surrounding the purported “microfossils” in the ~3.5 gyr
Apex Chert (e.g., Schopf, 1993; Marshall et al., 2011).
The acritarchs, an important microfossil group of uncertain and perhaps polyphyletic systematic position, first
occur in rocks from the Meso-/Neoproterozoic transition
(Knoll et al., 2006) and are the earliest age-diagnostic
microfossils, used for correlation in upper Proterozoic
and lower Paleozoic sediments (Mendelson, 1993).
Radiolarians and foraminifera (Figures 1 and 2), both
extant groups, are known from the Cambrian onward
(Casey, 1993; Boersma, 1998) and are important biostratigraphic indicators. For instance, the fusulinid and
schwagerinid benthic foraminifera commonly grew to large
sizes and underwent a particularly rapid evolution during
the Carboniferous and Permian, which makes them useful
zonal markers for the younger Paleozoic (Davydov
et al., 2012; Henderson et al., 2012). However, the toothlike
conodonts, which range from Cambrian through Triassic
and are believed by many to represent the feeding apparatus
of early chordates (Donoghue et al., 2000), provide the primary means of global age correlation of Paleozoic strata.
Simple vaselike chitinozoa, which are presumed to represent an ontogenic stage (i.e., eggs) in the life cycle of a cryptic animal group (Paris and Nolvak, 1999), first appeared in
the Ordovician, with morphologically more complex forms
evolving through the Paleozoic before the extinction of the
group in the late Devonian (Grahn and Paris, 2010).
Scolecodonts – fossil worm jaws – are a minor group which
occur from the Paleozoic onward but are of little biostratigraphic use despite being found most commonly in lower
Paleozoic rocks. Another biostratigraphically important
microfossil lineage in Paleozoic and younger marine sediments is the ostracods, first identified in the Ordovician.
Both the end-Permian and Triassic brought about major
turnovers in oceanic microbiota which led to the
development of three prominent phytoplankton groups during the Mesozoic, calcareous nannoplankton, dinoflagellates (Figure 4), and diatoms (Figure 3) (Falkowski
et al., 2004), and the rise of the zooplankton – planktic foraminifera (Hart et al., 2003). Although not as significant in
today’s oceans, other microfossil groups that appeared later
in the Mesozoic include silicoflagellates (McCartney
et al., 2014) and ebridians (Hoppenrath and Leander, 2006).
Microfossils in biostratigraphy
Microfossils can occur in marine sediments in large abundances (yielding up to several billion individuals per gram
of dry mass, e.g., Davies et al., 2009) and have widespread
distributions and relatively rapid evolutionary rates making them ideal targets for dividing up geological time
and correlating spatially disparate sediments. The advent
of deep-sea drilling in recent decades allowed microfossils
rather than macrofossils to form the basis for age determination in marine sedimentary sequences. However, since
most chronostratigraphic units were defined based on
onshore exposures where microfossils are absent or sparse
(Bolli et al., 1985), one of the major challenges of modern
micropaleontology has been to tie emerging microfossil
biostratigraphic zonations to the evolving chronostratigraphic framework (e.g., Cande and Kent, 1995;
Gradstein et al., 2012). For planktic foraminifera and calcareous nannofossils, most bioevents are now tied directly
to the geomagnetic polarity time scale (GPTS; Berggren
et al., 1995; Wade et al., 2011), and many dinoflagellate
bioevents are also so calibrated (e.g., Eldrett et al., 2004;
Brinkhuis et al., 2003). However, in the longer term the
aim is to fully calibrate microfossil-based data to the astronomical time scale, which provides the potential for much
higher temporal resolution than that based on magnetics
(e.g., Wade et al., 2011).
Here we focus on two examples from the Cenozoic to
illustrate how calcareous and siliceous microfossils are
used as zonal markers in low- and high-latitude sites.
