Mammal-like Reptiles Research Articles

Functional shape of the skull in vertebrates: Which forces determine skull morphology in lower primates and ancestral synapsids?

In order to determine the extent to which the shape of the synapsid skull is adapted for resisting the mechanical loads to which it is subjected, block- or simple plate-shaped finite-element models were constructed and loaded with external muscle and bite forces in locations estimated to resemble points of application of these forces. These 2D or 3D finite-element models were iteratively loaded and modified by removing elements that experience only low stresses, and the resulting morphologies of the models were compared with fossil skulls of synapsids and the skulls of extant mammals. The results suggest that the stress flows in these unspecific models are very similar to the arrangement of bone material in real skulls. Morphological differences between taxa depend on a few a priori conditions: length and position of the tooth rows in relation to the braincase, arrangement of muscles, position of the orbits, and position of the nasal opening. Given these initial conditions, finite-element analysis consistently reveals the close similarity between stress flows and real skulls. The major difference between mammal-like reptiles and primates is the size of the braincase. This difference accounts for most of the morphological divergence. The postorbital bar seems to be a constructional element of the skull, rather than a means to protect the eyes. The skull shapes of higher primates are determined mainly by masticatory forces and less by external forces acting on the head. This study demonstrates the utility of finite-element modeling for testing hypotheses regarding relationships between form and function in vertebrate skulls.

Serial 1 Development of jaws; over view--development of jaws in vertebrates

Mouth is entrance to the alimentary canal of the animal. One of the most significant evolutionary advances made in the evolution of vertebrate was the development of jaws. The presence of jaws manipulated by muscles allows an organism to grasp feed firmly. From primitive amphibian to primitive reptile, there had only primary oral cavity, but an advanced mammal-like reptile was divided primary oral cavity between oral and nasal cavity. An advanced mammal-like reptile was able to maintain breathing easily while the mouth is functioning in eating, fore get the development of a secondary palate. In the development of vertebrate it made many characteristic forms of jaws. In this series we will discuss that development and morphological character of the jaws.

Evolution of mammals: lactation helps mothers to cope with unreliable food supplies.

Lactation is a ubiquitous feature of mammalian reproduction. Because lactating females can draw on their nutrient reserves for milk production, it offers mothers and their dependent young independence from fluctuations in their food supplies. However, converting food to reserves and milk is relatively inefficient at delivering nutrients to offspring. We use dynamic programming to contrast the performance of mothers that provision dependent, refuge-bound offspring optimally from their nutrient reserves with otherwise equivalent mothers that do so directly from the food they find. In this way, we demonstrate formally that the selective advantage to lactating mothers, who can provision--at a cost--without having found food recently, can be substantial with uncertain food supplies and few opportunities for future reproduction under a wide range of circumstances. Hence, it is likely that unreliability associated with the lifestyles of the small, primitive mammal-like reptiles that evolved extended maternal care, selected for fully-developed milk production and consumption, prompting the evolution of true mammals. Moreover, this work suggests that selection for coping with unreliable food access during provisioning may underlie key life-history differences between birds and mammals because the mass constraints imposed by flight restrict the level of reserves that mothers can carry and provision from.

Temnospondyls from the Beaufort Group (Karoo Basin) of South Africa and Their Biostratigraphy

