Character evolution in the orbital region of the Afrotheria

The orbit or eye socket is a highly plastic area of the mammalian skull. There is significant variation within and between the different mammalian orders in the size and position of the bones and foramina that contribute to this region. For this reason, orbital characters are often used in attempts to determine the relationships of the various mammalian groups. This study describes the orbital region of the Afrotheria, the proposed group of endemic African mammals that comprises the paenungulates (elephants, manatees and dugongs, and hyraxes), elephant-shrews, aardvarks, golden moles and tenrecs. Evolution within the Afrotherian orbit is then explored by scoring 19 orbital characters in each Afrotherian genus, and plotting the character state changes on to previously existing phylogenies of the Afrotheria. These phylogenies were all produced from recent molecular work. It was found that there is a great deal of variation in the orbital region within the Afrotheria, most notably in the size of the lacrimal and its contacts with other bones, the appearance of the palatine in the orbit and the structure of the zygomatic arch. Overall, orbital characters strongly supported an elephant-hyrax clade over the more traditional grouping of elephants and sirenians (Tethytheria) within the paenungulates. There was also support for a monophyletic Tenrecoidea (a clade of tenrecs plus golden moles). Additionally, it was shown that there is a great deal of variation in the orbital region among the genera of the Tenrecidae and the Macroscelididae.     

Upper jaw in human cyclopia

In human cyclopia the upper jaw forms a solid bony mass between the median orbit and the oral cavity. The skeletal elements forming the upper jaw have been studied in serial sections through the median third of the head in 3 perinatal human specimens presenting with a median orbit and proboscis. One head was sectioned in the sagittal plane and 2 in the coronal plane. The upper jaw has also been studied in a dried cyclops skull and in a desiccated cyclops head in which the roof of the orbit had been removed. The data obtained demonstrate the particular contributions made by the lacrimal bones, the maxillae and the palatine bones to the upper jaw in human cyclopia. The effects of the absence of the frontonasal process contribution and of the absence of the nasal cavity on the upper jaw in cyclopia are considered.     

Function of the mammalian postorbital bar

Complete postorbital bars, bony arches that encompass the lateral aspect of the eye and form part of a circular orbit, have evolved homoplastically multiple times during mammalian evolution. Numerous functional hypotheses have been advanced for postorbital bars, the most promising being that postorbital bars function to stiffen the lateral orbit in taxa that have significant angular deviation between the temporal fossa and the bony orbit. Without a stiff lateral orbit the anterior temporalis muscle and fascia potentially would pull on the postorbital ligament, deform the orbit, and cause disruption of oculomotor precision. Morphometric data were collected on 1,329 specimens of 324 taxa from 16 orders of extant eutherian and metatherian mammals in order to test whether the orientation of the orbit relative to the temporal fossa is correlated with the replacement of the postorbital ligament with bone. The allometric and ecological influences on orbit orientation across mammals are also explored. The morphometric results corroborate the hypothesis: Shifts in orbit orientation relative to the temporal fossa are correlated with the size of the postorbital processes, which replace the ligament. The allometric and ecological factors that influence orbit orientation vary across taxa. Postorbital bars stiffen the lateral orbital wall. Muscle pulleys, ligaments, and other connective tissue attach to the lateral orbital wall, including the postorbital bar. Without a stiff lateral orbit, deformation due to temporalis contraction would displace soft tissues contributing to normal oculomotor function.     

Influence of Orbit Size on Aspects of the Tarsier Postorbital Septum

Explanations invoking the complex mechanical effects of orbital enlargement in Tarsius have been extended to several areas of the skull, including the conformation of the postorbital septum. The strong cline in the degree of relative orbital enlargement across tarsier species groups presents an unexploited opportunity to test such scenarios. Our goal is to evaluate hypotheses concerning the impact of orbital hypertrophy on the size of specific components of the postorbital region including the frontal, zygomatic, alisphenoid, and maxillary bones. The frontal process is almost always viewed as a functional projection whose bracing role requires a positive morphometric association with orbital hypertrophy. Conversely, the periorbital expansion of the zygomatic is often perceived as functionally unrelated to orbital enlargement and therefore is not expected to track increases in relative orbit size. Interpretations of the alisphenoid and maxillary periorbital processes range from vestigial remnants of once larger structures reduced because of ocular enlargement to structures large in tarsiers because of their functionally relevant role in supporting the enlarged ocular apparatus. We measured these attributes in an extensive sample of 4 tarsier species groups including Tarsius bancanus, T. syrichta, T. spectrum, and T. pumilus. In contrast to proposed functional interpretations, our results indicate that variation in most linear parameters might be better explained by differences in body size than intrageneric differences in orbit size. As expected, width of the zygomatic postorbital contribution does not parallel intrageneric variation in orbit size. However, morphometric relationships between relative orbit size and other parts of the septum are complex but not clearly associated with orbit size differences within Tarsius.     

