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Diachronous recovery patterns in Early Silurian corals, graptolites and
acritarchs
DIMITRI
KALJO
Institute of Geology, Estonian Academy o f Sciences, 7 Estonia Ave, EEOIO0 Tallinn, Estonia
Abstract: The extinctions that occurred in the latest Ordovician and earliest and latest
Wenlock, were the most impressive in the biotic history of this period. They were probably
caused by glaciations. Recovery processes, more or less, follow the classical scenario:
extinction-low-diversity survival interval-recovery through radiation events. Rather often
there occurs diachrony of the phases of this scenario in the different groups discussed.
Graptolite recovery was the most rapid with a diversity maximum in the mid-Llandovery.
Their main extinction and low-diversity interval were in the latest Ordovician. The evolution
of corals and acritarchs was slower - after a few lower-scale origination events a diversity
burst was reached in the late Llandovery. An analogous but lower-scale pattern was noted at
the very beginning and in the late Wenlock. The difference was caused by evolutionary and
ecological reasons. A good correlation between diversity changes and terrestrial environmental events (glaciations, sea-level movements, stable isotope records) is noted.
Biotic recovery is understood in this contribution as restoration of taxonomic diversity of
biota (or part of it) after a crisis, to a higher,
but not necessarily to a pre-crisis level. Biotic
progress, the acquiring or forming of new
characteristics (innovations), the adopting of
new habitats etc., are favourable factors for
recovery, but do not belong to the last concept.
Different general diversity data of Silurian
biota have been published repeatedly, e.g. by
Sepkoski 1986. In more detail the topic was
discussed by Kaljo et al. (1995) in the final
m o n o g r a p h of the IGCP Global Bio-event
project. In summary, the diversity of Silurian
biota was strongly influenced by extinctions that
occurred in the latest Ordovician, earliest and
latest Wenlock and late Ludlow.
All these and a number of less significant
Silurian bio-events are generally believed to be
caused by terrestrial, environmental (climate
including glaciations, oceanic conditions, sealevel and facies changes, nutrient supply) and
biotic reasons (Kaljo et al. 1995). However, a
remote or indirect influence of cosmic agents,
like Milankovitch cycles, cannot be neglected
(Jeppsson 1990).
The same set of environmental factors should
also be considered when analysing different
biotic recovery scenarios. In this paper the
diversity dynamics of corals (sessile macrobenthos), graptolites (macroplankton) and acritarchs (microphytoplankton) will be discussed,
compared with each other and with some
parameters of the early Silurian environment.
Coral scenario
Corals experienced a severe crisis at the end of
the Ordovician with 62 out of 90 genera, i.e.
nearly 70%, becoming extinct. At the species
level only a few (in Estonia only 5% of species;
Nestor et al. 1991) passed into the early Silurian.
The ensuing renaissance of the coral fauna
proceeded in successive phases, with some
accleration of origination in the second half of
the Rhuddanian and especially in the Telychian.
The latter can be considered some kind of a
burst of corals. This is well illustrated by more
detailed Baltic tabulate data (Klaamann 1986):
in the early Rhuddanian (lower Gx-2, stratigraphic indices see Fig. 1) only one new genus
(Macleodia) appears; a step higher (upper G1-2),
two more (Halysites, Ramusculipora); in the late
Rhuddanian and Aeronian (G3), four (Favosites,
Parastriatopora, etc.); and in the Telychian
(H), eight new genera (Thecia, Subalveolites,
Placocoenites, Angopora, etc.) come in. In
summary (Fig. 1), the most energetic diversity
rise occurred in the Llandovery (84 new genera
appeared, i.e. 75% of the total fauna). Later,
origination became slower, but due to only a
moderate extinction rate in the Wenlock, at that
period the Silurian coral fauna reached its
maximum diversity (Scrutton 1988, 1989; Kaljo
& M/irss 1991).
