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Chapter 70. Patterns and rates of evolution among Mesozoic Ostracoda
1022 R. WHATLEY
towards or against any part of the Mesozic nor for or against any one or more cytheracean families.
Thus we feel that these deficiencies do not substantially subtract from the value of our results”.
(1976, pp. 63, 64). The present paper demonstrates (Text-fig. 7) that, with respect to the Cytheracea,
Whatley and Stephens were certainly biased against the Triassic, although the updated figures for
both genera and species elsewhere in the Mesozoic generally confirm the trends they originally
reported. The present author is confident that, to the date of writing (April, 1985), he has
abstracted data from virtually all published works on the ostracods of the Mesozoic.
Of the 1267 references employed in the compilation of data for the present study, only 17 or 1.3 %
were published prior to 1850. Between 1850 and 1899, 83 papers or 6.5% were published and
between 1900 and 1850, 151 papers, representing 12% of the total. Between 1951 and 1975, no less
than 793 papers (63%) were published and since 1975 (the date of Whatley and Stephens’
presentation was 1976) 223 papers on Mesozoic ostracods have been encountered by the author,
representing 18% of the total.
Any study of this nature is rendered difficult to a degree by a variety of problems. For example,
it is often difficult to relate stratigraphical data on the ranges of species and genera to currently
accepted International Standards for the Mesozoic. This problem is particularly acute when using
older literature, but a surprisingly high number of more modern papers refer to the ranges of taxa
as being “Jurassic” or “Cretaceous”, for example. However in such cases it is usually possible to
refine the given ranges by reference to other publications on either the Ostracoda or the strata.
In the various figures and tables it can be seen that a number of stratigraphical compromises
have been made. For ease of illustration, the Upper Cretaceous stages Coniacian, Santonian and
Campanian have been incorporated within a single entity, the Senonian. Also, the Cretaceous is
herein regarded as a bipartite entity rather than separating the Aptian and Albian stages as
“Middle Cretaceous”. The stages of the Neocomian have been retained despite the difficulty of
dealing with the numerous records of species whose stratigraphical range is given as “Wealden”.
Tithonian and Volgian records have been accommodated within the “Portlandian” and “Purbeckian” although this has caused some difficulties particularly with respect to the occurrence of the
Jurassic/Cretaceous boundary within the latter “stage”. There are, in fact, many fewer records
which are “Tithonian” or “Volgian” than there are for the two “stages” employed here.
With respect to the Triassic, for ease of illustration, the divisions Lower, Middle and Upper
(which includes the Rhaetian) have been employed. Relatively little difficulty was experienced in
accommodating records of taxa from the “Bunter”, “Muschelkalk” and “Keuper“ into these
Without doubt, synonyms and homonyms created the greatest difficulty and consumed the
largest proportion of time during the preparation of this work. There may be some which have
Patterns and Rates of Evolution among Mesozoic Ostracoda 1023
slipped through the net, but the author is confident that he has eliminated the vast majority of them.
The time scale employed in the construction of the various figures is taken from Harland et al.
OF THE TOTAL
The total number of species encountered in the literature search was 6797 belonging to 739
genera (excluding subgenera). These taxa have been grouped into the following suprafamilial
categories : Cytheracea, Cypridacea, Bairdiacea, Darwinulacea, Platycopina, Metacopina, Cladocopina, Myodocopina and Palaeocopida. The classification employed is essentially sensu Moore
TABLEZ-NUMBER OF SPECIES AND GENERA
(First appearances of taxa in brackets).
91 ( 5 )
61 ( 5 )
1961 with certain necessary modifications. Although a more sophisticated classification could have
been employed, given the current somewhat fluid state of ostracod taxonomy, the author has
chosen not to pre-empt the Treatise revision.
Table 2 shows the distribution of all species and genera of ostracods in the Mesozoic in terms
of number of taxa per stage, division and system. The table also gives, as a measure of evolutionary
activity, the number of new taxa first appearing in each unit and also the percentage of new taxa.
