Scientists have identified a new species of tyrannosaur from fossils found in western New Mexico. The dinosaur has been named Tyrannosaurus mcraeensis. Although it lived many millions of years before T. rex, it was closely related to it and around the same size.

Tyrannosaurus mcraeensis

The study, published in “Scientific Reports” postulates that the ancestors of T. rex originated in southern Laramidia. Where and when the tyrannosaur lineage that includes T. rex and its closest relatives evolved remains unclear. It had been thought that these theropods originated in Asia, or perhaps at more northerly latitudes of Laramidia. The identification of fossils representing a giant, 12-metre-plus tyrannosaur suggests that large-bodied, apex predators evolved alongside other exceptionally large dinosaurs at lower latitudes.

The researchers examined a partial skull (NMMNH P-3698), that had been excavated from a location in Sierra County, New Mexico. The fossil material consisted of a right postorbital and squamosal, along with a left palatine, a fragmentary maxilla and elements from the lower jaws including the left dentary. The fossils come from Hall Lake Formation (McRae Group). Uranium to lead (U/Pb) isotope analysis of a layer some thirty metres below the tyrannosaur fossil site is dated to 73.2 mya plus or minus 0.7 million years. This indicates that Tyrannosaurus mcraeensis predates T. rex by approximately 6-7 million years.

The skull bones, previously assigned to T. rex are currently on display at the New Mexico Museum of Natural History & Science (NMMNHS).

Older and More Primitive than Tyrannosaurus rex

While the new discovery predates T. rex, the paper notes that subtle differences in the jaw bones make it unlikely that T. mcraeensis was a direct ancestor. However, it is assigned to the Tyrannosaurini tribe, which is defined by the authors as the last common ancestor of the Asian Tarbosaurus bataar and Tyrannosaurus rex and all its descendants.

Contributing authors on the study include researchers from the University of Bath (UK), NMMNHS, University of Utah, The George Washington University, Harrisburg University, Penn State Lehigh Valley, and the University of Alberta.

Ironically, it was the examination of horned dinosaur fossils from the same palaeoenvironment that led to the discovery of a new Tyrannosaurus species. In 2013, then-student Sebastian Dalman began to re-examine ceratopsian fossils, it led to a broader rethink about the dinosaur fauna associated with the McRae Group.

Dalman commented:

“I started working on this project in 2013 with co-author Steve Jasinski and soon we started to suspect we were on to something new.”

Careful Comparison with T. rex Skull Fossils

Analysis of the skull material revealed subtle, but unique traits relating to their morphology and articulation. Careful comparison with T. rex skull fossils led the research team to conclude that these bones did not represent Tyrannosaurus rex. This was something new.

As T. rex is known from multiple individuals, it is possible to show that T. mcraeensis lies outside of the range of individual variation seen in T. rex.

Co-author of the paper, Dr Anthony Fiorillo, Executive Director of NMMNHS explained:

“New Mexicans have always known our state is special, now we know that New Mexico has been a special place for tens of millions of years. This study delivers on the mission of this museum through the science-based investigation of the history of life on our planet.”

Size estimates for Tyrannosaurus mcraeensis put it in the same bracket as the famous and geologically younger T. rex. It is thought to have measured around twelve metres in length.

Fellow author of the paper, Dr Nick Longrich (Milner Centre for Evolution at the University of Bath) added:

“The differences are subtle, but that’s typically the case in closely related species. Evolution slowly causes mutations to build up over millions of years, causing species to look subtly different over time.”

Tyrannosaurus mcraeensis and the Origins of T. rex

The identification of a new Tyrannosaurus from New Mexico raises the intriguing possibility that there are several more new tyrannosaur discoveries yet to be made.

Co-author Dr Spencer Lucas (Palaeontology Curator at the NMMNHS) stated:

“Once again, the extent and scientific importance of New Mexico’s dinosaur fossils becomes clear. Many new dinosaurs remain to be discovered in the state, both in the rocks and in museum drawers!”

