Growth demands a temporary surrender of security. –Gail Sheehy
To grow, trilobites underwent ecdysis, or molting. This is because their calcitic exoskeletons were inelastic and could not accommodate growth. This is true of all members of the Superphylum Ecdysozoa, which includes arthropods, onychophorans (velvet worms like Peripatus), nematodes (round worms), and others.
Paleontologists break the development of trilobites into three stages: protaspid, meraspid, and holaspid. The protaspid stage is the earliest in development, during which the trilobite exoskeleton consisted of a single, tiny (often sub-millimeter) segment only. Once the exoskeleton exhibited two or more segments, the trilobite had entered the meraspid stage. During this stage, the pygidium “released” segments, one or a few at a time, into the thorax. Once the complete, adult number of thoracic segments existed within the thorax the trilobite had entered the holaspid stage. In contrast to growth during protaspid and meraspid stages, molting in the holapsid stage only accommodated an increase in size, not significant changes in appearance. All trilobites illustrated in this article are holaspid molts.
Molting (and shortly thereafter) is typically a time of vulnerability for arthropods because of physical incapacitation (function of some muscles is dependent upon attachment to a rigid exoskeleton) (Chatterton and Speyer, 1997) and a temporary lack of a hard, biomineralized coat of armor that protects against predators and the physical environment.
Field observations indicate that the great majority of trilobite body fossils occur as disarticulated or partly disarticulated exoskeletal sclerites. This is because each individual trilobite molted many times during its lifetime. Chatterton and Speyer (1997) estimated a range of eight to over thirty molts across the Trilobita. Also, once a trilobite died, scavengers and bioturbators (burrowing organisms like “worms” that churn sediments) usually saw to it that its exoskeletal elements were disarticulated and scattered. Add to this waves, tides, and currents, and it’s likely that preservation of whole, articulated trilobite specimens was a rare event. Perhaps equally rare are specimens that show molted sclerites in close association that give clues as to the details of the process of molting.
Many trilobites needed to disarticulate their cephalons along facial sutures (zones of weakness) in order to crawl free of their old exoskeletons. Some trilobites, like the phacopids above, were able to get free without disarticulating their cephalons, perhaps by rotating the cephalon backwards (and abandoning it upside-down on the sea floor) and crawling forward and free of the old exoskeleton. This is referred to as the “phacopid mode of ecdysis.” Some phacopids may have even had fused facial sutures. See Whittington (1997) for a summary of interpretations of how trilobites maneuvered out of old exoskeletons during exuviation (molting).
In addition to phacopids, I am suspicious that other trilobites that often occur as complete cephalons, but otherwise disarticulated from the rest of the body (like Ameura, above), were likewise able to molt without disarticulation of the cephalon.
The Wanneria specimen below is a molt that indicates that this species, too, was able to molt without disarticulating its cephalon. Interestingly, the genal spines may be anchored into the substrate, perhaps to get leverage to pull the animal from its old exoskeleton (see disclaimer below). I have been told by several trilobite collectors who frequent the Ordovician of Ohio that Isotelus genal spines are sometimes found vertically oriented in the rock, pointed end down. This could be for the same purpose, but with the usual caveat about bioturbation moving sclerites around.
One of the more interesting aspects of trilobite ecdysis are the mass occurrences of trilobite molts, like the Xenasaphus, Ellipsocephalus, and Eldredgeops specimens above. Do these occurrences mean that trilobites aggregated to molt, or are these simply hydraulic accumulations?
Visual inspection of the complete Xenasaphus slab of which the image above covers only a small part suggests to me that there may be a subtle orientation of long axes in one direction and sub-perpendicular to it–perhaps indicating some sort of wave action. A large orthoconic cephalopod shell is aligned with one of those directions. This pattern of orientation may be an artifact of specimens being so closely packed, though. See Figure 140 in Klikushin et al. (2009) for a similar visual impression.
Not an absolute, but a rule of thumb for carbonate facies, is that particles tend to be produced in place, and not transported far. I can well envision masses of Xenasaphus trilobites, gathering in response to some external stimulus, molting along a gently wave-buffeted shallow. The fact that masses of trilobite molts occur throughout the world, throughout the Paleozoic Era, and across many trilobite taxa suggest that these mass associations reflect a basic aspect of trilobite paleobiology and not sedimentology alone.
Finally, perhaps finding them too messy or unaesthetic, many collectors are not particularly interested in molts or partial trilobite specimens. Although I appreciate a perfect complete specimen as much as the next person, I tend to also be interested in specimens that clearly illustrate some aspect of trilobite paleobiology. Considerations of molting and other behavior really helps to bring trilobites—and the hobby of trilobite collecting to life.
Chatterton, Brian D. E., and Stephen E. Speyer. 1997. Ontogeny. 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, pp. 173-247.
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.
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.
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