Aspects of carbonate sedimentation in the Ordovician of Baltoscandia

In the Baltoscandian epicontinental sea the normal type of Ordovician carbonate sediments was a mixture, in various proportions, of skeletal sand and carbonate mud. In some marginal areas sediments of Bahaman type were deposited during the latest part of the Viruan Epoch, and such sediments were widely distributed from the very latest Ordovician onwards. It is suggested that deposition mostly took place in a temperate climatic zone and that the bahamitic deposits indicate an increase of temperature to subtropical or tropical level.
The succession of the Ordovician lithofacies belts in Baltoscandia is mostly roughly symmetrical with regard to grain size, but asymmetrical as to the distribution of terrigenous mud. The position of the boundary between the terrigenous and carbonate mud belts may have been a result of competition in the material transport from the west and from the east. The direction of the lithofacies belts is not parallel to the axis of the Caledonian geosynclinal zone. The predominance of carbonate sediments in the Baltoscandian Ordovician epicontinental sea probably does not reflect particularly good conditions for production of carbonate material, but rather, a very low supply of terrigenous matter.
In the Central Baltic area changes of sea level were probably largely opposite to those in Västergötland. This indicates differential isostatic movements in the Baltoscandian cratonic shield during parts of the Ordovician Period.     

Constituent analysis of an Ordovician limestone from Sweden

Quantitative data on the carbonate constituents of the early Viruan ('Middle Ordovician') Segerstad Limestone of Sweden are given, based on modal ana lyses of thin sections. The grain size of the sediment transgressed the boundary between carbonate mud mixed with skeletal sand (micritic calcarenite), and skeletal sand mixed with carbonate mud (calcarenitic calcilutite). Increase in grain size is largely due to the increase in the abundance of echinoderm particles.
The term ostraconzass is introduced for the total mass of the skeletal material produced by the organisms in a given area at a given time. The composition of the skeletal sand is completely different from that of the macrofossils in the same beds. The originally aragonitic grains constitute only one to ten per cent of the skeletal sand. This supports suggestions that during the early Palaeozoic the aragonite-calcite ratio of the ostracomass was low. Attention is drawn to some observations which indicate that the aragonite-calcite ratio of skeletal sand in the modern temperate marine shallow-water environment may be considerably lower than in tropical and subtropical seas.     

Functional morphology of the shell in platycope ostracodes - a study of arrested evolution

Shell morphology in relation to soft parts is described in the modem species Cytherella abyssorum G. O. Sars, based on microtome sections, serial peels and thin sections. Functionally important features of the cytherellids include the extensive development of the intervalvar cuticle ('ligament') along a strongly curved Line, the consequently very narrow ventral slit between valves in opened carapaces, and a special mode of egg care which is associated with a distinctive (domatial) type of sexual shell dimorphism. Platycope ostracodes represent a pronouncedly conservative evolutionary lineage; their essential characters are fully developed already in the earliest known true platycopes from the Silurian Period. The cause of the slow evolutionary change is suggested to have been the virtual impossibility to change their specific morphological organisation to fit some other mode of life or to diversify widely within the given morphological framework. The same obviously applies to the lingulaccan brachiopods and probably also other bradytelic groups.     

Balance of the head in hominid evolution

In primates it is useful to distinguish three basic types of bipedal posture: (1) agonial, with extended hips and knees as in modern humans, (2) monogonial, with flexed hips but extended knees. and (3) digonial, with flexed hips and knees as in pongids. Early hominids retained an ancestral, forwardly inclined posture of the neck and head. Therefore the body posture of australopithednes must have differed from that in modem man, in which the centre of gravity of the head can be aligned with that of the body, other major centra of gravity, and important axes of rotation in a single frontal plane. It is suggested that in australopithednes the gravitational tilt of the head was counterbalanced by bent hips in association with hyperextended knees (monogonial posture). In australopithecines the increase in brain weight would have counteracted an improvement in the balance of the head. After the neck had assumed a more vertical posture as a consequence of shortening of the face, selection for an improved balance system in the bipedal posture favoured an increase in the weight of the postcondylar portion of the head, accentuated by selection for a posterior shift of the superior nuchal line in order to minimise the force of the nuchal muscles. At this stage the evolutionary increase in brain weight may have been largely a by-product of the process towards perfecting the bipedal posture. When the centre of gravity of the head had first become aligned with that of the body, the conditions of balance of the head had become favourable for a dramatic increase of brain size, as a result of selection for greater learning and storage capacity of the brain.     

Morphological changes leading to hominid bipedalism

The adoption of habitual bipedal locomotion required a backward shift of the centre of gravity of the body, to a level relative to the supporting surface area of the body and pivotal axis of the hips at which walking with extended knees became practicable. In the morphology of the immediate ancestors to hominids, there were relatively few features whose change could have effectively affected the position of the gravity vertical of the body. Weight could be distributed posteriorly mainly by increasing the mass of the hindlimbs and the kyphotic curve of the vertebral column, by developing buttocks. and by flattening the thorax and abdomen. Estimates indicate that the adoption of a suitable curvature of the vertebral column alone was not sufficient for shifting the centre of gravity behind the 'threshold of bipedalism'. A considerable increase in the mass of the hindlimbs was also required, and the addition of the mass of the buttocks may have represented the decisive factor for crossing the threshold. A possible physical enviroment in which the change to bipedalism could have taken place was a mountainous terrain with long, steep slopes, transected by gorges and precipices. In such a terrain the increase in the power and mass of both the hindlimbs and the gluteus maximus proprius muscle could have been favoured by selection, leading ultimately to a condition in which the combined effect of heavy hindlimbs. a suitable curvature of the spine and the weight of the buttocks shifted the centre of gravity of the body backwards to a level at which habitual walking with extended knees became practicable. □ Hominid evolution. bipedalism, evolutionary thresholds.     

Functional thresholds in evolutionary progress

Analysis of some phylogenetically important stages in the evolution of graptoloids, articulate brachiopods, and the earliest mammals suggests that large scale evolutionary progress did not proceed linearly but that biological improvement was stepwise, achieved by rapidly crossing successive functional thresholds which were interpolated between gradual morphological changes (here termed proximations ) towards the condition at which the structures became predisposed to the next threshold crossing. Crossing of many functional thresholds involved morphological discontinuities or functional instabilities that had to be bridged abruptly, over dithyrial populations. The principle of scalariform evolutionary progress is believed to be of fairly general application, but our current ability to analyse many critical evolutionary events in functional terms is still at a primitive stage because the functional significance of many important morphological changes are unclear. It is stressed that the evolutionary success of any 'hopeful monster' is determined at the population level, and that therefore, if extreme cases are discussed, the emphasis must be on 'monstrous families'.