Bryozoan Growth Research Articles

BRYOZOAN MUD‐MOUNDS FROM THE UPPER ORDOVICIAN JIFARAH (DJEFFARA) FORMATION OF TRIPOLITANIA, NORTH‐WEST LIBYA

Abstract: A bryozoan fauna from carbonate mud‐mounds is described from subsurface well cores from the Upper Ordovician (Lower Ashgill) Jifarah (Djeffara) Formation of Tripolitania, north‐west Libya. Among a diverse assemblage dominated by trepostomes, nine species of bryozoans are identified, including Jifarahpora libyensis gen. et sp. nov. Delicate and robust branching, encrusting and nodular bryozoan growth forms are all common. The bryozoan‐rich limestones are mostly mudstones and wackestones, with bafflestone and floatstone textures, but the mounds apparently lack organic framework and microbial fabrics. Regional geophysical data indicate rapid thickness changes between wells, where mound complexes locally up to 100 m thick had limited topographic relief over the surrounding sea floor. The mounds formed in a high‐latitude, cool‐water carbonate belt that extended widely across the northern margin of Gondwana. Quaternary analogues from the Great Australian Bight suggest that these early Ashgill mounds may have developed in slope environments during an episode of glacial lowstand that preceded the late Ashgill, Hirnantian glacial event.

Growth rate and its variability in erect Antarctic bryozoans

Climate is altering rapidly in parts of the Arctic and Antarctic but we know little about how marine organisms are responding to, or might respond to such changes. Knowledge of within-taxon variability is the vital context (currently missing) to interpretation of environmental signals. We investigated growth in six species and three genera of erect Antarctic bryozoans, an ideal model taxon to investigate such response. Cellarinella margueritae, C. nodulata, C. rogickae, C. watersi, Melicerita obliqua and Stomhypselosaria watersi, extended 3.4, 5.2, 4.6, 4.1, 4.9 and 4.5 mm year−1 and synthesised 24, 55, 45, 176, 34 and 46 mg CaCO3 year−1, respectively. The maximum ages of these species ranged from 11 to 15 years except M. obliqua, which reached 32 years. This is the first investigation of growth rates of closely related Antarctic invertebrate species and reports the slowest growth rates of bryozoans known from anywhere to date. Our data coupled with that from literature shows that Antarctic bryozoan growth varies <<101 between species, 101 between genera, 102 between morphologies and is ∼101 slower than in tropical or temperate regions. However, within encrusting types the slowest growing species grow at similar rates from poles to tropics. Age was a strong confounding factor across our Antarctic study species but age-standardised data showed a possible decline in annual growth from 1992 to 2003. We identify several factors increasing this environmental signal strength, including (1) the importance of generic (though not necessarily species) identification and (2) use of dry-mass or ash-free dry-mass as the measures of growth.

Slow growth of Antarctic bryozoans increases over 20 years and is anomalously high in 2003

Some organisms are particularly appropriate models for investigation of variability in time and space for given environments. The erect bryozoan Cellarinella nutti, an endemic Antarctic species, is one such organism: it is extremely abundant, occurs from the polar front to the Antarctic continental edge, and preserves a clear macroscopic environmental record in its skeleton (growth check lines). We studied variability in the growth of 93 C. nutti individuals at depths between 247 and 414 m, at sites 1, 10, 100 and >1000 km apart in the Weddell Sea, Antarctica. Trawled C. nutti varied from 5 to 23 yr in age, and nearly half the colonies had grown asexually from fragments. We measured the annual growth increments, which ranged from ~18 zooids (8.6 mg dry mass, 0.8 mg ash-free dry mass; Age 1) to ~130 zooids (43 mg dry mass, 2.48 mg ash-free dry mass; Age 20). At 9 yr (the modal age), each C. nutti individual had precipitated >182 mg CaCO3, and by 20 yr nearly 580 mg. C. nutti grows slowly compared with other erect bryozoans, even those in the Antarctic. We found colony and site to be insignificant factors in its growth, despite the large range of distances between sites, but year was highly significant. We found a non-linear increase in growth spanning the last 2 decades that has no obvious relation to El Nino Southern Oscillation (ENSO)-associated environmental fluctuations. In particular we found that growth in 2003 was greater than in any other year measured by a factor of 2. Given its abundance, longevity, ubiquity in Antarctic waters, insignificant spatial variability in growth, but distinct annual variability, C. nutti may prove to be an important species to investigate whether benthic organisms, as well as krill, salps and fur seals, are beginning to develop major climate-related changes in phenology.

