Early Middle Ordovician scolecodonts from northwestern Argentina and the emergence of labidognath polychaete jaw apparatuses

ERIKSSON, MATS E.; TONAROVÁ, PETRA; DE LA PUENTE, G. SUSANA; HINTS, OLLE; RUBINSTEIN, CLAUDIA V.

PALAEONTOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2017 vol. 60 p. 583 - 593

0031-0239

Scolecodonts provide the fossil evidence of the evolution and diversification of jaw-bearing polychaetes from the latest Cambrian and onwards. Their record before the Darriwilian (Middle Ordovician) is, however, scarce worldwide, which limits our understanding of key evolutionary events. One of such event is the emergence of taxa possessing the asymmetrical labidognath-type jaw apparatus architecture, which became common in the Middle Ordovician and are often dominant throughout the Palaeozoic. In this paper we document a small collection of Dapingian scolecodonts from the Capillas section, Sierras Subandinas, northwestern Argentina. The isolated elements recovered allowed for the reconstruction of a distinctive jaw apparatus and introduction of a new taxon, Andiprion paxtoni gen. et sp. nov. The maxillary apparatus of Andiprion is intermediate between the symmetrognath type of the Early Ordovician Kadriorgaspis and the labidognath type such as those present in polychaetaspids and related taxa. The apparatus architecture of Andiprion corresponds best to the labidognath type, but the morphology of the individual jaws suggests that it may be the most primitive representative of this lineage currently known. We propose that Andiprion-like forms were ancestral to polychaetaspids, polychaeturids and ramphoprionids. The Capillas collection provides supporting evidence for the evolutionary homology of the ?basal plate? and the left first maxilla. Thus the labidognath type asymmetry, with an unpaired left maxilla III, developed on account of gradual diminishing of the first right jaw (?basal plate?) in front of the carriers, instead of loss or fusion of anterior maxillae.