Antiarchi

left|thumb|344x344px|Fossil fish slab, cast. Late Devonian, Mandagery Sandstone, [[Canowindra, New South Wales (Australia). Preserves 114 fish individuals, which died when their freshwater pond dried up. Most of the individuals in the slab are the antiarch placoderms Remigolepis walkeri and Bothriolepis yeungae. One sarcopterygian individual is present, Canowindra grossi, the largest fish in the slab. Two small and inconspicuous juvenile Groenlandaspis are also preserved.]]

Antiarchi ("opposite anus") is an order of heavily armored placoderms. The antiarchs form the second-most successful group of placoderms after the arthrodires in terms of numbers of species and range of environments. The order's name was coined by Edward Drinker Cope, who, when examining some fossils that he thought were armored tunicates related to Chelyosoma, mistakenly thought that the orbital fenestra (i.e., the hole in the headshield for the eyes, nose and pineal foramen) was the opening for the mouth, or oral siphon, 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.