Bessazoon Curtis and Lane, 1998

Sandford, Andrew C. & Holloway, David J., 2006, Early Silurian phacopide trilobites from central Victoria, Australia, Memoirs of Museum Victoria 63 (2), pp. 215-255 : 238-240

Bessazoon Curtis and Lane, 1998

Type species. Dalmanites weaveri var. tenuimucronata Whittard, 1938 from the Hughley Shales (upper Llandovery) of Shropshire, England, by original designation .

Remarks. Of the characters regarded by Curtis and Lane (1998: 62) as diagnostic of Bessazoon, Chatterton and Ludvigsen (2004: 47) questioned the taxonomic value of the size of the eyes and the loss of the pygidial mucro in large specimens. In the type species and B. tigerense ( Holloway and Sandford, 1993) the eyes are very large and occupy almost the entire length of the genal field, but in Curtis and Laneʼs species B. buttingtonense and the cephalon they illustrated as B. cf. B. tenuimucronatum (their pl. 9, fig. 1) the eyes are smaller and do not extend very close to the border furrows anteriorly and/or posteriorly. In regard to the loss of the mucro, Curtis and Lane illustrated three pygidia assigned to B. tenuimucronatum with the posterior termination preserved. Two of these (Curtis and Laneʼs pl. 8, figs 6, 8) have a mucro, that on the larger specimen being considerably shorter than the one on the smaller specimen. The third pygidium, which is very much larger that the others (Curtis and Laneʼs ʻtype 2ʼ pygidium, pl. 8, fig. 2), does not have ʻa shorter mucral spineʼ as stated by Chatterton and Ludvigsen (2004) but an embayment in the margin posteromedially. Because of this difference, as well as the differences in size and the much greater number of axial rings and pleural furrows, it is not possible to be confident that this pygidium is correctly assigned to B. tenuimucronatum . However, we note that it has a distinctive pleural structure in which the posterior bands expand distally and the anterior bands are pinched out, and that this structure appears to be shared by the smaller pygidium in pl. 8, fig. 6 (the structure is not clear in the third pygidium as the dorsal surface is broken away distally to reveal the doublure). Although we believe that these pygidia are correctly assigned to the Dalmanitidae , their pleural structure is unusual for Silurian and Devonian representatives of the family, in most of which it is the anterior rather than the posterior pleural band that is dominant distally ( Holloway, 1981: 710), as is also the case in B. buttingtonense ( Curtis and Lane, 1998, pl. 9, figs 2, 4, 7), B. tigerense ( Holloway and Sandford, 1993, fig. 6) and in the unnamed species from Victoria described below. The only pygidium of B. buttingtonense with the posterior termination preserved (Curtis and Lane, pl. 9, fig. 2a, b) is of about the same size as the ʻtype 2ʼ pygidium of B. tenuimucronatum but has neither a mucro nor a posteromedian embayment; instead the margin is rather truncated in dorsal view and arched upwards in posterior view. There is no evidence that the mucro is lost in B. tigerense but all known pygidia are smaller than that of buttingtonense and the ʻtype 2ʼ pygidium of tenuimucronatum .

Some of the other characters listed by Curtis and Lane as diagnostic of Bessazoon cannot in our view be used to distinguish Bessazoon either. The size of the palpebral area is determined by the size of the eye, and thus in comparison with other Silurian and Devonian dalmanitids is not particularly large in B. buttingtonense and B. cf. B. tenuimucronatum . We can see no difference from Dalmanites and other closely related genera in the form of the palpebral lobe, which rises steeply from the palpebral furrow and becomes flat towards the outer margin. Finally, a posterior cephalic border furrow that fails to meet the lateral border furrow distally, an epiborder furrow on the lateral cephalic borders and genal spine, and a straight-sided pygidial axis are not unusual features for dalmanitids but are present in most Silurian and Devonian representatives. The discrimination and composition of Bessazoon are in need of review, but in the meantime we apply the name here to dalmanitids differing from species commonly assigned to Dalmanites (e.g. see Ramsköld, 1985) in lacking a well-developed anterior cephalic process (although in B. tenuimucronatum the cephalic margin is deflected slightly forwards medially) and tubercles on the glabella, and in having a pygidium with a curved posterolateral outline, a slender, narrow-based mucro merging anteriorly with a strong postaxial ridge, and a very wide doublure extending adaxially beyond the distal ends of the pleural and interpleural furrows.