The end-Ordovician mass extinction occurred
in Estonia (Nestor et al. 1991) in two main steps
(within the limits of the precision available). The
first one was at the end of the pre-Hirnantian
From Hart, M. B. (ed.), 1996, Biotic Recoveryfrom Mass Extinction Events,
Geological Society Special Publication No. 102, pp. 127-133
127
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128
D. KALJO
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Fig. 1. Coral diversity changes. Solid line - origination
rate; percentage of new genera per unit. Dashed line extinction rate; percentage of genera in a unit which
are absent in a succeeding one. Dashed line with black
dots - the number of coral genera per unit. Circle with
a point - the number of tabulate genera per unit from
Estonia only. Scale for Estonian tabulates is the first
from the right. Units: O3, late Ordovician; $2, Ludlow.
Stages: RHUD, Rhuddanian; AER, Aeronian; TEL,
Telychian; SHEIN, Sheinwoodian; HOMER, Homedan. Estonian stages: G1-2, Juuru; G3, Raikkiila; H,
Adavere; Jl, Jaani; J2, Jaagarahu; K~, Rootsikfila; K2,
Paadla. Black arrows, levels of glaciations suggested
by isotope studies.
Pirgu Stage and the second at the very end of the
Ordovician (among tabulate corals the extinction rate was 60 and 67 per cent correspondingly). During the last time interval, the first
corals of 'Silurian type' (e.g. dissepimentate
Paliphyllum and Strombodes among Rugosa)
appear. This appearance can be considered a
pre-recovery or an innovation event, which
created morphological preconditions for the
following recovery. During the Rhuddanian
these new forms remain relatively rare and
therefore the genera of 'Ordovician type' play
an important role in the coral assemblage of the
early and partly also of the mid-Llandovery.
Among this early post-crisis fauna there are
many corals of small size (Densiphyllum, some
Paleofavosites etc.), which show the 'Lilliput'
phenomenon, as discussed by Urbanek (1993) in
graptolite evolution. In our case these dwarfs are
especially striking as they contrast with some of
the latest Ordovician gigantic rugose and tabulate corals, which were rather common: the wellknown Grewinkia buceros is about 30cm high
while a coraUum of Mesofavosites dualis from
the Porkuni quarry (Estonia) is more than 1 m in
diameter.
The early Rhuddanian coral survival period
with lilliputs was an interval of only relatively
low diversity, not comparable, for example, with
the severe diversity drop in graptolites in the
latest Ordovician.
Coral origination intensity was highest in the
Telychian and began to fall in the Wenlock
(Fig. 1; Kaljo & M/irss 1991). Scrutton (1988)
demonstrated that this was true owing to the
predominance of Rugosa; the tabulate coral
origination maximum occurred before the Late
Ordovician mass extinction and never recovered
to the pre-crisis level.
Origination--extinction patterns in corals during the transition from the Ordovician to the
Silurian, seems to be correlated with the changing environment (Hirnantian glaciation with
accompanying sea-level drop and succeeding
climate amelioration, transgression, etc.). Later
in the early Silurian succession, environmental
and coral changes are not so clearly connected.
Of course, insufficiently detailed global scale
data on coral distributions hinder making farreaching conclusions. Therefore, most observations of this kind are easier to explain by local
ecological conditions (e.g. in Fig. 1 a diversity
low of Estonian tabulates in the late Wenlock is
strongly influenced by local factors), though
there may also be a global component of the
process.
Graptofite scenario
Graptolites have a detailed biostratigraphy and
their rapid evolution might be much better
correlated with different environmental changes.
Morphological innovations and diversity dynamics of graptolites were recently summarized
by Koren (in Kaljo et aL 1995). Therefore,
without going into details and using only a
graptolite diversity curve compiled by her (Fig.
2A), we can note some interesting concurrent
changes in biota and environment.
1. Graptolite diversity increase was most
pronounced in the Rhuddanian after a short
post-crisis low-idversity interval (extraordinarius
and persculptus zones, latest Ordovician, see
Table 1). This process has roots in a fundamental innovation, the 'uniserial event' (origination of a monograptid colony; Rickards 1988) in
persculptus time and in a series of radiation
events in the earliest Silurian (Berry et al. 1990;
Koren in Kaljo et al. 1995). Strikingly the
graptolite diversity increase is correlated with
post-glaciation warming of the climate (Spiroden Secundo Episode of Aldridge et aI. 1993,
which may nevertheless have been relatively
cool) and rapid sea-level rise and corresponding
changes in the state of oceanic waters.
