Bobble-Nosed Trilobites

Cranidium and part of thorax of Onchonotellus sp., from Bao & Jago (2000).

While most fossil invertebrates manage to completely fly under the radar when it comes to popular culture (it's not as if we're drowning under cartoon depictions of euthycarcinoids or eldoniids), one group that will often get a passing nod is the trilobites. Any depiction of early animal life worth its salt is going to feature a couple of these crunchy bugs scurrying about. Nevertheless, the range of varieties of trilobite shown will generally be low, and will usually be something similar to Olenellus or Elrathia. Seeing as trilobites persisted for hundreds of millions of years, it should be no surprise that their actual diversity was much higher.

The fossil shown at the top of this post is a representative of the trilobite genus Onchonotellus. Remains have been assigned to this genus from the late Cambrian and the early Ordovician, though Adrain (2013) expressed some reserve about the genus' monophyly. It has been assigned to the Catillicephalidae, a mostly Cambrian group of trilobites, but again the coherence of this total group is uncertain.

Most known fossils of Onchonotellus are represented by isolated cranidia, the plates that in life covered the trilobites' head. Onchonotellus and other catillicephalids are characterised by an inflation of the glabella, the middle lobe of the cranidium. Generally, the glabella of Onchonotellus is barrel-shaped. In some Onchonotellus specimens, the glabella may almost look spherical in side-view, making this trilobite look like a Bubble O'Bill. In many trilobites, the glabella will bear a series of furrows along the sides, but in Onchonotellus these disappear so that the glabella surface is smooth. In other catillicephalids, the glabella extends right to the front margin of the head, but Onchonotellus does retain a distinct rim around its front. The cheeks on either side of the glabella are relatively broad (Öpik 1967; Shergold 1980). In their time, members of the genus were found around the world, and some species have been highlighted as index fossils, useful in determining the age of rock strata.

So what was the significance of the large glabella? Most researchers have suggested that it probably held some sort of expansion of the digestive system, such as a crop for storing food. Some trilobites in which the glabella became large enough that it actually overhung the front margin have been suggested to be predatory (Fortey & Owens 1999), with the glabella containing a large oesophagus that allowed the trilobite to swallow larger food items. Onchonotellus probably didn't take things that far: not only was its glabella just that little bit smaller, but I haven't found any indication of it possessing the enlarged eyes also found in the predatory forms. The furrows on the glabella of most trilobites may have marked the attachment positions for muscles associated with the oesophagus/crop/whatever, so did the reduction of these furrows in Onchonotellus indicate a correspondingly less muscular pharynx? Perhaps Onchonotellus was a detritus feeder, with an expanded crop allowing it to take in mouthfuls of sediment from which to sieve out tasty organic morsels. Or perhaps it was a scavenger, breaking off lumps from the carcasses of other animals. Whatever it was doing, it was something that involved a big nose that was in actuality a big mouth.

REFERENCES

Adrain, J. M. 2013. A synopsis of Ordovician trilobite distribution and diversity. Geological Society, London, Memoirs 38: 297-336.

Bao, J.-S., & J. B. Jago. 2000. Late Late Cambrian trilobites from near Birch Inlet, south-western Tasmania. Palaeontology 43 (5): 881-917.

Fortey, R. A., & R. M. Owens. 1999. Feeding habits in trilobites. Palaeontology 42 (3): 429-465.

Öpik, A. A. 1967. The Mindyallan fauna of northwestern Queensland. Commonwealth of Australia, Department of National Development, Bureau of Mineral Resources, Geology and Geophysics—Bulletin 74, vol. 1: 404 pp., vol. 2: 166 pp., 67 pls.

Shergold, J. H. 1980. Late Cambrian trilobites from the Chatsworth Limestone, western Queensland. Bureau of Mineral Resources, Geology and Geophysics—Bulletin 186: 1-111.

Sphaerexochus: A Possibly Predatory Trilobite

Sphaerexochus brittanicus, from Museum Victoria.

The fossil in the image above belongs to a genus of trilobites that lived from the Mid-Ordovician to the end of the Silurian. Distinguishing characters of Sphaerexochus include the massive inflation of the glabella, the central section of the trilobite head, which became almost spherical. The cheeks on either side of the glabella, in contrast, remained fairly small. The inflated glabella was marked on each side in the posterior part by a deep furrow that run in a curve from the side to the posterior margin. In the photo above, this furrow marks out the circular section that looks a bit like a large eye; the actual eye can just be made out in the photo (I think) as a crescent-shaped structure on the cheek below the glabella. The rear outer corners of the cheeks were more or less blunt in adult individuals, generally lacking the prominent cheek-spines of many other trilobites (small spines were present in juveniles). The spines on the trilobite's rear end (the pygidium) were also blunt and stout.

Přibyl et al. (1985) recognised four subgenera within Sphaerexochus, and this classification was largely supported by Congreve & Lieberman (2011) in a phylogenetic analysis (one of the subgenera, Onukia, is not known from well-preserved material and hence could not be analysed by Congreve & Lieberman). One of the subgenera, Korolevium, retained short cheek-spines as adults; they were absent in other subgenera. The remaining two subgenera, Sphaerexochus and Parvixochus, were distinguished by features of the hypostome, a plate on the underside of the head that covered the mouth in life. Korolevium and Parvixochus were both extinct by the end of the Middle Ordovician, but the Sphaerexochus subgenus sailed on and passed through the mass extinction event at the end of the Ordovician apparently unscathed (Congreve & Lieberman 2011).

Another view of Sphaerexochus brittanicus, from the Carnegie Institution.

Sphaerexochus was restricted to warmer waters in the Palaeozoic ocean (fortunately for Sphaerexochus, during the Silurian this was just about everywhere). Fortey & Owens (1999) interpreted its large glabellar furrows as providing an attachment site for the muscles of a correspondingly well-developed pair of limbs. Features of the hypostome indicate that Sphaerexochus had a relatively large oral cavity, and the combination of large anterior limbs and a big mouth lead Fortey & Owens to suggest that it may have been a raptorial predator. Přibyl et al. (1985) noted that Sphaerexochus specimens have often been found with the main body and head bent at an angle from each other, and suggested that they may have spent a lot of time with the body buried in the sediment with only the head above the surface. These observations add together to suggest Sphaerexochus living as an ambush predator, lurking in wait whilst largely hidden in the sand for any smaller animal unwary enough to get too close.

REFERENCES

Congreve, C. R., & B. S. Lieberman. 2011. Phylogenetic and biogeographic analysis of sphaerexochine trilobites. PLoS ONE 6(6): e21304. doi:10.1371/journal.pone.0021304.

Fortey, R. A., & R. M. Owens. 1999. Feeding habits in trilobites. Palaeontology 42 (3): 429-465.

Přibyl, A., J. Vaněk & I. Pek. 1985. Phylogeny and taxonomy of family Cheiruridae (Trilobita). Acta Universitatis Palackianae Olomucensis Facultas Rerum Naturalium Geographica-Geologica XXIV 83: 107-193.