Advantages and Limitations in the Use of Extant Xenarthrans (Mammalia) as Morphological Models for Paleobiological Reconstruction

Extant species of Xenarthra represent a severely restricted sample of the total diversity achieved by the group. Given their shared history, the extant representatives of the three major groups of xenarthrans (Cingulata, Folivora, and Vermilingua) provide a valuable basis for paleobiological inference. However, many extinct taxa are morphologically so dissimilar from their extant relatives that they suggest very different ways of life. In these cases, extinct forms do not have modern models within the group and the application of a simplistic and strict approach can produce nonsensical reconstructions. In this contribution, we evaluate the limitations of the use of extant xenarthrans as morphological models for paleobiological reconstructions. A database of linear dimensions of the appendicular skeleton of extant and extinct xenarthrans and other mammals (marsupials, carnivorans, rodents, primates, perissodactyls, artiodactyls, and proboscideans) was constructed. Exploratory analyzes were performed on general morphometric similarity between existing and extinct xenarthrans (PCA) and the accuracy of body mass estimates of extinct xenarthrans based on their close relatives and other mammals (simple and multiple linear regressions) were tested. Extinct xenarthrans occupy similar relative positions in the morphospaces as extant mammals other than their closest relatives. Most allometric equations, particularly those based only on xenarthrans, produced remarkable underestimates. This can be explained by dimensional differences (up to four orders of magnitude) and shape differences between most of the extinct and extant xenarthrans. This does not invalidate actualism and the use of analogues, but suggests the need to apply other approaches, such as mechanics, that address form-function relationships but are not necessarily based on known biological comparators.     

Comparative anatomy and histology of xenarthran osteoderms

Reconstruction of soft tissues in fossil vertebrates is an enduring challenge for paleontologists. Because inferences must be based on evidence from hard tissues (typically bones or teeth), even the most complete fossils provide only limited information about certain organ systems. Osteoderms ("dermal armor") are integumentary bones with high fossilization potential that hold information about the anatomy of the skin in many extant and fossil amniotes. Their importance for functional morphology and phylogenetic research has recently been recognized, but studies have focused largely upon reptiles, in which osteoderms are most common. Among mammals, osteoderms occur only in members of the clade Xenarthra, which includes armadillos and their extinct relatives: glyptodonts, pampatheres, and, more distantly, ground sloths. Here, I present new information on the comparative morphology and histology of osteoderms and their associated soft tissues in 11 extant and fossil xenarthrans. Extinct mylodontid sloths possessed simple, isolated ossicles, the presence of which is likely plesiomorphic for Xenarthra. More highly derived osteoderms of glyptodonts, pampatheres, and armadillos feature complex articulations and surface ornamentation. Osteoderms of modern armadillos are physically associated with a variety of soft tissues, including nerve, muscle, gland, and connective tissue. In some cases, similar osteological features may be caused by two or more different tissue types, rendering soft-tissue inferences for fossil osteoderms equivocal. Certain osteological structures, however, are consistently associated with specific soft-tissue complexes and therefore represent a relatively robust foundation upon which to base soft-tissue reconstructions of extinct xenarthrans.     

Reconstructing the Diet of an Extinct Hominin Taxon: The Role of Extant Primate Models

Modern humans represent the only surviving species of an otherwise extinct clade of primates, the hominins. As the closest living relatives to extinct hominins, extant primates are an important source of comparative information for the reconstruction of the diets of extinct hominins. Methods such as comparative and functional morphology, finite element analysis, dental wear, dental topographic analysis, and stable isotope biogeochemistry must be validated and tested within extant populations before they can be applied to extinct taxa. Here we review how these methods have and might be used to reconstruct the diet of a particular extinct hominin, Paranthropus boisei, which has no extant analogue for its highly derived masticatory morphology. Our review emphasizes the potential and limitations of using extant primates as models for the reconstruction of extinct hominin diets. We encourage paleoanthropologists and those who study the feeding behaviors of extant primates to work together to investigate and validate methods for interpreting the diets of all extinct primates, including hominins.     

