All in all, Cambrian Ocean World, is a wonderful source of information for anyone interested in the Paleozoic Era, even if it is used primarily as a reference and not read cover to cover. Having not worked in the geosciences for many years, this book reminds me just how much work, knowledge, and imagination is involved in trying to understand the fossil record and the life of the past. Foster’s book is certainly not for the vast majority who seek to skate around on the surface of existence, but for those seeking a fuller understanding of life on our planet it offers much to contemplate and appreciate.

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

Seeking Early Trilobites

The beginning is the most important part of the work. –Plato

Olenellus clarki, Latham Shale, Cambrian Period, Cadiz, California
Olenellus clarki, Latham Shale, Early Cambrian Epoch, Cadiz, California. Specimen is 1.7 cm across the genal spines.

I remember a field trip to the Cretaceous of Montana when I was an undergraduate geology student. The professor instructed the class to prospect the uppermost part of the Hell Creek Formation: He was interested in finding dinosaur fossils as close as possible to the Z Coal, the boundary with the overlying Paleocene Tullock Formation, to see if dinosaurs disappeared before the K/T extinction event. Wanting to find fossils, I kept drifting lower in the section. He noticed and yelled and waved me higher in the section. I yelled in reply, “But there’s nothing up there!” He glared back.

I had the same problem in reverse during childhood. When prospecting in the Cambrian of southeast Minnesota I usually found nothing. Occasionally a lingulid brachiopod or an isolated trilobite free cheek or pygidium would turn up. Prospecting the Ordovician or Devonian was an entirely different matter, however. Some localities were bristling with fossils.

Of course, there are highly fossiliferous Cambrian localities, the famous Burgess Shale around Mount Stephen in British Columbia, for example. Or Ruin Wash, Nevada in the Pioche Shale. This deposit straddles the Lower/Upper Cambrian boundary and is loaded with fossils, mostly olenellid trilobites–which were on the way out by this time.

Olenellus gilberti, Pioche Shale, Cambrian Period, Ruin Wash, Nevada
Olenellus gilberti, Pioche Shale, Cambrian Period, Ruin Wash, Nevada. Specimen is 4.0 cm across the genal spines.

But in general, the diversity (number of taxa) and abundance of shelly invertebrate fossils increase as you move up into the Ordovician–which is why I was so puzzled when I first read Wonderful Life (1989) by Stephen Jay Gould. One thesis of this book was that animal disparity (morphological variation) peaked during the Cambrian Period. Most of the evidence for this proposition came from Burgess Shale animals that Gould portrayed as surpassingly strange. The author, with few exceptions, concentrated on “phylum-level disparity.” Class-level disparity, such as the difference between a bat and whale or a Great Auk and a hummingbird mattered not.

In this context, Gould was often obsessed with the number of appendages coming from one or another body sclerite (or the presence of exotic appendages) in clearly arthropod-like animals. In some cases, the number and placement of appendages did not conform to the situation in later groups. In Gould’s mind, this meant that these Cambrian creatures didn’t belong to the Arthropoda sensu strictu.

But isn’t this is because Arthropoda was initially defined without knowledge of these Cambrian forms, without knowledge of the disparity they displayed? I felt that had some phyla, Arthropoda included, been defined with a full anatomical knowledge of Burgess and other Cambrian forms, taxonomists surely would have decided that these “weird” Cambrian animals belonged within more broadly defined higher-order taxonomic groupings, such as a different Arthropoda that could encompass an Anomalocaris or Opabinia. 

No matter your opinion of what constitutes “diversity” or “disparity,” the Cambrian is a fun place to visit, either in the mind’s eye or the field. But . . . something is to be said for places like Jbel Issoumour or just about any outcrop in the Pennsylvanian of Kansas where the fossils literally crunch beneath your feet . . . .

Democephalus granulatus, Weeks Formation, Late Cambrian Epoch, Millard County, Utah
Democephalus granulatus, Weeks Formation, Late Cambrian Epoch, Millard County, Utah. The Late Cambrian Epoch is the high water mark of family-level trilobite diversity. Many other major invertebrate groups such as gastropods, bivalves, and brachiopods continue their evolutionary radiations into the Ordovician Period, however. Specimen is 2.4 cm long.