Tyrannosaurus mcraeensis expands our understanding of tyrannosaurs in several ways. Firstly, it suggests that the apex predators lived in what is now the southern United States at least 72 million years ago. Secondly, the Tyrannosaurus genus likely originated in southern North America then later expanded into much of the western portion of the continent.

Phylogenetic analysis supports this hypothesis. The analysis places T. mcraeensis as sister taxon to T. rex and suggests the Tyrannosaurini tribe originated in southern Laramidia.

Tyrannosaurus mcraeensis Raises More Questions

The skull fossils assigned to T. mcraeensis suggest that larger, more robust and powerful tyrannosaurs evolved in the southern United States compared to the smaller and more primitive tyrannosaurs found further north.

For reasons as yet unknown, dinosaurs may have evolved to larger sizes in lower latitudes in North America. This body condition pattern is not seen in modern mammals. This newly described tyrannosaur was part of an ecosystem dominated by super-sized dinosaurs. For example, the giant chasmosaur Sierraceratops turneri was contemporaneous. In addition, the titanosaur Alamosaurus and an as yet, undescribed giant hadrosaur shared this palaeoenvironment.

Giant tyrannosaurs were able to spread north during the Maastrichtian stage of the Late Cretaceous. The reasons for this migration remain unclear. Perhaps the northward spread of giant herbivores such as Triceratops and Torosaurus created a food source that could be exploited by the very biggest tyrannosaurs.

Everything Dinosaur acknowledges the assistance of a media release from the University of Bath in the compilation of this article.

The scientific paper: “A giant tyrannosaur from the Campanian–Maastrichtian of southern North America and the evolution of tyrannosaurid gigantism” by Sebastian G. Dalman, Mark A. Loewen, R. Alexander Pyron, Steven E. Jasinski, D. Edward Malinzak, Spencer G. Lucas, Anthony R. Fiorillo,
Philip J. Currie and Nicholas R. Longrich published in Scientific Reports.

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Newly published research suggests that the Nanotyrannus genus is valid. Writing in the academic journal “Fossil Studies” researchers conclude that Nanotyrannus lancensis is a distinct species and that fossil specimens do not represent juvenile examples of Tyrannosaurus rex.

Nanotyrannus lancensis and Tyrannosaurus rex

The scientists, Dr Nick Longrich, from the Milner Centre for Evolution at the University of Bath and Dr Evan Saitta, from the University of Chicago propose that Nanotyrannus was probably not closely related to T. rex. Their research indicates that Nanotyrannus was a smaller, longer-armed tyrannosaur with a narrower snout.

The debate as to the validity of Nanotyrannus as a taxon has persisted for decades. The first skull assigned to Nanotyrannus was found in Montana in 1942. Analysis of a skull bone from a previously unrecognised T. rex fossil coupled with a detailed bone histology demonstrates that specimens of N. lancensis do indeed represent adult animals and not juveniles of another, already described species.

The research led Longrich and co-author Evan Saitta to a previous fossil discovery. The skull bone is a frontal, it was at a museum in San Francisco but had not been studied. The researchers were able to conclude that this frontal came from a juvenile T. rex, an animal that would have had a skull about 45 cm long and a body length of 5 metres.

Dr Longrich explained:

“Yes, it’s just one specimen, and just one bone, but it only takes one. T. rex skull bones are very distinctive, nothing else looks like it. Young T. rex exist, they’re just incredibly rare, like juveniles of most dinosaurs.”

Comparing Growth Rates

Measuring the growth rings in Nanotyrannus bones, the researchers demonstrated that they became more closely packed towards the outside of the bone – its growth was slowing. It suggests these animals were nearly full size, not fast-growing juveniles. Modelling the growth of the fossils showed the animals would have reached a maximum of around 900-1500 kilograms and five metres – about 15 per cent of the size of the giant T. rex, which grew to 8,000 kilograms and twelve metres long or more.

Dr Longrich commented:

“When I saw these results, I was pretty blown away. I didn’t expect it to be quite so conclusive.”

