Relative brain size has long been considered a reflection of cognitive capacities and has played a fundamental role in developing core theories in the life sciences. Yet, the notion that relative ...brain size validly represents selection on brain size relies on the untested assumptions that brain-body allometry is restrained to a stable scaling relationship across species and that any deviation from this slope is due to selection on brain size. Using the largest fossil and extant dataset yet assembled, we find that shifts in allometric slope underpin major transitions in mammalian evolution and are often primarily characterized by marked changes in body size. Our results reveal that the largest-brained mammals achieved large relative brain sizes by highly divergent paths. These findings prompt a reevaluation of the traditional paradigm of relative brain size and open new opportunities to improve our understanding of the genetic and developmental mechanisms that influence brain size.
River-channel and colluvial deposits, near Marsworth, Buckinghamshire, record a temperate-periglacial-temperate sequence during the late Middle Pleistocene. The deposits of a lower channel contain ...tufa clasts bearing leaf impressions that include
Acer sp., and
Sorbus aucuparia and containing temperate arboreal pollen attributed to ash-dominated woodland. The tufa probably formed at the mouth of a limestone spring before being redeposited in a small river whose deposits contain plant remains, Mollusca, Coleoptera, Ostracoda and vertebrate bones of temperate affinities. The sediments, sedimentary structures and limited biological remains above the Lower Channel deposits indicate that fluvial deposition preceded climatic cooling into periglacial conditions. Fluvial deposition recurred during a later temperate episode, as shown by the mammalian bone assemblage in stratigraphically higher channel deposits. The Upper Channel deposits are confidently attributed to Oxygen Isotope Sub-Stage 5e (Ipswichian) on the basis of their vertebrate remains. However, the age of the Lower Channel deposits is less clear. The mammalian and coleopteran remains in the Lower Channel strongly suggest correlation with Oxygen Isotope Stage 7 on the basis of their similarities to other sites whose stratigraphy is better known and the clear difference of the Lower Channel assemblage from well-established faunas of Ipswichian or any other age. By contrast, U–Th dating of the tufa clasts suggests an age post 160
ka BP, while Aile/Ile ratios on Mollusca point to an Ipswichian age and younger. Four ways of interpreting this age discrepancy are considered, the preferred one correlating the Lower Channel deposits with Oxygen Isotope Stage 7.
Sequences of calcareous slope and spring deposits (mainly tufa) are described from two localities in Kent (Folkestone and Wateringbury). They contain rich faunas of terrestrial Mollusca, allowing a ...detailed reconstruction of the local environments. The faunal changes also reveal an underlying common pattern, ascribed largely to the effects of climate and migration. Eight biostratigraphical assemblage zones are proposed, applicable to deposits of this general character in the area of southern England. The base of each zone is defined as follows: zone y first appearance of a molluscan fauna, dominantly of Pupilla, Vallonia and Vitrina; zone z expansion of Abida and Trichia; zone a decline of Pupilla, appearance of Carychium and Aegopinella; zone b expansion of Carychium and Aegopinella, occurrence of Discus ruderatus; zone c expansion of Discus rotundatus, suppression of Discus ruderatus; zone d expansion of Oxychilus cellarius; zone e re-expansion of Vallonia; zone f appearance of Helix aspersa. Botanical evidence (pollen and seeds) and some radiocarbon dates allow a partial correlation of the zones with the standard Godwin pollen zones of the Late Devensian (late-glacial) and the Flandrian.
The advantages of grid mapping in distribution studies are now widely accepted. A relatively even coverage is possible and comparison can be made between the faunas of areas of equal size. The ...organization of the British scheme is described. Similar schemes are now in progress in numerous countries and are gaining increasing importance in connection with problems of environmental conservation. An important long-term objective is the compilation of maps for large supra-national areas (e.g. the whole of Europe).