The hominoid proximal radius: re-interpreting locomotor behaviors in early hominins

Studies of fossil hominins are traditionally taxonomically narrow and often exclude comparisons with hylobatids. Hence, results of functional analyses of postcrania, interpreted as indicating that early hominins are "African-ape-like" in their postcranial skeletons and positional behaviors, may reflect an artifact of inadequate taxonomic and morphological breadth of the comparative sample. To address this problem and better understand early hominin positional behaviors, this study included hylobatids in a comparative analysis, focusing on the hominoid elbow joint. Specifically, morphometric variables of the proximal radius were derived from measurements from a sample of all genera of extant hominoids and casts of extinct hominin species. Univariate and multivariate analyses were performed on these data. Results show that early hominins are morphologically diverse and are not, as a group, similar to any one extant group. Instead, the fossils resemble Pan, Gorilla, and Hylobates, and are not like modern Homo sapiens or Pongo. This suggests that the morphology of Hylobates may reflect a morphotype for all later hominoids, thus complicating the functional interpretations of fossil hominins. The implications of these results are that the proximal radius is not a sensitive indicator of locomotor behavior among hominoids since the morphology in hylobatids and Gorilla and Pan is similar despite widely varying positional repertoires. Furthermore, inferences of function from form in extinct hominins can be drastically affected by the comparative outgroup selection. A re-evaluation of the functional morphology of the proximal radius in early hominins is addressed.     

牛科(哺乳纲:偶蹄目)动物与生境利用有关的适应形态模式

对广义牛科动物颅后骨骼的多元变量分析揭示了牛科生境利用和体型之间的骨学特征。利用逐步分辨分析方法和一个基于机器学习的决策树方法鉴别了每种生境中牛科动物颅后解剖结构的形态特征。从110个广义牛科动物测量了43个指标进行了这项分析。利用所有主要肢骨测量值和以单根肢骨测量为主的测量值获得的分辨函数和决策树可以完美地区分适应开阔生境、森林和山地的牛科动物(在所有分析中得到了100 %正确的再分类)。由于调整的函数仅涉及到很小的颅后骨骼测量集,这些函数可以应用于研究考古学和古生物学发掘物中保存的不完整标本。这些表征生境利用的生态适应函数与那些用颅齿部性状建立、用于推测牛科动物食物选择的函数结合,具有刻画已灭绝的分类类群的古个体生态学和重建古环境的潜力。我们还分析了多元回归是否较单一因子回归表现出较高的预测能力,并提出了从每一种单根主要肢骨测量的颅后形态变量得到的最好代数函数     

Estimating dinosaur maximum running speeds using evolutionary robotics

Maximum running speed is an important locomotor parameter for many animals-predators as well as prey-and is thus of interest to palaeobiologists wishing to reconstruct the behavioural ecology of extinct species. A variety of approaches have been tried in the past including anatomical comparisons, bone scaling and strength, safety factors and ground reaction force analyses. However, these approaches are all indirect and an alternative approach is to create a musculoskeletal model of the animal and see how fast it can run. The major advantage of this approach is that all assumptions about the animal's morphology and physiology are directly addressed, whereas the exact same assumptions are hidden in the indirect approaches. In this paper, we present simple musculoskeletal models of three extant and five extinct bipedal species. The models predict top speed in the extant species with reasonably good agreement with accepted values, so we conclude that the values presented for the five extinct species are reasonable predictions given the modelling assumptions made. Improved musculoskeletal models and better estimates of soft tissue parameters will produce more accurate values. Limited sensitivity analysis is performed on key muscle parameters but there is considerable scope for extending this in the future.     

