Tesi sul tema "Vertebrates Morphology"

Gnathostomes, or jawed vertebrates, make up the overwhelming majority of modern vertebrate diversity. Among living vertebrates, they comprise the chondrichthyans (“cartilaginous fishes” such as sharks, skates, rays, chimaeras) and the osteichthyans (“bony fishes” or bony vertebrates, inclusive of tetrapods). Gnathostomes appear to have originated in the early Palaeozoic Era, but their early fossil record is fairly scant. The best fossils appear first in the Late Silurian and Devonian periods. Much of gnathostome diversity owes to unique adaptations in the internal skeleton of their head (the endocranium). The endocranium is composed of the braincase, jaws, hyoid arch, and branchial arches, which sometimes fossilise when they are composed of bone or calcified cartilage. The purpose of this thesis is to describe and compare the fossilised cranial endoskeletons of a variety of different Palaeozoic gnathostomes. The objective is to test current conceptions of gnathostome interrelationships (i.e. phylogeny) and infer aspects of key morphological transformations that took place during the evolution of Palaeozoic members of this group. Two key areas are examined: the morphology and interrelationships of Palaeozoic gnathostomes and the morphology of the visceral arches in sarcopterygian fishes. New data on the visceral arches are described from the stem tetrapods Panderichthys and rhizodontids. These provide insight into the sequence of character acquisition leading to the tetrapod middle ear. Panderichthys shows key features of the tetrapod middle ear chamber were established prior to the origin fo digited limbs. New morphological data are described from the “acanthodian” fish Ptomacanthus. Ptomacanthus provides only the second example of a well-preserved braincase from any member of this group. It shows dramatic differences from that of its counterpart, Acanthodes, providing new evidence for acanthodian paraphyly. New interpretations of basal gnathostome and osteichthyan phylogeny are presented, challenging or enriching existing views of these problems.