Numbers of genera and species in the Triassic are higher than in the Jurassic, althaugh a significant increase in the number of taxa is shown to take place throughout the Jurassic. The number
of genera and species in the Lower Cretaceous is higher than in any part of the Jurassic. Upper Cretaceous ostracod diversity, however, is significantly higher than that of the Lower Cretaceous.
These general trends are very clearly illustrated by the mean number of species per stage (in the
case of the Triassic the tripartite divisions are used).
Mean No. of Species per Stage
As will be demonstrated later in this paper, the decline from a relatively high number of species
R E T A C EOUS
1-Histograms illustrating the simple species diversity and the inherited versus new component of all
ostracod species for each stage of the Mesozoic.
Patterns and Rates of Evolution among Mesozoic Ostracoda 1025
to the lowest number for the Mesozoic in the Liassic, is due to a number of factors. Among them
is the virtual extinction of the Palaeocopida in the Triassic. Other groups, such as the Cytheracea
and the Bairdiacea, which flourished subsequently in the Mesozoic, underwent a crisis at the
Triassic/Liassic boundary as witnessed by the sharp decline in their diversity. Several families of the
former superfamily became extinct at this time, an almost unique event in the Mesozoic.
The largest increment in the mean number of species per stage takes place from the Upper Jurassic to the Lower Cretaceous and the second largest increment is between the Lower and Upper
Cretaceous. The former is largely accounted for by the appearance in the Neocomian of large numbers of nonmarine cyprids; the latter is largely due to a major adaptive radiation of the Cytheracea.
The mean number of species per stage for each system also clearly reveals the same trend with a
mean number of 431 species for the Triassic, 296 for the Jurassic and 899 for the Cretaceous.
In Text-fig. 1, the total number of ostracod species per stage for the entire Mesozoic is plotted,
showing the relationship between new and inherited species. All species in the Lower Triassic
are assumed to be new. The following 12 stages have less new than inherited species:
The following 13 stages have more new than inherited species:
This is illustrated graphically in Text-fig. 2. Origination rates almost always increase notably in
the first stage of a system or major system division viz. Lower Triassic, Hettangian, Aalenian, Berriasian and Cenomanian. From such relatively high origination rates, there is an immediate and
often dramatic decline to much lower levels of first appearances of species, e.g. Middle Triassic,
Sinemurian, Bajocian, Valanginian and Turonian. This low level is generally continued until the
next major boundary. The only boundary which is not associated with an increase in origination
rates is the Middle/Upper Jurassic, there being a decline from the Callovian to the Oxfordian.
As Text-figs. 1 and 2 show, there is a complex relationship between the total number of species
per stage and the percentage of those species which are new. For example, although there is a high
percentage of new species in the Lower Triassic, the total number of species is the lowest of the
three Triassic divisions. Similarly, the low number of total species in the Hettangian and Aalenian
does not match the high percentage origination levels of these stages. However, the Berriasian has
the highest number of species and the highest proportion of new species of any of the Neocomian
stages. In the case of the Cenomanian, the number of species, although higher than at any previous
time in the Mesozoic and higher than the succeeding Turonian, is relatively low for the Upper
Cretaceous, despite the fact that 314 of its species are new.
Although the major system and intra-system boundaries for the Mesozoic were established on
thc: stratigraphical distribution of other fossil organisms, notably ammonoids, both Text-figs 1
and 2 demonstrate the validity of these boundaries and their ability to be recognized on the basis
of the distribution of Ostracoda.
percentage of newly appearing species in each stage of the Mesozoic for all Ostracoda.
Text-fig. 3 illustrates graphically the relationship between the number of species which first
appear and which become extinct in each stage.
In the following 12 stages, extinctions exceed originations:
The probably invalid assumption is made that all Mesozoic species became extinct at the Cretaceous/Tertiary boundary.
