Early Palaeozoic jawed polychaetes with focus on polychaetaspids and ramphoprionids from the Silurian of Gotland, Sweden

Abstract: Jawed polychaete annelid worms were abundant and diverse in the early Palaeozoic seas and their jaws (scolecodonts) are common microfossils in sedimentary rocks of that age. Despite their abundance, relatively little work has been focused on these fossils. One reason for this is that their taxonomy has been, and to some extent still is, in a state of disorder, mainly because of a parataxonomic treatment. In the present state of knowledge, however, parataxonomy and paranomenclatural names can and therefore must be rejected from the research field. Integration of element-based and apparatus-based taxonomies results in great progress and a more reliable picture of the fossil jawed polychaete faunas. Such a taxonomic approach has been applied here during the investigation of early Palaeozoic jawed polychaetes. In the Silurian succession (latest Llandovery to latest Ludlow) of the island of Gotland, situated east of the Swedish mainland in the Baltic Sea, scolecodonts are among the most common microfossils. Based on extensive collections, approximately one hundred polychaete species of a dozen families can be identified in the entire succession. Two families, Polychaetaspidae and Ramphoprionidae, were of prime interest in this study. Polychaetaspidae comprises one of the most abundant and taxonomically diverse families in the Silurian of Gotland. Ten species belonging to three genera, Oenonites Hinde, Kozlowskiprion Kielan-Jaworowska, and Dubichaetaspis Eriksson, have been identified and described here. Of these, seven species and one genus, Dubichaetaspis, were previously unknown. In contrast to the polychaetaspids, the ramphoprionids are neither particularly diverse nor abundant. Nevertheless, they comprise a characteristic part of the faunas, at certain localities constituting a major faunal element. Six named species belonging to four genera, Protarabellites Stauffer, Ramphoprion Kielan-Jaworowska, “Pararamphoprion” Männil & Zaslavskaya, and Megaramphoprion Eriksson, have been identified and described. Of these, five species and one genus, Megaramphoprion, were previously unknown. A few specimens were described under open nomenclature pending further investigations. Studies of smaller scolecodont collections from the Silurian of Skåne (Scania), southern Sweden, and western England indicate that most ramphoprionids had a fairly wide geographical distribution. Like extant polychaetes, many of the investigated polychaetes were environmentally controlled; many species were stenotopic, showing strong preference for particular environments. A new monogeneric family, Hadoprionidae, has been identified and described from rich collections from the Cincinnatian (upper Ordovician) of Indiana, North America. One species of the new genus, Hadoprion, was identified. The jaw apparatus of H. cervicornis (Hinde) has formerly been described under four generic and eight specific names. Hadoprionids have also been identified in Silurian strata of northern Europe. Chemical analyses applying a nuclear microprobe, Particle Induced X-ray Emission (PIXE) and Scanning Transmission Ion Microscopy (STIM) analytical techniques, demonstrated enrichments of heavy metals in jaws of fossil and extant polychaetes. The metals, particularly Fe, Zn, and Cu, were predominantly concentrated in the anterior region of the jaws. Closely similar enrichments have been reported in structures exposed to high stress and/or abrasional forces of various other, marine and terrestrial, Recent higher taxa. This indicates that the metals have functional significance. Occasionally, such metal accumulations seem to have been responsible for the present damaged condition of some fossil scolecodonts.