Direct evidence of megamammal-carnivore interaction decoded from bone marks in historical fossil collections from the Pampean region

Introduction

Reconstructing the biotic interactions between extinct organisms, including competition or predator–prey relationships, is an extremely difficult task, especially when the information available from living analogues is limited (Figueirido, Martín-Serra & Janis, 2016). This is particularly true in the case of ancient South American ecosystems, as members of the megafauna became extinct during the latest Pleistocene-early Holocene, and these groups of mammals have no living counterparts (Cione, Tonni & Soibelzon, 2009; Fariña, Vizcaíno & De Iuliis, 2013).

Megamammals from the southern portion of South America, or the Pampean (Argentinean) region, have fascinated scientists since the 18th century. Nevertheless, different studies performed to understand their palaeoecology are much more recent (e.g.,  Fariña, 1996; Bargo, 2003; Prevosti, Zurita & Carlini, 2005; Prevosti & Vizcaíno, 2006; Figueirido & Soibelzon, 2010; De Los Reyes et al., 2013; Fariña, Vizcaíno & De Iuliis, 2013; Scanferla et al., 2013; Soibelzon et al., 2014; Bocherens et al., 2016). To this respect, carnivore marks preserved on fossil bones of megaherbivores constitute an important source of information, as they represent direct evidence of predator–prey relationships, or alternatively, of scavenging activity by top predators such as strictly flesh-eating or bone-cracking hypercarnivores, respectively (e.g., Haynes, 1982; Marean & Ehrhardt, 1995; Pobiner & Blumenschine, 2003; Pickering, Egeland & Brain, 2004; Palmqvist et al., 2011; Espigares et al., 2013). Consequently, detecting the marks of biological activity preserved on the bone surfaces of Pampean megamammals, using detailed taphonomic investigations and next-generation techniques, is crucial for deciphering the ecological relationships between Pleistocene South American palaeocommunities.

Previous studies of bone surfaces performed on fossil collections housed in various museums in the Americas have revealed carnivore activity, and hence animal interaction (Haynes, 1980; Martin, 2008; Martin, 2016; De Araújo Júnior, De Oliveira Porpino & Paglarelli Bergqvist, 2011; Dominato et al., 2011; Labarca et al., 2014). Indeed, in South America, carnivore marks have been reported from different locations (Fig. 1). Specifically in the Pampean region, there is a neural apophysis of a glyptodont cf. Eosclerocalyptus lineatus (Glyptodontidae, Hoplophorini) from the Pliocene (Olavarría) with a clear carnivore tooth imprint, attributed to a giant Chapalmalania (Carnivora, Procyonidae) procyonid (De Los Reyes et al., 2013). Recently, a taphocoenosis from the margins of the Salado River, comprising remains of the equid Hippidion principale (Perissodactyla, Equidae) and some indeterminate bones with carnivore marks was associated with the dirk-toothed sabre cat Smilodon sp. (Carnivora, Felidae, Machairodontinae) (Scanferla et al., 2013). At the archaeological site Arroyo Seco 2, bones of extinct horses such as Equus sp. (Perissodactyla, Equidae) show carnivore marks (Politis et al., 2016). In Patagonia, the jaguar Panthera onca mesembrina (Carnivora, Felidae, Pantherinae) was reportedly responsible for interventions involving the ground sloth Mylodontidae (Xenarthra, Tardigrada) and Hippidion groups (Martin, 2008; Martin, 2016), and a member of Felidae produced marks on mastodont (Proboscidea, Gomphotheriidae) bones (Labarca et al., 2014) during the late Pleistocene. In Brazil, two sites have been described where the small canid Protocyon troglodytes (Carnivora, Canidae) presumably scavenged the carcasses of two mastodons, Notiomastodon platensis (Proboscidea, Gomphotheriidae), the giant ground sloths Eremotherium laurillardi (Tardigrada, Megatheriidae) and Glossotherium (Tardigrada, Mylodontidae) (De Araújo Júnior, De Oliveira Porpino & Paglarelli Bergqvist, 2011), and Haplomastodon waringi (Proboscidea, Gomphotheriidae) in the Pleistocene (Dominato et al., 2011).

In this article, we study for the first time, carnivore marks on megamammal (>1,000 kg; Cione, Tonni & Soibelzon, 2009) remains from different fossil collections recovered from the Pampean region and now housed in various institutions in Europe and Argentina. Our goal is to identify potential biological activity using taphonomic methods in order to understand predator-megaherbivore interaction within Pleistocene South American mammalian communities from the Pampean region.

