Deep-Sea Fans - Trace Fossils as Indicators of Sedimentary Environments - page 642

assemblages shows a distinct increase from the Cambrian to the Ordovician, but
later differences are considered as “not obvious”.
The diversity- versus -time graph ( Fig. 7 ; Uchman, 2004a ) indicates that the
diversity of deep-sea trace fossils markedly changed through the Phanerozoic,
with peaks in the Ordovician-Early Silurian and Early Carboniferous, a low in
the Permian to early Late Jurassic, a peak in the Tithonian to Aptian, a low in the
Albian, and a maximum peak in the Eocene. The contribution of graphoglyptids
gradually rose up to the end of the Mesozoic, with a peak in the Paleocene-
Eocene and a low in the Oligocene. The diversity changes were influenced
mostly by the availability of, and competition for, benthic food, the tempera-
ture, and oxygenation of bottom water, and also, indirectly, by the varying areas
of turbidite sedimentation (i.e., suitable deep-sea habitats). The diversity of
deep-water trace fossils was not severely affected by the major biotic crises,
such as those at the Ordovician/Silurian, Frasnian/Famennian, Triassic/Jurassic,
and Cretaceous/Tertiary boundaries (for details, see Uchman et al., 2005 ),
except for the minor Eocene/Oligocene crisis. However, the Ordovician/
Silurian, Cretaceous/Tertiary, and Paleocene/Eocene crises influenced the
graphoglyptid ichnodiversity and their relative abundance ( Uchman, 2003 ).
The decrease of diversity after the Late Carboniferous was probably caused
by the Gondwana glaciations and then reinforced by the Permian/Triassic mass
extinction. Recovery took a long time, and diversity remained low until the end
of the Jurassic, although the data from Oman show that the diversity in Tethyan
refuge areas was already high in the Late Triassic ( Wetzel et al., 2007 ).
The diversity of graphoglyptids ( Uchman, 2003 ) was low from the Cambrian
through the Middle Jurassic, with a minor peak in the Ordovician, followed by a
drop in the Silurian. In the Late Jurassic, there was a gradual increase. The
ichnologic radiation was probably delayed in the Early Cretaceous by the wide-
spread anoxia at that time. Then, a marked rise occurred in the Late Cretaceous,
probably during the Turonian. Graphoglyptids display an increase in complex-
ity with time ( Seilacher, 1977a ), showing a distinct acceleration in the infaunal
colonization during the Late Cretaceous, when the farming activity of their
tracemakers became a common feeding strategy. The Late Cretaceous radiation
of graphoglyptids may be correlated with global changes in the circulation and
oceanic settings, resulting in an increasing amount of plankton and organic
matter. In the Paleocene, the ichnodiversity decreased, but subsequently
increased (to a maximum) in the Eocene. The Eocene or Paleocene peak in gra-
phoglyptid diversity (total number of ichnogenera) and the Eocene peak in the
frequency of graphoglyptids may be related to the common occurrence of oce-
anic oligotrophy in the Late Paleocene and Early Eocene ( Uchman, 2003 ). The
Oligocene was marked by a sharp decrease in both graphoglyptid diversity and
frequency, followed by an increase in the Miocene ( Uchman, 2003 ). However,
the diversity trends are not applicable to every region, and some departures from
the model can be noted locally, viz. , where different phenomena can control
trace-fossil assemblages (e.g., L´pez Cabrera et al., 2008 ).