Palaeozoic suits of armour
Naturally, trilobites were not the only existing life form in the world’s ancient oceans. Apart from the threats from enemy predators they often faced hazardous environmental conditions: volcanic activities, storms, mud slides (to which we seem to owe a great amount of fossilized trilobites) and other rigours of nature. Both factors – natural enemies and treacherous habitats – made it necessary for our little bugs to develop appropriate survival strategies, the most effective of which undoubtedly being the capability of enrolment by the means of which all vulnerable parts of the trilobite body, namely all areas that were not covered by the protective exoskeleton, could be retracted into a sort of compact capsule.
Due to the very flexible construction of the thorax, consisting of several rigid segments, yet moveable against each other, many trilobites were capable of hermetically encapsulating all of the more fragile body parts like legs and antennae within the hard shell of the carapace whenever deemed necessary. In order to do that they activated internal muscular structures that controlled the elastic tissue between the inflexible thoracic segments in such a way that cephalon and pygidium converged towards each other. The defensive tactics of recent pill bugs are by no means a new invention!
When enrolment was successfully completed, predators and nature were left with a hard, rigid and sometimes, due to the peculiarities of some taxa, very spinose calcite capsule offering almost no weak points that could be deliberately attacked. No wonder many trilobite genera show a very similar shape in their head and tail shields.
Having said that: It would appear that not all members of the trilobite class were capable of using this defensive strategy. In particular the very early, so-called “primitive” trilobites of the Cambrian (e.g. the Paradoxides as shown on the right) display a morphology which places doubt on their capability of enrolment or at least makes it appear less likely, if not entirely impossible (see remarks on “partial enrolment” below). Many of the early Redlichiids were comparably large trilobites and predators in their own right which did not have to be afraid of a large spectrum of natural enemies. In the course of the Ordovician and Silurian, however, many taxa had adapted enrolment as an effective method of defence against a whole range of perils.
Some trilobites, in particular representatives of the Phacopidae group but not restricted to it, reached perfection in enrolment by developing specific morphological structures, the so-called vincular furrows, a groove to accommodate the edge of the pygidium and thoracic pleurae, pretty much in the way that modern tongue and groove panels work. A Palaeozoic “click”-system! ;-) These coaptative structures mark an evolutionary highlight in the morphological development of trilobites and in connection with the schizochroal eyes of the Phacopina account for the prominent position of this suborder within the class of the trilobita.
I freely admit that this statement may just be a reflection of my personal preference for the Phacopina. The often very large eyes with easily visible lenses, the prominent glabella and very convex carapace gives them a very unique character. Among the wide range of different and divergent trilobite taxa, the Phacopidae are the very dearest to my heart, even if for the most part they do not come as spectacular as some of the spiny Lichids. Phacops, Reedops, Drotops, Kainops and company are my best of friends! If you ever split a rock and were lucky enough to suddenly look into the eye of such a trilobite you might understand what I am trying to say. No offence to Richard Fortey, but I think they make for a more impressive encounter than early Redlichiid trilobites with their narrow, sickle-shaped, holochroal and often vertically compacted eyes, as described in his great book “Trilobite – Eyewitness to Evolution”. ;-)
Enrolment in the Phacopid trilobite Barrandeops granulops (CHATTERTON et al., 2006) from the Lower Devonian Timrhanrhart Formation of Zguilma, Southern Morocco. Cephalon and pygidium interlock perfectly by means of the vincular furrow, thereby sealing off all soft body parts inside a rigid capsule. This particular specimen is 0.6 inch wide. Found by Dieter Holland.
Fossilized remains of completely enrolled Phacopids like this beautifully preserved specimen from Devonian sediments at Jbel Zguilma, Morocco, are by no means rare. This can be accounted for not only by favourable environmental conditions during diagenesis or the Phacopids’ preference for enrolment but also by the overall abundance of these taxa in some local fossil records.
We can see countless specimens of enrolled or rather ‘collapsed’ Agnostids from Cambrian rock strata (with only two or three thoracic segments the term “enrolment” should not be stressed too much) and witness enrolment among common Ptychopariid trilobites like Ellipsocephalus, locally found in large numbers in the Barrandian of Bohemia.
As early as 1852, BARRANDE distinguished between three different types of enrolment and all of them made it to the 1959 Treatise on Invertebrate Paleontology and following publications: sphaeroidal , double, and discoidal. Differences can be found in particular in the morphological aspects that enable or disable a particular taxon to perform the process of enrolment to varying degrees of completion.
- Sphaeroidal enrolment denotes a process in which, by more or less equal participation of all available thoracic segments, the trilobite body bends in such a way that all vulnerable extremities are completely enclosed inside a ball-like capsule. Candidates for this type of enrolment can be found among isopygous and macropygous trilobites. Our Barrandeops shown above is a prime example for this type of enrolment, without doubt the most perfect.