The femur of extinct bunodont otters in Africa (Carnivora,Mustelidae, Lutrinae)

This study compares fossil femora attributed to extinct African bunodont lutrines with extant mustelids and ursids to reconstruct locomotor behavior. Due to the immense size differences among taxa, shape data were used to compare morphology. Based on morphological differences, the fossil femora are suggested to belong to different taxa with different locomotor abilities and habitat preferences. The Langebaanweg femur is the oldest and has a typical mustelid morphology suggesting that it was a locomotor generalist like most mustelids. The West Turkana form is more like extant nonbunodont otters, but much larger, and may have belonged to a semiaquatic taxon. The enormous Omo femur shares some features with truly aquatic taxa (e.g., Enhydra) and is the most likely to have been fully aquatic. The same may hold true for the Hadar species as it is most similar to that from the Omo. If these femora truly belong to bunodont lutrines, then they are more diverse in postcranial morphology than in dental morphology.     

PARALLEL EVOLUTION OF HAND ANATOMY IN KANGAROOS AND VOMBATIFORM MARSUPIALS: FUNCTIONAL AND EVOLUTIONARY IMPLICATIONS

Abstract:  The anatomy of the mammalian hand is exposed to an intriguing interplay between phylogeny and function, and provides insights on phylogenetic affinities as well as locomotory habits of extinct species. Within the marsupial order Diprotodontia, terrestrial plantigrade quadrupedalism evolved twice, in the mostly extinct vombatiforms and in extant macropodoids. To assess the influence of functional and phylogenetic signal on the manus in these two clades, manual anatomy and digital proportions in specimens of eight extinct and three extant vombatiforms were investigated and compared with extant macropodoids and extant possums. The results reveal extensive parallelisms in the carpal region of vombatiforms and macropodoids, including flattened distal metacarpal facets, reduction of the palmar process of the hamatum, reduction of mid-wrist joint curve, extensive hamatum/scaphoid contact, and absence of a lunatum. These transformations appear to be related to stabilization of the wrist for plantigrade locomotion. Vombatiforms are apomorphic in scaphoid and triquetrum anatomy and their metacarpals are much more gracile than in other Diprotodontia. Manual diversity is greater in vombatiforms than in macropodoids, as probably was locomotor diversity. Digital proportions as well as wrist anatomy divide the extinct vombatiforms into species resembling arboreal diprotodontians, whereas others group with terrestrial quadrupedal kangaroos and wombats. The latter is suggested to be owing to plantigrade locomotion and/or large size. Carpal anatomy and digital proportions suggest that a range of earlier diverging vombatiforms may have been arboreal or scansorial. As such, we propose that the ancestor of extant vombatiforms (koalas and wombats) may have been arboreal, an option that deserves consideration in the reconstruction of vombatiform evolution.     

A partial skeleton of the fossil great ape Hispanopithecus laietanus from Can Feu and the mosaic evolution of crown-hominoid positional behaviors

The extinct dryopithecine Hispanopithecus (Primates: Hominidae), from the Late Miocene of Europe, is the oldest fossil great ape displaying an orthograde body plan coupled with unambiguous suspensory adaptations. On the basis of hand morphology, Hispanopithecus laietanus has been considered to primitively retain adaptations to above-branch quadrupedalism-thus displaying a locomotor repertoire unknown among extant or fossil hominoids, which has been considered unlikely by some researchers. Here we describe a partial skeleton of H. laietanus from the Vallesian (MN9) locality of Can Feu 1 (Vallès-Penedès Basin, NE Iberian Peninsula), with an estimated age of 10.0-9.7 Ma. It includes dentognathic and postcranial remains of a single, female adult individual, with an estimated body mass of 22-25 kg. The postcranial remains of the rib cage, shoulder girdle and forelimb show a mixture of monkey-like and modern-hominoid-like features. In turn, the proximal morphology of the ulna-most completely preserved in the Can Feu skeleton than among previously-available remains-indicates the possession of an elbow complex suitable for preserving stability along the full range of flexion/extension and enabling a broad range of pronation/supination. Such features, suitable for suspensory behaviors, are however combined with an olecranon morphology that is functionally related to quadrupedalism. Overall, when all the available postcranial evidence for H. laietanus is considered, it emerges that this taxon displayed a locomotor repertoire currently unknown among other apes (extant or extinct alike), uniquely combining suspensory-related features with primitively-retained adaptations to above-branch palmigrady. Despite phylogenetic uncertainties, Hispanopithecus is invariably considered an extinct member of the great-ape-and-human clade. Therefore, the combination of quadrupedal and suspensory adaptations in this Miocene crown hominoid clearly evidences the mosaic nature of locomotor evolution in the Hominoidea, as well as the impossibility to reconstruct the ancestral locomotor repertoires for crown hominoid subclades on the basis of extant taxa alone.     

