Numerous vertebrates exhibit cranial kinesis, or movement between bones of the skull and mandible other than at the jaw joint. Many kinetic species possess a particular suite of features to accomplish this movement, including flexible cranial joints and protractor musculature. Whereas the musculoskeletal anatomy of these kinetic systems is well understood, how these joints are biomechanically loaded, how different soft tissues affect joint loading and kinetic capacity, and how the protractor musculature loads the skull remain poorly understood. Here, we present a finite element model of the savannah monitor, Varanus exanthematicus, a modestly kinetic lizard, to better elucidate the roles of soft tissue in mobile joints and protractor musculature in cranial loading. We describe the 3D resultants of jaw muscles and the histology of palatobasal, otic and jaw joints. We tested the effects of joint tissue type, bite point and muscle load to evaluate the biomechanical role of muscles on the palate and braincase. We found that the jaw muscles have significant mediolateral components that can impart stability across palatocranial joints. Articular tissues affect the magnitude of strains experienced around the palatobasal and otic joints. Without protractor muscle loading, the palate, quadrate and braincase experience higher strains, suggesting this muscle helps insulate the braincase and palatoquadrate from high loads. We found that the cross-sectional properties of the bones of V. exanthematicus are well suited for performing under torsional loads. These findings suggest that torsional loading regimes may have played a more important role in the evolution of cranial kinesis in lepidosaurs than previously appreciated.
Animals served important roles in the religious cults that proliferated during the Late (ca. 747–332 BCE) and Greco-Roman Periods (332 BCE–CE 337) of ancient Egypt. One result was the interment of animal mummies in specialized necropolises distributed throughout the country. Excavation of a rock-tomb that was re-used during the Ptolemaic Period (ca. 309–30 BCE) for the interment of animal mummies at the Djehuty Site (TT 11–12) near Luxor, Egypt, was carried out in early 2018 by a Spanish–Egyptian team sponsored by the Consejo Superior de Investigaciones Científicas, Madrid. The tomb burned sometime after deposition of the mummies, leaving behind abundant disassociated skeletal remains, primarily of avians, but also including two species of shrews (Soricidae): Crocidura olivieri and C. religiosa. To investigate possible intraspecific variation in morphology and locomotor function in these two species during the last two millennia, we measured morphological features of individual postcranial bones from the two archaeological samples and calculated indices that have been used to assess locomotor function. We compared the measurements to those from modern C. olivieri, C. religiosa, and C. suaveolens using principal components analysis, and we compared locomotor indices to those we calculated for the three modern species of Crocidura and to those from nine species of myosoricine shrews. Osteological features of the postcranial skeleton of conspecific Ptolemaic and modern samples of C. olivieri and C. religiosa are generally similar in character and proportion, and, skeletally, these shrews and modern C. suaveolens are consistent with soricids having a primarily ambulatory locomotor mode. One exception is the deltopectoral crest of the humerus, which appears to be longer in modern C. religiosa. Despite general conservation of form and function, Ptolemaic C. olivieri had larger body size than modern Egyptian populations and were more similar in size to modern C. olivieri nyansae from Kenya than to modern C. olivieri olivieri from Egypt.
The clade comprising the soricid tribes Blarinellini (Blarinella) and Blarinini (Blarina and Cryptotis) is notable within the Soricidae (Eulipotyphla) for the large proportion of reportedly semifossorial species. To better define locomotor modes among species in these two tribes, we quantified purported locomotor adaptations by calculating 23 functional indices from postcranial measurements obtained from museum specimens of Blarina and Blarinella and published measurements for 16 species of Cryptotis. We then analyzed relative ambulatory–fossorial function of each species using principal component analyses and mean percentile rank (MPR) analysis of the indices. Species within the Blarinellini–Blarinini clade exhibit a graded series of morphologies with four primary functional groupings that we classified as “ambulatory,” “intermediate,” “semifossorial,” and “fossorial.” To obtain a preliminary overview of evolution of locomotor modes in this group, we mapped MPRs on a composite phylogeny and examined the resulting patterns. That analysis revealed that the most recent common ancestor of the Blarinellini–Blarinini clade most likely had an intermediate or semifossorial locomotor morphology. Individual subclades subsequently evolved either more ambulatory or more fossorial morphologies. Hence, evolution of locomotor traits within this clade is complex. Multiple shifts in locomotor mode likely occurred, and no single directional tendency is apparent either among the major modes or in levels of complexity.
The divergent specializations of living archosaurs, crocodilians and birds, represent two of the great transformations in vertebrate evolution. Despite hailing from common ancestor with a tall skull, braced palate and relatively unspecialized jaw muscles, these two living clades followed disparate paths in cranial biomechanics and feeding functional morphology during the Mesozoic era, resulting in the flat, rigid skulls in crocodylians and the bulbous and beaked, flexible skulls of birds. Here we use new approaches in imaging, 3D modeling, morphometrics, lever mechanics, finite element modeling and phylogenetic comparative methods of living and extinct reptiles to show how morphological and functional changes in the palate, cranial joints and jaw musculature occurred within the fossil record of these lineages that were key features in acquisition of the modern forms. We show how jaw muscles reoriented their resultants and moments about jaw joints during skull flattening, palatocranial suturing and feeding specializations in Jurassic crocodyliforms. These morphological changes resulted in a decrease in mechanical efficiency of the feeding apparatus that was compensated for by an increase in jaw muscle sizes and canalized muscle architecture. Meanwhile, along the line to living birds, the acquisition of enlarged brains in coelurosaur dinosaurs resulted in a shifting of temporal and palatal muscle positions, a canalization of vertically‐oriented of muscle forces and consequently reorientation of their moments about palatocranial joints. These changes in loading environment, along with a concatenate increase in the propulsive and bending properties of the palate, facilitated cranial kinesis in neoavian birds. Together, these examples of evolutionary innovation in reptile cranial biomechanics reveal powerful new approaches to testing biomechanical and evolutionary hypotheses in any cranial or appendicular musculoskeletal system.
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