Eli Amson

Fossil Cetaceans (Mammalia, Cetacea) from the Neogene of Colombia and Venezuela

There are significant geographic gaps in our knowledge of marine mammal evolution because most fossils have been found and described from Northern Hemisphere localities and a few other high-latitude areas in the Southern Hemisphere. Here, we describe fossil cetacean remains from five geological units in the South American tropics (Urumaco, Codore, Castilletes, Cantaure, and Querales formations) generally representing marginal marine depositional environments (estuaries, deltas, and tidal flats). While fossil cetaceans from Venezuelan Neogene localities have been previously studied, this paper includes the first descriptions of fossil cetaceans from Colombia, including a diverse assemblage of mysticetes and odontocetes. We identified and provisionally referred fragmentary remains to the iniid Ischyrorhynchus vanbenedeni and to the platanistid Zarhachis flagellator. The latter suggests the presence of Platanistidae in the eastern coast of South America during the early-middle Miocene, representing the second record of Platanistidae in South America and the first record of Platanistidae in eastern South America. Other less-diagnostic specimens are characterized by features commonly seen in longirostrine odontocetes such as Iniidae, Platanistidae, Pontoporiidae, Lipotidae, Eoplatanistidae, and Squalodelphinidae.

Exceptional Skull of Huayqueriana (Mammalia, Litopterna, Macraucheniidae) From the Late Miocene of Argentina: Anatomy, Systematics, and Paleobiological Implications

ABSTRACT The Huayquerías Formation (Late Miocene, Huayquerian SALMA) is broadly exposed in west-central Argentina (Mendoza). The target of several major paleontological expeditions in the first half of the 20th century, the Mendozan Huayquerías (“badlands”) have recently yielded a significant number of new fossil finds. In this contribution we describe a complete skull (IANIGLA-PV 29) and place it systematically as Huayqueriana cf. H. cristata (Rovereto, 1914) (Litopterna, Macraucheniidae). The specimen shares some nonexclusive features with H. cristata (similar size, rostral border of the orbit almost level with distal border of M3, convergence of maxillary bones at the level of the P3/P4 embrasure, flat snout, very protruding orbits, round outline of premaxillary area in palatal view, and small diastemata between I3/C and C/P1). Other differences (e.g., lack of sagittal crest) may or may not represent intraspecific variation. In addition to other features described here, endocast reconstruction utilizing computer tomography (CT) revealed the presence of a derived position of the orbitotemporal canal running below the rhinal fissure along the lateroventral aspect of the piriform lobe. CT scanning also established that the maxillary nerve (CN V2) leaves the skull through the sphenoorbital fissure, as in all other litopterns, a point previously contested for macraucheniids. The angle between the lateral semicircular canal and the plane of the base of the skull is about 26°, indicating that in life the head was oriented much as in modern horses. Depending on the variables used, estimates of the body mass of IANIGLA-PV 29 produced somewhat conflicting results. Our preferred body mass estimate is 250 kg, based on the centroid size of 36 3D cranial landmarks and accompanying low prediction error. The advanced degree of tooth wear in IANIGLA-PV 29 implies that the individual died well into old age. However, a count of cementum lines on the sectioned left M2 is consistent with an age at death of 10 or 11 years, younger than expected given its body mass. This suggests that the animal had a very abrasive diet. Phylogenetic analysis failed to resolve the position of IANIGLA-PV 29 satisfactorily, a result possibly influenced by intraspecific variation. There is no decisive evidence for the proposition that Huayqueriana, or any other litoptern, were foregut fermenters.

Orientation of the lateral semicircular canal in Xenarthra and its links with head posture and phylogeny: COUTIER et al.

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.

The Postcranial Musculoskeletal System of Xenarthrans: Insights from over Two Centuries of Research and Future Directions

Xenarthrans stand out among mammals for various reasons, one of them being their musculoskeletal postcranial specializations. Extant armadillos, anteaters, and sloths feature archetypical adaptations to digging and/or diverse arboreal lifestyles. Numerous extinct xenarthrans dramatically depart in size and morphology from their extant relatives, which has sparked functional interpretations since the end of the eighteenth century. Here, we review the diverse methodological approaches that have been used to investigate functional aspects of the postcranial musculoskeletal system in extant and extinct xenarthrans. Specifically, we address qualitative and quantitative bone morphology (including geometric morphometrics), body size and allometry, bone inner structure, myology, as well as in vivo, ex vivo, and in silico experimentation. Finally, a short account is given on those analyses that included xenarthrans to gain insight into primate anatomy. This review helped to identify potential future directions for the functional analysis of the xenarthran anatomy, a tradition over two centuries old.

