Chirotherium
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Chirotherium (also known as Cheirotherium) or 'hand-beast', is the name of a creature which may be known only from fossil imprints of its tracks (ichnites). These look, by coincidence, remarkably like the hand of an ape/human or bear, with the outermost toe having evolved to extend out to the side like a thumb, although probably only providing a firmer grip in mud. Its tracks were first found in 1834, in red sandstone in Thuringia, Germany, dating from 225 - 195 million years ago (mya).
This creature was probably an archosaur, related to the ancestors of the dinosaurs.
Tracks left by this creature were found before dinosaurs were known and initial models of the animal proposed that it was a bear or ape, which walked with its feet crossed. This proposal was necessary to explain the toe on the outside. These tracks have now been found on North America, Africa, and Europe.
In reality, an external (lateral) 'thumb' was commonplace among archosauria of the creature's era, i.e. the Early Triassic Period.
Years later, the skeleton of a probably closely-related animal was found, called Ticinosuchus. It had the external toe on its hind feet but not on its front feet and was possibly a more advanced descendant, whose gait had improved enough to reduce the need for a stabilizing toe. Some have, on the other hand, proposed that it's the very same species, with what may have been a gender difference (sexual dimorphism).
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