In the following 13 stages, originations exceed extinctions:
Patterns and Rates of Evolution among Mesozoic Ostracoda 1027
relationship between originations and extinctions of species for all Mesozoic stages and all
High rates of extinction in either absolute or proportional terms can be seen to precede the
major divisions of the Mesozoic. The extinction rate rises in the Triassic to an Upper Triassic climax and this is succeeded by a low rate in the Hettangian, the lowest for the Mesozoic. Similar
phenomena can be seen when one compares extinction rates for the Pliensbachian and Aalenian,
Bathonian and Oxfordian, Tithonian and Berriasian, Barremian and Aptian, Albian and Cenomanian and the Maastrichtian high also probably precedes a very low extinction rate for the
As Text-figs. 1-3 show, in most of the instances cited above, the low level of extinction of species
which follow the absolute or proportional high levels correlate well with proportional or absolute
high origination rates. These have already been noted with respect to the Hettangian, Aalenian,
Berriasian, and Cenomanian.
The relationship between the level of origination and extinction is not only a measure of the
rate of evolution, it is also a reflection of mechanisms of regulation by which diversity levels are
maintained. Text-figure 1, however, demonstrates very clearly that simple species diversity fluctuated
considerably during the Mesozoic. In simple terms, it was higher in the Upper Triassic than at
any time in the JuraLsic and considerably higher throughout the Cretaceous than at any previous
time in the Mesozoic. The simple species diversity of the Neocomian, for example, is approximately
70% higher than that of the Upper Triassic, while that of the Maastrichtian is approximately
1028 R. WHATLEY
double that of the Upper Triassic and approximately 10.5 times greater than that of the Toarcian,
in which stage the lowest diversity of the Mesozoic is recorded.
There are many factors which can influence results such as these. Taphonomy must always be
considered, but this alone could not possibly be responsible for the differing diversites of Ostracoda
through the Mesozoic. Intensity of research is another possibility, but, as Table 1 shows, for
example, more studies have been carried out on Jurassic than on Triassic Ostracoda. Variable levels
of research activity may somewhat prejudice the results in favour of the Cretaceous but, as Whatley
and Stephens (1976, fig. 6) demonstrate for the Cytheracea, this is not likely to introduce major
discrepancies. The reasons for changes in diversity are discussed later.
For the most part, the distribution in terms of originations, extinctions and diversity of genera
reflect those of species. The greater longevity of genera being the principal difference between the
The same major trends in generic diversity as in species occur, as can be seen when the mean
number of genera per stage is considered.
&Histograms illustrating the simple generic diversity and the inherited versus new component of all
ostracod genera for each stage of the Mesozoic.
Inherited genera; m,New genera
Patterns and Rates of Evolution among Mesozoic Ostracoda 1029
Mean No. Genera per Stage Increment
A sharp decrease in mean generic diversity from the Triassic to the Liassic is followed by an
increase in mean diversity to the Upper Cretaceous. The trend for genera differs from that of
species only in that the incremental difference between Lower Jurassic/Middle Jurassic and Middle
Jurassic/Upper Jurassic is reversed. This difference is a function of the greater longevity of genera,
coupled with the fact that a large number of Jurassic genera make their first appearance in the
Aalenian-Bathonian interval (Table 2). When considered at the system level, the mean generic
diversity trend is 102 for each Triassic division and 81 and 173 for the Jurassic and Cretaceous
In Text-fig. 4, plotted as a series of histograms, the relationships between new and inherited
genera is given by stage. All genera in the Lower Triassic are, for convenience, assumed to be new.
When compared to Text-fig. 1 which demonstrates the same distribution for species, a number of
features become apparent :
5-The percentage of new genera in each stage of the Mesozoic for all Ostracoda.
--- - Originations
&The relationship between originationsand extinctions of genera for all Mesozoic stages and all Ostracoda.
1) The major trends for species are mirrored by genera.
2) Because of the greater longevity of genera, the inherited component of the histograms is larger.
3) Although generic diversity in the Hettangian-Aalenian interval is much lower than that of the
preceeding Triassic, that of the Bajocian-Purbeckian is more similar to that of the Triassic than
was the case with species.
