Apatosaurus

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha > Plateosauria > Massopoda > Sauropodiformes > Sauropoda > Gravisauria > Eusauropoda > Neosauropoda > Diplodocoidea > Diplodocidae > Apatosaurinae

Overview: Apatosaurus is but one of many large sauropod dinosaurs known from North America’s famous Morrison Formation. Other notable Morrison sauropods include genera like Camarasaurus, Brachiosaurus, Diplodocus, and Barosaurus. Apatosaurus was closely related to the latter two taxa, as a fellow member of the family Diplodocidae. Diplodocids often grew to remarkable lengths, but tended to be surprisingly lightly built. This was true for Apatosaurus to some extent, but it was much bulkier than either Barosaurus or Diplodocus, with a notably thicker neck, stockier limbs, and rather robust vertebrae. These traits appear to have been common to diplodocids within the subfamily to which Apatosaurus belonged – the Apatosaurinae. Diplodocus and Barosaurus, on the other hand, were in the subfamily Diplodocinae, which contained more slender diplodocids. Brontosaurus was another good example of an apatosaurine, also having a bulkier build, though it was on average a bit smaller than Apatosaurus itself. Apatosaurines and diplodocines often coexisted. Differences in their teeth morphology possibly suggest the apatosaurines ate tougher vegetation.

On top of being fellow apatosaurines, Apatosaurus and Brontosaurus were for a long time seen as the same genus. Apatosaurus was first described in 1877 by the renowned Othniel C. Marsh, based on fossils unearthed in Colorado. Its generic name, meaning “deceptive lizard”, is a reference to its tail vertebrae, which Marsh noted were easy to confuse with those of mosasaurs. Two years later, Marsh would describe Brontosaurus as its own genus, but a study published in 1903 would find the two were too similar to be distinct on a generic level. Apatosaurus was named first, so if the two were the same genus, its name would have priority, so the species referred to Brontosaurus were reassigned to Apatosaurus. The name Brontosaurus would fall out of use among scientists, but the name would enter the public consciousness thanks to some mounted skeletons labeled as such. One popular misconception is that Brontosaurus was renamed to Apatosaurus due to the wrong skull being assigned to said skeletons, namely the skull of Camarasaurus. This did happen, but it has no bearing on the validity of Brontosaurus. In 2015, a new study found that the two were distinct after all.

There is some debate over how sauropods, especially the diplodocids, held their necks. It seems to be obvious that a large herbivore would evolve an elongated neck to feed from tall trees, but some studies have found the neck was habitually held straight out on these animals. Recent studies have, however, cast doubt on this idea. Even if it did hold its neck high, Apatosaurus wouldn’t have been the tallest browser in its environment. Brachiosaurus, for instance, stood a lot taller than it, but the difference in feeding height allowed the Morrison sauropods to reduce competition for food. Rock layers studied from the Morrison Formation suggest Apatosaurus inhabited a vast floodplain, with a multitude of river systems lined by dense forests. Tall conifers would’ve dotted surrounding fern savannas. It appears to have been a semi-arid region with pronounced wet and dry seasons. Some notable dinosaurs from the Morrison, in addition to the aforementioned sauropods, included taxa like Dryosaurus, Camptosaurus, Stegosaurus, Ceratosaurus, Torvosaurus, and Allosaurus. The latter two theropods were probably the main threat to Apatosaurus.