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Re: Dinosaur diversity



>A while ago I asked how many species are currently recognised for the 
>genus _Triceratops_.  It's a fairly esoteric question, and probably 
>no-one really knows exactly.  But, it's probably more than just a 
>matter for taxonomy.

Well, it really is NOTHING more than a matter of taxonomy, but it has
implications for other fields, as mentioned below.  Incidentally, the
_Tricereatops_ systematics paper by Catherine Forster is forthcoming in the
summer issue (if I remember correctly) of the current volume of the Journal
of Vertebrate Paleontology.

>A lot of authors have pointed to a loss of dinosaur diversity in the 
>final few million years of the Mesozoic as evidence for a gradual 
>decline before their final extinction, rather than a single great
>catastrophe at the K/T boundary.

Incidentally, this pattern applies to other taxa, including some of the
important marine invertebrate groups (rudists, ammonoids, etc.).  As you
note, but as incorrectly interpreted by some, the pattern is on the scale of
millions of years, or (in other words) declines over one-and-a-half stages
(the late Campanian and the Maastrichtian).

>  In other words, late Maastrichtian
>sites (particularly in North America) are said by some to be dominated 
>by large numbers of fossils of a few species, while in
>stratigraphically earlier sites, species are represented by less 
>fossil material, but there larger numbers of species per site.
>
>But this "loss of diversity" could be an artifact of taxonomy.  For 
>example, in western North America, _Triceratops horridus_ is said to 
>be the dominant ceratopsian during the late Maastrichtian.  But this 
>taxon could embrace a number of _Triceratops_-like species (or even 
>genera, like _Diceratops_ or _Ugrosaurus_).

_Ugrosaurus_ has quite effectively been sunk.

In any case, if you are VERY generous with the species level taxonomy, you
still only get one _Diceratops_ species, one or two _Torosaurus_ species,
and two _Triceratops_ species in the late Maastrichtian among the
Ceratopsidae.  Compare this with the late Campanian, with one or two species
of _Styracosaurus_, one of _Centrosaurus_, three of _Chasmosaurus_, one of
_Anchiceratops_, one of _Pentaceratops_, one of _Ceratops_, and by the end,
_Einiosaurus_, _Achelousaurus_, etc.

Furthermore, this decline is not simply numbers of species: it represents
the loss of ecological diversity.  No centrosaurines have been reported out of
the literally hundereds of ceratopsian skulls from the Lancian age, in any of
the deposits from Saskatchewan, Alberta, Montana, Wyoming, the Dakotas,
Colorado, New Mexico, Arizona, Texas, or any of the other provinces/states
with upper Maastrichtian deposits.  Centrosaurine and ceratopsine (aka
chasmosaurine) snouts are very distinctive, and (it can be argued on the
basis of modern large herbivores) this reflects a difference in food
preference.  While the short-snouted centrosaurines are very common in the
late Campanian, and present in the early Maastrichtian, they have yet to be
found (or at least reported) in the late Maastrichtian rocks.

>  Similarly, there could be
>at least three _Tyrannosaurus_-like genera in the latest Maastrichtian 
>- _Tyrannosaurus_, _Dinotyrannus_, _Nanotyrannus_ - rather than just 
>one genus and species (_T. rex_).

The case for the distinctiveness of "Dinotyrannus" is still not established.

>Any ideas??  Is this a load of @#$@#?

Although one could argue that the depauparate nature of the late
Maastrichtian western North American fauna is an artifact of taxonomy, you
could use the same arguement to greatly inflate the (already large) number
of late Campanian dinosaurs.  No matter how you deal with the number of
species, the diversity of form is still greater in the Judithian than in the
Lancian.

(And, before someone brings up preservational bias, please note that the
greater diversity of late Campanian dinosaurs is found in a
sedimentologically *less* diverse number of formations.  It is the Lancian
dinosaurs which are already found from a greater number of environments and
sampled over an equally large (or arguably larger) geographic range.)

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661