Protective buttressing of the hominin face

When humans fight hand‐to‐hand the face is usually the primary target and the bones that suffer the highest rates of fracture are the parts of the skull that exhibit the greatest increase in robusticity during the evolution of basal hominins. These bones are also the most sexually dimorphic parts of the skull in both australopiths and humans. In this review, we suggest that many of the facial features that characterize early hominins evolved to protect the face from injury during fighting with fists. Specifically, the trend towards a more orthognathic face; the bunodont form and expansion of the postcanine teeth; the increased robusticity of the orbit; the increased robusticity of the masticatory system, including the mandibular corpus and condyle, zygoma, and anterior pillars of the maxilla; and the enlarged jaw adductor musculature are traits that may represent protective buttressing of the face. If the protective buttressing hypothesis is correct, the primary differences in the face of robust versus gracile australopiths may be more a function of differences in mating system than differences in diet as is generally assumed. In this scenario, the evolution of reduced facial robusticity in Homo is associated with the evolution of reduced strength of the upper body and, therefore, with reduced striking power. The protective buttressing hypothesis provides a functional explanation for the puzzling observation that although humans do not fight by biting our species exhibits pronounced sexual dimorphism in the strength and power of the jaw and neck musculature. The protective buttressing hypothesis is also consistent with observations that modern humans can accurately assess a male's strength and fighting ability from facial shape and voice quality.     

The erectile cheek-spine apparatus in the bristlenose catfish Ancistrus (Loricariidae, Siluriformes), and its relation to the formation of a secondary skull roof

In the South American catfish family Loricariidae, the opercle has been decoupled from the lower jaw, and has also lost its function in expiration. While many loricariid species have a small and slightly mobile opercle with reduced opercular musculature, within the hypostomine subfamily a novel opercular mechanism has developed that erects a tuft of enlarged odontodes anterior to the opercle. This defensive mechanism is examined in Ancistrus cf. triradiatus. The opercle has a prominent anterior process and the orientation of the reinforced articulation hinge to the hyomandibular bone has shifted. The opercular musculature is well developed, with a hypertrophied dilatator operculi that extends deep inside the skull roof bones and toward the midline, over the brain, but below the superficial skull roof. Hence the frontal, sphenotic, parieto-supraoccipital and compound pterotic bones consist of a dorsal, superficial part and a deeper part separating the brain from the muscle: two functional skull roofs are thus formed. The impact on the path of the cranial sensory canals is substantial, moving canals away from the skull surface. Hypertrophy of cranial muscles is known from many teleosts, but the invasion of such large muscles into the skull, which is drastically modified and literally hollowed out, has never been described before. These cranial modifications are greater in males than in females, related to the territorial behavior of the former, in which the erectile spines are usually used.     

Masticatory stress, orbital orientation and the evolution of the primate postorbital bar