Fossils, sediments, tectonics

Tectonic displacement and small-scale tsunamis apparently affected deposition of the Kolymbia limestone, Cape Vagia, Rhodes, Eastern Mediterranean. Coarse beds interrupt the sequential build-up of this Pliocene–Pleistocene bryomol limestone. Celleporid bryozoans, bivalves, and brachiopods dominate these beds. The palaeoecology of the thicket-forming Celleporaria palmata is re-evaluated and subsequently revised. The limestone comprises two parasequences in a transgressive systems tract, and deposition occurred at palaeodepths between 30 and 120 m. At intervals, tectonic movements lowered relative sea level and sent slumps of shallow-water fauna downslope. The depositional history was validated using independent sets of data: sediment structure and grain size, palaeobathymetry using bryozoan growth forms and occurrences of modern representatives of bryozoans and other taxa, basin configuration, and regional tectonics. Concordance of these lines of evidence provides a means of evaluating confidence in palaeoenvironmental inferences.

Oceanography, sedimentology and acoustic mapping of a bryomol carbonate factory in the northern Gulf of California, Mexico

Abstract Bryomol carbonates, composed of bryozoans and molluscs, are found in non-tropical shelf and upper slope settings where they are sensitive indicators of oceanographic conditions. Few modern bryomol carbonate settings have been investigated to date, despite their importance in the Phanerozoic rock record. Furthermore, no detailed facies mapping and long-term oceanographic observations have been undertaken in modern bryomol settings, even though this is important for accurately interpreting facies, climate and oceanography from fossil bryomol carbonates. A bryomol carbonate factory on the western margin of the northern Gulf of California, Mexico was selected for an integrated high-resolution in situ oceanographic monitoring, acoustic seafloor mapping, sediment and bryozoan growth morphology study. Molluscan- (28%), bryozoan- (25%) and barnacle- (14%) dominated carbonate production takes place under normal saline warm-temperate eutrophic conditions, with average near sea surface temperatures of 20°C. Even though temperatures are unusually warm for the formation of bryomol carbonates, they develop as a result of prevailing eutrophic conditions (average chlorophyll- a contents of 2.2 mg chl- a m −3 ). Eutrophic conditions provide ample food to heterotrophic calcifiers and largely exclude faster-growing phototrophic organisms by drastically restricting the depth of the euphotic zone and, therefore, water clarity. Thus, the presence of high amounts of nutrients can generate cool-water-type carbonate assemblages at temperatures where a warm-water association would be expected.

Rhodalgal/bryomol assemblages in temperate-type carbonate, channelized depositional systems: the Early Miocene of the Sarcidano area (Sardinia, Italy)

Abstract During Aquitanian-Burdigalian times, temperate-type carbonate-siliciclastic successions were deposited in basins located on the graben and half-graben along the Oligo-Miocene Sardinia Rift Basin (Sardinia, Italy). In one of these basins, the Isili Basin, rhodalgal/bryomol limestone was deposited in a temperate-type carbonate depositional system in which a shallow carbonate factory, a marginal tributary belt and a main channel were identified. In this paper, the Isili Limestone is described in terms of biogenic components and taphonomic characterization. To understand the palaeoecology of the depositional settings and the related palaeoceanographic controls, the coralline algal assemblages were differentiated according to coralline growth forms, rhodolith shapes, rhodolith inner structure and taxonomic composition. In addition, a first analysis was carried out in order to discriminate among the different bryozoan and mollusc assemblages present in different localities corresponding to different palaeoecological settings. The results obtained provide fundamental support for palaeoenvironmental reconstruction that combines palaeobiological analyses (i.e. rhodolith characteristics, bryozoan growth forms, taphonomic features) with previous physical (sedimentological and geometrical) observations. This allows a more detailed reconstruction of the complex relationships between the different sectors of the sedimentary system: the carbonate factory, the smaller erosive tributary channels and the deeper main channel.

