Thecodontosaurus antiquus ("Ancient Socket Tooth Lizard") was an herbivorous dinosaur which lived throughout the Late Triassic period (Norian and/or Rhaetian age). Its remains are known typically from Triassic "fissure fillings" in South England and Wales. On standard, it was 4 feet (1.25 meters) long, 1 foot tall (0.3 meters), and weighed 25 pounds (11 kilograms).

Although not actually the first member of the group (that honour belongs to as yet unnamed sauropodomorphs from Madagascar (Flynn and Wyss 2002)), Thecodontosaurus is the most prehistoric well-known representative of the sauropodomorph dinosaurs. At first it was included under the prosauropoda (Upchurch 1998) but more recently it has been optional that Thecodontosaurus and its relatives were prior to the Prosauropod-Sauropod split (Yates & Kitching 2003). New reconstructions demonstrate that its neck is proportionally shorter than in higher early sauropodomorphs.

The Thecodontosaurus was an injured party of World War 2 bombings by the Germans. The remains of this dinosaur and other fabric related to it were shattered in 1940. However, more remains have been found at a number of localities, counting Bristol. Some of this new material pertains to a young specimen that may belong to a separate species, Thecodontosaurus caducus Yates, and 2003