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

Abstract

Scolecodonts provide fossil evidence of the evolution and diversification of jaw‐bearing polychaetes from the latest Cambrian onwards. However, their record before the Darriwilian (Middle Ordovician) is scarce worldwide, which limits our understanding of key evolutionary events. One such event is the emergence of taxa possessing the asymmetrical labidognath‐type jaw apparatus architecture, which became common in the Middle Ordovician and is often dominant throughout the Palaeozoic. Here, we document a small collection of Dapingian scolecodonts from the Capillas section, Sierras Subandinas, north‐western Argentina. The isolated elements recovered allowed us to reconstruct the distinctive jaw apparatus, and to introduce a new taxon, Andiprion paxtonae gen. et sp. nov. The maxillary apparatus of Andiprion is intermediate between the symmetrognath type of the Early Ordovician Kadriorgaspis and the labidognath type that is 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 as a result of gradual reduction in size of the first right jaw (‘basal plate’) in front of the carriers, instead of loss or fusion of anterior maxillae.

New insights into the morphology and evolution of the ventral pharynx and jaws in Histriobdellidae (Eunicida, Annelida)

“…Within annelids, Eunicida is one of the most prominent jaw-bearing taxa, and numerous analyses of anatomical features of the jaw apparatus have been published thus far. Notably, evolutionary hypotheses regarding the jaw morphology of different eunicidan families, which are always based on complete apparatuses, have scarcely included extinct families [5][6][7][8]. However, the number of extinct families considerably exceeds the number of extant families [8].…”

Boreognathus pogorevichi, a remarkable new polychaete annelid from the lower Permian of the Pechora Basin, Russia

“…cornutus Kielan‐Jaworowska, 1962) maxillae labelled according to: A, neontological convention; B, palaeontological convention (see Paxton 2009; Hints et al . 2017). C, Skalenoprion alatus Kielan‐Jaworowska, 1962, MI and MII.…”

“…2016; Hints et al . 2017), the fossil record of Permian (and younger) scolecodonts and jaw apparatuses is still far from perfect (e.g. Tasch & Stude 1966; Szaniawski 1968; Taugourdeau 1978; Nakrem et al .…”

Indicadores de crecimiento y de uso para las colecciones paleontológicas del IANIGLA-CCT CONICET Mendoza (Argentina)

“…Las colecciones de Paleobotánica (IANIGLA-PB), Paleovertebrados (IANIGLA-PV), Paleoinvertebrados (IANIGLA-PI) e Icnología (IANIGLA-Icn) contienen en la actualidad unos 3000 ejemplares, colectados en secuencias paleozoicas, mesozoicas y cenozoicas fundamentalmente del centro-oeste de Argentina (provincias de Mendoza, San Juan y Neuquén) y en menor cantidad en secuencias paleozoicas del noroeste (provincias de Salta y Jujuy). Dichas colecciones incluyen 10 holotipos y 63 paratipos (Devincenzi, 2016), a los que se suman ocho holotipos y tres paratipos publicados recientemente (Gnaedinger et al, 2015, Agnolin et al, 2016Lara & Wang, 2016;Pirrone & Buatois, 2016;Beresi et al, 2017;Gouiric-Cavalli et al, 2017;Hints et al, 2017). Las tres primeras colecciones mencionadas están adheridas al Sistema Nacional de Datos Biológicos (SNDB) y publicadas parcialmente en el Global Biodiversity Information Facility (GBIF) (Devincenzi et al, 2018a, b, c).…”