Late Pleistocene bryozoan reef mounds of the Great Australian Bight: Isotope stratigraphy and benthic foraminiferal record

Cores from Sites 1129, 1131, and 1132 (Ocean Drilling Program (ODP) Leg 182) on the uppermost slope at the edge of the continental shelf in the Great Australian Bight reveal the existence of upper Pleistocene bryozoan reef mounds, previously only detected on seismic lines. Benthic foraminiferal oxygen isotope data for the last 450,000 years indicate that bryozoan reef mounds predominantly accumulated during periods of lower sea level and colder climate since stage 8 at Sites 1129 and 1132 and since stage 4 at the deeper Site 1131. During glacials and interstadials (stages 2–8) the combination of lowered sea level, increased upwelling, and absence of the Leeuwin Current probably led to an enhanced carbon flux at the seafloor that favored prolific bryozoan growth and mound formation at Site 1132. At Site 1129, higher temperatures and downwelling appear to have inhibited the full development of bryozoan mounds during stages 2–4. During that time, favorable hydrographic conditions for the growth of bryozoan mounds shifted downslope from Site 1129 to Site 1131. Superimposed on these glacial‐interglacial fluctuations is a distinct long‐term paleoceanographic change. Prior to stage 8, benthic foraminiferal assemblages indicate low carbon flux to the seafloor, and bryozoan mounds, although present closer inshore, did not accumulate significantly at Sites 1129 and 1132, even during glacials. Our results show that the interplay of sea level change (eustatic and local, linked to platform progradation), glacial‐interglacial carbon flux fluctuations (linked to local hydrographic variations), and possibly long‐term climatic change strongly influenced the evolution of the Great Australian Bight carbonate margin during the late Pleistocene.

Bryozoan growth mirrors contrasting seasonal regimes across the Isthmus of Panama

Two highly environmentally contrasting localities on either side of the Isthmus of Panama were chosen to investigate the response of zooid size to differing seasonal regimes within colonies of cupuladriid bryozoans. The Bocas del Toro region in the southern Caribbean experiences a very low mean annual range of temperature (MART) while, as a result of seasonal upwelling, the Gulf of Panama in the eastern Pacific experiences a high MART. Profiles of zooid size, from ancestrula to colony margins, were made on colonies from both localities. Results reveal that in colonies from the Gulf of Panama zooid size varies cyclically, while in colonies from the Bocas del Toro region no strong trends are observed, hence paralleling the respective seasonal temperature regimes. These data support previous evidence that zooid size varies inversely with temperature. We propose that given good taxonomic understanding, profiling zooid size in this way could provide important information on aspects of life histories in both fossil and Recent cupuladriids. We were also able to show that the amount of variation in zooid size within cupuladriid colonies correlates well with the MART which the colony experiences. This approach may be used to provide data on absolute MARTs over geological time by analysing zooid sizes in fossil cupuladriids. The abundance of cupuladriids in the Neogene of Central America suggests that this technique could provide important corroborative data on regional environmental changes associated with the closure of the Central American Isthmus.

Rhodalgal-bryomol temperate carbonates from the Apulian shelf (Southeastern Italy), relict and modern deposits on a current dominated shelf

Mound-shaped algal banks and associated bioclastic deposits have been investigated on the Apulian shelf, SE Italy, off Capo d’Otranto and Santa Maria di Leuca within a bathymetric range of 53–60m water depth. The bioclastic deposits form different sand-gravel skeletal assemblages; each one characterised by different associations and occurrence of prevailing organisms in relation to substratum types. These deposits lie on a lee shore and form rhodalgal and bryomol relict-modern deposits which originate from the banks’ erosion by means of bottom currents and storms. These play a significant role in reworking bottom preventing colonization by a sessile stabilizing bottom community, and mobile deposits prevent large encrustations. Sediments are thus characterised by a variable taphonomical preservation state and time-averaging as shown by the occurrence of abraded bryozoan growth forms. The banks, sub-circular in shape, display a maximum diameter of 70 m and 1–2m relief. Their position on the shelf coincides with the present day 50 m isobath and it is assumed to correspond to a paleoshore such as that which formed in the Holocene during the transgression following the last ice age. Living counterparts of these banks occur today north of the study area at 10–30 m water depth.

