Meniscopsia beebei (Robinson & Babcock 2011)


Exoskeleton ovate with low relief and somewhat variable width. Cephalon crescent shaped. Lateral cephalic borders widen rearward into stout, broad-based genal spines. Anterior sections of facial suture moderately divergent, posterior sections strongly divergent. Glabella unfurrowed, long, slightly tapered, bluntly rounded anteriorly, reaching anterior border furrow. Occipital ring short. Palpebral lobes slightly posterior to glabellar midpoint. Hypostome conterminant, with front margin broadly abutting laterally wide and sagittally narrow rostral plate along hypostomal suture.

Thorax containing 10 sagittally short, weakly falcate segments.

Pygidium large, semicircular, with broad, smooth border that widens rearward.Pygidial axis tapering to rounded terminus, length about 0.6 times that of pygidium; containing eight weakly defined rings and terminal piece. Pleural fields of pygidium triangular and very weakly furrowed.

Exoskeletal doublure significantly widens rearward beneath cephalon, showing slight abrupt increase beneath posterior cephalic border, then similar width beneath thorax, further slight broadening beneath pygidium, and slight medial constriction behind axial terminus.

After Matthew A. Beebe, for unpublished contributions (1990) to knowledge of the Weeks Formation and its fauna. Revision for publication of work on the Weeks Formation by Beebe ended with his untimely death in 1994.

During holaspid ontogeny, the pygidium of Meniscopsia beebei increased disproportionately in size relative to the cephalon. For example, the pygidium of a paratype, 15 mm in length (Fig. 29.4), is slightly shorter than the cephalon in sagittal length, whereas the pygidium of the holotype, 33 mm in length (Fig. 29.1), is almost 1.5 times longer than the cephalon in sagittal length. Variation in late holaspid exoskeletal width may be related to differences in postmortem compression.

Internal anatomy of trilobites is rarely preserved.This Meniscopsia beebei show a dark central feature that can be interpreted to be an early diagenetic filling of the alimentary tract and paired laterally projecting digestive glands (foregut and midgut glands), with one pair per segment, as in some modern chelicerate arthropods.