In recent years there have been multiple studies on the Quaternary South American Megamammals (Mammals with an estimated body mass at least 1000 kg), including Ground Sloths. Even so, there are several aspects of their life history that have not been addressed in detail, such as growth patterns, diseases, social behavior, habitat preference, feeding strategies, and trophic relationships with other mammals, both endemic to South America and immigrants from North America. Sloths (Xenarthra, Folivora), are one of the most conspicuous groups of Mammals, include representatives of, at least, five monophyletic families, Bradypodidae, Megalonychidae, Megatheriidae, Mylodontidae, and Nothrotheriidae. Representatives of this clade were very abundant and diverse in the Quaternary terrestrial ecosystems in South America. The extinction of Ground Sloths occurred in the late Pleistocene-early Holocene, along with that of the remaining Megamammals; it has been proposed that the main causes of the extinction could be related to climate and environmental changes, diseases, Human action, and combinations thereof. They have no ecological analogues living today, as extant sloths are only represented by obligatory arboreal species of Bradypus (Three-toed Sloths) and Choloepus (Two-toed Sloths), which are restricted to the Neotropical rain forests.
The ground sloth Lestodon armatus is the only valid species of the genus for the Quaternary; it is the largest representative of Mylodontidae, with an estimated body mass of about 3400–4100 kg for adult individuals. Remains assigned to this taxon are particularly abundant in late Pleistocene-early Holocene deposits of central Argentina, but there are also records in Brazil, Uruguay, Bolivia and Paraguay.
In a paper published in the journal Scientific Reports on 2 July 2020, Rodrigo Tomassini of the Departamento de Geología at the Universidad Nacional del Sur, Claudia Montalvo of the Facultad de Ciencias Exactas y Naturales at the Universidad Nacional de La Pampa, Mariana Garrone, also of the Departamento de Geología at the Universidad Nacional del Sur, Laura Domingo of the Departamento de Geodinámica, Estratigrafía y Paleontología at the Universidad Complutense de Madrid, and the Earth and Planetary Sciences Department at the University of California Santa Cruz, Jorge Ferigolo of the Museu de Ciências Naturais, Laura Cruz of the Museo Argentino de Ciencias Naturales, Dánae Sanz‑Pérez, also of the Departamento de Geodinámica, Estratigrafía y Paleontología at the Universidad Complutense de Madrid, Yolanda Fernández‑Jalvo of the Departamento de Paleobiología at the Museo Nacional de Ciencias Naturales, and Ignacio Cerda of the Instituto de Investigación en Paleobiología y Geología at the Universidad Nacional de Río Negro and Museo Carlos Ameghino, perform a multi-proxy analysis of a late Pleistocene assemblage constituted by several individuals of different ontogenetic stages assigned to the Giant Ground Sloth Lestodon armatus, from the Pampean Region of Argentina. Diverse taphonomic, pathological, osteohistological, and isotopic issues are evaluated in order to interpret and discuss palaeoecological and palaeobiological aspects of this species and the genesis of the assemblage.
A dating of 19 849 year before present (using a Lestodon armatus vertebra) places the studied Lestodon armatus assemblage at the end of the Last Glacial Maximum. Although the timing of Megamammal extinction in South America is not well-constrained, it seems that this phenomenon occurred since about 40 000 years ago, with an accelerated pace starting at about 13 500 years ago. Therefore, Tomassini et al.'s study provides novel information, based on multiple lines of evidence, on the life history of one of the largest members of the Quaternary fauna under an extinction scenario.
The Pampean Region of Argentina is characterised by several continental Pleistocene sites, some recognised from the nineteenth century by their palaeontological richness. Mammal assemblages recovered in this area are of great importance and have constituted the basis to define the biochronostratigraphical schemes used in several regions of South America. Playa del Barco is a fossiliferous locality in southwestern Buenos Aires Province, Argentina. There have been few studies on its geology and palaeontology due to the discontinuity of the outcrops, the reduced areal distribution, and the location in the current intertidal zone, which implies that they are usually covered by beach sand and are visible only during extreme low tides; however, several researchers highlighted the abundance and diversity of continental Vertebrate remains.
Most skeletal elements are represented in the sample, but calcaneus, pelves, and sacral vertebrae are absent. The average value of relative abundance is low, 20.6%. There are no skeletal elements with relative abundance values of 70% or higher. The atlas displays the highest value, with 69.2%, followed by hemimandibles, hemimaxillae, tibiae, thoracic vertebrae, and scapulae. The remaining elements have values are lower than 20%. All the indicators proposed to evaluate the susceptibility of the skeletal elements to be transported by water flows are recorded in the sample.
