It is impossible to describe the magnificent preservation of Eocene fossils from the Messel Pit Lagerstätten.
Fish, reptiles, birds and mammals were discovered lie down their side in the position of died, within coal layers, in a perfect structural integrity, and frequently with fur of feathers traces.
The site was declared a UNESCO World Heritage in 1995, and several perfectly conserved specimens are part of permanent exhibitions in many palaeontological museums worldwide.
One of such animals is Hallensia matthesi, exposed at Royal Belgian Institute of Natural Science, in the Gallery of Evolution. Hallensia matthesi was a small herbivore, with a diet based on leaves. It is related to the beginning of equoid radiation.
As shown in this photo, and similarly to other mammals from Messel, every anatomical part is completely preserved, and I used it in order to create and explain the anatomical characteristics, visible on bones of such tiny animal.
Hallensia had large upper canines (colored in white). Extant horses rarely develop similar homologous teeth and, in any cases, they are really small and atrophic. Teeth are not specialized for grazing, but each crown shows multiple cusps and premolars are monocuspids (in black), like other species of basal mammals.
Ulnae (turquoise) is as developed as much as the radius (in blue) and not completely fused with the latter. In modern horses, such couple of bones are strongly ossified, and diaphysis of the ulna ends before the animal wrist.
Metacarpals are multiple bones (in green) whereas they are reduced to one large bone in horses, like phalanxes (dark green) that create a single finger in the larger animal. In Hallensia, fingers are still three.
Evolution of horses was disclosed step-by-step, from the Eocene to the Holocene, with a progressive speciation of larger animals and the loss of lateral fingers, and a consequent change of mobile limbs to pendular movements during the trot.