Evolution__3rd_Edition

Tertiary
Lower Danian
Cretaceous
Upper Maastrichtian
Tri. Jur. Cret. Dan. Pal. Eoc. Olig. Mio. Pli. Rec.
(m)
13
12
11
10
9
8
7
6
5
4
3
2
1
0
1
2
3
4
5
6
(a) Brachiopods
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
8 9
6 7
5
4
3
2
1
(c) Bivalves
1 2 3 4 5 6 7 8
10 11
9
19 20
18
12 13 14 15 16 17
24 25
21 22 23
Figure 23.3
The mass extinction at the end of the Cretaceous affected all the
main taxa, but the evidence about whether they all went extinct
suddenly at the same time is controversial. Here are four
examples. (a) Brachiopods from Nye Kløv, Denmark. These
extinctions look synchronous. It has been argued that there
was a sedimentary hiatus here; others disagree, but note the
gap at the base of the Tertiary. (b) Dinosaurs from Hells Creek,
Montana. These look gradual. It has been argued that the
gradual extinctions, and persistence of dinosaurs into the
Tertiary, is due to secondary reworking of the fossils, and the
real extinction pattern is sudden and synchronous at the end
of the Cretaceous (see Smit & van der Kaars 1984; Sheehan et al.
1991). (c) Bivalves from Stevens Klint, Denmark. These look
synchronous. It has been argued that the sudden extinctions
26
35
33 34
28 29 30 31 32
27
Stratigraphic column (m)
0
20
40
60
80
100
120
140
160
180
200
220
(d) Ammonites
Time (Myr)
66.7
67
68
69
70
71
72
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74
75
76
77
Maastrichtian
Campanian
(b) Dinosaurs
0 40 80
Dinosaur genera
surviving (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
are only apparent, being due to a gap in the sedimentary record,
but most students of the site accept that the sedimentary record
is continuous and that the extinctions are real. (d) Ammonites
from the Zumaya section, northern Spain. The results are
shown for two seasons of collecting. Note the improved
evidence in the larger dataset (dashed lines) for synchronous
extinctions at the Cretaceous–Tertiary boundary. The nonsynchronous
pattern in the earlier data (solid lines) might
be due to incomplete evidence and the real pattern is
synchronous. (100 ft ≈ 39.5 m.) Redrawn, by permission of the
publishers: (a) from Surlyk & Johansen (1984), (b) from Sloan
et al. (1986), (c) from Alvarez et al. (1984), and (d) from Ward
(1990). (a–c) © 1984, 1986 American Association for the
Advancement of Science.
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