| Chirotherium Temporal range: 243 Ma |
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| Cheirotherium trace fossil, displayed in Oxford University Museum of Natural History | |
| Trace fossil classification | |
| Ichnoclass: | Reptilipedia |
| Ichnofamily: | †Chirotheriidae |
| Ichnogenus: | †Chirotherium Kaup, 1835 |
| Type ichnospecies | |
| C. barthii Kaup, 1835 |
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| Ichnospecies | |
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| Synonyms | |
Chirotherium, also known as Cheirotherium ('hand-beast'), is the name of a Triassic archosaur known only from fossil imprints of its tracks (trace fossils). These look, by coincidence, remarkably like the hands of apes, humans, and bears, with the outermost toe having evolved to extend out to the side like a thumb, although probably only functioning to provide a firmer grip in mud. Its tracks were first found in 1834 in red sandstone in Thuringia, Germany, dating from about 243 million years ago (mya).
This creature was probably a pseudosuchian archosaur related to the ancestors of the crocodiles. It likely belonged to a group of pseudosuchians called the rauisuchians, which were large carnivores with erect gaits.
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Chirotherium tracks were first found in German Triassic sandstones in 1834, and later in England in 1838. 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.[1] This proposal was necessary to explain the toe on the outside. The tracks were also proposed to be from a marsupial.[2] These fossil tracks have now been found on North America, Africa, and Europe.
British paleontologist Richard Owen suggested in 1842 that the tracks were made by a labyrinthodont amphibian.[2] Over the following years, new discoveries of archosaurian reptiles indicated that Chirotherium tracks were made by a pseudosuchian. The print's resemblance to mammals was only superficial; in reality, an external (lateral) 'thumb' was commonplace among Triassic archosaurs.
In 1965, the skeleton of a probably closely related animal was found, called Ticinosuchus.[2] 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 front toe. Chirotherium barthii and Ticinosuchus ferox have also been proposed to be the same species, with what may have been a gender difference (sexual dimorphism) visible in the tracks.[3]
Chirotherium trackways have been found in German sandstones that were likely deposited along shorelines. During the Triassic, much of Europe was a chain of islands surrounded by the shallow Tethys Ocean. In one location, Chirotherium trackways were found alongside those of early horseshoe crabs. The horseshoe crabs were likely breeding along the intertidal zone while the Chirotherium track maker preyed on them during low tide. Smaller reptiles like Macrocnemus, represented by the ichnogenus Rhynchosauroides, likely fed on the horseshoe crabs' eggs.[4]