ABSTRACT
The family Holectypidae includes some of the earliest and most primitive irregular echinoids. Two genera, Holectypus and Caenholectypus, are particularly common and widespread. They form a well-known Jurassic-Cretaceous lineage which, despite high recorded species diversity, persisted for over 100 million years with but little structural change. Other commonly supposed holectypids (Temoholectypus, Coptodiscus, Discholectypus, Metholectypus) have contrastingly low species diversity and restricted occurrence. They are relatively poorly known and of disputed holectypid status. The family Holectypidae thus essentially comprises a single morphologically conservative lineage, strikingly similar to the family Pygasteridae (Plesiechinus-Pygaster lineage) in both tempo and mode of evolution. “Conservatism” is a feature of these primitive Eognathostomata which contrasts with the more rapid and diverse morphological changes seen in other and later irregular echinoids. Holectypids and pygasterids were deposit-feeders but still relatively unspecialized as burrowers; they had therefore effectively achieved only the first of the two major adaptive advances which facilitated diversification of the irregulars as a whole.