Specimens of Lestodon armatus were distributed in a small area (about 500 m²), all of them disarticulated and isolated, but in close spatial proximity to one another. Broken specimens predominate in the assemblage. Complete specimens are mainly represented by metapodials and phalanges, but also include astragali, different long bones (humerus, ulna, tibia, and fibula), and thoracic and caudal vertebrae. Two almost complete skulls were recovered, although most of the cranial elements correspond to maxillae fragments. All the hemimandibles lack totally or partially the posterior portion. In most cases, both hemimaxillae and hemimandibles retain all teeth. Broken long bones mainly show smooth transverse fractures (about 90%), although stepped fractures (~ 10%) are also recorded. Ribs are mainly broken at the middle or distal portions, and show smooth transverse fractures, whereas most vertebrae do not preserve the processes. All scapulae are broken and represented by small portions.
Specimens with weathering show slight splitting parallel to the fiber structure, some of them reflect changes in the humidity, possibly related to water immersion and exposure events. Teeth (both isolated and retained in the alveoli) present slight splitting of dentine and orthodentine. Specimens with abrasion have slight rounding on the broken edges and ridges. Predation/scavenging marks, observed in vertebrae and hemimandibles, are represented by crenulated edges. Tomassini et al. did not recognise any trampling marks.
Pathological alterations are present in 41 skeletal elements (about 14% of the sample). The affected elements include cervical, thoracic, lumbar, and caudal vertebrae, ribs, metapodials, and phalanges. Evidences of palaeopathologies were also identified in long bones of Lestodon armatus from other fossiliferous localities of the Pampean Region. All types of vertebrae have osteophytes or bony spurs, which are mostly located in the margins of the vertebral body, but also in the costal articular facets of thoracic vertebrae; some osteophytes of thoracic and caudal vertebrae are very large and could have been part of intervertebral bone bridges. One cervical vertebra has subchondral erosion in the posterior vertebral endplate. Several thoracic vertebrae display reduced height, asymmetry in the shape/size of zygapophyses, transverse processes and neural arch, deformity of the spinous process, ossification of the ventral vertebral ligament, subchondral erosion in the vertebral endplates, and osteoporotic cancellous bone. Lumbar vertebrae show subchondral erosion in the vertebral endplates, deformity of the vertebral body (related to osteoporosis), and asymmetry of the neural arch. Caudal vertebrae also show destruction of the posterior surface of the vertebral body, and subchondral erosion in the vertebral endplates. Ribs display irregular bone surface and ossification of ligaments. Metapodials and phalanges present small osteophytes and irregular bone surface.
Spinous vertebral processes of juvenile, adult, and senile individuals show a compact cortex surrounding a medullary cavity constituted by trabecular tissue. Primary bone tissue is present in the compact cortex of both juvenile and adult individuals. The primary bone matrix grades from parallel fibered to lamellar bone tissue. Osteocyte lacunae exhibit elongated shapes. Vascularisation is reduced, characterised by the presence of randomly arranged longitudinal channels. Primary bone tissue is well developed in the middle and outer portions of the juvenile individual, while in the adult individual it is restricted to a thin subperiosteal layer. Primary bone tissue of the juvenile individual includes abundant Sharpey’s fibers bundles and three lines of arrested growth. Secondary remodeling of compact bone is recorded in both individuals, represented in some sectors by Haversian bone with at least three partly overlapping generations of secondary osteons and resorption cavities. Haversian bone is clearly more extensive in the adult individual than in the juvenile. The cortical bone of the senile individual is completely remodeled, formed by Haversian bone with, at least, three partly overlapping generations of secondary osteons and resorption cavities. Cancellous bone in all the individuals is formed by secondarily deposited lamellar bone tissue.
The mean proportional carbon¹³ value of the herbivore Mammal assemblage is -6.3 ± 2.6‰ compared to the Vienna Pee Dee Belemnite standard. The highest mean proportional carbon¹³ value occurs in Lestodon armatus (-3.8 ± 1.6‰), whereas the lowest mean value occurs in Morenelaphus sp. (-10.2 ± 0.8‰). Significant differences have been detected among taxa.
The mean bioapatite proportional oxygen¹⁸ carbonate values of the herbivore Mammal assemblage is 28.6 ± 0.9‰ (compared to the Vienna Standard Mean Ocean Water standard), with the highest mean proportional oxygen¹⁸ carbonate value recorded by Morenelaphus sp. (29.3 ± 0.4‰) and the lowest mean proportional oxygen¹⁸ carbonate value depicted by Scelidotherium leptocephalum (26.8‰). The mean proportional oxygen¹⁸ carbonate value of Lestodon armatus is 27.9 ± 0.5‰. Significant differences also occur among taxa.