A postorbital bar is one of a suite of derived features which distinguishes basal primates from their putative sister taxon, plesiadapiforms. Two hypotheses have been put forward to explain postorbital bar development and variation in circumorbital form: the facial torsion model and visual predation hypothesis. To test the facial torsion model, we employ strain data on circumorbital and mandibular loading patterns in representative primates with a postorbital bar and masticatory apparatus similar to basal primates. To examine the visual predation hypothesis, we employ metric data on orbit orientation in Paleocene and Eocene primates, as well as several clades of visual predators and foragers that vary interspecifically in postorbital bar formation.A comparison of galago circumorbital and mandibular peak strains during powerful mastication demonstrates that circumorbital strains are quite low. This indicates that, as in anthropoids, the strepsirhine circumorbital region is excessively overbuilt for countering routine masticatory loads. The fact that circumorbital peak-strain levels are uniformly low in both primate suborders undermines any model which posits that masticatory stresses are determinants of circumorbital form, function and evolution. This is interpreted to mean that sufficient cortical bone must exist to prevent structural failure due to non-masticatory traumatic forces. Preliminary data also indicate that the difference between circumorbital and mandibular strains is greater in larger taxa.Comparative analyses of several extant analogs suggest that the postorbital bar apparently provides rigidity to the lateral orbital margins to ensure a high level of visual acuity during chewing and biting. The origin of the primate postorbital bar is linked to changes in orbital convergence and frontation at smaller sizes due to nocturnal visual predation and increased encephalization. By incorporating in vivo and fossil data, we reformulate the visual predation hypothesis of primate origins and thus offer new insights into major adaptive transformations in the primate skull.     

Morphological evolution of the mammalian jaw adductor complex

The evolution of the mammalian jaw during the transition from non‐mammalian synapsids to crown mammals is a key event in vertebrate history and characterised by the gradual reduction of its individual bones into a single element and the concomitant transformation of the jaw joint and its incorporation into the middle ear complex. This osteological transformation is accompanied by a rearrangement and modification of the jaw adductor musculature, which is thought to have allowed the evolution of a more‐efficient masticatory system in comparison to the plesiomorphic synapsid condition. While osteological characters relating to this transition are well documented in the fossil record, the exact arrangement and modifications of the individual adductor muscles during the cynodont–mammaliaform transition have been debated for nearly a century. We review the existing knowledge about the musculoskeletal evolution of the mammalian jaw adductor complex and evaluate previous hypotheses in the light of recently documented fossils that represent new specimens of existing species, which are of central importance to the mammalian origins debate. By employing computed tomography (CT) and digital reconstruction techniques to create three‐dimensional models of the jaw adductor musculature in a number of representative non‐mammalian cynodonts and mammaliaforms, we provide an updated perspective on mammalian jaw muscle evolution. As an emerging consensus, current evidence suggests that the mammal‐like division of the jaw adductor musculature (into deep and superficial components of the m. masseter, the m. temporalis and the m. pterygoideus) was completed in Eucynodontia. The arrangement of the jaw adductor musculature in a mammalian fashion, with the m. pterygoideus group inserting on the dentary was completed in basal Mammaliaformes as suggested by the muscle reconstruction of Morganucodon oehleri. Consequently, transformation of the jaw adductor musculature from the ancestral (‘reptilian’) to the mammalian condition must have preceded the emergence of Mammalia and the full formation of the mammalian jaw joint. This suggests that the modification of the jaw adductor system played a pivotal role in the functional morphology and biomechanical stability of the jaw joint.     

The cranial arteries of turtles and their evolutionary significance

In this paper the cranial arteries, cranial arterial foramina, and bony canals of the Cheloniidae, Chelydridae, Pelomedusidae, and Chelidae are described in detail. From skull studies and published material, the general cranial arterial patterns of all the turtle families can be inferred. Sea turtles, the Cheloniidae and Dermochelyidae, possess both a large stapedial artery and a large artery supplying the orbit, which is possibly similar to the primitive cranial arterial pattern for turtles. From a primitive pattern in which stapedial and palatine arteries supply the orbit, the Chelydridae and Testudinidae retained a large stapedial artery and reduced the palatine artery, while the Kinosternidae and Dermatemydidae developed a large palatine artery and reduced the stapedial artery. The Trionychidae and probably the Carettochelyidae evolved a complex arterial pattern in which the stapedial artery was reduced somewhat and the pseudopalatine artery was substituted for the palatine artery. Pleurodires in general retained a large stapedial artery and reduced or eliminated the palatine artery. The Podocneminae, including the Madagascar species, developed a highly modified carotid canal, which is found in no other turtle group. The facts which have been presented should aid in fossil skull studies and in understanding the evolutionary background of recent turtles.     

