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Chapter 70. Patterns and rates of evolution among Mesozoic Ostracoda

Chapter 70. Patterns and rates of evolution among Mesozoic Ostracoda

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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.

TABLE1-STRATIGRAPHICAL

AND HISTORICAL

DISTRIBUTION

OF PUBLICATIONS

ON MESOZOIC

OSTRACODA.

Triassic

Pre 1850

1850-1899

1900- 1950

1951-1975

Post 1975



No

0

11

4

137

22

174



Jurassic



%



0

6.3

2.2

79.8

12.6



No

11

29

32

25 1

67

390



Cretaceous



%

2.8

7.4

8.2

64.3

17.1



No

15

43

115

405

134

712



Mesozoic



%

2.1

6.0

16.1

56.8

18.8



No

17

83

151

793

223



%

1.3

6.5

11.9

63.5

17.6



1267



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

divisions.

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.

(1982).



THEDISTRIBUTION

OF THE TOTAL

OSTRACOD

FAUNA

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

OCCURRING

(First appearances of taxa in brackets).



AND



APPEARING

FOR



THE



FIRSTTIMEIN EACHSTAGE.



SPECIES



m

5



su



3



;

6 8

~



g

&



3

2

m



8



&



2



9



2



8

5



S

&



2



%i



Maastrichtian

Senonian

Turonian

Cenornanian



1311

1181

794

907



(678)

(664)

(188)

(681)



Albian

Aptian

Barrernian

Hauterivian

Valanginian

Berriasian



864

669

786

825

789

860



(371)

(275)

(91)

(i63)

(117)

(757)



Purbeckian

Portlandian

Kirnmeridigian

Oxfordian



512

367

346

360



GENERA

233 (70)

213 (87)

171 (8)

190 (65)



1048

(2211)

(32.5%)



(26.1%)



167 (28)

154 (26)

143 (9)

156 (8)

139 (7)

162 (80)



(211)

(176)

(170)

(236)



396

(793)

(11.7%)



91 ( 5 )

95 (5)

97 (13)

105 (29)



97

(52)

(7.0%)



Callovian

Bathonian

Bajocian

Aalenian



340 (250)

396 (316)

235 (133)

131 (95)



276

(794)

(11.7%)



97

110

81

56



86

(99)

(13.4%)



Toarcian

Pliensbachian

Sinemurian

Hettangian



126 (59)

257 (51)

277 (76)

199 (177)



214

(363)

(5.3%)



Upper

Middle

Lower



552 (308)

456 (268)

286 (296)



431

(862)

(12.7%)



799

(1774)



899

(3985)

(58.6%)



202

(230)

(31.1%)



296

(1950)

(28.7%)



43 1

(862)

(12.7%)



(17)

(40)

(26)

(16)



154

(158)

(21.4%)



56 (12)

58 (3)

61 ( 5 )

59 (18)



59

(38)

(5.1%)



103 (39)

115 (35)

88 (88)



102

(162)

(21.9%)



173

(388)

(52.5 %)



81

(1 89)



(25.6%)



102

(162)

(21.9%)



R.WHATLEY



1024



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).

Upper Cretaceous

Lower Cretaceous

Upper Jurassic

Middle Jurassic

Lower Jurassic

Triassic



Mean No. of Species per Stage

Increment

1048

249

799

403

396

120

276

62

214

minus 217

43 1



As will be demonstrated later in this paper, the decline from a relatively high number of species

TRIASSIC



R E T A C EOUS

UPPER



K



w

a



3

P



1300

1200



Inherited species



;.:....



New species



1100

1000

900

VI



!?



800



CL

(Y



700

0



600



500

LOO



300

200

100



TEXT-FIG.

