Largest marine reptiles ever
Shastasaurid ichthyosaurs
20–25 metre(s)
Not Applicable ()

The largest marine reptiles ever to live on Earth were shastasaurid ichthyosaurs (family Shastasauridiae) of the Late Triassic period (237–201 million years ago). The fossilized remains of several species of these dolphin-like ocean giants indicate that they could have grown in excess of 20 m (66 ft) long, and perhaps up to 25 m (82 ft), on a par with the average length of a blue whale (Balaena musculus), the largest-ever marine mammal and largest animal alive today.

Stand-out examples of shastasaurid believed to have reached record-breaking proportions include Shonisaurus; the holotype specimen of Shonisaurus sikanniensis uncovered from the Pardonet Formation of north-east British Columbia, Canada, between 1997 and 2000 had an estimated total length of 21 m (69 ft). Its remains are now housed at the Royal Tyrrell Museum in Drumheller, Alberta, Canada. The front half of the S. sikanniensis skeleton is preserved at a length of 10 m (33 ft); only the posterior half of the skull was retrieved and is 1.5 m (5 ft) long, suggesting a total skull length of 3 m (10 ft).

A more recent example that could have equalled, and perhaps even exceeded, Shonisaurus in length is Ichthyotitan severnensis (“giant fish lizard of the Severn”), first described in PLoS ONE on 17 April 2024. The earliest traces of this particular ichthyosaur comprised a >2-m-long (>6-ft 6-in) jaw fragment recovered from the Westbury Mudstone Formation in Somerset, UK, in 2016 by professional fossil collector Paul de la Salle. That find was bolstered by the discovery of a more complete jawbone at Blue Anchor beach (also in Somerset) by amateur father-and-daughter fossil hunters Justin and Ruby Reynolds in 2020. Palaeontologist Dr Dean Lomax (UK) of the University of Manchester led the analysis that confirmed the two sets of jaw bones belonged to the same species, enabling the extrapolation of a total body length of up to 25 m (66 ft).

The Shastasauridiae family currently comprises seven genera, though some of these have been postulated to represent distinct groups: Shastasaurus, Besanosaurus, Guanlingsaurus, Guizhouichthyosaurus, Shonisaurus, Himalayasaurus and ‘Callawayia’ wolonggangensis. Ichthyotitan is proposed to be an eighth member of the family, and potentially represents the end of the dynasty before these giants disappeared in the Triassic-Jurassic extinction event 201.4 million years ago.