Tremichnus: a new ichnogenus of circular-parabolic pits in fossil echinoderms

The new ichnogenus Tremichnus is proposed to include simple circular-parabolic pits, with or without associated stereom swellings, on fossil echinoderms, primarily crinoids. Tremichnus is a common trace fossil that is largely confined to columns and calyces of Paleozoic crinoids; the ichnogenus ranges at least from Middle Ordovician to Permian, and perhaps into the Mesozoic. Four new ichnospecies are also defined: T. paraboloides, the type species, comprising deep circular-parabolic pits, 0.15-3.5 mm, without associated gall-like swellings: T. cysticus, similar, though smaller pits surrounded by cystose masses of stereomatic secretion; T. minutus, uniformly small, non-overlapping pits commonly surrounded by raised rims; and T. puteolus, very large, shallow pits generally with a concentric inner ring-like groove. A similarly large pit, T. sp. aff. T. puteolus occurs on diploporitan cystoids. Review of mode of occurrence of these pits suggests that Tremichnus was the work of a sessile, host-selective epibiont, probably a parasite or a commensalistic filter feeder. The pits were apparently produced by a combination of embedment (i.e., inhibition of stereom growth) and some true boring (i.e., removal of stereom).