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Ecology of End-Cretaceous "Omnivorous" Ornithischians



Thought I'd add my "piece" to this thread before it gets mired or lost ...


  With Justin Tweet's hypothesis of end-K theropod outcompeting ornithischians 
in the small- to
medium-sized omnivores or herbivores, I thought I'd help with some alternate 
work on some of the
taxa present at the end of the Cretaceous that will be used for comparison. 
However, to clarify
the search parameters, I decided to use [semi] cursioral ornithischians only 
for some particular
reasons:

  A] - theropods are all obligatory bipeds, and generally considered cursorial, 
with the expection
of the therizinosaurs which appear to have been mediportal bipeds. The body 
type is comparable to
the degree that a long hind leg, tail, short forelimb, long head, slender build 
make the types
into a sort of template;

  B] - the teeth in omnivores tends to be distinctively heterodont, with 
piercing or cutting
teeth, and shredding or "chewing" teeth (different types of [mammal] teeth 
acheive similar goals,
but I am excluding the grinding type of tooth), or plain ol' gripping teeth 
(conodont) ... this
type of dentition is incomparable to theropods except in the prescence of 
toothless theropods, or
the mesial (rostral, anterior) teeth of some theropods, especially 
therizinosaurids and
troodontids (also, *Microraptor*), which are typically "leaf-shaped" or 
phyllodont. Similar teeth
appear in prosauropods, with more rostral teeth being more blade-like.

  Using these criteria, with heterodonty and [semi] cursoriality, my list of 
end-K ornithischians
is limited:

  *Thescelosaurus* [upper Maastrichtian, Hell Creek/Laramie/Lance/Frenchman, 
~7-9ft]
  *Bugenasaura* [upper Maastrichtian, Hell Creek, ~6ft -- including 
*Thescelosaurus garbanii*]
  *Stegoceras* [upper Campanian/lower Maastrichtian, Dinosaur Park, ~6ft]
  *Gravitholus* [upper Campanian/lower Maastrichtian, Dinosaur Park, ~6ft]
  *Ornatotholus* [upper Campanian/lower Maastrichtian, Dinosaur Park, ~6ft]
  *Parksosaurus* [Maastrichtian, Belly River/?Kirtland, ~7ft -- premax teeth 
unknown]
  *Orodromeus* [upper Campanian, Two Medicine, ~6ft]
! *Pachycephalosaurus* [upper Campanian -- upper Maastrichtian, Hell 
Creek/Lance/Dinosaur Park,
~20ft]
? *Protoceratops* [upper Campanian, Djadokhta/Barun Goyot/Minhe/Iren Dabasu, 
10ft]
? *Breviceratops* [upper Campanian, Barun Goyot, 7ft -- possibly synonymous 
with *Bagaceratops*?]
  *Homalocephale* [lower Maastrichtian, Nemegt, 6ft]
  *Goyocephale* [upper Campanian, Barun Goyot, ~5ft]
  *Prenocephale* [upper Campanian -- lower Maastrichtian, Dinosaur Park/Nemgt, 
5-6ft]
  *Tylocephale* [upper Campanian, Barun Goyot, ~5ft]
  
! = this animal is pushing the upper limit of a medium-sized ornithischian
? = this is quadrupedal, and is included because it is heterodont and has been 
proposed as an
    omnivore (Barrett and Naish, 2001; Barret, 2001; Ford, 1999, 2000 [right?])

herbivorous theropods? (5-20):

  *Struthiomimus* (~15ft)
  *Dromeciomimus* (~15ft)
  *Ornithomimus* (~15ft)
  *Chirostenotes* (7ft)
  *Elmisaurus* (~7ft)
  *Oviraptor* (6--10ft)
  *Conchoraptor* (5ft)
  *Ingenia* (5ft)
  *Citipati* (6ft)
  *Khaan* (4--5ft)
  *Troodon* (6ft)
  *Saurornithoides* (5--6ft)
? [*Borogovia*] & [*Tochisaurus*] (both ~6ft)
  *Garudamimus* (~12ft)
  *Enigmosaurus* (~15ft)
  *Erlikosaurus* (~15ft)
! *Segnosaurus* (20ft)
! *Gallimimus* (15-20ft)
  *Shuvuuia* (3ft)
  *Mononykus* (3ft)
? [*Parvicursor*] (~2ft) & [*Patagonykus*] (~7ft)

