Four of the best: spiny trilobites

I visited the Oxford University Museum of Natural History a couple of days ago for the 9th Coleopterists' Day (more about that soon). While I was there, I had a wander round some of the exhibits and was distracted by a display of excellent trilobites. I have a couple of fossil trilobites at home, but nothing as spectacular as what was on show, so I felt compelled to take a few photos and find out a little more about them.

There are many good introductions to trilobites so I don't intend to spend long on this aspect, but to give alittle background, they are a group of marine arthropods that lived from the early Cambrian (about 526 mya) until the mass extinction at the end of the Permian (about 250 mya). They were incredibly successful and diverse with about 17,000 species currently known from the fossil record. Some scavenged, filter-fed or hunted on the sea bed while others swam and fed on plankton. Some, mainly in the family Olenidae, may have had a symbiotic relationship with sulphur-metabolising bacteria from which they derived nutrition (Fortey, 2000). Also, they grew as nymphs/instars through a series of moults, each becoming progressively larger, much like many modern-day invertebrates. Despite the rich fossil record, the taxonomy and phylogeny of trilobites remains somewhat uncertain - currently it seems plausible that they fit into the clade Mandibulata (i.e. Myriapoda, Crustacea & Hexapoda) (Scholtz & Edgecombe, 2005) although they have been popularly placed in the clade Arachnomorpha which includes all other arthropods. The jury remains out on which is correct (or at least the most appropriate).

However, rather than diving into evolutionary biology/cladistics which isn't my area, I would like to look at some examples from the OUMNH collection and highlight some of the (to me) unexpected aspects of their morphology and the possible functions of such structures. I will however give some links to factsheets which provide details of the characteristics placing these species/genera in their currently accepted orders. Firstly, the frankly bizarre Walliserops trifurcatus with its unmistakeable trident...

Walliserops trifurcatus showing the long trident at the front end.

This species is in the family Acastidae within the Order Phacopida (factsheet). I have written about a member of this order before as I have a good-quality fossil of Phacops sp., including some fine details such as eye structure, but nothing quite as impressively odd-looking as this. The genus is unusual in showing a departure from strict bilateral symmetry, particularly in W. hammii with its sideways-curled occipital spine. However, the most eye-catching feature is of course the trident. Its function is uncertain due to a lack of data (Chatterton et al., 2006), although the presence of horns strongly suggests sexual dimorphism rather than having a primarily sensory, protective or hydrodynamic function (Knell & Fortey, 2005), though one or more of these could have occurred secondarily. Instead it appears most likely that it was used in competition between males (much like the horns and antlers of certain modern-day male beetles) and/or as a feature involved in mate selection by females ("ooo, what a big getting-in-the-way-when-feeding trident you have"). More data may eventually tell, but I can certainly imagine the trident being used to flip rival males. A close relative in the same family is Comura bultyncki which is well armed with spines, including some which curve off-centre, but no trident.

A specimen of Comura bultyncki

Moving on to the family Styginidae within the order Corynexochida (factsheet), Kolihapeltis chlupaci is another spectacular spined species, this one being from a relatively rare genus characterised by long backward-curving spines which grow from behind the eyes and the back of the head. K. chlupaci also has a ribbed 'tail' (more technically the 'pygidium' formed of rear body segments and articulating with the rest of the body). The role of the pygidum in the addition of new segments during growth and development is discussed in some detail here, with large pygidia such as this having a protective function as a shield when the trilobite rolled up (like a modern woodlouse). Some styginids have spiny pygidia (Holloway, 1996) which again suggests a defensive function, although these spines are not present in K. chlupaci.

Specimen of Kolihapeltis chlupaci

Lastly I want to briefly look at the family Lichidae within the order Lichida (factsheet), in particular, the genus Ceratarges (the species was given in the museum but was obscured in my photo, so I only have the genus for certain though I think it is C. armatus). This is another genus with prominent and impressive spines and horns. Some, such as C. spinosus have secondary spines - short 'thorns' pointing out at right-angles from the main spines. These again are likely to have an defensive function and I have to wonder whether they have a role in mate selection and competition between males as suggested for the tridents in the genus Walliserops.

Specimen of Ceratarges sp.

That's enough about these splendid beasts (for now). Given the cost of good-quality fossils of spiny trilobites, I don't think any will be added to my curio shelves soon, but if you are shopping for such things, beware fakes and learn how to spot them!

References

Chatterton, B., Fortey, R., Brett, K., Gibb, S. & McKellar, R. (2006). Trilobites from the Lower to Middle Devonian Timrhanrhart Formation, Jbel Gara el Zquilma, southern Morocco. Palaeontographica Canadiana 25: 1-179.
Fortey, R. (2000). Olenid trilobites: The oldest known chemoautotrophic symbionts? Proceedings of the National Academy of Sciences 97(12): 6574-6578.
Holloway, D.J. (1996). New early Devonian styginid trilobites from Victoria, Australia, with revision of some spinose styginids. Journal of Paleontology 70(3): 428-438.
Knell, R.J. & Fortey, R.A. (2005). Trilobite spines and beetle horns: sexual selection in the Palaeozoic? Biology Letters 1: 196–199.
Scholtz, G. & Edgecombe, G.D. (2005). Heads, Hox and the phylogenetic position of trilobites. In: Koenemann, S. & Jenner, R.A. Crustacea and Arthropod Relationships. Crustacean Issues 16: 139–165. CRC Press, Boca Raton.