TABLE 2 Species richness, rates of origination, and rates of extinction for Proterozoic and early Cambrian acritarchs

Interval

(Ma ago)

N

FA

FA/Ma

FA/sp/Ma

LA

LA/Ma

LA/sp/Ma

M1 (1700–1400)

6

6

0.02a

0.003a

0

0.00

0.000

M2 (1400–1200)

8

2

0.01

0.001

2

0.01

0.001

M3 (1200–1000)

11

5

0.03

0.004

3

0.02

0.002

N1 (1000–900)

13

5

0.05

0.005

1

0.01

0.001

N2 (900–800)

28

16

0.16

0.009

3

0.03

0.002

N3 (800–750)

31

6

0.12

0.004

3

0.06

0.002

N4 (750–700)

35

7

0.14

0.004

12

0.24

0.008

N5 (700–650)

28

5

0.10

0.004

13

0.26

0.010

N6 (650–600)

17

2

0.04

0.002

1

0.02

0.001

N7 (600–575)

60

44

1.76

0.060

46

1.84

0.060

N8 (575–560)

16

2

0.13

0.008

4

0.26

0.017

N9 (560–545)

15

3

0.20

0.015

6

0.40

0.030

C1 (545–538)

11

5

0.70

0.070

4

0.60

0.060

C2 (538–531)

22

16

2.30

0.185

7

1.00

0.080

C3 (531–528)

48

32

10.70

0.400

12

4.00

0.150

C4 (528–524)

78

42

10.50

0.198

31

7.75

0.150

C5 (524–520)

54

7

1.75

0.035

18

4.50

0.089

N, total species richness; FA, first appearance; LA, last appearance; FA (LA)/Ma, first (last) appearance per Ma; FA (LA)/Sp/Ma, first (last) appearance per species per Ma. For the calculation of per species rates of origination and extinction, standing diversity was taken to be the geometric mean of diversity at the beginning and end of each interval, making the simplifying assumption that all extinctions took place at the ends of intervals.

a All species present in M1 are counted as first appearances, but some may have originated earlier. Thus, calculated rates of first appearance for M1 may be too high.

appearances also increase throughout these intervals and exceed first appearances at the end of the Early Cambrian.

Table 2 shows calculated rates of cladogenetic evolution for each of the intervals under consideration. For intervals M1 though N1, both total and per taxon rates of first and last appearances are low, indicating not only that diversity was low but also that constituent species were long lasting. (The calculated rates of first appearance for M1 may be misleading, in that all species are recorded as first appearances. The presence of simple acritarchs in rocks that may be older than 1700 Ma indicates that at least some of these forms may have originated earlier.) By 900–800 Ma ago (N2), total rates of origination had increased by an order of magnitude to a level at which they remained for the duration of the pre-Varanger Neoproterozoic. Interestingly, after an increase during interval N2, per taxon rates of origination returned to levels comparable to earlier intervals; both total and per taxon extinction rates increased toward the Varanger ice age.

Another order of magnitude increase in origination and extinction



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