Saturday 23 November 2013

A Late Jurassic lagerstätte from central Poland

The term lagerstätte is used by Palaeontologists to describe a particularly rich fossil source; a site where fossils are either exceptionally numerous or exceptionally well preserved (or, ideally, both). One of the best known of these is the Solnhofen Limestone, or Solnhofen Plattenkalk, which formed in the early Tithonian,  towards the end of the Jurassic, as a result of intense global warming that covered most of what is now Southern Germany with a shallow tropical sea. This was populated by abundant reef-forming Sponges and Corals, leading to the formation of numerous lagoons, in which the waters became cut of from the surrounding sea, then became hyper-saline due to evaporation.

In a paper published in the journal Lethaia in January 2013, Adrian Kin of the Institute of Geological Sciences at Jagiellonian University, Michał Gruszczyński of the Institute of Geography at Jan Kochanowski University, David Martill of the School of Earth & Environmental Sciences at the University of PortsmouthJim Marshall of the School of Environmental Sciences at the University of Liverpool and Błażej Błażejowski of the Institute of Paleobiology at the Polish Academy of Sciences, describe a newly discovered fossil lagerstätte from the Owadów-Brzezinki Quarry in central Poland, thought to be of late Tithonian origin, slightly younger than the Solnhofen Limestone.

The Owadów-Brzezinki Lagerstätte comprises three distinct units with a total thickness of about 13m. The lowermost unit (Unit I) comprises yellowish marly limestones grading upwards into massive fine-grained limestones and is roughly 6.6 m thick This unit contains numerous Zaraiskites Ammonites. The second unit (Unit II) comprises thinnly bedded, fine-grained limestones with occasional distinctive parallel lamination. Zaraiskites is also present in this unit, though less numerous, whch also contains a single thin horizon with a mass occurrence of Polychaete Worm tubes (serpulite). The uppermost unit (Unit III) comprises fine grained limestones with a variety of bedding structures. Zaraiskites is rare here, but the bed contains a wide variety of other fossils, notably the bivalve Corbulomima obscura as well as avariety of other marine invertebrates, including Bivalves, Brachiopods, Arthropods, Fish, Sharks, Sphenodonts and  (non-marine) Insects and Pterosaurs.

The quarry at Owadów-Brzezinki showing the successive sedimentary units I–III of the studied Late Tithonian (= Middle Volgian) strata. In the right column are details of the studied profile: (1) thick-bedded limestones of unit I; (2) thin-bedded limestones of unit II; (3) lithographic- type limestones of unit III; (4) boundaries dividing units; (5) serpulite; (6) Corbulomima horizon; (7) tsunamite or mega-stormite. Kin et al. (2013).

The sequence is thought to represent a progression from a shallow marine offshore environment to a nearshore environment, possibly coastal or lagoonal. Unit I appears to have formed bellow the storm wave base where sediments were heavily bioturbated by marine organisms, while Unit three shows signs of being influenced by numerous weather events, as well as horizons of tidal and evaporitic origin, suggesting that the waters were becoming shallower throughout the succession. The beds are believed to have been laid down in a shallow epicontinental (inland) sea which dominated central Europe during the Late Jurassic. The unit also contains a bed considered to be tsunamigenic in origin, which Kin et al. suggest may be related to Earthquake activity on the nearby Middle Polish Trough, an area of active subduction in the Late Jurassic.

Palaeogeographical map showing position of the Owado´w-Brzezinki quarry on the margin of the Middle Polish Trough. Kin et al. (2013).

The vast majority of fossils from the location come from a single bed within Unit III, referred to as the Corbulomima horizon, due to the presence of large numbers of the bivalve Corbulomima obscura. In addition to the bivalves his unit produces Ostracods, Ammonites, Brachiopods, Limulids (Horseshoe Crabs), Dragonflies, Beetles, Sharks, Boney Fishes, Sphenodonts (Squamate Reptiles related to Snakes and Lizards), Pterosaurs and a variety of other fossils.

Fossils from the Corbulomima horizon of Unit III. (A) Nearly complete exuvia of the Horseshoe Crab Crenatolimulus sp.. (B) Fragmentary preserved pycodontid fish skeleton (possibly tuberculate scales of Coelodus sp.). (C) Wing of the dragonfly Eumorbaeschna sp.. (D) Partial skeleton of indeterminate pycnodontid fish preserved in situ. Scale bars are all 10 mm. Kin et al. (2013).

Fossils from the Corbulomima horizon. (A) Bivalves:  
Mesosaccella sp. (1) and numerous representatives of Corbulomima  
obscura (2). (B) Indeterminate pterosaur tooth. (C) The only example  
of Zaraiskites zaraiskensis found from the Corbulomima  
Kin et al. (2013).

Examples of vertebrates from the Corbulomima horizon. (A) mandible of aquatic Sphenodontian Pleurosaurus goldfussi. (B) Nearly 
complete dentary bone of Actinopterygian (Ray-finned) Fish Caturus sp.. (C) Details of same specimen, showing teeth of the mid dentary in detail. Scale
bars are all 10 mm.
Kin et al. (2013).

See also A new species of Split-foot Lacewing from the Middle Jurassic of Inner MongoliaOpportunistic Bivalves during the Early Jurassic Toarcian Oceanic Anoxic EventA Caddisfly from the Middle Jurassic of Inner MongoliaChoristopsychid Insects from the Middle Jurassic of Inner Mongolia and A Pliosaur from the Jurassic Coast of Dorset.

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