Evolution__3rd_Edition

..
Incidence of repair
0.6
0.5
0.4
0.3
0.2
0.1
0.0
50
40
30
20
10
0
Late
Carboniferous
5–9 mm
10–19 mm
20–29 mm
Late Late
Triassic Cretaceous
Time interval
. . . in fossil–predator prey relations
...
Percentage of occurrence of
uncemented gastropods
Late
Miocene
Recent
CHAPTER 22 / Coevolution 635
Figure 22.12
Incidence of repair at five successive time periods, for shells
divided into three size classes. Note: (i) the incidence of repair is
higher in more recent times; and (ii) larger shells show a relatively
high incidence of repair compared with small shells in more
recent times, as compared with earlier times. Large shells tend to
be more resistant to breakage than small ones. One interpretation
of the second trend is that more recent predators have become
stronger, and therefore able to injure large-shelled animals.
(0.4 in ≈ 10 mm.) Redrawn, by permission of the publisher, from
Vermeij (1987).
limited evidence. Those broad patterns, as we shall see, have a wide scatter in the data.
Dietl et al.’s point is worth keeping in mind, because problems in the data are likely to
be one cause of the scatter.
Let us look at some of Vermeij’s evidence. The frequency of shell repair is one indicator
of predator–prey interactions in fossils. When a mollusk is non-lethally attacked,
it repairs the damage to its shell and the repair pattern can be observed in the shell.
Proportions of shells showing signs of repair have been measured in several fossil
faunas, and the trend appears to be toward increasing amounts of repair over time
(Figure 22.12). This Vermeij interprets as meaning that the prey have been suffering
higher frequencies of predatory attacks over evolutionary time. (Logically it could also
mean a though this is perhaps unlikely a that the predators have de-escalated from
forms that destroyed their prey to forms that sometimes merely injured them!)
The escalation of molluskan prey defenses is also suggested by a trend in the proportion
of different types of gastropod shells through time. The proportion of loosely
attached forms, which are relatively poorly defended, has decreased through time relative
to better defended types, such as burrowers and attached forms (Figure 22.13).
Vermeij also found limited evidence that the better defended burrowers increased from
being about 5–10% of genera in the late Carboniferous–late Triassic to about 37%
in the late Cretaceous and 62–75% in modern formations. Internal thickening or narrowing
of the aperture is another form of escalated defense and these types, too, have
(2) (3) (3)
(2)
Ord. Sil. Dev. E. L. Per. Tri. Jur. Cret. Neog.
Carb. Carb.
Time interval
Figure 22.13
The incidence of sessile or sedentary uncemented gastropods
through time. Note that the proportion decreases. Each point
is for one fossil assemblage, except where marked. For each
assemblage, Vermeij divided the gastropods into different types
(burrowers, sessile, attached forms, etc.), adding up to 100%. This
graph gives the proportion of gastropods that lie unattached on
the bottom surface. The Neogene (Neog.) includes the Miocene
and Pliocene (Figure 18.1, p. 526). Redrawn, by permission of the
publisher, from Vermeij (1987).