Geology Reference
In-Depth Information
increased. Whereas previously only soft-bodied animals burrowed into the sediments of
the sea floor, fossils indicate that in the Mesozoic, animals with shells also burrowed in-
to the sediments. Scientists think it is likely that these animals evolved this new way of
life to hide from predators that even their shells couldn't protect them from.
Single-celled organisms called
planktonic foraminifera
first appeared — and then evolved
increasing diversity — during the Mesozoic. Foraminifera are tiny animals with only one
cell that build mineral shells.
Benthic
foraminifera live at the bottom of the ocean and
have been present since the Cambrian period at the beginning of the Paleozoic (approx-
imately 540 million years ago). Planktonic foraminifera float near the surface of the
ocean and first appeared in the Jurassic. They were so diverse and abundant during the
Cretaceous period of the late Mesozoic that they are used as guide fossils. (I define
guide fossils in Chapter 19.) Planktonic foraminifera shapes are illustrated in Figure 20-3.
Figure 20-3:
Plank-
tonic foraminifera.
The fishes continued to diversify and evolve new species throughout the Mesozoic. Fish
evolution is so detailed and specific that it's easier to categorize the fish species as
prim-
itive, intermediate,
and
advanced.
The most obvious difference is that the advanced
group had a completely bony skeleton, unlike the earlier groups whose skeletons were
largely made of softer cartilage material (see Chapter 19). By the end of the Mesozoic,
the most complex fish group was the
teleosts.
Teleosts are ray-finned fish with bony skel-
etons that inhabit fresh and salt water. They are still the most diverse and abundant ver-
tebrate animal group on the planet!
The Symbiosis of Flowers
The changes seen in the fossil record for plants between 251 and 65.5 million years ago
lay the foundation for the majority of plant species on earth today. As the continents of