Low-latitude Eocene zonation (56.0–33.9 million
years ago)
Planktic foraminiferal and calcareous nannofossil biostratigraphy provide the basis for age control in most
low-latitude Cenozoic marine sediments. There are currently 16 planktic foraminiferal biostratigraphic zones recognized in Eocene low-latitude deep-sea sediments,
prefixed “E” by Berggren and Pearson (2005) (Figure 5).
Magnetochronologic calibration is available for all of these
zones, and the five uppermost zones are also astronomically
calibrated (Wade et al., 2011). Low-latitude siliceous microfossil schemes are considerably less advanced than those
based on calcareous and other microfossils largely due to
a preservational bias acting against siliceous microfossils
at low latitudes. However, the Eocene contains 13 radiolarian zones that are tied directly to the GPTS (Nigrini
et al., 2005). In contrast, diatom biozones are typically only
indirectly calibrated to the GPTS via correlation to planktic
foraminifer, calcareous nannofossil, and/or radiolarian
zones. Recently, Barron et al. (2015) have provided considerable amendments to the preexisting low-latitude diatom
zonation scheme of Fenner (1984, 1985), and 13 zones
are identified. But some diatom data remain uncertain, and
no information is currently available to evaluate diachrony
of key marker species, such as with foraminifera (e.g.,
Edgar et al., 2010) (Figure 5).
High-latitude zonation for the neogene
Siliceous microfossils offer better age control than their
calcareous counterparts in high-latitude marine records,
particularly in the Southern and equatorial Pacific Oceans
which are characterized by widespread deposition of
silica-rich sediments (Ragueneau et al., 2000) throughout
the Cenozoic, enabling the development of detailed zonal
schemes based on diatom bioevents (e.g., Harwood and
Maruyama, 1992; Censarek and Gersonde, 2002;
Kamikuri et al., 2012; Moore et al., 2015). Using data
from 32 Neogene deep-sea sites in the Southern Ocean,
Cody et al. (2008) compiled a computer-aided quantitative
synthesis of 116 diatom first and last occurrences relative
MARINE MICROFOSSILS
471
Apex Chert (e.g., Schopf, 1993; Marshall et al., 2011).
The acritarchs, an important microfossil group of uncertain and perhaps polyphyletic systematic position, first
occur in rocks from the Meso-/Neoproterozoic transition
(Knoll et al., 2006) and are the earliest age-diagnostic
microfossils, used for correlation in upper Proterozoic
and lower Paleozoic sediments (Mendelson, 1993).
Radiolarians and foraminifera (Figures 1 and 2), both
extant groups, are known from the Cambrian onward
(Casey, 1993; Boersma, 1998) and are important biostratigraphic indicators. For instance, the fusulinid and
schwagerinid benthic foraminifera commonly grew to large
sizes and underwent a particularly rapid evolution during
the Carboniferous and Permian, which makes them useful
zonal markers for the younger Paleozoic (Davydov
et al., 2012; Henderson et al., 2012). However, the toothlike
conodonts, which range from Cambrian through Triassic
and are believed by many to represent the feeding apparatus
of early chordates (Donoghue et al., 2000), provide the primary means of global age correlation of Paleozoic strata.
Simple vaselike chitinozoa, which are presumed to represent an ontogenic stage (i.e., eggs) in the life cycle of a cryptic animal group (Paris and Nolvak, 1999), first appeared in
the Ordovician, with morphologically more complex forms
evolving through the Paleozoic before the extinction of the
group in the late Devonian (Grahn and Paris, 2010).
Scolecodonts – fossil worm jaws – are a minor group which
occur from the Paleozoic onward but are of little biostratigraphic use despite being found most commonly in lower
Paleozoic rocks. Another biostratigraphically important
microfossil lineage in Paleozoic and younger marine sediments is the ostracods, first identified in the Ordovician.
