Ruslan I. Belyaev

In this study, we examine the dimensions of the nasal horns of woolly rhinoceroses and describe the longest rhino horn known to date, which was recently discovered in the permafrost of Yakutia. The extinct woolly rhinoceros is an iconic representative of the Late Pleistocene mammoth fauna of Eurasia, characterized by two huge keratinous horns. The nasal horn is considerably longer than the frontal one; it is laterally flattened and has a saber‐like shape, which is unparalleled among modern‐day rhinoceroses. The nasal horns of adult woolly rhinoceroses range in length along the anterior curvature from 68.5 to 134 cm (101.7 cm on average). Their mass varies between 2.5 and 11.4 kg (6 kg on average). Both of these values exceed those of all extant rhinoceroses. The record‐breaking nasal horn discovered in Yakutia last year reaches 164.7 cm along the anterior curvature and weighs ~9 kg. The relatively small size of the skull in this specimen indicates that it probably belonged to a rhinoceros female. This suggests that Coelodonta antiquitatis may have had the longest horns in females, similar to modern‐day African rhinoceroses. The number of periodic transverse bands (each of which consists of two, darker and lighter, keratin zones) along the length of the horn suggests that the discovered horn belonged to the oldest known woolly rhinoceros with an individual age of ≥40 years. Thus, this discovery not only provides insights into the morphological features of woolly rhinoceroses but also offers valuable information about their palaeoecology and sexual dimorphism.

The vertebral column of elephants has several unique features that distinguish them from all other modern‐day mammals. In this study, we examine various aspects of the functional morphology and intervertebral mobility of the elephant backbone, comparing it to that of other large herbivorous mammals, as well as to extinct Mammuthus primigenius , M. trogontherii , and Mammut americanum . The elephant vertebral column is characterized by a high degree of dorsostability. All three directions of intervertebral mobility (sagittal and lateral bending, and axial rotation), in all backbone modules, demonstrate the lowest amplitudes of motion known for mammals. In elephants, neck mobility is largely replaced by the mobility of the proboscis; the axial rotation in the thoracic region is not used for maneuvering, and the sagittal flexibility of the lumbar region is practically absent during locomotion. The mobility of the elephant cervical region in the sagittal and frontal planes is low; however, it is still responsible for movements that require significant force, such as tournament fights and breaking tree branches. The lumbosacral joint characterized by sagittal hypermobility in most terrestrial mammals is even less mobile than the intralumbar joints in elephantids. The thoracic‐lumbo‐sacral region of proboscideans is arch‐shaped in lateral view, resulting from the ventral‐ward tapering of the vertebral centra (the length along the spinal canal is longer than the length at the ventral side of the centrum). This tapering is most pronounced in the posterior thoracic and lumbar vertebrae. In contrast, the vertebral centra in the middle part of the trunk (T8–T14) are almost rectangular in lateral view. This arch‐shaped structure provides static support to the vertebral column, preventing sagging. The absolute length of the spinous processes in proboscideans is comparable to those of the largest bovines and exceeds the lengths of extant rhinoceroses. However, relative to the height of the vertebral centrum, the spinous processes in the withers area of elephants are 2–4 times shorter than those in bovines and rhinoceroses. The profile and inclination angle of the spinous processes differ drastically among different taxa. Despite these differences, we found no significant variations in the sagittal flexibility of the backbone among the different elephantids. We hypothesize that the observed differences may relate to how the arch shape of the backbone is maintained in the posterior part of the thoracolumbar region. In modern‐day elephants, dorsostability is primarily supported by a strong supraspinous ligament with short intersegmental portions. In mammoths, this is probably maintained by the linea alba and the abdominal muscles.

