Daspletosaurus horneri: a new species of Tyrannosaurid Dinosaur from the Late Cretaceous of Montana.

Tyrannosaurs are possibly the most iconic of all Dinosaurs, appearing in almost all Dinosaur-related fiction and among the most popular exhibits in museums of palaeontology. They were large, bipedal carnivorous Therapods from the Late Cretaceous of North America and Asia, noted for their reduced forearms and (in fictional accounts) presumed ferocity. Tyrannosaur remains are quite numerous in some North American deposits, and the group has been extensively studied, though these studies have concentrated largely on the diet and locomotion of the animals, with other areas of their biology often neglected.

In a paper published in the journal Scientific Reports on 30 March 2017, Thomas Carr of Carthage College, David Varricchio of the Department of Earth Sciences at Montana State University, Jayc Sedlmayr of the Louisiana State University Health Sciences Centre, Eric Roberts of Geosciences at James Cook University and Jason Moore of Honors College at the University of New Mexico describe a new species of Tyrannosaurid Dinosaur from the Late Cretaceous Two Medicine Formation of Glacier, Lewis and Clark and Teton counties in Montana.

The new species is placed in the genus Daspletosaurus, as it is thought to be closely related to the only previously described species in this genus, Daspletosaurus torosus, and given the specific name horneri, in honour of the distinguished American palaeontologist Jack Horner, for his extensive work on the Two Medicine Formation and mentoring of many students who went on to become leaders in the field. The species is described from a complete skull, partial pectoral limb, and nearly complete hind limb, thought to come from one animal, with several other specimens referred, including an incomplete skull, partial axial series, and partial pelvic girdle and hind limb, a nearly complete dentary of a small juvenile, a maxilla, partial postorbital, and parietal, a partial mandibular ramus, a left ectopterygoid and a right ectopterygoid. 

Skull and jaws of Daspletosaurus horneri; (A) photograph and, (B) labeled line drawing of skull and jaws in left lateral view; (C) photograph and, (D) labelled line drawing of occiput and suspensorium in caudal view; (E) photograph and, (F) labelled line drawing of skull in dorsal view. Scale bars equal 10 cm. Carr et al. (2017).

The largest specimen of Daspletosaurus horneri is estimated to have been 9 m in length and 2.2 m at the acetabulum (hip joint). The species has a distinctly wide dental arcade at the front of the snout compared to other Tyrannosaurids, and shows a reduction in tooth numbers in the oldest specimens, something that has been recorded in some other Tyrannosaurids, including Daspletosaurus torosus and Tyranosaurus rex (Tyrannosaurs, unlike Mammals, continuously shed and grew new teeth throughout their lives, so that the number and positioning of teeth was not fixed; this meant that as the animals grew and their jaws got longer the number of teeth tended to increase, but in some species, including Daspletosaurus horneri, the adults have a slightly reduced tooth count compared to the oldest subadults).

 The growth series of Daspletosaurus horneri, based on parsimony analysis. Unambiguously optimized derived phylogenetic characters were recovered as synontomorphies at two of the five growth stages, which are labeled at the corresponding numbers. Scale bar equals 10 cm. Abbreviations: AMNH FARB, American Museum of Natural History, Fossil Amphibians, Reptiles, and Birds; MOR, Museum of the Rockies. Carr et al. (2017).

Several of the specimens referred to the species show exceptional preservation of the facial bones (premaxilla, maxilla, nasal, lacrimal, jugal, postorbital, squamosal, dentary), enabling Carr et al. to make an assessment of the soft tissue covering of the face. These specimens show a course, hummocky texture to the surface of these bones, similar to that seen in extant Crocodylians, which is assumed to be indicative of a scaly outer integument (skin) like that of Crocodylians.

Also preserved are the neurovascular foramina, channels in the bone through which nerves and blood vessels run. In modern Birds these are limited to the jaw tips, the caudal end of the maxilla, and in a row along the side of the lower jaw, but in Crocodylians they are arranged in repeated rows associated with the positioning of scales; this is also the pattern seen in Daspletosaurus horneri, lending further support to the idea that these animals had a face covered by a Crocodile-like scaly skin.

