Shonisaurus

Credit: Gabriel Ugueto.

*Shonisaurus* is a massive extinct genus of ichthyosaur that lived during the Late Triassic in Nevada.

## History

*Shonisaurus* were first found in the Shoshone Mountains, Nevada in 1920, and were excavated 30 years later, bringing 37 individuals. In 1984, Nevada gave *Shonisaurus* the title of state fossil. Under the direction of Charles Camp and Samuel Welles, excavations begun in 1954 up to the 1960s and was finally named by Camp in 1976. These beds can be seen at the Berlin-Ichthyosaur State Park.

## Description

*Shonisaurus* occurs during the Carnian (Late Triassic). *S. popularis* is 15 meters (49 feet). *S. sikanniensis* from British Columbia was named in 2004, but phylogenetic studies find this species fits best in the genus *Shastasaurus*. However, a 2013 evaluation found the opposite. S. sikanniensis has been found in the Pardonet Formation 210 million years ago (Mid-Norian, Late Triassic), and was 21 meters (69 feet) long; one of the largest marine reptiles, aside from the Lilstock Monster.

*Shonisaurus* has a long snout with much longer and narrower flippers than in other ichthyosaurs, *Shonisaurus* was originally described as having socketed teeth that were present in the jaw tip in the smallest of juveniles. These suggest *Shonisaurus* was a specialized offshoot. However, recent finds show they had these teeth in all ontogenetic stages. Older depictions show a rotund animal, but studies from the 1990s and onward depict a slender body. Compared to other marine reptiles, *S. popularis* had a relatively deep body. Some paleoart shows a dorsal fin, but other shastasaurs lack this feature and there is no evidence to support such a structure. The upper fluke of the tail was likely less developed than in later species.

## Paleobiology

### Taphonomy

The large assemblage of individuals in Nevada is likely caused by many *Shonisaurus* dying at varying times, who all preserved on the sea bed and accumulated. The lack of invertebrates found near indicates they sank into somewhat deep, oxygen poor waters. In 2011, Mark McMenamin proposed Shonisaurus remains accumulated because of a giant predatory coleoid, which he dubbed the "Triassic Kraken". Him and Diana Schulte proposed many times that the arrangement of the bones resembled cephalopod suckers and was a "self portrait". Since, this hypothesis has been thoroughly debunked. Later evidence suggests that the large congregation of adults, few neonates/embryos and a lack of juveniles and subadults in a moderately-deep area suggests that the site was used for reproductive purposes.

### Diet

In the search of silver in 1863, Berlin, Nevada had become a mining-focused town. In the mines found large bones, today known as *Shonisaurus*. in 2021, Neil Kelley, Paige dePolo and Nick Pyenson investigated toothlessness in *Shonisaurus*. It had been previously assumed that the animal filter-fed purely based on large extant aquatic life, but actually had large deep-rooted teeth. They found *Shonisaurus* was more akin to a massive crocodile than to a whale. It was likely an opportunistic carnivore who crushed ammonite shells and fed on marine reptiles. The team also found this fit with observed patterns from the Triassic, a time when large aquatic vertebrates were predators. The bonebeds they accumulate in was also a migratory route, mating place or birthing area, since Shonisaurus outnumbers other fossil vertebrates from this locality.

## References

1. ↑ 1.0 1.1 1.2 https://en.wikipedia.org/wiki/Shonisaurus
2. ↑ https://www.cbc.ca/news/science/biggest-ichthyosaur-1.4612537
3. ↑ https://incertaesedisblog.wordpress.com/2021/07/03/the-triassic-kraken-revisited/
4. ↑ https://www.cell.com/current-biology/fulltext/S0960-9822(22)01761-4
5. ↑ https://www.hakaimagazine.com/news/shonisaurus-gets-a-makeover/