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Hypsodonty and body size in rodent-like notoungulates
Abstract and Figures
Four families of endemic rodent-like notoungulates of the
Suborder Typotheria (“Archaeohyracidae,” Interatheriidae,
Hegetotheriidae, and Mesotheriidae) show a high
degree of hypsodonty. We evaluate the evolutionary
pattern expressed by the variability of hypsodonty and
body mass in this group, and consider whether the traits
are correlated. We used hypsodonty and body mass data
for 36 species of Typotheria to test for an association
between these features. Major evolutionary changes in
hypsodonty and body size in Typotheria occurred during
the Eocene-Oligocene transition (EOT) and it is well
documented in the Patagonian fossil record at Gran
Barranca. At least two evolutionary trends are recognized
in two clades of Typotheria: (1) in Mesotheriidae an
increasing of the hypsodonty, associated with increasing
of the body mass in species with complex occlusal designs,
and (2) the Hegetotheriidae are characterized by possessing
high crown teeth with simplified occlusal designs
and small body sizes (Hegetotheriidae Pachyrukhinae).
The Hegetotheriidae Pachyrukhinae show the highest
indices of hypsodonty and were the smallest typotheres.
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... Within this order, Tremacyllus Ameghino, 1891 is a genus of small-sized herbivorous mammals belonging to the subfamily Pachyrukhinae (Hegetotheriidae). This clade comprises euhypsodont forms (i.e., with ever-growing dentition), ecomorphologically similar to leporids and rodents, with postcranial features frequently associated with fossorial habits, probably to build burrows (Reguero et al. 2010;Elissamburu et al. 2011;Sostillo et al. 2018). Pachyrukhines have been recorded in several mammal assemblages of southern South America from the late Oligocene (Loomis 1914;Simpson 1945;Reguero et al. 2007;Cerdeño and Reguero 2015) to the late Pliocene/early Pleistocene? ...
Tooth size variation within fossil assemblages can be associated with intra- or interspecific variation, functional, developmental, and geographical factors, and/or sexual dimorphism. Understanding these sources of variation is necessary to develop diagnoses for fossil mammals, where teeth are usually the most frequent remains. Tremacyllus (Ameghino, 1891) (Hegetotheriidae, Notoungulata) is a genus of small-sized herbivorous mammals abundant in late Miocene to Pliocene outcrops of southern South America. Its simplified, euhypsodont dentition and size variability have hampered systematics studies and led, for instance, to an overestimation of the number of species. I analyzed tooth size variations within assemblages of Tremacyllus in a quantitative framework to test three hypotheses: (1) magnitudes of size variation are different among tooth loci and assemblages; (2) tooth size follows a geographical pattern within the analyzed sample (Bergmann’s rule), but is also associated with taxonomy; and (3) there is a correlation between size variation and sexual dimorphism reflected in distinguishable subgroups. Results indicate that patterns of variation might be associated with eruption time and/or functional position. Northwestern forms are larger than southwestern-Pampean ones, not conforming to Bergmann’s rule but revealing a strong influence of latitude. Size differences between assemblages agree with dental features that distinguish T. incipiens and T. impressus, allowing expanded species diagnoses. Two size subgroups might reflect sexual dimorphism in the absence of biostratigraphic or morphological differences between them. This interpretation indicates that northwestern specimens referred to T. diminutus should be referred to T. incipiens.
... Nevertheless, many extinct mammalian groups lack much dental information due mainly to the scarcity of complete and well-preserved dental remains. Historically, the dental characteristics have been used to erect, describe, and contrast mammalian species, as well as to infer different aspects of their way of life, such as their habitat and feeding preferences (e.g., Patterson and Pascual, 1968;Fortelius, 1985;Janis, 1988Janis, , 1990Janis, , 1995Janis and Constable, 1993;Solounias et al., 1994;Pérez-Barbería and Gordon, 1998;Vizcaíno et al., 2006Vizcaíno et al., , 2011Townsend and Croft, 2008;Reguero et al., 2010;Cassini et al., 2012aCassini et al., , b, 2017Candela et al., 2013;Famoso et al., 2013Famoso et al., , 2016. In recent years, these works have been strongly enriched by the publication of other studies focused on deciduous dentition, reconstruction and description of the ontogenetic series, and tooth eruption/replacement patterns of fossil mammals, whose adaptive, taxonomic, and phylogenetic weight is recognized (Smith, 2000). ...
