in Jurassic and Cretaceous Stratigraphy

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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 schaften, Mathematisch-Naturwissenschaftliche Klasse, 1935, 22: 1-140. Stampfli G.M., Borel G.D., Cavazza W., et al. Palaeotectonic and palaeogeographic evolution of the western Tethys and PeriTethyan domain (IGCP Project 369), Episodes, 2001, 24, 4: 222-228. Ros S. Dinámica de la paleodiversidad de los Bivalvos del Triásico y Jurásico Inferior. Servei de Publi- 368 cacions de la Universitat de Valencia, 2009: 564. Virgili C. El Triásico de las Catalánides. Boletín del Instituto Geológico y Minero de España, 1958, 69: 1-856. Virgili C. The Permian-Triassic transition: Historical review of the most important ecological crises with special emphasis on the Iberian Peninsula and Western-Central Europe. Journal of Iberian Geology, 2008, 34(1): 123-158.
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Foraminiferal Response to the Toarcian Extinction Event M.B. Hart, M.D. Hylton, C.W. Smart School of Geography, Earth & Environmental Sciences, University of Plymouth, Drake Circus PL4 8AA, Plymouth, U K (E-mail: mhart@plymouth.ac.uk ) The Early Jurassic was a time of rapidly rising sea level associated with the extensive spread of anoxic bottom waters (Kemp et al., 2005; de Schootbrugge et al., 2005; Cohen et al., 2007). Sea level, in particular, rose across the Pliensbachian-Toarcian boundary, culminating in the Falciferum Zone of the Toarcian. The associated period of sea floor anoxia coincides with a notable period of mass extinction of the marine fauna (Hallam and Wignall, 1997). High resolution sampling and study of the microfaunas through several sequences in the United Kingdom (and elsewhere) confirm that benthic foraminiferal assemblages were similarly affected by an Early Toarcian Falciferum Zone event (Hylton, 2000; Hylton and Hart, 2000). Evidence for the foraminiferal extinction event in the Pliensbachian-Toarcian interval includes the eliminatiuon of the important Lower Jurassic Lingulina tenera, Frondicularia terquemi and Marginulina prima plexus groups, initiating a significant period of turnover. A marked change also occurred in the character of associated nodosariid assemblages: the uniserial forms of Nodosaria, Frondicularia and Lingulina, which dominate the Pliensbachian assemblages, were largely replaced by coiled Lenticulina in the Early Toarcian. A reduction in test size (Lilliput Effect?) and a decline in species diversity, compared with Hettangian to Sinemurian assemblages, reflect the development of low oxygen conditions followed by the subsequent renewal of the microfauna in the middle Toarcian. Little’s (1996) detailed biostratigraphic sampling of invertebrate macrofaunas through complete, expanded sequences in North Yorkshire, England and in southwest Germany shows that a species level event occurs in the early Toarcian. During the late Pliens- bachian there was a diverse range of benthic species including infaunal and epifaunal bivalves, brachiopods and crinoids. The equally diverse nekton included ammonites and belemnites. While there was significant ecological disruption resulting from the onset of anoxia, the biotic responses of groups of organisms were markedly different. Taxa inhabiting the upper water column were unaffected by anoxia and included ammonites and belemnites. In addition, epifaunal taxa adapted to low-oxygen conditions such as the posido- niids and inoceramids, flourished in the post-extinction environment during the survival interval. As conditions ameliorated, the biota became more diverse and gradually began to resemble the pre-extinction biota. This pattern was not reflected in the response of the microfauna and especially not in the case of benthic foraminifera. The post-extinction faunas were of a completely different character compared to the pre- extinction biotas and have greater affinities with Middle Jurassic foraminiferal faunas. The evidence from the North Yorkshire Coast, and other northwest European sections, confirmed the Hallam (1986) model that the main species extinctions during the late Pliensbachian–early Toarcian time interval occurred, not at the end of the Pliensbachian, but near the top of the early Toarcian tenuicostatum Zone. However, this event is only significant at species level; it cannot be recognised at genus level or at family level. This study shows a species extinction event with a 19% (9 from 47 species) reduction in diversity at the top of the tenuicostatum Zone. This was followed by an 8% (3 from 38 species) reduction at during the falciferum Zone. The loss of foraminiferal species is low compared to the figures of Little (1996) who records a significant macro-species extinction event at the top of the tenuicostatum Zone with a 77% (17 from 22 species) reduction in diversity. The benthos was most severely affected, with an 88% species drop including the disappearance of the total infauna. No major foraminiferal group suffered extinction over this interval but the important Lower Jurassic lageniids Lingulina tenera, Frondicularia terquemi and Marginulina prima plexi were eliminated; these seem to have been relatively shallow-water mid-shelf forms which were replaced by the very tolerant Lenticulina. The generic diversity of the Lagenina continued to increase through the Mesozoic and showed no signs of decrease through the early Toarcian events. Elevated, interconnected continents and restricted seaways in the Lower Jurassic resulted in the expansion of cosmopolitan smaller foraminifera. Common in the shales of the Jurassic (and Cretaceous), the flattened and elongate tests of the Lagenina were probably an adaption to burrowing in the upper few centimetres of organic-rich substrates, as detrital or bacterial sca- vengers. In contrast to these larger foraminifera of shallow water, the relatively small Nodosariidae, Vaginulinidae, as well as smaller Robertinina, thrived in the fine clastic sediments deposited in deeper waters. These foraminifera with flattened, lenticular, or low trochoid tests probably fed on bacteria or other proto- zoans, or were infaunal detrital feeders. As a result, the Lagenina were not greatly affected by the environmental changes and resulting extinctions during the early Toarcian, but continued to show a slow and steady increase in diversity. 369
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