Science history meets microtomography: Comparison of the world's oldest physical model of an extinct animal's inner ear with the 3D virtual counterpart

In 1936 in Basel, Johannes Hurzeler isolated the bony labyrinth out of a fossil petrosal bone of the small artiodactyl Cainotherium from the 24 million years old French locality of Branssat (Hurzeler, 1936). To remove it from the bone, he filled the hollow chambers of the bony labyrinth with celluloid and then dissolved the petrosal bone itself to get its endocast. To the best of our knowledge, this is the first cast of an extinct animal inner ear ever produced and published. Here we compare this analog model still preserved in the collection of the Natural History Museum Basel with a 3D virtual model generated by a microtomography scan of another petrosal bone of the same taxon from the same locality. A previous study published the first digital data on the ear region of a cainotheriid but without segmenting the bony labyrinth and another one visualized an inner ear without describing it. Our 3D digital model expands our knowledge of the inner ear anatomy of Cainotherium by showing the so far unclear extent and course of the vestibular and cochlear aqueducts as well as by enabling volumetric data of the different labyrinth parts to be measured.