Studies focused on deciduous dentition, ontogenetic series, and tooth eruption and replacement patterns in fossil mammals have lately increased due to the recognized taxonomic and phylogenetic weight of these aspects. A study of the deciduous and permanent dentition of Interatherium and Protypotherium (Interatheriinae) is presented, based mainly on unpublished materials. Deciduous cheek teeth are brachydont and placed covering the apex of the respective permanent tooth; in addition, some morphological and metrical differences are observed along the crown height. Five dental ontogenetic stages are distinguished among the juvenile specimens on the basis of the degree of wear, the replacement of the deciduous premolars, and the eruption of the molars. The crown height and the wear degree of different Interatheriinae taxa show: (1) eruption pattern of molars in an anterior–posterior direction (M/m1 to M/m3); (2) pattern of replacement of deciduous premolars and eruption of permanent premolars in a posterior–anterior direction (dP/dp4 to dP/dp2 and P/p4 to P/p2); and (3) eruption of M/m3 before the replacement of dP/dp4. Results allow evaluating the diagnostic dental characteristics used to describe some interatheriines, as well as reinterpreting some taxonomic assumptions: the holotype of Protypotherium diversidens Ameghino, 1891 is recognized as a juvenile of another species of the genus, and the species is not validated, considering it as Protypotherium sp.; the holotype of Eudiastatus lingulatus Ameghino, 1891 falls in the variability of Protypotherium , becoming P. lingulatus new combination, tentatively maintaining the species and implying the synonymy between Eudiastatus and Protypotherium ; and the holotype of Eopachyrucos ranchoverdensis Reguero, Ubilla, and Perea, 2003 is reinterpreted as bearing deciduous premolars.
Body mass (BM) is a fundamental variable for many paleobiological investigations that is challenging to accurately infer for species that lack living representatives and/or close morphological analogs. This study explores this issue using notoungulates, a diverse group of extinct South American herbivorous mammals with an extensive fossil record. We use a new dataset of 1,900+ extant mammal species (from ~80,000 specimens) to estimate notoungulate BM based on head-body length and a published dataset of 400+ species (~2,100 specimens) to estimate BM based on occipital condyle width. Condylobasal length, stylopod diameter and circumference, and neck length data are used to explore factors that can confound BM predictions. We estimate the following BM ranges for 10 osteologically well-characterized species and calculate similar ranges for 30 others known from less complete remains: Toxodontia: Thomashuxleya externa (80–120 kg), Homalodotherium cunninghami (250–350 kg), Scarrittia canquelensis (450–550 kg), Adinotherium ovinum (75–90 kg), Nesodon imbricatus (350–400 kg), and Toxodon platensis 1,000–1,200 kg); Typotheria: Interatherium robustum (1.9–2.0 kg), Miocochilius anomopodus (9–14 kg), Protypotherium australe (3.5–4.0 kg), and Pachyrukhos moyani (1.2–1.6 kg). We suggest that species such as these can be used as “calibration points” when inferring BM of species known from more limited remains. Discrepancies between our estimates and previously-published studies are primarily due to the distinctive craniodental morphology of notoungulates and the robust limb bones of toxodontians. There is significant, non-random error correlated with body habitus (i.e., being relatively robust or gracile) in many variables traditionally used to estimate BM, including femur circumference, and new methods are needed to compensate for this.