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BIOTIC RECOVERY, EARLY SILURIAN
2. The maximum diversity of Silurian
graptolites was reached at the beginning of the
Aeronian (gregarius Zone). Later, especially in
the early Telychian (turriculatus-crispus and
griestoniensis zones), the extinction rate was
much higher than origination (Fig. 2B) and
therefore the diversity curve (Fig. 2A) was
falling drastically. A low stand was reached in
the early Wenlock (riccartonensis Zone), and for
the second time in the nassa Zone after the
lundgreni extinction event.
Brenchley et al. (1994) gave bathymetric and
carbon and oxygen isotopic evidence, showing
that the end-Ordovician glaciation, confined to
the first half of the Hirnantian, was a short
episode (0.5-1 Ma) in a long greenhouse period.
The Hirnantian is usually correlated with the
extraordinarius and persculptus graptolite zones,
characterized by a low-diversity assemblage. The
main graptolite extinction occurred in the
preceding pacificus Zone (Koren 1987), which
should coincide with the very beginning of the
glaciation marked by sea-level drop in the
earliest Hirnantian. At least in this case we can
see clear environmental reasons for graptolite
diversity change. Berry et al. (1990), referring to
data from South China, suggested a diachronous
glaciation event starting in the pacificus time in
high latitudes and reaching a low latitude area
later in the extraordinarius time.
Brenchley et al. (1994) suggest that during the
glacial event the pre-Hirnantian warm saline
deep waters changed to those with a strong
circulation of cold well-oxygenated bottom
waters and upwelling bringing a rich influx of
nutrients to the surface waters. High bioproductivity and carbon sedimentation caused the high
6~aC values identified in a number of sections in
Baltoscandia, North America and South China
(Brenchley et al. 1994).
The correlation of these data with the
diversity curve of graptolites shows that they
prefer to live in warm waters, which are not welloxygenated and stratified. Berry et al. (1990)
added that optimal conditions for graptolites
were bacteria-rich waters.
Early Silurian glaciations have been under
discussion for some time. Recently Grahn &
Caputo (1992) gave evidence for four of them:
the O/S boundary interval, the gregarius Zone,
early Telychian (possibly starting in the late
Aeronian) and the most wide-spread tillites
marking a glaciation from the latest Llandovery
to earliest Wenlock (Fig. 2A). Johnson &
McKerrow (1991) also advocated a late Wenlock glaciation, which caused a considerable
lowering of global sea-level.
In order to find independent data, geochem-
129
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Fig. 2. Diversity dynamics of Silurian graptolites. (A)
The curve shows the number of species per zone; 013)
the histogram shows the percentage of summary
changes of diversity from one unit to the next. In
zones 5, 13, 14, 15 changes are shown separately from
the lower and upper parts of the zone. The curve is
taken from Koren in Kaljo et al. 1995. Standard
graptolite zones are shown by numbers: 1, extraordinarius--persculptus; 2, acuminatus; 3, vesiculosus; 4,
cyphus; 5, gregarius; 6, convolutus-sedgwickii; 7, turriculatus-crispus; 8, griestoniensis; 9, crenulata; 10,
centrifugus-murchisoni; 11, riccartonensis; 12, rigidusellesae; 13, lundgreni; 14, nassa-deubeli; 15, ludensis;
16, nilssoni. Other units see Fig. 1 Letters with arrows:
C, levels of isotope data (613C); G, levels of glaciations
(see text). Letters: oceanic state episodes according to
Aldridge et al. 1993: S, secundo and P, primo episodes;
E, events.
istry and stable isotopes of Baltic Silurian rocks
were also studied at the Institute of Geology,
Estonian Academy of Sciences (Kaljo et al.
