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 over 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. Still, if a paleontologist is lucky enough to find an almost intact fossil skeleton of an animal 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 exceptional compared to the amount of Hadrosaurine or even Sauropod material in the fossil record. They are amongst the rarest dinosaur fossils of all.
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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 a snooker table’s analogy. 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 not be easy 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. Scientists have to make educated guesses without the soft tissues, concluding 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, length, thickness, or composition. Left to make 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 about the hip bones, as in human beings. However, if the muscles were longer, then the thigh bone would have resembled the more horizontal position 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 palaeontological study, using powerful computers to model concepts and create 3-D images, is revolutionizing how 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 Manchester University team creates computer algorithms that carry out experiments to establish the most efficient locomotion method for prehistoric animals. At first, the programs cause the specimen to be modeled to fall over, but the computer programs gradually 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 locomotion form.
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 muscle activation pattern 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.
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 vertebrae’s height 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 that allowed it to store fat and water reserves to survive 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 an extensive crisscrossing of tendons and other structures.
Peter Falkingham specializes in using 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 commented that footprints could 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, several different scenarios can be tested. When this data is combined with anatomy, the locomotion and movement of a large dinosaur-like Acrocanthosaurus can be better understood. The Manchester University team has used computer algorithms to study how 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 on the peculiar structure and purpose of the tall neural spines and the bones’ structure making up the sacral vertebrae. The team has “fleshed out” this dinosaur creating a muscle map of this large, stocky meat-eater based on this work.
The neural spines seem to be broader immediately behind the sacrum; could they have supported thicker muscles that may have helped counterbalance 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 the long tendons found in kangaroos? Tendons as energy stores may not have enabled this animal to “bounce” along with like a kangaroo. Still, by tensing and relaxing, they may have helped this particular dinosaur maintain momentum and expend less energy as it moved about.
This dinosaur’s feet are also worthy of note; they look surprisingly small compared to the animal’s size; perhaps the peculiar lacerations and scars found on the sacral vertebrae and neural spines of the dinosaur provide a clue to this phenomenon also.
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