In this study, we aimed to achieve three objectives: (1) to precisely characterize the body plans of Elephantidae and other large herbivorous mammals; (2) based on this analysis, to determine whether the body plans of the extinct woolly mammoth (Mammuthus primigenius) and steppe mammoth (M. trogontherii) differ from those of modern‐day Elephantidae: the Asian elephant (Elephas maximus), the African bush (Loxodonta africana), and forest (L. cyclotis) elephants; (3) to analyze how the body plans have changed in extant perissodactyls and proboscideans compared with their Paleogene ancestors. To accomplish this, we studied mammoth skeletons from the collections of Russian museums and compared this data with a large number of skeletons of extant elephantids, odd‐toed, and even‐toed ungulates, as well as their extinct relatives. We showed that three genera of Elephantidae are characterized by a homogeneous body plan, which is markedly different from other large herbivores. Elephantids break the interrelationship, that exists in artiodactyls and perissodactyls, between the total length of the head and neck on one side and the limb's segments on the other. Their limbs are very tall (inferior in this regard among large ungulates only to the giraffe), and, contrary to the other large herbivorous mammals, elongated due to the length of the proximal segments. This allows them to effectively utilize the principle of inverted pendulum (straight‐legged walking) in locomotion. The biggest differences in the body plan of mammoths compared with extant elephants are a markedly larger pelvis, elongated fore‐ and hindlimbs (due to the increased relative length of their proximal segments), and different proportions of the skull. The body plans of plesiomorphic Paleogene proboscideans and perissodactyls differed markedly from their descendants in every body part; these differences are related, on the one hand, to the allometric growth, and on the other hand, to the advancement of the locomotor apparatus in the course of their evolution. The most notable difference in the body plan between Paleogene proboscidean Moeritherium and extant Elephantidae is the ~2‐fold increase in relative limb height.

Carnivorans are well‐known for their exceptional backbone mobility, which enables them to excel in fast running and long jumping, leading to them being among the most successful predators amongst terrestrial mammals. This study presents the first large‐scale analysis of mobility throughout the presacral region of the vertebral column in carnivorans. The study covers representatives of 6 families, 24 genera and 34 species. We utilized a previously developed osteometry‐based method to calculate available range of motion, quantifying all three directions of intervertebral mobility: sagittal bending (SB), lateral bending (LB), and axial rotation (AR). We observed a strong phylogenetic signal in the structural basis of the vertebral column (vertebral and joint formulae, length proportions of the backbone modules) and an insignificant phylogenetic signal in most characteristics of intervertebral mobility. This indicates that within the existing structure (stabilization of which occurred rather early in different phylogenetic lineages), intervertebral mobility in carnivorans is quite flexible. Our findings reveal that hyenas and canids, which use their jaws to seize prey, are characterized by a noticeably elongated cervical region and significantly higher SB and LB mobility of the cervical joints compared to other carnivorans. In representatives of other carnivoran families, the cervical region is very short, but the flexibility of the neck (both SB and LB) is significantly higher than that of short‐necked odd‐toed and even‐toed ungulates. The lumbar region of the backbone in carnivorans is dorsomobile in the sagittal plane, being on average ~23° more mobile than in artiodactyls and ~38° more mobile than in perissodactyls. However, despite the general dorsomobility, only some representatives of Canidae, Felidae, and Viverridae are superior in lumbar flexibility to the most dorsomobile ungulates. The most dorsomobile artiodactyls are equal or even superior to carnivorans in their ability to engage in dorsal extension during galloping. In contrast, carnivorans are far superior to ungulates in their ability to engage ventral flexion. The cumulative SB in the lumbar region in carnivorans largely depends on the mode of running and hunting. Thus, adaptation to prolonged and enduring pursuit of prey in hyenas is accompanied by markedly reduced SB flexibility in the lumbar region. A more dorsostable run is also a characteristic of the Ursidae, and the peculiar maned wolf. Representatives of Felidae and Canidae have significantly more available SB mobility in the lumbar region. However, they fully engage it only occasionally at key moments of the hunt associated with the direct capture of the prey or when running in a straight line at maximum speed.

Extinct woolly rhinoceroses were iconic representatives of the Late Pleistocene mammoth fauna of Eurasia. These animals were characterized by two huge keratinous horns. In adults, the length of the nasal horn often exceeded one meter. The nasal horn of Coelodonta was characterized by an unusual feature for rhinoceroses—the width of its base was considerably narrower than the width of the rugosity area on the nasal bones of the skull. In this study, a new discovery of woolly rhinoceros' nasal horn in the permafrost of Yakutia is described. This specimen shows that the shape of the base of the woolly rhino's nasal horn corresponds well to the shape (length and width) of the nasal rugosity area. The base of the nasal horn of Coelodonta was markedly elongated anteroposteriorly compared to extant rhinoceroses. Its length was about 150% of the width. We therefore suggest that the narrower shape of the nasal horn base in the majority of previously found specimens was associated with secondary damage after burial caused by maceration.