Thesis (MsCMedSc)--Stellenbosch University, 2012.<br>ENGLISH ABSTRACT: The gastrointestinal morphology and the distribution of the different types of mucin secreting goblet cells were investigated in three mammalian insectivorous species, namely A. spinosissimus, C. cyanea and A. hottentotus. The aim of the study was to provide a comprehensive morphological comparison between the different species. Another aim was to illustrate and compare the distribution of mucins (neutral, sulfo- and sialomucins) in the gastrointestinal tracts (GITs) of these species, in order to better understand the quality of the biofilm in the GIT. Mucins secreted onto the surface of the GIT have an effect on the colonisation of microflora in the mucosal layer, constructing a biofilm which protects the GIT surface from opportunistic pathogens. The shape, proportional length, and proportional surface areas of the different gastrointestinal regions were recorded and compared in the three species. Histochemical staining methods were used to detect and to distinguish between neutral, sulfo- and sialomucins. The number of goblet cells in the GIT containing each of the above mucins in the epithelium lining the surface or crypts was quantified, and the data expressed as the number of neutral, sulfo- or sialomucin containing goblet cells per mm2 of the surface or crypt epithelium. In all three species the stomach was uncompartmentalised. The internal aspect of the stomach in A. spinosissimus was hemi-glandular, containing stratified squamous epithelium in the fundus, with glandular epithelium in the body and pyloric region. However, C. cyanea and A. hottentotus had wholly glandular stomachs. A. spinosissimus was the only species studied that had a caecum which demonstrated transverse mucosal folds and V-shaped mucosal folds in the proximal colon. Both C. cyanea and A. hottentotus had villi up to the distal part of the GIT. Longitudinal mucosal folds were present in the distal colon. The GITs of both C. cyanea and A. hottentotus showed little morphological differentiation namely a simple, glandular stomach and the lack of a caecum. Mixed (neutral and acid) mucins and mixed acid (sulfo- and sialomucins) mucin secreting goblet cells were prominent mucin cell types in all three mammalian insectivorous species. Despite these general similarities, marked differences were observed in the qualitative expression and distribution of the three types of mucins throughout the GIT. The overall similarity between the three insectivores and other distantly related mammalian species suggests that mixed mucin secreting goblet cell types are prominent contributors to the maintenance of the intestinal biofilm in the majority of mammals, irrespective of their diet or taxonomy.<br>AFRIKAANSE OPSOMMING: Die bestudering van die morfologie van die spysverteringskanaal (SVK) en die verspreiding van die verskillende musien produserende bekerselle was in drie insek-etende soogdier spesies uitgevoer, naamlik in A. spinosissimus, C. cyanea en A. hottentotus. Die doel van die studie was om „n omvattende morfologiese vergelyking te maak tussen die drie spesies, sowel as om die verspreiding van die verskillende musiene te beskryf in die SVK. Kennis van die verspreiding van die verskillende tipes musiene (neutral, sulfaat en nie-sulfaat bevattende musiene) kan moontlik inligting verskaf aangaande die kwaliteit van die biofilm in the SVK. Die laasgenoemde musiene wat gesekreteer word op die oppervlak van die SVK, bepaal die kolonisasie van die mikroflora in die mukosale laag wat „n biofilm vorm en die SVK beskerm teen patogene. Die vorm, proportionele lengte en proportionele oppervlaks areas van die verskillende SVK gebiede is opgeteken, waarna dit vergelyk is tussen die drie insektivore spesies. Histochemiese kleurings tegnieke is gebruik om die musiene waar te neem en om te onderskei tussen die neutraal, sulfaat en nie-sulfaat bevattende musiene. Die aantal beker selle wat elk van die bogenoemde musiene bevat het, is getel in die oppervlaks epiteel- en kript areas van die SVK. Hierdie data is weergegee as die aantal neutraal, sulfaat en nie-sulfaat bevattende beker selle per oppervlaks epiteel- of kript area (mm2). Die vorm van die maag in al drie spesies was eenvoudig en nie gekompartementaliseer nie. Die interne aspek van die maag in A. spinosissimus het meerlagige plaveisel epiteel in die fundus gehad en klieragtige epiteel in die liggaam en pilorus gedeeltes. Daarbenewens het C. cyanea en A. hottentotus slegs klieragtige epiteel in die maag gehad. A. spinosissimus was die enigste spesie in hierdie studie wat „n sekum gehad het met dwars voue, asook V-vormige mukosale voue in die proksimale kolon. Beide C. cyanea en A. hottentotus het villi tot in die distale gedeelte van die SVK gehad. Longitudinale mukosale voue was teenwoordig in die distale gedeelte van die kolon. Die SVK van beide C. cyanea en A. hottentotus het min morfologiese differensiasie getoon deurdat die spesies „n eenvoudige, klieragtige maag gehad het en geen sekum nie. Gemenge (neutral en suur) musiene asook gemengde suur (sulfaat en nie-sulfaat bevattende) musiene was die dominante musien tipes in the SVK van al drie insek-etende soogdier spesies. Ten spyte van die algemene ooreenkomste, was daar merkwaardige verskille in die getalle en verspreiding van die verskillende musiene in die SVK. Die algemene ooreenkomste tussen die drie insektivore soogdier spesies met soogdiere van ander families, stel voor dat die gemende musien sekreterende beker selle „n prominente bydrae maak tot die onderhoud van die biofilm in the SVK in die meerderheid van soogdiere, ongeag van hul dieet of spesie klassifikasie.

The brain has been one of the central foci in studies of vertebrate evolution. Work in East African cichlids and other emerging fish models like the Mexican cavefish (Astyanax mexicanus) offer new insight on the role of patterning on brain evolution. These fish can be grouped into two major categories according to habitat; for cichlids it is rock-dwelling (known locally as mbuna) and sand-dwelling (non-mbuna) lineage. The brain development of mbuna versus non-mbuna is defined by changes in gene deployment working along the dorsal/ventral (DV) and anterior/posterior (AP) neuraxes, respectively. Comparison of disparate fish ecotypes offer a new perspective of the role of patterning on brain evolution; through the slight and early modification of signal pathways working across 3-D axes, and a subsequent magnifying effect across ontogeny, evolution can generate widespread changes in the brain. To illustrate this patterning model of brain evolution, two comparative studies were done between mbuna and non-mbuna, examining the action of gene pathways that work to pattern the cichlid forebrain. The first study found that non-mbuna cichlids have a more rapid AP expansion of a gene pathway (Wingless) into the presumptive midbrain and diencephalon versus mbuna. These forebrain structures are involved in sight processing and could be of ecological benefit to vision-focused non-mbuna. The second study described a difference within the developing telencephalon. The embryonic telencephalon is split into the pallium, which processes visual signals, and the subpallium, which develops into the olfactory bulbs. Mbuna possess a larger subpallium relative to non-mbuna, which have a larger pallium. This was correlated to a more rapid expansion of another gene pathway (Hedgehog) along the DV axis. The difference in size of the pallial vs. subpallialial comparments between cichlids can be correlated to expanded olfaction in mbuna and vision in non-mbuna adult brains. Overall, East African cichlids are an excellent system to investigate the role of patterning on brain evolution because they allow for the comparison of the earliest patterning events in brain ontogeny between distinct ecotypes. These fish systems link study in brain development to the brain morphology comparisons employed in classic studies of brain evolution.

