Leptictidium is an extinct genus of small mammals that were likely bipedal. Comprising eight species, they resembled today's bilbies, bandicoots, and sengis, and occupied a similar niche. They are especially interesting for their combination of characteristics typical of primitive eutherians with highly specialized adaptations, such as powerful hind legs and a long tail which aided in locomotion. They were omnivorous, their diet a combination of insects, lizards, frogs, and small mammals. Leptictidium and other leptictids are not placentals, but are non-placental eutherians, although they are closely related to placental eutherians. They appeared in the Lower Eocene, a time of warm temperatures and high humidity, roughly fifty million years ago. Although they were widespread throughout Europe, they became extinct around thirty-five million years ago with no descendants, as they were adapted to live in forest ecosystems and were unable to adapt to the open plains of the Oligocene.
Description
Leptictidium is a special animal because of the way its anatomy combines quite primitive elements with elements which prove a high degree of specialization. It had small fore legs and large hind legs, especially at the distal side (that further from the body). The lateral phalanges of its forelegs (fingers I and V) were very short and weak, finger III was longer and fingers II and IV were roughly equal in size, and slightly shorter than finger III. The tips of the phalanges were elongated and tapered.
The ankles and the sacroiliac joint were quite loosely fixed, while the pelvis had a flexible joint with only one coccygeal vertebra. The anteorbital muscle fenestrae in their crania suggest they probably had a long and mobile snout (or Proboscis), similar to that of sengis. Leptictidium had wide diastemata in the antemolar row, its upper molar teeth were more transverse than those of the North American leptictids and its fourth premolars were molariform. Its C1 canines were incisiviform. Its dentition was quite small in comparison to the size of the mandible and the animal as a whole. It varied between 60 and 90 cm (24 and 35 in) in length (more than half of which belonged to the tail), and 20 cm (7.9 in) in height. It weighed a couple of kilograms. These sizes could vary from one specimen to another. Leptictidium tobieni from Messel (Germany) is the largest known leptictid with skull 101 mm (4.0 in) long, head with trunk 375 mm (14.8 in) long, and tail 500 mm (20 in) long.
Locomotion One of the mysteries about Leptictidium is whether it moved by running or by jumping. Because there are very few completely bipedal mammals, it is difficult to find an appropriate living model to compare it with. If the kangaroo is used, it is probable that Leptictidium hopped along with its body tilted forward, using its tail as a counterweight. On the other hand, sengis combine both types of locomotion; they usually move on four legs, but they can run on two legs to flee from a predator. Studies of the bone structure of Leptictidium have yielded contradicting information: its leg articulations appear too weak to have supported the shock of repeated jumps, but its long feet were obviously adapted for jumping rather than running. Kenneth D. Rose compared the species L. nasutum with the leptictid Leptictis dakotensis. L. dakotensis had a series of traits which show it was a running animal which sometimes moved by jumping. Despite the marked similarities between Leptictis and Leptictidium, there are certain differences in their skeletons which prevent the example of Leptictis from being used to determine with certainty the way Leptictidium moved: the most important being that, unlike Leptictis, the tibia and the fibula of Leptictidium were not fused together.
Behaviour Perfectly preserved fossils of three different species of Leptictidium have been found in the Messel pit in Germany. The marks on their fur have been preserved, as well as their stomach contents, which reveal Leptictidium were omnivores which fed on insects, lizards and small mammals. The holotype of L. tobieni also had pieces of leaves and notable amounts of sand in its abdomen, but it cannot be determined with certainty if the animal swallowed it.
Habitat
Leptictidium lived in the European subtropical forests of the Eocene. From the beginning of this period, the temperature of the planet rose in one of the quickest (in geological terms) and most extreme episodes of global warming in the geological record, termed Paleocene–Eocene Thermal Maximum. It was an episode of quick and intense (of up to 7 °C in high latitudes) warming which lasted less than 100,000 years. The thermal maximum caused a great extinction which is used to distinguish the Eocene fauna from that of the Paleocene. The global climate of the Eocene was probably the most homogeneous of the Cenozoic; the temperature gradient from the equator to the poles was half that of today's, and the deep ocean currents were exceptionally warm. The polar regions were much warmer than today, possibly as warm as the present-day Pacific Northwest of North America. Temperate forests reached the poles themselves, while rainy tropical climates reached 45° N. The greatest difference was in temperate latitudes; nevertheless, the climate at the tropics was probably similar to today's. In the Eocene, most of what is now Europe, the Mediterranean and south-west Asia was submerged under the Tethys Sea. These two continents were separated by the Turgai Strait (an epeiric sea). Due to high humidity and temperatures, most of the European continent was covered in vegetation. The region which today is Germany was in a volcanically active zone during the Eocene. It is thought that the Messel pit could have been the old location of a volcanic lake saturated with CO2. The lake would periodically release the gas it contained, creating a lethal cloud which would asphyxiate any animal in its path. This would explain the great number of non-aquatic species which have been found in the old lake-bed of the Messel pit. In the lush forests of this region, Leptictidium shared its habitat with animals such as Godinotia, Pholidocercus, Palaeotis, or Propalaeotherium. There were also predators, the crocodilian Asiatosuchus, the hyaenodont Lesmesodon, and the Messel giant ant.
Species The genus Leptictidium includes eight species. These include:
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