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.

Habitat

Excavations at the Messel pit.

Main article: Eocene

Main article: Messel pit

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 7º 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 probablyy the most homogeneous of the Cenozoic; the thermal 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 more warm than today, maybe as warm as the Pacific Northwest nowadays. Temperate forests reached the poles themselves, while rainy tropical climates reached 45º 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 today is Europe, the Mediterranean and south-west Asia was submerged under the Tethys Sea. Both continents were separated by the Turgai Strait (an epeiric sea). Thanks to high humidity and temperatures, most of the European continent was covered in vegetation.

File:Lake nyos.jpg

The Messel pit zone included a volcanic lake satured with reactive gases, similar to Cameroon's Lake Nyos.

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: Asiatosuchus, Lesmesodon or the Messel giant ant. The alpha predator was Gastornis, a carnivorous bird almost two metres tall.

Species

Photograph of the MNHN's Leptictidium auderiense, with a full view of the long tail.

The genus Leptictidium includes five species:

Leptictidium auderiense

Main article: Leptictidium 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. It was the smallest species of all and was only sixty centimetres long. Several skeletons have been found at the Messel pit. 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

Main article: Leptictidium ginsburgi

Described by Christian Mathis in 1989. Fossils have been found in the lagerstätte at Robiac, Le Bretou, Lavergne, La Bouffie, Les Clapiès, Malpérié and Perrière (France), in Upper Ludian strata. The mesostyle typical of the Leptictidium genus is not developed in this species. The species is dedicated to Léonard Ginsburg, French paleontologist and deputy director of the Muséum national d'histoire naturelle in París.

Leptictidium nasutum

Main article: Leptictidium nasutum

Described by Adrian Lister and Gerhard Storch in 1985. It was a middle-sized species which was seventy-five centimetres long. Several skeletons have been found in the Messel pit, en estrats del Lutecià inferior. The tail of this species had 42-43 vertebrae, a number surpassed among mammals solely by the Long-tailed Pangolin. Its premolar and molar teeth were quite small in comparison to the dentition as a whole. The name of the species refers to the nose of the animal. The holotype is the complete skeleton of an adult specimen kept in the Forschungsinstitut Senckenberg in Frankfurt am Main.

Leptictidium sigei

Main article: Leptictidium sigei

Described by Christian Mathis in 1989. Fossils have been found in the lagerstätte at Sainte-Néboule, Baby, Sindou and Pécarel (France), and it has a more primitive appearance than L. nasutum. It is known mainly from isolated teeth. It has a P4 with a much reduced paraconid, as well as very distinct entoconids and hypoconulids on M1 and M2. The species is dedicated to Bernard Sigé, French paleontologist.

Leptictidium tobieni

Main article: Leptictidium tobieni

Fossil of Leptictidium tobieni at the
Hessisches Landesmuseum Darmstadt.

Described by Wighart von Koenigswald and Gerhard Storch in 1987. It was the largest species of all at ninety centimetres long. It is one of the species found in the Messel pit, in Lutetian strata. The species is dedicated to Heinz Tobien, descriptor of the genus Leptictidium and promotor of research in the Messel pit during the 1960s. The holotype is a complete and perfectly preserved skeleton of an adult specimen which was uncovered in September 1984 and which can be found at the Hessisches Landesmuseum Darmstadt. There is also a paratype; a non-complete and badly preserved specimen which can be found at the Institut Royal des Sciences Naturelles de Belgique.

It has a relatively robust mandible with a relatively large mesostyle. The molariform premolar teeth are a characteristic of the Leptictidium genus as a whole which is very marked in the P4 premolars of L. tobieni. The well-developed mesostyle and the transversal configuration of the upper molars are other typical traits of this species.

Comparison of the Messel species

By observing the clear morphological differences in the dentition of the three species found in Messel, the possibility can be discarded that either the discovered fossils are specimens of the same species but of different age, or that two of these forms belonged to the same species with a marked sexual dimorphism.

The Messel species developed very quicly a series of characteristical evolutionary traits, common to all of them, which separe them from the lagerstätte of Quercy.

