Wednesday, 1 February 2017

The Ya Ha Tinda Fossil Assemblage: An Early Jurassic Lagerstätten from North America.

Fossil Lagerstätten are sites of exceptional preservation, where organisms are preserved better and in greater numbers than in other parts of the fossil record. These sites provide unique insights into the history of life on Earth, preserving organisms that cannot be found in other places, or in more common organisms, tissues that are not otherwise seen. The Early Jurassic was a time of major ecological and environmental change, with a number of distinct events that influenced the change in the marine fauna that occurred during this time, most notably the Toarcian Oceanic Anoxic Event (about 183 million years ago). Sadly only three fossil Lagerstätten are known from the Early Jurassic, all of them from European deposits, so that we have almost no knowledge of many important groups outside of Europe during this period.

In a paper published in the journal Geology on 9 January 2017, Rowan Martindale of the Department of Geological Sciences at The University of Texas at Austin and the Department of Organismic and Evolutionary Biology at Harvard University, Theodore Them of the Department of Geosciences at the Virginia Polytechnic Institute and State University and the Department of Earth, Ocean and Atmospheric Science & National High Magnetic Field Laboratory at Florida State University, Benjamin Gill, also of the Department of Geosciences at the Virginia Polytechnic Institute and State University, Selva Marroquín also of the Department of Geological Sciences at The University of Texas at Austin and of the Department of Geosciences at the Virginia Polytechnic Institute and State University, and Andrew Knoll, also of the Department of Organismic and Evolutionary Biology at Harvard University, describe a new fossil Lagerstätten from the Early Jurassic of southwest Alberta, Canada.

The Ya Ha Tinda Fossil Assemblage comprises several outcrops of the Fernie Formation outcropping on the Ya Ha Tinda Ranch. These deposits have been dated using Ammonites and Coccoliths (groups with very high species turnover often used to date Mesozoic strata), carbon isotope chemostratigraphy (the ratios of different carbon isotopes incorporated into sedimentary rocks varies in direct relation to global atmospheric temperature; this means that the proportion of these elements rises and falls at the same rate in rocks all over the world, creating a fingerprint that can be used to date rocks) and uranium-lead zircon dates from intercalated ash beds (zircon is a mineral formed by the crystallization of cooling lavas.; when it forms it often contains trace amounts of uranium, which decays into lead at a known rate - since lead, which has a much lower melting point, will not have been present in the original lava, it is possible to calculate the age of a zircon crystal from the ratio between these elements.), giving a very high confidence to the dates assigned to these beds; which are calculated to span the boundary between the Pliensbachian and Toarcian stages, including the Toarcian Oceanic Anoxic Event.

This site is the first Early Jurassic Lagerstätten known from outside Europe, and the third Lagerstätten to include the Toarcian Oceanic Anoxic Event. It has yielded Vampyropod Cephalopods (Vampire Squid), Lobsters, Shrimps, Leptolepiform and Saurichthyiform Fish, Ichthyosaurs, Ammonites, isolated Dinosaur bones, Brachiopods, Gastropods, Bivalves, wood, and Coccolithophores.

Exceptionally preserved fossils of Ya Ha Tinda Lagerstätte (Alberta, Canada). RDM—Red Deer Member; PCS—Poker Chip Shale Member. (A) Articulated Ichthyosaur vertebrae and ribs (RDM, late Pliensbachian). (B) Skull of small Teleost Fish (PCS, within Toarcian Oceanic Anoxic Event [T-OAE] carbon isotope excursion [CIE]); note preservation of gills (arrow). (C) Seirocrinus subangularis (Crinoid) calyx collected by Russell Hall (RDM, late Pliensbachian). (D) Vampyropod gladius with mantle muscle (white arrow) and ink sac (black arrow) (RDM, early Toarcian). (E) Loligosepiid Vampyropod gladius with ink sac (arrow) (RDM, early Toarcian). (F) Shrimp body fossil (PCS, within T-OAE CIE). (G) Complete body fossil of Uncina pacifica, Lobster, proximodistally flattened (RDM, late Pliensbachian). (H) Complete body fossil of Eryonid Lobster, dorsoventrally flattened, ventral view (RDM, late Pliensbachian). Martindale et al. (2017).

These specimens are preserved in finely laminated clays (shales) laid down on a gently sloping shelf and basin. The articulation of the skeletons preserved and high organic carbon content of the shales suggests that the specimens were preserved under anoxic (or at least very low oxygen) conditions, though the presence of large benthic (bottom dwelling) invertebrates and patches of bioturbation suggests normal oxygen levels were present for at least some of the time. Pyritization (the conversion of organic material to iron pyrites, which can occur under anoxic conditions) is rare, with most specimens preserved as carbonaceous impressions or calcareous shells with occasional replacement by apatite or clay minerals. Only hard, mineralize tissues such as shells and bones are preserved in three dimensions, suggesting that mineralization occurred after tissue collapse. 

See also...

http://sciencythoughts.blogspot.co.uk/2015/09/exceptional-preservation-in-early.htmlhttp://sciencythoughts.blogspot.co.uk/2015/03/the-reaction-of-marine-invertebrates-to.html
http://sciencythoughts.blogspot.co.uk/2015/02/understanding-preservation-of-insects.htmlhttp://sciencythoughts.blogspot.co.uk/2013/11/a-late-jurassic-lagerstatte-from.html
http://sciencythoughts.blogspot.co.uk/2013/09/opportunistic-bivalves-during-early.html
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