Cryptic Strategies in Trilobites

Facts which at first seem improbable will, even on scant explanation, drop the cloak which has hidden them and stand forth in naked and simple beauty. –Galileo Galilei

Enrolled Flexicalymene meek inside bivalve shell, Arnheim Formation, Clinton County, Ohio
Enrolled Flexicalymene meeki inside bivalve shell, Arnheim Formation (Ordovician Period), Clinton County, Ohio. Did this trilobite seek a hidden place to molt, lay eggs or simply to evade predators or stormy weather? We’ll never know for certain. The trilobite is about 2.2 cm wide across the genals.

In response to the threat of predation and other challenges, trilobites seem to have developed a number of cryptic survival strategies. For example, there is evidence that trilobites sought hidden, protected places in which to molt, a time of vulnerability due to physical incapacitation and exoskeletal softness. Burrowing behavior is known or suspected for some groups, such as phacopids (below), perhaps not only in a quest for prey, but as a place to hide while molting, during routine threats from predators, or the destructive power of mother nature’s tide’s and gales. Cryptic textures (and colors) and encrustation by epibionts likely served as camouflage from predators, too.

Complete, articulated Paciphacops specimens with Huntonia pygidium on presumed paleosurface, Devonian, Oklahoma
Complete, articulated Paciphacops campbelli specimens (partly enrolled), Kainops raymondi (center, inclined to upper surface of slab) with Huntonia sp. pygidium on presumed paleosurface, Haragan Formation, Devonian Period, Coal County, Oklahoma. Undeniable burrows are identified when the rock filling the burrow is lithologically different than the substrate. That is not the case here, however. Burrowing behavior for these phacopid trilobites is inferred, however, from orientation of specimens: all three show their cephalon placed roughly parallel to a likely bedding surface containing a Huntonia pygidium. Hansen (2009) figured specimens in similar orientations interpreted as displaying burrowing behavior. Largest trilobite is 3.7 cm long.
The stalk-eyed trilobite Asaphus kowalewskii from the Ordovician of Russia.
The stalk-eyed trilobite Asaphus kowalewskii from the Ordovician of Russia. It’s easy to imagine this trilobite sitting buried in the sediment with only its eyes protruding, ever watchful for predators—or perhaps like a ghost crab, peering cautiously out of a burrow to make sure the coast is clear! This specimen is a small example of the species: the eyestalks are about 1 cm tall.
An Atlantic Ghost Crab (Ocypode quadrate) sits outside its burrow. East Beach, Galveston Island, Texas
An Atlantic ghost crab (Ocypode quadrata)  sits outside its burrow. East Beach, Galveston Island, Texas. This crab is about 5 cm across.

Because trilobites were such an evolutionarily long-lived group, the cast of potential predators changed throughout their history. Early in the Paleozoic Era (Cambrian Period) anomalocarids, other arthropods (e.g., Sidneyia), and predatory infaunal worms like priapulids were likely the greatest predatory challenges. See Conway Morris (1998) for descriptions of predator behaviors in Cambrian paleoenvironments.

By Ordovician times, cephalopods were likely a leading menace that persisted throughout the remainder of the Paleozoic Era. From the middle to the close of the Paleozoic Era, gnathostomes (jawed vertebrates) with durophagous crushing dentitions like some placoderms (e.g., ptyctodontids), chrondricthyans (holocephalians; hybodontoids), or osteichthyans (dipnoans) were present in many marine paleoenvironments that trilobites called home (see Brett and Walker, 2002 for a summary). A number of types of predator-inflicted injuries to trilobite exoskeletons are known, and these are discussed in “Evidence of Predator Injuries to Trilobites.”

Proetid molt inside Permian bivalve, Russia
Phillipsid Molt Inside Permian Bivalve, Russia. The presence of only disarticulated trilobite sclerites (with no other macroinvertebrate remains) inside this bivalve steinkern indicate that this phillipsid trilobite likely molted inside. Steinkern is 11 cm at its widest.
Ameura missouriensis cephalon inside mylinid steinkern, Pennsylvanian Period, Wyandotte County, Kansas
Ameura missouriensis cephalon inside myalinid steinkern, Pennsylvanian Period, Wyandotte County, Kansas. This trilobite may have molted inside this bivalve shell. Interestingly, Ameura specimens are often isolated cephalons (with free cheeks attached) as shown above. This may mean that Ameura was capable of molting without disarticulating its cephalon along the facial sutures, like some phacopids. Steinkern is about 8.5 cm at its widest.