This table compares the size of different specimens of each species found in the Messel pit (sizes in millimetres).

species	L. auderiense		L. nasutum			L. tobieni
specimens	LNK Me 418a	SMF 78/1	SMF ME 1143	LNK Me 576	HLMD Me 8059	HLMD Me 8011a	I. R. Sc. N. B. M1475
Cranium	67.4	67.2	88.2	89.8	88.6	101.1	-
Head+torso	220	215	300	295	328	375	-
Tail	(340)	375	450	445	454	500	-
Humerus	31.8	32.3	41.9	41.9	43.1	45.7	46.6
Ulna	29.4	31.8	45.6	44.2	45.3	51.2	-
Radius	21.5	23.9	31.7	31.1	33.2	37.0	36.2
Pelvis	54.5	56.5	77.8	77.1	-	85.7	(82)
Femur	54.2	57.5	75.3	71.5	75.1	84.8	(82)
Tibia	64.4	66.3	79.7	79.0	80.1	91.6	90.5
Fibula	57.4	58.7	75.2	72.3	-	86.6	-
Calcaneus	22.3	22.6	25.3	25.2	-	26.7	25.4
Neurocranium †	33.0	31.6	38.7	39.3	37.5	42.2	42.5
Height of the lower jaw below M2	5.7	5.6	6.9	6.6	6.4	9.0	9.5
Height of the mandibular ramus ††	14.3	-	20.8	23.3	23.0	29.2	28.5

† From the front edge of the orbit.
†† Above the incisura praeangularis.

Source: von Koenigswald, W. & Storch, G. (1987) Leptictidium tobieni n sp., ein dritter Pseudorhyncocyonide (Proteutheria, Mammalia) aus dem Eozän von Messel – Cour. Forsch.-Inst. Senckenberg, 91: 107-116, 9 Abb.

Evolutionary tendencies

In his work Quelques insectivores primitifs nouveaux de l'Eocène supérieur du sud de la France (1989), French paleontologist Christian Mathis studied the evolutionary tendencies of the genus Leptictidium, based on the comparison of the most primitive and the most recent species. From this observations, Mathis remarks:

• an increase in size;
• a precocious merging of the hypoconulid and the entoconid on M3;
• a slight reduction of the width of the talonid on M3 in comparison to the anterior molars;
• the formation and development of a mesostile on the molariform jugal teeth P4-M3;
• the reduction of the parametastylar and metastylar regions of these same teeth (with some exceptions), in particular with a reduction of the parastylar lobe which rises less in the anterior part of M3 and possibly P4;
• a transverse shortening of the upper molarised teeth, which become more square;
• the development of accessory conuli on the preprotocrista and postprotocrista;
• the development of the postcingulum.

Related species

Theodore Gill described the family Leptictidae in 1872.

Together with the genus Pseudorhyncocyon, the five Leptictidium species form the family Pseudorhyncocyonidae, one of the three which form the order Leptictida. These families and genera are:

Order Leptictida

• Family Gypsonictopidae (Van Valen, 1967)
  • Genus Gypsonictops (Simpson, 1924)
• Family Leptictidae (Gill, 1872)
  • Genus Leptictis (Leidy, 1868)
  • Genus Myrmecoboides (Gidley, 1915)
  • Genus Ongghonia (Kellner & McKenna, 1996)
  • Genus Palaeictops (Matthew, 1899)
  • Genus Prodiacodon (Matthew, 1929)
  • Genus Xenacodon (Matthew & Granger, 1921)
• Family Pseudorhyncocyonidae (Sigé, 1974)
  • Genus Leptictidium (Tobien, 1962)
  • Genus Pseudorhyncocyon (Filhol, 1892)

Because Leptictida is a paraphyletic clade, this list does not take into account the families or genera with an uncertain or disputed affiliation.

In documentaries

On Walking with Beasts.

A female Leptictidium and its young are the main characters in the first episode of the BBC's paleontology series Walking with Beasts. The episode narrates one day in the life of this animal of the Eocene.

The female Leptictidium appears for the first time after the fall of a micrometeorite, when she meets another specimen of the same species just before dawn. She is forced to flee from a Gastornis, hiding in the nest it has built at the base of a fig tree. Once the sun has risen, the mother goes out to hunt, looking for insects and cold-blooded animals which have not warmed up yet, and she hunts a frog. She reaches the shore of a lake, but she does not approach because an Ambulocetus has arrived from the coast. By midday she is forced to return to her nest despite the little success she has had, since it is the time there are more predators in the forest, and she sleeps until the afternoon.

When the hottest hours are over, the female goes out to hunt again. She uses her hindlegs to prepare the paths she will use to escape if a predator pursues her. This time, the young accompany the mother in her hunt. They are able to escape from the attack of a Gastornis, which ends up killing a Propalaeotherium. This time, they are successful and are able to feed well.

At nightfall, the Leptictidium are some of the few animals which are saved from a cloud of toxic gas, freed by the volcanic lake, thanks to the fact that their nest lay away from the path of the cloud. Instead, the Ambulocetus which had attacked them during the day dies asphyxiated by the carbon dioxide, and the following morning the Leptictidium pass next to it, knowing it is dead.

External references

BBC Online Science and Nature