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Bumastus is an extinct genus of Corynexochida which existed from the Early Ordovician period to the Late Silurian period. They were relatively large trilobites, reaching a length of . 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 America and South America, Europe, Asia, and Australia. They are classified under the family Styginidae in the order Corynexochida.
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. The axial lobe of Bumastus is also very broad in comparison to the pleural lobes.
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, with the suture ending along the hind cephalic margin. The angles of the cephalon - the edges where the lateral and rear margins of the cephalon meet - are rounded.Britannica Educational Publishing (2026). 9781615301966, The Rosen Publishing Group. . ISBN 9781615301966 The cephalon is effaced (smooth and mostly featureless), an evolutionary trend also seen in Illaenus and Homalonotidae, though not as pronounced as that of Bumastus. The glabella (the central lobe of the head) is almost fused to the fixigena.
The thorax has ten narrow segments while the pygidium (the tail) is smooth and very rounded. It is isopygous - that is, the pygidium is about the same size as the cephalon. The pygidium completely lacks any visible trilobation. It is usually semicircular in shape but can be pointed in some species like B. niagarensis.
The smooth are large and peculiarly well-developed. This, along with the rounded contours of their body, suggests that Bumastus may have spent most of its time buried in sediment with its eyes protruding.
The surface of the exoskeleton of most species is studded with minute punctures.
Bumastus is a Bottom feeder () trilobite. It was probably either detritivore, feeding on decomposing organic material drifting down in the currents, or carnivore.
Specimens have been recorded from the Silurian of Argentina, Canada, the Czech Republic, Estonia, Greenland, Kazakhstan, Norway, the Russian Federation, Sweden, Ukraine, the United Kingdom, and the United States.
They can also be found in the Ordovician formations of Australia, Canada, China, the Czech Republic, the Russian Federation, Spain, the United States; with specific occurrences from the Dobrotivian age/stage (Llandeilo age) of China and France, and the Whiterock Stage stage of the United States.
They are typically found in limestone, though they are sometimes found in limestone.
Murchison first believed that the specimens he discovered (including a large by specimen) belonged to the genus Isotelus because of the size, shape, and almost featureless cephalon. But he noted the almost absent trilobation of body and the difference in the number of segments in the thorax (10 in B. barriensis and 8 in Isotelus). He also recognized its close relationship with the genus Illaenus, but ultimately classified it as a new genus based on the extremely advanced state of effacement in the cephalon of Bumastus.
The genus is so named because of its curious resemblance to a large round grape. It comes from Latin būmastus (large grapes that resemble the of a cow), which in turn came from Greek language ( bous - cow) and ( mastós - breasts). The word was familiar in the English language during Murchison's time, being a word encountered in book two of Virgil's Georgics.
The specific name of the type species, barriensis, roughly meaning "of Barr", comes from its common name among collectors. It was then known as the "Barr trilobite" referring to the plentiful occurrence of B. barriensis in the limestone formations of Great Barr, Staffordshire.
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