with most species measuring long and weighing ; the smallest species, C. rostratus, was about long and weighed . It was a heavily armored swimmer that fed mainly on shellfish that it was specialized to uproot and crush with its powerful jaws. The body of Cyamodus, specifically the armor, has been described as possessing a turtle-like flatness. The shell was a two-part carapace on the upper surface of the body. The larger half covered Cyamodus from the neck to the hips and spread out flat, almost encompassing the limbs. The second, smaller plate covered the hips and the base of the tail. The shells themselves are covered in hexagonal or circular plates of armor. The skull is heart-shaped and broad. Distinct from Paraplacodus, the skull of Cyamodus had a shorter rostrum, a smaller orbit and a larger upper temporal fenestra that was rimmed by ossifications. The teeth were flat discs, only one tooth appeared on each premaxilla and only two teeth appeared on each maxilla, with the largest teeth on the pterygoid. The quadratojugal joined the squamosal and sealed up the lateral temporal fenestra from the ancestral species (Paraplacodus). The carapace of C. hildegardis has a series of similar-sized, enlarged lateral armor plates is rounder and less laterally expanded than was hypothesized. The separate pelvic shield, also carrying a smaller set of lateral armor plates that decrease in size with an anterior/posterior gradient covering mainly the pelvic girdle and the base of the tail. The short tail is armored by four series of armor plates that also show an anterior/posterior gradient of size reduction equivalent to the size reduction of the caudal vertebrae. Until further fossils are recovered, the internal organisation of dermal plates within the two armor shields of C. hildegardis remains little known. Cyamodus did not have any dorsal spines, although it did have a wing-like elongated flattened lateral spine that served to brace the overlying subdermal carapace. Presumably, the dorsal spines disappeared to provide a closer and better braced association between the vertebrae and the carapace. Paleobiology Although the shell would most likely have been too cumbersome for highly adept swimming, Cyamodus would still have been more agile than other single shelled placodonts like Henodus. The shells were also covered in hexagonal plates that not only increased the level of protection but also increased their weight, a typical placodont adaptation as additional weight just beyond the level