Research Article: An archosauromorph dominated ichnoassemblage in fluvial settings from the late Early Triassic of the Catalan Pyrenees (NE Iberian Peninsula)

Date Published: April 19, 2017

Publisher: Public Library of Science

Author(s): Eudald Mujal, Josep Fortuny, Arnau Bolet, Oriol Oms, José Ángel López, David Carrier.


The vertebrate recovery after the end-Permian mass extinction can be approached through the ichnological record, which is much more abundant than body fossils. The late Olenekian (Early Triassic) tetrapod ichnoassemblage of the Catalan Pyrenean Basin is the most complete and diverse of this age from Western Tethys. This extensional basin, composed of several depocenters, was formed in the latest phases of the Variscan orogeny (Pangea breakup) and was infilled by braided and meandering fluvial systems of the red-beds Buntsandstein facies. Abundant and diverse tetrapod ichnites are recorded in these facies, including Prorotodactylus mesaxonichnus isp. nov. (tracks possibly produced by euparkeriids), cf. Rotodactylus, at least two large chirotheriid morphotypes (archosauriform trackmakers), Rhynchosauroides cf. schochardti, two other undetermined Rhynchosauroides forms, an undetermined Morphotype A (archosauromorph trackmakers) and two types of Characichnos isp. (swimming traces, here associated to archosauromorph trackmakers). The Pyrenean ichnoassemblage suggests a relatively homogeneous ichnofaunal composition through the late Early Triassic of Central Pangea, characterized by the presence of Prorotodactylus and Rotodactylus. Small archosauromorph tracks dominate and present a wide distribution through the different fluviatile facies of the Triassic Pyrenean Basin, with large archosaurian footprints being present in a lesser degree. Archosauromorphs radiated and diversified through the Triassic vertebrate recovery, which ultimately lead to the archosaur and dinosaur dominance of the Mesozoic.

Partial Text

The earliest Mesozoic fossil record represents the life recovery and ecosystems turnover [1] after the loss of most species during the end-Permian mass extinction [2]. The environmental and climatic conditions hindered recovery of life on land, delaying the recovery until the Middle Triassic [3, 4]. Concerning the vertebrate non-marine fauna, several lineage pulses took place during the Early Triassic [4], but the role of the main tetrapod groups in the recovery, as well as their paleogeographic and environmental distribution, are still a matter of discussion.

The Catalan Pyrenees (NE Iberian Peninsula, S Europe) are a significant target to understand the late Paleozoic—early Mesozoic evolution in terms of paleontology and paleobiogeography, as long and continuous sedimentary successions ranging from the late Carboniferous to the Middle Triassic are recorded (Fig 1). The reference studies by Mey et al. [23] and Nagtegaal [24] defined the “post-hercynian” units containing the studied fossils. Séguret [25] and Zwart [26] performed the basic regional geology and mapping, while more recent works focused on tectonics and basin architecture and evolution [27–30]. Fortuny et al. [31] revised the vertebrate paleontological content, and Mujal et al. [8] revised and provided new data on the Permian-Triassic transition.

In order to provide a detailed geographical and geological framework, we mapped three fossil-bearing localities based on field tracking of strata and photointerpretation (Fig 1). We logged (by means of a Jacobs staff and a centimeter) and correlated bed-by-bed a total of nine stratigraphic sections (numbered I to IX from West to East; Fig 2). The sedimentary facies and architectural elements (Figs 3 and 4) were classified following nomenclature of Miall [33, 34] and further compared with those of Gretter et al. [30].

The fossil-bearing unit corresponds to the redbed Buntsandstein facies, analyzed in three different areas, from W to E: Erillcastell (section I, of >151 m thick, including the Tossal de Pollerini tracksite, Erillcastell-Estac basin), Buira (section II, of >226 m, Erillcastell-Estac basin, at 8.32 km from Erillcastell) and Port del Cantó (sections III-IX, of 240 to 350 m, including the Rubió and Argestues tracksites, Cadí basin, at 27.52 km from Buira) (Figs 1 and 2).

The studied sequences yield a diverse tetrapod ichnoassemblage (Figs 2 and 5–13), including abundant footprints with extramorphological features (precluding ichnotaxonomic assignation) related to locomotion and substrate conditions. Here we provide a detailed description of the identified tetrapod footprint morphotypes. We erect a new ichnospecies and emend the corresponding ichnogenus (monospecific until now), which was only known from the Central European Germanic Basin so far. Some ichnotaxa are equivalent to those of the Palanca de Noves tracksite (12 km Eastwards from Argestues tracksite) [8], thus here we provide additional remarks.

The Triassic Pyrenean Basin is a key region of Western Tethys to understand the early Mesozoic non-marine evolution in terms of paleoenvironment and (ichno-) faunal diversity. The red-bed Buntsandstein deposits of the Catalan Pyrenees correspond to a fluvial setting evolving from high-energy braided systems to low-energy meandering and floodplain systems that infilled the depocenters generated during the Triassic rifting. In our case, the Buntsandstein facies are arranged as a fining-upwards sequence culminated by the marine transgression of the Muschelkalk facies. A relatively diverse fossil record is yielded in the red-beds, and tetrapod footprints are especially abundant. We identified an ichnoassemblage composed by Prorotodactylus mesaxonichnus isp. nov., cf. Rotodactylus, at least two different chirotheriid morphotypes, Rhynchosauroides cf. schochardti and two other Rhynchosauroides forms, an undetermined Morphotype A, and two types of swimming scratches corresponding to Characichnos isp. The potential trackmakers of P. mesaxonichnus isp. nov. are archosauriform euparkeriids, suggesting, together with the other ichnotaxa, an ichnofaunal homogeneity at least along Central Pangea. Prorotodactylus and Rotodactylus may characterize the late Early Triassic continental deposits. The trackmakers of all these ichnotaxa probably correspond to archosauromorphs, suggesting that this group became dominant in the instable terrestrial settings after the end-Permian mass extinction. Archosauromorphs may represent the main pull of vertebrate recovery that lead to the further ecosystem stabilization of the Middle Triassic, with a turnover to larger archosaurian faunas and the radiation of this lineage.




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