Here, we studied several Pachyrukhinae (Mammalia: Notoungulata: Typotheria: Hegetotheriidae) fossils recovered from the late Early Miocene (Santacrucian SALMA) beds of the Cura-Mallín Formation at the Laguna del Laja (Biobío Region, Chile). The specimens are referred to a new species of the genus Pachyrukhos Ameghino, 1885, P. ngenwinkul sp. nov. The holotype of this taxon is a relatively well-preserved articulated skull and mandibles, which was investigated using 3D X-ray Microscopy. The new taxon is distinguished from other Pachyrukhos species by having a p2 smaller than p3, M3 mesiodistally shorter (around 30%) than M1, and upper premolars of a rather subtriangular contour. Pachyrukhos ngenwinkul is the most common mammal and the smallest-sized (with ~2 kg of body mass) notoungulate found in the Santacrucian assemblage at the Laguna del Laja. The discovery of this new taxon improves our understanding of the Neogene pachyrukhine diversity in southern South America and highlights the potential of the Chilean Andean Cenozoic deposits in revealing a hidden diversity of extinct continental mammals.
Mesotheres (Notoungulata: Typotheria) are among the most common mammals found in late Miocene to Pliocene deposits of central Argentina, including the classic type Monte Hermoso locality, which defines the Montehermosan South American Land Mammal “Age”. Nevertheless, the correct name for the mesothere species from this site has been shrouded in uncertainty for well over a century due to questions of taxonomic priority, specimen provenance, and ontogenetic changes in dental formula. Since Pseudotypotherium was named, three distinct species have been formally considered as the type species of the genus (i.e., in studies that have discussed the taxonomy of the genus): (1) Pseudotypotherium bravardi; (2) “Pseudotypotherium” maendrum; and (3) Pseudotypotherium exiguum. However, none of these species is a nominal species of the Pseudotypotherium genus; all three were originally referred to Typotherium. Article 67.2 of the International Code of Zoological Nomenclature (ICZN, 1999) indicates that only species considered as nominal species are eligible to set the type; in the case of Pseudotypotherium, these include: P. pulchrum, P. carlesi, P. hystatum, and P. carhuense. We conclude that Pseudotypotherium pulchrum F. Ameghino, 1904 (holotype MACN-A 10299, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Ameghino Collection), is the type species of the mesotheriid notoungulate genus from Monte Hermoso. According to Article 68.2, F. Ameghino fixed the type by original designation in 1904 when he described P. pulchrum and included “n. g., n. sp.”. Two of the other species previously considered as type species (P. (= T.) bravardi and P. (= T.) exiguum) are invalid according to Article 70.2, since their designations overlooked the previous type fixation. The third species (M. (= T.) maendrum) represents a different mesothere genus (Mesotherium) that only occurs in younger (Pleistocene) deposits. Our analysis puts an end to a historical debate that has been ongoing for more than a century regarding the identity of this well-represented late Miocene–Pliocene mesotheriine genus (Pseudotypotherium). This study provides a solid taxonomic foundation for future studies on intraspecific and ontogenetic variation of Pseudotypotherium pulchrum.
Timing and ecological steps of the rise and expansion of grasslands differ not only geographically but also according to the type of available information. Patagonia (Argentina) was largely considered the region where grasslands and grazers early evolved in the Eocene, mostly based on functional morphology of mammals. However, recent paleobotanical studies question this traditional view and maintain that this occurred after early Miocene. To tackle this discrepancy, we conduct a multidisciplinary and integrative approach using diverse lines of evidence obtained from middle Eocene - middle Miocene tuffaceous deposits of central and northern Patagonia. Gathered data correspond to sedimentary facies, paleosols, insect trace fossils, opal phytoliths and fossil mammals (tooth morphology, body size). Long-lasting and frequent volcanic ashfalls caused permanent disturbances in physical scenarios, biotic systems and grassy habitats; just like herbivory of mammals through coevolutionary grass-grazer mechanisms. The results indicate that soils, vegetation, insects and mammal herbivores begun synchronously to record diverse traits related to grasslands showing mosaic vegetation since middle Eocene (∼44 Ma). These traits include mollic paleosols, abundant and persistent dung beetle trace fossils (Coprinisphaera ichnofacies), phytolith assemblages attributable to savannas and medium to large-size hypsodont mammals. We also track the evolutionary history after the reduction of closed forests, recognizing alternating subhumid savannas with riparian forests, where the relative frequency between grasses, palms, trees and shrubs fluctuated in time and space; and desert vegetation (shrublands) dominated by palms and shrubs during dryer periods. The advent and expansion of grasslands can be linked to the cooling and drying period subsequent to the Early Eocene Climate Optimum.