Subaerial deposits of the Late-glacial Period (ca. 12000 to 8300 B. C.) of the Last Glaciation are described at a number of sites in Kent, Surrey and Sussex. The deposits are primarily stratified ...chalk muds and fine rubbles, produced by frost-shattering and the release of water from melting snow-fields and from frozen ground. The climatic improvement of zone II, or Allerod Oscillation (10000 to 8800 B. C.), is widely reflected stratigraphically by a rendsina soil, containing fragments of wood charcoal, separating two sheets of chalk muds referred to zones I and III. The age of the soil has been confirmed by radiocarbon dating. It is correlated with the Usselo Layer within the Younger Coversands of the Netherlands. There is evidence from two areas, Folkestone and the Medway Valley, that the climate of south-east England became sufficiently cold during zone III to produce fairly intense frost-heaving (cryoturbation). The deposits contain virtually no pollen, but yield a fauna of land Mollusca. Columns of samples were collected from six sections and the assemblages they yielded are presented in the form of histograms, showing the changing vertical abundance of each species. The fauna is a remarkable mixture of diverse zoogeographical elements; its relations are with the Alpine area rather than with the Arctic. The ecological and climatic significance of the changes in the assemblages is discussed. During zone II, the assemblages increase in variety and certain relatively thermophilous species were able to spread widely, most notably the West European and Alpine snail Abida secale. The climate of zone III was probably more humid than that of zone I, and also less cold. In Sussex, due to the proximity of the open sea to the south-west, the climate of zone I may have been relatively milder than in Kent and Surrey; this is suggested by the appearance of thermophilous species perhaps 1000 years before their general expansion on the North Downs. Evidence is put forward from several sites for a minor climatic oscillation within zone I ; this is equated with the Bolling Oscillation (zone I b) of north-west Europe. The Late-glacial Period is the last for which there is evidence of active erosion in the Chalk landscape.
This paper describes the morphology of a small piece of the Chalk escarpment near Brook in east Kent, and reconstructs its history since the end of the Last Glaciation. The escarpment contains a ...number of steep-sided valleys, or coombes, with which are associated deposits of chalk debris, filling their bottoms and extending as fans over the Gault Clay plain beyond. Here the fans overlie radiocarbon-dated marsh deposits of zone II (10 000 to 8800 B.C.) of the Late-glacial Period. The debris fans were formed and the coombes were cut very largely during the succeeding zone III (8800 to 8300 B.C.). The fans are the products of frost-shattering, probably transported by a combination of niveo-fluvial action and the release of spring waters; intercalated seams of loess also occur. The molluscs and plants preserved in the Late-glacial deposits give a fairly detailed picture of local conditions. The later history of one of the coombes, the Devil’s Kneadingtrough, is reconstructed. The springs have effected virtually no erosion and have probably always emerged more or less in their present position. In the floor of the coombe the periglacial chalk rubbles of zone III are covered by Postglacial deposits, mainly hillwashes. They are oxidized and yield no pollen, but contain rich faunas of land Mollusca, which are presented in the form of histograms revealing changing local ecological and climatic conditions. During most of the Post-glacial Period, from the end of zone III until about the beginning of zone VIII, very little accumulation took place on the coombe floor. But below the springs there are marsh deposits which span much of this interval. They yield faunas of considerable zoogeographical interest. The approximate beginning of zone VIIa (Atlantic Period) is reflected by a calcareous tufa, which overlies a weathering horizon, and represents an increase in spring flow. Two clearance phases are deduced from the molluscan record. The first may have taken place at least as early as the Beaker Period (Late Neolithic/earliest Bronze Age); the second is probably of Iron Age ‘A’ date. In Iron Age times the subsoil was mobilized and a phase of rapid hillwashing began. As a result the valley floor became buried by humic chalk muds. The prime cause of this process was probably the beginning of intensive arable farming on the slopes above the coombe; a possible subsidiary factor may have been the Sub-Atlantic worsening of climate. The muds yield pottery ranging in date from Iron Age ‘Kentish first A’ (ca. 500 to ca. 300 B.C.) to Romano-British ware of the first or second centuries A.D. Evidence is put forward for a possible climatic oscillation from dry to wet taking place at about the time of Christ. In the later stages of cultivation, possibly in the Roman Era, the valley floor was ploughed and given its present-day form.
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