The Echinoid Directory

Echinothurioids are almost exclusively found in the deep sea, although Asthenosma ijimai is an exception, being found not uncommonly at 18-20 m water depth off Japan. They are epifaunal benthic scavangers, although little is known about their diet. Mortensen (1935) reported that, from their gut contents, they are primarily ingesting bottom detritus, notably macroplant material. The few developmental studies that have been carried out show that echinothurioids are lecithotrophic and have yolky eggs that develop in, at, or near the surface (Young & Cameron 1987). The globiferous pedicellariae can be particularly toxic.

Young, C. M. & Cameron, J. L. 1987. Laboratory and in situ flotation rates of lecithotrophic eggs from the bathyal echinoid Phormosoma placenta. Deep-Sea Research 34, 1629-1639.

Echinothurioids are the rarest of all fossil echinoids. Yet the first representative of this group to be described was a fossil, the Upper Cretaceous Echinothuria. Only some years later, with the Challanger expedition and the first explorations of the deep sea, were living echinothurioids discovered. The imbricate and thin skeleton gives them a very low preservation potential and what few records of echinothurioids there are are mostly of isolated plates and spines.

The earliest definitive echinothurioid is Pelanechinus, from the Middle and Upper Jurassic. This is found preserved in carbonate shelf environments and, unlike later echinothurioids, this animal has a relatively thick test and well developed phyllodes.

The earliest crown group echinothurioid is Echinothuria, from the Upper Cretaceous of England. This is found in deep-water Chalk environments. As it nests high within the crown group, considerable diversification of crown group echinothurioids must already have taken place by this time.

Mortensen (1935) attributed isolated spines from the Maastrichtian and Danian to species of Araeosoma and Asthenosoma but in reality the spine morphology is not distinctive enough to place them more precisely than as Echinothuriinae gen. et sp. indet. There are, in addition, a handful of Miocene or Pliocene records from deep-sea sediments.

Mortensen, T. 1935. A monograph of the Echinoidea. Volume 2, Bothriocidaroida, Melonechinoida, Lepidocentroida and Stirodonta. C.A. Reitzel, Copenhagen.