Effects of nonnormality on studies of morphological variation of a Rhabdomesine bryozoan, Streblotrypa (Streblascopora) prisca (Gabb and Horn)

13 p., 4 fig.http://paleo.ku.edu/contributions.htm

Paleoenvironmental Significance of Celleporaria (Bryozoa) from Modern and Tertiary Cool-water Carbonates of Southern Australia

Certain members of the bryozoan genus Celleporaria form large, erect colonies of hollow branches ( 10–30 cm tall and 1–3 cm diameter). These are common and conspicuous in Pleistocene and Cenozoic neritic strata of the southern margin of Australia. Most of these basins are characterized by decimeter-scale cycles through subtidal, heterozoan, cool-water, carbonate sediments. Several intervals are further characterized by pervasive celleporarid bryozoan thickets (10–30 cm thick), individual units of which can be traced for many tens of kilometers. Study of modern, live celleporarid bryozoans from the continental shelf of the Great Australian Bight (GAB), Pleistocene celleporarid mounds stranded below the shelf break of the Australian margin, and Miocene celleporarid thickets from the Murray and Torquay Basins indicates that Celleporaria thickets form under specific environmental conditions: i.e., low-energy or sub-swell wave-base settings, mud-silt substrate, medium-high mesotrophy, and moderate sedimentation rate. These conditions, however, are not site specific. They are manifested here in two different environmental settings: (1) a deep-water (>200 m), upwelling nutrient source, below the shelf break during extreme sea-level low stands; and (2) shallow-water (<50 m), terrestrial nutrient source, in embayments, far inland from the shelf break during relative high stands of sea-level. The celleporarid growth habit (species specific—not an ecomorph) allows their populations to proliferate in spatially restricted, transitional, geologically short-lived environments where most other bryozoans cannot cope. Their environmental sensitivity means that Celleporaria have great potential for paleoenvironmental analysis of Cenozoic carbonate Tethyan sediments

Threshold Effects of Food Concentration on the Skeletal Morphology of the Bryozoan Electra Pilosa (Linnaeus, 1767)

Many palaeontological studies rely heavily on characteristics of the preserved phenotype, i.e. the morphology of skeletal hard parts. Although the potential for environmental influences on the phenotype is expected, rarely is the magnitude of the effects quantifiable relative to genetic factors. The clonal/colonial body plan of Bryozoa allows for the partitioning of morphological variance into its genetic and environmental factors addressing the question of, ‘how much phenotypic variation is induced in a population by changing a single environmental factor?’ The effects of variation of food concentration on whole-colony growth rate and on zooid size/morphology can be profound in bryozoans. Here we test experimentally food effects on the skeletal phenotype of the bryozoan Electra pilosa (Linnaeus, 1767), an encrusting sheet-like bryozoan. A threshold effect was observed for the relationship between zooecium size and food concentration. Very low concentrations resulted in stunted colonies with small zooecia, but at low to intermediate concentrations a close relationship existed with zooecium size. Maximum zooecium size occurred at submaximal food concentration and submaximum zooecium size occurred at higher food concentrations. Previous studies that have reported no effect of food availability on zooecium size assessed food concentration effects at higher concentrations than were effective in the present study. In the absence of other factors, variation in zooecium size is minimal and unchanging at moderate to high food concentrations. Greater variation in zooecium size is expected at and below threshold food concentrations. We show that the preservable phenotype of these specimens subjected to controlled and induced environmental variation also records information with genetic significance

Bryozoan colonial growth forms as paleoenvironmental indicators: evaluation of methodology and predictive utility

Bryozoans have played a significant ecological role in many shallow marine benthic communities since the Ordovician and are important contributors to carbonate sediment production in many modern cool-water marine environments. Correlation between bryozoan colonial growth forms and environments in which the organisms lived allows for the application of growth forms as paleoenvironmental indicators. This can be done as either (1) a characterization of regional environmental or distributional data within a comprehensive study; or (2) as a predictive tool applied in an unknown setting using limited data. A number of workers have demonstrated this potential in biological, paleontological, and sedimentological studies. Growth-form distributions established independently from, and later compared to, environmental factors provide for the greatest predictive utility. Problems encountered in methodology need to be addressed before bryozoan colonial growth forms can achieve their full potential as paleoenvironmental indicators. Methodological problems include those associated with specimen abundance versus species richness, numeric versus volumetric frequency, relative versus absolute abundance, and changes within growth forms among localities versus changes among growth forms within localities (facies). A procedure is proposed that combines species richness and specimen abundance, as well as information about distributions within growth forms and within localities, into a single, comparable data set. An example is provided using bryozoans from the cool-water Lacepede Shelf of southern Australia

Complexity Generated by Iteration of Hierarchical Modules in Bryozoa

Growth in colonial organisms by iteration of modules inherently provides for an increase in available morpho-ecospace relative to their solitary relatives. Therefore, the interpretation of the functional or evolutionary significance of complexity within groups that exhibit modular growth may need to be considered under criteria modified from those used to interpret complexity in solitary organisms. Primary modules, corresponding to individuals, are the fundamental building blocks of a colonial organism. Groups of primary modules commonly form a second-order modular unit, such as a branch, which may then be iterated to form a more complex colony. Aspects of overall colony form, along with their implications for ecology and evolution, are reflected in second-order modular (structural) units to a far greater degree than by primary modular units (zooids). A colony generated by modular growth can be classified by identifying its second-order modular (structural) unit and then by characterizing the nature and relationships of these iterated units within the colony. This approach to classifying modular growth habits provides a standardized terminology and allows for direct comparison of a suite of functionally analogous character states among taxa with specific parameters of their ecology

