Giant dinosaurs may have migrated

Evidence suggests sauropods sought greener pastures during summer

Recalling the trek made by the cartoon Apatosaurus Littlefoot in The Land Before Time, real sauropod dinosaurs in prehistoric western North America may have fled the summer drought conditions of lowland river floodplains for the lush vegetation of upland settings. Such migrations, if they occurred, might explain how long-necked sauropods reached their immense size, researchers from Colorado College suggest online October 26 in Nature.

By comparing the levels of a chemical tracer found in the enamel of sauropod teeth (shown above) and in sediment near the dinosaurs’ primary feeding grounds, geochemists now believe the mega-herbivores undertook seasonal migrations in search of other feeding sites. Henry Fricke

With fearsome Jurassic predators like Allosaurus about, the bigger sauropods grew, the safer they were.  “Once sauropods reached their full size, they were effectively immune to predation,” says study leader Colorado College geochemist Henry Fricke.  An allosaur attack would have been as harmless as “a bunch of hyenas trying to attack an elephant.”

Some paleontologists believe that sauropods grew so large because they had difficulty chewing and therefore needed huge stomachs to digest food. As the animals’ stomachs evolved to bigger sizes, so did the rest of them, the theory goes. While Fricke doesn’t discount this theory, he believes that seasonal sojourns to areas rich in vegetation also played a part in the evolution of gigantism in sauropods.

Fricke and his colleagues pursued his seasonal migration theory by studying chemical variations in the teeth of the chewing-challenged sauropod Camarasaurus. “When animals drink water, the oxygen in that water gets incorporated into the blood stream and eventually into tooth enamel,” Fricke explains.  That water takes on distinct chemical signatures based on where in the environment the dinosaur lapped it up. For example, water from a mountain tarn and water from a lowland swamp will have different amounts of a particular form, or isotope, of oxygen that has two extra neutrons in its nucleus.

Fricke and colleagues measured oxygen isotopes extracted from the tooth enamel of eight Camarasaurus fossil remains from the western United States, and then compared the enamel isotope levels to those of minerals found in nearby sediments. Because the levels differed between the enamel and sediments, Fricke believes that sauropods must have been leaving the basin itself and going to the adjacent highlands to eat and drink.

Though Fricke’s study suggests the sauropods were supping elsewhere, he and other paleontologists don’t rule out other possible reasons for the huge herbivores to take a hike. “Food may not have been the sole reason the sauropods moved,” says George Engelmann of the University of Nebraska at Omaha, who was not involved in the study. “But the isotopic variation suggests, at least, that they were moving around.”

Fricke plans to test the tooth enamel of other nonsauropod dinosaur species, such as Stegosaurus, so that he can provide a more complete picture of sauropod feeding behavior. If the oxygen isotope levels of smaller herbivores like Stegosaurus indicate that they remained in the lowlands year-round to feed, then Fricke will have more evidence that a higher nutrient demand was the central factor driving the larger sauropods to migrate.     

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