New giant carnivorous dinosaur reveals convergent evolutionary trends in theropod arm reduction

Giant carnivorous dinosaurs such as Tyrannosaurus
rex and abelisaurids are characterized by highly reduced
forelimbs that stand in contrast to their huge dimensions, massive skulls,
and obligate bipedalism.​ Another group that follows this pattern, yet is
still poorly known, is the Carcharodontosauridae: dominant predators that
inhabited most continents during the Early Cretaceous​​ and
reached their largest sizes in Aptian-Cenomanian times.​ Despite
many discoveries over the last three decades, aspects of their anatomy,
especially with regard to the skull, forearm, and feet, remain poorly
known. Here we report a new carcharodontosaurid, Meraxes gigas, gen. et
sp. nov., based on a specimen recovered from the Upper Cretaceous Huincul
Formation of northern Patagonia, Argentina. Phylogenetic analysis
places Meraxes among derived Carcharodontosauridae, in a
clade with other massive South American
species. Meraxes preserves novel anatomical information
for derived carcharodontosaurids, including an almost complete forelimb
that provides evidence for convergent allometric trends in forelimb
reduction among three lineages of large-bodied, megapredatory non-avian
theropods, including a remarkable degree of parallelism between the
latest-diverging tyrannosaurids and carcharodontosaurids. This trend,
coupled with a likely lower bound on forelimb reduction, hypothesized to
be about 0.4 forelimb/femur length, combined to produce this short-armed
pattern in theropods. The almost complete cranium
of Meraxes permits new estimates of skull length
in Giganotosaurus, which is among the longest for
theropods. Meraxes also provides further evidence that
carchardontosaurids reached peak diversity shortly before their extinction
with high rates of trait evolution in facial ornamentation possibly linked
to a social signaling role.