Leptictidium

Leptictidium is an extinct genus of small mammals that were likely bipedal. Comprising eight species, they resembled today's bilbies, bandicoots, and elephant shrews, 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

thumb|left|Illustration

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.

thumb|right|Artist's impression of a Leptictidium

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 elephant shrews.

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. (more than half of which belonged to the tail), and in height. It weighed a couple of kilograms.

Locomotion

thumb|upright|L. nasutum fossil

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, elephant shrews combine both types of locomotion; they usually move on four legs, but they can run on two legs to flee from a predator.

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. 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:

Leptictidium auderiense

thumb|L. auderiense

Described by Heinz Tobien in 1962 based on a series of lower jaws from the Lutetian faunal stage. Tobien also uncovered a small skeleton he defined as a paratype of the species, but Storch and Lister proved in 1985 that, in fact, the skeleton did not even belong to the genus Leptictidium. Mathis remarks the exceptional development of the paraconid (or mesiobucal cusp) of the lower P4 premolar. Its premolars and molars were quite small in comparison to the dentition as a whole. The name of the species refers to the Roman settlement of Auderia.

Leptictidium ginsburgi

Described by Christian Mathis in 1989. Fossils have been found in the Quercy Phosphorites lagerstätte at the Robiac, Le Bretou, Lavergne, La Bouffie, Les Clapiès, Malpérié and Perrière localities(France), within Upper Ludian aged strata. The mesostyle typical of the genus Leptictidium is not developed in this species.