Phosphatized echinoderm remains from upper Lower Ordovician strata of northern Öland, Sweden : preservation, taxonomy and evolution

Abstract: Phosphatization of late Early Ordovician echinoderm fragments from N Öland has resulted in excellent preservation of the echinoderm stereom ultra structure. The stereom investigated shows a close relationship to different types found in recent echinoderms. The several different types of phosphatic preservation found have been controlled by diagenetic relationships. The original stereom structures have also been a controlling factor in the distribution of three- dimensional preservational types. The stratigraphically oldest samples from the Gillberga quarry, N Öland, SE Sweden, are dominated by rhombiferans while the youngest samples are dominated by crinoids. This might reflect that the distribution of echinoderm classes in Gillberga has mainly been controlled by changing bottom conditions. The distribution and dominance of the different articulation types in echinoderm columnals closely follow the evolutionary pattern of columnal articulation as described by various other authors. It is also shown that amongst crinoids the species domination has mainly been controlled by articulational type. A major faunal shift is evident across the Arenig - Llanvirn boundary, and this could be linked to evolutionary and palaeoenvironmental controls on the echinoderm fauna. Asterozoans are rather common in some samples and they have obviously played a more important role in the echinoderm fauna at this time than has been recognised before. Amongst the asterozoans, the ophiuroids clearly dominate the samples. A possible occurrence of asteroids is also noted. The distribution of stylophorans could also most likely be linked to the depositional environment. Cyclocystoids, previously reported from other localities in Öland, have also been found. Thus the best-yielding levels, just below the Llanvirn boundary, contained fragments representing at least five different echinoderm classes, and also within these there is a surprisingly high diversity.