纖肢龍目

纖肢龍目是群小型爬行動物，身長不超過1公尺。外型類似現代蜥蜴，與現代蜥蜴的親緣關係相當遠。纖肢龍類的四肢、尾巴修長，具有兩個顳顬孔，使牠們不同於其他早期爬行動物。纖肢龍缺少下顳孔，只有上顳孔，形成類似闊孔亞綱的顱骨。

纖肢龍目的著名屬包含：纖肢龍、油頁岩蜥、齊椎蜥（Spinoaequalis），上述屬已發現較完整的骨骼。Aphelosaurus、Kadaliosaurus只有發現顱後骨骼部分，Zarcasaurus則只有發現一個下頜。

Dictybolos曾經被歸類於纖肢龍目，但該研究人員同時將原龍類、中龍類歸類於纖肢龍目，因此遭到不少的異議。

在2002年，美國奧克拉荷馬州曾發現一些新的纖肢龍類化石，但目前仍沒有經過科學敘述。

以下演化樹來自於deBraga與Reisz（1995年）、Falconnet與Steyer（2007年）的研究：

纖肢龍類的油頁岩蜥、齊椎蜥（Spinoaequalis）發現於石炭紀晚期的美國。Aphelosaurus生存於二疊紀早期的法國，Kadaliasaurus生存於二疊紀早期的德國，纖肢龍、Dictybolos、Zarcasaurus生存於二疊紀早期的美國。除了纖肢龍目，二疊紀早期的其他雙孔亞綱動物只有兩個屬，生存於德州的Apsisaurus、奧克拉荷馬州的山獵蜥。

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