Plutonides

Plutonides is a genus of trilobite, an extinct group of marine arthropods. Species occur in the middle Middle Cambrian of Russia (Siberia), Mongolia, England and Wales (Triplagnostus gibbus and Triplagnostus fissus Zones), Sweden (Triplagnostus fissus–Ptychagnostus atavus Zone), eastern Canada (eastern Newfoundland, Nova Scotia, Plutonides hicksi Zone),^[3] and the Czech Republic (Skrije - Týřovice Area).^[2] The frontal lobe of the central raised area (or glabella) of the headshield (or cephalon) overhangs the short frontal border and it is slightly pointed, rather than rounded or truncate. This character is shared with Anabaraspis, but in Anabaraspis there is a wide area in front of the glabella without a differentiated border and preglabellar field.^[3]

Description

The exoskeleton of Plutonides is relatively flat, oval to inverted egg-shaped. The glabella is shaped like a lightbulb, about 1⅓ to 1½× wider across the frontal lobe than across the occipital ring, and up to 40% of the length of the glabella. Its front is bluntly pointed and hangs over the short frontal border. From the back the glabella is divided in two rings by furrows that cross over the midline, although the frontal is shallow in the middle. Three sets of furrows to the front are restricted to the sides, and the frontal two sets may even be effaced. The articulate middle part of the body (or thorax) has 19 segments with short, sickle-shaped spines. The inner part of the ribs is slightly wider than the axis. The tailshield (or pygidium) (unknown in P. sedgwickii) is slightly hexagonal, and bluntly truncate at its back. The axis in the pygidium is convex, half as long as the pygidium, with a rounded tip and 1 axial ring. The surface of the dorsal exoskeleton has coarse granules and/or a meshlike pattern of fine, anastomosing ridges.^[3]

References