Alexander N. Kuznetsov

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.

The three‐dimensional configuration of the neck that produces extreme head turn in owls was studied using the Joint Coordinate System. The limits of planar axial rotation (AR), lateral, and sagittal bending in each vertebral joint were measured. They are not extraordinary among birds, except probably for the extended ability for AR. The vertebral joint angles involved in the 360° head turn do not generally exceed the limits of planar mobility. Rotation in one plane does not expand the range of motion in the other, with one probable exception being extended dorsal bending in the middle of the neck. Therefore, the extreme 360° head turn can be presented as a simple combination of the three planar motions in the neck joints. Surprisingly, certain joints are always laterally bent or axially rotated to the opposite side than the head was turned. This allows keeping the anterior part of the neck parallel to the thoracic spine, which probably helps preserve the ability for peering head motions throughout the full head turn. The potential ability of one‐joint muscles of the owl neck, the mm. intertransversarii, to ensure the 360° head turn was addressed. It was shown that the 360° head turn does not require these muscles to shorten beyond the known contraction limit of striated vertebrate muscles. Shortening by 50% or less is enough for the mm. intertransversarii in the middle neck region for the 360° head turn. This study has broad implications for further research on vertebral mobility and function in a variety of tetrapods, providing a new method for CT scan‐based measurement of intervertebral angles.

We present new reconstructions of subcephalic musculature for the stem chondrichthyan Pucapampella, the tetrapodomorph fish Eusthenopteron, and the Devonian tetrapod Ichthyostega. These reconstructions are based on macroscopic dissections of the head muscles of an archaic shark Heptranchias and an archaic actinopterygian Polypterus, that are combined with functional considerations and a reappraisal of not widely known theoretical concepts from the past. The subcephalic, as well as the supracephalic, musculature is formed by four anterior myomeres. They are continuous with subsequent myomeres of the trunk, but are innervated by ventral nerve roots of the medulla oblongata and thus belong to the head. The fourth subcephalic myomere ends with its posterior myoseptum on the occiput in osteichthyans, but on the first vertebra in chondrichthyans. The original function of subcephalic and supracephalic muscles in basal gnathostomes supposedly was to hold together anterior and posterior parts of the neurocranium during interaction with prey, such as the backward‐ripping prey dissection, hypothesized for Pucapampella. In sarcopterygian osteichthyans, subcephalic musculature is involved in active depression of the anterior part of the neurocranium; specialization of this mechanism resulted in a complete separation of m. subcephalicus from trunk myomeres in Latimeria. Fusion of anterior and posterior parts of the neurocranium has resulted in reduction of the subcephalic musculature in the majority of cartilaginous and bony fishes. However, hexanchid sharks retain three posterior subcephalic myomeres for backward‐ripping prey dissection. Polypterus and Chauliodus have retained the subcephalic musculature, but its function has shifted to a depression of the whole neurocranium.

Alternation of left and right antimeres of segmental structures, generally known as the glide-reflection symmetry, was found to be unexpectedly widespread in deuterostomes. It occurs in both somatic and visceral organs. Its distribution in echinoderms, acorn worms, cephalochordates, ascidians, hagfishes, and chimaeras might suggest its ancestral nature in deuterostomes. It could have been inherited from Precambrian ancestors. There is a lead of the left segments against their right counterparts in all deuterostomes with glide-reflection symmetry, where the leading side is known. This allows us to presume an inherent left-to-left-side and right-to-right-side homology across all deuterostomes, contradicting a hypothesis of the whole-body dorsoventral inversion in chordates. The features of inversion could be alternatively explained by a transformation of the gastrulation process, which has caused an inversion of dorsoventral polarity in some parts of the body, but did not affect the left–right polarity. It is suggested that this process had involved the animal-vegetal extension of the blastopore along embryonic dorsal side in basal deuterostomes that was followed by the shift of the left–right organizer from the dorsal lip of the blastopore to the anterior tip of archenteron in non-chordate deuterostomes, which has caused roof-to-floor relocation of the notochordal area in their archenteron.