The Permo-Triassic Beaufort Group (Karoo Basin) of South Africa is biostratigraphically subdivided into eight, temporally successive assemblage zones based on therapsids (‘mammal-like reptiles’). The Temnospondyli, fossil tetrapods usually regarded as extinct amphibians, are second only to therapsids in terms of diversity and abundance in these strata, with nine higher-level taxa (‘families’) known. Temnospondyls are also playing an increasingly important role in biostratigraphy and correlation of the Beaufort strata. The lower Beaufort Group (Late Permian) contains six of the eight biozones, but only one temnospondyl ‘family’, the Rhinesuchidae, whose record in the Karoo is the richest in the world. However, rhinesuchid taxonomy remains in flux and the group is thus of limited biostratigraphic utility. The Early Triassic Lystrosaurus Assemblage Zone (middle Beaufort Group) contains the Rhinesuchidae, Amphibamidae, Lydekkerinidae, Tupilakosauridae, Rhytidosteidae, Mastodonsauridae and Trematosauridae, although the biostratigraphy of temnospondyls within this biozone is poorly constrained. The uppermost reaches of the Lystrosaurus biozone contain a paucity of fossils but includes ‘Kestrosaurus’ (Mastodonsauridae) and ? Trematosuchus (Trematosauridae), taxa previously thought to pertain to the lower part of the overlying Cynognathus biozone. The late Early to Middle Triassic Cynognathus Assemblage Zone (upper Beaufort Group) hosts the Mastodonsauridae, Trematosauridae, Brachyopidae, Laidleriidae and, possibly, the Rhytidosteidae. Based largely on the spatial and temporal distribution of mastodonsaurids, this biozone has been biostratigraphically subdivided into a lower A, middle B and upper C subzones, characterised by differing ages and faunas.

Vertebrate Burrows from Triassic and Jurassic Continental Deposits of North America and Antarctica: Their Paleoenvironmental and Paleoecological Significance

Comparisons of recently identified Triassic and Jurassic continental trace fossils in North America and Antarctica to modern mammal and reptilian burrows facilitate the identification and interpretation of the ancient burrows as vertebrate in origin, indicating advanced behaviors. Hollow, bowl-shaped depressions in the Petrified Forest Member of the Upper Triassic Chinle Formation in Petrified Forest National Park, Arizona, are interpreted as nest-holes constructed possibly by phytosaurs, aeotosaurs, turtles, or rauisuchians. Large-diameter, multiple-branching, and interconnected burrows in the Owl Rock Member of the Chinle Formation in southeastern Utah are tentatively interpreted as vertebrate burrows indicating communal behavior. Complex, large-diameter burrows in the Salt Wash Member of the Upper Jurassic Morrison Formation near the Henry Mountains in southern Utah are interpreted as burrow systems of fossorial mammals. Large-diameter, gently dipping, simple, subhorizontal burrows in the Salt Wash Member are interpreted as possible dwelling burrows of sphenodontids. Other vertebrate trace fossils, such as the large-diameter burrows from the Lower Triassic Fremouw Formation in the Queen Maud Mountains of Antarctica, are reinterpreted as vertebrate burrows and were likely constructed by small mammal-like reptiles. These burrows were thought to have been enigmatic in origin and different from very large-diameter burrows interpreted as therapsid dwelling burrows. Descriptions and interpretations of all these trace fossils are important because most vertebrate ichnology research to date has focused on trackways or locomotion experiments with modern reptiles and birds. These Triassic and Jurassic ichnofossils represent fossorial and nesting behavior of several different groups of vertebrates. The Fremouw Formation burrows indicate fossorial behavior in several sizes of vertebrates, including small and large therapsids. The burrows were likely used for shelter, giving birth, raising young, and hibernation. During the early Mesozoic, the Fremouw landscapes were thought to have had cold winters due to their high-paleolatitude position. The bowl-shaped depressions from the Petrified Forest Member of the Chinle Formation likely represent the earliest known structures excavated by reptilians for the sole purpose of reproduction. Basic nest-hole architecture in extant reptiles with early Mesozoic ancestry has changed minimally in nearly 220 million years. Large-diameter, multiple branching, and interconnected burrows in the Owl Rock Member of the Chinle Formation were likely constructed by fossorial vertebrates that had communal family groups. Simple, gently dipping, subhorizontal burrows in the Morrison Formation were used by crocodiles or sphenodontids as dwelling structures constructed in firm, subaerially exposed substrata close to open bodies of water. Complex, large-diameter burrows in the Salt Wash Member suggest subsocial behavior of fossorial mammals, where the burrow was used for raising young, storage and disposal of food and wastes, and coping with episodic water inundation. Research demonstrates that fossorial behavior of reptiles, therapsids, and mammals was established by the beginning of the Mesozoic and prior to the break-up of Pangea. The basic architecture of vertebrate nest construction has changed little in nearly 280 million years. Fossorial burrowing behavior likely evolved several times in different vertebrate groups during this time. These basic burrow architectures are also used by invertebrate groups. This overlap in burrow architectures between vertebrates and invertebrates suggests strongly that paleoenvironmental and paleoclimatic organism-substrate relationships dictate the architecture used by the organism. These burrow morphologies indicate particular physicochemical conditions in terrestrial and freshwater settings that are unique to the continental realm.

