Known only from a head less than a millimetre long, Pyrenicocephalus jarzembowskii was a minute member of the Enicocephalidae, the distinctive “unique-headed” true bugs. Its long tubular cheek region, protruding antennal bases and deep constriction behind the eyes divide the head into conspicuous front and rear lobes. Small ocelli on the posterior lobe show that the fossil was an adult, overturning the earlier interpretation of the specimen as a juvenile. The original description estimated a complete body length of roughly 2.8–4.2 mm.

Living enicocephalids are cryptic predators that hunt tiny, soft-bodied arthropods in places such as leaf litter, soil spaces, moss, beneath bark and in rotten wood. A similar small-prey predatory role is the best ecological inference for the London Clay insect, although the feeding beak, legs and wings are not preserved. Its head anatomy also places it close to a living enicocephalid lineage that includes Hoplitocoris, giving this isolated pyritised head disproportionate evolutionary importance.