in Jurassic and Cretaceous Stratigraphy

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.
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