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Re: Tyrannotitan, new carcharodontosaurid from Argentina



My two cents (Euro-cents that is) on the new carcahrodontosaurid (congrats
to Novas et al. for this remarkable animal):

Tyrannotitan is known from two specimens found in close proximity (some 800
m from one another) from the ?Aptian Cerro Castaño Member of the Cerro
Barcino Formation close to the town of Paso de Indios in Chubut. It is the
same material that was mentioned by Rich et al (2000: Theropods from the
"Middle" Cretaceous Chubut Group of the San Jorge sedimentary basin, central
Patagonia. A preliminary note. Gaia 15:111-115) as a possible spinosaurid.
Vickers-Rich et al (1999. "Big tooth" from the Early Cretaceous of Chubut
Province, Patagonia: a possible carcharodontosaurid. National Science Museum
Monographs 15:85-88) already described possible carcharodontosaurid teeth
from the same unit. Otherwise, the fauna of the Lower Cretaceous of South
America is very poorly known, and further inferences on theropod diversity
and evolution are hampered by the poor fossil record. However, ceratosaurs
seemed to have played an important role: there is Ligabueno from the
Barremian-Hauterivian La Amarga Formation (Bonaparte, J.F. 1996. Cretaceous
tetrapods of Argentina. Münchener Geowissenschaftliche Abhandlungen (A)
30:73-130.), which is probably a noasaurid (Coria, R.A. and L. Salgado.
2000. A basal Abelisauria Novas, 1992 (Theropoda-Ceratosauria) from the
Cretaceous of Patagonia, Argentina. Gaia 15:89-102), some fragmentary
abelisaurid remains from the underlying La Paloma Member of the Cerro
Barcino Formation (Rauhut, O.W.M., G. Cladera, P. Vickers-Rich and T.H.V.
Rich. 2003. Dinosaur remains from the Lower Cretaceous of the Chubut Group,
Argentina. Cretaceous Research 24:487-497) and the probable ceratosaurid
Genyodectes, which most probably came from the same unit as Tyrannotitan
(Rauhut, O.W.M. 2004. Provenance and anatomy of Genyodectes serus, a
large-toothed ceratosaur (Dinosauria: Theropoda) from Patagonia. Journal of
Vertebrate Paleontology 24:894-902). Remains from other parts of South
America include spinosaurs (Sues, H.-D., E. Frey, D.M. Martill and D.M.
Scott. 2002. Irritator challengeri, a spinosaurid (Dinosauria: Theropoda)
from the Lower Cretaceous of Brazil. Journal of Vertebrate Paleontology
22:535-547; Kellner, A.W.A. and D.d.A. Campos. 1996. First Early Cretaceous
theropod dinosaur from Brazil with comments on Spinosauridae. Neues Jahrbuch
für Geologie und Paläontologie, Abhandlungen 199:151-166), the compsognathid
Mirischia (Naish, D., D.M. Martill and E. Frey. 2004. Ecology, systematics
and biogeographical relationships of dinosaurs, including a new theropod,
from the Santana Formation (Albian, Early Cretaceous) of Brazil. Historical
Biology 18:1-14) and a small coelurosaur of uncertain affinities
(Santanaraptor: Kellner, A.W.A. 1999. Short note on a new dinosaur
(Theropoda, Coelurosauria) from the Santana Formation (Romualdo Member,
Albian), northeastern Brazil. Boletim do Museu Nacional, Nova Série,
Geologia 49:1-8) [Now, before everybody jumps on that, I know that Holtz
(2004) tentatively referred this taxon to the Tyrannosauroidea, but this is
based on rather weak evidence]. Thus, the history of Lower Cretaceous
theropods from South America seems rather complex, but needs more
elucidation before we can draw major conclusions. However, this does not
necessarily mean that I disagree with the ideas put foreward by Novas et
al.;  it is always good to have some hypothesis out there that can be tested
by further evidence! (the discussion here already shows again that such
hypothesis can give very fruitfull food for thought)


> <IIRC teeth similar to those of Acrocanthosaurus were found in an
> Ethiopian
> unit of about the same age but this carcharodontosaurid seems more related
> to
> Giganotosaurus.>

These teeth are supposedly Late Jurassic in age, but the arguments for
referring them to Acrocanthosaurus are weak: "...possesses chisel-shaped
denticles very similar to ... Acrocanthosaurus" (Goodwin, M.B., W.A.
Clemens, J.H. Hutchinson, C.B. Wood, M.S. Zavada, A. Kemp, C.J. Duffin and
C.R. Schaff. 1999. Mesozoic continental vertebrates with associated
palynostratigraphic dates from the northwestern Ethiopian Plateau. Journal
of Vertebrate Paleontology 19:728-741). Since chisel-shaped denticles are
the plesiomorphic condition for archosaurs, this identification is highly
questionable.



> "chin" is not indicative of *Giganotosaurus*-only relationships. Teeth
> bearing
> "wrinkles" are known from the Aptian beds that Rich and Novas previosuly
> reported years back, and from Japan and Brasíl, but are absent in North
> American beds so far. Teeth from England appear to exhibit banding across
> the
> crown as in *"Megalosaurus" hesperis*, but this may be an artifact of
> coloration of the enamel surface, rather than surface texture.

I am somewhat uneasy with all that enamel wrinkle stuff - although I agree
that these wrinkles are always well-developed in carcharodontosaurids, they
are in no way exclusive to this group. Similar wrinkles occasionally occur
in other basal tetanurans (in fact, I once found a spinosaurid tooth that
had these wrinkles very well-developed), so a single tooth with enamel
wrinkles does not make a carcharodontosaurid. Working on the Tendaguru
theropod teeth at the moment, I found that several of the teeth of
"Megalosaurus" ingens have very well-developed wrinkles, very similar to
those found in carcharodontosaurids, but you have all grades in the
development of these wrinkles between this morphology and no wrinkles at all
(though this is maybe what you would expect from a very basal
carcharodontosaurid).  


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