Livro de Resumos - Dinossauros do Maranhão

Paleontologia 119
1 Georgian Academy of Sciences, A. Djanelidze Institute of Geology, M. Alexidze str. 1/9, 0193 Tbilisi, Georgia.
2 Institut für Geowissenschaften, Geologie-Paläontologie, Im Neuenheimer Feld 234, DE-69120 Heidelberg,
Germany.
Ammonite support for a three-fold division of the Cretaceous
E-mail: bengtson@uni-hd.de
Mikheil V. Kakabadze 1 & Peter Bengtson 2
Chronostratigraphic boundaries are generally selected at levels marked by significant biotic
turnovers (mass extinctions and/or radiation events) and other widespread and
conspicuous changes in the rock successions. During the 80 Ma long Cretaceous Period,
several significant global events left their traces in the rocks, thus providing potential
chronostratigraphic boundaries. The Cretaceous System is formally divided into a lower
and an upper series. This two-fold division has been a constant subject of debate ever
since it was introduced in 1885, with many authors favouring a three-fold division, as for
most other Phanerozoic systems. During the past decades, in particular, the terms “middle
Cretaceous” and “mid-Cretaceous” have been frequently used, although there has been no
general agreement on the boundaries of this informal unit [e.g., Reyment & Bengtson
1990, News letters on Stratigraphy 22(1), 1-6]. This has led to some confusion, and voices
have been raised for a revision of the subdivision of the Cretaceous, even including
specific proposals (Scott et al. 2004, 2005, 32nd International Geological Congress,
Abstract 69-3. Revised 2005 in: 7th International Symposium on the Cretaceous, Scientific
Program and Abstracts, 198-199). In order to settle the current nomenclatorial flux and
promote chronostratigraphic stability, the succession of ammonite taxa throughout the
Cretaceous was analysed on a global scale. Ammonites constitute the principal fossil group
for Cretaceous biochronostratigraphy, with inoceramid bivalves and Foraminifera playing
an important complementary role. The aim of the study was to identify the major
ammonite turnovers as a first step towards a revised subdivision of the Cretaceous
System. The current two-fold division of the Cretaceous takes the base of the Cenomanian
Stage as the boundary between the Lower and Upper Cretaceous series. However, the
most significant ammonite turnovers occur at the bases of the Albian and Coniacian
stages, which provides support for a three-fold rather than two-fold division, to wit Lower
Cretaceous (Berriasian–Aptian), Middle Cretaceous (Albian–Turonian), and Upper
Cretaceous (Coniacian–Maastrichtian), with durations of approximately 33 Ma, 23 Ma, and
24 Ma, respectively. This subdivision agrees with that proposed already by Haug (1910,
Traité de géologie: II. Les périodes géologiques, Fascicule 2, 929-1396) but disagrees with
that of Scott et al. (2004, 2005), who place the series boundaries one stage lower, at the
base of the Aptian and Turonian stages, respectively. The pattern of taxonomic turnovers
among Cretaceous ammonites should be integrated with data on other fossil groups and
with geological and climatic events to form a solid base for a formal proposal for a revised
subdivision. Besides the chronostratigraphic benefits, this work will lead to a better
understanding of the biotic, climatic and physico-chemical conditions on Earth during the
Cretaceous Period.
Paleoinvertebrados