[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: A Clutter of Duckbills

>From: RaptorRKC@aol.com
 > >Often little more than the opinion of the researcher.  ...
 > >  Granted, we can look at modern animals,
 > > compare the diversity shown in their populations, and try to apply that to=
 > > a fossil species, but this will inevitably lead to a wrong conclusion=
 > > somewhere, because we rarely will know what diversity existed within the=
 > > species in question. 
 > Why can't we set a criteria as to which species and genus goes where?

The criterion for species is effective reproductive isolation.
Even in *living* forms this cannot always be determined with certainty.
In this cases the judgement of an exert is usually deferred to as being
more likely to be a good guess than otherwise. (In living forms the
problem comes up mostly in forms that are geographically separated,
since there is no way to test the isolation of these under *natural*

All higher categories, including that of genus, are inherently
subjective (even if you use purely cladistic methods, you still
need to set the taxon boundaries somehow).

 >  There
 > has to be actual rules with this, not just opinions.  To correct the jumbled
 > taxonomy of dinosaurs we must first make a set of DEFINITE RULES.  Sort of
 > like "the law" when it comes to placing species and genera and such.

This cannot be done effectively without doing violence to the biological
species definition.  Only when there are enough fossils to identify
inter-group gaps is there a definate criterion.  Beyond that
paleontologists are in the same boat as modern biogeographers.
 > As far as I know, no such law exists. A law, however, does seem to exist for
 > giving dinosaurs names.  The International Code of Zoological Naming, or
 > something like that. 

International Code of Zoological Nomenclature.
 > Suppose someone makes a new finding of dinosaur bones.  What steps are taken
 > to determine where this specimen fits in? 

1. Identify the broad group it belongs to.

2. Compare it to all described species in that group.
   A. If it matches one, tentatively identify it as that species.
   B. Otherwise go to step 3.

3. Determine how it differs from its those described forms it is most
   similar to.

4. Based on the known patterns of variation in that group estimate
   whether it is likely to represent a distinct species (reproductively
   distinct) or not.

[Note, for a fragmentary specimen the best one can do may be step 1;
one should be willing to admit this - and label it 'cf.' or 'x indet.`].

 > What can lead the discoverer(s) to
 > place the specimen in a new species, genus, family, etc.? 

Distinctness and evolutionary relationships based on phylogenetic
analysis are used here.

 > How can we improve
 > upon the steps taken?

Careful use of phylogenetic analysis is one.

However I think a better awareness of the biological nature of
species, and a wider recognition that the biological species definition
*can* be applied (imperfectly) to fossils - especially when an entire
series of specimens is available.
 > Shouldn't the fossil collected, even if it were fragmentary, be compared with
 > already "definitively" classified taxa?  If, suppose, an isolated femur
 > matched the femur of Edmontosaurus, should the femur be classified as an
 > Edmontosaurus?  

This depends.  If it matches *only* E., and not several other hadrosaurs
as well - a hadrosaur femur is likely to match *several* genera of

So, depending on the level of distinctness of the part found, one
might cite it as:

Edmontosaurus sp.  (definately E., but not identifiable to species)
cf. Edmontosaurus  (probably E., but not clearly so)
Hadrosauridae indet. (well, at least its a hadrosaur).

These type of citation are quite frequent in faunal lists.
[Look at the faunal lists at the fron ot "The Dinosauria" for instance].

 >What would lead the femur to be classified otherwise?

See above.

swf@elsegundoca.attgis.com              sarima@netcom.com

The peace of God be with you.