©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.

 

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.

Merry Christmas!

Christmas is the day that holds all time together. –Alexander Smith

Proasaphiscus rigidus, Cambrian Period, Krashoiarsk Region, Russia
Proasaphiscus rigidus, Cambrian Period, Krashoiarsk Region, Russia. Specimen is about 3.5 cm long.

To all my readers and friends, I wish you all a merry Christmas!

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

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.

The Riddle of Genal Spines

In many families the presence or absence of genal spines is of significance only at the generic level, and in some families (e.g., Olenidae) there are genera that span the range between having very long (Olenus), short (Leptoplastus) or no (Jujuyaspis) genal spines . . . . (Fortey and Owens, 1997, p. 255)

Metaptychopyge truncate, Ordovician Period, Russia
Valdaites limatus(?), Ordovician Period, Russia. This conservative-looking pseudoasaphid trilobite lacks genal spines. Klikushin et al. (2009) described this species as having genal angles with “short spines,” but the figured specimen in that reference, as this one, lacks them. Many close relatives, though, have conspicuous genal spines, some quite formidable. Specimen is 6.6 cm long.

Genal spines are conspicuous structures present in many trilobite species. But they are also absent in many species. Indeed, they may be present or absent in many otherwise similar-looking, closely-related forms. The relative size of genal spines within a species can also decrease as trilobites increase in size (Fortey and Owens, 1997). The shape and orientation of genal spines, too, can change as trilobites increase in size. What do these facts indicate about the function of these structures?

Ptychopyge truncata, Ordovician Period, Russia
Ptychopyge angustifrons, Middle Arenigian, Ordovician Period, Russia. This trilobite, otherwise superficially similar to V. limatus (above), has pronounced, medially beveled, spike-like genal spines. Can we therefore conclude that these trilobites had significant differences in lifestyle? Specimen is 5.5 cm long.

There is a tremendous variety of form in genal spines from small and blade-like, to chisel-like and beveled, to long, cylindrical, and needle-like. If genal spines served a single, straightforward function, wouldn’t natural selection have settled on an optimal morphology?

Of course, organisms need not be perfectly adapted, merely adequately adapted to life. See Mayr (1983) for warnings about the “adaptationist program,” the search for (sometimes) adaptive explanations for individual structures in isolation, without due consideration of how development constrains the fit of those structures into the overall body plans of organisms.

Given the presence or absence of genal spines, along with the tremendous variety of form, I think it unlikely that these structures served a single function across all trilobites.

Cedaria minor, Upper Cambrian, Weeks Formation, Utah
An early example of enrollment: Cedaria minor, Weeks Formation, “upper” Cambrian, Utah. Specimen is 1 cm across the genals.

One standard interpretation of the function of genal spines is that they protected the “zone of weakness” between the cephalon and pygidium from predators in enrolled trilobites. Case in point: the Cedaria minor specimen above. This interpretation is logical, but difficult to prove.

Another proposed function for genal spines was anchoring the exoskeleton to the substrate during molting. There is direct evidence for this view in the form of Isotelus genal spines found vertically oriented in Ordovician rocks in Ohio. Presence or absence of functional facial sutures, multiple modes of molting, and presence or absence of genal spines, may indicate significant variation in ease of molting. Perhaps some species were “easy molters” and didn’t need leverage (or facial sutures or genal spines) to pry themselves free from old exoskeletons.

Dicranurus hamatus elegantus, Early Devonian Epoch, Oklahoma
Dicranurus hamatus elegantus, Haragan Formation, Early Devonian Epoch, Oklahoma. Specimen approximately 7 cm long.

The spines, genal and otherwise, of Early Devonian monsters like Dicranurus (above) are often thought to have had a defensive function. The “horns” and other cephalic protuberances of some trilobites, on the other hand, have been recently interpreted as being secondary sexual characteristics with a function much like that of superficially similar structures in rhinoceros beetles (see Knell and Fortey, 2005).

