基部四足动物是指位于四足动物进化树根部的四足动物（四条腿的动物），其中包括所有陆生脊椎动物，甚至一些海洋动物，如鲸类：鲸和亲属。 基部四足动物从石龙鱼类（叶鳍鱼）进化而来，今天的化石是浣熊鱼和肺鱼。

3.8亿年前，在泥盆纪时期，陆地上没有后来所有时代都有的茂密森林。 真正的树木刚刚开始进化，但原始的维管束植物随处可见，形成了矮小的灌木状森林。 这些森林中居住着节肢动物，如螨虫、蝎子和有蹄类动物，以及一些简单的软体动物，这些动物的化石效果并不好。 奇怪的真菌单体Prototaxites，高度可达9米（30英尺），是周围最高的东西。 地球很温暖，导致大陆架被淹没，低地上有营养丰富的沼泽。 这种环境非常适合鼓励一个小家族鱼进化出更强大的底鳍，用于推动自己穿过浅沼泽的植物。

早期的叶鳍鱼，如生活在3.8亿年前的Eusthenopteron，已知拥有许多今天已知的四足动物独有的特征：内鼻孔、带有颅间关节的两部分颅骨，最值得注意的是，前鳍有桡骨、尺骨和肱骨，盆鳍有腓骨、胫骨和股骨。 现在已知最后一个特征是所有鳞翅目动物共有的。

大约在同一时期，有一种浅水鱼Panderichthys，它具有更多类似四足动物的基本特征，特别是一个宽大的、类似四足动物的头部。 Panderichthys是一个关键的过渡性化石，显示了骨盆腰带进化的中间点，这使得基本的四足动物的腿成为可能。 它的头部甚至有一个螺旋桨，或氧气呼吸管，后来演变成四足动物耳朵的镫骨。 这使它在氧气不足的泥水中能够充分呼吸。

Living approximately 475 million years ago was the famous Tiktaalik, reconstructions and analysis of which most clearly show an intermediary form between a fish and amphibian. It is unknown whether Tiktaalik was a true tetrapod, venturing on land, although it does seem likely. Studies of the skeleton of Tiktaalik have shown it could have supported its own weight on hand. Its fins had a wrist structure not seen in any earlier fish. Its head had spiracles of substantial size which probably channeled oxygen to primitive lungs. This sarcopterygian even had a wide, crocodile-like head which it could turn to the left and right, independently of its body, an adaptation ideal for hunting on land and unlike any fish before it. Tiktaalik is the best-preserved sarcopterygians to be classified in the subclass Tetrapodomorpha. Tiktaalik, and earlier members of Tetrapodmorpha, like Kenichthys, are considered so similar to basal tetrapods that they have been called "fishapods," in reference to their combination of fish and

One of the first true tetrapods, which still probably didn't predominantly walk on land, but did use its primitive legs to navigate through swamps, was Ichthyostega, which lived 367-362.5 million years ago. Ichthyostega is one of a few basal tetrapods, a tetrapod that is neither an amphibian, synapsid (mammal and relatives), or sauropsid (reptile). These animals had a number of further evolutionary innovations for terrestrial life. For one, Ichthyostega primarily used its lungs to breathe, rather than its gills. It had a special skin covering which helped prevent it from drying out. It had a skeletal structure (especially spine and ribcage) that clearly shows this animal spent major time holding itself up. Ichthyostega and other ichthyostegoids (Acanthostega, Elginerpeton, Ichthyostega, and others) were eventually replaced by temnospondyls and anthracosaurs, including the famous Eryops. Unfortunately, there is a 20-30 million year gap in the fossil record between both groups, only bridged by a few specimens.