The evolution of modern corals and their early history

Scleractinians are a group of calcified anthozoan corals, many of which populate shallow-water tropical to subtropical reefs. Most of these corals calcify rapidly and their success on reefs is related to a symbiotic association with zooxanthellae. These one-celled algal symbionts live in the endodermal tissues of their coral host and are thought responsible for promoting rapid calcification. The evolutionary significance of this symbiosis and the implications it holds for explaining the success of corals is of paramount importance. Scleractinia stands out as one of the few orders of calcified metazoans that arose in Triassic time, long after a greater proliferation of calcified metazoan orders in the Paleozoic. The origin of this coral group, so important in reefs of today, has remained an unsolved problem in paleontology. The idea that Scleractinia evolved from older Paleozoic rugose corals that somehow survived the Permian mass extinction persists among some schools of thought. Paleozoic scleractiniamorphs also have been presented as possible ancestors. The paleontological record shows the first appearance of fossils currently classified within the order Scleractinia to be in the Middle Triassic. These earliest Scleractinia provide a picture of unexpectedly robust taxonomic diversity and high colony integration. Results from molecular biology support a polyphyletic evolution for living Scleractinia and the molecular clock, calibrated against the fossil record, suggests that two major groups of ancestors could extend back to late Paleozoic time. The idea that Scleractinia were derived from soft-bodied, “anemone-like” ancestors that survived the Permian mass extinction, has become a widely considered hypothesis. The 14-million year Mesozoic coral gap stands as a fundamental obstacle to verification of many of these ideas. However, this obstacle is not a barrier for derivation of scleractinians from anemone-like, soft-bodied ancestors. The hypothesis of the ephemeral, “naked coral”, presents the greatest potential for solution of the enigma of the origin of scleractinians. It states that different groups of soft-bodied, unrelated “anemone-like” anthozoans gave rise to various calcified scleractinian-like corals through aragonitic biomineralization. Although there is evidence for this phenomenon being more universal in the mid-Triassic interval, following a lengthy Early Triassic post-extinction perturbation, it appears to have occurred at least three other times prior to this interval. This idea suggests that, because of ephemeral characteristics, the skeleton does not represent a clade of zoantharian evolution but instead represents a grade of organization. In the fossil record, skeletons may have appeared and disappeared at different times as some clades reverted to soft-bodied existence and these phenomena could account for notable gaps in the taxonomic and fossil record. A fuller understanding and possible solution to the problem of the origin of modern corals may be forthcoming. However, it will require synthesis of diverse kinds of data and an integration of findings from paleobiology, stratigraphy, molecular biology, carbonate geochemistry, biochemistry and invertebrate physiology.     

Remote Sensing Of Three-Dimensional Winds With Elastic Lidar: Explanation Of Maximum Cross-Correlation Method

Maximum cross-correlation provides a method to remotely determine highly resolved three-dimensional fields of horizontal winds with elastic lidar throughout large volumes of the planetary boundary layer (PBL). This paper details the technique and shows comparisons between elastic lidar winds, remotely sensed laser Doppler velocimeter (LDV) wind profiles, and radiosonde winds. Radiosonde wind data were acquired at Barcelona, Spain, during the Barcelona Air-Quality Initiative (1992), and the LDV wind data were acquired at Sunland Park, New Mexico (N.M.), during the Border Area Air-Quality Study (1994). Comparisons show good agreement between the different instruments, and demonstrate the method useful for air pollution management at the local/regional scale. Elastic lidar winds could thus offer insight into aerosol and pollution transport within the PBL. Lidar wind fields might also be used to nudge or improve initialization and evaluation of atmospheric meteorological models.     

Measurements Of Thermal Updraft Intensity Over Complex Terrain Using American White Pelicans And A Simple Boundary-Layer Forecast Model

An examination of boundary-layer meteorological and avian aerodynamic theories suggests that soaring birds can be used to measure the magnitude of vertical air motions within the boundary layer. These theories are applied to obtain mixed-layer normalized thermal updraft intensity over both flat and complex terrain from the climb rates of soaring American white pelicans and from diagnostic boundary-layer model-produced estimates of the boundary-layer depth z_i and the convective velocity scale w_*. Comparison of the flatland data with the profiles of normalized updraft velocity obtained from previous studies reveals that the pelican-derived measurements of thermal updraft intensity are in close agreement with those obtained using traditional research aircraft and large eddy simulation (LES) in the height range of 0.2 to 0.8 z_i. Given the success of this method, the profiles of thermal vertical velocity over the flatland and the nearby mountains are compared. This comparison shows that these profiles are statistically indistinguishable over this height range, indicating that the profile for thermal updraft intensity varies little over this sample of complex terrain. These observations support the findings of a recent LES study that explored the turbulent structure of the boundary layer using a range of terrain specifications. For terrain similar in scale to that encountered in this study, results of the LES suggest that the terrain caused less than an 11% variation in the standard deviation of vertical velocity.     

