From dorsomobility to dorsostability: A study of lumbosacral joint range of motion in artiodactyls

Function of revolute zygapophyses in the lumbar vertebrae of early placental mammals

The unique morphology of mammalian lumbar vertebrae allows the spine to flex and extend in the sagittal plane during locomotion. This movement increases stride length and allows mammals to efficiently breathe while running with an asymmetric gait. In extant mammals, the amount of flexion that occurs varies across different locomotor styles, with dorsostable runners relying more on movement of long limbs to run and dorsomobile runners incorporating more flexion of the back. Although long limbs and a stabilized lumbar region are commonly associated with each other in extant mammals, many "archaic" placental mammals with short limbs had lumbar vertebrae with revolute zygapophyses. These articulations with an interlocking S-shape are found only in artiodactyls among extant mammals and have been hypothesized to stabilize against flexion of the back. This would suggest that archaic placental mammals may not have incorporated dorsoventral flexion into locomotion to the same extent as extant mammals with similar proportions. We tested the relative mobility of fossil lumbar vertebrae from two early placental mammals, the creodonts Patriofelis and Limnocyon, to see how these vertebrae may have functioned. We compared range of motion (ROM) between the original vertebrae, with revolute morphology and digitally altered vertebrae with a flat morphology. We found that the revolute morphology had relatively little effect on dorsoventral flexion and instead that it likely prevented disarticulation due to shear forces on the spine. These results show that flexion of the spine has been an important part of mammalian locomotion for at least 50 million years.

Are owls technically capable of making a full head turn?

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.

Running, jumping, hunting, and scavenging: Functional analysis of vertebral mobility and backbone properties in carnivorans

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.

Truly dorsostable runners: Vertebral mobility in rhinoceroses, tapirs, and horses

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.