Antiarchi

The Antiarchi ("Opposite anus") were the 2nd most successful order of placoderms known, after the Arthrodira. The order's name was coined by Edward Drinker Cope, who, when examining some fossils that he thought were armored tunicates related to Chelysoma, mistakenly thought that the eye-hole was the mouth, and that the opening for the anal siphon was on the other side of the body, as opposed to having both oral and anal siphons together at one end.

The front portions of their bodies were heavily armored, to the point of literally resembling a box with eyes, with the sometimes scaled, sometimes naked rear portions often becoming sinuous, particularly with later forms. The pair of pectoral fins were modified into a pair of caliper-like, or arthropod-like limbs. In primitive forms, such as Yunnanolepis, the limbs were thick and short, while in advanced forms, such as Bothriolepis, the limbs were long and had elbow-like joints. The function of the limbs are still not perfectly understood, but, most hypothesize that they helped their owners pull themselves across the substrate, as well as allow their owners to bury themselves into the substrate.