  These theropods have either leaf-shaped dentition, or none at all, and it is 
apparent that the
majority of forms are from the Late K of Mongolia. In this light, the two best 
formations to study
are the Hell Creek and the Nemegt, for this apparent flux that Justin notes. 
But we can also look
at the formations of 15my earlier, including the Campanian and lower, to study 
relative theropod
and ornithischian diversity.

  Hell Creek: *Thescelosaurus*, *Bugenasaura*, *Leptoceratops*, 
*Pachycephalosaurus*
           || "Troodon", *Ricardoestesia*, *Chirostenotes*, *Elmisaurus*;
  Nemegt: *Homalocephale*, *Prenocephale*
       || *Oviraptor*, *Elmisaurus*, *Gallimimus*, *Segnosaurus*, 
*Erlikosaurus*,
*Saurornithoides*

  There is a value disparity in bipedal dinosaurs below 20ft between 
Ornithischia and Theropoda.
Both formations also bear similar elements as end-K fauna: troodontids, 
high-domed
pachycephalosaurs, and caenagnathids. The Dinosaur Park Formation possesses 
more similarities to
the Nemegt than the Hell Creek does, including *Prenocephale*, and similar 
stegoceratine
pachycephalosaurs, troodontids, caenagnathids, tyrannosaurs, a possible 
segnosaur, ornithomimids
by the buttload (another Holtzism), and so forth.

  It is clear in the end K strata of the North American formations, that there 
is not a
significant decline in ornithischians compared to theropods of the same heft. 
Only two possible
herbivrous theropods appear to present, *Chirostenotes* and *Elmisaurus,* 
compared to four
herbivorous or omnivorous ornithischians. *Thescelosaurus* and *Bugenasaura* 
may be omnivores,
based on dentition, and in this, they may also compare to the previously 
mentioned theropods, plus
"Troodon". Thus the level appears to be balanced, if slightly favoring 
theropods. In the later
strata, the emphasis in the Edmontonian is on larger ornithischians, such as 
*Edmontosaurus*,
*Edmontonia*, ceratopids, etc.. Earlier, the *Maiasaura*/*Albertosaurus* & 
*Orodromeus*/"Troodon"
ecology of the Two Medicine is joined by the ceratopids, and diversity is 
restricted relatively.

  In the Mongolian strata, the theropod influence is strong, where small 
ornithischians are very
few and far between, predominant in *Pinacosaurus* and *Protoceratops* and 
other non-ceratopid
ceratopians; hadrosaurs are also fairly reduced to a few taxa, with one 
predominant and large
species, *Saurolophus*. The theropod diversity is exemplified by 
alvarezsaurians (mononykids),
troodontids, and oviraptorids, with a caenagnathid, and a few ornithomimids and 
tyrannosaurids,
shared by the contemporary North American species. Thus, the NA levels are 
reversed in the
Mongolian taxa, and there does not seem to be a global diversity drop in 
comparative hypothetical
competition. It would help in having an end Maastrichtian Mongolian level, but 
that doesn't seem
forthcoming.

  Other Maastrichitan levels bearing theropods and ornithischians, such as the 
Sinpetru Formation
of Romania, have a large diversity of large ornithischians relative to theropod 
size, but without
comparative size between the two to suggest competition [preserved] this level 
cannot be used in
comparison.

  So finally, while the Hell Creek looks to corroborate Justin's hypothesis, 
this is not so in the
Mongolian levels. So my preliminary conclusion is that any competitive 
replacement that may have
taken place was not global.


=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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