Monday, 4 July 2011

LH PV18, a New Mini-Tyranosaur, or an Immature Tarbosaur?

The Tyranosuarids are iconic dinosaurs, large bipedal predators with reduced forelimbs and powerful jaws. No dinosaur movie would be complete without an appearance by Tyranosaurus rex, the most famous member of the group, and species which gives the group its name. Tyranosaurids are relatively well known, there are quite a few good specimens, but almost all of these are from the Late Cretaceous; the earlier history of the group remains obscure.

In September 2009 a team lead by Paul Sereno of the Department of Organismal Biology and Anatomy at the University of Chicago published a paper in the journal Science in which they described a possible Early Cretaceous Tyranosaurid from northeastern China. This description was based upon a single specimen, LH PV18, given the Latin binomial Raptorex kriegsteini.

Specimen LH PV18, Raptorex kriegsteini.

Specimen LH PV18 had a dubious provenance. The specimen was purchased at the Tuscon Gem, Mineral and Fossil Showcase by Henry Kriegstein, an ophthalmologist and amateur fossil hunter (after who's father the specimen was named), who then donated it to Sereno's laboratory. Kriegstein bought the fossil from Hollis Butts, a Japan based dealer specializing in fossils from the Far East. Butts apparently sold the specimen as being from Mongolia, but there were persistent rumors that the fossil really came from China (it is illegal to export such specimens from China, so it is hardly surprising that Butts would deny this).

Sereno et al. examined the matrix in which the fossil was set for other fossils in an attempt to verify the provenance of the specimen. They claim to have found numerous smaller fossils, including 'fish bones, turtles, clam shells and other fauna', and on the basis of these concluded that the fossil originally came from the Lujiatun Beds of the Lower Cretaceous Yixian Formation in Liaoning Province, north-east China. Sereno was sufficiently convinced of this diagnosis to donate the specimen to the Long Hao Institute of Geology and Paleontology at Hohhot in Inner Mongolia, China.

Using small fossils to date rocks (or biostratigraphy) is very much the meat of palaeontology. Many small, shelly groups of creatures have very high rates of species turnover and palaeontologists can use this to date sediments very accurately - within hundreds of thousands of years on sediments hundreds of millions of years old. This information can be used to build up detailed models of the structure of sedimentary beds. These stratigraphic models are vital for the oil industry, as well as for engineering projects such as the Channel Tunnel.

A biostratigraphic section of part of the Neuqué Basin in the Argentine Andes, using nanofossils, ammonites and bivalves.

The actual specimen is a nearly complete skeleton roughly 2.5 m long, that probably came from a living animal 3 m long. The specimen was clearly not fully mature, as not all of its bones were fully ossified and fused, however several bones had fused which Sereno et al. interpreted to indicate that the animal was close to maturity.

This was two speculative jumps in a single paper; the dating and location of the original fossil, plus the age of the specimen, so it was unsurprising that the paper was duly challenged.

Peter Larson is co-owner of the Black Hills Institute of Geological Research, a private fossil hunting company based in Hill City, South Dakota. The Institute specializes in finding and preparing museum grade fossils; it was responsible for the discovery of 'Sue' the most complete Tyranosaurus specimen ever found, now housed in The Field Museum, Chicago.

Jørn Hurum is an associate professor of Paleontology at the University of Oslo. He specializes in Mesozoic Mammals and Dinosaurs, though recently he has been most famous for his description of Ida, an Eocene lemur.

In October 2010 Larson and Hurum spoke to the journal Nature about their concerns. To them LH PV18 looked like a juvenile specimen of Tarbosaurus, a well documented tyranosaurid dinosaur from Mongolia. Furthermore the provided supporting evidence for the age of the specimen didn't seem good enough. Despite having claimed numerous smaller fossils were found in the matrix, Sereno et al. provided images of only two, a pelycepod and a fish vertabrae which resembled one from a fish of the genus Lycoptera.

Pelycepod is another word for bivalve; the term is itself of little significance. Sereno et al. published a picture of a bivalve shell in their supporting material, without any description, and noted that pelycepods were common in the Lower Cretaceous Jehol fauna. To Larson & Hurum the specimen appeared to be a freshwater clam, a widespread and long surviving group, of little stratigraphic significance. To me it looks like a unionoid, or freshwater mussel, a group that appeared in the Triassic, and haven't changed much since. The fish vertebrae Sereno et al. described as probably from a fish of the genera Lycoptera, a fish abundant in the Jehol fauna, but with a fossil record dating from the Jurassic to the present. Thus even if the bone could be confirmed as coming from Lycoptera, this would be of no particular significance. Larson suggested that for a confident date far more reliable fossils would be needed, and suggested that the matrix material be analyzed for pollen, an excellent stratigraphic fossil.

In Jun this year (2011) a team lead by Denver W. Fowler of the Museum of the Rockies and the Department of Earth Sciences at Montana State University, published a more detailed review of LH PV18 in the journal PLoS One. Like Larson and Hurum they could not see any significance in the bivalve shell. They examined the fish vertebrae, and came to the conclusion that it did not come from Lycoptera, but was most probably from a ellimmichthyform fish, a group which survived from the Early Cretaceous till the Eocene. Fowler et al. suggest that in the absence of any stratigraphicly significant fossils it might be possible to date the sample using volcanic zircons in the matrix, or to analyze the bones for rare earth element content and try to correlate this with the rare earth content of bones from known localities.

In addition they performed an analysis of the osteology of the specimen and compared this to that of immature Tyranosaurus specimens from North America. From this they concluded that the level of development indicated a much younger animal than Sereno et al. implied. Tyranosaurus is very similar to Tarbosaurus, to the extent that some experts do not consider them to be separate species. If the two follow similar developmental patterns (which seems a reasonable conclusion), and LH PV18 really is a Tarbosaurus, then the pattern of development implies quite a young animal.

From this Fowler et al. conclude that there is no reason to conclude that LH PV18 is not an immature Tarbosaurus, and the name Raptorex is erroneous.

It is unlikely that Sereno, who is one of the world's leading dinosaur specialists, intended to deceive. Potentially a young palaeontologist striving to make a name for himself could be tempted to deliberately inflate the importance of their discoveries, but someone of Sereno's experience and reputation would have more to lose than to gain. It seems more likely that he allowed himself to be taken in by the romance of the apparently smuggled dinosaur, though Butts always claimed that the specimen came from Mongolia and was sold by him as an immature Tarbosaurus. This in itself would not be a bad discovery; no immature Tarbosuarus specimen has previously been described.

and Dinosaurs on Sciency thoughts YouTube.

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