- Double enrolment denotes a process in which the trilobite bends in such a way as to not only shield its pygidium but also some of the posterior thoracic segments under the cephalon, i.e. the pygidium does not interlock with the cephalon but curls up underneath it. This can be observed in many of the early Cambrian trilobites, for example among the Ellipsocephalidae which we have already mentioned. BERGSTRÖM (1973) had good reason to call this type “spiral enrolment”.
enrolment is defined as a process in which the trilobite bends but the anterior part of the thorax, using the posterior part and pygidium as a sort of inverted lid. Examples mentioned include in particular trilobites of the order Harpetida as well as some trinucleid species. Again, all vulnerable parts could be sufficiently shielded, the thorax performing an enormously narrow radius within a very restricted area. At first glance it would appear as if this put a big strain on the cephalic region (image no. 2 seems to show a downward arched cephalic border). But since we assume that the carapace was rigid and hardened, this may just be an optical distortion! ;-).
Enrolment in the Harpetid trilobite Harpes sp. from the Devonian of Southern Morocco. Despite the very peculiar morphology of the cephalic shield, the pygidium manages to attach itself to it perfectly. Trilobite prepared by Dieter Holland.
The main difference between the sphaeroidal and discoidal type of enrolment can be found in the fact that in the latter the posterior part of the thoracic region including the pygidium remains more or less unbent which can easily be seen from the first image of our Harpes. Drawing a line between two such similar types may look a little wiredrawn at first, but is justified by the fact that the different types of enrolment have been considered as a potential method of classification by prominent trilobite workers.
Another type as yet unmentioned is what we call incomplete enrolment. This type can frequently be found in early spinose Olenelloids and Paradoxidoids and denotes a process in which the pleurae are incapable of completely enclosing the ventral parts due to the trilobite’s overall morphology, leaving a breach in an otherwise very good line of defence. However, with the pointed pleural spines protruding outward, this vulnerable clearance is still defended properly against enemy predators. BERGSTRÖM used to call this variant cylindrical enrolment in following the shape of that classical headgear.
As already indicated, some workers considered focusing on the different types of enrolment a potential method of classification in trilobites, especially in the 1970s. However, with an almost unlimited variability within the aforementioned types, accounted for by the wide range of different morphological peculiarities within the class trilobita, specialists soon came to conclude that this very variability rendered the whole idea unsuitable for higher level classification. The differences observed being regarded as insignificant and therefore negligible, enrolment never made it past being considered as of just limited use. This opinion was reflected in the 1997 revision of the Treatise on Invertebrate Paleontology and to the best of my knowledge has not been seriously questioned in the last decade.
That fact notwithstanding, enrolment remains a fascinating aspect in trilobite research and it is always a great experience to unearth a perfectly enrolled specimen, regardless of what genus or species it belongs to.
In the opening paragraph of this site, we mentioned that trilobites had to face a range of perils. Apart from geological and meteorological hazards, natural enemies must have been the most decisive factor endangering the survival of our bugs. Now, do we know which animals preyed upon our beloved little creatures? It has to be said that we don’t know all too much yet about the predators roaming the water column or looming on the sea floors chasing after our multi-legged friends. Unfortunately the fossil record seems to be very scarce. (This could also indicate that different trilobite taxa may have preyed upon each other!)
Anomalocaris , a predator first found by Charles Doolittle Walcott in Cambrian rock strata of the Burgess Shale in British Columbia, a thick exposure of dark, occasionally fossiliferous shale exposed high in the Canadian Rocky Mountains near Burgess Pass, seems to have snacked on trilobites regularly (as a matter of fact it took quite some time until the impressive, roundish jaw apparatus was identified as what it actually was – Walcott originally thought it to be the remains of some sort of jellyfish).
Eurypterids, a group of sea scorpions related to arachnids, which include the largest known arthropods that ever lived (up to 6 ft 7 in) and which thrived in warm shallow water in the Ordovician to Permian, surely took their toll on trilobites. As we already noted, there also seems to have been a pattern of trilobites preying on trilobites. Paradoxides appears to have had a taste for smaller, benthic species. Substantial evidence for trilobites being prey can be found in a large amount of fossilized carapaces showing distinct bite marks, sometimes partially healed, indicating that these animals were capable of surviving even significant losses as long as no vital parts had been affected by the attack. All in all, our little bugs must have been a real tough kind of breed! Surely one more reason why they managed to survive for an unbelievable 300 million years.
Whether trilobites used to enroll in the course of natural death can not be proven beyond all doubt. It is regularly assumed that enrolment was a protective strategy to begin with rather than a position taken when death was imminent. Naturally, this assumption does not exclude our arthropods having been killed while following their natural instincts of self-protection, e.g. buried under tons of sediment in a submarine landslide set into motion across the continental shelves in the world’s ancient oceans.
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Last Update :
01/30/2010 5:02 PM