1994). The results of the study will be published
in full elsewhere, but here it would be appropriate to note the main conclusion. We have no
data on the isotopes in the Llandovery so far,
but in the Wenlock two very distinctive 613C
peaks occur in the Ohesaare borehole. One is in
the early Wenlock (riccartonensis Zone and the
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130
D. KALJO
beginning of the rigidus--ellesae Zone) and the
other in the late Wenlock (nassa Zone) reaching
+4.2 and 4.6%o levels respectively. Both peaks
were observed also in a graptolite mudstone
section in Latvia (Priekule). Jux & Steuber
(1992) recorded high 613C level values (+4.97
and 4.69%o) in the H6gklint and Tofta beds
(corresponds roughly to the riccartonensisrigidus peak), but not in the upper Wenlock.
Corfield et al. (1992) in turn established a 6~3C
peak and depletion in the upper Wenlock of
Britain (nassa Zone).
As mentioned above, the riccartonensis and
nassa zones are post-crisis low-diversity intervals
of graptolite evolution, which precede the
radiation events and diversity rise. The comparison of those with the described end-Ordovician-earliest Silurian e x t i n c t i o n - s u r v i v a l recovery scenario shows great similarity in many
aspects. This suggests that all three were directed
by the same factors - climate, oceanic state and
bioproductivity. Depending on the evolutionary
(phenotypic) situation, the resulting diversity
curve may differ to some extent.
Acritarch scenario
Acritarch diversity was recently analysed by Le
Herisse (in Kaljo et al. 1995). According to his
data (used below), near the Ordovician-Silurian
boundary there was no drastic acritarch extinction, but many genera and species disappeared
during a longer period in the Late Ordovician.
The same can be seen from data provided by
Uutela & Tynni (1991). Because of this the
Ordovician and Silurian acritarch floras differ
considerably and the few 'Silurian'-type genera
that appeared in the Hirnantian do not change
the above conclusion.
In the early Llandovery (acuminatus Zone) a
low-diversity interval occurred, where several
genera passing from the Ordovician are represented by small-sized species like 'lilliputs'.
The first radiation event among acritarchs
occurred in the middle Rhuddanian (vesiculosus
Zone), where six very distinctive genera appeared. Acritarch diversity then increases step
by step through the mid- and late Llandovery to
a maximum level in the Telychian and at the
very beginning of the Wenlock (centrifugusmurchisoni Zone), especially on the species level
(Fig. 3).
This was followed by a major extinction event
and a diversity low in the middle Sheinwoodian
(riccartonensis Zone and the beginning of the
rigidus-ellesae Zone). Analogous diversity
changes, but on the lower level, occurred also
in the late Sheinwoodian and Homerian with a
Total
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Fig. 3. Diversity of early Silurian acritarchs. Circle
with a point-the number of genera per unit; solid line the number of species per unit; dashed line - total rate
of appearing species (number of appearing taxa per
1 Ma). Both species level curves are based on data
from the Silurian of Gotland. Data for the figure are
taken from Le Herisse in Kaljo et al. 1995. Black
arrows - levels of glaciations suggested by isotope
studies. Standard graptolite zones are shown by
numbers, for explanation see Fig. 2.
maximum in the ellesae (species) and lundgreni
(genera) zones and with a minimum in the
ludensis Zone.
The acritarch diversity dynamics has much in
common with that of corals in the Llandovery,
i.e. the early Rhuddanian low diversity interval,
the first radiation in the middle Rhuddanian and
diversity burst in the Telychian. Wenlock curves
are different, but bearing in mind the falling
appearance rates in both groups, the general
process seems to be the same.
The most important difference between acritarchs and graptolites is the very early diversity
peak of the latter in the early Aeronian.
Wenlock curves of both groups are more or less
parallel.
Summary and conclusions
The above-discussed patterns of diversity dynamics of three ecologically different groups of
biota show some striking similarities and differences. In Table 1 these are correlated with
environmental parameters. Using these data we
may deduce the following.
