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A few weeks ago I visited the Natural Academy of Sciences in Philadephia to
photograph their new, fairly complete mounted cast skeleton of Giganotosaurus
(with the help of Bob Walters). Comparing it to the cast of the NY
Tyrannosaurus put up nearby a decade ago by Ken Carpenter, it was hard to say
which was larger - especially since the S American theropod is set in a
higher position. From the slides (using 400 film without flash and an
exposure time of a substantial chunk of a second) I've completed a skeletal
restoration, which allows the mass to be restored. 

Over all giganotosaurs were typical big theropods, less specialized than
tyrannosaurs. However, the arms are a little reduced, and the shoulder blade
was especially short (in tyrannosaurs the blade was slender, but remained
long). The short scapula suggests that the chest was a little shallower than
in other theropods. The body of Giganotosaurus appears to be a little less
massively constructed than Tyrannosaurus. 

The mass estimate for the Giganotosaurus with a 1430 mm femure and 1700 mm
skull is 6400 kg (assumes a specific gravity of 0.85, as per PDW). It seems
that there is a larger specimen that is about 8% bigger in dimensions (higher
values are probably incorrect, but I'm not sure), this works out to about
8000 kg. 

This compares to 5700 kg for both the NY Tyrannosaurus with a 1350 mm long
skull, and the Pittsburgh type with a 1300 mm femur. Sue with a 1380 mm femur
and a 1530 mm skull may have weighed 6.5-8 tonnes, a more accurate estimate
would require a skeletal restoration which of course is not possible now if
ever. The big Berkerely maxilla seems to be from a similar sized beast. There
may be somewhat larger fragments, but they may not be much larger. 

The mounted Giganotosaurus skull seems to be a few percent longer than the
very similar skull of Carcharodontosaurus, but this does not prove it weighed

It therefore seems that Giganotosaurus, Carcharodontosaurus and Tyrannosaurus
were about the same size. The first may have had the edge, but ironically we
do not know enough about the most complete Tyrannosaurus skeleton to be sure!
Of course, the size horse race can be expected to continue as new specimens
are found and described.