Middle Pleistocene revelations: unravelling taphonomic processes in mammals including Mesotherium cristatum (Mesotheriidae, Notoungulata), Corralito Site, Córdoba Province, Argentina

History, location & stratigraphy

Corralito is a fossil vertebrate site discovered after torrential rains in 1979–80s (see Argüello et al. 2006 for details), where numerous studies, stratigraphic and sedimentary analyses have been conducted (see Argüello et al. 2006; Frechen et al. 2009; Sanabria & Argüello 1999, 2011; Sanabria et al. 2013; Rouzaut et al. 2015; Rouzaut & Orgeira 2017). In addition, it is one of the palaeontological sites in Argentina with the highest number of dates (c. 15) obtained on its loess levels (see Frechen et al. 2003, 2009; Sanabria et al. 2006; Rouzaut & Orgeira 2017; Rouzaut et al. 2019; Fernández-Monescillo et al. 2023a), indicating an age range from the Middle Pleistocene to the Holocene (ranged from 220 ± 13 ka to 311/474 calibrated years before present (BP); or from Chibanian to Greenlandian; sensu Cohen et al. 2013 (v2023/06); marine isotope stage (MIS) 7-1).

The Corralito site is located 3 km northeast of Corralito village between the south of Departamento de Santa María and the north of Departamento de Tercero Arriba, Córdoba Province, Argentina (Fig. 1). Corralito site is a gully, c. 20 km long and oriented east–west, partly formed by anthropogenic factors such as extensive peanut and soybean cultivation and the type of plow used, combined with increased rainfall during the hydrologic year 1979–80 (see Argüello et al. 2006). It lies between the Xanaes (Segundo) and Ctalamuchita (Tercero) rivers. Geomorphologically, this area is known as the ‘Plataforma Basculada’ (Tilted Platform; see Capitanelli 1979).

The stratigraphy of the Corralito gully reveals massive loess walls ranging from 4 to 22 m high, interspersed with fossiliferous layers (Frechen et al. 2009; Rouzaut et al. 2015; Fernández-Monescillo et al. 2023a). In the western part of the gully, different levels of cross-bedding have been identified, including troughs with large clasts (20 cm diameter) and planar cross-stratification; indicating various fluvial levels (Tauber et al. 2014). Additionally, some sections feature crotovines (mammal burrows) with circular or elliptical cross-sections of varying length, width and orientation (Tauber et al. 2012). The most noticeable stratigraphic layer throughout the 20 km gully is the palaeosol (PS-2), with a variable thickness of 70 ± 30 cm, distinguishable by its colour and texture in the loess walls (Fernández-Monescillo et al. 2023a).

Previous studies of dating & temporal distribution of the faunal community

Three sections have been previously identified in Corralito: Corralito I (S 32°0′7″ W 64°11′8″, Rouzaut et al. 2015, feldspar luminescence dating (IRSL): 115 ± 21 to 13.8 ± 18.8 ka, thermoluminescence dating (TL): 99.8 ± 17.5 to 11.8 ± 1.8 ka, see Frechen et al. 2009); Corralito II (S 32°0′16″ W 64°7′21″, Rouzaut et al. 2015, IRSL: 43.3 ± 3.3 to 10.6 ± 1 ka, see Rouzaut & Orgeira 2017); and Corralito III (S 32°0′16.15″ W 64°8′16.93″, IRSL: 220 ± 13 ka, Fernández-Monescillo et al. 2023a). The exposed loessic sediments of the Corralito gully, along with the associated faunal assemblage, span from the Middle Pleistocene to the Holocene (Chibanian–Greenlandian; sensu Cohen et al. 2013; MIS 7-1) (Fig. 1C). Meanwhile that of the mesoterine (M. cristatum) stratigraphic level is currently identified in the Bonaerian Age in what has so far been identified as the most basal loess layer at Corralito site, designated as L-1 (see Fernández-Monescillo et al. 2023a, fig. 2).

