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. 

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The reaction of marine invertebrates to global warming during the Early Jurassic Toarcian Extinction Event.

About 183 million years ago a major eruptive episode in the Karoo-Ferrar Large Igneous Province of South Africa lead to an abrupt rise in global atmospheric and oceanic temperatures, leading to widespread oceanic anoxia which is reflected in the fossil record as the Early Jurassic Toarcian Extinction Event. Extinction events are common in the fossil record, and are widely used as proxies for environmental change. However it is far from clear if all organisms in the marine community were affected in the same way or at the same time, with the potential that sudden changes in fossil community makeup could be misleading if not interpreted correctly.

In a paper published in the journal Geology in March 2015, Silvia Danise of the School of Geography, Earth and Environmental Sciences at Plymouth University, Richard Twitchett of the Department of Earth Sciences at the Natural History Museum and Crispin Little of the School of Earth and Environment at the University of Leeds describe a study of benthic (bottom dwelling) and nektonic (water column dwelling) invertebrates across a 1.7 million year section in the Cleveland Basin, North Yorkshire, UK,  spanning the Early Jurassic Toarcian Extinction Event.

Danise et al. compared benthic and nektonic diversity levels to variations in oxygen, carbon, strontium, sulphur and molybdenum isotope ratios and total organic carbon levels in deposits of the Whitby Mudstone Formation, which was laid down in the Cleveland Basin during the Early Toarcian, when it was located at latitudes of between 30˚ and 40˚ north in part of the Laurasian Seaway.

Study area (Cleveland Basin, UK) and location map of the sections that form the composite stratigraphy. Maximum intertidal rock exposure shown in white. Danise et al. (2015).

Molybdenum isotope levels are a proxy for ocean anoxia, while carbon isotope levels reflect changes in the rate at which organic matter is produced and buried and strontium isotope levels reflect weathering of rocks on land. Oxygen isotope ratios are used to determine temperatures, and sulphur isotope levels reflect productivity by sulphate reducing Bacteria, which thrive under anoxic conditions, but also require phosphorus from organic matter.

Danise et al. found that extinction and diversity levels in benthic and nektonic communities across the Toarcian Extinction Event responded to different isotope changes, suggesting that they were driven by different processes. Following the initial extinction levels diversity levels recovered most quickly in nektonic communities, reaching maximum diversity when the temperature was warmest and sea levels were highest, which is likely to reflect high phytoplankton productivity, followed by a recovery in oxygen levels in the water column.

Benthic communities took longer to recover and were dominated first by Bositra radiata, which is interpreted as an epifaunal (surface-dwelling, non-burrowing) filter feeder tolerant of low oxygen levels, then by a slightly more diverse fauna dominated by Pseudomytiloides dubius with occurrences of Bositra buchii and Meleagrinella substriata, which are all also is interpreted as an epifaunal filter feeder tolerant of low oxygen levels.

Bositra radiata shells from the Toarcian Oceanic Anoxic Event. Palaeoenvironmental Change.

Prior to the extinction event the most abundant nektonic group were Belemnites (an extinct group of Squid-like Cephalopods with internal shells) of the genus Passaloteuthis, which is interpreted to have lived in deep, cool waters, while following the event these were replaced with members of the genus Acrocoelites, interpreted to have lived in shallower, warmer waters, which may also be a reaction to low oxygen levels in deeper waters. These Belemnites were eventually replaced by Ammonites (Cephalopod Molluscs with chambered external shells).

Specimen of the deep water Belemnite Passaloteuthis bisulcata from before the Toarcian Extinction Event. Wikimedia Commons.

The strongest correlation between isotope ratios and extinctions was seen for molybdenum, with 40% of benthic species turnover relating directly to this variable, suggesting that in the Early Jurassic as now the main factor determining diversity in benthic marine communities was dissolved oxygen levels. Following the main extinction event diversity was also correlated with variations in strontium and carbon isotope levels, which correlate to levels of erosion and organic matter burial.

For Nektonic communities molybdenum isotope ratios are still important, but strontium isotope ratios become more important, suggesting that diversity is more strongly related to terrestrial erosion levels. During the Toarcian atmospheric carbon dioxide levels are thought to have risen sharply due to emissions from the volcanoes of the Karoo-Ferrar Large Igneous Province, leading to an increase in dissolved carbon dioxide in rainwater (acid rain), which would in turn lead to higher rates of weathering on land. This would lead to increased levels of nutrients entering the marine environments, driving productivity in surface waters. However the same atmospheric carbon dioxide would also have raised temperatures in both the atmosphere and oceans (global warming) which would have led to lower oxygen levels in the water column (warm water can hold less dissolved oxygen than cool water), particularly in deeper waters which cannot replenish their oxygen from the atmosphere.