Trabecular architecture in the forelimb epiphyses of extant xenarthrans (Mammalia)

Background

Bone structure has a crucial role in the functional adaptations that allow vertebrates to conduct their diverse lifestyles. Much has been documented regarding the diaphyseal structure of long bones of tetrapods. However, the architecture of trabecular bone, which is for instance found within the epiphyses of long bones, and which has been shown experimentally to be extremely plastic, has received little attention in the context of lifestyle adaptations (virtually only in primates). We therefore investigated the forelimb epiphyses of extant xenarthrans, the placental mammals including the sloths, anteaters, and armadillos. They are characterised by several lifestyles and degrees of fossoriality involving distinct uses of their forelimb. We used micro computed tomography data to acquire 3D trabecular parameters at regions of interest (ROIs) for all extant genera of xenarthrans (with replicates). Traditional, spherical, and phylogenetically informed statistics (including the consideration of size effects) were used to characterise the functional signal of these parameters.

Results

Several trabecular parameters yielded functional distinctions. The main direction of the trabeculae distinguished lifestyle categories for one ROI (the radial trochlea). Among the other trabecular parameters, it is the degree of anisotropy (i.e., a preferential alignment of the trabeculae) that yielded the clearest functional signal. For all ROIs, the armadillos, which represent the fully terrestrial and fossorial category, were found as characterised by a greater degree of anisotropy (i.e., more aligned trabeculae). Furthermore, the trabeculae of the humeral head of the most fossorial armadillos were also found to be more anisotropic than in the less fossorial species.

Conclusions

Most parameters were marked by an important intraspecific variability and by a size effect, which could, at least partly, be masking the functional signal. But for some parameters, the degree of anisotropy in particular, a clear functional distinction was recovered. Along with data on primates, our findings suggest that a trabecular architecture characterised by a greater degree of anisotropy is to be expected in species in which the relevant epiphyses withstand a restricted range of load directions. Trabecular architecture therefore is a promising research avenue for the reconstruction of lifestyles in extinct or cryptic species.

     

An oreopithecid proximal humerus from the middle miocene of Maboko Island,Kenya

A proximal humerus, recently recovered from the middle Miocene of Maboko Island, Kenya, provides the earliest evidence of postcranial structure and adaptation of Oreopithecidae. Provisionally attributed toNyanzapithecus pickfordi (Harrison, 1986), the specimen manifests a globose head, subequally large tuberosities, and a board, shallow bicipital groove. Although readily distinguished from the fundamentally cercopithecoid proximal humeral morphology ofVictoriapithecus (Senut, 1986), the Maboko Island oreopithecid, shows none of the derived features that are characteristic of the proximal humeri of extant hominoids. It is inferred from proximal humeral anatomy that the Maboko Island oreopithecid was an active arboreal scansor with moderate mobility at the shoulder but lacking adaptations for circumduction of the arm. In combination with craniodental evidence, proximal humeral morphology indicates that Oreopithecidae was a clade of hominoids which originated before the last common ancestor of extant apes and went extinct, without issue, in the later Miocene.     

