Icthyostega stensioei (Säve-Söderbergh, 1932) Late Devonian ~365 mya

Acanthostega gunnari (Jarvik 1952) Late Devonian ~365 mya

The transition from fins to limbs begins with these two derivations of osteolipiform fish. Both had paddle-like limbs and more than five toes.

Both Ichthyostega and Acanthostega had nares (nostrils) that opened low, close to the jaw line and orbits (eye sockets) high on the skull. As in their more fish-like ancestors, both retained large palate fangs in addition to the sharp teeth that rimmed the jaws. The stapes was a large bone connecting the braincase to the palate, bracing the two.

The vertebral column consisted of ~28 vertebrae composed of two parts. The lower part wrapped around the notochord. The upper part included a robust neural spine. The fish-like tail included unjointed, long and numerous lepidotrichia to increase the tail height and depth. The dorsal ribs were shorter in Acanthostega, the more primitive (fish-like) of the two.

The pectoral girdle was largely coossifled. The humerus was L-shaped. The radius was much longer than the ulna, proportions otherwise seen only in more primitive taxa. Only the intermedium is identifiable in the carpus (wrist). There are eight digits in the manus (hand).

The pelvis was largely coossified and relatively much larger in Ichthyostega. The femur was hourglass-shaped. The knee and ankle were not suitable for much bending in Acanthostega, but more bendable in Icthyostega. Eight toes appear in Acanthostega, six in Ichthyostega.

Proterogyrinus and Eoherpeton are more derived sister taxa.

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This is a segment from the larger tree upon which this ordering of the taxa presented in this site is based - plus several additional pre-amniote microsaurs.

This study found the Microsauria to be diphyletic with Utaherpeton and Tuditanus nesting within the Reptilia and the remainder (Sauropleura through Rhynchonkos) nesting within the Amphibia (caecilians, frogs, salamanders, their last common ancestor and all of its descendants).

Ichthyostega is at the base of this tree. Reptilomorphs are tetrapod taxa leading toward reptiles (not including Eryops and kin). Silvanerpeton and Gephyrostegus are reptile sister taxa with the latter considered a protoreptile.

In past studies a longer list of parareptiles that included Diadectes, Chroniosuchus and Solenodonsaurus were considered reptilomorphs. That is an error resulting from a small taxon list. Here those taxa nest deep within the reptiles.