Bryozoan provinces and patterns of generic evolution and extinction in the Late Ordovician of North America

The Late Ordovician bryozoan genera of central and southeastern North America are geographically distributed in three biotic provinces, separated by boundaries reflecting major lithofacies differences. The central Cincinnati Province contains most of the North American endemic genera, and represents a narrow ecological zone separating the clastic wedges of the marginal Reedsville-Lorraine Province from the cratonic carbonate platform of the Red River-Stony Mountain Province. The provinces provided major life zones, or biomes, for each of the five bryozoan orders. Genera comprising the provinces differed as well in morphologic complexity, geochronologic survivorship, tiering, endemism and eurytopy. Regions on either side of the Cincinnati Province were dominated by inferred immigrants from Baltoscandia. Al-logenic provincial succession produced time-averaged mixed faunas in regions near the provincial boundaries. Although most generic originations took place within the Cincinnati Province, evolutionary novelties are associated with the Reedsville-Lorraine Province. The loss of the diverse Cincinnati Province, connected with global cooling and a eustatic lowering of sea level, may have been a chief factor in the Late Ordovician extinction of bryozoan genera. Genera from the Red River-Stony Mountain Province differentially survived into the Silurian.     

AUTECOLOGY AND THE FILLING OF ECOSPACE: KEY METAZOAN RADIATIONS

Abstract:  All possible combinations of six tiering positions in relation to the substratum/water interface, six motility levels and six feeding strategies define a complete theoretical ecospace of 216 potential modes of life for marine animals. The number of modes of life actually utilized specifies realized ecospace. Owing to constraints of effectiveness and efficiency the modern marine fauna utilizes only about half the potential number of modes of life, two-thirds of which (62 of 92) are utilized by animals with readily preserved, mineralized hard parts. Realized ecospace has increased markedly since the early evolution of animal ecosystems. The Ediacaran fauna utilized at most 12 modes of life, with just two practised by skeletal organisms. A total of 30 modes of life are recorded in the Early and Middle Cambrian, 19 of which were utilized by skeletal organisms. The other 11 are documented from soft-bodied animals preserved in the Chengjiang and Burgess Shale Konservat-Lagerstätten. The number of modes of life utilized by skeletal organisms increased by more than 50 per cent during the Ordovician radiation to a Late Ordovician total of 30. Between the Late Ordovician and the Recent the number of utilized modes of life has doubled again. The autecological and taxonomic diversity histories of the marine metazoa appear to be broadly parallel, and future studies of theoretical ecospace utilization should provide more detailed tests of pattern and process in the ecological history of the metazoa.     

Infaunal or semi-infaunal bellerophont gastropods: analysis of Euphemites and functionally related taxa

Burrowing and plowing gastropods may have existed prior to the Mesozoic, contrary to the conservative tendency of categorizing Paleozoic archaeogastropods as herbivorous hardground dwellers. We propose that Euphemites and other bellerophontiform molluscs such as Praematuratroph and Beyrichidiscus make functional and paleoecological sense only if reconstructed with an internal shell adapted to an infaunal mode of life. Attributes diagnostic of a probable infaunal mode of life in fossil bellerophontiform molluscs include: (1) lack of apertural flare; (2) restricted aperture; (3) low rate of whorl expansion; (4) semi-radial aperture; (5) extensive secondary shell deposits; (6) absence of extensive collabral ornament; (7) dorsolateral or umbilical furrows; (8) color patterns indicative of mantle extension and retraction capabilities; (9) absence of shell breakage and repair; and (10) associated lithologies indicative of penetrable substrates such as mudstones. Niche diversity in the bellerophontacean molluscs was probably much greater than previously suspected. This is entirely consistent with their long geologic range. Most bellerophontaceans, particularly the Late Paleozoic forms, were probably either 'nestling' or semi-infaunal with the apertural margins enwrapped by mantle or mantle and foot. As such they may have been particularly susceptible to predation during the postulated Mesozoic increase in number of infaunal predators- a factor which may have been contributory to the bellerophontacean demise.     