Surficial Sediments of the Great Australian Bight: Facies Dynamics and Oceanography on a Vast Cool-Water Carbonate Shelf

ABSTRACT The Great Australian Bight (GAB), the largest sector of the southern Australia continental margin, is a site of cool-water carbonate sedimentation throughout, ranging from locally warm-temperate inboard to cool-temperate outboard. Surficial sediments are a mixture of calcareous Pleistocene skeletal and lithic intraclasts (relict grains), and Holocene biofragments, with minor amounts of quartz inboard. The inner is an area of abundant macrophytes and seagrasses, active carbonate sediment production and accumulation, and little relict sediment. The huge middle portion is a shaved shelf with active sediment winnowing and mostly relict sediment. The outer and upper slope is a variably productive sediment factory characterized by prolific calcareous epibenthic growth on hard substrate subaqueous islands shedding particles into surrounding sands and muds. Patterns of Holocene sedimentation are linked to modern oceanographic parameters in this high-energy setting characterized by overall downwelling. Prolific rhodoliths occur on the NW inner shelf, where shallow summer waters are the warmest in the GAB. These warm, saline, nutrient-depleted waters then drift eastward across the shelf, suppressing heterozoan carbonate production on the central and eastern mid-shelf. This arrested production in the eastern GAB is countered locally by summer coastal upwelling along western Eyre Peninsula, with bryozoan-rich sediment extending well inboard onto the mid-shelf. The outer and upper slope is an area of prolific bryozoan growth, likely linked to upwelling, except in the central GAB, a region of year-round downwelling, where the area is one of off-shelf fine sediment transport and carbonate mud deposition. These patterns, in the central GAB at least, are present in the underlying Holocene and Pleistocene, suggesting that the general modern oceanographic dynamics and resultant carbonate sedimentation have persisted throughout the Quaternary.

Quaternary bryozoan reef mounds in cool-water, upper slope environments: Great Australian Bight

Bryozoan reef mounds are common features in the geological record, occurring within mid-ramp, slope paleoenvironments, especially in Paleozoic carbonate successions, but until now have not been recorded from the modern ocean. Recent scientific drilling in the Great Australian Bight (Ocean Drilling Program Leg 182) has confirmed the existence of shallow subsurface bryozoan reef mounds in modern water depths of 200–350 m. These structures have as much as 65 m of synoptic relief, and occur both as single mounds and as mound complexes. They are unlithified, have a floatstone texture, and are rich in delicate branching, encrusting and/or nodular-arborescent, flat-robust branching, fenestrate, and articulated zooidal bryozoan growth forms. The muddy matrix is composed of foraminifers, serpulids, fecal pellets, irregular bioclasts, sponge spicules, and calcareous nannofossils. The 14C accelerator mass spectrometry dates of 26.6–35.1 ka indicate that the most recent mounds, the tops of which are 7–10 m below the modern seafloor, flourished during the last glacial lowstand but perished during transgressive sea-level rise. This history reflects changing oceanographic current patterns; strong upwelling during lowstands, and reduced upwelling and lowered trophic resources during highstands. Large specimens of benthic foraminifers restricted to the mounds confirm overall mesotrophic growth conditions. The mounds are similar in geometry, scale, general composition, and paleoenvironments to older structures, but lack obvious microbial influence and extensive synsedimentary cementation. Such differences reflect either short-term local conditions or long-term temporal changes in ocean chemistry and biology.

