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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.
Is much known about trilobite internal anatomy?
ReplyDeleteSurprisingly little, as it turns out. There's some references given in this 2012 paper, but the juvenile described therein appears to have had the first well-preserved trilobite digestive system discovered. Even the limbs (which, these being arthropods, do allow a fair bit of inference of other anatomy) are only known from a handful of forms, such as the Cambrian taxa Olenoides serratus and Agnostus pisiformis.
ReplyDeleteThanks.
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