A few months ago, a team of paleontologists was privileged to be shown a life-size cast of the sacral vertebrae and hip structure of a large North American Theropod Acrocanthosaurus. It is a cast of the fossil specimen known as NCSM 14345 from the Black Hills Institute. The cast is from a paratype (a specimen not related to the holotype – the original material from which the animal was named and described). They were all rather blown away by the sheer size and scale of the animal. It is only when you get up close to cases like these and the real fossils themselves that you get an appreciation of just how sizeable some of these creatures were. There are only one species of Acrocanthosaurus recognized at present (A. tokens), and it was certainly an apex predator, reaching lengths in excess of 12 meters and weighing more than 4 tons.
Rare Dinosaur Fossil Material
Complete articulated skeletons of dinosaurs are extremely rare, associated bones are like hen’s teeth but if a paleontologist is lucky enough to find an almost intact fossil skeleton of an animal like Acrocanthosaurus, the fossil bones only tell half the story. A couple of new specimens of Acrocanthosaurus were discovered in the 1990s, more complete than the original holotype specimen excavated forty years earlier. Even so, fossils of large Theropods are very special as when compared to the amount of Hadrosaurine or even Sauropod material in the fossil record they are amongst the rarest dinosaur fossils of all.
Learning About Long Extinct Prehistoric Creatures
The fossilized bones of a complete skeleton can tell scientists a lot about the animal, but they do not make up the complete picture. With very little soft tissue such as skin, muscle, tendons, and organs preserved scientists remain very much in the dark over key aspects of Dinosauria anatomy. When visiting classrooms to discuss what paleontologists actually know about the Dinosaur, it is useful to explain using the analogy of a snooker table. Imagine you came across a snooker table, never having seen one before. You would see a large flat table, covered in green baize with six pockets spaced around it. It would be difficult to work out what the table was used for unless you found the balls, snooker cues, spiders, triangle and all the other elements associated with the game as well. Without the soft tissues, scientists have to make educated guesses, drawing conclusions on muscle size and fixation by studying the scars on bones that indicate muscle attachments.
Studying the Anatomy of the Dinosauria
The fossil bones may indicate where muscles were attached but they do not reveal information about the relative sizes, their length, thickness or composition. Left to making educated guesses paleontologists can make widely varying assessments regarding dinosaur locomotion, gait, physical size, and velocity.
For instance, if the muscles connected to the femur of a Tyrannosaurid were short, this would suggest that the femur would have been angled more vertically in relation to the hip bones, as in human beings. However, if the muscles were longer, then the thigh bone would have resembled the more horizontal position as seen in Aves (birds).
Although the femur in humans and birds are basically the same shape the orientation with the hip girdle and related muscles can drastically change stance. Humans are upright, whilst Aves have a semi-upright standing position.
Powerful Computers Assist Palaeontologists
A relatively new field of palaeontological study, the use of powerful computers to model concepts and create 3-D images is revolutionizing the way in which soft tissue structures are visualized. Such work is being pioneered by a team of researchers based at Manchester University. Researchers such as Bill Sellers and paleontologist Phil Manning create virtual muscles on scanned images of dinosaur bones to calculate how muscles worked and the anatomy of these long dead creatures.
The team from Manchester University create computer algorithms that carry out experiments to establish the most efficient method of locomotion for prehistoric animals. At first, the programs cause the specimen being modeled to fall over but gradually the computer programs learn from their mistakes, correct them and come up with the most likely solution.
Explaining how the process works, computer paleontologist Peter Falkingham of Manchester University stated that the computer program learns to walk through trial and error deciphering the data until it finds the most stable and appropriate form of locomotion.
However, the scientists employ “genetic algorithms,” or computer programs that can alter themselves and evolve, and so run pattern after pattern until they get consistent improvements.
Eventually, they evolve a pattern of muscle activation with a stable gait and the dinosaur can walk, run, chase or graze, Falkingham added.
Assuming the Manchester team’s computer software is mimicking the process of natural selection, then the computer generated animal should move similar to its now extinct counterpart. By comparing their cyberspace results with real measurements of extant species such as humans and emus the Manchester team can be confident in the results computed for extinct prehistoric animals such as dinosaurs.
Now back to Acrocanthosaurus, this giant meat-eating dinosaur from the mid-Cretaceous (Aptian to Albian faunal stages). Acrocanthosaurus was named after the tall neural spines that ran along the backbone, the function of these spines, some of which measure nearly three times the height of the vertebrae from which they project, is not known. Scientists have speculated that the spines are similar to those found in modern bison, these spines are used to support a hump that stores fat. Perhaps this large meat-eating dinosaur had a hump which allowed it to store fat and water reserves to help it survive times when food was scarce.
Bizarre Neural Spines
The tall, spatula-shaped neural spines can be clearly seen in pictures of this dinosaur’s skeleton, running from the cervical vertebrae along the spine. Over the sacral vertebrae, there is extensive crisscrossing of tendons and other structures.
Peter Falkingham specializes in using the computer algorithms to interpret fossilized trackways, unlike fossil bones that may be transported a long way from where the animal lived before being deposited, footprints and trackways are preserved “in situ”.
He went on to comment that footprints can tell so much about a dinosaur that the body fossils (skeleton) cannot. They can tell you how the animal moved about, how it walked or ran.
Using Computer Models to Explore Dinosaur Locomotion
Recreating fossil trackways as manual models and experimenting on them to calculate how the animal actually walked would be very time to consume and accurate, consistent results would be difficult to achieve. However, by using the computer software a number of different scenarios can be tested and when this data is combined with aspects of anatomy, the locomotion and movement of a large dinosaur like Acrocanthosaurus can be better understood. The Manchester University team have used the computer algorithms to study the way in which Acrocanthosaurus walked (fortunately there are some extensive trackways in the United States attributed to Acrocanthosaurus to assist in this research).
In this way, the team hopes to shed some light as to the peculiar structure and purpose of the tall neural spines and structure of the bones making up the sacral vertebrae. Based on this work the team has “fleshed out” this dinosaur creating a muscle map of this large, stocky meat-eater.
The neural spines seem to be broader immediately behind the sacrum, could they have supported thicker muscles that may have helped counter balance the creature as it walked? Could the structures have acted as shock absorbers to steady the animal as it ran, or could they have stored some energy in the large tendons associated with this part of the skeleton and reduced energy expenditure as the animal moved in a similar way to the long tendons found in kangaroos. Tendons as energy stores may not have enabled this animal to “bounce” along like a kangaroo but by tensing and relaxing they may have helped this particular dinosaur maintain momentum and expend less energy as it moved about.
The feet of this dinosaur are also worthy of note, they look surprisingly small when compared to the size of the animal, perhaps the peculiar lacerations and scars found on the sacral vertebrae and neural spines of this dinosaur provide a clue to this phenomenon also.
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