Sequence stratigraphy of a Middle to Upper Ordovician foreland succession (Ottawa Embayment,central Canada): Evidence for tectonic control on sequence architecture along southern Laurentia

The Middle to Upper Ordovician foreland succession of the Ottawa Embayment in central Canada is divided into nine transgressive‐regressive sequences that defines net deepening of a platform succession over ~15 m.y. from peritidal to outer ramp settings, then a return to peritidal conditions over ~3 m.y. related to basin filling by orogen‐derived siliciclastics. With a backdrop of net eustatic rise through the Middle to Late Ordovician, there are several different expressions of structural influence on sequence development in the embayment. During the Middle Ordovician (Darriwilian), foreland‐basin initiation was marked by regional onlap with abundant synsedimentary deformation across a faulted trailing‐margin platform interior; subsequent craton‐interior uplift resulted in voluminous influx of siliciclastics contemporary with local structurally influenced local channelization; then, a formation of a platform‐interior shale basin defines continued intrabasin tectonism. During the Late Ordovician (Sandbian, early Katian), structural influence was superimposed on sea‐level rise as indicated by renewed local development of a platform‐interior shale basin; differential subsidence and thickness variation of platform carbonate successions; abrupt deepening across shallow‐water shoal facies; and, micrograben development coincident with foreland‐platform drowning. These stratigraphic patterns are far‐field expressions of distal orogen development amplified in the platform interior through basement reactivation along an inherited buried Precambrian fault system. Comparison of Upper Ordovician (Sandbian‐lower Katian) sequence stratigraphy in the Ottawa Embayment with eustatic frameworks defined for the Appalachian Basin reveals greater regional variation associated with Sandbian sequences compared to regional commonality in base level through the early Katian.     

Modelling subsurface flow conditions in a salinized catchment in south‐western Australia,with a view to improving management practices

Finite element modelling of the saturated–unsaturated surface–subsurface flow mechanisms operative in a small salinized catchment in south‐western Australia was used to help define the flow system and explain the causes of waterlogging and salinization there. Data available at the site from a previous study were used to obtain a first approximation to the flow system. Altering the properties of some of the strata gave a closer calibration. It was found that the modelled saturated hydraulic conductivity of the B horizon in the duplex soil zone needed to be at least an order of magnitude lower than that measured in order to reproduce the perching conditions observed in the field. Also, the model indicated the influence of a doleritic dyke, whose presence was confirmed by field measurement. Our analysis showed that there were two main flow systems operating in the hillslope. The first, and most dominant, was the recharge occurring through the upslope gradational soil zone and percolating down to both the deeply weathered regolith and the basal aquifer. The second flow system is an unsaturated flow system operating in the high permeability A horizon in the downslope duplex soil zone. The first system is primarily responsible for the saline seepage zone in the valley bottom. The second contributes to the waterlogging and perching occurring upslope of the seepage zone. Vertical flow through the higher permeability B horizon in the gradational soil zone in the upper slopes is a major contributor of recharge. Recharge by flow through macropores occurs where, but only where, perched aquifers develop and allow the macropores to be activated. Areas with perched aquifers occurred in downslope locations and near a doleritic dyke located upslope. Thus, the area where macropore recharge occurred was not large. The recharge rate required to maintain the piezometric levels at present values is only about 30 mm/yr (about 5% of the annual rainfall). The piezometric levels under the upper part of the catchment varied greatly with only small changes in recharge rate. A 50% reduction in recharge rate had the effect of reducing the length of the seepage zone at the end of winter by 40%. Changes in recharge rate had little effect on the extent of the perched aquifer at the end of winter. Deep‐rooted perennial forages, shrubs or trees on the gradational soil zone in the upper part of the catchment and on the zones upslope of geological barriers to flow would be required to reduce the recharge and to allow for rehabilitation of the saline valley floor. Waterlogging associated with the perched water table in the bottom part of the catchment would be best addressed by tree plantations located just upslope of the salinized zone in the valley floor. Copyright © 1999 John Wiley & Sons, Ltd.