Within Pleistocene cave deposits, snake fossils tend to be relatively common and generally occur as isolated vertebrae. However, the specific, and sometimes generic, identification of isolated snake vertebrae is often hindered by the significant amount of intra- and interspecific variation along the precloacal vertebral column, a subject that has largely been neglected in many fossil snake identifications. Identifications are typically based on the vertebrae of disarticulated modern specimens with preference given to mid-trunk vertebrae. This study utilizes 2-D geometric morphometrics to determine the extent of intervertebral variation along the precloacal vertebral column of North American pit vipers of the genera Crotalus (rattlesnakes) and Agkistrodon (copperheads and moccasins), two closely related genera geographically sympatric in northeast Tennessee and much of the eastern United States with similar vertebral morphologies. The focus of this study is to determine the need for identifying the morphological regionalization of the precloacal vertebral column and determining the regional position of isolated vertebra prior to identification. Using one individual from each genus, every third vertebra was chosen and analyzed in anterior view using geometric morphometrics and relative warp analyses. A discriminant function analysis was then performed to distinguish between the two genera. Viperid fossils from Hickory Tree Cave in northeast Tennessee underwent the same geometric morphometric and discriminant function analyses as a means of identification. It is expected that the relative warp analyses will show some morphological regionalization of the precloacal vertebral column, but may prove to not be significant enough for use in the identification of isolated vertebrae. In that case, comparison to mid-trunk vertebrae for identification may prove accurate. This study also shows the need for more modern skeletal specimens in herpetological collections and the need for preserving the vertebral order of those specimens.

Snake vertebrae are common throughout the Cenozoic fossil record, but identification of isolated vertebrae often proves difficult due to inter- and intra-columnar variability. Most fossil identifications are based on comparisons with disarticulated modern specimens, with a focus on mid-trunk vertebrae. One focus of this study was to determine the necessity of identifying a true mid-trunk vertebra prior to identification and to develop a method of locating the columnar position of an isolated vertebra for both modern and fossil identifications. Colubrid genera Farancia and Heterodon were chosen for the analysis because they share distinct morphological similarities, articulated modern specimens were available, and fossil species in these genera need to be reassessed. Every third pre-cloacal vertebra was selected from each specimen to undergo geometric morphometric analysis on its anterior face. Relative warp analyses detailed the inter-columnar variation of each specimen and found that the only significant difference in the column was between the anterior most vertebrae, which are already identifiable, and the remainder of the pre-cloacal vertebrae. Despite concern, the convention of using mid-trunk vertebrae for identification may prove accurate for these genera. Due to Farancia and Heterodon’s vertebral similarities, a discriminant function analysis was utilized to distinguish the two genera from one another. To evaluate this method’s utility in paleontology, vertebrae of two extinct species, Heterodon brevis and Paleofarancia brevispinosus, will undergo identical morphometric and discriminant analyses. This study also emphasizes the need for more modern snake skeletons in collections and the necessity of stringing the vertebral column prior to disarticulation.

Chez les teleosteens on definit quatre grands modes de nage axiale (anguilliforme, subcarangiforme, carangiforme, thunniforme). Chacun de ces quatre modes de nage se caracterise par une morphologie differente des poissons et des specificites hydrodynamiques reposant sur la proportion du corps soumise aux ondulations motrices. L'etude biometrique (quatre mesures pour chaque vertebre) et histomorphometrique (compacite osseuse vertebrale) du rachis chez l'anguille, la truite, le chinchard et la bonite revelent une organisation particuliere et des caracteristiques structurales liees aux differents modes de nage qui se traduisent par une regionalisation du rachis en deux regions chez l'anguille et en trois regions chez les autres. L'emplacement des valeurs maxima des parametres vertebraux sur le rachis n'est pas le meme pour chaque modele. L'etude biometrique montre aussi un phenomene de croissance allometrique de certains secteurs vertebraux. La pertinence des descripteurs morphologiques est testee chez d'autres teleosteens avec un mode de nage axiale ou paraxiale grace a l'etude de leurs profils vertebraux et par une comparaison d'indices de position des maxima. Cet indice permet effectivement de regrouper dans un meme mode de nage certaines especes eventuellement issues de familles differentes, surtout dans le cas des modes de nage axiale. En revanche, pour des especes comme le brochet et pour les modes de nage paraxiale si certaines especes ont des caracteristiques morphologiques de leur rachis, il semble plus difficile pour d'autres d'utiliser les seuls resultats biometriques pour decrypter leur mode de nage

