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Raptorex reanalysis: probable juvenile Tarbosaurus (PLoS ONE)

From: Ben Creisler

New in PLoS ONE:

Fowler, D.W., Woodward, H.N., Freedman, E.A., Larson, P.L., & Horner, J.R.
Reanalysis of ?Raptorex kriegsteini?: A Juvenile Tyrannosaurid Dinosaur
from Mongolia. 
PLoS ONE 6(6): e21376. doi:10.1371/journal.pone.0021376

The carnivorous Tyrannosauridae are among the most iconic dinosaurs:
typified by large body size, tiny forelimbs, and massive robust skulls with
laterally thickened teeth. The recently described small-bodied
tyrannosaurid Raptorex kreigsteini is exceptional as its discovery proposes
that many of the distinctive anatomical traits of derived tyrannosaurids
were acquired in the Early Cretaceous, before the evolution of large body
size. This inference depends on two core interpretations: that the holotype
(LH PV18) derives from the Lower Cretaceous of China, and that despite its
small size, it is a subadult or young adult. Here we show that the
published data is equivocal regarding stratigraphic position and that
ontogenetic reanalysis shows there is no reason to conclude that LH PV18
has reached this level of maturity. The probable juvenile status of LH PV18
makes its use as a holotype unreliable, since diagnostic features of
Raptorex may be symptomatic of its immature status, rather than its actual
phylogenetic position. These findings are consistent with the original sale
description of LH PV18 as a juvenile Tarbosaurus from the Upper Cretaceous
of Mongolia. Consequently, we suggest that there is currently no evidence
to support the conclusion that tyrannosaurid skeletal design first evolved
in the Early Cretaceous at small body size.

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