Taphonomic analysis of Batallones-10, a Late Miocene drought-induced mammalian assemblage (Madrid basin, Spain) within the Cerro de los Batallones complex

Highlights

• Batallones-10 (Late Miocene) hosts three drought-induced mammalian assemblages.

• Recurrent phenomenon in modern ecosystems, but rarely identified in fossil record.

• Batallones-10 provides detailed taphonomic information on such unusual assemblages.

• It completes the taphonomic histories of the whole Cerro de los Batallones complex.

Abstract

The Late Miocene Batallones-10 site (Madrid Basin, Spain) contains several authochthonous multitaxic assemblages. A total of 15 large-mammal species are documented, including hipparionine horses (Hipparion sp.), giraffes (Decennatherium rex) and moschids. Three discrete fossiliferous levels have been identified with the aid of artificial intelligence systems. Although there are some statistical differences in the faunal composition of each level, paleontological, geologic and taphonomic data from all three Batallones-10 levels point to similar taphonomic histories. Mortality data, with abundant young individuals and the presence of pregnant females, indicates a catastrophic process of bone accumulation mainly driven by prolonged drought. Relatively high proportions of partially preserved or fully-articulated skeletons, and absence of carnivore feeding marks on bones, suggest that carcasses were undisturbed by predation. Rapid desiccation of soft tissues kept many of the joints intact, resulting in a high proportion of articulated remains. During drought episodes, large mammalian herbivores congregated around a shrinking water-hole, depleting the local vegetation, such that weakened individuals died of starvation and miring rather than dehydration. When rainfall resumed, the lack of vegetation and presence of low-permeability surficial sepiolite-rich marls, resulted in rapid run-off. This buried most of the unweathered, unabraded remains, whilst slightly orienting or completely removing others. After burial, faulting and slickensides produced diagenetic fractures, which favoured the precipitation of manganese oxides on bone surfaces and the diagenetic corrosion of remains, which is more pronounced in the uppermost Level III. This work, which focuses on the taphonomy of a herbivore-dominated site, improves our overall knowledge of the formation of the Cerro de los Batallones palaeontological complex.

Introduction

Batallones-10 is one of the nine palaeontological sites hosted in the Cerro de los Batallones (Batallones Butte) complex, located 30 km south of Madrid (Spain, Fig. 1A), in the municipality of Torrejón de Velasco (Fig. 1B). These sites are composed of exceptionally rich and diverse faunal assemblages (Morales, 2017), including herbivorous mammals, such as proboscideans (Alberdi et al., 2017), perissodactyls (Romano et al., 2017; Sanisidro and Cantalapiedra, 2017; Domingo et al., 2018), artiodactyls (Sánchez et al., 2009; Sánchez et al., 2011; Pickford, 2015; Ríos et al., 2017; Ríos and Morales, 2019), as well as carnivorans, including sabretooth cats (Antón et al., 2004; Salesa et al., 2006, Salesa et al., 2008, Salesa et al., 2010; Monescillo et al., 2014; Siliceo et al., 2014; Antón et al., 2020), smaller felids (Salesa et al., 2012; Salesa et al., 2017), hyaenids (Fraile, 2016, Fraile, 2017), amphicyonids (Peigné et al., 2008; Siliceo et al., 2015; Morales et al., 2017; Siliceo et al., 2017, Siliceo et al., 2020), ailuropod ursids (Abella et al., 2012, Abella et al., 2013, Abella et al., 2015; Abella and Valenciano, 2017), ailurids (Peigné et al., 2005; Antón et al., 2006; Salesa et al., 2006), mustelids and mephitids (Valenciano et al., 2015; Valenciano, 2017; Valenciano et al., 2020; Valenciano and Govender, 2020) and a rich assemblage of small mammals, including insectivores, rodents and lagomorphs (López-Antoñanzas et al., 2010, López-Antoñanzas et al., 2014; Álvarez-Sierra et al., 2017; Medina-Chevarrías et al., 2019). Other vertebrates identified at the Cerro de los Batallones sites include birds, fishes, amphibians, and reptiles, including large and small tortoises and lizards (Morales, 2017).

