Morrison Sauropods\etc. (long) (was: Re: Sauropods [was: Iguanodontidae and Theropod Boredom])

[Justin Tweet <thescelosaurus@juno.com>]

On Mon, 02 Jul 2001 10:28:07 -0400 Darryl Jones <dinoguy@sympatico.ca>
writes:

[snip]
> I know that this doesn't seem very scientific, but I am bothered by
the
> amount of diversity of such large creatures.  For example, the
Morrison
> Formation (which I realize covers about 10 million years, but still)
has
> Apatosaurus, Barosaurus, Diplodocus, Brachiosaurus, Haplocanthosaurus,
> Supersaurus, Camarasaurus, and Seismosaurus at very minimum.  There
> are others named (Amphicoelias for example, which is probably not
> distinguishable from Diplodocus) and others that are not yet described
(a
> primitive diplodocid described as Amphicoelias sp. at the SVP meeting
in
> New York?).  These are just the genera.  When you start adding in
species,
> the number of sauropods in the Morrison Formation starts topping 18 or
> so.  How is this possible?  These were massive creatures that must have

> required an enormous amount of sustenance.  Either they were extremely
> successful as organisms (sauropods that is) or we have far too many
> named.

        (Forgive me in advance for a rather lengthy discussion, hopefully not
too misinformed or misdirected)

        I've pondered the same thing myself.  The Morrison especially is amazing
for the known diversity (check out the thread starting at
http://www.cmnh.org/fun/dinosaur-archive/1999May/msg00704.html for a
massive list of Morrison dinosaurs); however, I strongly suspect that if
more formations were known as well as the Morrison, we would see similar
diversity all over the world.  For example, Tendaguru has dicraeosaurids,
brachiosaurids, diplodocids, and remains that may belong to
"camarasaurids" and titanosaurids.  The Chinese MJ-LJ faunas also have a
high number of separate species, although the classification of these
taxa is currently uncertain (and it seems, from what I've read, that the
Chinese sauropods were more similar to each other than the sauropods in
the Morrison or Tendaguru were to their contemporaries).  Even in less
well-sampled formations we see great diversity; Lourinha has
camarasaurids and diplodocids, and the middle Jurassic English formations
have "cetiosaurids", diplodocoids, and macronarians (likely of the
brachiosaurid persuasion).
        So, I don't think that, in the Morrison at least, we have too many
named.  Even the sinking of a few of the questionable species still
leaves you with quite a few well supported taxa.

> I do think that there was a good diversity of these Morrison sauropods.
 There are often a minimum of 4 genera in
> given quarries.  Were sauropods more prone to specific variation?  Was
the Morrison Formation a
> perfect place and time for sauropods (although sauropod finds in China
lead
> me to believe that this was not the case)?  Is there more of a
progression
> along the Morrison that I am not seeing that has no more than 3 or 4
genera
> present at any given point in time?

        I don't know if sauropods were more prone to specific variation than
other dinosaurs; I think that if small dinosaurs ("hypsilophodonts",
"coelurids") were better known, they would show a higher specific
variation, because there tends to be more species of small genera than
large.  We just have a lot more sauropod material than "hypsilophodont"
material.
        The well-supported genera and species have morphological differences
that indicate different niches.  Of the Big Six (*Apatosaurus*,
*Barosaurus*, *Brachiosaurus*, *Camarasaurus*, *Diplodocus*, and
*Haplocanthosaurus*), each is very distinctive.  For example, in feeding:
        *Apatosaurus* has peg-like teeth but relatively the shortest neck and
stoutest build of the Morrison diplodocids;
        *Barosaurus* has peg-like teeth and the longest neck of the Morrison
diplodocids;
        *Brachiosaurus* has spoon-like teeth with a long, upwardly-directed neck
and long forelimbs;
        *Camarasaurus* has spoon-like teeth but is smaller and stockier than
*Brachiosaurus*, pointing to a different feeding strategy;
        *Diplodocus* fits between the morphologies of *Apatosaurus* and
*Barosaurus*;
        *Haplocanthosaurus* likely had spoon-like teeth, combined with a less
derived body plan than the others (probably overlapping *Camarasaurus* in
its niche due to size).
        Of the less common genera, we have the super-elongated *Seismosaurus*
and *Supersaurus*, mysterious *Dystrophaeus*, apatosaur-like
*Eobrontosaurus*, *Amphicoelias*, *Dystylosaurus*, and titanosaurid-like
"Apatosaurus" *minimus* (and possibly *Dyslocosaurus* as well).  These in
turn are also all distinctive.
        This is not like *Triceratops* taxonomy, which at one point had
something like fifteen virtually indistinguishable species at the same
time and almost the same place.  These sauropods, for the most part, all
appear to have had a distinct niche.  In addition, there are some
temporal concerns in the Morrison; certain genera like *Eobrontosaurus*
and (possibly) *Haplocanthosaurus* are only known from the early Morrison
(and *Dystrophaeus may predate them all), while (I believe)
*Seismosaurus*, *Dystylosaurus*, and *Supersaurus* are late Morrison.
Some species are only upper Morrison; the largest species of
*Camarasaurus* and *Apatosaurus* are uppermost Morrison, along with the
type of *Amphicoelias*.  The faunal list doesn't tell the whole story.

> This actually applies to more than just sauropods in the Morrison.
There
> is the same incredible diversity in stegosaurs (with at least four
species
> present) and (sorry) theropods (with at least 8 genera).

        I'm not sure about stegosaurid diversity; I know that the well-known
ornithopods separate well by size:
*Drinker* as teeny-tiny (if Bakker is right);
*Othnielia* in the one to two meter range;
*Dryosaurus* in the three plus meter range;
*Camptosaurus* and unnamed iguanodontians at the top, probably up to
seven plus meters.
Both known ankylosaurs were fairly small.

        The theropods also separate well by size.  At the top we have
*Torvosaurus* and the "super" allosaurs; next is regular *Allosaurus
fragilis*, followed by *Ceratosaurus*, then *Marshosaurus*.
*Stokesosaurus* and the "coelurosaurs" (*Ornitholestes*, *Coelurus*,
*Koparion*, *Palaeopteryx*, unidentified animals, etcetera) brought up
the rear.  It's not as though they were all competing for the same food
source.

> With this in mind, I wondered if there were not a land bridge formed at
some point
> during this period that allowed a MAJOR migration of animals from, say,

> Africa that would have placed many more genera and species into one
> place than would normally develop on their own.

        On the subject of land bridges, as the members of Pangaea had not yet
fully separated, there was probably a lot of faunal mixing (see Morrison
similarities with Tendaguru and Lourinha); however, I would think this
mixing would have spread everyone out more evenly, rather than
concentrating unusual diversity in a single area.
        As I said above, I suspect that the Morrison probably preserves the kind
of diversity fairly typical of a late Jurassic dinosaur community, and we
just don't have anything comparable.-*Thescelosaurus*

> Darryl Jones  <dinoguy@sympatico.ca>
>
> For information on tyrannosaurids and
> cool activities and information for kids,
> visit my webpage at:
>
> http://www3.sympatico.ca/dinoguy/
>

---------------------------------------------------------------
Justin Tweet, *Thescelosaurus*
See "Thescelosaurus!": http://personal2.stthomas.edu/jstweet/index.htm

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