Evolutionary trends and the origin of the mammalian lower jaw

The single bony element forming the lower jaw of living mammals, the dentary, has been interpreted as representing the culmination of a long and gradual evolutionary trend. Numerous fossil nonmammalian synapsids (“mammal-like reptiles”) show varying degrees of enlargement of the dentary and concomitant reduction in the postdentary bones. To quantitatively reexamine patterns of morphological change in the evolution of the mammalian lower jaw, measurement and discrete character data were collected from 322 fossil synapsid mandibles spanning Late Carboniferous through Jurassic time. Measurements confirm that the relative contribution of the dentary increased in theriodont (advanced therapsid) evolution with regard to both stratigraphic and phylogenetic position. However, dentary enlargement and postdentary reduction failed to typify all therapsid subclades. Qualitative characters of the mandible were used to quantify morphological similarity with regard to the early mammal Morganucodon . Analyses contrasting stratigraphic and phylogenetic position with mammalian similarity indicate that mandibular evolution was primarily conservative, with only anomodont therapsids evolving substantial morphological novelty. Scaling analyses comparing the area of the dentary and postdentary regions to jaw length uniformly show isometry or slight positive allometry, although cynodont therapsids have a smaller postdentary region than any other therapsid subgroup. These results suggest that body size decreases cannot fully explain the reduction of the postdentary bones. Finally, step size bias was tested as a mechanism for explaining long-term trends. Qualitative data reveal no significant difference in the magnitude of character changes occurring in mammalian and nonmammalian directions.

Mammalian skin evolution: a reevaluation.

A 1972 model for the evolutionary origin of hair suggested a primary mechanoreceptor role improving behavioral thermoregulation contributed to the success of late Paleozoic mammal-like reptiles. An insulatory role appeared secondarily subsequent to protohair multiplication. That model is updated in light of new data on (a) palaeoecology of mammalian ancestors; (b) involvement of HRPs in keratinization; (c) lipogenic lamellar bodies that form the barrier to cutaneous water loss; and (d) growth factors involved in hair follicle embryogenesis and turnover. It is now proposed that multiplication of sensory protohairs caused by mutations in patterning genes initially protected the delicate barrier tissues and eventually produced the minimal morphology necessary for an insulatory pelage. The latter permitted Mesozoic mammals to occupy the nocturnal niche 'in the shadow of dinosaurs'. When the giant reptiles became extinct, mammals underwent rapid radiation and reemerged as the dominant terrestrial vertebrates.

Fungal abundance spike and the Permian–Triassic boundary in the Karoo Supergroup (South Africa)

The most severe mass extinction of marine species and terrestrial vertebrates and plants is associated with the Permian–Triassic boundary (∼251 Ma). The extinction interval is also marked by the disappearance of most Late Permian gymnosperm palynomorphs at a layer containing solely the abundant remains of fungi. This ‘fungal spike’ apparently represents widespread devastation of arboreous vegetation. Stratigraphic and palynological study of the Carlton Heights section in the southern Karoo Basin of South Africa revealed a 1-m-thick fungal spike zone that occurs simultaneously with the last appearance of typically Late Permian gymnosperm pollen. The plant extinction and fungal spike zone are found above the last occurrence of Late Permian mammal-like reptiles of the Dicynodont Zone at other Karoo sections. Using the fungal event as a time line in marine and non-marine sections allows placement of the marine extinctions and the extinction of terrestrial plants and reptiles within a brief crisis interval of less than about 40 000 years at the end of the Permian.