Both the end-Permian and Triassic brought about major
turnovers in oceanic microbiota which led to the
development of three prominent phytoplankton groups during the Mesozoic, calcareous nannoplankton, dinoflagellates (Figure 4), and diatoms (Figure 3) (Falkowski
et al., 2004), and the rise of the zooplankton – planktic foraminifera (Hart et al., 2003). Although not as significant in
today’s oceans, other microfossil groups that appeared later
in the Mesozoic include silicoflagellates (McCartney
et al., 2014) and ebridians (Hoppenrath and Leander, 2006).
Microfossils in biostratigraphy
Microfossils can occur in marine sediments in large abundances (yielding up to several billion individuals per gram
of dry mass, e.g., Davies et al., 2009) and have widespread
distributions and relatively rapid evolutionary rates making them ideal targets for dividing up geological time
and correlating spatially disparate sediments. The advent
of deep-sea drilling in recent decades allowed microfossils
rather than macrofossils to form the basis for age determination in marine sedimentary sequences. However, since
most chronostratigraphic units were defined based on
onshore exposures where microfossils are absent or sparse
(Bolli et al., 1985), one of the major challenges of modern
micropaleontology has been to tie emerging microfossil
biostratigraphic zonations to the evolving chronostratigraphic framework (e.g., Cande and Kent, 1995;
Gradstein et al., 2012). For planktic foraminifera and calcareous nannofossils, most bioevents are now tied directly
to the geomagnetic polarity time scale (GPTS; Berggren
et al., 1995; Wade et al., 2011), and many dinoflagellate
bioevents are also so calibrated (e.g., Eldrett et al., 2004;
Brinkhuis et al., 2003). However, in the longer term the
aim is to fully calibrate microfossil-based data to the astronomical time scale, which provides the potential for much
higher temporal resolution than that based on magnetics
(e.g., Wade et al., 2011).
Here we focus on two examples from the Cenozoic to
illustrate how calcareous and siliceous microfossils are
used as zonal markers in low- and high-latitude sites.
Low-latitude Eocene zonation (56.0–33.9 million
years ago)
Planktic foraminiferal and calcareous nannofossil biostratigraphy provide the basis for age control in most
low-latitude Cenozoic marine sediments. There are currently 16 planktic foraminiferal biostratigraphic zones recognized in Eocene low-latitude deep-sea sediments,
prefixed “E” by Berggren and Pearson (2005) (Figure 5).
Magnetochronologic calibration is available for all of these
zones, and the five uppermost zones are also astronomically
calibrated (Wade et al., 2011). Low-latitude siliceous microfossil schemes are considerably less advanced than those
based on calcareous and other microfossils largely due to
a preservational bias acting against siliceous microfossils
at low latitudes. However, the Eocene contains 13 radiolarian zones that are tied directly to the GPTS (Nigrini
et al., 2005). In contrast, diatom biozones are typically only
indirectly calibrated to the GPTS via correlation to planktic
foraminifer, calcareous nannofossil, and/or radiolarian
zones. Recently, Barron et al. (2015) have provided considerable amendments to the preexisting low-latitude diatom
zonation scheme of Fenner (1984, 1985), and 13 zones
are identified. But some diatom data remain uncertain, and
no information is currently available to evaluate diachrony
of key marker species, such as with foraminifera (e.g.,
Edgar et al., 2010) (Figure 5).
High-latitude zonation for the neogene
Siliceous microfossils offer better age control than their
calcareous counterparts in high-latitude marine records,
particularly in the Southern and equatorial Pacific Oceans
which are characterized by widespread deposition of
silica-rich sediments (Ragueneau et al., 2000) throughout
the Cenozoic, enabling the development of detailed zonal
schemes based on diatom bioevents (e.g., Harwood and
Maruyama, 1992; Censarek and Gersonde, 2002;
Kamikuri et al., 2012; Moore et al., 2015). Using data
from 32 Neogene deep-sea sites in the Southern Ocean,
Cody et al. (2008) compiled a computer-aided quantitative
synthesis of 116 diatom first and last occurrences relative
MARINE MICROFOSSILS
471