The evolution of hyoid apparatus in Xenarthra (Mammalia: Eutheria)

The hyoid apparatus reflects aspects of the form and function of feeding in living and extinct organisms and, despite the availability of information about this structure for Xenarthra, it remains little explored from an evolutionary perspective. Here we compare the morphology of the hyoid apparatus in xenarthrans, describing its general morphology and variation in each major clade and score these variations as phylogenetic characters, which were submitted to ancestral states reconstructions. The general hyoid morphology of Xenarthra consists of a v-bone (basihyal fused with the thyrohyals) and three paired bones (stylohyal, epihyal and ceratohyal), which are unfused in the majority of taxa. The clade-specific morphology observed here, allowed us to obtain additional synapomorphies for all major clades of Xenarthra (Cingulata, Pilosa, Folivora and Vermilingua), for Glyptodontididae, and for Nothrotheriidae. The fusion of hyoid elements are convergentelly achieved among the diphyletic extant tree sloths, some extinct ground sloths and glyptodontids. Despite the heavy influence of adaptive evolution related to feeding habits, the morphology of the hyoid apparatus proved to be a valuable source of phylogenetic information.     

Orientation of the lateral semicircular canal in Xenarthra and its links with head posture and phylogeny

The orientation of the semicircular canals of the inner ear in the skull of vertebrates is one of the determinants of the capacity of this system to detect a given rotational movement of the head. Past functional studies on the spatial orientation of the semicircular canals essentially focused on the lateral semicircular canal (LSC), which is supposedly held close to horizontal during rest and/or alert behaviors. However, they generally investigated this feature in only a few and distantly related taxa. Based on 3D‐models reconstructed from µCT‐scans of skulls, we examined the diversity of orientations of the LSC within one of the four major clades of placental mammals, that is, the superorder Xenarthra, with a data set that includes almost all extant genera and two extinct taxa. We observed a wide diversity of LSC orientations relative to the basicranium at both intraspecific and interspecific scales. The estimated phylogenetic imprint on the orientation of the LSC was significant but rather low within the superorder, though some phylogenetic conservatism was detected for armadillos that were characterized by a strongly tilted LSC. A convergence between extant suspensory sloths was also detected, both genera showing a weakly tilted LSC. Our preliminary analysis of usual head posture in extant xenarthrans based on photographs of living animals further revealed that the LSC orientation in armadillos is congruent with a strongly nose‐down head posture. It also portrayed a more complex situation for sloths and anteaters. Finally, we also demonstrate that the conformation of the cranial vault and nuchal crests as well as the orientation of the posterior part of the petrosal may covary with the LSC orientation in Xenarthra. Possible inferences for the head postures of extinct xenarthrans such as giant ground sloths are discussed in the light of these results.     

Sexual dimorphism in postcranial skeletal shape suggests male-biased specialization for physical competition in anthropoid primates

Sexual dimorphism often arises as a response to selection on traits that improve a male's ability to physically compete for access to mates. In primates, sexual dimorphism in body mass and canine size is more common in species with intense male–male competition. However, in addition to these traits, other musculoskeletal adaptations may improve male fighting performance. Postcranial traits that increase strength, agility, and maneuverability may also be under selection. To test the hypothesis that males, as compared to females, are more specialized for physical competition in their postcranial anatomy, we compared sex-specific skeletal shape using a set of functional indices predicted to improve fighting performance. Across species, we found significant sexual dimorphism in a subset of these indices, indicating the presence of skeletal shape sexual dimorphism in our sample of anthropoid primates. Mean skeletal shape sexual dimorphism was positively correlated with sexual dimorphism in body size, an indicator of the intensity of male–male competition, even when controlling for both body mass and phylogenetic relatedness. These results suggest that selection on male fighting ability has played a role in the evolution of postcranial sexual dimorphism in primates.     

