肿肋龙亚目（Pachypleurosauria）是种原始的鳍龙超目爬行动物，外表稍微地类似水生蜥蜴，只生存于三叠纪。牠们的体型修长，体型从20公分到1公尺长，并拥有小头部、长颈部、鳍状肢、长尾巴。牠们的肢带（Limb Girdles）缩小许多，所以这群动物不太可能移动到陆地上。牠们的颌部前部布满钉状牙齿，显示这群动物以鱼类为生。

肿肋龙亚目被归类于幻龙目。在有些亲缘分支分类法里，肿肋龙类被认为是真鳍龙类的姐妹分类单元，真鳍龙类演化支包括幻龙类与蛇颈龙类。

Pachypleurosaurs were primitive sauropterygian reptiles that vaguely resembled aquatic lizards, and were limited to the Triassic period. They were elongate animals, ranging in size from 20 cm to about a metre in length, with small heads, long necks, paddle-like limbs, and long deep tails. The limb girdles are greatly reduced, so it is unlikely these animals could move about on land. The widely spaced peg-like teeth project at the front of the jaws, indicate that these animals fed on fish.