Leptictidium

Leptictidium Temporal range: Lutetian – Ludian
Leptictidium auderiense skeleton at the Muséum national d'histoire naturelle (Paris)
Conservation status
Extinct
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Subclass:	Theria
Order:	Leptictida
Family:	Pseudorhyncocyonidae
Genus:	Leptictidium
Species
L. auderiense L. ginsburgi L. nasutum L. sigei L. tobieni

Leptictidium ("[[graceful weasels" in latin) is an extinct genus of small prehistoric mammals; together with macropods and humans, they are the only known completely bipedal mammals. They were one of the first branches to split from basal eutherians, appearing in the Lower Eocene, roughly fifty million years ago. Despite the fact that they were widespread throughout Europe, they became extinct roughly thirty-five million years ago leaving no descendants, probably due to the fact that they were adapted to live in forest ecosystems and were unable to adapt to the open plains of the Oligocene.

Physical characteristics

Leptictidium ressembled today's elephant shrews.

Leptictidium is a special animal because of the way its anatomy combines quite primitive elements with elements which prove a high degree of specialisation. It had small forelegs and large hindlegs, 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 iliosacral articulation 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, 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 canine were incisiviform. Its dentition was quite small in comparison to the size of the mandible and the animal as a whole.

Its size varied between roughly sixty and ninety centimetres in length (more than half of which belonged to the bald tail), and twenty centimetres in height. It weighed a couple of kilograms. This sizes could vary from one specimen to another.

Locomotion

One of the questions about Leptictidium which have not been solved yet is whether it moved by running or by jumping. Because there are very few completely bipedal mammals, it is difficult to find an appropriate animal to compare it with Leptictidium. On one hand, if kangaroos are used as a model, it is probable that Leptictidium ran 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. Studies of the bone structure of Leptictidium have yielded contradicting information: on one hand, it seems its leg articulations were too weak too support the shock of repeated jumps, but on the other hand it is obvious that its long feet were not adapted for running, but for jumping.

Kenneth D. Rose did a comparison of 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.

While some paleontologists argue that Leptictidium was obviously a running animal, others reply that what is obvious is that it was cursorial. The fact is that the way this eutherian moved remains a controverted subject.

Behaviour

Leptictidium was a diurnal animal which went out to hunt twice a day, once in the morning and once in the afternoon. One of the first things it did after waking up was grooming intensively. It is believed that it showed a particular habit when it hunted; it cleared paths in the jungle where it lived. Every morning and every afternoon it kept itself busy patrolling these paths, catching any prey it found and clearing them from any object which hindered the way. Keeping these paths clear was of utter importance, as they were the way it used to flee if ever a predator attacked it.

Since Leptictidium young were very vulnerable to attacks of predators, it is probable that they were born quite developed and that they became independent when they were still quite young. From the behaviour of today's elephant shrew, the possibility can be suggested that Leptictidium young went out to hunt with their mother, and that they learned to distinguish prey by licking their mother's mouth when she had just caught an animal, in order to know the flavour of food.

Perfectly preserved fossils of three diferent 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.

External references

BBC Online Science and Nature