Values of proportional oxygen¹⁸ phosphate are used here as a control for diagenetic alteration of biopatite, particularly in Xenarthran orthodentine. The difference between proportional oxygen¹⁸ carbonate and proportional oxygen¹⁸ phosphate values obtained for all taxa from Playa del Barco are close to the obtained values from modern unaltered bioapatite. Thus, stable isotope data from Playa del Barco sample can be used to assess past conditions.
Playa del Barco is a very rich fossiliferous locality of the Pampean Region, represented by several extinct large and Megamammal taxa, both endemic to South America and immigrant from North America. An age of 19 849 years before present allows assigning the studied assemblage from Playa del Barco to a period of Marine Isotope Stage 2 related to the end of the Last Glacial Maximum, already within an extinction scenario of the South American megafauna. It is a multitaxic, with high diversity, and monodominant bonebed, being Lestodon armatus the most abundant taxon in terms of number of specimens and individuals. The sample of Lestodon armatus includes several cranial and postcranial elements belonging to at least 13 juvenile, adult, and senile individuals, possibly both males and females. Tomassini et al. performed a detailed analysis of the Lestodon armatus specimens from Playa del Barco site following several lines of evidence. This evaluation allowed them to interpret the origin and the possible taphonomic history of the assemblage.
The low average value of relative abundance reflects an important loss of skeletal elements; part of this loss could be linked with pre-burial processes. Based on the environmental context and the taphonomic evidence, Tomassini et al. consider that most of the bones exposed in the surface would have been winnowed out towards other sectors by the high-density sheetflood generated during the flooding (i.e. events of overbank floods or rainfall) of the plain. All groups proposed to evaluate the susceptibility of skeletal elements to be transported by water flows were recorded; however, the values of relative abundance evidence a lower representativeness of the elements with high susceptibility (e.g. ribs, vertebrae, phalanges) with respect to the elements with low susceptibility (e.g. mandibles, maxillae), which suggest hydrodynamic sorting and differential loss of the elements.
The record of isolated specimens, but in close spatial proximity to one another, suggests that, during the exposure lapse in the surface, the carcasses became disarticulated and the bones sparsely mobilised and scattered, probably by water flows. The marked predominance of unweathered specimens (and weathered specimens showing very slight modifications), the scarcity of marks attributed to predation/scavenging, and the absence of clear trampling evidences would indicate that, in general, the exposure time was relatively short. Minimal abrasion in a few specimens reflects that the time of interaction between bones and sedimentary particles was short or that the intensity was very low, which is concordant with the environmental context of accumulation. The absence of anthropic activity signs is consistent with the proposed age (about 13–12 thousand years ago) for the arrival of the first Human groups to the Pampean Region of Argentina. Taking into account the location of the fossiliferous site, in the current intertidal zone, Tomassini et al. consider that the high degree of breakage obtained (with a clear predominance transversal fractures) could be related to the sea action during systematic current re-expositions of the outcrops occurred in extreme low tides.
The record of a high density of specimens distributed in a thin stratigraphic level restricted to a small area (about 500 m²), belonging to several individuals of the same taxon, Lestodon armatus, with different ontogenetic stages and possibly different sex, together with the observed taphonomic features, supports the interpretation of a local single event of catastrophic mortality to explain the origin of the Lestodon armatus assemblage studied by Tomassini et al. This event would have been associated to a relatively short time of exposure in surface of elements that remained close to the place of death. It was not possible to determine the cause of death of the individuals, but an ontogenetic selective phenomenon can be discarded. Assemblages of extant Vertebrates originated by catastrophic death events are, in general, representative of living social groups. This type of monodominant assemblage is very useful to the study of palaeobiological and palaeoecological issues of a particular taxon.
Different pathologies are identified in several postcranial elements. The asymmetry degree of neural arches, zygapophyses, and transverse processes reflects discrete osteoarthrosis. Marked height reduction, deformities, and high porosity of the vertebral body are modifications indicative of severe osteoporosis. The record of osteophytes and subchondral erosion in the vertebral bodies reflects the development of intervertebral discopathies. The presence of osteophytes in the costal articular facets of a thoracic vertebra suggests osteoarthrosis at the level of the costovertebral joints. The identified intervertebral bone bridges would have prevented the sliding of a vertebra over another, avoiding injuries in the spinal medulla and nerves. On the other hand, this situation would also have significantly reduced the movements of the individual. Destruction of the posterior surface of the vertebral body in some caudal vertebrae is interpreted as osteochondritis dissecans. Both discopathies and osteochondritis dissecans of the caudal vertebrae could be related to a habit proposed for some Ground Sloths, which involves the use of the tail as a 'third limb' to sit; this situation would produce an overload on that segment of the spine.
