Partial Theropod Femur – Right People, in the Right Place at the Right Time

News broke this week of the discovery and subsequent description of a partial left femur from a theropod dinosaur that had been found on the south-west tip of Sucia Island, which is part of Washington State.  This is the first time that a dinosaur fossil has been recorded from Washington State.  Sucia Island is part of the San Juan archipelago group of islands, it is a conservation area, part of a marine park (Washington State Marine Park).  Sucia Island is very small, it covers approximately 0.88 square miles, making the island about the size of the Queen Elizabeth Olympic Park in London, the site of the 2012 Olympic Games.

The Shoreline where the Theropod Fossil was Discovered (Sucia Island State Park)

Picture credit: Burke Museum

The First Dinosaur

The fossil was collected by a team of palaeontologists from the Burke Museum of Natural History and Culture (Seattle) in May 2012.  The partial femur was located in very hard, silty sandstone rocks which represent a shallow marine environment.  The discovery of the fossil had been made earlier by Burke museum field workers who had been excavating ammonite fossils and other invertebrates from the same bed.  It has taken three years to prepare the fossil for further study by carefully removing the extremely hard matrix and then to identify what type of animal the partial thigh bone might represent.

Based on comparative studies the bone has been ascribed to a tyrannosaur, but more about that later.

Scientists Proudly Show Off Washington State’s First Dinosaur Fossil

Picture credit: Burke Museum

Right People at the Right Time

This really is an example of the right people, in the right place at the right time.  Occasionally vertebrate fossils are found in marine deposits, in the United Kingdom, the Dorset coast provides an example of this.  The “Jurassic Coast” around Charmouth and Lyme Regis in Dorset represent a shallow marine environment but the fossilised bones of a dinosaur (Scelidosaurus) have been found.  It seems likely that the corpses of dinosaurs were washed out to sea and once the body cavity had ruptured the carcase sank to the bottom and became covered in sediment permitting the fossilisation process to begin.

The picture above shows Dr Sidor holding the partial femur fossil, Brandon is holding a cast of femur from a tyrannosaur called Daspletosaurus (D. torosus) to allow a visual comparison to be made.  Both scientists are showing the back view of the objects (the posterior view), the photographer is looking at the back of the fossil, this is important to note as an anatomical feature located at the back of the fossil bone is helping palaeontologists to classify the sort of dinosaur the Washington State bone represents.

A Rare Find

To find a dinosaur fossil in an marine environment is very rare and given the strong waves and tides associated with Sucia Island the fossil material could have been quickly washed out to sea and lost forever, so it really is a question of the right people being in the right place at the right time.  The analysis and identification of the femur was undertaken by Dr Sidor (Burke Museum of Natural History and Culture) and Brandon Peecock (University of Washington), their paper has just been published in the on line journal PLOS One.  It is highly unlikely that other fossils related to this individual will be found at the site.

Why a Tyrannosaur?

To answer this question we first have to take a step back.  How do we know that this is a dinosaur bone in the first place?  The fossils are found in a horizon that makes up a member of the Cedar District Formation, Upper Cretaceous strata that outcrops along the north-western coast of the United States and the extreme southern portion of the Canadian Pacific coast around Vancouver.

The exact age of these rocks is debated, but the rocks were probably laid down in the Campanian faunal stage and are perhaps around 80 million-years-old.  During this time, in the Late Cretaceous, North America was divided into several landmasses by the Western Interior Seaway.

Laramidia

The island of Laramidia lay to the west and this strip of land is represented by strata found in Alaska all the way down to Mexico on the western seaboard of this continent.  Dinosaurs were the dominant terrestrial megafauna and much is known about the dinosaurs that roamed the eastern side of Laramidia.  Less is known about those from the western provinces of this landmass, this is due to the relative paucity of fossil material found to date on America’s Pacific coast.  Hence the significance of the Sucia Island discovery.

Dinosaur and Prehistoric Bird Fossil Locations (Western Laramidia)

Picture credit: PLOS One

Three Tell-tale Signs

There are three tell-tale signs that indicate that this fossil fragment represents a potential tyrannosaur, let’s briefly go through these:

• The Size of the Fossil (UWBM 95770)

Although only a fragment of the top part of the femur (see picture below to gain an understanding of which part of the thigh bone has been preserved), the fossil measures a whopping 42.5 cm long and 22.4 cm wide at its widest part (the top end of the fossil, this represents that portion of the femoral head that articulates with the hip socket.  This fossil is from a very large animal, hence the confident dinosaur diagnosis.

The researchers writing in PLOS One undertook a comparative analysis study, comparing this fragment to the bones of theropod dinosaurs.  Based on this analysis, the projected actual size of this thigh bone would be about 1.17 metres long (1.167 metres +/- 63 mm with 95% degree of confidence in the result).

