Apatosaurus

*Apatosaurus* is an extinct genus of sauropod that lived during the Late Jurassic in North America.

## Description

*Apatosaurus* lived in small-medium sized herds and used its long neck to forage for low plants. Each forelimb had a claw and three on its hind limbs. As shown by an *Apatosaurus* ischium discovered, large theropods often attacked *Apatosaurus*. *Apatosaurus* had spins down its neck and back, reaching the tip of its tail where it may have had a barbed tip. *Apatosaurus*' brain had a similar morphology to other diplodocids. *Apatosaurus* had deeply bifurcated neck vertebrae, forming a depression in the center with neural spines on each side of the trough. A. louisae has about 15 cervical, 10 dorsal, 5 sacral and 82 caudal vertebrae, though it has been noticed that the number of vertebrae varies between species and individuals. *Apatosaurus* vertebrae is short and robust, closely resembling vertebrae from the unrelated *Camarasaurus*. The cervical vertebrae support rows of cervical ribs, which become narrower at the top of the neck; nearly making a triangular cross-section. *A. louisae* has an atlas-axis complex, and the structure is nearly fused to the first cervical.

Like birds, *Apatosaurus* had vertebrae that had hollow air sacs in order to reduce weight. In *Apatosaurus*, the function worked to make the animal less heavy, and more nimble. In birds, the function serves as weight reduction in order for powered flight. Like *Supersaurus*, *Apatosaurus* has tall neural spines; making-up nearly half of the vertebrae's height. The animals ribs are very long, giving *Apatosaurus* a characteristically deep chest compared to other members of diplodocidae. Apatosaurus had very robust limb bones, with stout arm bones. It's humerus resembles *Camarasaurus **and*** **Brontosaurus**. It is noted, however, that the humerus in *Brontosaurus* and *A. ajax* bear more similarities with each other than to *A. louisae*. Charles Gilmore proposed in 1936 that the radius and the ulna would have crossed, though it was revealed later that the bones would have layed parallel in the animals body. *Apatosaurus* bore a large claw on its forelimbs, a feature all sauropods more evolved than *Shunosaurus* had. Each hindlimb had claws on the first three toes. The phalangeal formula is 2-1-1-1-1, meaning the phalanx comprises of two bones, and the next only has one. The single ungual is slightly curved in is squared and truncated at its anterior end. The pelvic girdle is made of a robust ilia, fused and co-ossified pubes and ischia. The femora is very short, representing one of the most robust femora in all of known sauropoda. Both the tibia and the fibula are different from those in Diplodocus, and indistinguishable from *Camarasaurus*. The fibula is is both longer and most slender than the tibia. The foot has three claws on the innermost toes, making the digit formula 3-4-5-3-2. The first metatarsal is shortest, like in many other diplodocids.

Studies in 2020 revealed that *Apatosaurus* likely had a neck more vertically inclined than previously thought. This applies with all diplodocids.

## Paleobiology

### Tail

The end of *Apatosaurus*' tail is often depicted with barbs at the tip of its tail to further injure predators. Further studies concluded that the bullwhip-functionalities of the tail would hurt the animal, but instead the tail could make sounds of up to 200 decibels. Further studies say that such an action would have injured the animal, due to its vertebrae being so delicate. *Apatosaurus*' tail was most of its body and was so long and fast, it could break the sound barrier like a whip. However, they likely did not use their tail for defense, as it would have been incredibly painful for the animal. *Apatosaurus* likely used the long, stiff tail as a whip to ward off predators, such as packs of hungry *Allosaurus'*. The tail could have also created loud noises, not only for scaring predators, even for territorial disputes. Unlike many other diplodocids, the animals tail had decreasingly tall vertebrae, at the smallest at the tip at its farthest from the hips.

### Teeth and Digestion

*Apatosaurus* had forelimbs which were just shorter than its hindlimbs. Its jaws were lined with rake-like teeth, packed into rows to mulch tough plants. It is possible it may have ingested gastroliths, stones which help break down food located in an animals gut, which are then defecated out. Diplodocids like *Apatosaurus* have been documented with gastroliths in their intestinal tracts and their stomachs. Some *Apatosaurus* have articulation in their teeth, and teeth that expose an enamel surface lack scratches, only wear and a 'sugary' texture.

### Growth

One study placed the growth rate of *Apatosaurus* at about 5000 kilograms (11,000 pounds) per year, reaching full size at bout 10 years of life, using growth lines in the fossils. However, another study states *Apatosaurus* grew roughly 520 kilograms (1150 pounds) in mass per year, reaching full size before 70 years of age, using limb lengths and body mass. However, this method of estimation may not be accurate, as growth lines in *Apatosaurus*' bones may be destroyed by bone remodeling. Another growth factor for *Apatosaurus* is the total number of sacral vertebrae, which increases by five by maturity. This was noted in 1903, and again in 1936. Specimens can be histologically analyzed, showing scientists that several specimens reached maturity at different times and grew to different amounts.

*Apatosaurus* is known from a lot of juvenile specimens. Multiple specimens in the OMNH collection are from an indeterminate *Apatosaurus* species. The material includes: partial shoulder and pelvic girdles, vertebrae and several limb bones, The A. sp. material comes from overlapping material, from different ages, from at least three different individuals. The specimens likely belong to the genus. Juveniles usually have shorter necks and tails, and a greater difference in limb sizes.

## Classification

Many *Apatosaurus* species were named from fragmentary remains, especially since Marsh named as many as possible during the Bone Wars. Paul Upchurch et al. (2005) analyzed species and relationships in *Apatosaurus*, finding A. louisae to be the most basal, with FMNH P25112 following, then a polytomy of *A. ajax*, *A. parvus* and *A. excelsus*. This was revised and expanded on by many more specimens in 2015, resolving relations differently and splitting *Brontosaurus*. Tschopp, Mateus, and Benson (2015) find by identifying remains individually:

## Species

* *A. ajax*
* *A. louisae*

### Reassigned

* *A. grandis*
* *A. excelsus*
* *A. parvus*
* *A. minimus*
* *A. alenquerensis*
* *A. yahnahpin*

## Paleoecology

*Apatosaurus* lived in the Morrison Formation during the Late Jurassic. The animal lived in large familial groupings and were often pursued by *Allosaurus*, *Saurophaganax*, *Torvosaurus*, and other large carnivores of the Morrison. *Apatosaurus*' habitat was ruled by many large predators, as well as many smaller coelurosaurs. Despite the Morrison being chalk-full of predators, *Apatosaurus* coexisted with *Brachiosaurus*, *Brontopodus*, *Camarasaurus*, *Cathetosaurus*, *Diplodocus*, *Dinheirosaurus*, *Amphicoelias* and countless other diverse sauropods.

## Gallery

### Mounts

A juvenile mount; *A.* sp. 1.BYU 17096 "Einstein", a complete specimen. possibly representing *A.* sp. 3.
## References

* https://en.wikipedia.org/wiki/Apatosaurus
* https://www.nature.com/articles/s41598-022-21633-2