Casieria casieri was a small catshark that probably spent much of its life close to the seafloor. Living catsharks occupy a wide range of shelf and slope habitats and commonly prey on small fishes, crustaceans, cephalopods and other invertebrates; a broadly similar ecological role is plausible for this Eocene species.

Its small pointed teeth formed a grasping dentition rather than a heavy cutting or crushing apparatus. This would have suited an opportunistic predator taking prey small enough to be swallowed whole or with limited processing.

The species is known almost entirely from isolated teeth, so details such as colour pattern, body proportions and behaviour are unknown. Its presence helps show that the London Clay fauna included numerous small bottom-associated sharks as well as the much larger predators that tend to dominate fossil-shark displays.