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Dinosaur diversity



Tim Williams recently (3/14/9) wrote:

> 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.

Here are all the species that have ever been referred to the genus
_Triceratops_:

Triceratops Marsh, 1889
        Triceratops albertensis C. M. Sternberg, 1949
        Triceratops alticornis (Marsh, 1887)
        Triceratops brevicornus Hatcher, 1905
        Triceratops calicornis Marsh, 1898
        Triceratops elatus Marsh, 1891
        Triceratops eurycephalus Schlaikjer, 1935
        Triceratops flabellatus Marsh, 1889
        Triceratops galeus Marsh, 1889
        Triceratops hatcheri (Lull, 1907)
        Triceratops horridus (Marsh, 1889)
        Triceratops ingens Lull, 1915
        Triceratops maximus Brown, 1933
        Triceratops montanus (Marsh, 1888)
        Triceratops mortuarius (Cope, 1874)
        Triceratops obtusus Marsh, 1898
        Triceratops prorsus Marsh, 1890
        Triceratops serratus Marsh, 1890
        Triceratops sternbergii Forster, 1993
        Triceratops sulcatus Marsh, 1890
        Triceratops sylvestris (Cope, 1872)

Most of these are junior synonyms of _Triceratops horridus_, as noted by
Ostrom & Wellnhofer, 1986 (in a paper on _T. brevicornus_). This makes for a
slight problem, since the name _T. alticornis_ is senior to _T. horridus_ and
would have to take priority if it were formally synonymized with it. Thus,
_T. alticornis_, based on those horn cores originally referred to the genus
_Bison_, is usually retained as a separate species for bookkeeping reasons.
The same problem confronts _T. sylvestris_, the type species of _Agathaumas_.
If this is considered a synonym of _T. horridus_, then the genus _Agathaumas_
would have priority over _Triceratops_, and, even though this was bandied
about in the early years of the 20th century, nobody wants to change
_Triceratops_ to _Agathaumas_, not just for nomenclatural stability reasons
but because the type material of _A. sylvestris_ is postcranial and probably
not diagnostic.

Also, _T. galeus_ is based on a _Torosaurus_ nasal horn core, so it's not in
the synonymy of _T. horridus_ either. And _Triceratops montanus_ is based on
horn cores too short to belong to the genus _Triceratops_ and is thus
retained in its own genus _Ceratops_, the type genus of the family
Ceratopidae and subfamily Ceratopinae. (Bookkeeping reasons again; Scott
Sampson recently noted, pers. comm., that "the genus _Ceratops_ is toast.")

Cathy Forster now doubts the distinctness of her species _T. sternbergii_ (in
her doctoral dissertation, published by UMI) and considers it a synonym of
_T. horridus_.

This leaves _T. hatcheri_, the type species of _Diceratops_, which still
seems to be generically distinct from _Triceratops_, and _T. eurycephalus_,
which Cathy has suggested is a species of _Triceratops_ distinct from _T.
horridus_.

_Ugrosaurus olsoni_ is universally considered to be an indeterminate species
of _Triceratops_, probably referable to _T. horridus_.

Continuing through the posting, Williams wrote:

>  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_).

And Tom Holtz replied (3/15/96):

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

Ralph Molnar considered it distinct from _Tyrannosaurus_ in his original
paper on the specimen in 1980, as did Greg Paul when he gave it the species
name _Albertosaurus megagracilis_. You should also account for numerous
isolated tyrannosaurid teeth from the Lance too small to belong to
_Tyrannosaurus_. Are they _all_ from juveniles?

> [Williams]>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.)

Another problem is extrapolating the Lance and Hell Creek of western North
America to the entire world at the end of the Mesozoic. The seeming lack of
dinosaur diversity in the Lance and Hell Creek does not necessarily extend to
equivalent formations of South America, Asia, or Europe. Indeed,
Lance-equivalent formations from anywhere outside North America are pretty
difficult to find; we need to sample more such formations before we can admit
that there was a general, worldwide drop in dinosaur diversity from Campanian
to Maastrichtian time. (There's actually a publishable paper there for
someone to write, if one is so inclined.)