Largest marine reptiles ever
21 metre(s)
Canada ()

Based on total length, the largest marine reptiles ever to live on Earth were the shastasaurid (family Shastasauridiae) marine reptiles, a family of Triassic ichthyosaurs that ranged from 6 m (19 ft) to in excess of 20 m (66 ft) long. The largest members of the family found to date were Shonisaurus sikanniensis, uncovered in the Late Triassic Pardonet Formation of north-east British Columbia, Canada. This record is based on the holotype specimen (TMP 1994.378.0002), at an estimated total length of 21 m (69 ft). The specimen was found in 1994, collected between 1997 and 2000 by crews from the Royal Tyrrell Museum, and was described in the Journal of Vertebrate Paleontology in December 2004. The specimen is now housed at the Royal Tyrrell Museum in Drumheller, Alberta, Canada, and is currently on public display.

The Shastasauridiae family currently comprises six genera: Shastasaurus, Besanosaurus, Guanlingsaurus, Guizhouichthyosaurus, Shonisaurus and ‘Callawayia’ wolonggangensis. A seventh genus, Himalayasaurus, has been tentatively included by some but has yet to be universally accepted.

The front half of the holotype S. sikanniensis skeleton is preserved at a length of 10 m (33 ft). Only the posterior half of the skull is preserved and is 1.5 m (5 ft) long, suggesting a total skull length of 3 m (10 ft).

A potentially even larger shastasaurid (as yet identified/named) has been described based on a jaw fragment unearthed from the Westbury Mudstone Formation located in Lilstock, Somerset, UK. Extrapolated using the same physiology of the much more complete remains of S. sikanniensis, this ichthyosaur may have been as long as 26 m (85 ft), as reported in the journal PLOS ONE on 9 April 2018.