Page 435 - Introduction to Paleobiology and The Fossil Record
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422 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
and diplograptid evolutionary faunas (Chen suggest that, far from being a quiet period,
et al. 2006). These faunas not only help us the Silurian was punctuated by a number of
understand better the Ordovician radiation significant extinction events associated with
of the graptolites but add an additional large climatic and environmental fl uctuations.
dimension to the zonal framework of the Critical is the accurate correlation between
Ordovician System. sections in the Barrandian basin with sections
elsewhere, otherwise we cannot show these
Biostratigraphy: graptolites and time changes were indeed global. Higher in the
Silurian, Lennart Jeppsson and Mikel Calner
Graptolites are among the best zone fossils (2003) have reported that the Mulde Secundo-
(see p. 28) and are excellent for biostrati- Secundo Event (Wenlock), fi rst identifi ed in
graphic correlation. Traditionally, four the Silurian platform carbonates of the
sequential graptolite faunas have been recog- Swedish island of Gotland, includes three
nized through the Early Ordovician to Early extinctions, widespread deposition of carbon-
Devonian interval (Fig. 15.29). The anisograp- rich sediments, and wild sea-level fl uctuations
tid fauna, with Rhabdinopora and allied together with a glaciation event. These extinc-
genera, characterizes the Tremadocian; tions are related to a severe reduction in
although Upper Tremadocian graptolite primary plankton productivity. Amazingly,
faunas are rare, the genera Bryograptus, such precision and the recognition of these
Kiaerograptus and Aorograptus are impor- important events was only made possible by
tant and have been described in detail from the accurate graphic correlations of sections
western Newfoundland (Williams & Stevens based on the rapid evolution of these small,
1991). The appearance of the Floian dichograp- beautiful creatures.
tid fauna is signaled by Tetragraptus, associ-
ated with didymograptids and some relict
anisograptids. The later diplograptid
fauna contains four smaller units: the Biogeography: graptolites in space
Glyptograptus-Amplexograptus (Darriwil- Since the majority of graptolites lived either in
ian), Nemagraptus-Dicellograptus (Sand- the water column or within the plankton, quite
bian), Orthograptus-Dicellograptus (Katian) different factors infl uenced their distribution
and Orthograptus-Climacograptus (Hirnan- in contrast to, say, that of the coeval benthos.
tian) subfaunas. The monograptid fauna con- Provinciality was most marked in the earlier
tains a variety of evolving single-stiped forms. Ordovician (Darriwilian) when two main
The last graptoloids disappeared in the provinces, the Atlantic and Pacifi c, were recog-
Pragian (Lower Devonian). nized. The Atlantic province, including the
In some parts of the world graptolites have then high-latitude regions of Avalonia and
provided the basis for some high-resolution Gondwana, was characterized by pendent
stratigraphy. The Upper Ordovician–Lower Didymograptus species. The Pacifi c province,
Silurian clastic succession in the Barrandian including low latitude, tropical regions such as
basin in the Czech Republic was deposited on the Laurentian margins, was more diverse with
an outer shelf, influenced by the end-Ordovi- isograptids, cardiograptids and oncograptids.
cian Gondwanan glaciation and an aftermath The isograptid biofacies, itself, was more pan-
that included a persistent post-glacial anoxia demic, occupying deeper water and associated
related to upwelling systems. High-resolution with the world’s continental margins. During
graptolite stratigraphy based on some 19 the end-Ordovician extinction events (see p.
biozones has provided a framework to link 169), the Pacific province graptolites suffered
sedimentary environments with graptoloid particularly badly, and although graptolites
faunal dynamics and fl uctuations in organic again diversified during the Silurian their pro-
ˇ
content (Storch 2006). The resulting analyses vinciality developed a different if less obvious
have provided an accurate time line through pattern. During the Early Silurian, the Gond-
four major transgressive cycles. On these are wanan province was characterized by endemic
superimposed glacial and interglacial events, taxa while later, in the Mid Silurian, the
intervals of upwelling and oceanic perturba- equatorial region hosted taxa not known from
tions (Fig. 15.30). Moreover these new data elsewhere.
