Tuesday 31 December 2013

A new species of Leptictid Mammal from the Late Eocene of South Dakota.

Leptictid Mammals appeared early in the post-Cretaceous radiation of Mammals, and persisted till the Oligocene. They may have been distant relatives of the Rodents and Primates, though some authors regard them as being the earliest group to break of from the other Placental Mammals. They were small Mammals, the largest reaching about 90 cm, and appear to have been primarily insectivorous in diet. Many Leptictids had reduced forelimbs, and are thought to have had a hopping gait similar to Kangaroos or Jerboas.

In a paper published in the journal Acta Palaeontologica Polonica on 17 August 2011, Terence Meehan of the Section of Vertebrate Paleontology at the Carnegie Museum of Natural History and Larry Martin of the Division of Vertebrate Paleontology at the Natural History Museum and Biodiversity Research Center at the University of Kansas, describe a new species of Leptictid Mammal from the Late Eocene White River Group of South Dakota.

The new species is named Megaleptictis altidens, where 'Megaleptictis' means 'Large Leptictid' and 'altidens' means 'tall-toothed'. The species is described from a nearly complete skull and mandibles, with most of the teeth intact. The skull of Megaleptictis altidens is large for a Leptictid, the snout broad, and the teeth higher and more heavily worn than in other comparable species. 

Megaleptictis altidens (A1), skull in dorsal view, note the postorbital constriction of the parasagittal crests; (A2) lateral view, the teeth and rostrum are tall; (A3) stereo pair, ventral view; (A4) occipital view. A portion of the supraoccipital region is broken away. Scale bars are 10 mm. Meehan & Martin (2011).

Megaleptictis altidens. (B1) Left dentary in lateral view; i2–m2 are well preserved, except for the  fractured crown of p3; the base of i1 and posterior talonid of m3 are present; the posterior ramus is broken away, except for a portion of the condyloid region shown in its approximate position. (B2) Occlusal view of mandibles placed in articulation. (B3). Right dentary in lateral view; complete p3–m2 and the fractured bases of i2–p2 and m3 are present. Meehan & Martin (2011).

The higher teeth and increased dental wear of Megaleptictis altidens compared to other Leptictid Mammals, and indeed other insectivorous Mammals in general, suggests tat that their diet may not have been the same as that of other members of the group. The modern Sengis (or Elephant Shrews) of Africa are often considered to be a good ecological model of for the Leptictids, and also have high teeth. However the fossil record of Sengis suggests that they have only adopted an insectivorous lifestyle quite recently, and that the higher teeth may be a relict from a previous plant-based diet. 

High teeth and heavy dental wear are typically associated with a diet high in plant matter, and the teeth of Megaleptictis altidens show adaptations apparently converging on those of Lagomorphs (Rabbits). This strongly suggests that Megaleptictis altidens may have had a large amount of plant matter in its diet. The late Eocene was a time of drying climate, and increased seasonality, and Megaleptictis altidens probably had to contend with a long dry season when Insects would have been scarce. Meehan & Martin suggest that this increased seasonality and scarcity of Insect food may have caused Megaleptictis altidens to change its diet, consuming a higher proportion of plant matter than earlier Leptictids.