My personal suspicion is that long, bristling spines likely functioned as sensory organs, probing the immediate environment for dangers or obstructions and determining the direction of incoming disturbances in the watery medium in which trilobites lived. Such sprawling genal and pleural spines would also have spread the weight of the animal out over a large surface area and allowed it to walk out over soft, water-saturated sediments.

Isotelus gigas, Cobourg Formation (Upper Ordovician), bowmanville, Ontario, Canada
Isotelus gigas, Cobourg Formation (Upper Ordovician Epoch), Bowmanville, Ontario, Canada. The genus Isotelus shows a huge range in variation of genal spines from absent (in I. latus), to long and needle-like (in small I. maximus). The above specimen of I. gigas has what I would term “incipient genal spines,” tiny blade-like nubs. Specimen is 10 cm long.
Enrolled Isotelus maximus, Ordovician Period, Ohio
Enrolled Isotelus maximus, Ordovician Period, Ohio. What was so different about the lifestyles of I. gigas and I. maximus that their genal spines were so disparate? One could certainly make the case that these formidable genal spines protected the zone of weakness between the cephalon and pygidium. This trilobite would have been about 8-9 cm long if stretched out. In larger I. maximus, the genal spines are relatively shorter, stouter, and more laterally directed.

In conclusion, it is logical to assume that genal spines had some sort of defensive function, at least in some trilobites. Anyone who has tried to pick up a spiny marine arthropod knows that the defensive thrashing of such animals can inflict damage to the hands, and presumably the mouth or tentacles of any potential attacker. On the other hand, I, as one who routinely observes birds easily ingesting spiny marine arthropods (blue crabs), doubt that spines would be much of a deterrent to a specialized predator. The great variety of form, and the presence/absence of spines across the the group also gives me pause. The defensive explanation can not be the whole story.

Other explanations, no matter how logical, will likely remain a matter for speculation. I like to imagine trilobites with sharp, flattened genal spines (perhaps like the Ptychopyge above) sitting with their cephalons on the sediment-water interface and genal spines lodged into the substrate for stability. In my mind’s eye, the rest of their bodies would have been slanted downward into a burrow. Mouths facing into a current, these trilobites would have been waiting for prey or detritus to be carried toward them. Dreaming up such “just-so  stories” is one of the many things about fossil collecting that make it such a charming and engaging pastime.

References

Fortey, Richard A. and Robert M. Owens. 1997. Evolutionary history. in Roger L. Kaesler (ed.), Treatise on Invertebrate Paleontology, O Arthropoda 1, Trilobita, Revised. Geological Society of America and University of Kansas Press, Lawrence, Kansas, pp. 249–287.

Klikushin, V, A. Evdokimov, and A. Pilipyuk. 2009. Ordovician Trilobites of the St. Petersburg Region, Russia. Griffon Enterprises Inc., St. Petersburg, Russian Federation. 541 p.

Knell, Robert J. and Richard A. Fortey. 2005. Trilobite spines and beetle horns: sexual selection in the Palaeozoic? Biology Letters 1 (2): 196-199.

Mayr, Ernst. 1983. How to carry out the adaptationist program? The American Naturalist 121 (3): 324-334.

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

Evidence of Predator Injuries to Trilobites: A New Article

Don’t become a mere recorder of facts, but try to penetrate the mystery of their origin. –Ivan Pavlov

Altiocculus harrisi with "bite mark," Wheeler Shale Formation, Cambrian Period,Millard County, Utah
Altiocculus harrisi with apparent “bite mark” on right pleural lobe, Wheeler Shale Formation, Cambrian Period, Millard County, Utah. Conventional wisdom is that anomalocarids caused this type of injury . . . but not so fast! Trilobite is 3.3 cm long.

Among the more fascinating aspects of trilobite paleoecology are considerations of predator-prey relationships. In this field much less is known than unknown, perhaps adding to its allure. In “Evidence of Predator Injuries to Trilobites,” I explore this subject. Enjoy! Thanks to reader and friend M.P. for pointing out that the article page was not loading.

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