The single known adult of Pactopus avitus was a small, elongate throscid or false click beetle about 6 mm long. Its head sat partly inside a forward-tapering prothorax, and deep grooves beneath the body received the antennae and folded legs. That arrangement produced a streamlined, protected underside, while the unusually faint surface sculpture distinguishes the fossil species within its genus.

Its way of life is less certain than its anatomy. Modern throscids occupy soil, litter and decaying-wood microhabitats, and living Pactopus has been encountered beneath logs; these observations make a forest-floor or deadwood association plausible, but they do not demonstrate that P. avitus was itself a wood-borer. The biology of the London Clay animal therefore remains largely inferred from the family, because the species is represented by one pyritised adult.