Paleobiology, 31(1), 2005, pp. 73±97 Early Jurassic climate change and the radiation of organic- walled phytoplankton in the Tethys Ocean Bas van de Schootbrugge, Trevor R. Bailey, Yair Rosenthal, Miriam E. Katz, James D. Wright, Kenneth G. Miller, Susanne Feist-Burkhardt, and Paul G. Falkowski Abstract.ÐDuring the Early Jurassic, cyst-forming dino¯agellates began a long-term radiation that would portend ecological importance of these taxa in the pelagic plankton community throughout the rest of the Mesozoic era. The factors that contributed to the evolutionary success of dino¯a- gellates are poorly understood. Here we examine the relationship between oceanographic and cli- matic conditions during the Hettangian±Toarcian interval in relation to the radiation of dino¯a- gellates and other organic-walled phytoplankton taxa in the Tethys Ocean. Our analysis is based on two data sets. The ®rst includes d13Ccarb, d13Corg, total organic carbon (TOC), and quantitative palynological observations derived from the Mochras Core (Wales, U.K.), which spans the complete Early Jurassic. The second is a coupled Mg/Ca and d18O record derived from analyses of belemnite calcite obtained from three sections in northern Spain, covering the upper Sinemurian to Toarcian. From these two data sets we reconstructed the in¯uence of sea level, trophism, temperature, and salinity on dino¯agellate cyst abundance and diversity in northwest Europe. Our results suggest that organic-walled phytoplankton (acritarchs, prasinophytes, and dino¯agellates) diversity in- creased through the Early Jurassic. The radiation coincides with a long-term eustatic rise and over- all increase in the areal extent of continental shelves, a factor critical to cyst germination. On shorter timescales, we observed short bursts of dino¯agellate diversi®cation during the late Sinemurian and late Pliensbachian. The former diversi®cation is consistent with the opening of the Hispanic Corridor during the late Sinemurian, which apparently allowed the pioneer dino¯agellate, Liasi- dium variabile, to invade the Tethys from the Paleo-Paci®c. A true radiation pulse during the late Pliensbachian, with predominantly cold-water taxa, occurred during sea level fall, suggesting that climate change was critical to setting the evolutionary tempo. Our belemnite d18O and Mg/Ca data indicate that late Pliensbachian water masses cooled (DT ø 268C) and became more saline (DS ø 12 psu). Cooling episodes during generally warm and humid Early Jurassic climate conditions would have produced stronger winter monsoon northeast trade winds, resulting in hydrographic instability, increased vertical mixing, and ventilation of bottom waters. During the late Pliensba- chian, dino¯agellates replaced green algae, including prasinophytes and acritarchs, as primary pro- ducers. By producing benthic resting cysts, dino¯agellates may have been better adapted to oxi- dized ocean regimes. This hypothesis is supported by palynological data from the early Toarcian ocean anoxic event, which was marked by highly strati®ed anoxic bottom water overlain by low- salinity, warm surface waters. These conditions were advantageous to green algae, while cyst-pro- ducing dino¯agellates temporarily disappeared. Our results suggest that the rise in dino¯agellate diversity later in the Jurassic appears to correspond to deep water ventilation as a result of the opening of the Atlantic seaway, conditions that appear to have simultaneously led to a loss of pra- sinophyte dominance in the global oceans. Bas van de Schootbrugge, Trevor R. Bailey, Yair Rosenthal, and Paul G. Falkowski. Institute for Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901. E-mail: [email protected] Trevor R. Bailey, Yair Rosenthal, Miriam E. Katz, James D. Wright, Kenneth G. Miller, and Paul G. Fal- kowski. Department of Geological Sciences, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854 Susanne Feist-Burkhardt. Natural History Museum, Paleontology Department, Cromwell Road, London SW7 5BD, United Kingdom Accepted: 7 April 2004 Introduction web underwent extensive reorganization; The Early Jurassic (Hettangian±Toarcian) predators such as ammonites, and especially witnessed major marine faunal and ¯oral belemnites, ¯ourished in open waters, provid- turnovers in the aftermath of the Triassic/Ju- ing useful biostratigraphic markers and ``fast rassic boundary mass extinction (Little and food'' for a highly diverse community of ma- Benton 1995). All levels of the marine food rine reptiles, such as the ichthyosaurs (Gus- q 2005 The Paleontological Society. All rights reserved. 