|Diadematoids are a successful group of epifaunal regular echinoids that can be found subtidally in protected shallow water environments, such as back-reef lagoons, but also down to 4000 m. Shallow water forms feed primarily on encrusting organisms and small organisms on the sea floor. When on level open bottoms they are often found herded together, presumably because of the added protection provided by the mass of spines.|
There is a major problem in correctly ascertaining which fossil taxa belong to the Diadematoida. A number of fossil genera, previously grouped together in the Pseudodiadematidae, have perforate, crenulate tubercles and closely resemble Centrostephanus in test morphology. These were originally thought to be closely related to the diadematids, but it is now known that Pseudodiadema, Eodiadema, Tiaromma and Allomma all have keeled teeth rather than grooved teeth. Without information on the lantern structure it is impossible to place fossil pseudodiadematids with any degree of confidence.
The poor state of knowledge concerning most Jurassic regular echinoids is a severe hindrence to understanding the early history of this group. Never the less, there are some fossils that can be assigned to the diadematoids with confidence, extending their range back to the Middle Jurassic. It seems highly likely that others will eventually be proven. Pedinoids and diadematoids probably split in the Late Triassic or earliest Jurassic.