192 Evolution? The Fossils Say YES!
korotnevi). Two lineages that had been distinct for millions of years (Pterocanium praetextum
and Pterocanium charybdeum trilobum) appeared to hybridize during the late Pleistocene. The
diagram in figure 8.6 does not capture how the change can be traced gradually centimeter
by centimeter through many different cores, so we could see every transitional step between
one extreme shell shape and another. However, it was possible to measure many different
features, such as the length of the thorax, and see the gradual shift of sizes in each population
spanning 7 million years.
This is just one example of evolutionary changes among the radiolaria, and there are
hundreds more that could be cited. In fact, radiolaria are still not that well studied (especially
compared to forams), so there could be hundreds more as yet unknown. Let us look at just
one more classic example, probably the most extreme change in morphology ever docu-
mented in the fossil record. If you look at samples of microfossils from the middle Eocene
(50 million years ago), you will find distinctive spongy ball-shaped radiolarians known as
Lithocyclia ocellus (fig. 8.7). As you trace the spongy balls up through the sediments spanning
FIGURE 8.7. Evolutionary transformation in the cannartid-ommatartid lineage of radiolaria over the past
50 million years, from spongy balls to four- and then three-armed and finally two-armed bipolar structures,
with further variations in the spongy caps later in their evolution. Taxa are as follows: 26, Lithocyclia ocellus;
27, Lithocyclia aristotelis; 28, Lithocyclia angusta; 30, Cannartus tubarius; 31, Cannartus violina; 32, Cannartus
mammiferus; 33, Cannartus laticonus; 34, Cannartus petterssoni; 35, Ommatartus hughesi; 36, Ommatartus
antepenultimus; 37, Ommatartus penultimus; 38, Ommatartus avitus; 39, Ommatartus tetrathalamus. (Modified
from Haq and Boersma 1978)
26272830313233363738
39
(Middle Pliocene- Pleistocene
Middle Pliocene3435