Today, salenioid sea-urchins are confined to the deep-sea, living at depths of between 100 and 3000 m, with most species found at between 100 and 500 m. They are exclusively epifaunal and have long needle-like spines to defend themselves against predators. Their diet is mixed, as deduced from gut contents, which comprises sediment with benthic organisms; so it would appear that they are best classified as scavangers and opportunistic carnivores. The function of the often prominent pitting and sculpturing of the apical disc remains a mystery.
They first appeared in the late Jurassic, approximately 160 million years ago, and were shallow water inhabitants. Some groups started moving into deeper water continental shelf settings in the mid Cretaceous and by the late Cretaceous many lineages had specialized for low-nutrient conditions of the chalk seas. After the early Palaeocene Salenioida become rare as fossils; they apparently moved off the continental shelf into the deep sea environment, where they are confined to today.
Illustration: Salenocidaris cincta (Agassiz) from Sagami Bay, Japan (from Shigei, M. 1986 The sea urchins of Sagami Bay. Maruzen Co, Tokyo, pl. 12, fig.3).