Trilobite Seas: On Hiatus

Dear Readers:

Hurricane Harvey destroyed our house in late August, and we’ve been trying to get our lives back on track ever since. I don’t know when, if ever, I’ll be able to get Trilobite Seas up and running again. All my specimens (and much of my equipment) are in storage, and we’re having to deal with so many other issues it will likely be many months before I am back to anything resembling normal. Cheers, Chris

 

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.

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.

Homalonotids: Large Weird Trilobites

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

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

Homalonotids are well-known fossils of Silurian and Devonian age from around the world. Despite occurring in deposits alongside other more typical-looking trilobites, they have a number of unusual features.

Many specimens show “indistinct trilobation,” giving them a streamlined torpedo-like appearance. They generally lack spines, although some “Burmeisteria armatus” (aka “Elvis”), widely faked and composited specimens from Morocco, apparently have short, stout spines. Such streamlining (in all but Elvis) could be used to make a case for a burrowing lifestyle.

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

What gives pause to the notion of burrowing, however, is the pitted orange-peel texture exhibited by some species. There are a variety of types of pores and canals, often associated with bumps or pustules, that perforate the exoskeletons of trilobites. Interpretations of the functions of these structures vary and include openings for the diffusion of oxygen, a chemosensory function, secretion, and most often setae (hair-like filaments or bristles) that could have had a protective or sensory function.

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

Presence of bristles over the surface of the body would seem to be at crossed purposes with a burrowing lifestyle where smoothness would most helpful. Perhaps the pores of such animals as Dipleura just allowed easier diffusion of oxygen through the shell to the gills below and are unrelated to setae. Maybe a secreted slime layer flowed through the pores and allowed easy movement through a gritty substrate. Or perhaps they were for setae–but allowed a buried, often immobile, animal to sense prey or predators in the surrounding sediment. We will likely never know.

In the seascape of my imagination, though, homalonotid trilobites like Dipleura were covered in hairs like giant asp caterpillars wandering the seabed. Perhaps, like asps, these trilobites, too, were venomous–offering up the most unpleasant possible mouthful for any passing monster cephalopod or placoderm.

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

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

The Uniquely Spiny Thysanopeltis

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

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

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

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

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

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

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

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

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

The “Horns” of Dicranurus

From the exterior face of the wall towers must be projected, from which an approaching enemy may be annoyed by weapons, from the embrasures of those towers, right and left. –Vitruvius

Dicranurus hamatus elegantus, Devonian Period, Oklahoma
Dicranurus hamatus elegantus, Haragan Formation, Early Devonian Epoch, Oklahoma. Specimen approximately 7 cm long. Specimen prepared by Maestro Bob Carroll.

Among the spiny trilobite monsters of the Devonian Period, Dicranurus stands out as one of the most spectacular “horned” forms. Emlen (2005) blithely considered the horns of this trilobite (as well as a variety of spines and exoskeletal projections in other trilobite taxa) as “weapons,” likely used by males in infraspecific combat. A more cautious discussion of the evidence and reasoning used to draw this type of conclusion (but in the case of raphiorids) can be found in Knell and Fortey (2005).

I find the interpretation of the horns of Dicranurus as analogous to the horns of ungulates or even horned beetles to be unconvincing. The notion that animals covered in fine, delicate, and easily breakable spines would purposely engage in pushing, shoving, or wrestling matches seems unlikely. Further, the horns of Dicranurus are simply an extreme example within odontopleurids. Ceratonurus and Miraspis, for example, both have similar, although more gracile horns.

These other horned odontopleurids, however, also have stalked eyes anterior to the horns. This would seem to inevitably lead to losing an eye or two if the horns were used to attack each other! Use of horns as weapons in stalk-eyed forms would seem even less likely than in Dicranurus, and the idea that the horns in Dicranurus had a function different from that in other horned trilobites stretches credulity further.

I tend to be of the opinion that the spines in the spiniest Devonian trilobites played a role in gathering sensory information about the environment. As they crawled through their reefy habitats the spines would have mapped out a corridor of clear navigation. If they encountered a soft-bodied predator, it would  be delivered an unpleasant poke. The curling around of the rams-horns of Dicranurus may simply be an adaption to crawling around in patches of habitat with lots of overhangs, such as branching bryozoans or corals.

For those of us willing to entertain non-adaptationist interpretations, the possibility exists that the extreme horns of Dicranurus and others served no particular function in and of themselves. The gene(s) responsible for horn development may have been linked to other genes that did have adaptive significance, perhaps spininess in general.

Until sexual dimorphism is clearly demonstrated in these trilobites, and evidence is found of battles (one trilobite’s spine lodged in another or two specimens entangled in each other’s horns), I remain a skeptic of the spines and horns as weapons concept.

Dicranurus mostrosus, Devonian Period, Morocco
Dicranurus monstrosus, Devonian Period, Morocco. Did the horns curl under simply to avoid entanglements from above? Trilobite is approximately 5.5 cm across genals.

References

Emlen, Douglas J. 2008. The Evolution of Animal Weapons. The Annual Review of Ecology, Evolution, and systematics 39: 387-413.

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

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

Crusher Teeth, Spines, and the Fall of Trilobites

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Agnostida: Tiny, Blind (Mostly) Trilobites

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

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

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

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

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

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

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

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

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

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

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

Trilobite Multiples

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

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

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

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

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

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

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

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

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

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

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