The Garstang Hypothesis
Modern Ascidians
(Subphylum Urochordata)
typically have a motile tadpole larva that undergoes metamorphosis to a
sessile adult. The tadpole has the typical characteristics of Chordata,
including a notochord, pharyngeal gill slits, and a post-anal
tail. According to the Garstang Hypothesis,
development
of
sexual maturity in a non-metamorphosing lineage of
tunicates
might provide the immediate proto-chordate ancestors of more typical
chordates
such as Amphioxus (Subphylum Cephalochordata),
or
even
of early Vertebrata (Ostracoderms) as shown here.
Development
of sexual maturity in a larval form is an example of Progenesis
(Garstang called the process Neoteny, which today is used for a
different but related differnetial growth phenomenon).