Mass extinction of corals (El) and graptolites
(Emax) occurred at the end of a warm greenhouse
period or at the very beginning of a glacial event
(end of the pacificus Zone). At that time (and
until the end of the Ordovician) acritarchs
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BIOTIC RECOVERY, EARLY SILURIAN
131
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Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on February 18, 2016
132
D. KALJO
experienced normal background or stepwise
extinction. The extinction of corals accelerated
in the Hirnantian icehouse period concurrently
with the cooling of the ocean, although the water
was rich in oxygen and nutrients. The shallow
shelf sea, which was a habitat for giant corals,
might still have remained warm owing to
tropical sunlight in the equatorial areas (e.g.
Baltica). During this period graptolites survived
at a low diversity level. All three groups,
however, displayed different morphological innovation and origination of a few new taxa. At
the very beginning of the Silurian, graptolites
demonstrated a very rapid diversity rise (Fig. 2),
whereas corals and acritarchs experienced a brief
low-diversity period and much slower origination of new taxa.
This correlation allows us to conclude the
following.
1. As is commonly known, corals preferred to
live in well-aerated waters rich in food. Their
diversity drop therefore seems to have been
caused by the lowering of temperature at the
outset of glaciation (E 1 extinction; Table 1) and,
perhaps, by the reduction or even loss of suitable
habitats (E2) due to a rapid sea-level rise at the
very beginning of the Silurian. The giants
recorded in the Porkuni Stage in Estonia and
elsewhere were evidently inhabiting warmer
niches.
2. Graptolites were expected to show low
diversity in the glacial period, because as shown
by Berry et al. (1990), they usually occur in
oxygen-poor waters.
3. Most biomass is usually produced by
planktic, especially microplanktic organisms.
Owing to their relatively low diversity in the
latest Ordovician, the share of the groups
discussed in the summary bioproduction seems
to have been insufficient for ensuring high values
of 613C (Brenchley et al. 1994; Table 1). The real
reason for this still remains obscure, especially
because there is no direct dependence between
diversity and bioproduction. A good example is
the Monograptus riccartonensis low-diversity
assemblage producing a considerable amount
of biomass due to the mass occurrence of the
index-species.
The end-Ordovician-Early Silurian sequence
of bioevents was discussed in more detail in
order to get an idea of the pattern followed by
different groups through the extinction-survival-recovery scenario. An analogous diachronous pattern is observed at the LlandoveryWenlock junction (the extinction of graptolites
began just before or at the very beginning of the
glaciation; Fig. 2; Melchin 1994; a strong
diversity drop of acritarchs followed a step later,
etc., see above) and in a less pronounced form
also in the late Wenlock.
In general, proceeding from the above data
and discussion, we can draw the following
conclusions:
1. After the Late Ordovician, early and late
Wenlock, probably glaciation- (climate-) triggered extinctions, the recovery processes of
corals, graptolites and acritarchs took place
more or less according to the classical scenario:
extinction-survival-recovery, but are different in
detail as explained above.
2. Diachrony of phases of this scenario in
different groups discussed occurs frequently:
main extinctions might be coinciding (sometimes
partly) or successive or show a stepwise pattern:
survival or low-diversity interval and recovery of
these groups began correspondingly at slightly
different time levels. The latter might be rapid in
a short interval (graptolites) or occur as a slow
prolonged rise of diversity (corals), sometimes
with a burst in a certain time (acritarchs).
3. The differences in diversity dynamics seem
to depend on evolutionary and ecological
characteristics of the group of organisms involved.
This could account for the rapid response of
graptolites (early diversity peak) to the changing
environment with the profound innovations as
pointed out above. Surprising is the parallelism
in coral and acritarch dynamics.
4. For understanding mutual relations and
influence in an ecosystem (organisms + environment), exact dating of different events, isotope
excursions, etc., seems to be crucial.
The author thanks P. J. Brenchley for his helpful
comments and suggestions. His colleagues T. Kiipli, T.
Martma and A. Noor are thanked for their help. The
study was partly supported by International Science
Foundation (grant No. LC 4000) and Estonian Science
Fund (grant No. 314).
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