Background: Rhabdodontid ornithopod dinosaurs are characteristic elements of Late
Cretaceous European vertebrate
faunas and were previously collected from lower Campanian to Maastrichtian continental
deposits. Phylogenetic analyses
have placed rhabdodontids among basal ornithopods as the sister taxon to the clade
consisting of Tenontosaurus,
Dryosaurus, Camptosaurus, and Iguanodon. Recent studies considered Zalmoxes, the best
known representative of the clade,
to be significantly smaller than closely related ornithopods such as Tenontosaurus,
Camptosaurus, or Rhabdodon, and
concluded that it was probably an island dwarf that inhabited the Maastrichtian Hat¸eg
Island.
Methodology/Principal Findings: Rhabdodontid remains from the Santonian of western
Hungary provide evidence for a
new, small-bodied form, which we assign to Mochlodon vorosi n. sp. The new species
is most similar to the early Campanian
M. suessi from Austria, and the close affinities of the two species is further supported
by the results of a global phylogenetic
analysis of ornithischian dinosaurs. Bone histological studies of representatives
of all rhabdodontids indicate a similar adult
body length of 1.6–1.8 m in the Hungarian and Austrian species, 2.4–2.5 m in the subadults
of both Zalmoxes robustus and
Z. shqiperorum and a much larger, 5–6 m adult body length in Rhabdodon. Phylogenetic
mapping of femoral lengths onto
the results of the phylogenetic analysis suggests a femoral length of around 340 mm
as the ancestral state for
Rhabdodontidae, close to the adult femoral lengths known for Zalmoxes (320–333 mm).
Conclusions/Significance: Our analysis of body size evolution does not support the
hypothesis of autapomorhic nanism for
Zalmoxes. However, Rhabdodon is reconstructed as having undergone autapomorphic giantism
and the reconstructed small
femoral length (245 mm) of Mochlodon is consistent with a reduction in size relative
to the ancestral rhabdodontid
condition. Our results imply a pre-Santonian divergence between western and eastern
rhabdodontid lineages within the
western Tethyan archipelago.
