Stan the Tyrannosaurus rex

Thursday, February 9, 2017

Stan the Tyrannosaurus rex

The skeleton of Stan the Tyrannosaurus rex (T. rex) was excavated by the Black Hills Institute in 1992, after its discovery by an amateur palaeontologist, Stan Sacrison. The fossil remains were found in an outcrop of Badlands (rough terrain) near Buffalo (South Dakota) in the USA. The excavation required the removal of the rock above the specimen, called overburden, with the help of a Bobcat (a small earth moving machine), a jackhammer, picks and shovels. Knives, scribes and brushes were used to expose the fossil bones and teeth, while glues and other chemicals were applied to stabilise and preserve the surface of the fossils. Before removal of the fossils, each one was drawn on a ‘map’ using a grid set over each section of the site. The fossils were then covered with aluminium foil and encased in a field jacket, layered burlap (sack-cloth) and plaster, so they could be moved without breakage. All excavated material was then transported to the lab for specialised cleaning, called preparation.

“Sue”, AMNH 5027, “Stan”, and “Jane”, to scale with a human. By Matt Martyniuk

Abundant well-preserved leaves and a few palm fronds found with Stan have given us new information about the environment and climate in which he lived. Other evidence from the site shows that Stan’s body probably remained exposed for a few months before a flood scattered a variety of bones and buried his skeleton, entombing it for 65 million years. The excavation was carefully documented with photographs, video and scientific notations for future study of the specimen and public education. Since the initial excavation, the Black Hills Institute teams have returned to the site twice to search further into the hillside for bones. The continuing excavation has yielded several more bones and additional material. To date 199 of Stan’s bones have been found – 70% of a complete skeleton – making Stan the second most complete T.rex found so far.

Stan ‘n Sue comparison by ScottHartman on DeviantArt

When Stan was collected in north-western South Dakota in 1992, it was clear that Stan Sacrison had made a significant discovery. It was only after the bones were prepared, however, that it became clear that this specimen included the best skull of all the Tyrannosaurus rex specimens ever collected. Not only was Stan’s skull virtually complete, but the bones were preserved in pristine condition. Perhaps even more importantly, the many bones of the skull were found mostly disarticulated (separated), allowing for a more detailed study than any other T.rex skull.

Stan’s teeth

The teeth of Tyrannosaurus rex were specialised: they had tiny serrations to help them cut through flesh and are round in cross-section for breaking through bone. The crown of the tooth was covered with hard enamel. The long dentine roots were dissolved and re-used by new teeth as they grew. Eventually, the old tooth, worn from use and weakened at the root, would break-off while feeding (called a shed tooth), making room for the new germ tooth to grow into position. Unlike mammals, dinosaurs grew new teeth throughout their lifetimes.

Stan Tyrannosaurus in Manchester Museum

Many of Stan’s teeth were found loose in the rock around the bones. Because each tooth ‘socket’ produces a different shaped tooth, it was possible to put those teeth back into the appropriate sockets of the 58 available positions. Wear surfaces on the teeth show that the lower jaws closed inside the upper, like giant shears, enabling T. rex to bite through even the leg bone of another T. rex.


Pathologies are evidence of injury or disease. Stan’s fossil skeleton exhibits various pathologies. Several healed puncture wounds in his skull and on a rib suggest that T.rex fought each other as he had certainly not bitten himself! One terrible injury gave Stan a very stiff and painful neck and, probably, horrible headaches. Two of his neck vertebrae were fused together and a great deal of extra bone had grown around the injury. In other words, Stan survived!

Stan’s skull preserves a number of pathologies, or healed injuries. The cheeks (jugals) and lower jaws reveal the presence of irregular openings that are not duplicated on the opposite side. One possible explanation for these holes is that they were puncture wounds received in battles with another T. rex. Smoothed edges around the holes indicate that the wounds healed in time. Perhaps the most interesting pathology is found at the back of the skull on the bones that make up part of the braincase. A portion of the top of the crest, where the powerful neck muscles once attached is missing. Below this we find a round hole that is a perfect match for a tooth from the lower jaw of another T.rex. Despite very serious injuries, Stan healed and lived to fight another day.

Cranial kinesis

Cranial kinesis is the technical term for movement of the joints between the bones in the skull. It is cranial kinesis that allows a snake to swallow things larger than its head, and keeps a woodpecker from addling its brain. Careful examination of Stan’s disarticulated skull shows that T.rex had a kinetic skull. There are hinge, screw, and slip joints that allowed Stan to open wide both the upper and lower jaws, greatly increasing bite area. These joints also acted as shock absorbers, protecting both the bones of the skull and Stan’s teeth from breakage when biting through the bones of prey.

Reconstructing dinosaurs

Of course, no-one has ever seen a living dinosaur! What we know of them comes from careful study before, during and after cleaning and conservation of their fossil remains. Skilled workers known as preparators spent 25,000 hours using small air-powered chisels and air-abrades (miniature sand-blasters) to take away the matrix that had encased Stan’s fragile bones since their burial. Interesting clues about Stan’s life emerged during this process. After moulding the bones using special silicone rubber to ensure that even the smallest detail would be preserved, a polymer was poured into the moulds to create an accurate copy of Stan’s skeleton, which was then mounted on an internal steel framework known as an armature.

Stan’s cast in a dynamic hunting pose at dinopolis, Teruel’s museum of paleontology (Spain).

Are birds living dinosaurs?

Scientists who study meat-eating dinosaurs, known as theropods, have noted their similarity to living birds. Stan’s feet, for instance, are very bird-like in appearance. Recent discoveries of wishbones, called furculas, in theropods (including T.rex), and feather-like structures preserved with theropod skeletons from China, provide further evidence. In addition, the bones in Stan’s neck and body are also thin-walled and ‘airy’ with an inside ‘honeycomb’ structure. These same features are found today only in birds, and double as part of their respiratory system (consisting of lungs and air-sacs). Birds breathe in a very different way from other animals. They have a network of air-sacs which occupy the bones and one-way lungs with two openings. Birds’ lungs are constantly bathed with fresh oxygen, giving them energy for a very active life. These similarities lead many scientists to conclude that birds are living theropod dinosaurs.

Male or female?

Over thirty incomplete skeletons of T.rex have been discovered so far. They can be divided into two body shapes, called morphotypes. One type has much larger and heavier bones and is the robust morphotype. The other has smaller and lighter bones and is the gracile morphotype. A wider pelvis with a larger passageway for eggs, a difference in the first chevron, and other clues in the robust morphotype lead scientists to believe that these skeletons were female. Stan’s skeleton is gracile, so we believe he was a male.

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