FUNCTIONAL MORPHOLOGY AND PALEOECOLOGY OF STELLEROIDS OF THE HUNSRÜCK SLATE (EMSIAN, LOWER DEVONIAN)

Interpretation of life modes of extinct animals is problematic, as is exemplified by differences of opinion over familiar, well-studied examples: Was Tyrannosaurus a fierce predator or a scavenger depending on sense of smell? Was the beak of Diatryma used in predation or instead to chop coarse vegetation? Interpretation of generalist invertebrates, such as Paleozoic stelleroids, is still more problematic, but the taxonomic and morphologic diversity found among Hunsrück Slate stelleroids and their similarities with modern taxa invite evaluation of ancient life modes.

Detailed taxon-to-taxon comparisons are likely unreliable because of generalized behavior and the potential for use of structures for different functions, but features and suites of features can be compared. For example, authors have suggested that the specialized arm skeleton of extant ophiuroids is unlike that of many Paleozoic representatives, whose large tube foot basins suggest an ecology more like that of living asteroids. Discovery of asteroid-like fossilized tube feet in the Hunsrück ophiuroid Bundenbachia supports their interpretation, which then can be carried a step further: Because modes of life of extant ophiuroids is expressed by characters that are absent from many Paleozoic species, functional analogs for Hunsrück ophiuroids must be sought among extant asteroids.

Function of disk size has not been analyzed in detail, but a large disk is found in asteroids living on soft, particulate substrates and in sediment-swallowers; Luidia and Astropecten, however ingest shells but disk size is fairly small. Small disks occur in asteriids that wrap their arms about their prey, but also in some small-particle bottom feeders. Disk sizes and specific skeletal analogies, such as dense arrays of spines, are similar in Devonian and modern species. Multiarmed construction has been related to active predation in extant asteroids, and similar forms are found in both asteroids and ophiuroids of the Hunsrück. Modern suspension-feeding asteroids are marked by long, slender but strong and quite robust arms with prominent protective spines; the disk is small and sturdily constructed. Hunsrück Urasterella is similarly arranged, although it lacks pedicellariae found in the modern species. Much is available for analysis but understanding is preliminary.