The fossil record of echinoids is poor because soon after death they break apart into isolated plates. In the present seas regular echinoid species outnumber irregular species; whereas, in the Tertiary only 20% of the known echinoid species are regular. This suggests that regular echinoids are less likely to be preserved than irregular echinoids. The tests of regular echinoids are exposed to scavengers and currents upon their death, but irregular echinoids generally live buried in the sediment and are protected from these destructive forces. Furthermore, the tests of the regular echinoids lack the calcareous supports found in some irregular echinoids. The gut is not filled with sediment and its apical system is generally larger and more fragile. Finally, many regular echinoids live in environments less likely to be preserved in the sedimentary record.

Although the irregular echinoid is more likely to be fossilized, its record is poor during some periods in the past. Although 1,014 irregular echinoid species are known from the Eocene, only 83 species are known from the Paleocene and only 343 are known from the Oligocene. Is this reduction because fewer species lived then or because they have not been preserved?