Focal palatine erosion associated with dental malocclusion in captive cheetahs

A severe palatine disorder caused by maloccluded molars was discovered in captive adult cheetahs at the San Diego Wild Animal Park. This defect has been labeled focal palatine erosion (FPE). Subsequently, a total of 59 cheetahs from several institutions have been examined to evaluate the occurrence and etiology of this disorder. Maloccluded lower molars contacting the palatal mucosa appear to be the primary source of irritation. Infection develops when decaying food and grass particles become lodged in the resulting defect. Eventually, complete palatine perforation results which can extend into the nasal passages and eventually lead to systemic disorders. Of 59 cheetahs studied, 15 displayed various stages of FPE. Examination of 26 museum skull specimens revealed four cases of palatine perforation. These four were from a group of eight animals that were zoo raised. Focal palatine erosion appears to be a result of dietary factors. All cheetahs with FPE received a commercially prepared soft diet while in captivity. The lack of normal biting, tearing, and pulling action associated with natural prey capture and consumption could result in malocclusion caused by atrophy from disuse of the masticatory apparatus. Improper occlusion could also stem from insufficient wear or unsynchronized development of the opposing dental arches. All but two cases of FPE were found in cheetahs imported from a limited area in southwest Africa in 1970, or their offspring. Since molar size and jaw structure can be inherited, there may be genetic factors involved but more data are needed to support this idea. Treatment includes changing diet, reducing the height of the offending molar, controlling infection, eliminating irritants, and in some cases, surgical reconstruction.     

Osteological correlates and phylogenetic analysis of deep diving in living and extinct pinnipeds: What good are big eyes?

Marine tetrapods have evolved different sensory solutions to meet the ecological challenges of foraging at depth. It has been proposed that pinipeds, like ichthyosaurs, evolved large eyeballs for such demands. Here, we test this hypothesis using morphological and diving data from a comprehensive data set (n= 54 species; 435 individual specimens), including living and extinct pinnipeds and other select carnivorans as outgroup taxa. We used bony orbit size as a proxy for eyeball size, and recorded associated skull measurements to control for relative changes in orbit size; for diving depth, we used the deepest dive depth reported in the literature. Our analyses included both standard regressions and those corrected for phylogeny (i.e., independent contrasts). Standard regression statistics showed orbit size was a significantly good predictor of diving depth for phocids and for pinnipeds overall, although there was no correlation for otariids. In contrast, independent contrasts showed little support for a relationship between orbit size and diving depth for any group broader than family level, although this approach did demonstrate deep diving has evolved multiple times in crown Pinnipedia. Lastly, using select fossil taxa, we highlight the need to test adaptive hypotheses using comparative data in an evolutionary context.     

Limitations of a morphological criterion of adaptive inference in the fossil record

Experimental analyses directly inform how an anatomical feature or complex functions during an organism's lifetime, which serves to increase the efficacy of comparative studies of living and fossil taxa. In the mammalian skull, food material properties and feeding behaviour have a pronounced influence on the development of the masticatory apparatus. Diet‐related variation in loading magnitude and frequency induce a cascade of changes at the gross, tissue, cellular, protein and genetic levels, with such modelling and remodelling maintaining the integrity of oral structures vis‐à‐vis routine masticatory stresses. Ongoing integrative research using rabbit and rat models of long‐term masticatory plasticity offers unique insight into the limitations of functional interpretations of fossilised remains. Given the general restriction of the palaeontological record to bony elements, we argue that failure to account for the disparity in the hierarchical network of responses of hard versus soft tissues may overestimate the magnitude of the adaptive divergence that is inferred from phenotypic differences. Second, we note that the developmental onset and duration of a loading stimulus associated with a given feeding behaviour can impart large effects on patterns of intraspecific variation that can mirror differences observed among taxa. Indeed, plasticity data are relevant to understanding evolutionary transformations because rabbits raised on different diets exhibit levels of morphological disparity comparable to those found between closely related primate species that vary in diet. Lastly, pronounced variation in joint form, and even joint function, can also characterise adult conspecifics that differ solely in age. In sum, our analyses emphasise the importance of a multi‐site and hierarchical approach to understanding determinants of morphological variation, one which incorporates critical data on performance.     