Materials & Methods

In order to identify those bones showing evidence of carnivore intervention, we examined 1,976 bones belonging to the following four collections (Table 1): (i) 1,478 bones from the Rodrigo Botet collection, housed at the Museo de Ciencias Naturales de Valencia (MCNV; Spain), result of the excavations undertaken by Enrique de Carles in the northeast of the Buenos Aires province (Belinchón et al., 2009); (ii) 30 bones from the Dupotet collection, housed at the Muséum National d’ Histoire Naturelle (MNHN; Paris, France) of Pampean age from Luján City; (iii) 330 bones from the Krncsek collection, housed at the Naturhistorisches Museum Wien (NMW; Austria) that proceed from the Luján River in Mercedes City, and are identified as “Diluvium-Upper Pampean”; and (iv) 138 bones from the Ameghino collection, housed at the Museo de La Plata (MLP; Argentina), and which were extracted from a 20 m stretch along both sides of a water channel in the Canal de Conjunción (La Plata) (Ameghino, [1889] 1916 :128–129).

These collections were gathered during various non-systematic excavations carried out in the eastern region of what is currently Buenos Aires province, in the Pampean region (Argentina), during the 19th and early 20th centuries. This is an extensive, flat geomorphological unit located in the central area of Argentina. The Quaternary was characterised by loess deposition, with different regressive and transgressive events (Fucks & Deschamps, 2008; Cione, Tonni & Soibelzon, 2009). The early and middle Pleistocene corresponds to the Ensenadan and Bonaerian Stages/Ages that were characterised by a cold and arid environment (Fucks & Deschamps, 2008; Cione, Tonni & Soibelzon, 2009). An important faunal turnover marks the boundary between the two stages, at ca. 0.5 Ma (Cione, Tonni & Soibelzon, 2009). The late Pleistocene-early Holocene corresponds to the Lujanian Stage/Age. Significant palaeoenvironmental oscillations, aeolian pulses, fluvial process and various pedogenetic events influenced this period (Tonni et al., 2003; Fucks & Deschamps, 2008; Cione, Tonni & Soibelzon, 2009). When the collections analysed in this study were originally collected, these units were included in the “Pampean Formation” (Tonni, 2011). Current biostratigraphical information (Tonni, 2009) allows the material from MCNV to be assigned to the Ensenadan to Lujanian Stage/Age and the material from MNHN and NMW to the Bonaerian and Lujanian Stages/Ages. Furthermore, in the NMW collection, the old reference to Upper Pampean is currently equivalent to the Bonarian Stage/Age (Tonni, 2011). The last record of these mammal groups comes from the Guerrero Member of the Luján Formation, deposited between 21,000 and 10,000 ¹⁴C years BP (Tonni, 2009). In the case of the MLP assemblage, the presence of the notoungulate Mesotherium cristatum (Notoungulata, Mesotheriidae) among the identified species means this material can be dated as Ensenadan (Cione, Tonni & Soibelzon, 2009) (Fig. 2 and Table 1).

To understand the natural burial conditions of the remains, we considered different types of bone surface modifications such as post-depositional fractures, the presence of original sediment or concretions, fluvial erosion, trampling, weathering, root growth, manganese spots and burning traces (e.g., Behrensmeyer, 1978; Binford, 1981; Shipman, 1981; Olsen & Shipman, 1988; Lyman, 1994; Fernández-Jalvo & Andrews, 2003; Fernández-Jalvo & Andrews, 2016). These allowed us to discard any type of intervention that could simulate carnivore activity or, if superimposed onto carnivore marks, could have indicated a previous carnivore intervention.

We follow the literature to identify whether bone marks were the result of carnivore activity (e.g., Haynes, 1980; Haynes, 1982; Haynes, 1983; Binford, 1981; Capaldo & Blumenschine, 1994; Lyman, 1994; Domínguez-Rodrigo & Piqueras, 2003; Pickering, Egeland & Brain, 2004; Domínguez-Rodrigo et al., 2012; Delaney-Rivera et al., 2009; Sala, Arsuaga & Haynes, 2014; Sala & Arsuaga, in press). As large mammal bones are too large to be ingested (Fernández-Jalvo & Andrews, 2016), we did not considered this effect as a possible agent of the marks. Furthermore, small bones tend to be splintered by the teeth of predators, making them impossible to classify either anatomically or taxonomically (Fernández-Jalvo & Andrews, 2016). Therefore, this type of fragmented material was not included in our review. The only exception was the case of the indeterminate and medium-sized bones from the MLP collection where part of the original association was conserved. Coprolites were absent in the reviewed collections.

