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Body plan and shield composition

On this page we would like to deliver an insight into the general morphology of trilobites by way of several line drawings created by Dr. Sam Gon III of Hawaii, to whom we are very grateful for his kind generosity in allowing us to use his superb drawings and sketches. Therefore, should you come across images you remember having seen somewhere before, chances are the place was Sam’s own fantastic website on trilobites: www.trilobites.info. In addition to this visual overview, we would like to give supplemental information on the actual structure of the trilobite exoskeleton in reference to both chemical composition and biomechanical capabilities.

The upper side of the trilobite exoskeleton (dorsal side)

Upper side of trilobite exoskeletonStructure and function:

The chemical composition of the trilobite exoskeleton, first and foremost a dorsal shield which protected the animal’s vulnerable inner organs and tissues, has been examined and described by various scientists. It was found to show a very high degree of calcification, i.e. primarily consisting of low-magnesium calcite, a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO³) with only a small portion of organic material to it. (WILMOT & FALLICK , 1989 - Original mineralogy of trilobite exoskeletons. Paleontology, 32, 297-304). It is safe to say that this composition was the most important precondition in permitting fossilization in the first place. As organic material in its original structure is scarcely preserved, we cannot rule on its former composition, but there is a good chance that it consisted of proteins and chitin fibrils (bundled micro-filaments, i.e. long chains of protein subunits). In its unmodified form, chitin is translucent, pliable, resilient and quite tough. In arthropods, however, it is often modified, becoming embedded in a hardened proteinaceous matrix, which forms much of the exoskeleton.

As with all exoskeletons the trilobite carapace served mainly as a protective shield with supporting functions to the animal’s inner organs. It had to be able to withstand both tractive and compressive forces, the former resulting from the shield’s structure itself and its supportive function in reference to the inner organs, the latter due to the forces imposed by currents and natural enemies. To cope with such a variety of requirements, the trilobite exoskeleton did not only feature flexible organic parts, (the chitin fibrils mentioned above) which countered tractive forces but also very short calcite crystals, resistant to pressure.

Apart from some Agnostida showing but a single-layer construction, the overall majority of trilobites featured a multi-layered carapace, consisting of a very thin prismatic outer layer built of the short calcite crystals we just mentioned and a much thicker main or primary layer. The prismatic outer layer, which in some trilobites display a very fine lamination (MUTWEI , 1981 - Exoskeletal structure in the Ordovician trilobite Flexicalymene. Lethaia, 14, 225-234), is formed by very small crystals of approx. 1 μm in diameter (0.000001 m or the thousandth part of a millimeter) which, following their C-axis, are arranged vertically in reference to the shield’s surface. The underlying primary layer represents between 85 to 95 % of the entire thickness of the exoskeleton. The relation in strength between the outer prismatic layer and the primary layer can therefore be set at an average ratio of 1:10.

It has to be noted that the unmineralized exoskeletons of insects and other arthropods, primarily constructed from purely organic materials, regularly outclass the mineralized shields of trilobites and other marine life forms in terms of resilience against destructive forces (they usually do not crack; a spider falling from a window sill usually survives its rapid descent completely unharmed). This is one reason why the latter have to grow much thicker to even the odds. Fortunately, the waters of the world’s oceans are rich in soluted calcium ions, and it is far more rational for a marine arthropod to afford a thick mineralized shield by absorbing what is available in abundance than to invest a disproportionate amount of energy into building a construction entirely based on organic compounds.

All line drawings on this page ©1999, 2007 by S. M. Gon III

The bottom side of the trilobite exoskeleton (ventral side)

Bottom side of trilobite exoskeleton

While the prismatic layer forms but the thin surface of the carapace, the major part of the protective shield is formed by the underlying primary layer, a fine-grained substance in which the calcite crystals fail to stick to the extraordinary homogenous grid that is usually found in the prismatic layer. The primary layer at times shows parallel structures which may be attributed to organic material in the living animal. However, as we stated before, the amount of organic material in the trilobite exoskeleton seems to have been very small indeed and if these structures are indeed what we assume them to be, they must have been way below 1 μm in strength.