• Anatomical Features Indicate a theropod dinosaur

Although the bone is fragmentary, there is evidence of matrix infill in the medullary cavity (the middle of the bone, where marrow was present).  This indicates that this bone was hollow in the middle, this feature is only seen in theropod dinosaurs in this geological period. Thus, the hollow middle cavity is diagnostic of this bone having come from a theropod dinosaur.

The Fourth Trochanter

• The Clincher!  The Fourth Trochanter

An anatomical feature on the back of the thigh bone (posterior), called the fourth trochanter suggests a theropod dinosaur.  The fourth trochanter is a prominent bump or ridge of bone that indicates muscle attachment.  In this specimen, the fourth trochanter is located in the top third portion of the thigh bone, close to the hip socket.  Its shape and position on the bone suggests Theropoda.  All dinosaurs (all archosaurs for that matter), have this characteristic, but in other big, Late Cretaceous dinosaurs such as ceratopsians and hadrosaurs, the fourth trochanter is shaped differently and in a different position at the back of the femur.

In hadrosaurids for example, the fourth trochanter is located near the middle of the femur and is triangular in shape.  In horned dinosaurs (ceratopsians) the fourth trochanter is typically a long, low, straight ridge, which is also positioned towards the middle portion of the thigh bone.

An Illustration of the Fossil Fragment and Comparison with Left Femur of Daspletosaurus

Picture credit: Burke Museum of Natural History and Culture with additional annotation by Everything Dinosaur

 If you look carefully at the picture above, the bottom end of the fourth trochanter is wider and roughened (distal end).  The top portion (proximal) appears smooth and thinner.  A similar morphology is found on the fourth trochanter of the Late Cretaceous tyrannosaur known as Daspletosaurus (D. torosus).

Daspletosaurus spp.

This explains why the research team have tentatively assigned this fossil fragment to the tyrannosaur family and used Daspletosaurus fossil material as a direct comparison.

An Illustration of Daspletosaurus (D. torosus)

Picture credit: Everything Dinosaur

Commenting on the significance of the fossil discovery, Dr Sidor stated:

“This fossil won’t win a beauty contest.  But fortunately it preserves enough anatomy that we were able to compare it to other dinosaurs and be confident of its identification.”

The First Dinosaur from Washington State – A Tyrannosaur?

The partial thigh bone, now tentatively assigned to the same dinosaur clade as Tyrannosaurus rex is on display at Burke Museum.  It is worth noting that based on the thigh bone’s total estimated size, the individual meat-eater the fossil material represents would have been over eight metres in length.  The dinosaur would have been approximately the same size as a Daspletosaurus, a Gorgosaurus or an Albertosaurus (all tyrannosaurids).  Although the tyrannosaurid diagnosis cannot be confirmed with absolute certainty and a definition down to genus level is not possible, the tyrannosaurs dominate the apex predator positions within the Laramidia fossil sites.

Perhaps as many as nine genera have been identified to date, the Late Cretaceous seems to mark the peak in the Superfamily Tyrannosauroidea diversity.

Tyrannosaurs Present in Laramidia

Team members at Everything Dinosaur have tried to list the nine genera of tyrannosaurs identified thus far from strata that represent Laramidia.

1. Albertosaurus
2. Gorgosaurus
3. Tyrannosaurus
4. Daspletosaurus (to read an article about face biting in potential Daspletosaurs): Cannibal Daspletosaurus?
5. Lythronax (to read an article about this dinosaur discovery): The King of Gore.
6. Nanotyrannus (Nomen dubium) – to read more about this potential new genus of tyrannosaurid and the debate surrounding it: Duelling Dinosaurs.
7. Bistahieversor (for a review on the CollectA Bistahieversor model: CollectA Bistahieversor Reviewed.
8. Teratophoneus (refer to the Lythronax article for more information): “Monstrous Murderer” of a Tyrannosaur.
9. Nanuqsaurus – the polar tyrannosaurid: Nanuqsaurus hoglundi.

Dinosaur Provinciality

Palaeontologists have proposed that there was a great deal of dinosaur provinciality within the landmass of Laramidia.  The idea of different parts of the landmass being home to different types of dinosaur has been studied for the last two decades.

Unfortunately, due to the tectonic activity on the Pacific coast of western North America, it is difficult to determine where exactly in Laramidia this new tyrannosaur came from.  It has been suggested that since the end of the age of dinosaurs there has been around 1,800 miles of tectonic displacement in roughly a northerly direction.  This tyrannosaur could have lived as far south as what is now southern California.

CollectA have introduced a wide selection of tyrannosaur models into their not-to-scale range of replicas including a model of Daspletosaurus: CollectA Prehistoric Life Figures.