Material

Four fossil specimens corresponding to two individuals of the mesotheriine Mesotherium cristatum were analysed. Specifically, the right hemimandible fragment (CORD-PZ 4456) with m1–3 was identified based on the morphological characteristics outlined by Fernández-Monescillo et al. (2022b) and Fernández-Monescillo & Tauber (2024).

We examined the remains of the appendicular skeleton of another specimen of M. cristatum, with identified taphonomic traces, from Corralito site, primarily focusing on the forelimb, which included a fragmented right humerus (stylopod), ulna + radius (zeugopod) in anatomical position (MRFA 0836-1; elbow flexion; c. 38° of flexion between humerus and ulna; and c. 25° humerus–radius). This specimen also includes hind limb remains with identified taphonomic traces, encompassing a left proximal femur fragment (MRFA 0836-2) and a left tibia (MRFA 0836-3), all of them of the same individual. The taxonomic identification of both fore and hind elements as belonging to M. cristatum is based on the morphological characteristics identified and detailed in previous works (Serres 1867; Gervais 1867; Cattoi 1943; Fernández-Monescillo et al. 2018).

Furthermore, we incorporated into the analysis distal fragments of metacarpals of an indeterminate camelid (MRFA PV 1236). Based on the presence of camelids in the Pampean region in the Middle Pleistocene and Holocene (Ensenadan–Bonaerian–Lujanian Age; Chibanian–Greenlandian) we consider that these remains could belong to various camelid taxa such as Lama guanicoe or Hemiauchenia paradoxa, previously identified in the western Pampean region (Castellanos 1944; Menegáz 2000; Cruz et al. 2012; Tauber et al. 2014); the latter also identified at Corralito (Tauber et al. 2014).

Methods

We acknowledge that in the majority of studies concerning the taphonomy of structures, whether abiotic or biotic in nature, present on the surface of current or fossilized bones, the term ‘marks’ is commonly employed. However, in accordance with Vallon (2015) and other authors (Bertling et al. 2006, 2022; Mikuláš et al. 2013), we consider the term ‘trace’ to be more appropriate. This preference is grounded in the specific definition of trace as a morphologically recurrent structure resulting from the life activity of an individual organism (or homotypic organisms) modifying the substrate, encompassing dwelling traces, feeding traces, and bite traces (see Bertling et al. 2006, p. 266).

A taphonomic analysis was conducted on the superficial traces observed on a fragment of the right hemimandible of Mesotherium cristatum (CORD-PZ 4456). These traces were analysed using confocal laser scanning microscopy (CLSM), at millimetre or nanometre scale. This procedure was performed at the Laboratorio de Análisis de Materiales by Espectrometría de Rayos X (LAMARX) of the Facultad de Matemáticas, Astronomía, Física y Computación (FaMAF) of the Universidad Nacional de Córdoba (UNC), Argentina. Macroscopic traces were measured in length and width using a digital calliper to the nearest 0.01 mm, for comparison with previous studies recognized in the Argentinian Pampean region for the Pleistocene–Holocene periods to identify productors (e.g. Chichkoyan et al. 2017a). Moreover, the fossil material here analysed was photographed in the institutions where the material is housed (MRFA and Museo de Paleontología, UNC) with a Canon 550D camera, with a fixed focal length lens of 50 mm.

Biostratigraphic & chronostratigraphic context

Mesotherium cristatum is the final representative of the Typotheria suborder and the only taxon of this clade to reach the Quaternary. It is the only mesotherine taxon found at the Corralito site, thus becoming the only palaeontological site with the presence of this taxon in the Bonaerian Age (see Tauber 1991, 2008; Fernández-Monescillo et al. 2023a; Fernández-Monescillo & Tauber 2024). This contrasts with the historical and traditional consideration of identifying it as a taxon from the older Ensenadan Age (Rusconi 1936; Bond et al. 1995; Cione & Tonni 1995a, 1995b, 1999, 2005; Bond 1999; Pomi 2008; Soibelzon et al. 2008a, 2008b, 2009, 2019; Cione et al. 2015). Krapovickas & Tauber (2016) and Krapovickas et al. (2017, p. 232) identified possible latest faunal records at several Upper Pleistocene (Lujanian Age) sites in the Sierras Pampeanas of Córdoba (e.g. Atos Pampa, Atum Pampa, Copina Bosque Alegre, Pampa de Olaen, Pampa Vaca Corral). This suggests that this area could serve as an important faunal reservoir compared to other areas of the Pampean region and the Chaco-Pampa plains.

