Pushing the boundary? Testing the "functional elongation hypothesis" of the giraffe's neck

The elephant backbone: Morphological differences, dorsostability, and vertebral fusions

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.

Covariation in the Craniocervical Junction of Carnivora

The craniocervical junction is the transition between the skull and the vertebral column that provides mobility while maintaining sufficient stability (i.e., protection of the brainstem and the spinal cord). The key elements involved are the occiput, the first cervical vertebra (CV1, atlas) and the second cervical vertebra (CV2, axis). The two vertebrae forming the atlas-axis complex are distinct in their morphology and differences in form have been linked to differences in ecological function in mammals. Here, we quantified the morphological diversity of the cranium, CV1 and CV2 in a sample of Carnivora using 3D geometric morphometrics to reveal phylogenetic and ecological patterns. Our results indicate that the observed variation in CV2 is related to the taxonomic diversity (i.e., strong phylogenetic signal), whereas variation in CV1 appears to be decoupled from species diversity in Carnivora and, thus, is likely to reflect a functional signal. The phylogenetically informed correlation analyses showed an association between the CV1 morphology and diet. Taxa that primarily feed on large prey tend to have larger transverse processes on CV1 which provides larger muscle attachment areas and may correlate with stronger muscles. The latter needs to be verified by future quantitative covariation analyses between bone and muscle data. Morphological peculiarities within Pinnipedia and Mustelidae could be explained by differences in terrestrial locomotion between Phocidae and Otariidae and the exceptional defensive behavior (i.e., handstanding) in Mephitidae. Despite differences in the degree of morphological diversity, covariation between cranium, CV1 and CV2 morphology is consistently high (≥ 0.82) highlighting that overall, the craniocervical junction is an integrated structure, but there are traits that are not constrained.

The functional significance of aberrant cervical counts in sloths: insights from automated exhaustive analysis of cervical range of motion

Besides manatees, the suspensory extant 'tree sloths' are the only mammals that deviate from a cervical count (CC) of seven vertebrae. They do so in opposite directions in the two living genera (increased versus decreased CC). Aberrant CCs seemingly reflect neck mobility in both genera, suggesting adaptive significance for their head position during suspensory locomotion and especially increased ability for neck torsion in three-toed sloths. We test two hypotheses in a comparative evolutionary framework by assessing three-dimensional intervertebral range of motion (ROM) based on exhaustive automated detection of bone collisions and joint disarticulation while accounting for interacting rotations of roll, yaw and pitch. First, we hypothesize that the increase of CC also increases overall neck mobility compared with mammals with a regular CC, and vice versa. Second, we hypothesize that the anatomy of the intervertebral articulations determines mobility of the neck. The assessment revealed that CC plays only a secondary role in defining ROM since summed torsion (roll) capacity was primarily determined by vertebral anatomy. Our results thus suggest limited neck rotational adaptive significance of the CC aberration in sloths. Further, the study demonstrates the suitability of our automated approach for the comparative assessment of osteological ROM in vertebral series.

Preliminary ontogeny of the giraffe neck

The giraffe juvenile has different proportions of head to neck from the adult. The head just about doubles in size from the juvenile to adult, whereas the neck increases almost 4.5× (roughly four times) in length. The T1 posterior dorsal vertebral width of the newborn is clearly wider than in the adult where it is narrow. In the okapi, the dorsal vertebral width is narrow in both juvenile and adult. The giraffe neck changes in ontogeny anisometrically. In the okapi the changes are more isometric. The giraffe juvenile vertebrae are shorter and do not have fused the cranial epiphyseal plates. That facilitates anterior elongation-growth. The ventral tubercles are undeveloped. The juvenile T1 is wide caudally unlike the adult. This may be a similarity to a gelocid (Gelocidae) ancestor of the giraffe.