La peau des vertebres forme des annexes tegumentaires caracteristiques de chaque region du corps, un contraste net existant entre les phaneres des regions mediane et caudale du tronc ou des segments median et distal du membre. La formation des plumes ou des ecailles des oiseaux, des poils ou des pelotes plantaires des mammiferes est le resultat d'interactions demoepidermiques susceptibles d'etre modifiees par l'acide retinoique. Deux familles de facteurs de transcription pouvant etre impliquees dans l'etablissement de cette specificite regionale sont d'une part les recepteurs de l'acide retinoique (rar), d'autre part les homeoproteines, les premiers etant des regulateurs potentiels des secondes. Nous avons montre que chez le poulet, comme chez la souris, les transcrits rar sont caracteristiques de la differentiation epidemique. Nous avons etudie l'expression d'un gene appartenant a un paralogue median, hoxc-8, ainsi que celles de trois genes situes a l'extremite 5' du complexe d: hoxd-9, d-11 et d-13. Chez le poulet, comme chez la souris, l'expression de ces genes dans la peau apparait suivre la regle de colinearite spatio-temporelle: les transcrits du gene hoxc-8 caracterisent la peau de la region mediane du tronc et des membres, ceux des genes hoxd-9 et d-11 la peau caudale de souris et ceux du gene hoxd-13 la peau plantaire. L'expression du gene hoxd-13 peut etre inhibee chez le poulet par l'acide retinoique, alteration qui resulte en l'apparition des plumes sur certaines ecailles. La recombinaison heterotopique de derme et d'epiderme dorsaux et plantaires demontre que la competence epidermique a former ou non des plumes est etablie tot au cours du developpement et semble liee respectivement a l'expression entre autres des genes hoxc-8 et hoxd-13. En conclusion, l'expression regionalisee des homeogenes apparait correlee a l'etablissement de la specificite regionale du tegument

Description de plourdosteus canadensis, un arthrodire du frasnien inferieur de la formation d'escuminac, quebec. Revision de l'environnement sedimentologique. Comparaison de cette espece avec les autres brachythoraci, revision de la systematique des dunkleosteoidea, des plourdosteidae et des coccosteidae

Le role biologique des gangliosides a ete etudie en utilisant des cultures primaires d'embryon de poulet en presence et/ou en absence d'un melange de gangliosides purifies. Les gangliosides s'inserent tres rapidement dans la membrane neuronale et produisent un effet neurotrophique et neutrogenique. Une legere stimulation de la synthese proteique est observee. Les gangliosides stimulent la methilation des proteines et des acides nucleiques, representant un mecanisme de regulation de l'activite cellulaire

Etude des lésions spontanées de l'intima, caractérisée principalement par une rupture de la limitante élastique interne, dans certaines artères de rat. Il semble qu'il existe chez le rat de laboratoire une anomalie héréditaire, se traduisant par un défaut mineur au niveau de la structure des fibres élastiques qui entrainerait sous l'effet de forces hémodynamiques la formation de ces lésions spontanées.

L'etude comparative du caryotype des differents lepilemur a permis de reconstituer le caryotype ancestral des lepilemuridae et d'etablir une phylogenie chromosomique entre les differentes especes. L'evolution chromosomique de ce taxon s'est faite a partir de ce caryotype ancestral d'une facon lineaire dichotomique avec accumulation de translocations chromosiques, ce qui a eu pour consequence de reduire de facon importante le nombre de chromosomes et de bras chromosomiques et a donc entraine une forte diminution des recombinaisons genetiques favorisant vraisemblablement la speciation dichotomique des lepilemur

Importance des facteurs endoluminaux d'origine testiculaire dans la différenciation et le maintien de la structure de l'épididyme de rat, notamment au segment initial. Une androgénodépendance différentielle des cellules épithéliales a été mise en évidence. Étude de la lame basale et du métabolisme oxydatif