Macroscopic & microscopic biological identification of taphonomic traces

The most identifiable traces on the ventral border of the hemimandible of M. cristatum (CORD-PZ 4456) are macroscopically observable, particularly in lateral (Fig. 2A–C) and medial view (Fig. 2D–G). Both pseudo-parallel trace patterns present in ventral view a longitudinal symmetry axis marked on the ventral hemimandibular edge (Fig. 2G, H).

On the lateral side of the hemimandible (CORD-PZ 4456), we identified several traces. Using CLSM, we were able to distinguish pseudo-parallel and linear traces exhibiting a U-shaped cross-section (identified as carnivore traces, Haynes 1980b, 1982; Fiorillo 1984) without internal striations (Fig. 3A–E). This pattern is characteristic of traces made by premolars/molars and is indicative of carnivoran feeding behaviour (see Fernández-Jalvo & Andrews 2016, p. 32). Besides these, we also identified canine puncture traces recognized by their deep, rounded cone shapes (two traces visible in lateral view; Fig. 3G, H). Additionally, there are deep root etching traces in two distinct areas also analysed using CLSM (Fig. 3F, I, J), and other slightly marked or irregular coarse and elongated traces recognized as the result of trampling (Fig. 3K, L).

In addition to the carnivory traces indicated above (Fig. 2E, F), the medial side of CORD-PZ 4456 was also analysed using CLSM (Fig. 4A–D), revealing an evident root trace largely situated ventral to the molar line (m1–3) (Fig. 4E). Towards the caudal end of the ventral carnivoran traces, two flat impressions with distinct ridges characterized by parallel and straight edges are visible, and perpendicular to the longitudinal hemimandible axis, that we interpret as rodent gnawing traces (see Pokines et al. 2016; Indra et al. 2022; Figs 2E–G, 4B, F, G). Additionally, within the medial area of these markings, there seems to be a convergence axis between the upper and lower incisors (Fig. 4H, dotted black lines; without being able to know whether a/a′ or b/b′ are produced by an upper or lower incisor, see Fig. 4H). Particularly noteworthy is the presence of a discernible structure resembling a small central ridge in the central part (Fig. 4H, yellow dotted lines), interpreted as the diastema between the right and left incisors.

In the appendicular skeleton, we observed numerous traces. For example, on the right limb in anatomical position (humerus, ulna and radius), there are small traces similar to those found on the hemimandible. Traces are present on the caudal edge of the ulna, the proximal part of the radius, and the proximolateral edge (epicondylar crest) of the humerus (Fig. 5). Additionally, deep traces, probably caused by large carnivorans, appear on the proximal part of both the ulna and the humerus (Fig. 5E, F). In the case of the left femur fragment (MRFA 0836-2), we found small but numerous traces on the cranial part of the diaphysis, with the most surprising being a deep and transverse bite trace (Fig. 6C, G, I, J). On the tibia (MRFA 0836-3), we observed three deep and transverse traces on the cranial surface (Fig. 7). Finally, on the metacarpal specimen of a camelid (Hemiauchenia?/Paleolama?; MRFA-PV 1236), we found small longitudinal traces on the distal part (Fig. 8), similar to those found on the hemimandible and parts of the humerus, ulna and radius of the appendicular skeleton of M. cristatum.