The observed postcranial pathologies are frequent in large and Megamammals, including extinct Folivora. As it is mentioned previously, particularly in the case of the vertebrae, these pathologies are compatible with individuals of advanced age and a huge body mass. This evidence improves the knowledge on the diverse diseases that affected the skeletal elements of the extinct Ground Sloths.
The recorded microstructural features are consistent with the published descriptions on both extant and extinct Folivora, including Lestodon. Some minor differences are fundamentally linked with the type of skeletal element analysed, as most osteohistological studies are based on long bones and ribs.
The poorly vascularised parallel fibered to lamellar primary bone tissue, present in both juvenile and adult (as a remnant) individuals, indicates a slow apposition rate; this tissue is not recorded in the senile individuals because of the profuse Haversian remodeling. The record of dense Haversian bone in individuals of different ontogenetic stages suggests remodeling in progress of the primary bone tissue, until it is complete in the senile individual.
A previous study mentioned that long bones of adult Xenarthra individuals are characterised by an important development of dense Haversian bone and highlighted that secondary remodeling is more important in large taxa than in small taxa, and particularly for Folivorans, in extinct species more than in extant species. A proposal to explain this situation is that extinct Folivora had a more prolonged life span than extant representatives, which would favour the increase of size and loading, resulting in an extensive remodeling. This idea is compatible with Tomassini et al.'s results, which reflect a relative increase of the remodeling degree throughout the ontogeny; however, as they evaluated a different skeletal element (thoracic vertebrae instead of long bones), other possible causes to explain the observed ontogenetic histological variations cannot be discarded.
According to several authors, lines of arrested growth would allow estimating the minimum age of an individual at the time of death. Following this proposal, the identification of three lines of arrested growth in the juvenile individual suggests an age of, at least, three years; however, the age of this individual is possibly underestimated due to the loss of primary bone tissue by secondary remodeling and to the expansion of spongy tissue. The absence of lines of arrested growth in both adult and senile individuals would be linked with the extensive secondary remodeling.
The obtained results reflect significant changes in the osteohistology of Lestodon armatus during the ontogeny. In this sense, Tomassini et al. observe a remarkable correspondence between the ontogenetic stages determined on the basis of macroscopic anatomical characters and the microstructural features.
The obtained values of proportional carbon¹³ from the Playa del Barco point to a preferential intermediate C₃–C₄ diet by most herbivorous taxa, Morenelaphus sp. being the only taxon consuming exclusively C₃ Plants. Concerning Lestodon armatus, proportional carbon¹³ data (-6.3±1.6‰, compared to the Vienna Pee Dee Belemnite standard) reflect a mixed C₃–C₄ diet or an intermediate diet between open C₃ vegetation and mixed C₃–C₄ vegetation, which depends on the proportional carbon¹³ bioapatite-diet enrichment applied to Xenarthrans (+14.1‰ or +15.6‰); in any case, there is a large variability among individuals, probably indicating some extent of dietary flexibility. These results agree with proportional carbon¹³ values obtained for Lestodon armatus from Uruguay, which point to foraging in relatively open areas, and support the proposal that considers Lestodon armatus as a wide-muzzled Sloth, mostly bulk-feeder, with a diet probably based on Grass and herbaceous Plants.
When comparing different ground sloths from the Playa del Barco locality, Megatherium americanum depicts significantly lower proportional carbon¹³ values than Lestodon armatus, pointing to a more browsing behavior for the former and the incorporation of food items from mixed C₃–C₄ areas for the latter. In turn, the only analysed sample of Scelidotherium leptocephalum shows a proportional carbon¹³ value (− 3.9‰, relative to the Vienna Pee Dee Belemnite standard) similar to that depicted by Lestodon armatus. It has been suggested that the marked hypsodonty of Scelidotherium leptocephalum would support a digging behavior and the ingestion of abrasive food items. Differential dietary preferences and/or strategies among sympatric Ground Sloths may have eased the competition pressure and facilitated their coexistence. On the other hand, the dietary flexibility depicted by Lestodon armatus from Playa del Barco may have as well enabled the sympatry with endemic (e.g. Toxodon platensis) and immigrant (e.g. Stegomastodon platensis) herbivorous taxa, as no significant differences have been pinpointed among them.