He added:

“If they were young T. rex they should be growing like crazy, putting on hundreds of kilograms a year, but we’re not seeing that. We tried modelling the data in a lot of different ways and we kept getting low growth rates. This is looking like the end for the hypothesis that these animals are young T. rex.”

No Evidence of Fossils with Combined Traits

In addition, the researchers found no evidence of fossils combining features of both the Nanotyrannus and T. rex, which would exist if the one transitioned into the other. Every fossil they examined could be confidently identified as one species or the other. Neither did the patterns of growth in other tyrannosaurs fit with the hypothesis that Nanotyrannus fossils were juvenile T. rex.

The picture (above) shows a replica of Nanotyrannus lancensis in the PNSO model range.

To view this range of dinosaur models: PNSO Age of Dinosaurs.

Nanotyrannus lancensis – Strong Evidence in Support of this Genus

Dr Longrich said:

“If you look at juveniles of other tyrannosaurs, they show many of the distinctive features of the adults. A very young Tarbosaurus – a close relative of T. rex – shows distinctive features of the adults. In the same way that kittens look like cats and puppies look like dogs, the juveniles of different tyrannosaurs are distinctive. Nanotyrannus just doesn’t look anything like a T. rex. It could be growing in a way that’s completely unlike any other tyrannosaur, or any other dinosaur- but it’s more likely it’s just not a T. rex.”

The researchers argue these findings are strong evidence that Nanotyrannus is a separate species, one not closely related to Tyrannosaurus. It was more lightly-built and long-limbed than its thick-set relative. It also had larger arms, unlike the famously short-armed T. rex.

Furthermore, the authors suggest that, given how difficult it is to tell dinosaurs apart based on their often-incomplete skeletons, palaeontologists may be underestimating the diversity of dinosaurs, and other fossil species.

Everything Dinosaur acknowledges the assistance of a media release from the University of Bath in the compilation of this article.

The scientific paper: “Taxonomic Status of Nanotyrannus lancensis (Dinosauria: Tyrannosauroidea) — A Distinct Taxon of Small-Bodied Tyrannosaur” by Nicholas R. Longrich and Evan T. Saitta published in Fossil Studies.

Visit the Everything Dinosaur website: Everything Dinosaur.

A new species of giant titanosaur has been scientifically described. The dinosaur, known from fossils from Neuquén Province, (Argentina) has been named Bustingorrytitan shiva. This dinosaur may have weighed more than sixty-seven tonnes! Although the body mass estimates are prone to error, it is likely that this huge herbivore weighed at least fifty tonnes.

The fossil material was collected from the base of the Huincul Formation and consists of a relatively complete skeleton and the partial remains of three others. The strata have been dated to the upper Cenomanian (95 mya). The fossils were collected from the surroundings of Villa El Chocón. The genus name was erected to honour Manuel Bustingorry, who permitted the excavation work to take place.

The species name is from the Hindu deity Shiva, which transformed the universe. This alludes to the extensive faunal turnover that occurred in the Cretaceous towards the Cenomanian/Turonian boundary.

Calculating the Weight of a Giant Titanosaur

Both cranial and postcranial material was recovered. The fossil material includes right and left humeri and fragmentary thigh bones (femora). From these bones (humerus and the femur) the minimum circumference of these limb bones can be established. A formula (Campione and Evans, 2012) can then be applied to estimate the body mass of the animal. These calculations suggest that B. shiva was heavier than Dreadnoughtus schrani and perhaps comparable to the original body weight calculated for Patagotitan mayorum.

Intriguingly, the holotype material from which some of the limb bone measurements originate, suggests that the holotype specimen was not fully grown when it died. Bustingorrytitan shiva, may have been much larger.

The scientific paper: “A new gigantic titanosaurian sauropod from the early Late Cretaceous of Patagonia (Neuquén Province, Argentina)” by María Edith Simón and Leonardo Salgado published in Acta Palaeontologica Polonica.