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:

Valanginian

15%

Sinemurian

27 %

Hauterivian

20%

Pliensbachian

20 %

Toarcian

46 %

Barremian

12%

Aptian

41%

Kimmeridgian

49 %

Portlandian

47 %

Albian

43 %

Purbeckian

41%

Turonian

24 %

The following 13 stages have more new than inherited species:

Lower Triassic

100%

Callovian

74 %

Middle Triassic

59 %

Oxfordian

66%

Upper Triassic

56%

Berriasian

88 %

Hettangian

89 %

Cenomanian

75 %

Aalenian

73 %

Senonian

56%

Maastrichtian

52%

Bajocian

57 %

Bathonian

79 %

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.



1026 R.WHATLEY



z

z



9

0



z



w

v)



10L



9080-



;70-



._u



?L



60-



I



50-



z



f



LO-



M%



3 0-



2010-



TEXT-FIG.2-The



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:

Upper Triassic

Sinemurian

Pliensbachian

Toarcian

Bathonian

Kimmeridgian



+274

f

l

+128

f

5

20

10



+

+



Purbeckian

Valanginian

Barremian

Albian

Turonian

Maastrichtian



+110



+ 61

4-343

+270

43

f694



+



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:

Lower Triassic

Middle Triassic

Hettangian

Aalenian

Bajocian

Callovian

Oxfordian



+182



+ 74

+I53

+ 62

+ 15

+ 79



+ 69



Portlandian

Berriasian

Hauterivian

Aptian

Cenomanian

Senonian



+

+

+

+



98

$672

17

91

+375

50



Patterns and Rates of Evolution among Mesozoic Ostracoda 1027

'RIASSIC



-



'PER



a



w

a



I)



1300



-Extinctions



----Originations



12 00



11W

1000



900

111



:,



800



a



7j 700

0



z



600



500

L 00



300



200



1



too.



TEXT-FIG.

3-The



relationship between originations and extinctions of species for all Mesozoic stages and all



Ostracoda.



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

Danian.

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.



THEEVOLUTIONARY

DISTRIBUTION

OF MESOZOIC

GENERA

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

taxa.

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.



<

!



i



-



RETAC

R



:ous

UPPER



a

w

a

=

I



240220200-



180160-



4

g 140

"120



TEXT-FIG.

&Histograms illustrating the simple generic diversity and the inherited versus new component of all

ostracod genera for each stage of the Mesozoic.



0,

Inherited genera; m,New genera



Patterns and Rates of Evolution among Mesozoic Ostracoda 1029



Upper Cretaceous

Lower Cretaceous

Upper Jurassic

Middle Jurassic

Lower Jurassic

Triassic



Mean No. Genera per Stage Increment

48

202

57

154

97

11

86

27

59

minus 53

102



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

stages respectively.

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 :



a Y



S n0



0



E



a

w

P

n



3



100-



9080700

C



603



-50%



z

LO-



30-



2010-



TEXT-FIG.

5-The percentage of new genera in each stage of the Mesozoic for all Ostracoda.



1030



R.WHATLEY



I



TRIASSIC



I



I



<



-Extinctions

--- - Originations

100-



0070-



e



60-



01

(Y



50-



z0

L 0-



302010-



TEXT-FIG.

&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

Mesozoic.

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.



THEEVOLUTIONARY

DISTRIBUTION

OF THE MAJOR

GROUPS

OF MESOZOIC

OSTRACODA

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.



I

-



TRIASSIC



z



0

U



-?



a



W



a

a



z



i!



3



5

z

0



z



In

W



-Total Cytheracea

1985



........



Total Cytheracea

Whatley 8 Stephens

1976



900.



800700D



.$ 600.

a

(Y



6 5000



z



/.......



LOO-



300200/



100-



c



TEXT-FIG.

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



t

c



!



i

110-2



W



2 : g



,



n

3



75-



.fj



50-



z

0

25-



Oa



-Bairdiacea



...... .". ........Platycapina



100-



?



b:a



UPPER



//#f



a



E



E



RETACEOUS



......



....



.........



a..*



..



\...



b'



-_



-Cytheracea



........ Cypridacea



800700-



600-



.s



500-



-



LOO-



yl



8b.



0,



a



'



.-



300200100-



0-



..................



.....

..



e

.



I



TEXT-FIG.

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.



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