Bumastus is an extinct genus of corynexochid trilobites which existed from the Early Ordovician period to the Late Silurian period.[1] They were relatively large trilobites, reaching a length of 6 in (15 cm). They were distinctive for their highly globular, smooth-surfaced exoskeleton. They possessed well-developed, large compound eyes and were believed to have dwelled in shallow-water sediments in life.
Bumastus fossils have been found in North and South America, Europe, Asia, and Australia. They are classified under the family Styginidae in the order Corynexochida.[2]
Bumastus is a large trilobite, reaching a length of 6 in (15 cm).[3][4] The body is oblong-oval, about twice as long as it is wide,[5] It had a strongly convex profile, giving it its distinctive globular appearance.[6]
Like all trilobites, the body is divided into three functional segments known as tagmata (singular: tagma), which in turn are divided into three lobes - the central lobe (axial) and two lateral lobes (pleural). Aside from faint depressions in the thorax, Bumastus is unusual in that the three lobes are barely discernible from each other.[7][8] The axial lobe of Bumastus is also very broad in comparison to the pleural lobes.[6]
The cephalon (head segment) is very large and strongly convex. The facial sutures (the divisions by which the cephalon splits when the trilobite molts) is opisthoparian,[9] with the suture ending along the hind cephalic margin.[10] The genal angles of the cephalon - the edges where the lateral and rear margins of the cephalon meet - are rounded.[11] The cephalon is effaced (smooth and mostly featureless), an evolutionary trend also seen in Illaenus and Trimerus, though not as pronounced as that of Bumastus. The glabella (the central lobe of the head) is almost fused to the fixigena.[9]
The thorax has ten narrow segments[5] while the pygidium (the tail) is smooth and very rounded.[5] It is isopygous - that is, the pygidium is about the same size as the cephalon.[12] The pygidium completely lacks any visible trilobation. It is usually semicircular in shape but can be pointed in some species like B. niagarensis.
