Palaeoecology and taphonomy of spatangoid-dominated echinoid assemblages: A case study from the Early-Middle Miocene of Sardinia, Italy

Abstract

The Early-Middle Miocene marine sedimentary succession of the Borutta Formation cropping out at Porto Torres (northern Sardinia) represents one of the most diverse and well-preserved spatangoid-dominated sedimentary successions in the Miocene of the Mediterranean area. The echinoid-bearing succession is studied with the twofold purpose of firstly reconstructing the palaeoenvironmental setting in which these echinoids occurred, and secondly investigating factors influencing the preservation potential of spatangoid echinoids in the fossil record. These goals are accomplished by 1) detailed logging of sedimentary facies, 2) applying functional morphological interpretations of echinoid skeletons in order to reconstruct their behaviour and life habits, 3) quantifying the relative abundance of taxa and taphonomic features of test preservation, and 4) analyzing associated trace fossils within the beds.The sedimentary succession is composed mainly of fine-grained sandstones which are intensely bioturbated by Thalassinoides-like burrows. Spatangoid abundance is high throughout the profile declining only in intercalated dm-thick rhodolith beds. A 20-cm-thick bed with a bedding surface of approximately 250m2 was investigated in detail with respect to relative abundance of spatangoid taxa and taphonomic signatures. The echinoids consist almost exclusively of deposit-feeding irregular forms with a dominance of the spatangoids Ova and Brissopsis and the minute clypeasteroid Echinocyamus. The spatangoids Opissaster, Holanthus, Metalia and Hemipatagus as well as the clypeasteroid Clypeaster marginatus are also present.A low energy, moderately deep, outer sublittoral environment with high rates of bioturbation and episodes of sediment deposition by storms is inferred. Interspecific competition among deposit-feeding echinoids appears to be limited by their different burrowing depths, feeding strategies and resource partitioning. The gregarious behaviour of spatangoid echinoids, their habitat in quiet, deeper environments and episodes of rapid deposition seem to represent the main factors leading to this spatangoid-bearing deposit. Since both shallow- and deep-burrowing spatangoid echinoids are well represented as complete specimens, mode of life does not appear to play a major role in their preservation potentials.