0094-8373/05/3101-0006/$1.00 74 BAS VAN DE SCHOOTBRUGGE ET AL. tomesov 1978). Contemporaneously, new true evolutionary event. On the basis of this groups of photosynthetic eukaryotic phyto- ®nding, one might hypothesize that the ``in- plankton taxa, including cyst-forming dino- vention'' of benthic resting cysts was a major ¯agellates and calcareous nannoplankton, contribution to the evolutionary success of di- which ®rst appeared in the Middle±Late Tri- no¯agellates during the Mesozoic. Hence, un- assic, radiated rapidly during the Early Juras- derstanding environmental changes control- sic (Bown et al. 1992; Stover et al. 1996; Fen- ling cyst-forming dino¯agellates can be used some et al. 1999). Modern members of these to further our understanding of the group as algal groups contain ``red'' plastids (charac- a whole. terized by the presence of chlorophyll a and c In this paper, we present a multi-proxy data in their photosynthetic organelles). These set that includes long-term d13Ccarb and ``red'' phytoplankton appear to have dis- d13Corg isotope and total organic carbon placed ``green'' plastid eukaryotic algae (char- (TOC) records obtained from the Llanbedr acterized by the presence of chlorophyll a 1 b) Mochras Farm Borehole core in Wales (U.K.). as dominant primary producers from the Me- These data permit analysis of changes in car- sozoic to the present (Falkowski et al. in bon cycling in surface waters, trophism, and press). Here we examine the early evolution of water stability (ventilation of bottom waters) one of these groups, the dino¯agellates. from the northern reaches of the Tethys Ocean The evolution of dino¯agellates is recon- during the Early Jurassic. Our palynological structed from the taxonomy of cysts, which analyses from the same core include dino¯a- are dominantly nonmotile (hypnozygotic) gellates, acritarchs, and prasinophytes (rep- resting stages of motile pelagic cells. The cysts resentatives of the green line). To assess the in- are made of extremely durable biopolymers ¯uence of temperature and salinity on dino- (dinosporin), sometimes coated with silica or ¯agellate abundance and diversity, we gener- carbonate. Whereas motile (free-swimming) ated detailed d18O and Mg/Ca records from dino¯agellates appear to prefer well-strati®ed Early Jurassic belemnites sampled in nearby waters, cyst formation (although still poorly Cantabria, northern Spain. Our data set is understood) has been related to changes in used to address the following questions: water column stability, such as increased tur- 1. When did the founder species of cyst-forming bulence during late spring and autumn (Dale dino¯agellates ®rst appear in the Tethys Ocean and 1983). In the contemporary ocean, cysts tend what were their origins? to be most abundant in seas of temperate lat- In the early late Sinemurian, almost exclu- itudes (Wall et al. 1977). Encystment has also sively cysts of the dino¯agellate Liasidium var- been shown by many neontologists (e.g., iable Drugg 1978 are found in the fossil record. P®ester and Anderson 1987) to be strongly re- This species shares some morphological sim- lated to nutrient depletion (N and P). Other ilarities with Late Triassic Rhaetogonyaula- triggers for encystment include decreasing caceae and appears to have been the progen- temperatures, high population density and itor of Early and Middle Jurassic dino¯agel- low light (Wall et al. 1977; Dale 1976, see or- lates. We examine what tectonic and paleoen- view in Tyson 1995). vironmental conditions may have contributed Although there is no body fossil evidence to to the rise of L. variabile in the Tethys in the suggest that dino¯agellates originated before Early Jurassic. the Middle Triassic (except for a handful of ac- ritarchs with dino¯agellate af®nities), organic 2. What paleoenvironmental factors contributed geochemical (dinosteranes) and molecular to the radiation of dino¯agellates in the late Pliens- phylogenetic evidence indicates that dino¯a- bachian? gellates were present since at least the Cam- The late Pliensbachian was a crucial time for brian (Moldowan and Talyzina 1998). How- dino¯agellates, because it shows the appear- ever, Fensome et al. (1999) showed that the ra- ance of another missing link from the extinct diation of cyst-forming dino¯agellates from order Nannoceratopsiales. Nannoceratop- the Late Triassic to the Early Jurassic was a siales combine characters of dinophysioid tab- EARLY JURASSIC DINOFLAGELLATE EVOLUTION 75 FIGURE 1. Palaeogeography for the Early Jurassic (Pliensbachian; 195 Ma). Studied sections are indicated: 1, Mo- chras Borehole. 2, Playa de la Griega. The Playa de la Griega (PLG) section is located toward the northeast of Oviedo along the Cantabrian shoreline and was previously studied by Suarez Vega (1974). 3, Castillo de Pedroso. The Cas- tillo de Pedroso (CDP) section was studied previously by Comas-Rengifo