Dissertation submitted to the Faculty of Science, University of the Witswatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. May 2015 in Johannesburg<br>Claudins are a large gene family found in all vertebrates. Claudins encode tetraspan membrane proteins, involved in the structure and function of the tight junctions. This association of cells leads to the formation of the epithelial sheet which is involved in many functions such as embryo morphogenesis. The NCBI database shows 27 claudins identified in humans; 23 in mice and 17 in Xenopus. This suggests that an increase in gene family size may correlate with the evolution of more complex vertebrates. In this study claudins from the most basal extant vertebrate, the sea lamprey, were investigated. RNA used to build up the lamprey genome by Jeramiah Smith (Smith et al., 2012), was used for lamprey claudin sequences. Additionally this study identified 2 more claudins (Cldn B & Cldn F). The phylogenetic tree constructed using claudins from higher vertebrate model organisms and the invertebrates Ciona intestinalis and Drosophila melanogaster; showed that lamprey claudins are evolutionarily more distantly related to their orthologs in higher vertebrates. Furthermore some claudins in lamprey did not show any homologs in higher vertebrates and vice versa, indicating the emergence of novel members in higher vertebrates. However lamprey Cldn A was found to be homologous to CLDN 3 in higher vertebrates. This is interesting since CLDN 3 is involved in the development of two vertebrate specific traits; one of which is the ear placode. Thus Cldn A (renamed Cldn 3B), was made a focus of this study. RNA in situ hybridization using probes designed from individual UTRs showed localised expression of Cldn 3B in the ear placode, pharyngeal pouch, pericardial cavity and the fin fold whereas Cldn B (renamed Cldn 8B) was mostly expressed in the pharyngeal pouch and ear placode much like its orthologs in higher vertebrates. Knockout experiments showed that Cldn 3B is involved in sealing and expansion of the ear placode and pharyngeal arches during development whereas Cldn 8B is involved in determining ear placode development. Thus claudins are seen to be heavily involved in the morphology of vertebrate specific traits therefore an expansion in this gene family would affect the complexity of vertebrates during evolution.

The thoracic skeletal morphology of homininae is poorly known and understood. As a result of the representative fossil record of ribs and vertebrae being rare, distorted, fragmentary or unrecognised even when recovered, very little is known about the variability of rib and vertebral morphology when compared to the other cranial and postcranial elements in this lineage. Yet the costal skeleton forms a substantial part of the postcranial skeleton and thus ribs and vertebrae are therefore potentially numerous in the fossil record; but in comparison with other skeletal elements, and for the reasons mentioned above, very little is known about vertebrate and especially hominin rib morphology. The assessment of the structure of the thoracic skeletal elements and its evolutionary and ecological significance, particularly in the Homininae, poses a challenge but is still important as the shape and form of the rib cage has numerous functional and behavioural implications. The present study analysed the ribs of selected primate and non-primate mammalian species by examining fifteen variables, seven indices and eight osteological non-metric features. These observations and measurements were compared to ribs found in the fossil record in order to determine if there are any structural correlates between the extant and the extinct hominin and mammalian species and in order to create a template for the identification of hominin ribs within an abundant and diverse mammalian assemblage. The results suggest that the 1st rib, due to its unique morphology, may be considered most diagnostic in differentiating various taxa. In addition, a template for the morphology of the proximal end of the first rib has been created to be used for both the general as well as the specific identification of fossilised fragments, and to determine thoracic shape. The recently recovered costal elements of the Australopithecus sediba fossils were also examined as one of the most abundant assemblages of the elements in the early hominin record in order to add to our understanding of the morphology, and evolution of this poorly known area of hominin anatomy. The thorax of Australopithecus sediba demonstrates a medio-laterally narrow, ape-like upper thoracic shape, which is different from the broad upper thorax of Homo that has been associated with to the locomotor pattern of endurance walking and running. The lower thorax, however, is less laterally-flared than that of apes, and more closely approximates the morphology found in humans. This indicates a mosaic morphology of the thorax during the human evolutionary linage.

