Describer

Osi, Prondvai, Butler, Weishampel 2012

Time

Cretaceous Late Santonian

Classification

Ornithischia Ornithopoda Iguanodontia Rhabdodontidae

Diet

Herbivore

Fossilsite

Csehbanya Formation, Iharkut, Veszprem County, Bakony Mountains, Transdanubian Range, western Hungary

Fall under

Rhabdodon

Info

Rhabdodon (Matheron, 1869) = ?Mochlodon (Seeley, 1881 / Bunzel, 1871)

Rhabdodon (Matheron, 1869) > Rhabdodon priscus (Matheron, 1869) >> Oligosaurus adelus (Seeley, 1881) > Ornithomerus gracilis (Seeley, 1881) >> Mochlodon suessi (Bunzel, 1871) > Iguanodon suessi (Bunzel, 1871)

Rhabdodon (Matheron, 1869) > Rhabdodon septimanicus (Buffetaut& Le Loeuff, 1991)

Rhabdodon (Matheron, 1869) > Mochlodon vorosi (Osi, Prondvai, Butler, Weishampel, 2012)

Abstract

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 Haţeg 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.

Holotype

Left complete dentary with four broken teeth (MTM V 2010.105.1).

Etymology

In honour of Dr. Attila Vörös, palaeontologist and full member of the Hungarian Academy of Sciences who founded the Paleontological Research Group of the Hungarian Academy of Sciences.

Type locality

Iharkut, Veszprem County, Bakony Mountains, Transdanubian Range, western Hungary.

Type horizon

Csehbanya Formation, Santonian.

Referred specimens

Left postorbital (MTM 2012.14.1); two right quadrates (MTM V 2010.110.1, V 2010.111.1), two left (MTM V 2010.105.1., 2012.15.1) and two right (MTM V 2010.107.1., V 2010.109.1.) dentaries, all four of which are almost complete, six fragmentary dentaries (MTM V 2010 106.1, V 2010 107.1, V 2010 108.1, V 2010 109.1, V 2010.112.1, 2012.16.1), 15 maxillary and 23 dentary teeth (MTM V 2000.01., V 2000.32., V 2000.33., V 2003.10., V 01.161., V 2003.14,– V.2003.16, V 01.64., 2012.17.1, 2012.18.1), isolated cervical (MTM 2012.19.1), dorsal (MTM 2010.118.1.), and caudal (MTM 2012.20.1, 2012.21.1) vertebrae, almost complete but compressed sacrum (MTM V 2010.121.1.), three coracoids (MTM V 01.53., V 2010.122.1., V 2010.123.1.), one fragmentary scapula (MTM 2012.22.1), one fragmentary (MTM 2012.23.1) and one complete humerus (MTM V 2010.128.1.), one complete ulna (MTM 2012.24.1), two almost complete femora (MTM V 01.225., V 2010.126.1.), one fragmentary femur (MTM 2012.25.1), one complete tibia (MTM V 2010.127.1.), two fragmentary tibiae (MTM V 01.101., 2012.26.1), and two phalanges (MTM 2012.27.1, 2012.28.1).

Diagnosis

Mochlodon vorosi n. sp. differs from Mochlodon suessi in having a dentary with a markedly deeper depression just below the coronoid process that becomes transversely shallower but dorsoventrally wider toward the dentary–surangular suture. The rostral tip of the dentary is directed rostrally (rather than being rostroventrally directed as in M. suessi), such that the dorsal margin of the symphyseal region is horizontal and thus close to the level of the alveolar margin.

This difference can also be observed between the smallest dentary of M. vorosi and the lectotype of M. suessi confirming a genuine taxonomical rather than ontogenetic feature. The groove on the dorsal margin of the symphyseal region is bordered caudally by a dorsally rounded vertical wall that separates the first alveolus from the symphyseal region.

Mochlodon vorosi can further be distinguished from species of Rhabdodon and Zalmoxes in that the proximal end of the quadrate of M. vorosi is strongly curved caudally (directed caudodorsally at c. 60u to the vertical plane) compared to that of Zalmoxes robustus (c. 45u), Zalmoxes shqiperorum (c. 20u) and Rhabdodon sp. (c. 25u in specimen MC 397).

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