Influences of predatory polychaetes and epibenthic predators on the structure of a soft-bottom community in a maine estuary

Field manipulations were used to determine the importance of two predatory polychaetes, Nereis virens Sars and Glycera dibranchiata Ehlers, and epibenthic predators in structuring an intertidal soft-bottom community in Maine. Epibenthic predators were excluded from portions of the soft bottom using cages which also enclosed elevated densities of the predatory polychaetes. The experiments ran 10 wk and 20 wk beginning in June 1979. Exclusion of epibenthic predators had no effect on infaunal densities after 10 wk but produced ≈ 1.5-fold increase in total density after 20 wk. Since gulls (Larus spp.) avoided all cages, including those not designed to exclude epibenthic predators, the effect of gull predation on infaunal abundances was not tested using exclusion cages. Crabs, Carcinus maenas (Linnaeus) and Cancer irroratus Say were observed in cages not designed to exclude predators.Densities of Nephtys incisa Malmgren, Polydora ligni Webster, Streblospio benedicti Webster, Scoloplos robustus Verrill, phyllodocids, and bivalves were highest in cages containing elevated Glycera dibranchiata density and lowest in cages containing elevated Nereis virens density. N. virens was the only taxon whose abundance was reduced in the presence of Glycera dibranchiata which may account for high infaunal densities in the G. dibranchiata treatment. Laboratory experiments demonstrated that G. dibranchiata are capable of preying on Nereis virens. Additional field experiments revealed that the presence of N. Virens reduced the abundance of some taxa within the first 10 days of colonization. N. virens may have reduced infaunal densities by predation and/or disturbance at the sediment surface. These results suggest that complex interactions within the infauna are important in structuring marine soft-bottom communities.     

遗迹化石Chondrites的指相意义和阶层分布

介绍和评述Chondrites的基本特征和流行的成因解释 ,记述最近发现于美国犹他州中西部北美下 中奥陶统之交层型剖面上的Chondrites遗迹组构的不寻常特征和属性 :细小的Chondrites被所有与之共生的遗迹化石交切 ,形成于水与沉积物界面附近缺氧的早期软底质中 ,Chondrites是形成最早和最浅的遗迹阶层。流行的遗迹相和遗迹阶层模式的不足是 :前者在操作上 ,简单地将遗迹化石及其组合处理为同沉积或准同沉积的物理沉积构造 ,忽略了造迹活动的多阶段和多阶层的特点 ;后者的资料基础仅依据中生代以来的地层记录 ,对重大生物 环境事件和造迹生物生态习性演化对遗迹阶层形成和分布的影响注重不够。指出Chondrites的阶层分布和对古氧相的示踪在古生代与中生代有重要差别     

Retinal mosaics of the principal eyes of some jumping spiders (Salticidae: Araneae): Adaptations for high visual acuity

Summary Retinal mosaics of the four tiers of receptors (layers I–IV) in the principal eyes of two species of salticid spider known to exhibit high visual acuity are described at the ultrastructural level. The four tiers of receptors have been previously classified byLand (1969 a) andBlest et al. (1981). Only the tier (layer I) farthest from the dioptric system has a mosaic quality capable of sustaining the visual discriminations that have been demonstrated by behavioural experiments. More distal layers are of poor to negligible mosaic organization.It is shown that rhabdoms of layer I are disposed with respect to their surrounds in such a way as to enable them to act as light-guides: the ultrastructure of layer I receptors appears to be designed to minimize optical cross-talk between receptors. The cytoplasm of the more distal layers is packed with mitochondria which have been shown to equilibrate the refractive indices of the rhabdoms and the material that surrounds them. It is argued that the mitochondria show no evidence of metabolic activity, and that an optical role may be their only one. Their effect will be to minimize the degradation of images received by layer I.     