Evolutionary Patterns in the History of Permo-Triassic and Cenozoic Synapsid Predators

Synapsids include modern mammals and their fossil ancestors, the non-mammalian synapsids, or ‘mammal-like reptiles' of old classifications. The synapsid fossil record extends from the Late Carboniferous to the present, a span of nearly 300 million years. However, it can be broken into two distinct phases of diversification, separated by about 150 million years. The first phase extends from the Late Carboniferous to the mid-Triassic, includes the first large land predators on Earth, and is almost entirely non-mammalian. The second phase begins about 65 million years ago after the demise of the dinosaurs, includes only mammals, and extends to the present. In this overview of synapsid predators, we emphasize terrestrial species of large size, and their adaptations for killing and feeding, rather than locomotion. Despite fundamental differences in jaw mechanics and tooth morphology, there are significant parallels in the non-mammalian and mammalian radiations of synapsid predators. Both groups evolve sabertooth forms more than once, and both evolve short-snouted, powerful biting forms. In addition, both the Late Carboniferous—Triassic and Cenozoic phases are characterized by repeated patterns of clade replacement, in which one or a few clades evolve large size and seem to dominate the carnivore guild for several million years, but then decline and are replaced by new taxa. Moreover, within both ancient and Cenozoic predator clades, there are parallel trends over time toward increased body size and hypercarnivory that likely result from a combination of interspecific competition and energetic constraints.

Where is the Fossil Evidence for Gondwanan Crayfish?

Crayfish body- and ichnofossils recently reported from Lower Permian and Lower Triassic strata of Antarctica have been reinterpreted to be the body fossils of euthycarcinoid arthropods and the burrows of small- to large-sized therapsids (mammal-like reptiles). The earliest fossil evidence of crayfish presence and activity in continental deposits is still from the Laurasian continents of Pangea. This evidence is in the form of body fossils and burrows from the Upper Triassic and burrows from the Lower Permian of the American southwest. For that matter, nearly all ichno- and body fossils have been described from Laurasian rocks with the exception of the occurrence of an isolated claw of a parastacid crayfish from Paleogene deposits in Queensland, Australia. Where is all the fossil evidence for Gondwanan crayfish? There is a rich extant diversity of parastacid crayfish in Australia, and they are dispersed between other Gondwanan continents, with the exception of Africa and Antarctica. Paleontological and ichnological evidence, when taken into consideration with paleogeographic reconstructions, indicates that they originated early, during, or slightly after the formation of Pangea. Thus, at the very least, there should be trace and body fossil evidence in Australia indicating the long evolutionary history of the parastacids. Evidence for ancient Gondwanan crayfish in terrestrial and freshwater ecosystems should occur in rocks of similar ages to those in North America where ichno- and body fossils have been identified.

Evolution of hindlimb posture in nonmammalian therapsids: biomechanical tests of paleontological hypotheses

Analyses of limb joint morphology in nonmammalian therapsid “mammal-like reptiles” have suggested that among many lineages, individual animals were capable of shifting between sprawling and upright hindlimb postures, much like modern crocodilians. The ability to use multiple limb postures thus might have been ancestral to the generally more upright posture that evolved during the transition from “mammal-like reptiles” to mammals. Here I derive a biomechanical model to test this hypothesis through calculations of expected posture-related changes in femoral stress for therapsid taxa using different limb postures. The model incorporates morphological data from fossil specimens and experimental data from force platform experiments on iguanas and alligators.Experimental data suggest that the evolutionary transition from sprawling to nonsprawling posture was accompanied by a change in the predominant loading regime of the limb bones, from torsion to bending. Changes in the cross-sectional morphology of the hindlimb bones between sphenacodontid “pelycosaurs” and gorgonopsid therapsids are consistent with the hypothesis that bending loads increased in importance early in therapsid evolution; thus, bending stresses are an appropriate model for the maximal loads experienced by the limb bones of theriodont therapsids. Results from the model used to estimate stresses in these taxa do not refute the use of both sprawling and more upright stance among basal theriodont therapsids. Thus, the hypothesis that the use of multiple postures was ancestral to the more upright posture typical of most mammals is biomechanically plausible. Model calculations also indicate that the axial rotation of the femur typical in sprawling locomotion can reduce peak bending stresses. Therefore, as experimental data from alligators and iguanas suggest, the evolution of nonsprawling limb posture and kinematics in therapsids might have been accompanied by increased limb bone bending stress.

