Size and Weight:
When you think about it, other marine reptiles deserve the name Thalassiodracon ("sea dragon") a lot more than this genus does--to name just two examples, the gigantic, awkwardly named Liopleurodon and Cryptoclidus. Still, paleontology operates by a strict set of rules, and the name Thalassiodracon has stuck to this small (only about six feet long), unassuming, and (judging from the size of its head) not very bright plesiosaur.
Thalassiodracon is such an ancient reptile--dating back to the end of the Triassic period--that experts can't tell if it was technically a plesiosaur or pliosaur. It's possible that this was one of the "basal" forms that later gave rise to these two types of aquatic reptiles, one group characterized by their long necks and small heads and the other by their short necks and long heads.