The vertebral column is a hallmark of vertebrates; it is the structural basis of their body and the locomotor apparatus in particular. Locomotion of any vertebrate animal in its typical habitat is directly associated with functional adaptations of its vertebrae. This study is the first large‐scale analysis of mobility throughout the presacral region of the vertebral column covering a majority of extant odd‐toed ungulates from 6 genera and 15 species. In this study, we used a previously developed osteometry‐based method to calculate available range of motion. We quantified all three directions of intervertebral mobility: sagittal bending (SB), lateral bending (LB), and axial rotation (AR). The cervical region in perissodactyls was found to be the most mobile region of the presacral vertebral column in LB and SB. Rhinoceroses and tapirs are characterized by the least mobile necks in SB among odd‐toed and even‐toed ungulates. Equidae are characterized by very mobile necks, especially in LB. The first intrathoracic joint (T1–T2) in Equidae and Tapiridae is characterized by significantly increased mobility in the sagittal plane compared to the typical thoracic joints and is only slightly less mobile than typical cervical joints. The thoracolumbar part of the vertebral column in odd‐toed ungulates is very stiff. Perissodactyls are characterized by frequent fusions of vertebrae with each other with complete loss of mobility. The posterior half of the thoracic region in perissodactyls is characterized by especially stiff intervertebral joints in the SB direction. This is probably associated with hindgut fermentation in perissodactyls: the sagittal stiffness of the posterior thoracic region of the vertebral column is able to passively support the hindgut heavily loaded with roughage. Horses are known as a prime example of a dorsostable galloper among mammals. However, based on SB in the lumbosacral part of the backbone, equids appear to be the least dorsostable among extant perissodactyls; the cumulative SB in equids and tapirs is as low as in the largest representatives of artiodactyls, while in Rhinocerotidae it is even lower representing the minimum across all odd‐toed and even‐toed ungulates. Morphological features of small Paleogene ancestors of rhinoceroses and equids indicate that dorsostability is a derived feature of perissodactyls and evolved convergently in the three extant families.

This study is the first analysis of mobility in the lumbosacral joint of even‐toed ungulates covering the full range of body masses and running forms. In this study, we modified a previously developed osteometry‐based method to calculate the available range of motion (aROM) in the lumbosacral joint in artiodactyls. We quantified all three directions of intervertebral mobility: sagittal bending (SB), lateral bending (LB), and axial rotation (AR). This research covers extant artiodactyls from 10 families, 57 genera, and 78 species. The lumbosacral joint in artiodactyls is on average almost twice as mobile in SB as the average intralumbar joint (aROM 15.68° vs 8.22°). In all artiodactyls, the first sacral prezygapophyses are equipped with postfacet fossae determining the available range of lumbosacral hyperextension. SB aROM in the lumbosacral joint in artiodactyls varies almost sevenfold (from 4.53° to 31.19°) and is closely related to the body mass and running form. An allometric equation was developed for the first time, for the joint angular amplitude of motion, exemplified by the artiodactyl lumbosacral SB aROMs, as a power function of body mass, the power coefficient value being close to −0.15. High SB aROM at the lumbosacral joint is characteristic of artiodactyls with at least one of the following characteristics: high cumulative and average SB aROM in the lumbar region (Pearson r = 0.467–0.617), small body mass (r = −0.531), saltatorial or saltatorial‐cursorial running form (mean = 16.91–18.63°). The highest SB aROM in the lumbosacral joint is typical for small antelopes and Moschidae (mean = 20.24–20.27°). Among these artiodactyls SB aROMs in the lumbosacral joint are on par with various carnivores. Large and robust artiodactyls, adapted predominantly to mediportal and stilt (running on extremely tall limbs) running forms, have 2–3 times smaller SB aROMs in the lumbosacral joint. Adaptation to endurance galloping in open landscapes (cursorial running form) is accompanied by smaller lumbar and lumbosacral SB aROMs compared to that in saltatorial‐cursorial artiodactyls of the same body mass. The wide range of species studied makes it possible to significantly expand the knowledge of relations of the mobility of the lumbosacral joint in artiodactyls to body mass and running form.