Identification of biotic taphonomic agents & bone modifications stages in fossil bones from the Corralito site

The hemimandible fragment (CORD-PZ 4456) is the most remarkable fossil due to the abundance of traces identified on its surface. The most characteristic and abundant traces are those of carnivoran teeth, identified both macroscopically and microscopically. The affected area is possibly linked to the area of the powerful masseter muscle (m. masseter) in mesotheriids (Fernández García 2018; Sosa & García López 2018), whose insertion is located caudally to the predominant area in ventral edge of the hemimandible. These suggest the active acquisition of meat or carrion presumably by mammalian carnivorans, leaving bite traces from premolars or molars imprinted on the cortical layer of the bone (Fig. 2G). At the time when these traces were made, the bone still retained its associated muscular tissue, indicating its freshness (Haynes 1980a; Cáceres et al. 2002) and suggesting the recent demise of the specimen, in this case M. cristatum.

Among the carnivorans discovered in Corralito, some are notably large, like Smilodon populator (220–360 kg; Christiansen & Harris 2005; Chimento et al. 2019), which we ruled out as a likely producer of the traces here analysed here due to their small size. Potential candidates for the producers of the traces on CORD-PZ 4456 include fossils identified in Corralito such as canid Cerdocyon sp. (MRFA 0899) (5–8 kg, Berta 1982) and the mustelids Galictis sp. (1.2–3.8 kg, Yensen & Tarifa 2003) and the mephitid Conepatus sp. (MRFA 0735) (1.1–4.5 kg, Dragoo & Sheffield 2009). The distance between the two small punctures seen in the lateral view of the mandible, here inferred to be the upper canines (Fig. 3A, G) is 16.2 mm, a measurement comparable to the alveolar distance between the upper canines of the lesser grison Galictis sp. (13–19 mm, see Yensen & Tarifa 2003, p. 1, or 16.75 ± 1.41 mm, see Bornhold et al. 2013, table 4), but also to that of Conepatus sp. (16–24 mm; see Dragoo & Sheffield 2009), and Cerdocyon sp. These punctures traces are likely to have been made during movement or transport of the hemimandible. While we cannot definitively determine the specific mammal responsible for creating the carnivoran bite traces, or conclusively assert whether they are exclusively attributable to a single taxon (there are at least two of different trace sizes; see Table 1), the morphology and shape enable us to confidently eliminate the involvement of other scavengers, such as reptiles, raptors or vultures, which are also well-known for their scavenging behaviours (see Naves-Alegre et al. 2020). Even so, of the small to medium carnivorans present at the site (the mephitid Conepatus sp., the mustelid Galictis sp. and the canid Cerdocyon sp.) it is probable that scavenging activity was primarily undertaken by the former (see Prugh & Sivy 2020). Analysis of the scavenging traces of the extant lesser grison G. cuja (<2 mm in length the bone specimens analysed, and most of them appear to have been previously digested; see Gutiérrez et al. 2021 for details) shows a large size difference compared with the traces identified here; this taxon is therefore ruled out as the producer. Conepatus sp. is also a small taxon for which, to our knowledge, there are no studies on possible tooth traces on bones with which to compare. However, there is current evidence of scavenging on carcasses (Castillo & Schiaffini 2019), including instances of kleptoparasitism on larger carnivorans (see Valenzuela & Leichtle 2015). None of the taphonomic traces found on the hemimandible of the M. cristatum specimen allow us to ascertain the cause of death of that specimen.

