Paul Drez speaks of the lack of juvenile shells in the fossils record.

It is true that some shell beds have few juveniles, probably because they
are less dense (and therefore winnowed away by currents), because they are
fragile (and therefore broken), or because they are thin-shelled (and
therefore readily dissolved by ground water). The main shell beds of the
Gosport Sand (middle Eocene of Alabama), for instance, are winnowed storm
deposits. Adults are relatively common (though juveniles are by no means
absent) and the lips of gastropods tend to be chipped.

But other deposits may have abundant juveniles, and they can comprise the
majority of shells in some deposits, e.g., the early Eocene Bashi Marl
Member of Alabama and Mississippi. Adults are generally also present in the
same deposits because they are denser, larger, and thicker-shelled. So most
paleontologists describe the adults, with the convention that the juvenile
is described as the early part of the shell.

The juveniles of closely related species commonly look alike; to give a
famous example, human and chimpanzee babies look a lot more like each other
than the adults do. Most biologists prefer to name species based on adults,
whether fossil or modern.

Of course, with only a juvenile shell in hand, it may be mistaken for an
adult and named as a new species. And some malacologists can't seem to stop
themselves from naming new species even when the material is inadequate
(e.g., broken, discolored, unique, freakish, or juvenile). Usually, I
suspect, this is due more to a sense of tidiness than to a desire for glory.

Most of the time, the lack of published work on juveniles is not a problem.
But when we work with small samples, such as those from dredgings or cores,
the abundance of fossil juveniles is all too evident, with many species
being represented only by juveniles or by fragments of adults. Then, we wish
that people had published more about juveniles.

Andrew K. Rindsberg
Geological Survey of Alabama