Sauropod dinosaurs - the largest land animals of all time - weighed up to 80-100 tons and reached 40-50 meters in length. Summarizing all the available data, paleontologists named five main factors that made possible the development of gigantism in sauropods: refusal to chew food, rapid reproduction by laying many small eggs, rapid growth, a perfect respiratory system, and a decrease in metabolic rate in adult animals compared to young ones
Science magazine published an article by paleontologists Martin Sander of the University of Bonn and Marcus Clauss of the University of Zurich, in which the authors summarize the results of years of study of giant sauropod dinosaurs. In terms of their mass, these animals were an order of magnitude larger than the largest land mammals, as well as representatives of other groups of dinosaurs (theropods and ornithischids).
Sauropods appeared at the end of the Triassic period (about 210 million years ago). To date, paleontologists have described about 120 genera of sauropods. These herbivorous giants dominated many terrestrial ecosystems from the mid Jurassic to the end of the Cretaceous, that is, for about 100 million years - twice as long as the heyday of large herbivorous mammals. Thus, the sauropods were a very successful and thriving group, not an "evolutionary mistake" at all.
Quite naturally, scientists, as well as the general public, are concerned with the question: why did sauropods become so huge?
It is not possible to explain the gigantism of sauropod by any external reasons, although such attempts have been made repeatedly. For example, they tried to find a correlation between the evolutionary dynamics of the size of sauropods and such factors as the concentration of oxygen and carbon dioxide in the atmosphere, changes in climate, sea level and land area - and in all cases it turned out that there were no significant correlations. From this, the authors conclude that the key to the mystery of the gigantism of sauropods must be sought in their biology.
Diagram illustrating the relationship between primitive and advanced characteristics in the largest herbivorous animals: mammals, sauropods, ornithisch dinosaurs, and modern reptiles. From left to right: mammals (African elephant, giraffe, fossil rhinoceros indricotherium), sauropods (Argentinosaurus, Brachiosaurus), ornithischian dinosaurs (Shantungosaurus, Triceratops), giant Galapagos tortoise. Figure: from the discussed article in Science
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The structure of the teeth, mouth, and neck of sauropods is quite diverse, which indicates that they did not have a once and for all established diet (although they all undoubtedly ate only plant foods). However, all sauropods have one primitive feature that sharply distinguishes them from other large phytophages - mammals and ornithischian dinosaurs. Sauropods never chewed food: their teeth and jaws were small and completely unsuitable for chewing. They also did not have the habit of swallowing stones in order with their help to grind the swallowed vegetation in the stomach, as some other dinosaurs and birds did. Apparently, the absence of a chewing apparatus was compensated by the fact that the digestive tract of sauropods, due to their monstrous size, was very long, so that even unchewed food had time to digest in it.
Avoiding chewing food allowed the sauropod head to remain small, which in turn made it possible to develop a very long neck, thanks to which sauropods could effortlessly reach food sources inaccessible to other animals. It is assumed that the long neck could also be used to attract sexual partners; perhaps the male sauropods fought with their necks like modern giraffes. In this case, sexual selection should have contributed to the lengthening of the neck.
In contrast, most large herbivorous mammals and ornithischian dinosaurs have developed very powerful teeth, jaws, and chewing muscles, resulting in a dramatic increase in head size. This imposed significant restrictions on the acceptable neck length.
Large body sizes give rise to a number of physiological problems, the most important of which is the problem of overheating. In addition, the presence of a long neck makes it difficult for fresh air to enter the lungs (you need to inhale very deeply so that not only the air that remained in the long windpipe after the previous exhalation gets into the lungs). The sauropods seem to have solved both of these problems through the development of a very sophisticated and sophisticated system of air sacs.
Air sacs, most likely, appeared in the common ancestors of all dinosaurs lizard-like, that is, sauropods and carnivorous theropods, and from the latter they were inherited by birds. In birds, thanks to air sacs connected to the lungs by a complex system of pipes and valves, fresh air is forced through the lungs both on inhalation and exhalation. In addition to intensifying breathing, air sacs provide effective cooling of the body. The fact that lizard-like dinosaurs also had air sacs is evidenced by the results of studying their bones (many air sacs penetrate into the bones or leave characteristic imprints on them). For the latest information on air sacs in dinosaurs, see a recently published article: Sereno PC, Martinez RN, Wilson JA, Varricchio DJ, Alcober OA, et al.(2008) Evidence for Avian Intrathoracic Air Sacs in a New Predatory Dinosaur from Argentina // PLoS ONE 3 (9): e3303.
For the gigantic size to be of real benefit to sauropods and to be supported by selection, sauropods had to grow as fast as possible. If the animal has already relied on gigantism - in order to protect itself from predators or to access the crowns of tall trees - then it is not profitable for it to be small, and the period of childhood must be passed as soon as possible. Sauropods, meanwhile, had to increase their mass by 100,000 times to reach their maximum size - a record figure for reptiles, not to mention birds and mammals (a hatched calf weighed only a few kilograms, and an adult, hardened dinosaur - many tens of tons). The histological analysis of the bones of sauropods generally confirms the assumption of rapid growth, although these data are not entirely unambiguous. Apparently, sauropods reached sexual maturity in the second,and the maximum size - in the third decade.
Such a rapid growth is possible only under the condition of a very intensive metabolism - the same as in modern birds and mammals. However, if the metabolic rate in adult sauropods remained as high, they would inevitably overheat, and no amount of air sacs could save them. In addition, they would need a completely unimaginable amount of food. The authors see only one way out of this contradiction: apparently, the metabolic rate in sauropods was high in youth, and significantly decreased with age. Unfortunately, paleontologists do not yet have direct data on the metabolic rate of sauropods.
All dinosaurs are known to be oviparous. Their cubs were born small and unprotected, but there were many of them. In contrast, large herbivorous mammals give birth to large and well-protected young, but in very small numbers. Because of this, the development of gigantism in mammals sharply increases the likelihood of extinction of the species: the larger the animal, the less it produces cubs, and the longer it takes to restore the population after its temporary decline. Sauropods were not bound by this limitation: gigantism in them did not lead to a decrease in fertility, and the population size could grow very quickly in favorable conditions.
Thus, according to the authors, gigantism in sauropods became possible due to a special combination of primitive and advanced characteristics. The primitive traits that contributed to gigantism are inability to chew food and laying eggs; advanced - fast growth, perfect respiratory system and metabolic rate changing with age.