Masticatory muscle features allow for an understanding of how dietary habits and masticatory functions have evolved across mammalian lineages. Herbivorous mammals were traditionally classified as pertaining to either ‘ungulate-grinding’ or ‘rodent-gnawing’ morphotypes, but those classes might not adequately encompass the diversity of past and present herbivores. We compiled data for 104 herbivorous (nine orders, including new data for 31 species) and ten non-herbivorous (five orders) extant species and for four extinct taxa, in order to construct a more comprehensive scheme of masticatory morphotypes. A myological phylomorphospace revealed 15 morphotypes, which were tested statistically and interpreted morphofunctionally. Extant herbivorous morphotypes diversified along two main pathways that did not reflect the traditional types. Additionally, some extinct herbivores adapted along alternative pathways to those of extant forms. Early mammaliaforms and mammals possessed muscle proportions almost identical to those of living carnivores. Several more plesiomorphic herbivorous morphotypes were recognized, with some of them retaining generalized myological proportions. The rodent disparity was remarkable and could not be encompassed by a single category or pathway. Features of some derived euungulates and diprotodonts resembled rodents, but only a few extinct euungulates and extant wombats converged closely with them. Criteria for selecting models for palaeobiological reconstructions are discussed.

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.

The popular hypothesis that the flapping flight of birds evolved through the stage of wing-assisted incline running (WAIR) is tested through calculations of external forces acting on the wings in the downstroke and upstroke during WAIR activity of an adult chukar partridge (Alectoris chukar). Contrary to previous studies of WAIR, it is found that the total vertical impulse provided by the wings is negative (i.e. downward directed). Thus, the wings cancel out about half of the positive (upward-directed) impulse provided by the legs. The function of the wings in WAIR is not to lift the body, but to push it towards the steep slope. It is like a wing, or aerofoil, of a race car, which is tilted at a negative angle of attack to press the car to the ground for greater friction and adhesion of the wheels and to prevent the race car from becoming airborne. The calculated force pattern suggests that the dominant external forces acting on the wings in the sagittal plane during WAIR are not the virtual forces of inertia but the aerodynamic forces. More specifically, it is only the aerodynamic lift that can be responsible for the wing force component acting at right angles to the wingbeat plane in both the downstroke and the upstroke during WAIR. The lift towards the slope during the WAIR upstroke is ensured by separation of the primary flight feathers and their setting at a negative angle of attack. Analysis of the force pattern in the anatomical coordinate space of a bird’s body shows that the main wing muscles in the WAIR downstroke are the humeral retractor muscles and in the WAIR upstroke the humeral elevator muscles. The humeral protractor muscles are not required at all during WAIR. Therefore, the morphologically necessary protractor state of the supracoracoideus muscle in a ‘semi-flying’ ancestor, which is logically explained by the classical hypothesis of a gliding ancestor, does not fit the WAIR hypothesis of flapping flight origin. Wing-assisted incline running should be regarded as a crown locomotor specialization of birds and is not an appropriate model for locomotion in avian ancestors.

Comparative study of terrestrial locomotion of 4 fish genera including Anabas, Channa, Clarias, and Monopterus, was performed in experimental setting with the substrate surface of wet clay. No special adaptations for terrestrial locomotion were found. Every fish uses for propulsion on land what it already has. Eel‐shaped Monopterus crawls by body undulations in a serpentine or sidewinding technique, the latter of which was not previously observed beyond snakes. The other 3 fish genera walk by body oscillations using stiff appendages as propulsors. When they are located anteriorly, as the serrate operculum in Anabas and the preaxial spine of the pectoral fin in Clarias, the propulsion is termed prolocomotor, when posteriorly, as the spiny anal fin in Channa—metalocomotor. Channa is the heaviest fish walking out of water in our days, quite comparable in size with first Devonian tetrapods Acanthostega and Tulerpeton. A theoretical calculation is suggested for the upper size limit of a fish capable of terrestrial walking without special locomotor adaptations. It should be roughly 20 cm in the vertical dimension of the trunk, which is just a little above the known size of Devonian tetrapodomorph fishes Panderichthys and Elpistostege. The metalocomotor walking technique of Channa is suggested as the closest extant model for terrestrial locomotion at the fish‐tetrapod transition. The major difference is that the metalocomotor propulsor in Channa is represented by the anal fin, while in tetrapodomorphs by the pelvic fins. The sprawled pelvic fins were advantageous in respect of reduced requirement for side‐to‐side tail swinging.

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.