Holectypoids are an extinct clade showing a mixture of primitive and derived features. They have a large and powerful lantern with wedge-shaped teeth, like regular echinoids, but their periproct is displaced out of the apical disc as in irregular echinoids. The arrangement of oral tubercles suggests that holectypoids excavated sediment radially out from underneath themselves and thus presumably lived buried in sediment. However, the lack of respiratory tube-feet aborally and the rather sparse spine canopy indicate that they must have been restricted to living within relatively coarse substrata. Given these observations it seems probable that Jurassic taxa such as Holectypus or Pygaster (figured above) lived buried for part of the time, emerging, possibly at night, to feed like a regular echinoid. A number of more derived genera probably lived entirely on the surface of the sediment. It is possible that they covered their test in debris picked up from the sea floor much like many present-day regular echinoids.
The group first appears in the Early Jurassic and became an important and commonly encountered member of shallow marine communities from the Middle Jurassic onwards. Several lineages specialised for life in the chalk seas in the Late Cretaceous, with shallow-water representatives rare after the Cenomanian. They became extinct at the end of the Cretaceous.
Smith, A. B. 1984. Echinoid Palaeobiology. George Allen & Unwin, London.