Leptictidium is anextinctgenus of smallmammals that were likely bipedal. Comprising eight species, they resembled today'sbilbies,bandicoots, andelephant shrews, and occupied a similar niche. They are especially interesting for their combination of characteristics typical of primitiveeutherians with highly specialized adaptations, such as powerful hind legs and a long tail which aided in locomotion. They were omnivorous, their diet a combination ofinsects,lizards,frogs, and smallmammals.Leptictidium and other leptictids are notplacentals, but are non-placentaleutherians, although they are closely related to placental eutherians. They appeared in the LowerEocene, a time of warm temperatures and high humidity, roughly fifty million years ago. Although they were widespread throughoutEurope, they became extinct around thirty-five million years ago with no descendants,^[1] as they were adapted to live inforestecosystems and were unable to adapt to the open plains of theOligocene.

Leptictidium Temporal range: Early to Late Eocene,50–35 Ma PreꞒ Ꞓ O S D C P T J K Pg N
FossilL. auderiense skeleton,Muséum national d'histoire naturelle
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	†Leptictida
Family:	†Pseudorhyncocyonidae
Genus:	†Leptictidium Tobien, 1962
Type species
†Leptictidium auderiense Tobien, 1962
Species
L. auderienseTobien, 1962 L. nasutumLister & Storch, 1985 L. tobieniKoenigswald & Storch, 1987 L. ginsburgiMathis, 1989 L. sigeiMathis, 1989 L. listeriHooker, 2013 L. proutiHooker, 2013 L. storchiHooker, 2013

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 lateralphalanges 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.^[2]

Theankles and thesacroiliac joint were quite loosely fixed, while thepelvis had a flexible joint with only onecoccygeal vertebra. The anteorbitalmusclefenestrae in their crania suggest they probably had a long and mobilesnout (orProboscis), similar to that of elephant shrews.

Leptictidium had widediastemata in the antemolar row,^[3] its uppermolar teeth were more transverse than those of the North Americanleptictids and its fourthpremolars were molariform.^[4] Its C1canines were incisiviform. Itsdentition was quite small in comparison to the size of themandible and the animal as a whole.^[3]

It varied between 60 and 90 cm (24 and 35 in) in length^[5] (more than half of which belonged to the tail),^[6] and 20 cm (7.9 in) in height. It weighed a couple of kilograms.^[7] These sizes couldvary from one specimen to another.Leptictidium tobieni fromMessel (Germany) isthe 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.^[8]

One of the mysteries aboutLeptictidium is whether it moved by running or by jumping. Because there are very few completelybipedal mammals, it is difficult to find an appropriate living model to compare it with. If thekangaroo is used, it is probable thatLeptictidium hopped along with its body tilted forward, using its tail as a counterweight.^[7] On the other hand,elephant shrews combine both types oflocomotion; they usually moveon four legs, but they can run on two legs to flee from a predator.^[6] Studies of the bone structure ofLeptictidium have yielded contradicting information: its legarticulations 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 speciesL. nasutum with theleptictidLeptictis dakotensis.L. dakotensis had a series of traits which show it was a running animal which sometimes moved by jumping. Despite the marked similarities betweenLeptictis andLeptictidium, there are certain differences in their skeletons which prevent the example ofLeptictis from being used to determine with certainty the wayLeptictidium moved: the most important being that, unlikeLeptictis, thetibia and thefibula ofLeptictidium were not fused together.^[9]

Perfectly preservedfossils of three different species ofLeptictidium have been found in theMessel pit inGermany. The marks on their fur have been preserved, as well as theirstomach contents, which revealLeptictidium wereomnivores which fed oninsects,lizards and smallmammals.^[10] Theholotype ofL. tobieni also had pieces ofleaves and notable amounts of sand in its abdomen, but it cannot be determined with certainty if the animal swallowed it.^[3]

Leptictidium lived in theEuropeansubtropicalforests of theEocene. From the beginning of this period, the temperature of the planet rose in one of the quickest (ingeological terms) and most extreme episodes of global warming in the geological record, termedPaleocene–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.^[11] The thermal maximum caused a great extinction which is used to distinguish the Eocenefauna from that of thePaleocene.