Possible latitudinal clines in Antarctic intertidal and subtidal zone communities encrusting ephemeral hard substrata

Summary Aim Encrusting faunal communities on rocks were examined from Southern Ocean intertidal and subtidal (6 m) zones to investigate potential change with latitude. Location The site locations were South Georgia (54°S [sub‐Antarctic]), Signy Island (60.5°S [maritime Antarctic]) and Adelaide Island (68°S [high Antarctic]). Methods The number of taxa, degree of colonization and bryozoan growth and mortality were measured. Results The communities present were relatively simple in terms of composition and interactions. Spirorbid polychaetes (Annelida) and cheilostomatid bryozoans were the principal components. The community simplicity and dominance of unitary (compared to colonial) fauna increased with latitude. Mean annual mortality of subtidal cheilostomatids also significantly increased with latitude. The number of bryozoan species and potential aspects of community complexity increased with latitude. The growth of the bryozoan Inversiula nutrix increased with latitude reversing the general trend of decreased growth rate. Conclusions These findings suggest that disturbance may significantly increase with latitude within the Southern Ocean. The apparent reversal of typical latitudinal growth trends is probably a result of classical island biogeography, with species decreasing away from a centre of high diversity.

A new ctenostome bryozoan ectosymbiotic with terminal‐moult paddle crabs (Portunidae) in New Zealand

Triticella capsularis sp. nov. (Bryozoa, Ctenostomata) is newly described as an ectosymbiont of paddle crabs (Ovalipes catharus) in central New Zealand (Bay of Plenty to northern South Island). The bryozoan produces the longest zooids known in the genus Triticella, with colonies forming a “fur” up to almost 10 mm thick on large crabs, mostly males. The densest area of colonisation is the ventral anterior half of the crab. The bryozoan lives only on O. catharus and probably benefits from its “messy” and voracious feeding habits, opportunities for gene exchange during crab swarming behaviour, and dispersal. There is no synchrony between reproduction of the bryozoan and moulting cycles of the crab, as the bryozoan achieves reproductive colony size only on old terminal‐moult crabs. Although visually striking when dense, the bryozoan growth is only superficial and affects neither the behaviour of the crab nor the quality of its meat.

Bryozoan growth habits: Classification and analysis

Bryozoans are an important part of the benthic marine fauna in a wide variety of modern environments and are found in rock forming abundance in a number of settings throughout much of the Phanerozoic. Bryozoologists and nonspecialists have grouped taxa into colonial growth forms (e.g., erect fenestrates or encrusting sheets), both to simplify analyses and because correlations exist between some colony growth forms and the environmental conditions in which the organism lived. These correlations allow for the possibility of paleoenvironmental analyses based on skeletons alone. Existing bryozoan colonial growth form classifications do not, however, fully exploit the ecological information present in colony form.A new scheme is proposed here (Analytical Bryozoan Growth Habit Classification), which provides a list of colony-level morphological characteristics for bryozoan growth habits. This differs from previous approaches to bryozoan growth form analysis in that it is a classification of growth habit characteristics rather than a classification of morphological groups as such. The classification is based on eleven character classes, which describe the orientation of the colony and its occupation of, and placement in space. The overall colony shape is described based on the arrangement of modules in colonial growth. This classification provides a common ground for systematic comparison of character states among varied bryozoan growth habits. This approach allows for the evaluation of correlations among observed morphological character states and specific environmental conditions in which they develop. In addition, these growth habit characters can be used to recognize, characterize, evaluate, and apply more traditional growth form groups in broader studies.

The western Pacific brachyuran (Hemigrapsus sanguineus: Grapsidae), in its new habitat along the Atlantic coast of the United States: geographic distribution and ecology

The Asian shore crab Hemigrapsus sanguineus (de Haan, 1853), discovered on the coast of New Jersey in 1988, is now known to be distributed in the western Atlantic from Massachusetts (south of Boston) to Oregon Inlet, North Carolina. Living in the mid to upper rocky intertidal zone, it exploits a niche mostly unoccupied by native brachyurans. In the northern part of its range there is some mid-intertidal overlap with the green crab Carcinus maenas. In North Carolina H. sanguineus becomes sympatric in the high intertidal with another grapsid, the wharf crab Armases cinereum. Distributional evidence indicates that H. sanguineus was probably introduced via ballast water at one or more major shipping centres south of Cape Cod (i.e. the New York Bight, Delaware Bay, Chesapeake Bay), perhaps in the early 1980s. Its present latitudinal range in the western Atlantic is only about one-fifth of that in the western Pacific. H. sanguineus is now the most abundant brachyuran at the intertidal monitoring site in southern New Jersey, and apparently in some areas of Long Island Sound to the north. In New Jersey, crabs range in carapace width from 2.3 to 43.9 mm. The breeding season is from late April through September, and recruitment to the intertidal population begins in June and continues through the fall and winter. Some crabs become subtidal, particularly during the winter months, as evidenced by the growth of bryozoans, mussels and barnacles on their carapaces. There is no evidence in this population of parasitism with metacercariae, nemerteans, rhizocephalans or bopyrid and entoniscid isopods.