Spinosaurus is not an aquatic dinosaur

A predominantly fish-eating diet was envisioned for the sail-backed theropod dinosaur Spinosaurus aegyptiacus when its elongate jaws with subconical teeth were unearthed a century ago in Egypt. Recent discovery of the high-spined tail of that skeleton, however, led to a bolder conjecture that S. aegyptiacus was the first fully aquatic dinosaur. The 'aquatic hypothesis' posits that S. aegyptiacus was a slow quadruped on land but a capable pursuit predator in coastal waters, powered by an expanded tail. We test these functional claims with skeletal and flesh models of S. aegyptiacus. We assembled a CT-based skeletal reconstruction based on the fossils, to which we added internal air and muscle to create a posable flesh model. That model shows that on land S. aegyptiacus was bipedal and in deep water was an unstable, slow-surface swimmer (<1 m/s) too buoyant to dive. Living reptiles with similar spine-supported sails over trunk and tail are used for display rather than aquatic propulsion, and nearly all extant secondary swimmers have reduced limbs and fleshy tail flukes. New fossils also show that Spinosaurus ranged far inland. Two stages are clarified in the evolution of Spinosaurus, which is best understood as a semiaquatic bipedal ambush piscivore that frequented the margins of coastal and inland waterways.

Topology-Based Three-Dimensional Reconstruction of Delicate Skeletal Fossil Remains and the Quantification of Their Taphonomic Deformation

Taphonomic and diagenetic processes inevitably distort the original skeletal morphology of fossil vertebrate remains. Key aspects of palaeobiological datasets may be directly impacted by such morphological deformation, such as taxonomic diagnoses and phylogenetic hypotheses, interpretations of the shape and orientation of anatomical structures, and assessments of interspecific and intraspecific variation. In order to overcome these ubiquitous challenges we present a novel reconstruction workflow combining retopology and retrodeformation, allowing the original morphology of both symmetrically and asymmetrically damaged areas of fossils to be reconstructed. As case studies, we present idealised three-dimensional reconstructions of the sternum of the crownward stem-bird Ichthyornis dispar, and cervical vertebrae of the diplodocid sauropod Galeamopus pabsti. Multiple Ichthyornis sterna were combined into a single, idealised composite representation through superimposition and alignment of retopologised models, and this composite was subsequently retrodeformed. The Galeamopus vertebrae were individually retrodeformed and symmetrised. Our workflow enabled us to quantify deformation of individual specimens with respect to our reconstructions, and to characterise global and local taphonomic deformation. Our workflow can be integrated with geometric morphometric approaches to enable quantitative morphological comparisons among multiple specimens, as well as quantitative interpolation of “mediotypes” of serially homologous elements such as missing vertebrae, haemal arches, or ribs.

Covariation between the cranium and the cervical vertebrae in hominids

The analysis of patterns of integration is crucial for the reconstruction and understanding of how morphological changes occur in a taxonomic group throughout evolution. These patterns are relatively constant; however, both patterns and the magnitudes of integration may vary across species. These differences may indicate morphological diversification, in some cases related to functional adaptations to the biomechanics of organisms. In this study, we analyze patterns of integration between two functional and developmental structures, the cranium and the cervical spine in hominids, and we quantify the amount of divergence of each anatomical element through phylogeny. We applied these methods to three-dimensional data from 168 adult hominid individuals, summing a total of more than 1000 cervical vertebrae. We found the atlas (C1) and axis (C2) display the lowest covariation with the cranium in hominids (Homo sapiens, Pan troglodytes, Pan paniscus, Gorilla gorilla, Gorilla beringei, Pongo pygmaeus). H. sapiens show a relatively different pattern of craniocervical correlation compared with chimpanzees and gorillas, especially in variables implicated in maintaining the balance of the head. Finally, the atlas and axis show lower magnitude of shape change during evolution than the rest of the cervical vertebrae, especially those located in the middle of the subaxial cervical spine. Overall, results suggest that differences in the pattern of craniocervical correlation between humans and gorillas and chimpanzees could reflect the postural differences between these groups. Also, the stronger craniocervical integration and larger magnitude of shape change during evolution shown by the middle cervical vertebrae suggests that they have been selected to play an active role in maintaining head balance.

How the even-toed ungulate vertebral column works: Comparison of intervertebral mobility in 33 genera

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.