We identified two bite traces on the ventral portion of the hemimandible fragment CORD-PZ 4456 (Fig. 4H), characterized by parallel lateral edges and a predominantly flat base, not particularly deep (Fig. 4F, G). These features suggest that they are likely to have been produced by rodents (see Bunn 1981; Shipman & Rose 1983; Fernández-Jalvo & Andrews 2016; Pokines et al. 2016; Indra et al. 2022). Rodent gnawing traces not related to the location of muscle insertions in mesotheriines (see Sosa & García López 2018; Fernández García 2018) are very commonly positioned on the ventral border of mandibles (Fernández-Jalvo & Andrews 2016). Furthermore, the presence of a small longitudinal ridge in the middle of each bite trace (a–a′) and (b–b′) is notable (Fig. 4H, dotted yellow lines), which we infer to be the incisive (I1 right – I1 left) diastema. Additionally, by observing the ‘upper’ (a and b) and ‘lower’ (a′ and b′) shape of the traces we can infer the transversal location point of the bite in complete occlusion between ‘upper/lower’ incisors (Fig. 4H, transverse black dotted lines; however, due to the similar width of the ‘upper’ and ‘lower’ counterparts (see Table 1), we cannot determine which trace corresponds to which of the first incisors). The bite traces identified on the hemimandible are 4–5.69 mm wide (Table 1), which is consistent with the buccolingual width of either the I1 or i1 of specimens recognized as Ctenomys (MRFA PV 0567, MRFA PV 0018) from Corralito, but exclude Lagostomus as a producer due to its larger incisive width. There is little evidence of this type of bite trace on bones produced by rodents of the genus Ctenomys, but it has been suggested as the potential source of such traces on the bones of camelids (archaeological sites: KCH56, Iroco Region, Departamento de Oruro, Bolivia; Capriles & Tripcevich 2016). The potential causes of rodent bite traces on bones can be attributed to nutritional requirements, such as the intake of minerals (phosphate), or the necessity to gnaw in order to maintain the proper length of continuously growing incisors (Froberg-Fejko 2014).

Additionally, irregular surface traces have been observed on the hemimandible specimen (CORD-PZ 4456) identified using CLSM analysis as indicative of trampling (Fig. 3K, L). This suggests that the mandible had previously been broken, becoming a hemimandible fragment with the lateral side facing upward, before being stepped on by fauna sympatric to Mesotherium cristatum.

The other Mesotherium cristatum remains analysed belong to a single individual preserving postcranial elements from both the right forelimb (MRFA 0836-1, humerus–ulna and radius in anatomical position and in flexion) and the left hind limb (MRFA 0836-2, femur fragment, and MRFA 0836-3, tibia fragment). We identified the traces of small and medium–large carnivorans, and root etchings, but no evidence of trampling or rodent bite traces.

The right humerus maintains its anatomical position alongside the radius and ulna, and displays a series of elongated and fine traces, similar to those previously found in two regions on the hemimandible. Specifically, on the distal side of the lesser tubercle that is anterolaterally placed to the head of the humerus, we observed traces (n = 4) arranged in a transverse pattern, close to the insertion zone of the deltoid crest where the m. deltoideus pars spinalis, pars scapularis, and the m. brachioradialis are located (see Fernández-Monescillo et al. (2018) for a myological forelimb reconstruction of the late Miocene mesotheriine Plesiotypotherium achirense; see Fernández-Monescillo et al. (2019) for dating). In addition, the caudal edge is completely bitten (along a length of c. 9 cm) resulting in the complete loss of the bony edge of the crista supracondylaris lateralis (epicondylar crest) where the m. brachioradialis (m. supinator longus), digital extensor muscles (m. extensor digitorum lateralis, m. extensor digitorum communis, and m. extensor digitorum radialis) are inserted (see Fernández-Monescillo et al. 2018, fig. 4) (Fig. 5). On the cranial edge midway along the diaphysis, there is a parallel series of narrow linear traces (n = 10) which we also infer, from their similarity in size and shape to those identified on the hemimandible, to be produced by small carnivorans (Fig. 5C, D; Table 1). In addition, we observed small traces (n = 4) on the medial aspect of the radius in the middle part of the diaphysis (Fig. 5C; Table 1). In the distal region, there are two oval punctures (3.39 and 5.78 mm in maximum transverse length, and 2.95 and 3.47 mm in perpendicular width, respectively) probably caused by a large carnivorans (Fig. 5E, F; Table 1).