We classified the bone marks potentially produced by carnivores into four categories (Table S1): (i) pitting and/or punctures, (ii) u-shaped elongated scratches or scores, (iii) furrowing; and (iv) spiral fractures. To investigate the body size of the potential carnivores that inflicted the marks, we used a box-plot diagram (Hammer, Harper & Ryan, 2001) to compare the size of the pitting and/or punctures from the MCNV, MNHN and MLP specimens with those published by Pickering, Egeland & Brain (2004) (various bones), De Los Reyes et al. (2013) (bone specimen Xen 30-12), and Martin (2016) (various bones); the material from NMW was excluded for the small sample size (Tables S2–S5). We follow the studies mentioned above as they allowed us to compare palaeontological and archaeological cases from the Pampean region, Patagonia, and one African case, and appreciate any similarities and/or differences with African ecosystems. Even though this information was still statistically poor, it allowed us to make some preliminary assumptions. Additionally, assigning a pit or puncture to a specific taxa is always problematic given the different factors involved (e.g., the part of the bone marked and the bite force of an animal) (Delaney-Rivera et al., 2009). Nevertheless, the overlapping of our data with the comparative cases allowed us to ascribe the marked bones to general carnivore size categories. Even though some authors have also included scores in their studies of body size (Delaney-Rivera et al., 2009; Labarca et al., 2014; De Araújo Júnior, De Oliveira Porpino & Paglarelli Bergqvist, 2011), we agree with Domínguez-Rodrigo & Piqueras (2003) that score marks relate not only with teeth size, but also the effect of the teeth being dragged over the bone surface; variability can therefore be expected from this type of marks.

We also reviewed actualistic studies describing the marks that different carnivore taxa leave when feeding and, more specifically, recent research into marks made by the members of the large carnivore guild, such as ursids (Carnivora, Ursidae), felids (Carnivora, Felidae) and canids (Carnivora, Canidae) (Table S1). Specialised bone-breaking hyenas were not considered because they were not present in South America at that time. Various studies report that ursids leave scarce to abundant teeth marks (Haynes, 1980; Haynes, 1983; Burke, 2013; Saladié et al., 2013; Arilla et al., 2014; Sala & Arsuaga, in press). In contrast, felids tend to make fewer marks on the bones since they feed exclusively on meat (Christiansen & Wroe, 2007; Sala & Arsuaga, in press), although they can leave important signs of predation (Haynes, 1983; Marean & Ehrhardt, 1995; Martin, 2008; Martin, 2016; Domínguez-Rodrigo et al., 2012; Kaufmann et al., in press; Sala & Arsuaga, in press). Finally, canids can make a great number of intervention marks (Haynes, 1982; Haynes, 1983; Yravedra, Lagos & Bárcena, 2011; Burke, 2013; Domínguez-Rodrigo et al., 2012; Sala, Arsuaga & Haynes, 2014; Sala & Arsuaga, in press). Furthermore, while felids (including Smilodon) and ursids have straighter incisive arcades, canids have curved arcades (Biknevicius, Van Valkenburgh & Walker, 1996). This shape is useful when analysing pitting and/or puncture arrangements on bone surfaces (e.g., linear or curved rows of tooth impressions).

We examined the fossil remains of the megaherbivores present in the collections with 3.5× and 12× magnifying glasses. We also used a Dino-Lite Microscope AD4113T (at magnifications of 20×–45×) and the software Dino-Lite 2.0. Both the length and breadth (major and minor axes) of the scores, pits and punctures were measured. Larger marks were measured using a caliper, and smaller ones were recorded with the measurement tool installed in the Dino-Lite. For each collection, high-resolution digital images were taken, in each museum, using a Panasonic Lumix DMC-TZ35 camera.

For the MLP assemblage we also applied the well-established archaeozoological variables MNI (Minimum Number of Individuals) and NISP (Number of Identified Specimens), as all the specimens are part of the same taphocoenosis (Lyman, 1994). While MNI was used to account for the minimum number of mammals with carnivore marks represented in the sample, the second informed the counting per taxa or skeletal part categories.