Ghost lineages and “mammalness”: assessing the temporal pattern of character acquisition in the Synapsida

Abstract The origin of mammals has been characterized as a gradual process, a claim based primarily on a well-preserved series of extinct nonmammalian synapsids (“mammal-like reptiles”) that span some 200 million years. In contrast to the origin of many other higher taxa, the origin of mammals from within cynodont-grade therapsids is not considered to coincide with a major morphological change, but rather to be simply the culmination of a series of more and more mammal-like transitional forms. To test these assertions, an asymmetrical cladogram extending from primitive “pelycosaurs” to morganucodontid mammaliaforms was created. Three different methodologies were then used to compare the amount of morphological change between nodes on this cladogram with the minimum missing time interval between each node, as inferred from sister taxon-based ghost lineages. In general, a statistically significant positive relationship was found, indicating that greater numbers of derived features tend to be correlated with longer ghost lineages. A significant correlation between the number of accumulated apomorphies and branching events was also found. Although the rate of character change was variable, in no case was a long ghost lineage associated with few apomorphies. These correlations are consistent with the hypothesis that rapid accumulation of derived features occurred relatively infrequently within the synapsid lineage leading toward mammals and that gradual character evolution predominated.

The jaw function and adaptive radiation of the dicynodont mammal-like reptiles of the Karoo basin of South Africa

The structure and functioning of the dicynodont jaw system are described. A pivoting action of the lower jaw around the palate posteromedian to the caniniform processes is as basic to the jaw movement as is the well-known double-convex jaw articulation. The sequence of origin of the characteristic features of the dicynodonts is analysed cladistically; this demonstrates the patterns of association of these characters into functional character-complexes. The structures of the palate and lower jaw, and their functional integration in feeding, are described and illustrated in standard format. Five different lineages of dicynodont can be identified:Eodicynodon;the robertoids (includingDiictodon); the dicynodontoids (includingDicynodonand the majority of the large dicynodont genera of the Permian and Triassic); the endothiodontoids (includingProdicynodon [=‘Chelydontops’]andPristerodon), and the emydopoids (includingCistecephalus, MyosaurusandKingoria).Eodicynodonor a similar form could have been ancestral to the other four lineages. The robertoids probably fed upon the stems and rhizomes of equisetaleans, while the varied dicynodontoids probably fed upon the varied glossopterid seed-ferns. The endothiodontoids, too, were probably herbivorous, but many, perhaps all, of the small emydopoids were burrowing and may have been omnivorous. The dicynodonts were probably ectothermal, and the dicynodontoids may have migrated to warmer latitudes in the winter. Onlyc. 20 genera of Karoo dicynodont are now recognized as valid, and it is suggested that this fauna is now almost completely known. Their distribution in the Karoo biozones is reviewed and correlated with environmental changes. The Permian ancestors of the Triassic dicynodonts, includingLystrosaurus, probably lived on higher, drier ground, and were therefore already adapted to the more fibrous food that spread into the basins as the climate became drier in the Triassic.

