února 24, 2009

Změny v dinosauří fauně z Cedar Mountain zkoumány pomocí onkoidů

Originální způsob, jak lépe porozumět změnám v dinosauří fauně z Cedar Mountain Formation (Utah), které se udály ve spodní křídě, přináší jedna z mnoha dalších nových studií. Zkoumá tyto události za pomoci onkolitů - tělísek složených z uhličitanu vápenatého, vzniklých postupným přirůstáním vrstev sinic. Studie se konkrétně zaměřila na oblast Woodside Anticline. Pro důkladné pochopení změn, ke kterým ve spodní křídě u dinosaurů docházelo, je nutná nejen důkladná analýza horninových jednotek, ale i porozumění rozsáhlejším vzorcům druhohorního fosilního záznamu a tektoniky. Onkolity se sice v Cedar Mountain Formation vynořují na povrch, mimo Woodside Anticline ale nejsou příliš rozšířené. Populaci onkolitů zde tvoří dva hlavní typy: hojnější útvary s průměrem 2 - 5 cm a menší populace s průměrem menším než 25 cm (právě větším onkolitům se říká onkoidy, což je slovo, které abstrakt studie používá častěji). Jako jádra, ke kterým později vrstvičky přirůstali, sloužil nejen vápenec, ale i dinosauří kosti. Petrografie onkoidů naznačuje, že tyto útvary vznikaly na břehu jezera, kam byly splavovány dinosauří kosti. Ty byly následně obalovány vrstvami bakterií a vznikly onkolity, resp. onkoidy. Charakteristiky těchto útvarů také naznačují, že výška břehů jezera byla ovlivněna kolísající hladinou vody. Dinosauři zde tedy žili kolem sladkovodního, tzv. efemérního jezera - takové jezero bylo naplněno vodou jen po určitou část roku.



Abstrakt studie:

Shapiro, R.S., Fricke, H.C., and Fox, K. 2009. Dinosaur-bearing oncoids from ephemeral lakes of the Lower Cretaceous Cedar Mountain Formation, Utah. Palaios 2(4):51-58. doi: 10.2110/palo.2008.p08-013r.

Here we document the occurrence of locally common oncoids in the Cedar Mountain Formation of Utah in the Woodside Anticline area of the San Rafael Swell and use them to understand changes in the Early Cretaceous landscape and their effects on the dinosaur fauna. Detailed facies analysis is required to understand the context of these changes within the broader patterns of Mesozoic tectonics and the fossil record. Oncolite crops out in the Cedar Mountain Formation, directly overlying the Buckhorn Conglomerate. Oncolite is not widely distributed outside of the Woodside Anticline area. The oncoids are found in a bimodal population with the majority in the 2-5-cm-diameter range and a smaller population >25 cm in diameter. Nuclei are mostly rounded chert clasts and also include litharenite, polymict conglomerate, limestone, and both abraded and nonabraded dinosaur bone and wood fragments. Cortices are 3-5 mm thick with distinct, penecint (laminae that completely enclose a body; Hofmann, 1969), low-relief laminae. Some laminae are crenulated and comprise ministromatolites. The petrography of the oncoids suggests formation along lake margins where large fragments of reworked sedimentary clasts and dinosaur bones came to rest and were coated by bacterial mats. Caliche cements and coats some of the oncolite; these define a lake shoreline affected by fluctuating lake level. The isotope geochemistry indicates a combination of primary and diagenetic signals consistent with oncoid formation in open, ephemeral freshwater lakes.

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