The phacopid trilobiteEchidnops taphomimusn. sp. from the Lower Devonian of Argentina: insights into infaunal molting, eye architecture and geographic distribution

Abstract

Some Silurian-Devonian Argentinian trilobites characterized by infaunal behavior during molting are considered. After a taxonomic reappraisal, a species previously referred to a lineage of the phacopidPaciphacopsis proposed asEchidnops taphomimusnew species, from the Lower Devonian (probably late Lochkovian) of the Talacasto Formation, Argentine Precordillera. The visual surface ofE.taphomimusindicates that a irregular pattern of lens arrangement, typical of early phacopids such as the OrdovicianOrmathops, can also be recognized in more derived Devonian relatives, providing new insights on some evolutionary aspects of visual development.Echidnopsis recognized in Australia and Argentina, recording an unusual distribution pattern in trilobites from the Lower Devonian of southern South America, otherwise mostly linked to faunas of related Gondwanan, austral circum-polar Devonian basins of the Malvinokaffric Realm. In accordance with proposed increasing predation pressure in the context of the Mid Paleozoic Marine Revolution, evidence indicates that the infaunal molting behavior, as defensive strategy, arose in several trilobite groups during Silurian-Devonian times, rather than in a single endemic lineage ofPaciphacopsspecies.

First report of Devonian bactritids (Cephalopoda) from South America: paleobiogeographic and biostratigraphic implications

“…This interval contains corals preserved in life position within large lenticular calcareous nodules (up to 50 cm wide), which are equivalent to the Lochkovian records just 70 m above the base of the Talacasto Formation in the nearby Quebrada de los Algarrobos, as reported by Carrera et al (2013). Specimens PULR-I 1–3 were also collected from the lower argillaceous interval of the same unit at Sierra de las Minitas, in La Rioja Province from a shaly marker unit bearing fossiliferous nodules containing Lochkovian trilobites such as Echidnops taphomimus Rustán and Balseiro, 2016 and Talacastops zarelae Edgecombe, Vaccari, and Waisfeld, 1994 (Rustán et al, 2011a, b). In turn, the specimen CEGH-UNC 27089, assigned to Bactrites gracilis , was collected 30 meters below Keidel’s bed at the Quebrada de Talacasto locality, of probable Pragian age.…”

“…In the case of fossils coming from isolated outcrops corresponding to the argillaceous lower part of the Talacasto Formation in the Sierra de Las Minitas, La Rioja Province, the distinctive association of trilobites (including the rare calmoniid Talacastops zarelae Edgecombe et al, 1994) and its taphonomic signature (infaunal molting in the phacopid Echidnops taphomimus Rustán and Balseiro, 2016) allow a precise correlation with an equivalent low position at the Las Aguaditas section in San Juan Province (Edgecombe et al, 1994), thus supporting a Lochkovian age for the fossiliferous interval (Rustán et al, 2011a, b).…”

Palynomorphs from the Devonian Talacasto and Punta Negra Formations, Argentinean Precordillera: New biostratigraphic approach

“…2), interpreted as an external muddy platform with no influence of wave action and a low sedimentation rate (Astini, 1991). Towards the top, it passes to sandrich deposits influenced by wave action (Astini, 1991;Carrera and Rustán, 2015;Rustán and Balseiro, 2016). The Talacasto Formation was considered as a muddy-shelf depositional system, developed during a high stand (Astini, 1991).…”

Devonian bryozoans from Argentina: new cosmopolitan components of Southwestern Gondwanan basins

“…In this scenario, new contributions on poorly known taxonomic groups from the Devonian of the Precordillera, when compared to records from closely related basins, called attention to a number of contrasting biogeographical patterns in Southwestern Gondwana, which in many cases challenge the idea of strongly endemic faunas. Remarkably, trilobites, although exhibiting overall endemic signatures, record unexpected extra-Malvinokaffric affinities (Rustán and Vaccari, 2010, 2012; Holloway and Rustán, 2012; Rustán and Balseiro, 2016). Ostracodes show wide distributions during Lochkovian while Emsian–Middle Devonian ones record frequent Malvinokaffric endemism (Salas et al, 2013; Salas, 2016).…”