Palaeoaplysina

Palaeoaplysina is a genus of tabular, calcified fossils that are a component of many Late Palaeozoic reefs. The fossil acted as a baffle to trap sediment. Historically interpreted as a sponge or hydrozoan, recent studies are converging to its classification in the coralline stem group, placing it among the red algae.
Morphology.
The thalloid organism had a series of internal canals opening on one side of the body (presumably the upper side), and volcano-like protuberances on that same side inviting comparison to filter-feeding organisms. On the other hand, it seems to have had a calcified cellular make up akin to that of the coralline reds, suggesting that it was either a stem-group coralline or a coralline-encrusted filter feeder.
Distribution.
The organism is widespread in the tropical and near-tropical margin of the Laurentian continent (45–15°N), but is not found elsewhere. Its oldest reported occurrence is Middle Pennsylvanian (mid- to late Moscovian) and youngest is the late Sakmarian. It acts as an important reservoir rock for oil deposits.