WAS MELONECHINUS A PELAGIC ECHINOID?

The Paleozoic echinoids represented by the family Palaechinidae were peculiar even in comparison with other Paleozoic echinoids. They were cosmopolitan and locally abundant during a time when echinoids were rare. Specimens of Melonechinus are among the largest known echinoids from any period. The thickness of the coronal plates, exceeding plate diameter by 2-4 times, set them apart from all other echinoids. They were covered in short, fine miliary spines that could not have offered much protection against predators. They were in general very well preserved in death; fossils are commonly composed of the entire test – another rarity among the imbricate and flexible Paleozoic forms. In addition, many fossils preserve the spherical shape of the living animal, although any particular slab of specimens may contain both inflated and deflated forms in various orientations.

Details of constructional morphology and taphonomy can be compared with that of floating crinoids like the Mesozoic Uintacrinus or the loboliths of the floating Paleozoic crinoid Scyphocrinites, suggesting that Melonechinus (and other palaechinids) may have been buoyant. There are a number of significant differences between Melonechinus and large modern benthic echinoids such as Echinus melo or deep-sea echinothurioids (very motile flexible echinoids). A critical examination of the assumption that Paleozoic echinoids were benthic because modern echinoids are benthic shows that many of the odd traits of palaechinids can be explained by a pelagic (or semi-pelagic) lifestyle. Such a lifestyle could also explain how these echinoids were able to grow to such large size, maintain a spherical shape, and still manage to move, feed, and protect themselves from predators.