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
I generally wade in blind and trust to fate and instinct to see me through. –Peter Straub
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?
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.
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.
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.
Very few species have survived unchanged. There’s one called lingula, which is a little shellfish, a little brachiopod about the size of my fingernail, that has survived for 500 million years, but it’s survived by being unobtrusive and doing nothing, and you can’t accuse human beings of that.–David Attenborough
Some may tend to think of trilobites as small animals, but in the context of their times a few species were large animals. This is because the largest animals of the Paleozoic generally were not giants by Recent standards. Some orthoconic cephalopods (e.g. Cameroceras) grew to perhaps 5 meters in length, and some fishes (Dunkleosteus, Titanichthyes) grew to similar sizes. But these were outliers, the vast majority of Paleozoic animals were very much smaller.
The largest known complete trilobite specimen, Isotelus rex from the Ordovician of the Canadian Arctic, is about 72 cm in length and dwarfs most Ordovician invertebrate species. Known only from fragmentary remains, Terataspis grandis from the Devonian of New York achieved similar, but likely slightly smaller, sizes. It’s important to note that because of plate tectonic processes what we know of the life of Paleozoic Era is confined to species that inhabited the epicontinental seas, not the open oceans. The sizes achieved by the denizens of those vast open waters remains completely unknown. Likely some creatures were large, perhaps very large. The largest animals of today, the baleen whales, are creatures of the ocean basins.
It’s notable that the relative size of trilobites compared with the largest creatures of the time also changed throughout the Paleozoic Era. During the Cambrian Period, for example, the largest trilobites were a significant fraction of the size of the largest known animals. The largest trilobites of that time approached half a meter in length, and the largest known mobile animals, like Anomalocaris, reached about a meter. Some sponges likely grew to well over a meter in height.
By the middle Paleozoic, the very largest known trilobites were over half a meter in length (Tetrataspis, Uralichas), and the largest predatory fishes were about ten times that long. But by the late Paleozoic the largest trilobites were very much smaller than the largest animals and probably tried to go about their business as unobtrusively as possible. By the time trilobites became extinct at the end of the Permian Period, the land and water teemed with monsters, and a really large trilobite was about 10 cm long . . . .
And why, since these be changed enow,
Should I change less than thou.–Elizabeth Barrett Browning, Change Upon Change
The Unicorn in association with heraldry is usually drawn as a horse with a single long twisted horn, lion’s tail and the legs of a stag. The Unicorn symbolizes extreme courage, strength and virtue.—clancunninghamglobal.com
Rostral protuberances are common in trilobites, but a handful of families (Raphiophoridae, Alsataspididae; Hapalopleuridae) of generally similar morphology contain members with a single, needle-like, forward-projecting glabellar spine. Many trilobites with this spine are blind or have greatly reduced eyes (the atheloptic condition), and are usually considered to have inhabited an offshore, deep water, low light, benthic paleoenvironment. Often, they occur in siliciclastic rocks.
Most trilobite spines are interpreted to have had some sort of defensive function. In the case of the unicorns, however, many think that the glabellar spine, in conjunction with the long genal spines, acted to spread the trilobite’s weight over a larger area thus allowing them to live at the surface of soft, soupy sediments, perhaps as filter feeders in a fashion similar to the trinucleids that were discussed in the last post.
In any case, unlike “real unicorns,” the trilobitic ones are quite common, and the trilobite enthusiast can easily assemble a nice little collection of them!
Congratulations, you have a sense of humor. And to those who didn’t: Go stick your head in the mud. –Jesse Ventura
Trilobites are thought to have pursued a variety of feeding strategies. Some may have been burrowing predators, and others are thought to have been scavengers or detritus feeders, perhaps wandering the bottom in search of whatever they could find. On the other hand, species with large cephalic chambers may have been filter feeders. A large number of specimens in the collection fall into this common general morphology, and just a few examples are shown here to illustrate.
In general, these likely filter feeders have large, broad cephalons, presumably to house a filtration apparatus. Also, they tend to have long genal spines, which in forms like some brachymetopids (e.g. Cordania) and harpetids (e.g. Aristoharpes) are deep and blade-like.
Filter feeding trilobites may have plowed head-first into the sediment, their massive cephalons balanced on genal spines. Beating legs may have either churned through sediments or generated currents that pushed stirred up detritus or small organisms into the filtration apparatus.
Restricted to Ordovician rocks, the trinucleids are perhaps the most specialized of the filter feeders and had pitted, bilaminar cephalic margins that acted like strainers. Pits may have allowed water to flow through the margin leaving food particles stranded behind. Specially adapted limbs may have swept these particles into the mouth, but this is speculative. This biomechanical interpretation is figured nicely in Gon (2003).
It’s fun to think of trilobites as wandering boldly around the Paleozoic sea-floor looking for prey, or perhaps carving out territories for mating or egg-laying purposes. In many cases, however, trilobites probably lived far less exciting lives than we imagine. Head-first into the mud, the filter feeders probably picked through the sediment as quietly and unobtrusively as they could.
Gon, Samuel M., III, 2003. A Pictorial Guide to the Orders of Trilobites. 88p.
The Ancients understood the omnipotence of the underside of things. ― Louis Pasteur
Perhaps the most interesting calcitic ventral structure in trilobites is the hypostome (or hypostoma). Although not completely understood, this exoskeletal element is usually interpreted as a mouthpart.
In the majority of trilobite species, specimens with the hypostome in life position are not known. There are several reasons for this. In some trilobites, the hypostome was attached to the animal by the ventral membrane only (the natant condition).
In some trilobites, the hypostome was fused to the rostral plate, a separate anterior element that functioned as part of the doublure, or the doublure itself. Sometimes a flexible(?) suture existed between the hypostome and the doublure. Sometimes a stalk existed between the hypostome and the rest of the exoskeleton. In these two latter cases, given the vagaries of preservation, it’s easy to understand why the hypostome is not often found in association with the rest of the exoskeleton.
Further, the hypostome was typically shed during molting along with the rest of the exoskeleton when it became just another particle in the sedimentary rock record.
Given the position of the hypsotome, it’s logical to suppose that it functioned in feeding. If this is the case, the wide variety of sizes, shapes, and manner of attachment to the dorsal exoskeleton likely means that trilobites exhibited a wide variety of specific feeding strategies. The details of these, of course, will likely never be known.
The not-infrequent discovery of a trilobite hypostome in the field is usually a happy moment. For even if articulated specimens remain elusive, the presence of these strange and mysterious little elements means that trilobites were around, and hope can remain for the discovery of the rest of the animal!
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!