>The recent mention of the carrier shell Xenophora mekranensis konoi
>Habe raises some interesting questions.  The name konoi apparently
>indicates that this shell is a subspecies of Xenophora mekranensis.
>However, X. mekranensis is an extinct form, known only from fossils.
>A couple of weeks back there was a Conch-L discussion concerning the
>question whether subspecies necessarily have to be geographically
>isolated, and the prevailing opinion seemed to be that they do.
>However, the above example appears to be a case where subspecies are
>isolated not geographically but temporally.  Are there any ICZN rules
>governing such situations?  Is this a common practice?  Would it be
>reasonable to say that many recent fossil forms which resemble extant
>forms should  be described as subspecies, rather than distinct
>species?  Does the nominate species have to be the "older" (extinct)
>form (as in the case above)?  What if the living form is named first?
> Can a fossil form that preceded it be named as a subspecies of an
>extant species?
 
Whatever is named first becomes the nominate form.  If the modern one was
named first, the fossil will be the subspecies.  Older or younger fossils
may be subspecies of the other, too.