Results

We found four bones (0.2% of the total) of megaherbivores and 24 bones (1.24% of the total) of medium-sized and indeterminate species with potential carnivore intervention. In addition, a detailed description of the marks is given in Data S1. Below, we give a general overview of the most important damage found in each collection (Table 2 and Table S5) and provide general observations from the box-plot diagram (Fig. 3):

1. A right tibia from the MCNV (no 64-492) that corresponds to the ground sloth cf. Scelidotheriinae gen (Tardigrada, Mylodontidae). This bone presents important furrowing on both epiphyses and pits and scores on the distal epiphysis, as well as on the posterior and medial faces of the diaphysis (Fig. 4). In the box plot diagram it can be observed that the measurements of these pits slightly overlaps with the maximum sizes of large carnivores (and outliers) from Pickering, Egeland & Brain (2004) and falls within the measurements presented by De Los Reyes et al. (2013), but are slightly bigger than the Pampean case (De Los Reyes et al., 2013). Nevertheless, this discrepancy could be due the bigger pit from MCNV that seems to be enlarged by post-depositional process (Data S1 and Fig. S1). They also coincide with the smaller sizes from Cueva del Milodón (Martin, 2016);

2. A left humerus of Glossotherium robustum labelled MNHN.F. PAM 119 from MNHN, with pits, scores and furrowing (Fig. 5). Comparing this with the other samples reveals the same trend as for MCNV. It matches with the log area of the tibia from MCNV, but also overlaps more with the specimens in Pickering, Egeland & Brain (2004) because of the presence of smaller pits on the MNHN bone. It also coincides with the range of Xen 30-12, but has bigger and smaller log area extremes than the Pampean case (De Los Reyes et al., 2013). In addition, it compares well with the smaller marks from Cueva del Milodón (Martin, 2016);

3. A left distal humerus of Mylodon robustum (no. 1908.XI.110) housed at MNW with furrowing and a possible puncture (Fig. 6). The furrowed border is scalloped and part of it is flaked. This species is considered to represent Glossotherium robustum (McAfee, 2009). Although not plotted, Table S5 shows that the log area coincides with the range for the rest of the sampled material; and

4. At the MLP, one femur condyle from the notoungulate Toxodontidae (MLP 15-I-20-32) (Notoungulata; Toxodonta) was found with scratches (Fig. 7). Moreover, in this collection 22 long bones of medium-sized species and two further indeterminate bones have fresh fractures, scratches, punctures/pits and crenulated edges (details of these marks are shown in Table S6) (Figs. 8–10). The box plot reveals the same trend for these pits and punctures as seen in the other cases. Nevertheless, the presence of smaller marks on this sample results in greater coincidence with the Swartkrans specimens (Pickering, Egeland & Brain, 2004), and there is partial overlap with Xen 30-12 (De Los Reyes et al., 2013). However, only the outliers from MLP coincide with the smaller sizes from Cueva del Milodón (Martin, 2016), and the plot partially overlaps with those of the material from MCNV and MNHN. The smaller pits on the MLP specimens were considered together with the bigger punctures on the two indeterminate bones. Large carnivores can generate both small and large pits and/or punctures (Delaney-Rivera et al., 2009), and this may explain the variability in the marks observed here.

Discussion

The information presented above suggests that the different types of bone marks found on both megamammal and the other mammal remains were most likely inflicted by some large-sized carnivores that inhabited the Pampean region during the Pleistocene. Considering the limited evidence available from this region, the data presented here is crucial for exploring different predator–prey and/or scavenging scenarios, at a coarse scale.

The agents: pleistocene mammalian predators from the Pampean region

Several species of Quaternary carnivores have been recorded from the Pampean region. In the supplementary information, we offer a general description of these, along with some ecological characteristics (Data S2). These carnivores include ursids, felids and canids. The ursids comprise Arctotherium angustidens from the Ensenadan Stage/Age and Arctotherium vetustum, Arctotherium bonariense and Arctotherium tarijense from Bonarian and Early Lujanian times (Soibelzon et al., 2014; Figueirido & Soibelzon, 2010). In particular, the first species would have had an important capacity to feed on meat (Figueirido & Soibelzon, 2010). Felids are represented by three hypercarnivorous species: Smilodon populator, Puma concolor and Panthera onca (Christiansen & Harris, 2006; Prevosti & Vizcaíno, 2006; Bocherens et al., 2016). While the first two had some bone marking capacity, the third would have been capable of inflicting more damage (Van Valkenburgh & Hertel, 1993; Marean & Ehrhardt, 1995; Antón et al., 2004; Martin, 2008; Martin, 2016; Muñoz et al., 2008; Binder & Van Valkenburgh, 2010; Domínguez-Rodrigo et al., 2015; Kaufmann et al., in press). Finally, several pack-hunting and/or scavenging canids were present at the time, including Theriodictis platensis (and its sister taxon “C”. gezi) in the Ensenadan (Prevosti & Palmqvist, 2001; Prevosti, Tonni & Bidegain, 2009), various Protocyon species throughout the Pleistocene (Prevosti, Zurita & Carlini, 2005; Prevosti & Schubert, 2013; Bocherens et al., 2016), Canis nehringui (currently recognised as a junior synonym of C. dirus, (Prevosti, Tonni & Bidegain, 2009), and Dusicyon avus in the late Pleistocene (Prevosti & Vizcaíno, 2006).

