Once considered among the biggest and most impressive of all dinosaurs, most experts now agree that the house-sized Seismosaurus was probably an unusually large species of the much better-known Diplodocus. Not to further disillusion you, but there’s also a distinct possibility that Seismosaurus wasn’t quite as big as once believed. Some researchers now say this late Jurassic sauropod weighed as little as 25 tons and was considerably shorter than its stated length of 120 feet, though not everyone agrees with these drastically scaled-down estimates. By this accounting, Seismosaurus was a mere runt compared to the gigantic titanosaurs that lived millions of years later, such as Argentinosaurus and Bruhathkayosaurus.
Seismosaurus has an interesting taxonomic history. Its type fossil was discovered by a trio of hikers, in New Mexico in 1979, but it was only in 1985 that the paleontologist David Gillette embarked on a detailed study.
In 1991, Gillette published a paper announcing Seismosaurus halli, which in a burst of reckless enthusiasm he said may have measured over 170 feet long from head to tail. This certainly generated impressive newspaper headlines, but one imagines it didn’t do much for Gillette’s reputation, as his fellow scientists re-checked the evidence and calculated much more petite proportions (in the process, of course, stripping Seismosaurus of its genus status).
The (indisputably) extreme length of Seismosaurus’ neck–at 30 to 40 feet, it was much longer than the necks of most other sauropod genera, with the possible exception of the Asian Mamenchisaurus–raises an interesting question: could this dinosaur’s heart possibly have been strong enough to pump blood all the way to the top of its head? This may seem like an arcane question, but it bears on the controversy of whether or not plant-eating dinosaurs, like their meat-eating cousins, were equipped with warm-blooded metabolisms. In any case, it’s most likely that Seismosaurus held its neck roughly parallel to the ground, sweeping its head back and forth like the hose of a giant vacuum cleaner, rather than in the more taxing vertical position.