Cranial ontogeny, diet, and ecogeographic variation in African lorises

A series of 22 craniodental measurements were obtained for the three subspecies of potto (Perodicticus) and angwantibos (Arctocebus). To describe patterns of variation in Perodicticus, a discriminant function analysis (DFA) was performed with adult data. To investigate the ecogeographic correlates of size variation in Perodicticus, adult cranial dimensions were compared with field data on latitudinal and longitudinal coordinates for available specimens as well as altitudinal data for a more limited sample. Ontogenetic series for larger-bodied Perodicticus and smaller-bodied Arctocebus were compared to test the hypothesis that inter- and intrageneric variation in skull form results from the differential extension/truncation of shared patterns of relative growth, and to assess morphological variation in the masticatory complex of sister taxa with differing dietary habits. Analyses of relative growth indicate that skull proportions in Perodicticus subspecies are largely ontogenetically scaled. In comparisons between Perodicticus and Arctocebus, most facial dimensions also are ontogenetically scaled, with all but one of the seven divergent comparisons (interorbital breadth) representing a feature of the masticatory apparatus. The DFA provided independent support for prior classifications of Perodicticus into three taxa. Size differentiation in African lorises appears to be correlated with altitudinal variation (Bergmann's Rule) as well as character displacement. The smallest pottos, P. p. potto, occupy low-lying coastal habitats in western Africa, whereas the larger, eastern forms inhabit higher, presumably colder elevations. The largest potto, P. p. edwardsi, is sympatric throughout most of its range with the smallest and most insectivorous African lorises (Arctocebus). A basis for intrageneric taxonomic variation in Perodicticus is supported by such nonclinal size variation, as well as divergences in the ontogeny of masticatory proportions corresponding to interspecific variation in dietary proclivities.     

Cranial anatomy and phylogenetic position of Suminia getmanovi, a basal anomodont (Amniota: Therapsida) from the Late Permian of Eastern Europe

Suminia getmanovi , a recently discovered basal anomodont from the Late Permian of Russia, is characterized by robust, 'leaf-shaped' teeth, and a masticatory architecture that is similar to that of the highly diverse and cosmopolitan group of Permo-Triassic herbivores, Dicynodontia (Anomodontia). Based on new material, the skull is reconstructed in three dimensions and described in detail. A cladistic analysis of the basal anomodonts, Patranomodon, Galeops, Otsheria, Ulemica , and Suminia , using 37 cranial characters, resulted in a single most parsimonious tree, in which Suminia is united with the Russian taxa, Ulemica and Otsheria. This clade, diagnosed by four unambiguous characters, is designated as Venyukovioidea. The South African anomodont, Galeops , appears as the sister taxon to Dicynodontia. Patranomodon is the most basal anomodont. The cladistic analysis suggests that a 'dicynodont-type' masticatory architecture, with an expanded adductor musculature and sliding jaw articulation, may have originated prior to the advent of the (Venyukovioidea + ( Galeops + Dicynodontia)) clade.     

Characters of multituberculates neglected in phylogenetic analyses of early mammals

Multituberculate anatomy is compared with that of other mammals, with an emphasis on the characters that have either been neglected or misinterpreted in previous analyses of early mam mal relationships. These are: brain structure, backward masticatory power stroke (along with aspects of cranial design), and foot structure. New data on ear ossicles and a controversy con cerning multituberculate posture are also discussed. The following characters of multitubercu late skull and lower jaw are interpreted to be related to the backward masticatory power stroke: anterior orbital area roofed dorsally and without a floor (characteristic of advanced multituber culates), parietal postorbital process, lack of the angular process and a more anterior position of the coronoid process and masseteric fossa than in all other mammals. It is argued that the parallel development in the cranial structure of multituberculates and other mammals was lim ited by the backward masticatory power stroke of multituberculates that resulted in different configuration of the masticatory musculature and related osteology. In the postcranial skeleton the parallelism was limited by the structure of the multituberculate foot, in which the calca-neum contacts the fifth metatarsal (MtV) and the middle metatarsal (MtIII) is abducted 30° from the longitudinal axis of the tuber calcanei. Backward masticatory power stroke and related skull design do not show unequivocally whether multituberculates originated from some ‘tri-conodonts’ (a polyphyletic group), or independently from all other mammals from cynodonts. The foot structure refutes the origin of multituberculates from the Morganucodontidae. The brain structure allies the multituberculates with the Triconodontidae, the postcranial skeleton of which remains unknown. New data on ear ossicles suggest close relationships of multituber culates to all modern mammals. Lack of uncontested pre-Kimmeridgian multituberculates dis proves the separate origin of multituberculates from cynodonts.     