The jaw function and adaptive radiation of the dicynodont mammal-like reptiles of the Karoo basin of South Africa

The jaw function and adaptive radiation of the dicynodont mammal-like reptiles of the Karoo basin of South Africa

Evolution der akustischen Kommunikation (Phylogenese)

The evolutionary transition from mammal like reptiles to mammals is accompanied by two major phylogenetic changes that affected the development of acoustic communication: First, changes in the jaw joint freed the later middle ear ossicles from participating in the feeding apparatus and thereby creating a functional middle ear. This functional middle ear is a basic requirement for perceiving air borne sound and was of fundamental importance for small nocturnal mammals to survive in a world dominated by dinosaurs. Second, a major increase of the neocortex occurred. This increase of the neocortex induced significant changes in the basic anatomy of the central sensory, but affected them in different ways. A topographic somatosensory representation of the body surface in terms of a highly ordered map only exists in the neocortex and has no precursor in other brain structures, and thus has most likely been established in the context the development of the neocortex. The dominating sensory system of the mammal-like reptiles, the visual system, has been fundamentally chang-ed: The primary visual center, the tectum opticum in the midbrain, has been replaced by a new neocortical center, the visual cortex. In contrast, in the auditory system the original basic subcortical integration center, the inferior colliculus has been preserved. The inferior colliculus has multiple connections to premotor and motor centers as well as to centers of major importance for motivation and mental status. Higher centers of auditory processing of speech and semantic information have been established behind the inferior colliculus but did not replace it. This conservation of the direct connection of a subcortical integration center with centers of motivational control that in turn can affect the neocortex might be the reason for the successful use of music and familiar voices as treatment in persistent-vegetative-state-rehabilitation.

Palaeoentomology of the kap københavn formation, a plio-pleistocene sequence in Peary Land, North Greenland

Natural selection and the development of new taxa are associated with ecological replacement and the increase in number of niches with time. Continental faunal interchange was possible globally because of the existence of the super-continent Pangaea during much of the Upper Palaeozoic and Mesozoic. Figures of tetrapod niches vs. time and discussion of this concept for that period are presented for the first time. Four habitat divisions are used, namely marine, fresh-water, lowland and upland.The marine habitat was colonised rather late by tetrapods and these may have been the first predators on the early bony fishes which had diversified in the Permian. The radiation of bony fishes in the Jurassic was followed by a further increase in variety of their reptilian predators. Predators seem to develop some time after the radiation of a new potential prey group.Most early amphibians occupied fresh-water habitats in “crocodile” or “frog” niches, but from the Triassic tetrapods moved from fresh-waters and lowlands into the uplands also.In terrestrial habitats, the replacement of mammal-like reptiles by dinosaurs is tentatively explained in terms of palaeoclimatology and thermoregulatory physiology. Ornithischians capable of dealing with tough vegetation evolved to occupy the new niches produced by the radiation of conifers in the Jurassic. The extinction of dinosaurs appears to have been connected with temperature and habitat changes.Conclusions are supported by a summary of published opinions on the palaeoecological roles of early tetrapods.

A complement factor B-like cDNA clone from the zebrafish ( Brachydanio rerio)

An important molecule in the activation of the complement system in vertebrates is factor B, a serine protease with a molecular mass of 95,000. Factor B and the complement component C2 are thought to have arisen by gene duplication. In mammals and in Xenopus the factor B gene is linked to the major histocompatibility complex (MHC), whereas in domestic fowl it segregates independently of the MHC. Here we describe the isolation of a cDNA clone coding for factor B in the zebrafish, Brachydanio rerio. The deduced protein sequence exhibits a characteristic mosaic structure consisting of the short consensus repeat (SCR), the von Willebrand factor, and the serine protease domains. The estimated time of factor B and C2 divergence (approximately 350 million years ago), combined with the fact that C2 has thus far been found only in mammals, suggest that the factor B-C2 gene duplication occurred after the divergence of mammal-like reptiles from other reptiles and hence also birds. After the duplication, the C2 component evolved significantly faster than factor B.

TURBINATES IN THERAPSIDS: EVIDENCE FOR LATE PERMIAN ORIGINS OF MAMMALIAN ENDOTHERMY.

