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The force of gravity – together with certain physiological and ecological constraints – holds in check the evolution of ever larger body-sizes among mammals on land. By becoming secondarily adapted to life in water, however, whales have been able to circumvent at least some of these size restrictions.
The largest extant land mammal – the African Elephant – is considerably outweighed by sea-going baleen whales of even middling proportions. In all of the Cenozoic era (the 65 million year period following the extinction of the dinosaurs), no terrestrial mammal ever grew to match the modern Gray Whale, let alone the Blue whale, in body dimensions. The reduced weight constraints of an aquatic medium accounts for this apparent difference in maximum attainable size.
Outside of the mammals, however, there is one group of extinct land creatures that did approach, and in some cases surpass, the awe-inspiring lengths of the largest baleen whales* pushing the evolutionary envelope in terms of height, length, weight and girth in a way that no other terrestrial animal group ever did.
Together with whales, the sauropods are examples of animal gigantism par excellence.
What are Sauropods?
The term “Sauropoda” refers to group of quadrupedal, megaherbivorous dinosaurs that existed for a span of over 135 million years – from the close of the Triassic period to the very end of the reign of the dinosaurs. Their highly distinctive body plan was characterized by:
1) An elongate neck. One that, in some genera, grew to double the length of the trunk.
2) A small skull relative to body size, with enlarged eye-orbits and highly placed nasal openings
3) A massive body with a long tail
4) Stout, columnar limbs positioned directly below the body. The bones of the hands/forefeet were arranged into a roughly tubular configuration (vertical with respect to the ground), with the phalanges (finger-bones) reduced. Only the first digit bore a claw – and this too was lost in some of the later groups. The structure of the hind foot was notably different from that of the fore foot – the phalanges were larger and three of the digits were typically claw bearing. The bones of the hindfoot were not arranged vertically with respect to the ground, as was the case with the hand bones, but appear to have assumed a “flatter” posture (semi-plantigrade). A cushioning “pad” of tissue seems to have been present at the base of the hindfoot. Reconstructions of sauropod hands and feet as either elephant-like, with nail-like hooves, or lizard-like, with clawed fingers splayed out every which way, are equally incorrect.
There was limited deviation from this general body plan over the rather lengthy course of sauropod evolution. Paleontologists have puzzled for decades over the ecological, biomechanical and physiological implications of sauropod size and anatomy. How big did they get? What sort of diet fueled those enormous bodies? How did the sauropod heart pump blood across those serpentine necks, all the way to the brain? This article shall consider some of these questions.
How big did Sauropods get?
I have seen books quantify the dimensions of sauropods in feet, meters, cars, double-decker buses, building stories, elephants and bulldozers. The longest of them (Diplodocus and Supersaurus) hit a length of about 33-35 meters (longer than a blue whale). Even conservative body mass estimates suggest that the heaviest sauropods (Argentinosaurus) weighed over 70 tonnes (10 times the weight of a male African elephant).
There were, of course, examples of much smaller sauropods – Eoparasaurus, for example, was only 6 meters long from snout to tail.
The immense size of sauropods would have served as a deterrent against predators (and there was no shortage of large, powerful predators in many of the times and places in which sauropods existed). The long neck would have given the animal a wide sphere of access to vegetation.
What did sauropods feed on? How did they process food?
Sauropod teeth – which ranged from pencil shaped to spatulate, depending on the species – were designed primarily for grabbing and tearing vegetation off shoots and branches (‘cropping’) rather than grinding down tough plant matter. There is nothing analogous to the chewing apparatus of modern mammalian herbivores built into the oral anatomy of sauropods – no large, flattened, squarish teeth positioned at the back of the jaw to pulverize ingested food items. The head was small and the dentition weak. We may infer that very limited mechanical breakdown of food took place in the oral cavity before it was swallowed.
It has been proposed that sauropods utilized large stones in the stomach (called gastroliths) to grind down food. This digestive adaptation is called a “gastic mill” and is observed in modern birds. But the small sizes of fossilized ‘gizzard stones’ relative to body dimensions as well as the possibility that they are simply a result of sedimentary processes, has led a number of researchers to dismiss the idea that this form of food reduction played major role in sauropod digestion. But, without a gastric mill or significant oral processing, how did sauropods physically reduce ingested plant matter into smaller, more digestible bits?