Interestingly oxygen isotope levels did not appear strongly correlated with extinction rates, despite the fact that these are thought to reflect atmospheric temperature, the factor which is thought to have driven ocean anoxia. Danise et al. suggest that this is because oxygen isotope levels are obtained from Belemnite shells, and we may not fully understand how their physiology and behaviour changed in response to environmental changes during the extinction event.

Finally Danise et al. suggest that this decoupling between factors affecting benthic and nektonic communities may be found in other extinction events where it has not previously been looked for, and that it may also be seen in modern faunas faced with similar environmental stresses, given that Jurassic marine invertebrate faunas are thought to be quite close to modern faunas in composition.

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Opportunistic Bivalves during the Early Jurassic Toarcian Oceanic Anoxic Event.

The Toarcian Oceanic Anoxic Event is an extinction event that took place in the Early Jurassic, about 183 million years ago. It took place in four phases, thought to have been related to Milankovitch Cycles. During each phase the temperature of the global ocean is thought...

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Opportunistic Bivalves during the Early Jurassic Toarcian Oceanic Anoxic Event.

The Toarcian Oceanic Anoxic Event is an extinction event that took place in the Early Jurassic, about 183 million years ago. It took place in four phases, thought to have been related to Milankovitch Cycles. During each phase the temperature of the global ocean is thought to have risen abruptly by as much as 13℃, leading to a depletion in oxygen levels in the oceans, followed by an extinction event. Warmer waters are less able to retain oxygen, and this is thought to have been made worse by an increase in runoff from the continents due to higher rainfall, and a breakdown in ocean currents caused by the warming of deep oceanic waters. Each of these phases is marked by a distinct shift in Carbon, Oxygen and Strontium isotope ratios, an extinction event in the fossil record and the deposition of vast amounts of organic matter which has led to extensive hydrocarbons deposits at these levels in many places around the world. The Early Jurassic was considerably warmer than today, and it is thought that the warmest points on the Milankovitch Cycles (which are driven by cyclic variations in the Earth's orbit) raised global temperatures above a tipping point which led to runaway warming.

In a paper published in the journal Geology on 6 September 2013, Bryony Caswell of the School of Environmental Sciences at the University of Liverpool and Angela Coe of the Department of Environment, Earth and Ecosystems at the Centre for Earth, Planetary, Space and Astronomical Research at the The Open University, examine the behavior of populations of two species of opportunistic Bivalve, Bositra radiata and Pseudomytiloides dubius during the Toarcian Oceanic Anoxic Event in deposits at Whitby near North Yorkshire, England.

During the run up to the initial event the deposits were dominated by the Bivalve Bositra radiata, a Posidoniid Clam related to modern Scallops, quickly came to dominate to fauna, forming monospecific pavements and growing to sizes not achieved prior to the onset of the event. 

Bositra radiata shell pavement from Hawsker Bottoms, North Yorkshire. Scale bar is in milimeters. Caswell et al. (2009).

During the remaining three events the Inoceramid Clam Pseudomytiloides dubius (also related to modern Scallops) quickly came to dominate the faunas, again reaching larger sizes during the event. The two species appear to compete during the onset of the third event, but Pseudomytiloides dubius quickly came to dominate, suggesting that the two species competed for some resource (probably food) and that Pseudomytiloides dubius was able to outcompete Bositra radiata.

Pseudomytiloides dubius from Port Mulgrave in North Yorkshire. Scale bar is in milimeters. Caswell et al. (2009).

Bositra radiata and Pseudomytiloides dubius are both thought to have been opportunistic species. Both were small (even when reaching exceptional sizes), and are thought to have had short generation times and high larval production rates, probably comparable to the modern Mulinia lateralis, a small Surf Clam which can breed at two months old and seldom lives more than two years, which does well in anoxic conditions. Such organisms can thrive in environments even where they are exterminated each year, since they will have time to reproduce and can recolonize from adjacent areas.

The approximate location of the study area. Google Maps.

The Toarcian Oceanic Anoxic Event is of particular interest today as we are living in time when rising global temperatures, combined with local anoxic zones in parts of the oceans caused by excess runoff due to human activities, for example in the Black Sea and the Gulf of Mexico. This makes it important to understand how organisms cope (or don't) with such events, and what the long-term impact on the global biosphere is likely to be, particularly if these events become larger and more frequent.

See also A new species of Pea Clam from the Miocene of Bosnia and Herzegovina, Iceberg damage in an Antarctic Clam, The evolution of Galeommatoid Bivalves, Symbiosis and the success of Galeommatoid Bivalves and A new species of Scallop from Western Australia.

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