According to Janine WILMOT, who researched extensively into the properties of the trilobite exoskeleton while working with London’s Natural History Museum, cracks in the trilobite carapace most likely ran along these organic structures rather than directly through individual calcite crystals. Apart from the trilobite eye, which due to optical requirements was made of calcite of very high purity (crystals with a considerable length to their C-axis), no cuticular crystals of equal length could be identified. As a matter of fact, none could be found that exceeded 3 μm in dimension. A possible reason for this can be found in the observation that serious defects in crytalline structures are more likely to happen when the individual crystals are very large and less likely in structures with smaller crystals.

The reconditioning of the trilobite exoskeleton after moulting to its hardened state appears to have been an accelerated process. For all arthropods, this 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 - The post-ecdysial development of the cuticle and the eye of the Devonian trilobite Phacops rana milleri STEWART, 1927).

This seems to indicate that the prismatic outer layer might have been the more easy part to build. Apart from being a reasonable protection against drilling organisms, it must have been very resilient against pressure applied in a vertical direction. 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.

All line drawings on this page ©1999, 2000 by S. M. Gon III

A reconstruction of the complete animal’s ventral side
(Olenoides serratus)

Reconstruction of Olenoides serratusWhen judging upon the value of an exoskeleton’s particular design, emphasis should be placed on the specific advantages it gives to its bearer. In trilobites the carapace seems to have served mainly as a protection against natural enemies, not so much as a framework for the attachment of muscular structures, although such a function seems to be evident in many preserved specimens. The thickness of the average trilobite exoskeleton in conjunction with its high grade of mineralization speaks in favour of evolution’s affinity to develop stronger shields for better protection. As a matter of fact, there are examples where trilobites, when living in an oxygen-poor evironment unfavourable to a large predator fauna, e.g. the “Olenoid Sea”, developed but relatively thin shields (FORTEY, 1985).

Further indications as to its main purpose as a protective shield can be found in the capability of enrollment with many trilobites and the fact that trilobite exoskeletons seem to show a more or less identical thickness throughout the whole shield (except for those points that appear to have served as attachments for muscular structures as mentioned above – these regions are usually thicker). This circumstance allowed for a very effective defensive position when assaulted by a predator, without any weak points that could have been selectively attacked.

As to the subject of enrolment we are glad to refer you to this subpage.

You may also be interested in the following works:
HARRINGTON, H. J. 1959 - General description of Trilobita. O38-O117 in MOORE, R. C. (ed.) Treatise on invertebrate paleontology. Part O. Arthropoda I. Geological Society of America and University of Kansas Press, New York and Lawrence, Kansas, 560 pp.
BERGSTRÖM, J. 1973 - Organisation, life and systematics of trilobites. Fossils and Strata, 2, 1-69, pls 1-5.
FORTEY R. A. & OWENS R. M. 1979 - Enrolment in the classification of trilobites. Lethaia, 12, 219-226.

All line drawings on this page ©1999, 2000 by S. M. Gon III

Note on the image to the right: Please observe that the living animal possessed both antennae (evident in all trilobites) and posterior appendages (peculiar to Olenoides!) This reconstruction by Dr. Sam Gon is not rooted in pure speculation. Fossilized trilobites showing detailed preservation of the soft body parts, although replaced by minerals, can be found in some world-famous locations like the Burgess Shale in British Columbia, Canada, the Cambrian of Chenjiang in China or the Devonian Hunsrück Slates in Germany where favourable environmental conditions allowed such preservation.

Ventral reconstruction of Olenoides serratus
This figure ©2005 by S. M. Gon III

Soft body parts preservation in Chotecops sp.

Soft parts and cross section

Image above: Evidence of the appearance of the extremities in the Lower Devonian phacopid trilobite Chotecops sp. from the dark Hunsrück Slates at Bundenbach, Germany. This extroardinary preservation as pyrite impressions shows the biramous legs of the animal (segmented legs and gills). The second image shows a simplified cross section of the trilobite body that shows the position of the biramous legs with its two branches in relation to the trilobite carapace. Every thoracic segment bears a pair of legs. When exposed to X-rays, the trilobite portrayed in the first image reveals very delicate details in the gills. Antennae can be identified in front of the trilobites head shield (cephalon).

Preparation and photographic image: W. Stürmer (†)

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Last Update : 01/30/2010 5:06 PM