Based on the large and small mammal faunal association, the Cerro de los Batallones sites are inferred to have a late Vallesian age (ca. 9.1 Ma; early Late Miocene; Domingo et al., 2007; Morales et al., 2008; López-Antoñanzas et al., 2010; Gómez-Cano et al., 2011), which places them in the middle of the Vallesian Crisis, a period of profound faunal reorganization in the Iberian peninsula associated to global changes in climate seasonality (Domingo et al., 2014; Gómez-Cano et al., 2014; Azanza et al., 2017; Blanco et al., 2018; Blanco et al., 2021). Small mammal studies place the Batallones localities either in the J2 local subzone (9.71–9.48 Ma) or in the J3 local subzone (9.34–9.95 Ma), with Batallones-10 being the oldest of the sites, followed by Batallones-1 and with Batallones-3 being the youngest site (López-Antoñanzas et al., 2010).

The Cerro de los Batallones sites are hosted in cavities with an hourglass-shaped vertical profile (Fig. 1C), formed due to pseudokarstic processes in the Late Miocene (Pozo et al., 2004; Calvo et al., 2013). These cavities were scattered throughout a woodland landscape with wooded grassland patches (Domingo et al., 2013b, Domingo et al., 2016). Cavities have been historically separated into two distinct fossil assemblages, one in the upper part and another in the lower part of the hourglass, separated by nonfossiliferous deposits in the connecting conduit (Fig. 1C). Deposits in the lower cave host carnivoran-dominated assemblages, formed by carnivores entering the cave and becoming trapped (Abella et al., 2017, Domingo et al., 2011, Domingo et al., 2012, Domingo et al., 2013a; Martín-Perea et al., 2021), whereas the upper deposits contain more typical, herbivore-dominated assemblages. Additionally, a recent study has confirmed the existence of discrete fossiliferous levels in both of these upper and lower fossil assemblages, using artificially intelligent systems (Martín-Perea et al., 2020b).

The most important of these herbivore-dominated assemblages in Cerro de los Batallones is Batallones-10. This site was discovered in 2007 in the context of mining works for the extraction of sepiolite, which were carried out in the area. Subsequent geophysical research showed that this site had a larger extension than other known herbivore-dominated localities at Cerro de los Batallones. Besides, very promising, well-preserved, abundant material was unearthed during its discovery. This is why the Cerro de los Batallones team decided to start excavating the site in the same year that it was discovered. Since then, Batallones-10 has undergone 14 excavation campaigns between the years 2007 and 2020 and together with the carnivoran-dominated sites of Batallones-1 and Batallones-3, constitute the Cerro de los Batallones localities that have been excavated for a longer period of time.

Batallones-10 contains three discrete fossiliferous levels (Martín-Perea et al., 2020b). The assemblages are hosted in marls deposited disconformably in a small water-hole that developed over sepiolite-rich mudstones, marls and limestones (Pozo et al., 2004; Calvo et al., 2013). The cavity is inferred to have formed due to erosion, along faulting that was caused by deep dissolution processes. As indicated above, Batallones-10 is an exceptionally rich and diverse herbivore-dominated assemblage with bones generally showing excellent preservation. It contains the best assemblages of hipparionine horses (Hipparion sp.), giraffes (Decennatherium rex) and moschids of the Iberian Peninsula, based on the quantity and quality of their remains. Although formation processes of carnivoran-dominated sites have been widely studied in the Cerro de los Batallones complex (Domingo et al., 2011, Domingo et al., 2012, Domingo et al., 2013a; Martín-Perea et al., 2021), an exhaustive taphonomical study of the herbivore-dominated sites is still lacking. A single study, based on the analysis of Hipparion sp. dental ontogeny sequences at Batallones-10, offered some taphonomic inferences (Domingo et al., 2018). According to the mortality profiles inferred at Batallones-10, this study proposed the assemblage was the result of a gradual, attritional mortality profile. At the time, however, Batallones-10 was still believed to contain only a single level. In light of this, the Batallones-10 Hipparion sp. sample was considered as a single unit. Subsequently, three separate levels were identified by Martín-Perea et al. (2020b). Attritional mortality is often observed in sites frequented by taxa over long periods of time, such as carnivore dens (Marean and Ehrhardt, 1995) or watering holes (Agenbroad, 1978, Barnosky, 1985). Given that Batallones-10 was deposited in a small water-hole (Calvo et al., 2013), we propose as our starting hypothesis that the Batallones-10 assemblage conforms to an attritional model, where animals died of natural causes around the waterhole that they habitually visited.

This study presents a thorough taphonomical analysis of each of the discrete fossiliferous levels of Batallones-10, to further understand the biostratinomic and diagenetic processes which led to the formation of this assemblage. By focusing on the most representative locality containing these poorly-researched herbivore-dominated upper assemblages, this study is key to obtaining a more complete picture of the taphonomic histories of the Cerro de los Batallones sites.