Evolution What the Fossils Say and Why it Matters

Fish Tales 213

FIGURE 9.4. A diagrammatic family tree showing how chordates evolved from more primitive forms, as
postulated by Walter Garstang and Alfred S. Romer. In many cases, such as the transition from hemichordates
to tunicates (“sea squirts”), or tunicates to higher chordates, the larval form with its swimming tail enables
them to escape the dead end of their highly specialized adult body forms. (After Romer 1959, redrawn by
Carl Buell)

Primitive filter-feeding

vertebrate

Advanced chordate; sessile

adult stage lost

Ancestral tunicate with

free-swimming larva

Shift from arm feeding

to gill filter feeding

Pterobranchs

Primitive sessile arm feeder

Primitive echinoderm

Acorn worms

Tunicates

Amphioxus

from organisms that retain the features found in the juveniles but never metamorphose into
adults. And indeed the next step (fig. 9.4) is not much more advanced than the sea squirt
larva. Known as the lancelet or amphioxus, it looks more and more like the primitive jaw-
less fish (fig. 9.6). This little sliver of flesh is usually only a few inches in length and swims
much like an eel, although it does not have a true head, jaws, teeth, or bones. Adult lancelets

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