Belongs within: Sphaerexochinae.
Sphaerexochus is a cosmopolitan genus of trilobites known from the Middle Ordovician to the Late Silurian (Přibyl et al. 1985).
Sphaerexochus: a possibly predatory trilobite
Published 28 October 2013
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).
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.
Systematics of Sphaerexochus
Characters (from Přibyl et al. 1985): Glabella arched into a spherical shape, lateral glabellar furrows 3S and 2S shortened; lobe 1L limited to terminal margins of glabella. Hypostome with deep notch at posterior border. Pygidium with 3(+1) axial rings and three pairs of pleurae; pleurae narrow, lathy.
<==Sphaerexochus Beyrich 1945PVP85 |--+--S. (Sphaerexochus)PVP85 | | |--*S. (S.) mirus Beyrich 1845PVP85 | | |--S. (S.) bohemicus Barrande 1872PVP85 | | |--S. (S.) bradleyi Chatterton & Perry 1984PVP85 | | |--S. (S.) bridgei Cooper & Kindle 1936PVP85 | | |--S. (S.) calvus McCoy 1846PVP85 | | |--S. (S.) centeo Lane & Owens 1982PVP85 | | |--S. (S.) costabilis Dean 1971PVP85 | | |--S. (S.) dimorphus Perry & Chatterton 1977PVP85 | | |--S. (S.) discrepans Raymond 1925PVP85 | | |--S. (S.) eurys Tripp 1962PVP85 | | |--S. (S.) glaber Holloway 1980PVP85 | | |--S. (S.) guizhouensis Wu 1977PVP85 | | |--S. (S.) hiratai Kobayashi & Hamada 1974PVP85 | | |--S. (S.) johnstoni Chatterton & Perry 1984PVP85 | | |--S. (S.) laciniatus Lindström 1885PVP85 | | |--S. (S.) latens Barrande 1872PVP85 | | |--S. (S.) latifrons Angelin 1854PVP85 | | |--S. (S.) lorum Chatterton & Campbell 1980PVP85 | | |--S. (S.) molongloensis Chatterton & Campbell 1980PVP85 | | |--S. (S.) orientalis Kobayashi & Hamada 1970PVP85 | | |--S. (S.) paramirus Šnajdr 1980PVP85 | | |--S. (S.) planirachis Kobayashi & Hamada 1974PVP85 | | |--S. (S.) romingeri Hall 1868PVP85 | | `--S. (S.) scabridus Angelin 1854PVP85 | `--S. (Onukia Kobayashi & Hamada 1976)PVP85 | `--S. (*O.) sugyiamai Kobayashi & Hamada 1976PVP85 |--S. (Korolevium Přibyl & Vaněk in Přibyl, Vaněk & Pek 1985)PVP85 | |--S. (*K.) arenosus Chatterton & Ludvigsen 1976PVP85 | |--S. (K.) arcuatus Tripp 1976PVP85 | |--S. (K.) atacius Ludvigsen 1979PVP85 | |--S. (K.) balclatchiensis Reed 1924PVP85 | |--S. (K.) bilobatus Whittard 1958PVP85 | |--S. (K.) conusoides Koroleva 1959PVP85 | |--S. (K.) filius Tripp 1967PVP85 | |--S. (K.) hapsidotus Whittington & Evitt 1954PVP85 | |--S. (K.) hisingeri Warburg 1925PVP85 | |--S. (K.) pulcher Whittington & Evitt 1954PVP85 | `--S. (K.) tuberculatus Warburg 1925PVP85 `--S. (Parvixochus Přibyl & Vaněk in Přibyl, Vaněk & Pek 1985)PVP85 `--S. (*P.) parvus Billings 1863PVP85
*Type species of generic name indicated
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.
[PVP85] 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.