The effect of age on the function of the masticatory system--an electromyographical analysis

Background: Old age is the last stage of human life and, unfortunately, the ageing rhythm of the oral cavity and masticatory system seems to be rather accelerated. Hence, there is a reduction in the amount of food ingested, establishing the imbalance of nutritional phenomena. Objective: To assure a better quality of life for the elderly, it is necessary to understand their masticatory system. Materials and methods: An electromyographical analysis of the masticatory system in 10 individuals aged between 60–75 years (group 1) and 10 between 23–30 years old (group 2) was carried out. The analysis was performed using a MyoSystem‐Br1 electromyographer with differential active electrodes. The test was registered during positioning and functional conditions, such as chewing, and the muscles assessed were the temporalis and masseter. Data were normalised by maximum voluntary contraction (MVC), and the results were analysed statistically using independent t‐test during the comparison between groups. Results: Statistical significance (p ≤ 0.05) was found in the analysis of various activities, in which the elderly showed greater electromyographical activity values relative to their MVC during the different mandibular positions, such as left laterality. While the elderly subjects revealed values of 30% relative to their MVC, the young revealed 10%. However, during masticatory activities, the elderly revealed values of up to 79%, and the young of 108%, relative to their MVC, such as when they are chewing peanuts. Conclusion: It could be concluded that the elderly show hyperactivity of masticatory musculature during posture maintenance and a slight hypoactivity of this musculature during chewing when analysed side by side with young individuals.     

The craniomandibular mechanics of being human

Diminished bite force has been considered a defining feature of modern Homo sapiens, an interpretation inferred from the application of two-dimensional lever mechanics and the relative gracility of the human masticatory musculature and skull. This conclusion has various implications with regard to the evolution of human feeding behaviour. However, human dental anatomy suggests a capacity to withstand high loads and two-dimensional lever models greatly simplify muscle architecture, yielding less accurate results than three-dimensional modelling using multiple lines of action. Here, to our knowledge, in the most comprehensive three-dimensional finite element analysis performed to date for any taxon, we ask whether the traditional view that the bite of H. sapiens is weak and the skull too gracile to sustain high bite forces is supported. We further introduce a new method for reconstructing incomplete fossil material. Our findings show that the human masticatory apparatus is highly efficient, capable of producing a relatively powerful bite using low muscle forces. Thus, relative to other members of the superfamily Hominoidea, humans can achieve relatively high bite forces, while overall stresses are reduced. Our findings resolve apparently discordant lines of evidence, i.e. the presence of teeth well adapted to sustain high loads within a lightweight cranium and mandible.     

The role of tissue expanders in an anophthalmic animal model

A study of orbital bony expansion using a custom tissue expander was performed in the anophthalmic cat model. Twelve 6-week-old kittens underwent right unilateral enucleations. Six kittens had immediate insertion of a tissue expander into the orbit. The remaining six served as controls. Every 2 weeks 0.5 cc saline was injected into the expander to a maximum of 5 cc. External horizontal and vertical orbital dimensions were obtained by palpation technique weekly. All animals had preoperative and study conclusion head CT scans with three-dimensional reconstructions performed. Dry skull preparations were done at the study conclusion at 24 weeks. Results demonstrated that tissue expanders were successful in maintaining normal orbital growth and size relative to the contralateral control orbit. The animals with enucleation only had an average difference in vertical and horizontal orbital measurements of -27 and -13 percent when compared with the contralateral normal orbit. In contrast, the enucleation and tissue-expansion animals had vertical and horizontal measurements of +4 and +2 percent (p less than 0.05) when compared with the contralateral orbit. Head CT scans with three-dimensional reconstructions demonstrated normal orbital geometry and volume for the animals with tissue expanders, whereas animals with enucleation only had small hypoplastic orbits. In conclusion, orbital tissue expanders offer a promising new technique in the treatment of anophthalmos.