Members of the Order Notoungulata are among the most diverse and common mammals in South America during the Cenozoic. Several lineages within notoungulates (e.g., suborders Typotheria and Toxodontia) show a tendency for increased body sizes and hypsodonty during the last 50 Myr. However, the timing, evolutionary mode, and drivers of such tendencies are not entirely understood. In this paper, we use an extensive database of notoungulate fossil occurrences and body mass and hypsodonty estimates to characterize the evolutionary mode of these two phenotypic traits over time, test the extent to which several factors (e.g., development of open environments in the south of South America) have influenced it through time, and investigate whether large trait values were selected through elevated origination or reduced extinction rates. Our results demonstrate that most of the major notoungulate clades evolved toward larger body sizes (up 1500 kg) and higher tooth crown, from a small and low-crowned tooth ancestor, in a punctuated mode. We also show that body mass and the hypsodonty in typotherians and toxodonts had a coupled evolutionary history. Species sorting was a relevant macroevolutionary process in some notoungulate clades, as taxa with high teeth crown and body mass had lower extinction rates. Finally, the development of the hypsodonty in notoungulates must reflect repeated and quick instances of adaptive responses to the increased availability of volcanic or other terrigenous particles, within the broad context of the SSA Cenozoic Andean mountain building.
The caudal cranium and occipito-cervical region, although usually overlooked, are informative about the paleobiology of fossil mammals, allowing inquiry into vision and hearing abilities, as well as head and neck postures. Particularly for Pachyrukhinae, some
related features remain unexplored. In this contribution, 22 specimens of Paedotherium and Tremacyllus were analyzed in a mainly qualitative and comparative framework. Pachyrukhines are characterized by having large orbits and hearing cavities, moderate to
short necks with generalized morphologies, and S-shape postures. These features allow rejecting some types of specialized digging habits, and support the preference of open or low-humidity environments. Paedotherium typicum is distinguished by the presence of
laterally oriented eyes,marked vaulted cranium and predominant short extensor and stabilizing neckmuscles, and cervico-occipitalhyoid configurations suitable for ventro-flexed resting posture. These features indicate accentuated frontation and panoramic-vision, upward head postures enhancing substrate perception, and the resistance of impacts during leaping-cursorial locomotion. Conversely, P. bonaerense, and to lesser degree Tremacyllus spp., show less frontation and probably adopted more horizontal head postures. More particularly, stronger ventral and lateral neck and head flexors and extrinsic arm musculature are reconstructed for P. bonaerense, compatible with generalist or scratch-digging habits. Its smaller auditory cavities and stronger ear musculature,
compared to the contemporary P. typicum and especially Tremacyllus spp., would indicate larger ears and microhabitat segregation. The integrative analysis proposed here and in the accompanying contribution aims to shed light on convergences with extant models, paleobiology, niche partitioning, and external appearance of the latest rodent-like ungulates.
Inquiring into the paleoecology of extinct forms is always a challenge, particularly when the taxa under study correspond to derived ecomorphs of ancient and completely extinct clades. In this contribution, the configuration of the masticatory apparatus and
associated features of the Neogene pachyrukhines Paedotherium and Tremacyllus are studied in a detailed, mainly qualitative, comparative analysis of 36 specimens. Tooth morphology and the reconstructed muscular configuration of pachyrukhines indicate an important mediolateral component during chewing, and predominant crushing over grinding, as well as anteroposterior movements for the coupling and action of stronger gnawing incisors. These actions are more compatible with hard and brittle or turgid fruit food consumption than specialized folivorous, and particularly grazing, habits. The infraorbital and palatal foramina morphology and other rostral features indicate increased touch sensibility for object recognition and are congruent with the presence of infoldings of the lips protecting the gingiva during gnawing on hard foods. Additionally, there was a morphological gradient between Tremacyllus and P. bonaerense, from high selection of relatively soft and small food items, to specialized hard item consumption and higher resistance for abrasion and masticatory efforts (e.g., in eventual association with digging habits), respectively.