These neurovascular foramina are very numerous and densely packed, which is taken to be indicative of a highly sensitive snout. This is also seen in modern Crocodylians, where the facial skin plays an important role in a number of behaviours, most notably detecting movements in water that could indicate potential prey, and monitoring the temperature of nesting sites (very important for Crocodylians, which have a temperature dependent method of sex determination). Tyrannosaurids are thought to have been terrestrial hunters, making it highly unlikely that they would have used sensitive facial skin to detect movements in the water, and though Carr et al, consider it quite likely that they had a temperature dependent sex determination system and a consequent need to asses the temperature of nesting sites, they feel that this in itself would be unlikely to be the sole reason for the high levels of  neurovascular development in the face of Daspletosaurus horneri. Instead they suggest that this area of skin may have been important in tactile communication, possibly with Tyrannosaurs touching or rubbing one another's faces during courtship.

The craniofacial epidermis of Daspletosaurus horneri, based on comparison with its closest living relatives, Crocodylians and Birds. Bone texture indicates large zones of large, flat scales and subordinate regions of armor-like skin and cornified epidermis; integumentary sense organs occur on the flat scales that cover the densest regions of neurovascular foramina. The region outside of the Crocodylian-like skin is reconstructed with small scales after fossilized skin impressions of Tyrannosaurids. Dino Pulerà in Carr et al. (2017).

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A review of Jurassic Tyrannosauriods leads to the description of a new genus.

The giant Tyrannosaurs of the Late Cretaceous are some of the most iconic of all dinosaurs, but the group has more humble roots, originating as roughly human-sized predators in the Middle Jurassic, and only growing to large sizes in the last 20 million years of the Mesozoic. These early Tyrannosaurs have not been well understood until recently, when a series of new discoveries has greatly increased our understanding of the group's origin.

In a forthcoming paper in the journal Acta Palaeontologica Polonica Stephen Brusatte of the Division of Paleontology at the American Museum of Natural History and Roger Benson of the Department of Earth Sciences at the University of Cambridge carry out a systematic review of the early Tyrannosaurs, and as a result move one species into a new genus.

Stokesosaurus langhami was described in 2008 by Roger Benson, in a paper in the Journal of Vertebrate Paleontology, from a partial skeleton from the Late Jurassic of Dorset, England. It showed a number of similarities to Stokesosaurus clevelandi, an American species described in 1974. Subsequently a number of new species of Jurassic Tyranosaurs have been discovered, which share many of the same features as S. langhami and S. clevelandi, but which do not otherwise seem to be closely related, suggesting that these features were widespread within early members of the group. On the basis of this Brusatte and Benson conclude that there is no good reason to keep the species in the genus Stokesosaurus and assign it to a new genus, Juratyrant (Jurassic Tyrant).

(A) Right ilium of Juratyrant langhami (image reversed). (B) Left ilium of Stokesosaurus clevelandi. (C) Left ilium of Eotyrannus langhami. Arrows indicate position of lateral ridge, previously thought to be diagnostic of the genus Stokesosaurus. Scale bars are all 5 cm. From Brusatte and Benson (2012).

(A) Right pubis of Juratyrant langhami. (B) Left ischium of Juratyrant langhami. (C) Pubic peduncle of the left ilium of Stokesosaurus clevelandi. Scale bars are all 5 cm. From Brusatte and Benson (2012).

See also The fore-limbs of Majungasaurus, How did raptors use their claws? (and did it help them learn to fly?), A new Troodontid Dinosaur with an injured toe, LH PV18, a New Mini-Tyranosaur, or an Immature Tarbosaur? and Dinosaurs on Sciency thoughts YouTube.

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.

See also Head Butting in a Pachycephalosaur,

An Australian Spinosaurid,

The Ashdown Maniraptoran,

and Dinosaurs on Sciency thoughts YouTube.