Retinal mosaics of a primitive jumping spider,Spartaeus (Salticidae: Araneae): A transition between principal retinae serving low and high spatial acuities

Summary Previous papers (Blest andPrice 1984,Blest andSigmund 1984) have described the receptor mosaics in the tiered principal retinae of jumping spiders with high and low visual acuities, respectively. The latter paper offers a phylogenetic model for the later stages in the evolution of high acuity retinae in which layer I, farthest from the dioptrics, is alone required to sustain fine visual discriminations. Nevertheless, current evidence does not establish that the low acuity principal retinae of primitive jumping spiders were modified to yield the high acuity retinae of advanced forms: each might have evolved independently from a common precursor.We now show that the principal retina ofSpartaeus (the type genus of a primitive subfamily, theSpartaeinae) is a true intermediate between the low acuity retina ofYaginumanis and the high acuity retina ofPortia (also in theSpartaeinae).InSpartaeus, foveal layer I receptors at the anatomical outer side of the retina each bear a single rhabdomere designed as a light guide. Inner layer I receptors each have two rhabdomeres, and rhabdomeres of horizontally adjacent receptors are contiguous, so that optical pooling limits their acuity. Horizontal staircasing of the tips of layer I receptors was shown byBlest et al. (1981) to cause outer receptors to receive focused images from remote planes in object space, and inner receptors from nearer planes. In the high-acuity retina ofPortia all foveal layer I receptors are light-guides, and in the low-acuity retina ofYaginumanis none are. Layer I inSpartaeus, therefore, is a true intermediate step in the evolution of a high-acuity retina, and the local region of the foveal layer I mosaic modified to that end is concerned, appropriately, with the resolution of distant objects.The layer II mosaic is degraded, although less so than inPortia. Layer III is conservative throughout theSalticidae. Foveal layer IV exhibits greater homogeneity inSpartaeus than in other forms.     

MANAGING THE ST. LUCIA ESTUARY—MESSAGES FROM THE MUD

Summary

The infaunal macrobenthic community of the St Lucia estuary has been exposed to natural disturbance in the form of salinity fluctuations arising from floods and hypersaline conditions, and human-induced interference arising from dredging and beam trawling. The responses of the benthos to these disturbances have been recorded between 1983 and 1994 and are discussed in the light of management options for the estuary. A simple benthic sampling strategy is suggested to monitor the important benthic species.     

Intrastratal trace fossil zonation,Cretaceous flysch of northern California

Dissection of fine‐grained turbidites within the Franciscan complex at Trinidad Harbor reveals intrastratal zonation of biogenic structures. The pre‐depositional association comprises only three ichnotaxa (preserved in convex hyporelief) and is less diverse than the tiered post‐depositional association (comprised by at least seven ichnotaxa preserved in full relief). The latter association is dominated by Melatercichnus (n. ichnogen.). Post‐depositional trace‐makers exploited abundant plant detritus deposited with the turbidite sand. Temporal alternation of trace fossil associations resulted from the infrequent, but catastrophic, disturbance of pre‐turbidite communities and local replacement by assemblies of opportunistic organisms. Although probably short‐lived at specific sites, the post‐turbidite infauna was tiered, enabling a variety of deposit feeders to harvest phytodetritus of varied size or density simultaneously at different depths within newly deposited turbidite sand layers.     

Ichnology of the Winnipeg Formation,southeast Saskatchewan: a glimpse into the marine infaunal ecology of the Great Ordovician Biodiversification Event

The ichnology of the Middle Ordovician Winnipeg Formation has been analysed based on the study of cores from five wells drilled in southeast Saskatchewan (Canada). Six sedimentary facies, ranging from upper shoreface to lower offshore settings in a shallow‐marine environment, have been characterized. Ichnological attributes are consistent with those in currently proposed models for shallow‐marine wave‐dominated settings, but ichnodiversity is lower than in post‐Palaeozoic settings. Low ichnodiversity in the Winnipeg Formation most likely reflects evolutionary factors rather than environmental controls. Interestingly, low‐energy, distal deposits of the Winnipeg Formation display intense degree of bioturbation, reflecting a well‐developed mixed layer and underscoring the importance of the Great Ordovician Biodiversification Event in terms of sediment mixing.