Paedotherium typicum presents intermediate characteristics, with incisors designed for better cropping action or poorer selectivity during feeding. This more profound understanding of the feeding habits of pachyrukhines further allows the suggestion of paleoecological factors that could have contributed to niche segregation between these long-term coexisting rodent-like taxa.
Large herbivorous mammals have a long history of adaptation to changing environmental circumstances. Many groups of mammalian herbivores started as omnivores and opportunistic browsers of fruits and other plant parts, later adapting to increasingly specialised leaf browsing, and finally to grazing as open grass-dominated environments spread following climatic cooling and drying during the Neogene. Changes in global climate led to vegetational changes in terrestrial ecosystems, which resulted in changes in the proportions of browsing and grazing species in the ungulate guilds. There is currently a range of proxy methods to assess diets and feeding ecology of large extinct herbivorous mammals, including dental microwear and mesowear analyses and stable isotope analyses. Together these methods have enabled an increasingly diverse and fine-scale understanding of the dietary variation of herbivorous mammals throughout the Cenozoic, providing a more detailed picture than traditional comparative ecomorphology approaches alone. This chapter will provide an up-to-date assessment of the analytical methods of determining the diet of extinct large herbivorous mammal taxa, and provide insights into changes in the assemblages of browsing and grazing mammals and how these relate to changes to climate and the evolution of different plant forms.
Cenozoic South American land mammals demonstrated to be the richest and most eloquent known testimonies to infer the geobiotic evolution of the continent throughout the geological span. The concept and practice followed to recognize by them the South American Land Mammal Ages (SALMAs) are explained. Likewise, it is explained the analytical method used to recognize four hierarchical ranks of organization of the 16 SALMAs, which are distinguished as “faunal cycles” (from higher to lower, Megacycles, Supercycles, Cycles and Subcycles). Each of these cycles is defined by an unique association of mammal families, and on the whole are apparently correlated to a hierarchy of world-wide and regional changes in environmental conditions. The Faunistic Cycles are used as “standard units” because they appear to be more closely related to physical and biological regional phenomena. The long-standing autochthonism showed by the South American Late Cretaceous-Cenozoic mammals, as well as the peculiar South American biogeographical history, is briefly analyzed as associated to the climatic shifts related to eustatic falls and geodynamic processes. The successive Late Cretaceous-Cenozoic range pattern and facies of land mammal-bearing deposits is also analyzed in association to eustatic falls and geodynamic processes. Finally, using this background, and as base for the seven Faunistic Cycles so far recognized, the general pattern of evolution of land mammals is chronologically and synthetically analyzed, and particularly, the trends and features of each of the Faunistic Cycles.
Key Words: Land mammals, Cenozoic, South America. geobiotic evolution.
Addresses problems of coronal morphogenesis, amelogenesis, food comminution and digestion, amastication, tooth eruption and wear, in order to identify functional intrrelations and developmental constraints in the evolution of cheek tooth morphology. Many aptive features probably or certainly did not arise for their current functions, but are one-time constraints which have become incorporated into functional systems (exaptations rather than adaptations).-from Author
Paedotherium is recorded from the late Miocene to the early Pleistocene. Two Plio-Pleistocene species, P. typicum Ameghino and P. bonaerense Ameghino are recognised. Paedotherium is frequently registered within paleoburrows of the Chapadmalal Formation (Upper Pliocene), suggesting possible fossorial habits for these species. The fossorial habits of these species are evaluated through morphometric and biomechanical analyses. Comparison with extant caviomorph rodents and other small notoungulates are presented. Seven functional indices are calculated: humerus and ulna robustness, humerus deltoid and epicondyle development, olecranon proportion, and femur and tibia robustness. Principal component, discriminate, Pearson correlations and Mann-Whitney mean difference analyses are used to interpret data. Humerus robustness, epicondyle development, olecranon proportion and ulna robustness, are clearly correlated with limb function in living caviomorph rodents and become useful to understand limb function in Paedotherium. Both species are better prepared to develop force rather than speed, and a fossorial habit can be assigned to them, although P. typicum was probably more cursorial. The capacity of the limbs for developing force is present in different lineages of typothere notoungulates, as Hegetotheriidae and Interatheriidae.