Cool-Water Carbonate Production from Epizoic Bryozoans on Ephemeral Substrates

Bryozoan skeletons are a dominant constituent of cool-water carbonate sediments in the Cenozoic of southern Australia. The primary substrate on much of the modern continental shelf is loose sediment that is reworked intermittently to 200+ m water depth by storm waves. Availability of stable substrate is a limiting factor in the modern distribution of bryozoans in this setting. As a result, a significant proportion of the sedimentologically important modern bryozoans (30–250 m water depth) live attached to sessile, benthic invertebrate hosts that possess organic or spicular skeletons. Hosts such as hydroids, ascidian tunicates, sponges, soft worm tubes, octocorals, and other lightly-calcified and articulated bryozoans provide ephemeral substrates; after death, host skeletons disarticulate and decay, leaving little or no body fossil record. The calcareous sediments produced by these epizoic bryozoans from ephemeral substrates result in loose particles that rarely preserve substratal relationships, but potentially retain diagnostic basal attachment morphologies. Although the best known examples of epizoic carbonate production on ephemeral substrates are from the southern Australian margin, this may be an important phenomenon both globally and in the fossil record. Bryozoan sediment production from epizoans on ephemeral substrates would seem, however, to have a scant record prior to the Cretaceous

Stable Isotope Profiles Of Skeletal Carbonate Validate Annually-Produced Growth Checks In The Bryozoan Melicerita chathamensis From Snares Platform, New Zealand

The bryozoan Melicerita chathamensis Uttley and Bullivant, 1972 produces colonies that exhibit visible growth segments defined by narrow growth checks. If these growth checks are annual, then colony age, and seawater variations among seasons and years can be quantified. The purpose of the present study was to use stable isotope profiling to evaluate whether the segments between growth checks represent annual temperature cycles in this species. We applied three independent methods to determine colony age of six colonies from 168 m depth on the Snares Platform located south of New Zealand. First, each colony was X-rayed to determine the location of the growth checks based on skeletal density. Second, branch width was measured for each zooid generation along the growth axis to locate the growth checks. Third, we measured stable C and O isotope values along the colony axis. Branch width patterns corresponded broadly with X-ray patterns, suggesting colony ages of 1.5–7.5 yrs (mean: 4.0 yrs). d18O profiles suggested colony ages of 4.0–6.5 yrs (mean: 5.3 yrs). This species does precipitate its skeleton in isotopic equilibrium with seawater, such that annual growth checks correlate with cooler winter temperatures. A conceptual model is proposed for the annual growth cycle in this species. In the most complete colony, the d 18 O-derived temperatures correlated with inter-annual variations related to the El Niño–Southern Oscillation Index

The Oldest Hyolithids (Cambrian Series 2, Montezuman Stage) From The Iapetan Margin Of Laurentia

The recent description of the nevadioid trilobite Buenellus chilhoweensis Webster and Hageman, 2018 established the presence of early Cambrian Montezuman Stage (Cambrian Series 2, Stage 3) faunas in the Murray Shale of Chilhowee Mountain, Tennessee. The description recognized the oldest known age-diagnostic Cambrian trilobite from the Laurentian margin of the former Iapetus Ocean since Buenellus Blaker, 1988 is known otherwise only from the Sirius Passet Lagerstatte on the Innuitian margin of North Greenland. The bivalved arthropods Isoxys chilhoweanus Walcott, 1890 and Indota tennesseens is (Resser, 1938a) have also been described from the Murray Shale, but hyolithids appear to be the dominant body fossils in terms of diversity and abundance. Although poorly preserved, the hyolithids occurring together with Buenellus chilhoweensis are described to improve understanding of the Murray Shale biota. The hyolith assemblages of the Murray Shale and Sirius Passet Lagerstatte are not closely similar, although the poor preservation of both hinders comparison

Microevolutionary Implications Of Clinal Variation In The Paleozoic Bryozoan Streblotrypa

Ideally, studies of microevolution, including those of patterns and rates of speciation, need to account for features of geographic and ecophenotypic morphologic variation. These factors can provide primary sources of variation for evolution to act upon. They can also, however, produce variation among characters that are not directly related to speciation events, thus confounding recognition of larger evolutionary patterns. Because of limitations of geologic settings, features of morphologic variation associated with geographic and ecophenotypic variation are difficult to constrain and are often neglected. Consequences of ignoring potential geographic variation in microevolutionary studies are varied. The likelihood of observing stasis or gradation may increase or decrease depending on initial assumptions. A series of tests that place reasonable limits on potential errors, however, can strengthen conclusions from empirical microevolutionary studies. Late Paleozoic cyclothems of the North American Midcontinent provide an excellent opportunity to study patterns of microevolution and concomitant ecophenotypic and geographic variation