W. D. Liddell

Preservation and paleoecology of a Middle Ordovician hardground community

-Limestone beds in the Middle Ordovician (Trentonian) Bobcaygeon Formation, exposed near Kirkfield, Ontario, exhibit irregular, bored and encrusted surfaces indicative of early lithification. These hardgrounds were formed in extensively burrowed carbonate sediments and their hummocky surface topographies were inherited, in part, from a pre-existing pattern of burrow tunnels. A diverse community, including bryozoans, brachiopods, crinoids, edrioasteroids, and paracrinoids, colonized these hard substrates. In addition, most surfaces are riddled with small, cylindrical boreholes (Trypanites) which represent dwellings of softbodied organisms. Some hardgrouind surfaces were inhabited by multiple generations of organisms. Remains of the older generations of encrusters were strongly abraded and nearly obliterated. Superimposed upon these worn remnants are well-preserved remains of rather fragile organisms, e.g. complete hybocystitid crinoids and edrioasteroids. Evidently, certain hardground surfaces were rapidly buried by muds, resulting in the in situ preservation of the last generation of attached organisms. Slightly differing subcommunities of organisms inhabited various microhabitats provided by the irregular hardground surfaces. Thus, the relative abundance of bryozoans and echinoderms encrusting the roofs of small crevices differs from that on the exposed upper surfaces of the hardgrounds. This is the geologically oldest known hardground community in which microhabitat subdivision can be recognized. However, polarity between the subcommunities is not as pronounced as in geologically younger hard substrate faunas. Carlton E. Brett. Museum of Paleontology, University of Michigan, Ann Arbor, Michigan 48109 W. David Liddell. Museum of Paleontology, Univer.sity of Michigan, Ann Arbor, Michigan 48109 Accepted: March 18, 1978

Hard substrata community patterns, 1-120 M, North Jamaica

Tropical marine communities from shallow-water (<30 m) carbonate environments are often dominated by hermatypic scleractinian corals with lesser amounts of crustose coralline algae and endolithic demosponges. Living cover is typically high (80-100%7p). Along the north-central coast of Jamaica and at many western Atlantic sites, communities existing below 55 m inhabit a vertical to overhanging wall of reef limestone, the deep fore reef, which extends to approximately 130 m. At 60 m the community resembles that of shallower water, although scleractinians are less abundant and encrusting and erect demosponges are much more abundant. Coralline algae and macroalgae are also important space occupants at 60 m and living cover approaches 65%. Encrusting sponges and coralline, filamentous, and macroalgae predominate in the middle region of the deep fore reef. A low-diversity assemblage occupying 40% of the substratum and dominated by diminutive encrusting and endolithic? demosponges and largely endolithic filamentous algae occurs from 100-130 m, the lower limit of the deep fore reef. Community structure and zonation on the shallower reefs is controlled by a number of biotic and abiotic factors, most notably predationlgrazing, light intensity, and turbulence. On the deep fore reef, grazing and turbulence are greatly reduced. While reduced in intensity, light continues to exert a strong influence on community bathymetric zonation. Sedimentation also exerts an important control on the spatial distribution of the deep fore-reef biota with the most diverse assemblages flourishing in areas protectedfrom sediment. Despite a regime of reduced disturbance in deep water, community diversity remains relatively constant to a depth of 90-100 m.

Back reef and fore reef analogs in the Pleistocene of North Jamaica: Implications for facies recognition and sediment flux in fossil reefs

The 125,000 y.b.p. (Sangamon) Falmouth Formation is an emergent fringing-reef complex exposed along the north coast of Jamaica. Q-mode cluster analysis using constituent composition of Falmouth Formation rocks collected near Discoveiy Bay was employed to differentiate two distinctive facies. These are a dense, well-lithified skeletal packstone containing abundant calcareous algae (Halimeda and coralline algae) and molluscs, and a poorly lithified skeletal grainstone composed primarily of sand-sized coral fragments and coralline algae with only minor amounts of Halimeda. The composition of sediments within these facies is comparable to the composition of back-reef and shallow (5-8 m deep) fore-reef sediments of the Holocene Jamaican fringing-reef system. These results contradict the pervasive idea that sandsized, reef-derived sediments are ineffective as environmental indicators owing to post-depositional transport away from the reef system. Petrographic and X-ray analyses of the mineralogy of Falmouth Formnation limestones reveal that back-reef packstones retain much of their original aragonite and high-Mg calcite. In fore-reef grainstones, however, better sorting (increased permeability) results in dissolution and leaching of these metastable phases and reprecipitation of low-Mg calcite. Thus, variability in original sediment texture may create a preservational bias in the fossil record against the more permeable fore-reef deposits.

Oxygen consumption in two shallow-water comatulid crinoids

Rates of oxygen consumption in the two shallow-water comatulid crinoids Antedon bifida bifida Pennant from Ireland and Nemaster rubiginosa Pourtales from Jamaica are similar to that in other marine poikilothermic invertebrates when expressed in terms of dry tissue weight. The exponent b in the power function Y = a·Xb, relating weight-specific respiration to body weight, averaged 0.6. The Q10 of the weight-independent metabolic rate of both species suggests thermal adaptation to their respective environmental temperatures.