Holocene carbonate sedimentation on the west Eucla Shelf, Great Australian Bight: a shaved shelf

The southern continental margin of Australia is a cool-water carbonate sedimentary province located in a high-energy, swell-dominated oceanographic setting. A vibrocore transect of 14C-dated sediments across the centre of the Eucla Shelf is the first record of Holocene shelf deposition in the Great Australian Bight. Much of the seafloor shallower than 70 m water depth, the base of wave abrasion, is bare Cenozoic limestone, in some places encrusted by (?) Late Pleistocene, coral-rich, limestone that is cemented by high-magnesium calcite (12 mole% MgCO 3). The areally extensive, 100 km-wide, hard, bored substrate supports an epibiota of coralline algae, minor bryozoans and soft algae or is covered by patches of Holocene sediment up to 1.5 m thick; generally a basal bivalve lag (< 3 ka) overlain by quartzose-bioclastic palimpsest sand. This pattern of active carbonate production but little accretion on the wave-swept mid- to inner-shelf is similar to that on other parts of the southern Australian continental margin. The term shaved shelf is proposed for this style of carbonate platform, formed by alternating periods of sediment accretion, cementation and erosion. The palimpsest sand is typically rich in bivalves, coralline algae and locally, detrital dolomite. Outer shelf Holocene sediment, below the base of wave abrasion but inboard of the shelf edge, is a metre-thick unit of fine, microbioclastic muddy sand with minor delicate bryozoans overlying a 9–13 ka rhodolith gravel. Some of this outer shelf sediment appears to have been resedimented. The shelf edge is a sandy and rocky seafloor with active bryozoan growth and sediment production. The Holocene sediments are enriched in coralline algal particles and conspicuous large foraminifers (cf. Marginopora) and depleted in bryozoans, as compared to coeval deposits on the Lacepede and Otway shelves off southeastern Australia. These differences are interpreted to reflect warmer waters of the Leeuwin Current and prevalent downwelling in this area as opposed to the general upwelling and colder waters in the east.

Indications for microbial clues for bryozoans when settling

A correlation between skeletal growth of recent bryozoans and external environmental factors (e.g. abrasion, suspended sediment load) exists in ahermatypic tropical shallow water associations of the Philippines. Yet, the spatial zonation pattern of bryozoan species in a modern Philippine reef exhibits an amazingly high level of stability and order which cannot be readily correlated with external environmental factors. This is due to the observation that zonation correlates with the bryozoan taxa but not with their growth form categories (e.g. flexible-erect). Bryozoans are closely linked with their reef (micro)environment and reef microbiota by feedback control: Interacting biofilms, microbial mats, bryozoans and bryozoan larvae illustrate how such internal functions in a Philippine reef may stabilize the biotope structure as a whole

Branching patterns, generating rules, and astogenetic trajectories in arborescent bryozoans

The terminal growth of bryozoans allows a reconstruction of their growth history and generating rules from the developed pattern. Three species of arborescent bryozoans share a bias in growth rate that favors reverser branches (those whose direction of growth is opposite that of their parent branch); this bias produces a common hummocky appearance to the top margin of the colony. Other differences in the growth rules, however, result in markedly different colony forms. For two species, the cue for splitting of branches seems to be the length of the branch; for the other, the cue seems to be the time since the last splitting. Simulations based on the distinction between reversing and nonreversing branches reproduce the hummocky pattern, but offer little insight into the underlying mechanism. Simulations based on occlusion, the line-of-sight blocking of the branches by each other, give similar patterns with fewer assumptions. These simulations also suggest that the pattern is due to a limited availability of nutrients to occluded growing tips, and that nutrients remain relatively localized within colonies. The actual determinants of colony form are likely to combine both nutrition and geometry.