In this study, we used a previously developed osteometry‐based method to calculate available range of motion in presacral intervertebral joints in artiodactyls. We have quantified all three directions of intervertebral mobility: sagittal bending (SB), lateral bending (LB), and axial rotation (AR). This research covers 10 extant families of artiodactyls from 33 genera and 39 species. The cervical region in artiodactyls is the most mobile region of the presacral vertebral column in SB and LB. Mobility is unevenly distributed throughout the joints of the neck. The posterior neck joints (C4–C7) are significantly more mobile (on average by 2.5–3.5°) to anterior joints (C2–C4) and to the neck–thorax joint (C7–T1) in SB and LB. An increase in the relative length of the cervical region in artiodactyls is accompanied by an increase in the bending amplitudes (SB: Pearson r = 0.781; LB: r = 0.884). Animals with the most mobile necks (representative of Giraffidae and Camelidae) are 2–3 times more mobile in SB and LB compared to species with the least mobile necks. The thoracic region in artiodactyls, as in other mammals, is characterized by the greatest amplitudes of AR due to the tangential orientation of the zygapophyseal articular facets. The lowest AR values in the thoracic region are typical for the heaviest artiodactyls—Hippopotamidae. The highest AR values are typical for such agile runners as cervids, musk deer, pronghorn, as well as large and small antelopes. SB mobility in the posterior part of the thoracic region can be used by artiodactyls during galloping. The highest values of SB aROM in the posterior part of the thoracic region are typical for small animals with high SB mobility in the lumbar region. The lumbar region in mammals is adapted for efficient SB. Both the cumulative and average SB values in the lumbar region showed correspondence to the running type employed by an artiodactyl. The greatest SB amplitudes in the lumbar region are typical for small animals, which use saltatorial and saltatorial–cursorial running. An increase in body size also corresponds to a decrease in lumbar SB amplitudes. The lowest SB amplitudes are typical for species using the so‐called mediportal running. Adaptation to endurance galloping in open landscapes is accompanied by a decrease in lumbar SB amplitudes in artiodactyls. The consistency of the approach used and the wide coverage of the studied species make it possible to significantly expand and generalize the knowledge of the biomechanics of the vertebral column in artiodactyls.

In this paper, we develop and validate an osteometry‐based mechanistic approach to calculation of available range of motion (aROM) in presacral intervertebral joints in sagittal bending (SB), lateral bending (LB), and axial rotation (AR). Our basic assumption was the existence of a mechanistic interrelation between the geometry of zygapophysial articular facets and aROM. Trigonometric formulae are developed for aROM calculation, of which the general principle is that the angle of rotation is given by the ratio of the arc length of motion to the radius of this arc. We tested a number of alternative formulae against available in vitro data to identify the most suitable geometric ratios and coefficients for accurate calculation. aROM values calculated with the developed formulae show significant correlation with in vitro data in SB, LB, and AR (Pearson r = 0.900) in the reference mammals (man, sheep, pig, cow). It was found that separate formulae for different zygapophysial facet types (radial (Rf), tangential (Tf), radial with a lock (RfL)) give significantly greater accuracy in aROM calculation than the formulae for the presacral spine as a whole and greater accuracy than the separate formulae for different spine regions (cervical, thoracic, lumbar). The advantage of the facet‐specific formulae over the region‐specific ones shows that the facet type is a more reliable indicator of the spine mobility than the presence or absence of ribs. The greatest gain in calculation accuracy with the facet‐specific formulae is characteristic in AR aROM. The most important theoretical outcome is that the evolutionary differentiation of the zygapophysial facets in mammals, that is the emergence of Tf joints in the rib cage area of the spine, was more likely associated with the development of AR rather than with SB mobility and, hence, with cornering rather than with forward galloping. The AR aROM can be calculated with the formulae common for man, sheep, pig, and cow. However, the SB aROM of the human spine is best calculated with different coefficient values in the formulae than those for studied artiodactyls. The most suitable coefficient values indicate that the zygapophysial articular facets tend to slide past each other to a greater extent in the human thoracolumbar spine rather than in artiodactyls. Due to this, artiodactyls retain relatively greater facet overlap in extremely flexed and extremely extended spine positions, which may be more crucial for their quadrupedal gallop than for human bipedal locomotion. The SB, LB, and AR aROMs are quite separate in respect of the formulae structure in the cervical region (radial facet type). However, throughout the thoracolumbar spine (tangential and radial with lock facets), the formulae for LB and AR are basically similar differing in coefficient values only. This means that, in the thoracolumbar spine, the greater the LB aROM, the greater the AR aROM, and vice versa. The approach developed promises a wide osteological screening of extant and extinct mammals to study the sex, age, geographical variations, and disorders.