Conodonts may be useful for dating fossil mollusks.  There are also mollusk
or mollusk-like organisms in the same deposits as some of these forms.
After the development of jaws, some of these early chordates may have fed
on mollusks (even before, they could have filtered larvae), and cephalopods
could have fed on the early chordates.  Apart from that, I'm afraid there
is little molluscan connection.

>A classic example of an organ that may have developed by a drastic
>evolutionary step is the chordate jaw, based on various threads of evidence
>(anatomical, embryological, paleontological), it seems as though the lower
>jaw may have derived from the misplacing of the forward pair of ribs. This
>would have looked like a birth defect, as Paul Monfils said. But THIS birth
>defect would have enabled the fish to capture prey by grasping it, instead
>of sucking or rasping it--a tremendous advantage. If something like this
>conjectural scene really happened, then we have an example of the advanced
>fishes arising in one generation from the more primitive fishes. That will
>do nicely for an example of MACROevolution.

Actually, it is the anterior gill arch rather than ribs.  The first use may
have been to enhance water pumping for respiration, with prey capture as a
later innovation.  The jaw may have evolved twice, as the extinct, armored
placoderm fish have jaws rather different from other jawed vertebrates.

>Cathaymyrus is the oldest known chordate at 535 million years old and
>Pikaia from the Cambrian Burgess Shale is slightly younger; Yunnanozoon from
>the lower Cambrian of China is also thought to be an early chordate.

The identity of one of the Chinese forms as a chordate is uncertain; I have
forgotten whether it was Cathaymyrus or Yunnanozoon, but the other two seem
to be good chordates.  In addition, there are Upper Cambrian scales similar
to those of certain Ordovician jawless fishes, and conodonts are relatively
abundant Cambrian to Triassic chordates.  Conodont-like forms are known
from the latest Precambrian; however, there remains some uncertainty
whether these are true conodonts or possibly chaetognath (arrow worm) jaws.


David Campbell

"Old Seashells"

Department of Geological Sciences
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315
USA

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"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug