Change of suit
How trilobites grew
During their lifetime, trilobites not only went through three different stages of distinctive morphologies (protaspid, meraspid and holaspid stages) but, with age, also grew considerably in size. Just as modern crustaceans with their chitinous carapace need to shed their old armours every once in a while to allow their bodies to develop in size and shape, our Palaeozoic friends were forced to periodically dispose of their rigid calcite exoskeleton and construct an entirely new and bigger one. In order to allow such a “change of suit” the trilobite’s head shield was equipped with a series of predetermined breaking points, the so-called facial sutures.
Once the old carapace became too tight for the animal to proceed in its bodily development it was time to moult. The scientific name for this process is ‘ecdysis’. Just like today’s crustaceans, the animal would try to find a protected place in which it could shed its armour without facing too big a risk of its temporary vulnerability being exploited by enemy predators. Specific hormones seem to have played a major part in jump-starting the process. The facial sutures, which have been classified into three different types (proparian, gonatoparian and opisthoparian - defined by where the sutures end, relative to the genal angle of the cephalic shield) started to break open. The following line drawing portrays the opisthoparian sutures in the trilobite genus Xystridura, the suture type being determined by the fact that its end is clearly positioned along the hind cephalic margin. In a proparian type, the suture would end forward of the genal angel while in a gonatoparian suture the end would be positioned right at the genal angle of the cephalic shield (see image to the right)..
The reconditioning of the trilobite exoskeleton after moulting seems to have been a rather accelerated process. As we already noted in another chapter on this website: For all arthropods shedding their armour is a time of extreme vulnerability. Some scientists assume that the trilobite secreted a thin prismatic layer first, with a very thin primary layer underneath, in order to build a first line of defence. As secretion proceeded, the primary layer became thicker and thicker (MILLER & CLARKSON, 1980). This seems to indicate that the prismatic outer layer might have been the more easy part to build. More information on the structure and composition of the trilobite carapace can be found on our morphology page.
Apart from being a reasonable protection against drilling organisms, it must have been very resilient against pressure applied in a vertical way. On the other hand it may have been vulnerable to shearing forces and distortions. Resulting cracks would have run all through the prismatic layer until stopped by the underlying primary layer. It therefore appears imperative that with every reconditioning of the exoskeleton after moulting, both the prismatic and the primary layer had to be rebuilt simulatenously. The “anti-crack” function of the primary layer is usually preserved even in the fossilized animal – every preparator has seen parts of the outer layer disintegrate under an unfortunate hit whilst the primary stayed unimpressed and intact.
Disintegration of the cephalic shield in the typical Redlichioid trilobite Xystridura during moulting (ventral view). The shield breaks up into cranidium, free cheeks (librigenae), rostral plate (pink) and hypostome. The latter may have stayed attached to the rostral plate in this genus. Line drawing courtesy of Dr. Sam Gon , © 1999, 2000 by S. M. Gon III
As shown in Sam’s superb line drawing above, the cephalic shield disintegrates into a whole range of individual parts, namely a pair of free cheeks or librigenae (actually, that is why they are called “free cheeks” in the first place) and the cranidium (comprised of the glabella and fixed cheeks) as well as the ventrally positioned rostral plate and hypostome. It is evident that the surface of the visual system was renewed like the rest of the carapace, allowing the trilobite to recuperate from any previously impaired eyesight (development of a new cornea). Depending on the running of the sutures, the visual surface stayed with the free cheeks or remained with the cranidium. The following two images are good examples of disintegrating cephalic shields in some common trilobites.
We cannot be absolutely sure that these particular fossils actually represent disposed of shields. However, they show the different runnings of the facial sutures and the integration of the visual surface in the process of moulting.
left: Xenaspahus (Delphasaphus) delphinus (LAWROW, 1856), Ordovician, Wolchow River, St. Petersburg: In this specimen the right free cheek has completely detached, taking along the eye’s visual surface. The running of this suture classifies as opisthoparian as its end is clearly positioned along the hind cephalic margin.
right: Flexicalymene retrorsa (FOERSTE, 1910), Ordovician, Arnheim Formation, Cincinnati, Ohio: The facial sutures are easy to recognize.Both free cheeks have begun to separate from the cranidium. The running of the suture classifies as gonatoparian as it ends right at the genal angle.
Once the cephalic shield had disintegrated, the trilobite used this “exit” to completely dispose of its old carapace by squeezing its entire body, including its legs, forward through the opening. If the animal was equipped with genal and/or pygidial spines, it may have used the latter to support this arduous effort by planting them into the sea floor and likewise anchoring downward the anterior part of the cephalic shield. Dr. Sam Gon has created a simplified but nevertheless very vivid animation of this process, using an early Redlichiid trilobite as an example. Of course, there may be other explanations for the existence of spines in general and genal spines in particular, apart from their potential use of assisting in moulting (see this subpage).
Even if a spiny morphology was conducive to the process of moulting, common trilobites like Phacops and Asaphus would not have been able to make any use of it as these genera lacked spines in the first place. In addition their facial sutures were inseparably fused. Therefore they must have found other ways to shed their carapaces and used other routines of movement to support their escape from the obsolete exoskeleton. The 1959 Treatise contains relevant sketches of how the popular Phacops might have exited from its old shell. (Salterian Mode).
After shedding its old exoskeleton the trilobite most likely used to hide away in its retreat until the new carapace had achieved an adequate state of hardness by absorbing a sufficient amount of calcium carbonate from the surrounding sea water (mineralisation).
Animation courtesy of Dr. Sam Gon
© 1999, 2000 by S. M. Gon III
Adapted from Whittington, 1990 via the 1997 Treatise (p. 156-7)
It would appear that trilobites had to go through the process of moulting quite frequently. Chances are that a huge amount of fossilized trilobite remains are but the shed exoskeletons (excuvia) rather than the remains of the actual animal at its time of death. However, this consideration does not necessarily have an effect on the value of a specific fossil unless significant and essential parts of the carapce are completely missing.
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