It is clear that carnivores with an important capacity for bone modification and/or consumption would have been responsible for the various marks observed. Even though felids such as Smilodon or Puma could have produced some bone-damage, as observed in some studies (Van Valkenburgh & Hertel, 1993; Marean & Ehrhardt, 1995; Muñoz et al., 2008; Kaufmann et al., in press), their reduced bone-breaking potential rules them out as the principal generator of the feeding traces recorded. Furthermore, it is worth mentioning that the highly specialised viscera-eating dentition of the dirk-toothed Smilodon would have prevented this animal from feeding on carrion unlike other scimitar-toothed predators (e.g., Homotherium) (Palmqvist et al., 2007).

Conclusions

Four megaherbivore fossil bones, 22 bones of medium-sized species, and two indeterminate bones with carnivore marks were studied from European and Argentinean collections of Pleistocene remains from the Pampean region, collected during the 19th and early 20th centuries. The marks were predominately identified on appendicular bones. After internal organs and muscles are consumed, limb bones are the richest parts with regard to within-bone nutrients, and in particular, the epiphyses are the easiest to penetrate by gnawing (Binford, 1981; Dominato et al., 2011; Labarca et al., 2014). Analysis of the punctures and pitting shows that these partially overlap with the range of bigger marks made by large carnivores from African environments, the smaller markings of Panthera onca mesembrina, and they are comparable with the giant Chapalmalania from the Pliocene of the Pampean region (Pickering, Egeland & Brain, 2004; De Los Reyes et al., 2013; Martin, 2016). Moreover, our measurements generally agree with the information reported from other South American sites (Martin, 2008; Dominato et al., 2011; Labarca et al., 2014; Politis et al., 2016). Consequently, it is likely that different members of the Pampean large-carnivore guild produced the marks described in this study. We interpret the data presented here as indicating the fact that ursids, canids, and possibly felids would have consumed the soft and hard tissues, inflicting various tooth marks, including pits, punctures, and scratches, furrowing bone epiphyses, and even breaking the diaphyses of long bones in order to access the marrow. These latter represent the final stages of carcass exploitation, given that the marks described on the epiphyses and diaphyses were not inflicted when bone still held large quantities of meat.

Considering that there is little information on carnivore marks from the region, as this type of evidence is still scarce, the few remains presented here significantly increase our knowledge of palaeoecological relationships in the Pampean region. The marked bones indicate that the megamammal carcases were fully exploited. This type of evidence has been recorded in the Pliocene (De Los Reyes et al., 2013) and, according to the evidence presented here, continued periodically throughout the Pleistocene. Consequently, temporal shifts in prey availability would have influenced predator–prey and/or scavenging dynamics, increasing competition for carcasses and resulting in the consumption of bone and within-bone nutrients by the same or multiple taxa. Pleistocene large mammal communities would have developed different trophic levels with multiple competitive species, allowing them to persist through time and overcome different palaeoclimatic fluctuations. This situation lasted until the late Pleistocene-early Holocene when many megafaunal extinctions occurred (Van Valkenburgh et al., 2016).

Current taphonomic methods allow new results to be obtained from historical collections. In this study, different types of carnivore marks inflicted on megamammal and other mammal bones were measured and categorised. Interpreting these with the help of current ecological information sheds light onto the palaeoecological relationships of native Pampean mammal communities from the Pleistocene. This novel perspective offers new insights into the development of future systematic fieldwork. Both collection and field-based research will provide crucial information on the evolution of the Pleistocene ecosystems of the South American Southern Cone.

Supplemental Information