Perhaps such processing was not necessary. Like modern vertebrate herbivores, Sauropods almost certainly relied on a community of symbiotic microbes to break down (otherwise-indigestible) cellulose present in the cell walls of ingested plant material. This microbe-mediated process, involving the enzymatic breakdown of cellulose (and other carbohydrates) to short chain fatty acids that can be absorbed by the host, is called fermentation. The tremendous sizes of sauropods might have permitted the retention of food in the digestive tract for long periods of time. Prolonged food retention times and extensive exposure to microbial fermentation may have actually compensated for the limited mechanical reduction of food in the mouth and gut.
The lengthy necks of sauropods gave them an enormous foraging range. They fed primarily on gymnosperms (conifers), sphenophytes (eg. Horsetails) and pteridophytes (ferns). As flowering plants diversified rapidly during the mid-cretaceous, they too were no doubt incorporated into the sauropod diet.
Bird lungs and long necks
The vertebrae and ribs of sauropods have well-developed air-spaces, which have been interpreted as emanating from the respiratory system. These air spaces are similar in nature to those found in birds (their closest living relatives), suggesting that sauropods may have sported an avian-style respiratory system – with air-sacs distributed throughout the body. The presence of these air spaces lightened the enormous skeletons of these animals without compromising strength. In addition, the presence of air sacs may have permitted the evolution of one of the signature features of sauropods: an elongate neck. As one increases the length of the pathway of air-conduction from the nostrils to the lungs, the amount of so-called anatomical “dead space” increases. Dead space refers to inhaled air, located in the conducting areas of the respiratory system, which does not participate in gas exchange. The large dead space present in the incredibly long tracheas of sauropods (also seen in the much smaller necked giraffe) would have severely lowered breathing performance were it hooked up to a mammalian-type lung. Under an avian model of respiration, however, the additional air-storage capacity provided by the air sacs would allow the trachea to overcome this dead space and maintain respiratory efficiency.
The high rates of growth determined from histological analysis of sauropod bone tissues appear to indicate that, for at least part of their life span, sauropods had high basal metabolic rates far more comparable to large mammals than to modern ectothermic reptiles. This high BMR may have slowed down later in the life of the animal. Adult sauropods would have retained heat energy and maintained a relatively stable body temperature by mere virtue of their size (gigantothermy). Muscular activity, metabolic reactions and digestive processes, such as fermentation in the gut, can be assumed to have produced heat internally. The air sacs described earlier would have served as surfaces for heat exchange.
How did the sauropod heart pump blood to the head?
The vertical distance between the heart and the head of sauropods is dependent on neck posture. If large sauropods did hold their necks upright, the vertical heart-brain distance in many cases would be over 8 meters. Scientists infer that huge blood pressures (over 700 mm Hg) – unheard of among modern animals – would be necessary to supply the head with oxygen and nutrients. The enlarged, highly muscular heart that would be necessary to produce this astonishing hydrostatic pressure would be grossly energy inefficient, cause space problems and suffer from a number of mechanical disadvantages. And so, various cardiovascular adaptations have been hypothesized to exist in sauropods to get around the hypertension issue.
Some workers have suggested that the sauropod circulatory system featured multiple ‘hearts’ in series, each accessory heart capable of pumping blood to the next valved pump in the circulatory pathway (thereby allowing effective blood-flow from the primary heart to the brain). However, no such system has been observed to exist in modern vertebrates and it is unclear how the nervous co-ordination of this congo-line of secondary hearts would have operated. Perhaps sauropod blood had a higher viscosity and erythrocyte count, increasing its oxygen carrying capacity.
The neck posture of sauropods is still widely debated, but if the head were habitually positioned at low-to-medium heights – as appears to be the case in Diplodocus – then there is no need to invoke the presence of a grossly hypertrophied heart or outrageously high blood pressures. Browsing at high elevations for limited periods of time, though costly in terms of cardiac output – may have given sauropods access to critical food resources unavailable to other animals.
Could sauropods rear up?
Kinetic-dynamic modeling of the skeletons of sauropods indicates that at least some of them were capable of briefly rearing up on their hind legs and utilizing their tails as a “third leg” of sorts (a kind of tripodal stance) before dropping back down to a quadrapedal stance. This would have allowed for browsing at great heights. A rearing diplodocus would have been a sight to behold indeed.
One of my favorite television depictions of sauropods was in a BBC production called The Ballad of Big Al. This clip involves a pack of Allosaurus’ launching a concerted attack on a Diplodocus herd. Enjoy!
* These same whales do still have the sauropods safely beat in terms of sheer tonnage.
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