小頭龍的外型與體格類似橡樹龍,但頸部比例上較長。身體全長估計不超過4公尺,[6]不過奎格利·保羅(英语:Gregory S. Paul)給出較高的估計值身長4.7公尺及體重300公斤。小頭龍與更進化禽龍類的不同之處在於,其喙狀嘴前端仍保有牙齒(前上頜齒);還有第一腳趾的存在,更進化的禽龍類第一趾消失僅剩中間三趾。肱骨具有供肌肉附著的凹陷處,此特徵也出現在其他南美洲鳥腳類如南方稜齒龍、阿納拜斯龍。上述及南美鳥腳類彼此呈現出的相似特徵,暗示牠們可能屬於一個南半球特有的鳥腳類演化支,但研究者告誡這些特徵並不足以證實演化支存在的理論。2015年的莫羅龍研究發現這個演化支確實存在。[6]
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