The Early Cambrian Experiment in Reef-Building by Metazoans : 無料・フリー素材/写真
The Early Cambrian Experiment in Reef-Building by Metazoans / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | These are my personal notes taken during a geology presentation at the "Neoproterozoic-Cambrian Biological Revolutions" Paleontological Society short course, which was held in Denver, Colorado, USA in fall 2004. I give the notes here because they may be of some interest. Do not expect them to always be in complete sentences, etc.-----------------------------------The Early Cambrian Experiment in Reef-Building by MetazoansPresented by: Melissa Hicks & Stephen Rowland (Department of Geoscience, University of Nevada, Las Vegas, Nevada, USA)(geoscience.unlv.edu/people/stephenmrowland.html)6 November 2004----------Who are the Neoproterozoic players?Grotzinger, Walter & Knoll (2000) described Namacalathus.Wood et al. (2002) described Namapoikia, which was large-sized. Who are the Early Cambrian players?1) archaeocyathans (2 single occurrences in the post-Early Cambrian - one in the Middle Cambrian, and one in the Late Cambrian).(dinoera.com/sites/default/files/archaeocyatha04_cc.jpg)(dinoera.com/sites/default/files/archaeocyatha03c_cc.jpg)(www.infosyslab.fr/archaeocyatha/images/schema_complet.png)2) radiocyaths (similar to receptaculitids; may be algae, rather than having sponge affinities).3) corals/corallimorphs (occur within archaeocyathan reefs; most are late Atdabanian to Botomian in age; they make up a low percentage of the reefs - they make up 5% of fossil reefs in Esmeralda County, Nevada, USA).4) calcimicrobes(earthsurfaceprocesses.com/3c-E-MassExtnFig7.jpg)(en.wikipedia.org/wiki/Calcimicrobe) There are 2 varieties of archaeocyathans - regulars and irregulars.Regular archaeocyathans have thin borders with inner and outer walls.Irregular archaeocyathans have much less symmetry, similar to modern sphinctozoan sponges; they have chambers; they are more common in reef environments. During their maximum diversity, there are 180 genera of archaeocyathans. Archaeocyathans had an 11 million year duration of reef building activity (~same duration as the Paleocene). Archaeocyathans are traditionally classified as incertae sedis, but they are now considered to be a class of Phylum Porifera. Archaeocyathans had aspiculate, originally high-Mg calcite skeletons. If they’re aspiculate, they can’t be sponges, right? But, 10-15 years ago, a living aspiculate sponge was found, similar to archaeocyathans. So, this objection isn’t valid anymore. Archaeocyathans can be sponges.Archaeocyathans were put in Kingdom Archaeata for a while (also known as Kingdom Inferibionta).The uncertain status of archaeocyathans is no longer uncertain. The problem is now resolved. Scuba allowed observation of a modern aspiculate archaeocyathan-like sponge with chambers - the demosponge Vaceletia crypta (it is known to be a demosponge from embryological studies). It is possible to have a sponge without spicules. Vaceletia is not a descendent of archaeocyathans, however. Archaeocyathans are a class-level clade of sponges. Archaeocyathan reefs - occur from the Tommotian to Toyonian Stages in the Cambrian. Reefs vary greatly in size from 1 meter high & 1 meter wide to 100 meters in width and 50 meters in height. Archaeocyathan reefs are loaf-shaped to lens-shaped, and range from patch reefs to compound reefs. The reefs are a consortium of archaeocyathans and calcimicrobes. Tommotian reefs - 520-521 million years ago; generally small patch reefs (~2 meters wide); examples are in the Anabar-Sinyaya Basin of Siberia; reefs include archaeocyathans and Renalcis. They formed rigid, cavernous frameworks. They were the first true metazoan reefs. They were low diversity, but contained all guilds (bafflers, dwellers, constructors). Tommotian reefs did not contain “simpler” forms. Atdabanian-Botomian reefs - 511-520 million years ago; known in Morocco, Sardinia, Yukon, Nevada, northern Mexico, China, Australia. Apex reefs - 180 genera of archaeocyathans (= high diversity of reefal & peri-reefal organisms); occurred in low to high energy environments; occurred in shallow to deep shelf settings. Dolomite-filled primary cavities have trilobites, echinoderm plates, calcimicrobes, and archaeocyathans.The Sinsk (mid-Botomian) and Toyonian regression extinctions hit archaeocyathans.The Sinsk regression has not yet been seen in Laurentia. The Sinsk event is recorded in non-bioturbated black shale. Toyonian reefs - 510-511 million years ago; see a global decline in reef building; Siberian reefs are basically gone; a few scattered reefs around the world; 35 genera of archaeocyathans; 1 or 2 genera of archaeocyathans per reef, commonly; reefs still can be fairly large & complex; high diversity of reef dwellers - hyoliths, Salterella, ostracods, trilobites, corals, brachiopods, chancelloriids, echinoderms; reefs got hit hard by the Toyonian regression (= Hawke Bay event) - a global regression is seen in Morocco, Laurentia, Baltica, South China, Iran; sessile organisms were hit (such as archaeocyathans) & vagrant organisms were hit (such as trilobites).Why didn’t reefs recover?There is a 30 to 40 million year hiatus in metazoan reef building after this. See lots of microbialites instead.(www.flickr.com/photos/jsjgeology/21021361776)See the book: "Phanerozoic Reef Trends" Why was there such a long hiatus? Hypotheses:1) post-extinction lag - however, 40 million years is longer than it should take to recover.2) photosymbiosis recovery - archaeocyathans probably did have photosymbionts (there’s no evidence, though).3) reduced grazing - cyanobacteria + microbialites went crazy; the fossil record doesn’t support this hypothesis.4) nutrient deficiency - there was a lack of much exposed terrestrial rocks on the continents (Example: high transgression on Laurentia).5) high level of atmospheric carbon dioxide (CO2) gas - doubling CO2 gives corals trouble (it changes the pH of the water); the Berner (1997) curve shows that atmospheric CO2 was 20 times higher in the Cambrian than now.6) global warming7) Mg/Ca seawater chemistry - seafloor spreading resulted in increased Ca and decreased Mg levels; therefore, calcite organisms do well and aragonite or high-Mg calcite organisms (such as archaeocyathans) have trouble; this is a time of changeover from aragonite seas to calcite seas.-----------------Audience comment: there are problems with some of these hypotheses - reefs rebound when temperatures and sea level are still high; so, causative factors have to be more complex.----------------- |
| 撮影日 | 2015-09-11 20:53:00 |
| 撮影者 | James St. John |
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