Examination of the interior spaces of large mollusk shells is a highly recommended field method for the trilobite enthusiast even if the the collector has no interest in the mollusk itself. Such examination sometimes reveals a variety of taphonomically different fossil assemblages containing trilobite remains. Complete articulated trilobite specimens do occur, such as the Flexicalymene specimen above. See Meyer and Davis (2009) for additional figured examples. Clusters of disarticulated, likely molted sclerites in the absence of other invertebrate remains occur as well. In both these cases, it’s clear what happened: trilobites sought sheltered places to molt or simply to hide. Sometimes the story is not so clear, however.

Living chamber of Silurian nautiloid containing assemblage of invertebrates, Henryhouse Formation (Silurian), Arbuckle Mountains, Oklahoma
Living chamber of Silurian nautiloid containing assemblage of invertebrates, Henryhouse Formation (Silurian Period), Arbuckle Mountains, Oklahoma. Cheirus infensus (pygidium, cephala) and Scotoharpes sp. (cephalon). Fenestrate, crustose, and ramose bryozoans and a brachiopod are also present. Living chamber about 7.5 cm across.

In the case of the Silurian nautiloid living chamber containing an assemblage of taxonomically diverse invertebrate remains shown above, one could come to the conclusion that we are seeing skeletal remains that were transported into the cavity, the remains of organisms that were living there, or a mixture. In this example, a case can be made that the trilobites were molting in the living chamber, rather than being washed inside. Specifically, both trilobite species are quite rare and the sclerites are well preserved with spines and other surface features crisp and intact and showing no evidence of transport. Other interpretations are possible, however.

More often than the cases described above one sees large mollusk shells, especially cephalopods, in which the living spaces contain fragmentary and abraded invertebrate debris. In the Ordovician of Ohio, for example it is common to find bits and pieces of Isotelus mixed with brachiopod and bryozoan debris. These accumulations are clearly hydraulic.

Nautiloid living chamber filled with invertebrate skeletal debris, Ordovician Period, Ohio
Nautiloid living chamber filled with invertebrate skeletal debris, mostly bryozoan and brachiopod fragments, Ordovician Period, Ohio. The dark-colored fragments are scraps of Isotelus. Living chamber about 5 cm across at its widest.

A variety of fouling epibionts such as bryozoans, brachiopods, cornulitids, and corals are known to encrust a wide variety of species of trilobite exoskeletons. Such encrustations would be a logical natural source of camouflage, and extant arthropods are commonly covered in epibionts. The extent to which trilobites willingly submitted themselves to epizoan colonization for purposes of camouflage must, of course, remain a mystery. Perhaps in some (or all?) cases the trilobites may simply not have lacked mechanisms to remove epizoans, except molting.

Proving that a particular example of encrustation occurred during the life of the trilobite and not postmortem is not so easy, though. It is makes sense that dorsal surfaces of complete, articulated specimens near the maximum species size (presumably when molting was less frequent) bearing epizoans were colonized in life.

One would expect that scavenging or bioturbation would quickly disarticulate exoskeletons. On the other hand, submarine cementation, particularly in carbonate environments can be extremely rapid and would quickly incorporate even articulated trilobite exoskeletons into a stable substrate suitable for colonization by encrusting organisms.

Isotelus mafrtizae with Stigmatella bryozoan epibiont, Cobourg Formation, Upper Ordovician, Port Hope, Ontario
Isotelus mafritzae with Stigmatella bryozoan epizoan, Cobourg Formation, Upper Ordovician, Port Hope, Ontario. If attachment occurred in life, such an epizoan could clearly help camouflage a trilobite such as this. At some point, though, encrustation must have become a hindrance, even a hazard. Note that if this bryozoan was attached in life it would have blocked the trilobite’s view. It also would have been an encumbrance if the trilobite tried to slip through the sediment. This trilobite is 7 cm long.
Flexicalymene sp. with bryozoan growing primarily on elevated parts of axial lobe. Corryville Member, Grant Lake Formation, Ohio
Flexicalymene sp. with bryozoan growing primarily on elevated parts of axial lobe (rings). Corryville Member, Grant Lake Formation, Ohio. Minor bryozoan colonization has also occurred on pleural lobes as well. Specimen is about 3 cm across the genals.