In the ulna we identified small longitudinal traces (n = 4) on the caudal edge of the medial side at the mid-height of the diaphysis and also on the same caudal edge, though more proximally located (n = 2) (Fig. 5C). In the proximal first third of the ulna, on the medial aspect, we can observe an oval-shaped and large-sized trace (11.95 mm maximum transverse length, 5.59 mm of perpendicular width; Fig. 5E, F; Table 1) that possibly corresponds to the canine of a large carnivoran, as the previous canine punctures identified close together on the humerus (Fig. 5E, F). In addition, in the distal portion of the ulna we observed a large area on which deep root etchings are traced (Fig. 5C); these are not present on the radius or humerus, despite being close to these elements. The identified modification of these forelimb bones (humerus, ulna and radius) is low to moderate based on the classification proposed by Sala et al. (2014), and Sala & Arsuaga (2018). Damage to the humerus is moderate; the whole of the lateral deltoid crest and crista supracondylaris are affected, as well as the medial entepicondyle (epicondylus medialis) (Fig. 5I, J). We can even suggest some carnivoran activity in the areas of the greater trochanter and the entepicondyle, which are completely missing and the resulting fractures filled with sediment. The ulna lacks its distal epiphysis, and the most distal part of the olecranon. However, we cannot ascertain whether the absence of these parts is a result of scavenging activity by carnivorans or for other reasons. The same uncertainty applies in the distal part of the radius (Fig. 5G, H). There is no evidence of the radial sesamoid located between the ulna and the humerus, previously recognized in different taxa of the family Mesotheriidae (Trachytheriinae + Mesotheriinae; Sydow 1988; Shockey et al. 2007; Fernández García 2018).

In the left fragment of the femur (MRFA 0836-2), we have identified abundant and deep root etching traces, some of which exhibit oval or rounded shapes (Fig. 6A–H). On the cranial side, small rounded to oval traces (n = 6) can be observed, probably caused by tooth contact from scavengers (Fig. 6F, K, L; Table 1). In size, these resemble those observed on the hemimandible (CORD-PZ 4456) discussed above (Fig. 3G, H). This suggests that both sets of punctures were made by small carnivorans (e.g. Galictis sp., Cerdocyon sp., Conepatus sp.), recognized by fossil evidence at the site. In the medial half portion of the femur, a pronounced longitudinal and transverse trace (17.5 mm) can be observed, extending deeply (2–3 mm) from the superficial surface of the femur (Fig. 6I, J). This is direct evidence of femoral shaft fracture caused by a large carnivoran bite, as described in the prey of extant lion (Panthera leo; Domínguez-Rodrigo et al. 2021), or other scavengers (Diedrich 2012, figs 8, 7b). There are two other transverse traces distally, spanning the entire cortical width (c. 4 mm), probably also caused by premolar/molar teeth during attempts to break the bone and access the bone marrow. The femoral diaphysis, MRFA 0836-2, was undoubtedly cracked by a large carnivoran, to gain access to the bone marrow (see Blumenschine 1986). Among the large carnivorans present in the Pampean region at that time (post-Ensenadan Age) were Canis nehringi at 30–38 kg (Prevosti & Vizcaíno 2006; Lujanian Age), Aenocyon (= Canis) dirus at 30–70 kg (Prevosti & Forasiepi 2018; Prevosti 2023), Lujanian Age, and Arctotherium sp. (Cruz et al. 2012; Soibelzon et al. 2014). Although so far, to the best of our knowledge, no direct evidence of diaphyseal cross-bite breakage of a medium–large mammal (megafauna; >44 kg; Cione et al. 2009) has been reported from for the Pampean region. Accordingly, it is important to note that the femur diaphysis crack documented here (Fig. 6I, J) or the attempt to crack the tibia (Fig. 7A–F) represents the first direct evidence of bone-cracking feeding behaviour in the Quaternary of South America in native South American lineage (Notoungulata, Typotheria). A potential scavenger that may have had the strength to break a femur of M. cristatum, and was present in the Middle Pleistocene of the Pampean region, is the bear Arctotherium sp. (Soibelzon et al. 2014; Cruz et al. 2012), for which the possibility of bone-breaking has been suggested (Soibelzon et al. 2014) (Fig. 6I, J). We consider that Smilodon populator, with its highly specialized biting action focusing on the soft neck region of its prey, would avoid another feeding action such as tooth–bone contact to break the diaphysis (see Turner et al. 2011). Based on the classification scheme of Sala & Arsuaga (2018, table 2) the femur shows moderate to heavy modification.

