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?
Fear has many eyes and can see things underground. –Miguel de Cervantes
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.
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.
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.
Always remember that you are absolutely unique. Just like everyone else. –Margaret Mead
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.
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.
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.
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
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.
Emlen, Douglas J. 2008. The Evolution of Animal Weapons. The Annual Review of Ecology, Evolution, and systematics39: 387-413.
Knell, Robert J., and Fortey, Richard A. 2005. Trilobite spines and beetle horns: sexual selection in the Palaeozoic? Biology Letters1 (2): 196-199
If you have food in your jaws you have solved all questions for the time being. —Franz Kafka, Investigations of a Dog
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.
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
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.
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!
We do not see things as they are, we see things as we are.—Vikas Runwal
One of the most persistent things you notice when examining a large collection of different species of phacopids is that the schizochroal eyes are usually tilted downward by 30-40 degrees. This is true for phacopids of every description. Large species covered in pustules like Drotops (above) or much smaller smooth forms like Crythops (below)–and every morphology in between–show this feature.
Such an orientation would seem to waste part of the field of view given that it means the anterior third or so of the lenses looked down into the mud if the animal was walking level across the bottom. Likewise, the posterior one-third of lenses would be looking up into the water column behind the animal as it moved across the sea floor.
It’s possible to imagine a functional significance for such an orientation of the eyes with the animal extended on the sea floor, though. Perhaps these trilobites were keeping an eye out for nasty piscine or cephalopod predators that came swimming down from behind and above. Perhaps they were simultaneously inspecting the sea floor at the ten- and two-o’clock positions for prey or detritus.
Much more likely, in my view, is that these were infaunal or semi-infaunal animals, and perhaps spent a significant amount of time with their heads poking out of the sediment. Phacopids in just the position one would expect for an animal peering out of a burrow are known from the Devonian of Oklahoma. In this posture, the orientation of the eyes makes sense: The sea floor could be surveyed with maximum efficiency, the lenses probing the widest possible scene.
Trilobites are known burrowers, and they are also known to have exhibited cryptic behaviors. Cruziana, the trace fossil associated with trilobitic burrowing, is a common fossil. In some deposits, such as the those at the famous Ichnological Park at Penha Garcia (Lower to Middle Ordovician), Portugal, Cruziana specimens compose the rock.
Definitive examples of trilobites preserved in burrows are rare, however, and limited to a hand-full of examples. The taphonomic requirements for the preservation of trace fossils and body fossils are different. Although it has happened, finding an example of a trilobite that died in its tracks in a burrow is highly unlikely. All we can do is keep looking!
Write it on your heart that every day is the best day in the year. –Ralph Waldo Emerson
For the past few years I’ve made it a point to reflect upon the year’s events during winter breaks. This is a time to plan for the future and consider mistakes made and lessons learned. As always, I hope the new year is a mix of the old and the new. I resolve to hold on to the positive aspects of the past, reject the negative and dysfunctional, and continue looking for new ways of thinking and doing.
Last year, the persistent El Niño weather gave us fits, nearly flooding our house repeatedly again and spoiling many hopes for field work. Although the weather has been (and continues to be) terrible this winter, there is hope for the new year as the El Niño pattern has fallen apart. This change may be a big improvement and could lead to more productive time in the field. I resolve to make better use of good weather (and really, any opportunity).
Continued work on birds and trilobites will mean working on something interesting every day. It’s always tempting to push life further down the road. I resolve to continue working on these interests with renewed energy, but be more in the moment. I resolve not to have just another look-alike year.
Finally, I resolve to reach out to old friends more, and perhaps rekindle some connections and mutual interests that will make 2017 a year to remember . . .
And to all my readers and friends, I wish you a happy and productive new year!
If dimorphism in the exoskeleton of a species of trilobites does occur, it has not yet been satisfactorily demonstrated. If it is shown, the decision on whether to regard it as evidence of sexual dimorphism or to rank it as a subspecific or specific difference will be arbitrary. Whittington (1997, p. 161-162)
Because some trilobites are common, collectors might tend to overlook them and not consider how strange and interesting they really are. The trilobite Paralejurus is a case in point. For those willing to take a second look, there are several forms or morphotypes of these odd, likely burrowing thysanopeltid (or styginid or scutelluid depending on author) trilobites that are readily available commercially from Morocco. This variation brings up a number of questions.
Paralejurus hamlagdadicus (“Type A”) specimens show the same kind of dull, blackish-brown exoskeleton and occur in dense, dark gray carbonate with dusty orange oxide staining on weathered surfaces, irrespective of alleged provenance. These specimens are more interesting to me than the typically encountered, smaller “Type B” specimens because they show two distinct morphologies, shown above and below.
The significance of difference in pygidial shape in these animals is, of course, a matter for pure speculation. It’s easy to imagine that tail shape is a sexually dimorphic feature and relates to mating or egg laying (or scooping out holes to lay eggs in?). Making a compelling case for this, however, would be of monumental difficulty given that it would involve collecting a statistically significant sample from precisely the same horizon in a hostile land. Even if this were done, some would, no doubt, simply assert that the differences were taxonomic (subspecific?) rather than sexually dimorphic (see quotation at opening of post).
No matter one’s opinion of the meaning of “Type A” forms, “Type B” specimens are clearly a different animal. Much smaller and bearing stout, blade-like genal spines, these trilobites exhibit spectacular (but enigmatic) terrace line prosopon. Further, although they occur in a dense, dark gray carbonate, the preserved exoskeleton is jet black and glossy—aggravatingly so from a photographic standpoint.
Not being an expert on the taxonomy of these animals, it’s hard enough to know exactly what to call each of these Paralejurus morphotypes—let alone what they mean. Experts and enthusiasts seem to have called them by a variety of names over the years, and the taxon containing them (Bronteus, Scutellum . . . . ) has been subject to significant taxonomic revision since the discovery and description of these trilobites in the mid-19th Century.
Yet another case that encourages the trilobite enthusiast to sit back and enjoy their specimens . . . while getting comfortable with the idea of not knowing exactly what they hold in their hands.
Whittington, H. B. 1997. Mode of life, habits, and occurrence. 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, 137-169.