The genus was described by Christian Erich Hermann von Meyer in 1863, based on specimens found in Germany.

Like some other placodonts, Cyamodus has an armoured carapace composed of irregular hexagonal plates, with the mouth containing a small number of large, rounded teeth that were likely involved in crushing hard shelled organisms (durophagy).

However further investigation resulted in its reclassification as a placodont, and it is closely related to other turtle-like reptiles of the Triassic period such as Henodus and Psephoderma.

[3] Similar to these other placodonts, Cyamodus lived hovering close to the sea floor, vacuuming up various shellfish, and crushing them between its blunt teeth.

[4] Historically, the first Cyamodus remains were found in Upper Muschelkalk shallow marine limestones at near Bayreuth in Bavaria (Germany).

A complete Cyamodus skeleton, including its skull, is known for C. hildegardis, which was found outside the Germanic Basin in the northern Tethys in Switzerland.

[1][7][8][9] It was a heavily armored swimmer that fed mainly on shellfish that it was specialized to uproot and crush with its powerful jaws.

[2] 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.

Other adaptations for protection besides the heavily armored shell include the strongly built rear of the skull and limbs that do not protrude too far.

[4] C. hildegardis from the Besano Formation (Middle Triassic) of the Alpine area of Switzerland and northern Italy has been reconstructed with a broad, laterally expanded main armor (carapace) and a separate smaller pelvic shield, giving it a sprawling appearance.