The structure and function of the nasal conchae of extant reptiles, birds, and mammals are reviewed, and the relationships to endothermy of the mammalian elements are examined. Reptilian conchae are relatively simple, recurved structures, which bear primarily sensory (olfactory) epithelium. Conversely, the conchae, or turbinates, of birds and mammals are considerably more extensive and complex, and bear, in addition, nonsensory (respiratory) epithelium. Of the mammalian turbinates, only the exclusively respiratory maxilloturbinal has a clear functional relationship with endothermy, as it reduces desiccation associated with rapid and continuous pulmonary ventilation. The other mammalian turbinates principally retain the primitive, olfactory function of the nasal conchae. The maxilloturbinates are the first reliable morphological indicator of endothermy that can be used in the fossil record. In fossil mammals and mammallike reptiles, the presence and function of turbinates are most readily revealed by the ridges by which they attach to the walls of the nasal cavity. Ridges for olfactory turbinals are located posterodorsally, away from the main flow of respiratory air, whereas those of the respiratory maxilloturbinals are situated in the anterolateral portion of the nasal passage, directly in the path of respired air. The maxilloturbinal is also characterized by its proximity to the opening of the nasolacrimal canal. Posterodorsal ridges, for olfactory turbinals, have long been recognized in many mammallike reptiles, including early forms such as pelycosaurs. However, ridges for respiratory turbinals have not been identified previously in this group. In this paper, the presence of anterolateral ridges, which most likely supported respiratory turbinals, is reported in the primitive therocephalian Glanosuchus and in several cynodonts. The presence of respiratory turbinals in these advanced mammallike reptiles suggests that the evolution of "mammalian" oxygen consumption rates may have begun as early as the Late Permian and developed in parallel in therocephalians and cynodonts. Full mammalian endothermy may have taken as much as 40 to 50 million yr to develop.

Fibre Architecture of the Deltoid Muscles in the Japanese Macaque, with Special Reference to the Relationship with That of the Pectoral Muscles

Spatial fibre architecture of the deltoid muscles in five Japanese macaques was observed and functionally analysed to compare it with that of the adjoining pectoral muscles. The fibre bundles of the deltoids possess a twisted and radiating architecture, as do those in the pectorals, and are basically arranged symmetrically about the central fibre of the acromiodeltoid. Unlike those of the pectorals, the bundles of deltoids do not exhibit any crossing structure. These structural features of the deltoids are thought to have been formed through evolutionary modifications in the mammal-like reptiles and primitive mammals, which have disposed the humerus more perpendicularly while simultaneously preserving the deltoids as a single-joint muscle. The lateral shift in the cleidodeltoid and the accompanying clavicular origin of the pectoralis in brachiators and human are suggested to have been formed concomitantly with the flattening of the thorax as one of the evolutionary traits in primates.

Heterochronical patterns of evolution in the transitional stages of vertebrate classes

Transitional forms of the recent classes of vertebrates are only known in paleontology. The well described examples are: Eusthenopteron foordi (Crossopterygii), Ichthyostega and Acanthostega (Labyrinthodontia) between Osteichthyes and Amphibia, Seymouria baylorensis (Amphibiosaria) between Amphibia and Reptilia, Archaeopteryx lithographica (Archaeornithes) between Reptilia and Aves, and the mammal-like reptiles Pelycosauria, Therapsida and Cynodontia between Reptilia and Aves, and the description of their phylogenetical heterochronies in terms of peramorphosis and paedomorphosis shows the progressive role of the motorial, especially the locomotorial organ systems and their functions in comparison with the retarded evolution of the axial system, especially the skull and central nervous system. The evolution of the Hominidae shows the same rule. The evaluation of these transitional forms in their fossil context reveals them as inhabitants of biotopes situated in the border areas of coastal and shore landscapes of marine, brackish or fresh water. These biotopes have obviously favoured the innovations on the high taxonimic level of macro-evolutionary characteristics.