In 1883, O. C. Marsh provided the first, classic appraisal of Apatosaurus, which at that time was known more widely as Brontosaurus:
A careful estimate of the size of Brontosaurus ... shows that when living the animals must have weighed more than twenty tons. The very small head and brain, and slender neural cord, indicate a stupid, slow-moving reptile. The beast was wholly without offensive or defensive weapons, or dermal armature. In habits, Brontosaurus was more or less amphibious, and its food was probably aquatic plants or other succulent vegetation.
Beginning with this description, the perceptual legacy for sauropods was of large, stupid, slow, defenseless animals that frequented water bodies and ate soft foods. However, knowledge gained since 1883 about sauropods and their sauropodo-morph relatives, the prosauropods, has resulted in radical revisions to most of the concepts in Marsh's original assessment of Brontosaurus. Nonetheless, sauropodo-morphs are still not regarded as the most intelligent of vertebrates, especially when their brain size is compared to their overall body size. Regardless, they were certainly intelligent enough for evolutionary success in their respective environments for about 140 million years.
Marsh is still correct as far as size is concerned: some sauropodomorphs evolved into the largest animals that ever dwelled on land, and some may have weighed more than 50 tonnes. Because all sauropodomorphs seemingly were vegetarians, they would have included the largest terrestrial herbivores that ever lived. Their unparalleled gigantism presents an interesting puzzle for evolutionary scientists. What sorts of genetic and environmental factors caused selection for this body size in some lineages, beginning in the Late Triassic and continuing through the Late Cretaceous? Additionally, the impact of these huge herbivores on Mesozoic plants and their ecosystems must have been considerable. What types of plants could they have eaten that would have grown back quickly enough to sustain subsequent generations? Lastly, their mere movement over the land would have caused noticeable changes to the habitat. The closest modern model to the ecological and physical impact of sauropods is a herd of elephants, but this analogy seems to pale in comparison to the vision of herds of 50-tonne herbivores walking together across a Mesozoic landscape and dining on its flora.
Movement is another revised and refined concept in the study of sauropodo-morphs. This applies not just to locomotion but also to other aspects of their anatomy, specifically the neck and tail. The relatively tiny heads of some large sauropods make complete sense as adaptations to their extremely long necks (one species had 19 cervical vertebrae!), but why their necks were so long is a valid evolutionary mystery to ponder. Likewise, the tails of some were as long as their necks, which collectively made some of them the lengthiest animals. So why did they also have such an extreme number of caudal vertebrae? Recent and not-so-recent research on sauropodomorph necks and tails resulted in some hypotheses that partially answer these questions. These hypotheses then encourage continuing debate on the role sauropodomorphs played in Mesozoic ecosystems and how they interacted with plants, their predators, and one another.
Sauropodomorphs thus represent evolutionary experiments on a grand scale through both time and space, because of their unparalleled sizes coupled with the longevity of their lineages. Adaptations in their skeletal architecture were astonishing, resulting in a balance of lightening and strengthening of a support system for massive muscles and organs. The fact that they not only moved easily with these adaptive features but also mated, made nests, laid eggs, browsed for food, and possibly migrated long distances is borne out of their widespread record of both bones and trace fossils. Paleontologists today examine these fossils to better understand their subsequent, lasting effect on the Earth.
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