Unlike extant Sloths, restricted to the Neotropical rain forest, extinct Ground Sloths were able to diversify in climates with arid and cool conditions, pointing to some mode of body temperature regulation. It has been suggested that Ground Sloths may have coped with climatic fluctuations by developing a hairy covering and by reaching large body sizes, which may have allowed them to better maintain a constant body temperature. This is supported by the difference between proportional oxygen¹⁸ carbonate and proportional oxygen¹⁸ phosphate values obtained for Ground Sloths from Playa del Barco, similar to the difference observed in extant homeotherm Mammals. This means that their bioapatite proportional oxygen¹⁸ carbonate and proportional oxygen¹⁸ phosphate values are likely related to their body water proportional oxygen¹⁸ signal under a constant body temperature and therefore, it may have been directly routed from the proportional oxygen¹⁸ value of ingested water. If so, the lack of significant differences with other herbivorous taxa from Playa del Barco would mean that Lestodon armatus may have ingested water from the same hydrological sources.
Gregariousness is a common behavior among living Mammals, which favors the survival of the most vulnerable members against adverse intrinsic and extrinsic natural factors (e.g. predator attack, diseases, scarcity of resources such as food or water, adverse environmental and climatic conditions), parental care, and territory/resource defense, among others issues. For that reason, several species of herbivorous Negamammals form large aggregations.
There are diverse biological and ecological traits of the extant Megamammals that were also considered for extinct species, in order to shed more light on the relevance of this behavior in the past. Gregory McDonald suggested, based on the remarkable differences in size and anatomy, that the extinct Ground Sloths probably had a more complex social structure than their extant relatives, which are solitary Animals. However, specific studies including considerations on the social structure of extinct South American Ground Sloths are very scarce.
In this context, Tomassini et al. highlight the significance of the Lestodon armatus sample from Playa del Barco site. The identification of an assemblage formed by several individuals of different ontogenetic stages, possibly belonging to both males and females, likely originated by a local single event of catastrophic mortality, constitute and evidence of an intrinsic biogenic concentration that reflects a social behavior. Therefore, Tomassini et al. interpret here that this Mylodontid had, at least sometimes, gregarious habits forming intergenerational herds. Other assemblages dominated by Lestodon armatus have been reported for the Pleistocene of southern South America, but in these studies there are not references on the possible social behavior of this species.
It is difficult to evaluate when and why gregarious behavior was acquired, because pre-Pleistocene record of Lestodon, and Mylodontidae in general, is very scarce. Although speculative, it could have been adopted or have been relevant during the Pleistocene in response to: (1) the occurrence of several glacial-interglacial cycles, which involved continuous and pronounced climatic and environmental changes, and the consequent loss of habitats and/or temporal lack of resources; and (2) the impact linked with the most important pulses of the Great American Biotic Interchange. This event implied, on the one hand, the arrival of several herbivorous large and Megamammals that would have generated competition for resources and habitats. On the other hand, the interaction with new predators; isotopic analyses suggest that Lestodon armatus was a probable prey of the carnivores Smilodon populator (present at Playa del Barco) and Protocyon troglodytes.
Tomassini et al. report and analyse a bonebed, from the late Pleistocene of Pampean Region of Argentina, where the Ground Sloth Lestodon armatus represents the dominant component. Sedimentological characteristics of the host level, density of specimens, number of individuals, ontogenetic representation, possible sexual dimorphism, and taphonomic features, allow Tomassini et al. to interpret that the Lestodon armatus accumulation was originated by a local single event of catastrophic mortality and represents, at least part of, a social group. This record is the first accurate evidence of gregariousness for this taxon, providing novel information on a poorly-known behavior among extinct Folivora.
This comprehensive multi-proxy study sheds new light on several palaeobiological and palaeoecological issues (e.g. social behavior, ontogenetic changes, sexual dimorphism, osteological diseases, resource and habitat use, trophic relationships) of a giant Ground Sloth endemic to southern South America. In an extinction scenario that began about 40 000 years ago, the age of the assemblage from Playa del Barco (dated to 19 849 years before present) results interesting to evaluate the state of the megamammal communities, because it represents a moment: (1) linked with a key climatic event, the end of the Last Glacial Maximum; (2) posterior to the massive arrival of North American immigrants (herbivorous and carnivorous Mammals); and (3) previous to the arrival of the first Human groups. Finally, this type of analysis integrating different lines of evidence on bonebeds is critical to understanding the ecological relevance of the late Pleistocene megafauna and the possible impact of its extinction in the late Pleistocene-early Holocene.
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