Mobility of the vertebral column is important for many ecological aspects of vertebrates, especially in the cervical series, which connects the head to the main body. Thus, fusion within the cervical series is hypothesized to have ecological and behavioural implications. Fused, anterior cervical vertebrae have evolved independently over 20 times in ecologically disparate amniotes, most commonly in pelagic, ricochetal, and fossorial taxa, suggesting an adaptive function for the ‘syncervical.’ Fusion may help increase out-force during head-lift digging or prevent anteroposteriorly shortened vertebrae from mechanically failing during locomotion, but no hypothesis for syncervical function has been tested. The syncervical of neoceratopsian dinosaurs is hypothesized to support large heads or aid in intraspecific combat. Tests of correlated character evolution within a ceratopsian phylogeny falsify these hypotheses, as the syncervical evolves before large heads and cranial weaponry. Alternative functional hypotheses may involve ancestral burrowing behaviour or unique feeding ecology in early neoceratopsians.

All living hominoids are characterized by taillessness and adaptations to orthograde (upright) trunk posture. Accordingly, these features have importance for our understanding of ape origins, evolutionary relationships and positional behaviors. Despite extensive study of the hominoid postcranial skeleton, researchers continue to face difficulty identifying taillessness and orthogrady from fossil material. In part, difficulties persist because although the Miocene fossil record indicates that the evolution of tail loss and orthogrady was decoupled, previous research has focused primarily on how the skeletal anatomy of extinct apes resembles that of living apes, in which these traits appear in conjunction. The remarkable diversity in tail lengths and positional behaviors exhibited by other mammals presents a valuable opportunity to employ the strength of the comparative method for testing functional hypotheses. The goal of this dissertation is to identify anatomical correlates of tail length and positional behaviors from sacral and caudal vertebral morphology among primates and other mammals in three studies. The first study examines the relationship between trabecular structure in the first sacral vertebra and positional behaviors (N= 78 primates). The second study quantifies aspects of internal (N=78 primates) and external (N= 472 mammals) sacral anatomy for correlates of relative tail length. The third study evaluates the functional morphology of caudal vertebrae among nonprehensile-tailed primates and other mammals that vary in relative tail length, offering additional insight into the anatomy associated with tail loss (N=333). The relationship between trabecular structure in the proximal sacrum and positional behaviors among living primates is somewhat unclear. Some trabecular parameters in the distal sacrum appear to have a relationship with tail length. Results support the functional links between previously and newly defined metrics from the external morphology of sacra and caudal vertebrae, and relative tail length, among primates and other mammals. Identified anatomical correlates from the extant primate sample are used to reconstruct the tail lengths of extinct primates. From the sacral data, Proconsul is reconstructed as tailless, Archaeolemur likely possessed a long tail, and Palaeopropithecus, Megaladapis and Epipliopithecus had short tails. From the caudal vertebrae data, Archaeolemur is reconstructed as possessing a long tail and Palaeopropithecus is reconstructed as having a short tail.<br>text

abstract: Despite the critical role that the vertebral column plays in postural and locomotor behaviors, the functional morphology of the cervical region (i.e., the bony neck) remains poorly understood, particularly in comparison to that of the thoracic and lumbar sections. This dissertation tests the hypothesis that morphological variation in cervical vertebrae reflects differences in positional behavior (i.e., suspensory vs. nonsuspensory and orthograde vs. pronograde locomotion and postures). Specifically, this project addresses two broad research questions: (1) how does the morphology of cervical vertebrae vary with positional behavior and cranial morphology among primates and (2) where does fossil hominoid morphology fall within the context of the extant primates. Three biomechanical models were developed for the primate cervical spine and their predictions were tested by conducting a comparative analysis using a taxonomically and behaviorally diverse sample of primates. The results of these analyses were used to evaluate fossil hominoid morphology. The two biomechanical models relating vertebral shape to positional behaviors are not supported. However, a number of features distinguish behavioral groups. For example, the angle of the transverse process in relation to the cranial surface of the vertebral body--a trait hypothesized to reflect the deep spinal muscles' ability to extend and stabilize the neck--tends to be greater in pronograde species; this difference is in the opposite of the direction predicted by the biomechanical models. Other traits distinguish behavioral groups (e.g., spinous process length and cross-sectional area), but only in certain parts of the cervical column. The correlation of several vertebral features, especially transverse process length and pedicle cross-sectional area, with anterior cranial length supports the predictions made by the third model that links cervical morphology with head stabilization (i.e., head balancing). Fossil hominoid cervical remains indicate that the morphological pattern that characterizes modern humans was not present in Homo erectus or earlier hominins. These hominins are generally similar to apes in having larger neural arch cross-sectional areas and longer spinous processes than modern humans, likely indicating the presence of comparatively large nuchal muscles. The functional significance of this morphology remains unclear.<br>Dissertation/Thesis<br>Ph.D. Anthropology 2013