Chapter 18 Ordovician and Silurian polychaete diversity and biogeography

Abstract

Eunicidan polychaetes formed a significant part of Early Palaeozoic marine invertebrate communities, as shown by the abundance and diversity of scolecodonts (polychaete jaws) in the fossil record. In this study we summarize the early radiation and biodiversity trends and discuss the palaeobiogeography of these fossils. The oldest (latest Cambrian–Early Ordovician) representatives had primitive, usually symmetrical, placognath/ctenognath type jaw apparatuses. The first more advanced taxa, possessing labidognath-type jaw apparatuses or placognath apparatuses with compound maxillae, are first recorded in the Middle Ordovician. The most significant increase in generic diversity occurred in the Darriwilian, when many common taxa appeared and diversified. The Ordovician and Silurian scolecodont occurrences allow some palaeobiogeographical units and distribution patterns to be explored and outlined. The most robust data presently at hand derive from successions in Baltica and Laurentia. That information, together with new records from other palaeocontinents, reveals a wide distribution for the most frequent and species-rich genera and families, similar to the biogeographical patterns of extant polychaetes. Like many other benthic and pelagic fossil groups, scolecodont-bearing polychaetes show an increased cosmopolitan character in the Silurian as compared with the Ordovician. Species-level endemism appears to be relatively common, inferring a potential for scolecodonts as biogeographical tools in the future.

South-west Sri Lanka: a floristic refugium in South Asia

“…This makes it difficult to find clear-cut evidence for 'outof-India' dispersal. However, with the palynological evidence (Morley, 2000;Prasad et al, 2009) and findings from phylogenetic reconstruction studies, a far greater role for an 'Out of India' hypothesis for migration of plant taxa from Indian Gondwana to Asia is emerging Ashton, 2014;Conti et al, 2002;Eriksson et al, 2004;Datta-Roy and Karanth, 2009). South West Sri Lanka where an ever-wet climate still prevails represents a potential refuge for hyper-rich biota of perhumid climates and consequently, a unique phylogeographic oasis in an otherwise seasonal region in South Asia.…”

Jaw-bearing polychaetes of the Silurian Eramosa Lagerstätte, Ontario, Canada

“…Although the collection at hand is limited, their abundance in the Park Head residues, together with the bedding plane specimens from Wiarton, indicates that the Paulinitidae was the most common polychaete family in the Eramosa environments. This is not surprising considering their abundance and diversity in Silurian strata globally (Bergman, 1989;Eriksson et al, 2004Eriksson et al, , 2013Tonarová et al, 2012).…”

Early Middle Ordovician scolecodonts from north‐western Argentina and the emergence of labidognath polychaete jaw apparatuses

“…; Hints & Eriksson ; Eriksson et al . ). However, the Early and early Middle Ordovician record of scolecodonts, and hence that of jaw‐bearing polychaetes, is incomplete and only a handful of reports are available worldwide (Underhay & Williams ; Cooper et al .…”

“…; Eriksson et al . ). This largely coincides with the main phase of the Great Ordovician Biodiversification Event (GOBE; see e.g.…”