Some authors have made a case for Flexicalymene being fouled by bryozoans in life based on detailed examination of placement of epizoans on significant numbers of trilobite specimens. This trilobite has been interpreted as having a “ploughing” lifestyle in which bryozoans preferentially attached to the most dorsal parts of exoskeleton, such as thoracic axial rings, as it pushed through sediments. Bryozoans, as filter-feeders, may have benefitted from the motion of the trilobite through the water (Brandt, 1996; Key et al., 2010).

Encrinuroides capitis, Ordovician Period, Oklahoma
“Encrinuroides” capitis, Ordovician Period, Oklahoma. Although this trilobite appears to bear dark stripes and dots, this coloration is likely the result of staining by secondary iron-bearing minerals. Specimen is about 1.5 cm long.

Cryptic coloration likely existed in trilobites just as it does in extant organisms. Preservation of original color patterns does exist in trilobites, but is rare. Detailed examination of many putative cases of preservation of original color patterns often leads to the conclusion that patterns are secondary, not original. Schoenemann et al. (2014) reviewed some cases for preservation of original color in a variety of trilobite taxa.

In addition to color, exoskeletal texture would seem to also be a logical vehicle for trilobites to use in order to conceal themselves. It’s easy to imagine trilobites sending ripples down their bodies to dust themselves with sediments the way stingrays and flounders do today.

Phacops speculator, Devonian, Morocco
“Phacops speculator”, Devonian Period, Morocco. Could the pebbly surface texture have provided camouflage in a sandy paleoenvironment? Trilobite about 2.8 cm in diameter.

Brezinski (1988) suggested that some Mississippian trilobites with coarse tubercles on their exoskeletons blended with the coarse sediments of their habitat. Rough and “pebbly,” pustulose, or “warty” surface “ornamentation” is common across many groups of trilobites. Detailed examination of some of these features indicates that rather than being simply bumps on the surface of the exoskeleton, these features are actually spine bases.

Some trilobites, like Haragan phacopids (see Hansen, 2009), then, were covered in a coat of extremely fine, hair-like spines. These could have functioned as sensory organs, a mechanism to trap fine particles for camouflage, or simply to make themselves an unappetizing asp-like mouthful for predators—or perhaps all these things.

Finally, such paleoecological considerations of trilobites as these can help us flesh them out as once-living creatures that lived in a world every bit as rich and interesting as our own. The collector of trilobites, may appreciate them aesthetically as collectibles, as carriers of a kind of sympathetic magic that puts their owner in touch with a bygone period in earth’s history, or as creatures that were well-adapted to the many challenges of their times.

Eothinites akastensis, Lower Permian, Aktjubinsk Region, South Ural Mountains, Russia
Eothinites akastensis, Lower Permian, Aktjubinsk Region, South Ural Mountains, Russia. Just as squids and octopuses prey on crustaceans today, ammonites such as this were likely predators of trilobites and other marine arthropods. Specimen is 7.5 cm across at its widest.

References

Brandt, D.S. 1996. Epizoans on Flexicalymene (Trilobita) and implications for trilobite paleoecology. Journal of Paleontology 70 (3): 442-449.

Brett, Carlton E., and Walker, Sally E. 2002. Predators and predation in Paleozoic marine environments. Paleontological Society Papers 8: 93-118.

Brezinski, David K. 1988. Trilobites of the Gilmore City Limestone (Mississippian) of Iowa. Journal of Paleontology 62 (2): 241-245.

Conway Morris, Simon. 1998. The Crucible of Creation: The Burgess Shale and the rise of animals. Oxford University Press. 242p.

Hansen, George P. 2009. Trilobites of Black Cat Mountain. iUniverse, Inc., New York. 404 p.

Key, Marcus M. Jr., Gregory A. Schumacher, Loren E. Babcock, Robert C. Frey, William D. Heimbrock, Stephen H. Felton, Dan L. Cooper, Walter B. Gibson, Debbie G. Scheid, and Sylvester A. Schumacher. 2010. Paleoecology of Commensal Epizoans Fouling Flexicalymene (Trilobita) from the Upper Ordovician Cincinnati  Arch Region USA. Journal of Paleontology 84 (6): 1121-1134.

Meyer, David L., and Davis, Richard A. 2009. A Sea Without Fish: Life in the Ordovician Sea of the Cincinnati Region. Indiana University Press. 346p.

Schoeneman, Brigitte, Euan N.K. Clarkson, and Ewe Ryck. 2014. Color Patterns in Devonian Trilobites. The Open Geology Journal 8: 113-117.

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