Tropidosteus

Tropidosteus curvatus is a large, extinct holonematid arthrodire placoderm from the Givetian-aged Crinoidenmergel stratum of Middle Devonian Rheinland, Germany. T. curvatus is known from primarily from a slender, 42 centimeter long, arched median dorsal plate, where the two sides meet at a sixty to ninety degree angle, which would have given the live animal a humped appearance. The median dorsal plate is very similar to the median dorsal plates of Rhenonema and Belemnacanthus, and is the primary reason for T. curvatus ' placement within Holonematidae. After stating this reason, Denison, 1978, then questions Tropidosteus' placement within the family, noting that the dorsal plate lacks ridged ornamentation, which is a key diagnostic trait of the family. The ornamentation otherwise consists of an external covering of small tubercles.