Evolution__3rd_Edition

..
(a)
(b)
Planktotrophic
larvae
Non-planktotrophic
larvae
35
Cumulative
percentages of
developmental
types
Species/time
interval ( N )
Buccinidae
Olividae
Mitridae
Nassarlidae
Fasciolariidae
Volutidae
Epoch
Midway Wilcox Claiborne Jackson
5 2 4 2 0 3 1 1 1 1 7 27
40
40
15 5 8 9 6 1 2 2 2 1 3 9 63
Paleocene Eocene
45
45
50
Time (Myr)
6 6 1 3 2 1 2 1 1 1 2 26
50
55
55
60
Paleocene Eocene
Early Late Early Middle Late
Time (Myr)
5 3 6 1 1 2 2 2 7 28
60
2 4 1 6 0 0 1 4 18
65
0 50 100 0 50 100 0 50 100 0 50 100
Percentages
65
Direct development Planktonic development
Figure 23.9
(a) The proportion of gastropod species with direct, rather than planktonic, development increases through the Paleocene and Eocene. The effect exists within several of
the six families, and as an average for all six families. (b) Detailed observations for Volutidae, from the Gulf Coast of the USA. (Volutidae are furthest left of the six
families in (a).) Note the proliferation of directly developing species. The extinction rate of species with planktonic development appears to be lower a which counters
the trend toward more directly developing species. Small discrepancies between the numbers in (a) and the number of lineages in (b) may be due to extra data in the later
paper (used to compile (a)). Redrawn, by permission of the publisher, from Hansen (1978, 1983). © 1983 American Association for the Advancement of Science.