Scaling effect on the mid-diaphysis properties of long bones—the case of the Cervidae (deer)

How skeletal elements scale to size is a fundamental question in biology. While the external shape of long bones was intensively studied, an important component of their organization is also found in their less accessible inner structure. Here, we studied mid-diaphyseal properties of limb long bones, characterizing notably the thickness of their cortices (bone walls), in order to test whether body size directly influences bone inner organization. Previous examinations of scaling in long bones used broad samplings to encompass a wide range of body sizes. To account for the effect of confounding factors related to different lifestyles, we focused our comprehensive sampling on a mammalian clade that comprises various body sizes but a relatively uniform lifestyle, the Cervidae. Positive allometry was found in femoral cross-sectional shape, indicating greater directional bending rigidity in large-sized taxa. None of the compactness parameters scaled allometrically in any of their bones. The cortices of sampled zeugopodial bones (tibia and radius) were found as significantly thicker than those of stylopodial bones (femur and humerus). Furthermore, while the mean relative cortical thickness values for both stylopodial and zeugopodial bones are close to mass-saving optima, the variance for the stylopodial bones is significantly lower. This suggests that mass saving is less intensively selected in zeugopodial bones. Finally, the long-legged Elk (Alces) and the short-legged dwarf Cretan deer (Candiacervus) featured rather thin and thick cortices, respectively, suggesting that the acquisition of a different limb proportion is accompanied by a modification of the relative mid-diaphyseal cortical thickness.

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.

Palaeobiological inferences based on long bone epiphyseal and diaphyseal structure - the forelimb of xenarthrans (Mammalia)

Trabecular architecture (i.e., the main orientation of the bone trabeculae, their relative number, mean thickness, spacing, etc.) has been shown experimentally to adapt with extreme accuracy and sensitivity to the loadings applied to the bone during life. However, the potential of trabecular parameters used as a proxy for the mechanical environment of an organism’s organ to help reconstruct the lifestyle of extinct taxa has only recently started to be exploited. Furthermore, these parameters are rarely combined to the long-used mid-diaphyseal parameters to inform such reconstructions. Here we investigate xenarthrans, for which functional and ecological reconstructions of extinct forms are particularly important in order to inform our macroevolutionary understanding of their main constitutive clades, i.e., the Tardigrada (sloths), Vermilingua (anteaters), and Cingulata (armadillos and extinct close relatives). The lifestyles of modern xenarthrans can be classified as fully terrestrial and highly fossorial (armadillos), arboreal (partly to fully) and hook-and-pull digging (anteaters), or suspensory (fully arboreal) and non-fossorial (sloths). The degree of arboreality and fossoriality of some extinct forms, “ground sloths” in particular, is highly debated. We used high-resolution computed tomography to compare the epiphyseal 3D architecture and mid-diaphyseal structure of the forelimb bones of extant and extinct xenarthrans. The comparative approach employed aims at inferring the most probable lifestyle of extinct taxa, using a phylogenetically informed discriminant analysis. Several challenges were identified, and no extinct sloths were here ascribed to one of the extant xenarthran lifestyles. Differing from that of the larger “ground sloths”, the bone structure of the small-sized Hapalops (Miocene of Argentina), however, was found as significantly more similar to that of extant sloths, even when accounting for the phylogenetic signal.

The Southernmost Occurrence of the Aquatic Sloth Thalassocnus (Mammalia, Tardigrada) in Two New Pliocene Localities in Chile

Abstract. Thalassocnus is a sloth (Mammalia, Tardigrada) adapted to an aquatic lifestyle. It was first described from the Neogene deposits of the Pisco Formation of Peru, from where most of the specimens come. The genus is represented by five species ranging from the late Miocene to the late Pliocene, occupying successive stratigraphic levels. Morpho-functional studies of the cranial and postcranial skeleton of Thalassocnus have demonstrated the progressive adaptation of these sloths to a marine environment, establishing gradual differences from from the geologically oldest to the youngest species of the genus. The first records of Thalassocnus outside the Pisco Formation have been referred to the Neogene Bahía Inglesa Formation, in northern Chile, where older species were recovered. In this paper, we describe materials from two new Pliocene localities in Chile: the Coquimbo and the Horcón formations, in northern and central Chile, respectively. The Coquimbo Formation material was collected from the Lomas del Sauce locality and consists of a partial skeleton of a single individual. Detailed comparisons of the elements with diagnostic features enabled the referral of this specimen to T. carolomartini. The material from the Horcón Formation was collected from the Playa La Luna locality and consists of an isolated phalanx, which is attributed to one of the species of Thalassocnus younger than T. natans. Thus, we present the first record of younger species of Thalassocnus in Chile and the southernmost occurrence of the genus.