Evolution What the Fossils Say and Why it Matters

Fish Tales 219

to protect them and “chain mail” down to their tails. None, however, had a strong pectoral
or pelvic fin for steering, as do most modern fish, because they lacked bone to support the
fin. Instead, armor covered much of their bodies, although no one is sure what predator they
needed all that armor for. Some of these jawless fish, the cephalaspids or ostracoderms, had a
large horseshoe-shaped head shield with a flat bottom, and apparently filtered out food from
the seafloor. The heterostracans, thelodonts, and anaspids, on the other hand, had simple slit-
like mouths and tails with the main lobe pointed downward to keep their heads up while
they swam. These fish apparently sucked water through their mouths and filtered it through
the gills, as many living fish do today.
Thus, from the primitive acorn worms, we can trace a series of transitional forms that
become more and more like vertebrates. Add the notochord and we have lancelets (fig. 9.9).
Add the head and the paired fins and we have the early Cambrian Chinese forms Haikouella
and Haikouichthys (figs. 9.9 and 9.10). Finally, by making the body more streamlined and
adding bony scales, we have a jawless fish (fig. 9.8). Thus the transition from invertebrates
to vertebrates is documented not only by living fossils but also by an increasingly good
fossil record.

FIGURE 9.9. The steps in the evolution from primitive chordate relatives to the first vertebrates. (Redrawn by
Carl Buell, based on Shu et al. 1999)

Acorn "worm"

Lancelet

Amphioxus (recent)

Pikaia

(Middle Cambrian)

Burgess Shale

Haikouella

(Middle Cambrian)

Chengjiang

Haikouichthys

(Middle Cambrian)

Chengjiang

Anaspid (Early Silurian)

Notochord

Head

Paired fins, vertebrae

Fusiform body scales

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