Cainotheriidae
Extinct European artiodactyls with rabbit-like traits and large ear bullae.
Cainotheriidae were a family of small artiodactyls that lived in Europe from the Late Eocene through the Middle Miocene. Their fossils are most often recovered from karstic deposits. These animals stood no more than about 15 centimeters at the shoulder, with body sizes ranging from that of a rabbit to that of a tragulid. Their skulls were small, with a short snout and orbits that were closed at the back and positioned near the center of the skull. They had a full set of teeth, and the premolars and molars were highly selenodont—meaning the cutting edges were curved and crescent-shaped, much like those of modern ruminants.
One of the most distinctive features of the group was their auditory bullae, the bony structures protecting the ear. These were exceptionally large, similar to those seen today in small mammals that inhabit open, dry environments. This specialized cranial anatomy contrasted sharply with the rest of the skeleton, which remained relatively primitive. Cainotheriids still had four non-reduced toes on each foot—an ancestral artiodactyl trait—though the outer toes were shorter. The toes ended in long claws, reminiscent of modern rabbits. Their hind legs were much longer than the front ones, suggesting strong adaptations for running and jumping.
Classification has shifted over time. Early research placed them near Tylopoda, but more recent work puts them closer to Ruminantia. The genus *Robiacina*, from the middle to upper Eocene, was once included in this family but is now considered a separate sister taxon. During the Upper Eocene, primitive genera like *Oxacron* and *Paroxacron* appeared, regarded as the first true cainotheriids and grouped in the subfamily Oxacroninae. In the Oligocene, the subfamily Cainotheriinae underwent a modest evolutionary radiation, producing rabbit-sized genera such as *Plesiomeryx* and *Caenomeryx*. The most specialized genus, *Cainotherium*, was also the last to survive, disappearing in the Middle Miocene. At the start of the Miocene, these animals were still common across Europe, with several species known. They went extinct when the climate grew colder during the Miocene.
For a long time, their combination of small size, long hind legs, and large auditory bullae led researchers to view them as ecological parallels to rabbits—presumed to move by leaping. However, fossil trackways from the Lower Miocene at Salina
- temporal_range
- Late Eocene to Middle Miocene
- geographic_range
- Europe
- size
- Up to 15 cm (5.9 in) at the shoulders
- diet
- Herbivorous (selenodont dentition)
- known_for
- Large auditory bullae and long hind legs adapted for running and jumping
- classification
- Primitive artiodactyls, placed near Tylopoda or Ruminantia
Lore & Background
Cainotheriidae first appeared in the Late Eocene of Europe, with primitive genera such as Oxacron and Paroxacron considered the first true members. During the Oligocene, the subfamily Cainotheriinae underwent a discrete evolutionary radiation, producing rabbit-sized genera like Plesiomeryx and Caenomeryx. The most specialized genus, Cainotherium, persisted until the Middle Miocene and was the last to disappear. Even at the beginning of the Miocene, these animals were quite common in various parts of Europe, with numerous species such as C. laticurvatum, C. miocenicum, and C. bavaricum.
Reader's Guide
Cainotheriidae are significant for their bizarre combination of primitive and specialized traits. Their large auditory bullae, similar to those in modern small mammals of open, dry environments, contrast with a primitive skeleton retaining four non-reduced toes and long claws. The hind legs were much longer than the front ones, indicating adaptation to running and jumping. For a long time, they were considered to have a lifestyle similar to hares and rabbits, but fossil footprints from the lower Miocene of Salinas de Anana in Spain, left by Cainotherium, showed locomotion quite similar to that of small ruminants. Their extinction during the Miocene coincided with a cooling climate. The family's classification has been debated: earlier research placed them near Tylopoda, while more recent work places them closer to Ruminantia. The genus Robiacina was previously classified within Cainotheriidae but has been split into a separate sister family.
Did You Know?
- Cainotheriidae are mostly found preserved in karstic deposits.
- They possessed four non-reduced toes, an ancestral condition of artiodactyls.
- Their auditory bullae were very large, similar to those in small mammals living in open, dry environments.
- Fossil footprints from Spain show Cainotherium locomotion was similar to small ruminants, not rabbits.
More in Extinct Cenozoic Mammals 1-24
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