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

Diversity Curves and Clumping

That the Recent is a very special case that needs to be
treated differently from the other times has been realized
since the beginning of Raup's/Sepkoski's work and, I assume,
Simpson's as well (can't remember, must reread) When I was a
graduate student at Rochester with Raup and Sepkoski in 1975
they were constantly talking about the "pull of the Recent"
and how the Recent has effects even extending back into the
record - recognizability of taxa, possible inflation of
higher taxa earlier due to the fact that earlier taxa will
seem more exotic than more recent ones that are also extreme
morphologies, we just understand the current ones better and
how they fit in phylogenetically. That's why Raup's (first)
and Raup & Sepkoski's (middle) and Sepkoski's work selected
the specific taxa they did for study of Phanerozoic
diversity patterns. The trends that show increases towards
the Recent are quite real. They are not an artifact of
paleontologists leaning on morphology and neontologists
having a broader range of stuff to study, as almost all taxa
within the sampling of these studies (well-skeletonized
metazoa and relevant single-cells and plantae) have been
studied relatively little, except morphologically and, at
least for the molecular work, not until very recently after
all those curves were assembled.

The use of the various taxonomic levels in these studies is
indeed controversial, with ardent cladists hating them
because the hierarchy is becoming less and less important -
or even worthless to their thinking -  and others suggesting
they are indeed useful because these higher taxa are
consistent samples - statistical samples -  of the species
level and some, at least in the fossil record, may be more
consistent in what they represent. This may especially be
true for the genus level. In fact, neontologists doing
biodiversity work have in recent years finally started using
some of these approaches to estimating abundances in areas
that will take many years to sample appropriately, if they
manage to stay around that long. So perhaps we can not throw
away the baby with the bath water and use these data for the
neat information they can provide us, while not being led by
the nose by them relative to the importance, or lack
thereof, of higher taxon relative to the biological

Now as far as lumping and splitting goes, there is an awful
lot of variation in the varying effects of morphology and
non-morphological aspects. Yes, pure morphological studies
will underestimate the apparent species diversity, and there
are morphological variants that will certainly look like
different taxa that are the same so the opposite effect will
happen. Dimorphs in ammonites take lots of care. There are 2
conflicting aspects towards whether clumping and splitting
is the preferred way. Some state that any reason for
splitting is a good one because no harm is done by
oversplitting - the unneeded taxa can just be eliminated.
Others would suggest that science is best done
parsimoniuosly - and I don't mean just phylogenetic stuff
here - That if you have no excellent reason for splitting,
then it is not justified. I have spent a good amount of time
killing old, bad, oversplit taxa because the reason they
were erected did not hold up. 

So simply, if you have a solid reason for erecting a new
taxon, do so. If it falls apart, oh well, but it will then
be clumped in with something else. However, if there are
well known limits of variation within a "population" of one
type of animal from a major locality - perhaps such as
Allosaurus - and a new specimen from another fits within
that variation, then there is no justifiable reason to name
a new taxon. If a sample of material from a new location
shows variation that overlaps the original taxon but shows
significant population differences from the original, then
it is justified to address whether a new taxon, or just
regional/ecophenotypic variation is the cause. 

Distance is a VERY bad reason to do this. In Upper Cambrian
trilobites, especially ptychoparioids, Charles Resser had
stated in publications that he considered any specimen found
more than, I believe 75 miles (although the distance I think
varied with the pub and when he was talked to) from the type
outcrop MUST be a new species. He then managed to oversplit
Cambrian trilobites to a level where they are still pretty
messed up despite the best efforts of lots of people over
the past 80 years or so. He would name 12 species of the
same genus from a single limestone lens in an outcrop.
Basically you must have a good reason for naming a new
taxon. Period. Distance won't do it. We are paying the price
for having so many taxa named based on crappy material and,
at times, a lousy understanding of biology and variation.
Yes, we can indeed kill the new taxa but it is, in many
cases, a waste of time and effort and money.

Part of the problem is in the process we now use - there is
just no way of easily referring an odd tooth, for example,
to a unit other than a new species despite the fact that a
single tooth is a lousy way to erect a new taxon of any
type. Heck, conodonts are the extreme end of this in that
you have individual elements named, as well as element
suites named, although we know the latter actually are the
more reasonable for representing an organism. So the
paleontologists who visits East Blitcho somewhere for the
first time, has 25 minutes to sift 5 pounds of Mesozoic
sediment, gets 6 different dinosaur type teeth that are not
particularly equivalent to anything else is confronted with
the problem of what to do. If he wants to be associated with
this first find, he names each to a new taxon. In my opinion
- and as seen by many paleontologists - it is important
certainly describe the teeth and refer to them as unknown
theropod type 1, basal marginocephalian (I wish) type 6,
etc. and wait for the excitement of having someone else (or
them) go back and find enough to justify a new taxon. In the
better, latter case, the first person may not be part of the
name and that can be an ego deflator for them but, tough.
Better to have better taxa named. Hell, the genus Anatotitan
was named in Chapman & Brett-Surman in an appendix by Mike
and is properly called Anatotitan Brett-Surman, although
emotionally I would have loved to be a part of the naming of
it because it is a wonderful animal. However, Mike managed
to do about 100.1% of that work and it was not justifiable
for me to be part of that and we chose to make sure it was
put in in such a way as to recognize that it is clearly
Mike's taxon and work. Just have to remove the emotional
attachment that comes with naming new taxon because it
promotes too many bad ones.

So, yes there are times when a more splitiferous reasoning
may work best, but I think, relative to what has been done
historically, we need to move more towards the lumping

Ralph Chapman

Ralph E. Chapman
Applied Morphometrics Laboratory
National Museum of Natural history
ADP, EG-15  NHB, 10th & Constitution, NW
Smithsonian Institution
Washington, DC 20560-0136
(202) 786-2293, Fax: (202) 357-4122