Historical record of erect bryozoan growth forms

Information compiled from literature on marine bryozoans suggests that erect taxa have had a limited range of repeatedly evolved growth forms, each with characteristic branch widths or surface morphology, through the nearly 500 Ma history of the phylum. The range of erect growth forms among the Bryozoa has been limited to six (with only rare exceptions) that have co-occurred constantly throughout the history of the phylum. Ciliated tentacles of bryozoan lophophores are located just above the surface of colonies and draw a flow of food-bearing water. Outflow of the filtered water is confined to the space between the tentacles and the colony surface, until some point of escape is reached. In almost all erect bryozoans, lophophores are closely spaced and, in extant species, feed simultaneously. Therefore, for broad surface areas to develop, colony margins must be supplemented by other points of outflow (chimneys), commonly situated on surface disruptions, to keep local volume and velocity within manageable bounds. Presence of chimneys apparently permits branch widths to increase beyond certain size limits into less tightly constrained morphospace, and branch widths have been narrowly limited in the various growth forms except where regularly spaced chimneys can be demonstrated or reasonably inferred. It is hypothesized that the observed limits on form, size and surface morphology are derived primarily from the phylogenetic constraint of byrozoan lophophore size and attendant limitations on flow of filtered water across colony surfaces.

Sedimentology, Facies and Depositional Environments of the Rochester Shale (Silurian; Wenlockian) in Western New York and Ontario

ABSTRACT In western New York and Ontario, the Rochester Shale (Silurian; Wenlockian; Clinton Group) is everywhere divisible into lower (Lewiston) and upper (Burleigh Hill-Stoney Creek) members; thin units within these members are traceable east to west for distances exceeding 100 km, without substantial change in lithology, or fossil content. In contrast, abrupt lateral facies changes occur within the Rochester Shale along short north-south sections (for example, Niagara Gorge). Such stratigraphic evidence suggests that Rochester shale facies belts are elongate east-west, parallel both to a northern paleoshoreline and also to the present outcrop trend. The Rochester Shale consists of sparsely to richly fossiliferous, gray, shaley mudstone with abundant interbedded carbonates, including intrasparrudites, lenticular biosparites and biomicrites (= calcarenites) and laminated pelmicrites (= calcisiltites). Such interbeds provide evidence for episodic concentration and transport of carbonate sediments by storm-wave action on a gently southwards-sloping shelf. Calcarenite lenses were formed as erosion-lag deposits in regions within storm-wave base. Finer debris, winnowed from such areas, formed suspension clouds which flowed downslope, depositing storm silt- (calcisiltite) and mud layers. A crinoidal bank facies (Wiarton) accumulated in the area of the Algonquin Arch during Rochester deposition. Adjacent shelf areas, to the south, received a mixture of terrigenous muds from southeastern source areas (Taconics) and detrital carbonate sediments from the Wiarton shoal, forming argillaceous carbonates and/or calcareous shales (Stoney Creek-upper Burleigh Hill). During times of low sedimentation this area became the site of abundant bryozoan growth--bryozoan belt (Lewiston B, D and E). Farther southward (basinward), facies comprise a mixture of sparsely fossiliferous mudstones and storm-silt layers (Lewiston C, lower Burleigh Hill). Vertical facies changes in the Rochester Shale are attributed to north-south shifting of environmental belts, due to migration of the northern paleoshoreline. As a whole, the formation comprises two (eustatic?) trangressive-regressive cycles. The Lewiston Member represents a nearly symmetrical cycle of deepening (units A-C) and shallowing (units C-E), while the upper units (Burleigh Hill, Stoney Creek) reflect aspects of an asymmetrical, shallowing-upward hemicycle. During the last event, allodapic carbonate sedimentation was considerably increased, as a result of migrating crinoidal bars, inhibiting the growth of bryozoans and other organisms. The Rochester Shale provides a paradigm for interpreting numerous marine units in western New York which exhibit layer-cake arrays of facies.