Regarding the proximal fragment of the left tibia (MRFA 0836-3) of M. cristatum (see Cattoi 1943; Fernández García 2018), only deep traces (n = 3; see Table 1) possibly made by premolars/molars were found located on the cranial edge of the diaphysis (Fig. 7A–F). This signifies a distinctive pattern indicative of activity carried out by carnivorans (Fernández-Jalvo & Andrews 2016). This taphonomic scenario occurs when individuals perish in close proximity to the location where they are scavenged, preserving soft tissues (e.g. ligaments) that remain intact. No evidence of abrasion by transport (e.g. wear traces) or weathering (e.g. desiccation traces) has been observed in the specimens (see Behrensmeyer 1978, 1991; Shipman 1981).

Sequence of taphonomic processes in hemimandible CORD-PZ 4456

Taphonomic trace analysis of Mesotherium cristatum postcranial remains

The evidence suggests that the skeletal remains of a single individual of M. cristatum (MRFA 0836-1, 0836-2, 0836-3) underwent distinct stages of scavenging activity. Firstly, an extensive scavenging phase was carried out by small-sized carnivorans (e.g. Cerdocyon sp., Conepatus sp.) A later phase of carcass consumption entailed the fracture of the femur bone diaphysis followed by the consumption of its medulla (see Blumenschine 1986). We unequivocally identified a large–medium carnivoran as the originator scavenger of these bone-cracker traces (Arctotherium?, Protocyon?) (Fig. 5). The fact that some of the appendicular remains of the forelimb (humerus, radius and ulna) still retain their anatomical connection suggests that the carnivory/scavenging event occurred shortly after death.

Taphonomic trace identification & vertebrate tooth ichnotaxonomy

From an ichnological perspective, we found several specimens at the Corralito site of ichnotaxa Machichnus bohemicus, Nihilichnus clavus and N. nihilicus (see Mikuláš et al. 2006; LaBarge & Njau 2024). We also describe two new ichnotaxa: Corralitoichnus conicetensis and Katagmichnus myelus. Corralitoichnus conicetensis is inferred to be produced by the upper and lower incisors of rodents (probably Ctenomys; Fig. 9A; Table 1); Katagmichnus myelus comprises massive bite traces (probably made by medium–large carnivorans) causing diaphysis bone breakage to access the bone narrow in large bones (Fig. 9B).

We are aware that ichnotaxonomic classification is mainly based on trace morphology and therefore often lacks biological (taxonomic) and phylogenetic support, which complicates the biological, palaeobiogeographical or temporal tracking of such classifications (e.g. Ma. bohemicus is identified in the Pleistocene of Brazil, Miocene of the Czech Republic, Middle Pleistocene of the Pampean region of Argentina, and in the present day; see Haynes 1983a, 1983b, Mikuláš et al. 2006; Araújo-Júnior et al. 2017). However, it is still useful to characterize a type of trace based on a common or similar biological function or behaviour of the producers, to provide direct evidence of an interaction between the two taxa (‘producer’ and ‘substrate’) (see Zonneveld et al. 2022). Nevertheless, we can still categorize these palaeontological ichnotaxa based on their resultant shape, size and mode of production. We acknowledge that different recognized ichnotaxa may represent a wide range of behaviours exhibited by a single carnivoran taxon, for example including cutting, catching, nibbling and cracking, depending on the specific function of the carnivore tooth (see Diedrich 2012) or function/behaviour (Cassini et al. 2021) involved. These actions (such as biting to feed on surrounding muscle tissue, transporting bone and associated flesh, or breaking bone to access marrow) can collectively indicate a single scavenging event.

The significance of taphonomic studies lies in their ability to deepen our understanding of ecological interactions between the trace producers and other sympatric taxa within shared environments. This study reveals previously unknown biological, faunal and ecological interactions among sympatric species during the Early Pleistocene, specifically in the western Pampean region near the Sierras Pampeanas de Córdoba.