Osteology Supports a Stem-Galliform Affinity for the Giant Extinct Flightless Bird Sylviornis neocaledoniae (Sylviornithidae,Galloanseres)

The giant flightless bird Sylviornis neocaledoniae (Aves: Sylviornithidae) existed on La Grande Terre and Ile des Pins, New Caledonia, until the late Holocene when it went extinct shortly after human arrival on these islands. The species was generally considered to be a megapode (Megapodiidae) until the family Sylviornithidae was erected for it in 2005 to reflect multiple cranial autapomorphies. However, despite thousands of bones having been reported for this unique and enigmatic taxon, the postcranial anatomy has remained largely unknown. We rectify this deficiency and describe the postcranial skeleton of S. neocaledoniae based on ~600 fossils and use data from this and its cranial anatomy to make a comprehensive assessment of its phylogenetic affinities. Sylviornis neocaledoniae is found to be a stem galliform, distant from megapodiids, and the sister taxon to the extinct flightless Megavitiornis altirostris from Fiji, which we transfer to the family Sylviornithidae. These two species form the sister group to extant crown-group galliforms. Several other fossil galloanseres also included in the phylogenetic analysis reveal novel hypotheses of their relationships as follows: Dromornis planei (Dromornithidae) is recovered as a stem galliform rather than a stem anseriform; Presbyornis pervetus (Presbyornithidae) is the sister group to Anseranatidae, not to Anatidae; Vegavis iaai is a crown anseriform but remains unresolved relative to Presbyornis pervetus, Anseranatidae and Anatidae. Sylviornis neocaledoniae was reconstructed herein to be 0.8 m tall in a resting stance and weigh 27–34 kg. The postcranial anatomy of S. neocaledoniae shows no indication of the specialised adaptation to digging seen in megapodiids, with for example, its ungual morphology differing little from that of chicken Gallus gallus. These observations and its phylogenetic placement as stem galliforms makes it improbable that this species employed ectothermic incubation or was a mound-builder. Sylviornis neocaledoniae can therefore be excluded as the constructor of tumuli in New Caledonia.     

Proximal femoral anatomy of a sivaladapid primate from the late middle Eocene Pondaung formation (central Myanmar)

The postcranial anatomy of the Asian sivaladapid adapiforms is still virtually undocumented, whereas dental remains of these primates have been known for several decades. Little is known about their positional behavior as a result. In this article, we describe a partial left femur of a medium-sized primate preserving its entire proximal portion and a significant length of its shaft. This fossil was recently recovered from the fossiliferous locality of Thamingyauk in the late middle Eocene Pondaung Formation (central Myanmar). This femur is considered to pertain to the same individual as two tarsal elements (fragmentary talus and calcaneus) from the same locality (same location), and attributed to a medium-sized sivaladapid adapiform primate (Kyitchaungia takaii). This new postcranial element provides the first documentation of femoral anatomy among Sivaladapidae from Asia. The mechanical implications deriving from the musculoskeletal interpretation of this bone indicate an animal that probably engaged in a kind of active arboreal quadrupedalism with some degree of proficiency in leaping. Even though many musculoskeletal aspects suggest that branch walking and running were important parts of its locomotor repertoire, in other details it appears that relatively complex movements at the hip joint were actually possible and probably associated with climbing or some hindlimb suspensory activities.     

Look in the trees: Hylobatids as evolutionary models for extinct hominins

Studying extant apes is of central importance to paleoanthropology. This approach is informative in inferring how hominin skeletal morphology reflects phylogeny, behavior, development, and ecological context. Traditionally, great apes have dominated the paleoanthropological literature as extant analogs for extinct hominins, to the exclusion of their phylogenetic sister group, the hylobatids. Phylogenetic proximity, large body size, and high encephalization quotients may have contributed to decisions to use great apes as models for hominins. However, if we reexamine hylobatids as extant models for extinct hominins—using modern phylogenetic, behavioral, and ecological data—this clade is uniquely poised to inform future frameworks in paleoanthropology. The following features make hylobatids strong analogs for extinct hominins: taxonomic diversity, the timing of diversification, hybridization between species, small body size, and reduced sexual dimorphism. Based on these shared features, hylobatids offer future opportunities to paleoanthropology, and provide a much richer extant analog than is currently recognized.