The global climate of the Eocene was probably the most homogeneous of theCenozoic; thetemperature gradient from theequator to thepoles was half that of today's, and the deepocean currents were exceptionally warm. The polar regions were much warmer than today, possibly as warm as the present-dayPacific Northwest of North America. Temperate forests reached the poles themselves, while rainy tropical climates reached45° N. The greatest difference was in temperate latitudes; nevertheless, the climate at thetropics was probably similar to today's.^[12]

In the Eocene, most of what is nowEurope, theMediterranean and south-westAsia was submerged under theTethys Sea. These twocontinents were separated by theTurgai Strait (anepeiric sea).^[13] Due to high humidity and temperatures, most of the European continent was covered invegetation.

The region which today isGermany was in avolcanically active zone during the Eocene. It is thought that theMessel pit could have been the old location of a volcanic lake saturated withCO₂. 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.^{[citation needed]}

In the lush forests of this region,Leptictidium shared its habitat with animals such asGodinotia,Pholidocercus,Palaeotis, orPropalaeotherium. There were also predators, the crocodilianAsiatosuchus, thehyaenodontLesmesodon, and theMessel giant ant.^[14]

The genusLeptictidium includes eightspecies. These include:

Described byHeinz Tobien in 1962 based on a series oflower jaws from theLutetian faunal stage. Tobien also uncovered a small skeleton he defined as aparatype of the species, but Storch and Lister proved in 1985 that, in fact, the skeleton did not even belong to thegenusLeptictidium.^[3] It was the smallest species of all and was only sixty centimetres long. Several skeletons have been found at the Messel pit.^[15] Mathis remarks the exceptional development of the paraconid (or mesiobucal cusp) of the lower P4premolar.^[16] Its premolars andmolars were quite small in comparison to the dentition as a whole. The name of the species refers to theRoman settlement ofAuderia.

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), inUpper Ludian strata. The mesostyle typical of the genusLeptictidium is not developed in this species.^[16] The species is dedicated toLéonard Ginsburg,Frenchpaleontologist and deputy director of theMuséum national d'histoire naturelle inParís.

Described byAdrian Lister andGerhard Storch in 1985. It was a middle-sized species which was seventy-five centimeters long. Several skeletons have been found in the Messel pit,^[15] inLower Lutetian strata. The tail of this species had 42-43vertebrae,^[2] a number surpassed amongmammals solely by thelong-tailed pangolin. Itspremolar andmolar teeth were quite small in comparison to the dentition as a whole. The name of the species refers to thenose of the animal. Theholotype is the complete skeleton of an adult specimen kept in theForschungsinstitut Senckenberg inFrankfurt am Main.^[2]

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 thanL. 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.^[16] The species is dedicated toBernard Sigé,Frenchpaleontologist.

Described byWighart von Koenigswald andGerhard 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,^[15] inLutetian strata. The species is dedicated toHeinz Tobien, descriptor of the genusLeptictidium and promoter of research in the Messel pit during the 1960s. Theholotype 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.^[3] 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 robustmandible with a relatively large mesostyle. The molariform premolar teeth are a characteristic of the genusLeptictidium as a whole which is very marked in the P4 premolars ofL. tobieni. The well-developed mesostyle and the transversal configuration of the upper molars are other typical traits of this species.^[3]

By observing the clear morphological differences in the dentition of the three species found in Messel, the possibility can be discarded that either the discoveredfossils are specimens of the same species but of different age, or that two of these forms belonged to the same species with a markedsexual dimorphism.^[3]

The Messel species developed very quickly a series of characteristic evolutionary traits, common to all of them, which separate them from the lagerstätte ofQuercy.^[16]

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

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

In his workQuelques insectivores primitifs nouveaux de l'Eocène supérieur du sud de la France (1989),Frenchpaleontologist Christian Mathis studied the evolutionary tendencies of the genusLeptictidium, based on comparison of the most primitive and the most recent species. From his observations, Mathis remarks:^[16]

• 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.