Una biota transicional Ediacárica–Cámbrica en el anticlinal de Abenójar (Macizo Ibérico, España)

J Simón

doi:10.3989/egeol.43012.470

Abstract

Se describe una secuencia conteniendo una biota transicional ediacárica–cámbrica, en el valle del arroyo de las Navezuelas, anticlinal de Abenójar (Grupo Ibor, Zona Centroibérica, España). La secuencia empieza con capas siliciclásticas de grano fino conteniendo Vendotaenia antiqua y moldes de Cloudina cf. hartmannae, uno de los primeros animales con esqueleto mineralizado. Continúa con carbonatos con Cloudina fosfatizadas, coincidiendo en muestra de mano con Vendotaenia antiqua y con protoconodontos semejantes a Protohertzina unguliformis, lo cual sugiere una edad ediacárica terminal o cámbrica basal. Las capas detríticas sobre este nivel contienen pistas fósiles simples de organismos sedimentívoros. Sobre un estrato superior de carbonatos con pequeños vendoténidos se ha hallado un conjunto de pistas fósiles de aspecto cámbrico, incluyendo Treptichnus pedum y madrigueras ornamentadas semejantes a la superficie ventral de Psammichnites. El hallazgo de protoconodontos es nuevo para la transición Ediacárico–Cámbrico en Europa, y esta es la tercera vez a escala mundial que este tipo de fósiles típicamente cámbricos son hallados junto con cloudínidos, típicamente ediacáricos, lo cual sugiere que la transición Ediacárico–Cámbrico fue biológicamente más gradual de lo que se pensaba anteriormente.

Highlights

In the time of Darwin, one of the biggest challenges for his theory of evolution was the apparently sudden occurrence of the first fossils in the stratigraphic column, at the point currently recognized as the Lower Cambrian (Chapter X in Darwin, 1859)
This seeming jump in the fossil record has been tempered by a huge amount of data showing that the early animal evolution was a much more gradual process than previously thought, with simple multicellular organisms preceding the first skeletal animals, and these appearing before the Cambrian rise of shelly fossils (McMenamin, 1987; Knoll, 2003; Fedonkin et al, 2007)
A drastic evolutionary discontinuity is still widely thought to have occurred exactly at the Ediacaran–Cambrian boundary, where many Ediacaran organisms seem to disappear from the fossil record (Amthor et al, 2003; Fedonkin et al, 2007; Buatois et al, 2014)

Summary

Introduction

In the time of Darwin, one of the biggest challenges for his theory of evolution was the apparently sudden occurrence of the first fossils in the stratigraphic column, at the point currently recognized as the Lower Cambrian (Chapter X in Darwin, 1859). A drastic evolutionary discontinuity is still widely thought to have occurred exactly at the Ediacaran–Cambrian boundary, where many Ediacaran organisms seem to disappear from the fossil record (Amthor et al, 2003; Fedonkin et al, 2007; Buatois et al, 2014) In this usual view, that crisis preceded the rapid unfolding of the diversity of bilateral animals that characterizes the beginning of the Cambrian (the “Cambrian explosion”, Gould, 1989). That crisis preceded the rapid unfolding of the diversity of bilateral animals that characterizes the beginning of the Cambrian (the “Cambrian explosion”, Gould, 1989) According to this scheme, the Ediacaran to Cambrian transition signals a mass extinction followed by an evolutionary jump. This would explain why Ediacara-type fossils are absent from the Cambrian onwards, with a few exceptions (for example, Jensen et al, 1998; Hagadorn et al, 2000), they could have been declining before the Ediacaran–Cambrian boundary (Darroch et al, 2015)

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