4) The generic diversity for all Cretaceous stages is higher than at any preceeding interval in the
The percentage of new genera per stage is plotted in Text-fig. 5. Although in Text-fig. 2, which
plots the same percentage for species, no less than 13 stages have more than 50 % new species, apart
from the Lower Triassic (where all genera are assumed to be new) no other Mesozic stage has more
than 50 % new genera. With respect to species, highest levels of origination occur in the first stage
after a system or major infra-system boundary. This effect is less clear with respect to genera, but
the generic origination level is higher in the Hettangian than in any other Liassic stage and also that
of the Berriasian, with 49%, is higher than at any subsequent stage in the Cretaceous.
The relationship between originations and extinctions of genera is given in Text-fig. 6 . As with
species (Text-fig. 3), enhanced extinction rates take place immediately prior to major boundaries
within the Mesozoic viz. Upper Triassic, Toarcian, Callovian, Purbeckian, Barremian, Albian and
Maastrichtian. As Text-figs. 5 and 6 show, in many instances, these elevated extinction rates are
Patterns and Rates of Evolution among Mesozoic Ostracoda 1031
followed by enhanced origination rates ; the best examples are the Berriasian, Aptian and Cenomanian.
OF THE MAJOR
The distribution of genera and species of 9 suprafamilial groups of Ostracoda has been analysed.
These comprise the superfamilies Cytheracea, Cypridacea, Bairdiacea and Darwinulacea, the
suborders Platycopina, Metacopina, Myodocopina and Cladocopina and the order Palaeocopida.
The largest Mesozoic category (by both species and genera) is the Cytheracea. Text-fig. 7 compares graphically the results produced by Whatley and Stephens (1976, fig. 6 ) with those calculated
to April 1985 for the distribution of cytheracean species. The 1976 figures for the Triassic were
much too low. This was due to the fact that the authors of that paper did not have access to a
number of important papers on Eastern European Triassic faunas. The 1985 figures for Triassic
species are also a reflection of a considerable degree of research activity in the system post 1975
(Table 1). For the remainder of the Mesozoic, the graph for 1985 closely follows that for 1976, the
only divergence being in the Cenomanian. In all stages, the 9 years since the publication by
Whatley and Stephens has seen the description of new species.
Whatley 8 Stephens
7-Graphs showing the simple species diversity of Cytheracea for each Mesozoic stage for 1976(WhatIey
and Stephens) and 1985.
1032 R. WHATLEY
The distribution of cytheracean species departs somewhat from that of all Mesozoic ostracod
species (Text-fig.1). The principal difference is that, although Triassic diversity levels are higher
than those of the Hettangian-Bajocian interval, those of the Upper Jurassic are as high or higher
than those of the Triassic and also higher than those of the Neocomian. The Liassic was the time of
lowest species diversity among the Cytheracea. Although many of the Triassic taxa described in the
literature are thought to have been brackish, given the restricted extent of marine environments
in the Triassic as opposed to the extensive marine transgressions of the Liassic, it is surprising that
an essentially marine superfamily such as the Cytheracea should not have proliferated in the Lower
Jurassic. Elsewhere in the Mesozoic, an overall relationship between cytheracean diversity and
palaeoenvironment can be seen. A good example is the decline in diversity from the marine dominant Upper Jurassic to the Neocomian, a period of widespread global regression. This is followed
by a major transgressive event initiated in the Aptian, at which time most Wealden non-marine
basins were eliminated, and culminating in the Upper Cretaceous. The response to this event is
clearly seen in Text-fig. 7 with (apart from a decline in the Turonian which will be discussed later)
a fairly regular increase in cytherid diversity from Aptian to Maastrichtian. Cytherid species
diversity for this last stage of the Mesozoic is 7 times higher than that of the Berriasian, the least
diverse part of the Cretaceous and 15 times greater than that of the Toarcian, the least diverse part
of the Jurassic (and the Mesozoic). Maastrichtian cytherid diversity is also 2.7 times higher than
2 : g
...... .". ........Platycapina
8-a. The numerical distribution of species of Bairdiacea and Platycopina through the Mesozoic.
b. The numerical distribution of species of Cytheracea and Cypridacea through the Mesozoic.