Louisae

A. louisae fossils are rare, known only from one site in the upper Brushy Basin Member; they date to the late Kimmeridgian stage, about 151 mya.

Artistic interpretation of A. louisae The limb bones are also very robust.

However, the humeri of Brontosaurus and A. ajax are more similar to each other than they are to A. louisae.

In 2015, Tschopp et al. found the type specimen of Apatosaurus laticollis to nest closely with CM 3018, meaning the former is likely a junior synonym of A. louisae.

Neck posture further Artistic interpretation of an individual of A. louisae arching its neck down to drink Diplodocids like Apatosaurus are often portrayed with their necks held high up in the air, allowing them to browse on tall trees.

Othniel Charles Marsh described and named the first-known species, A. ajax in 1877, and a second species, A. louisae, was discovered and named by William H. Holland in 1916.

The skull of Apatosaurus was confused with that of Camarasaurus and Brachiosaurus until 1909, when the holotype of A. louisae was found, and a complete skull just a few meters away from the front of the neck.

The vertebral formula for the holotype of A. louisae is 15 cervicals, 10 dorsals, 5 sacrals, and 82 caudals.

They found that A. louisae was the most basal species, followed by FMNH P25112, and then a polytomy of A. ajax, A. parvus, and A. excelsus.

This specimen, CMC VP 7180, was found to differ in both skull and neck features from A. louisae, but shared many features of the cervical vertebrae with A. ajax.