Big Bend Dinosaurs [was: Re: Texas tyrannosaurs (was Re: paleoart idea!)]

["Jonathan R. Wagner" <znc14@TTACS.TTU.EDU>]

        The dinosaurian fauna of Big Bend National Park and the surrounding
reason is currently under review. Let me give just a brief summary of what
is there. I'll try not to spoil anyone's work by giving away inside dope, so
this may not be all that helpful. Besides which I am doing this from memory,
so I may leave something out.

        The terrestrial Mesozoic fossiliferous strata of Big Bend National
Park are devided into two formations, the Aguja Fm. and Javelina Fm.

        The Aguja formation consists of alternating mudstones and sandstones
interpreted as having been deposited by a prograding delta complex and to
some extent more proximal fluvial facies. It records the last two major
regressions of the Cretaceous epiric sea across the region, and the
intervening transgression, and exhibits an intertonguing relationship with
the marine Pen Formation. Marine fossils in the transgressive sand unit in
the lower third of the formation suggest a "middle" Campanian age for the
lower part of the section, and vertebrate biostratigraphy of the upper
portion and the overlying Javelina Fm. suggest a Late Campanian to Early
Maastrichtian (Judithian to Edmontonian) age for the upper part. This
formation is considered partially equivalent to the nearby San Carlos Formation.
        The dinosaurian fauna is similar in character to northern Judithian
faunas, although strikingly less diverse. It is dominated by hadrosaurs,
although ceratopsians appear to have been common as well. One important
feature of the Aguja Formation, and indeed the overlying Javelina Formation,
is that fossils are RARE and on the whole very poorly preserved. Articulated
material is exceedingly rare. Most taxa are known from small bonebed
accumulations and cranial material usually consists of one good skull per
group ("family", not genus). Theropod material is exceedingly rare, possibly
influenced by the presence of the giant alligatoroid _Deinosuchus) (also
poorly known).
        Hadrosaurs: Reported as _Kritosaurus (=Gryposaurus)_ and
_Kritosaurus cf. K. navajovius_ in previous literature. In review.
        Ceratopsians:   _Chasmosaurus mariscalensis_
        Ankylosaurs:    One genus?
        Pachycephalosaurs: One genus?
        Theropods:      Teeth of at least one large theropod, numerous small
theropod teeth running the usual range seen in Canada. Some fragmentary
ornithomimid material.

        The Javelina Formation consists of sand and mudstone interpreted as
fluvial meandering channel and overbank deposits. The Javelina was
apparently depostied concurrently with the onset of Laramide tectonism.
Vertebrate biostratigraphy suggests a Late Maastrichtian (Lancian) to
Paleocene age for this formation, and it is correlated with the nearby El
Picacho Formation. The Javelina is particularly noteworthy for its paleosol
horizons, which give the formation its distinct purple and grey banding. It
is overlain gradationally by the Paleocene Black Peaks Formation, and some
have suggested that these two units should be combined (along with another)
into the "Tornillo Formation". However, the boundary is mappable, and some
workers insist on retaining the distinction. Within the Upper Javelina
Formation lies the Cretaceous/Tertiary systemic boundary, which has been
identified at at least one site in Big Bend.
        One especially important note should be made: do not trust that
fossils have always been attributed to the correct formation in Big Bend!
The contact between the Aguja and Javelina is particularly difficult to pin
down, and some confusion has resulted. One important thing to note, however,
is that, to the best of my knowledge, this contact is neither gradational
nor truly arbitrary. The upper Aguja Fm. sometimes looks alot like the lower
Javelina Fm., just as apparently the top of the non-Naashoibito Kirtland
Shale looks a lot like the Naashoibito Member in some areas. So there are
fossils running around with tags that say "Javelina" but which should say
"Aguja."
        Vertebrate fossils are also rare in the Javelina Formation, and tend
to be encrusted in especially unpleasant concretions. The Javelina is
perhaps best known for the giant pterosaur _Quetzalcoatlus_, but most of the
bone found in it belongs to the poorly known sauropod _Alamosaurus_.
        Sauropods:      _Alamosaurus sanjuanensis_
        Hadrosaurs:     Reported as _Edmontosaurus_, under review
        Ceratopsians:   At least one genus? A new animal has been described,
but not named, from the roughly coeval El Picacho Formation.
        Theropods:      Not sure... There is the one "_T. rex_" maxilla,
which was shoehorned to fit _T. rex_ by ignoring the absolute size of the
specimen. Needs further work.

        Ok, did I forget anything?

        Later,

        Wagner
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     Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
  "Why do I sense we've picked up another pathetic lifeform?" - Obi-Wan Kenobi