Gephyrostegus bohemicus (Jaeckel 1902) Upper Carboniferous (~310 mya)~22 cm snout-vent length, is the basalmost amniote from Nyran in the Czech Republic, 30 milion years younger than other, more derived amniotes.. Derived from a sister to Silvanerpeton, Gephyrostregus phylogenetically preceded the basal reptiles, Eldeceeon and Urumqia.
Distinct from Silvanerpeton, the skull of Gephyrostegus descended posteriorly. The lacrimal and quadratojugal were shorter. The otic notch was reduced.
The presacral vertebral count was reduced to 24. The humerus was more gracile. An olecranon process (elbow) developed on the ulna. Manual digits IV and V were shorter. The pelvis was shorter. The dorsal process of the ilium was shorter. Certain anklebones fused together to create a larger bone, the astragalus, otherwise found only on reptiles.
Gephyrostegus bohemicus has no traditional amniote characters, but it does nest at the base of the two reptile clades, the Archosauromorpha and the Lepidosauromorpha. Therefore, by phylogenetic bracketing, this taxon must have laid amniotic eggs, the key trait of the Amniota. A sister to G. bohemicus must have preceded the other amniotes living 30 million years earlier in the Viséan.
While most early tetrapods lived their lives in water, Gephyrostegus was among the few that preferred land (= moss covered coal forest logs). Tiny circular scales covered the body except beneath where large V-shaped scales were present.
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