Hard Substrate Communities Research Articles

Paleoecology and Evolution of Marine Hard Substrate Communities: An Overview

Hard substrates, including hardgrounds, cobbles/boulders, and skeletons of organisms, present unique opportunities for the study of community paleoecology and evolution. Because remains of hard substrate encrusting and endolithic organisms are buried or embedded in their life sites, problems of postmortem transport and assemblage mixing, which complicate many paleocommunity studies, are reduced. Thus, hard substrate fossil assemblages permit examination of ecological details, such as spatial distribution, overgrowth competitive interactions, and community succession. This theme issue of PALAIOS encompasses a series of papers on hard substrate paleocommunities and the biotic and physical factors controlling their preservation, ecology, and evolution. Certain of these contributions were first presented at a symposium on this general subject at the Fourth North American Paleontological Convention, held August 13, 1986, in Boulder, Colorado. This paper summarizes the general evolutionary history of hard substrate communities through the Phanerozoic, and reviews some of the factors appearing to have controlled their diversity and structure that are discussed by authors of the present symposium. Obviously there have been major advances in knowledge of hard substrate biotas in the past 15 years, and this research paves the way for broader synthetic studies of paleoecology and evolution of these communities.

Deep‐Sea Hydrothermal Vent Mussels: Nutritional State and Distribution at the Galapagos Rift

Rich faunal communities have been discovered around active deep—sea hydrothermal vent sites. The mussel Bathymodiolus thermophilus (Bivalvia, Mytilidae) is one dominant member of this hard—substrate community along the Galapagos Rift, east of the Galapagos Islands in the equatorial eastern Pacific. These mussels are most dense around the vent effluent and decrease in number with increased distance away from the discharge site. At the Mussel Bed vent, at 2490 m depth, the nutritional state of mussels (oxygen consumption and chemical composition) from a densely populated site °1 m from a warm—water fissure was compared to that of mussels from a sparsely populated peripheral site to examine the importance of food supply on mussel distribution. The submersible Alvin was used to collect bottom—water samples for chemical analysis, to manipulate in situ respirometers, and to collect specimens for chemical composition analysis. The quantity of particulate organic carbon immediately above the bottom (<10 cm) was twice as high (207.0 mg/L) at the dense site at the peripheral site (106.4 @m/L). Other parameters measured showed no significant variation (P > .05) between sites, including water temperature, current speed, and concentrations of dissolved oxygen, nitrate, and dissolved organic carbon. Concentrations of ammonium, nitrite, and hydrogen sulfide were below the detection limits of the assay used. Oxygen comsumption rates (R) as a function of mussel dry mass (M) were higher at the dense site (R = 0.23M038) than at the peripheral site (R = 0.098M0.48). Dense—site mussels had robust soft—tissue structure whereas peripheral mussels had an emaciated appearance. Water content was lower but condition index values and glycogen content were higher in the dense mussels. Less distinct differences were noted in total lipid and protein content between populations. Mussels examined 312 d after reciprocal site—to—site transplanting conformed in water and glycogen content to their adopted populations. I conclude that the nutritional state of the dense—site mussels is considerably higher than that of the peripheral mussels and is probably related to increased nutritive supply at the dense site. Possible causative factors are differences in the nutritive quality of organic matter and influences of symbiotic chemoautotrophs in the gills of these mussels.

Late Cambrian hard substrate communities from Montana/Wyoming: the oldest known hardground encrusters

Hardground surfaces from the Late Cambrian Snowy Range Formation in Montana/Wyoming are the oldest known non-reefal hard substrates exhibiting encrusting fossils. These surfaces range in age from Early Franconian to early Trempealeauan. Hardgrounds were developed on slightly hummocky to planar, truncated surfaces of glauconite-rich, carbonate, flat pebble conglomerates, which were deposited during episodes of storm scouring in shallow subtidal environments of the Montana/Wyoming shelf. Snowy Range hardgrounds are encrusted by a low diversity assemblage of fossils dominated by simple discoidal holdfasts of pelmatozoans, probably crinoids, and including small conical spongiomorph algae? and probable stromatolites. Macroborings (e.g. Trypanites) are notably absent from all hardground surfaces, although sharp-walled, vertical, cylindrical holes (borings?) occur in micrite clasts imbedded in certain flat pebble conglomerates. No evidence of faunal succession or microecologic partitioning of irregular surfaces was observed on these Cambrian hardgrounds.

Late Cambrian hard substrate communities from Montana/Wyoming: the oldest known hardground encrusters

Late Cambrian hard substrate communities from Montana/Wyoming: the oldest known hardground encrusters