Please note that you are viewing the old Science in Africa website. Please see www.scienceinafrica.com for our new site!

Science in AfricaLogo Merck: Distributors of fine chemicals and apparatus. Enter here for more information.
March 2004

Feature

 


Ancient giants unearthed in South Africa


Dr Adam Yates


The archetypal dinosaur image that most of us hold is very much that of a sauropod - the classic, tiny-headed, long-necked, elephantine-limbed giant. Despite this there are vast gaps in our knowledge about the sauropod group. Fortunately a series of discoveries in South Africa is starting to fill in some vital missing pieces.


Antetonitrus ingenipes A bed of bones belonging to a type of sauropod called Vulcanodon, was recently identified in South Africa and an excavation is currently underway. Just as tantalising was the discovery of a truly ancient sauropod in the famous Karoo Basin of South Africa, near the town of Ladybrand. Named Antetonitrus in 2003, this sauropod is 215 million years old (Late Triassic Period), which is close to the beginning of the history of this group.

What is already known about sauropods? The group includes the largest of all dinosaurs, indeed the largest of all terrestrial animals ever to have existed. The very biggest may have reached 40 metres in length and weighed close to 100 metric tons.

Sauropods were ecologically significant and dominated the large terrestrial herbivore niches in most parts of the world from 170 million years ago (Middle Jurassic) to the end of the dinosaur age - 65 million years ago (Late Cretaceous).

The closest relatives of the sauropods, a group of primitive dinosaurs known as prosauropods, extends back to the Late Triassic (about 220 million years ago) implying that the sauropods may also have had a history extending back as far. Thanks largely to a series of spectacular discoveries in South America palaeontologists are now confident that the common ancestor of all dinosaurs was a small to medium sized (no more than 2 metres long and weighing less than 50 kg), carnivorous biped.

Sauropods were very much transformed from this common ancestor. They were strictly herbivorous and had wide semicircular, or rectangular cropping jaws packed with spoon-shaped teeth for taking big mouthfuls of fodder. Unlike the teeth of most reptiles, including those of the ancestral dinosaurs, the teeth of sauropods contact the teeth in the opposite jaw in precise occlusion. Sauropod teeth have prominent wear-facets indicating that they chewed their food to a substantial degree. Other features of the sauropod skull that are unique include their greatly enlarged nostrils, which are retracted away from the snout tip so that they occupy a place high on the skull roof. The biological significance of this change is not well understood.

The sauropod neck is extremely elongated and includes at least two more vertebrae than its ancestors, with a total of at least 12. One genus of sauropod has a staggering 19 vertebrae in the neck, the largest number found in any vertebrate. Clearly sauropods had a long reach and thus had a very large 'feeding envelope' that they could exploit without having to walk a step.

The bipedal nature of the ancestral dinosaur indicates that sauropods were only secondarily quadrupedal animals, which dropped back onto all fours early in their evolution. This meant that their grasping hand had to be modified for walking upon. This required two significant changes. Firstly the hand had to be rotated so that it swung in a fore-aft plane (we call such hands 'pronated') as opposed to the transverse plane of bipedal dinosaurs. To achieve this, the bones of the forelimb (radius and ulna) twisted their positions relative to one another, and their shape at the wrist joint is such that they were locked into position, preventing rotation of the hand. Secondly the hand had to be modified to bear the huge weight of a sauropod body. To achieve this the palm of the hand was modified into a semi-tubular colonnade of equal-length bones. This colonnade bore all of the weight, consequently the fingers were not needed and faded away.

Other limb modifications include straightened column-like posture, reduced muscle attachments and broad, spreading hind feet. All these indicate that sauropods held their limbs straight under their body and moved slowly, never taking big steps and never taking more than two feet off the ground at any one time.

Clearly sauropods present us with many questions on how such prodigious animals functioned as living organisms and how such dramatic evolutionary changes occurred. Unfortunately the earliest sauropod fossils are substantially incomplete, telling us little about the early members of the group. The most informative of these early sauropod fossils is Vulcanodon, from the shores of Lake Kariba in Zimbabwe. This fossil is 180 million years old, which places it in the later part of the Early Jurassic.

The most informative parts of Vulcanodon are its limbs. (The skull, neck, trunk vertebrae and hands are all missing.) Although the limbs are substantially more primitive than later sauropods, they are already highly specialised and show that the animal was a large, fully quadrupedal animal with columnar limbs and a short foot, indicating that it had adopted the slow, stiff-legged gait of later sauropods. The radius and ulna of the forelimb are twisted indicating that the hand was pronated, but without the hand bones we cannot tell if it was modified into the tubular, fingerless structure of later sauropods.

The Karoo Basin's Antetonitrus skeleton (mentioned in the first paragraph) is by no means complete, but there are bones from all major regions of the skeleton. These show a small, heavily-built sauropod (perhaps 10 metres long and weighing 1.5 tons) caught in the evolutionary transition from a prosauropod-like body to a typical sauropod. Most intriguingly the evidence shows that although Antetonitrus had evolved full quadrupedalism (for instance its forelimb was nearly as long as its hind limb), modification of the hand lagged behind and it remained a primitive grasping organ. Unfortunately none of the skull and very little of the neck was found so we cannot yet know what changes had occurred in these regions.

Nevertheless the Karoo is far from exhausted and preliminary investigations show that there are many significant fossils still to be excavated, including more bones at the Antetonitrus site. Analysis of some of the Karoo's previously named dinosaurs, for example Melanorosaurus, suggests that they too are primitive sauropods, rather than prosauropods as previously identified. The Karoo may be viewed as the 'cradle of sauropods' and we can look forward to many questions about the origin of these dinosaurian titans being answered by palaeontologists in South Africa.


More information:

Dr Adam Yates Email: YatesA@geosciences.wits.ac.za 

Dr Yates will be delivering a lecture at the Sasol Scifest 2004: "South Africa and the origins of the dinosaur titans". More information on the Science festival and Dr Yates talk at: www.scifest.org.za 

 

Science in Africa - Africa's First On-Line Science Magazine

Return to Home PageReturn to the TopYour FeedbackRegister with "Science in Africa" 

Copyright  2002, Science in Africa, Science magazine for Africa CC. All Rights Reserved

Terms and Conditions