Thursday, June 27, 2013

[Paleontology / PalaeoIchthyology • 2013] Teleost centrum and jaw elements from the Upper Cretaceous Nemegt Formation (Campanian – Maastrichtian) of Mongolia and a re-identification of the fish centrum found with the theropod Raptorex kreigsteini

RAPTOREX REBUTTAL: A new paper by Newbrey et al 2013 adds even stronger evidence that the small tyrannosaurid skeleton named "Raptorex" was illegally removed from the Late Cretaceous Nemegt Formation of Mongolia, and not from the Early Cretaceous of China (as originally claimed).
http://facebook.com/photo.php?fbid=682305425119461
via Horner Paleo Lab, Museum of the Rockies


Abstract
Isolated centra and a premaxilla of a teleost from the Upper Cretaceous Nemegt Formation (Late Campanian - Early Maastrichtian) of Mongolia are described and aligned with the hiodontids and the Late Cretaceous teleost Coriops from North America. The atlas of the Nemegt taxon has an anterior articular surface with the dorsal half being subdivided into two flat articulator surfaces as in those of the hiodontids. In more posterior abdominal centra, the centrum is strongly constricted at the notochord foramen, the rib loosely articulates in a facet on the lateral wall of the centrum posterior to the parapophysis as in hiodontids, and parapophyses are fused to the centrum. Neural arch articular facets are small and round. Distinct mid-dorsal foramina are absent or small and poorly developed. A single stout premaxilla is relatively straight and has a low rounded dorsal margin on the posterior end. There are two rows of strong conical teeth and the tooth bases of the lateral row protrude laterally. The Nemegt centra are then used to re-identify a teleost centrum associated with the Asian theropod, Raptorex kreigsteini. Initially the fish centrum found with R. kreigsteini was assigned to Lycoptera. The stratigraphic range of Lycopteridae, ~120-135 Ma, was used to infer an age of deposition for the basal taxon Raptorex. Subsequently this centrum was re-identified as a clupeomorph centrum. However, centra of Lycoptera are mainly comprised of the chordacentrum surrounded by a very thin autocentrum, thus giving the appearance of being tubular with an unconstricted notochordal foramen; they are thin-walled, small (≤ 2 mm diameter), and may have a broad bar (presence depends on the species and ontogenetic development) extending the length of the centrum in lateral view. Parapophyses are not fused with the autocentrum and articulate with the centrum at large facets as in those of lower teleosts. Pleural ribs in Lycoptera articulate with the parapophyses. The fish centrum found with R. kreigsteini is of a higher teleost with a well-developed autocentrum strongly constricting the notochord, thereby giving the centrum an amphicoelous shape. This centrum has several aspects in common with the Nemegt Formation teleost centra: poorly developed mid-dorsal foramen; shape and position of the facets, where the arch articulates, being circular and located near the anterior end of the centrum; presence of short, fused parapophyses at the ventro-lateral corner of the centrum; lateral surface of the centrum bearing a series of foramina of small to moderate size that are generally organized into rows. Thus we reject the hypotheses that the fish centrum found with R. kreigsteini has affinities with the Lycopteridae or the Clupeomorpha and reassign the centrum to the hiodontids. The morphological characteristics of the fish centrum found with R. kreigsteini suggest a Late Cretaceous hiodontid-like taxon and thus its co-occurrence with Raptorex suggests that dinosaur is an Upper Cretaceous theropod.

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RAPTOREX REBUTTAL: A new paper by Newbrey et al 2013 adds even stronger evidence that the small tyrannosaurid skeleton named "Raptorex" was illegally removed from the Late Cretaceous Nemegt Formation of Mongolia, and not from the Early Cretaceous of China (as originally claimed). 

The new paper shows that a fossil fish vertebra (backbone) found with the "Raptorex" skeleton does not belong to a "lycopterid" fish, as originally claimed, but instead is indistinguishable from a type of fish only found in the Nemegt Formation of Mongolia. This supports the findings of a previous rebuttal by MOR researchers (Fowler et al., 2011b), which disputed the major claims of the original species description (Sereno et al., 2009).

The findings are important as if "Raptorex" was from the Early Cretaceous (~ 130 million years old, as originally claimed) then it would mean that many of the distinctive features that we associate with tyrannosaurids evolved much earlier in time than previously thought. However, the new paper further supports the view that Raptorex is actually from the late Cretaceous (~ 70 million years ago), and that tyrannosaurs evolved their distinctive features more gradually.

NEWBREY, M.G., BRINKMAN, D.B., WINKLER, D.A., FREEDMAN, E.A., NEUMAN, A.G., FOWLER, D.W., & WOODWARD, H.N. 2013. Teleost centrum and jaw elements from the Upper Cretaceous Nemegt Formation (Campanian – Maastrichtian) of Mongolia and a re-identification of the fish centrum found with the theropod Raptorex kreigsteini. 291-303. in Arratia, Schultze & Wilson (eds), Mesozoic Fishes 5 - Global Diversity and Evolution. Verlag, Munchen, Germany. ISBN 978-3-89937-159-8. http://www.pfeil-verlag.de/07pala/pdf/4_59d11.pdf

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