BackgroundProtocetidae are middle Eocene (49–37 Ma) archaeocete predators ancestral to later whales. They are found in marine sedimentary rocks, but retain four legs and were not yet fully aquatic. Protocetids have been interpreted as amphibious, feeding in the sea but returning to land to rest.Methodology/Principal FindingsTwo adult skeletons of a new 2.6 meter long protocetid, Maiacetus inuus, are described from the early middle Eocene Habib Rahi Formation of Pakistan. M. inuus differs from contemporary archaic whales in having a fused mandibular symphysis, distinctive astragalus bones in the ankle, and a less hind-limb dominated postcranial skeleton. One adult skeleton is female and bears the skull and partial skeleton of a single large near-term fetus. The fetal skeleton is positioned for head-first delivery, which typifies land mammals but not extant whales, evidence that birth took place on land. The fetal skeleton has permanent first molars well mineralized, which indicates precocial development at birth. Precocial development, with attendant size and mobility, were as critical for survival of a neonate at the land-sea interface in the Eocene as they are today. The second adult skeleton is the most complete known for a protocetid. The vertebral column, preserved in articulation, has 7 cervicals, 13 thoracics, 6 lumbars, 4 sacrals, and 21 caudals. All four limbs are preserved with hands and feet. This adult is 12% larger in linear dimensions than the female skeleton, on average, has canine teeth that are 20% larger, and is interpreted as male. Moderate sexual dimorphism indicates limited male-male competition during breeding, which in turn suggests little aggregation of food or shelter in the environment inhabited by protocetids.Conclusions/SignificanceDiscovery of a near-term fetus positioned for head-first delivery provides important evidence that early protocetid whales gave birth on land. This is consistent with skeletal morphology enabling Maiacetus to support its weight on land and corroborates previous ideas that protocetids were amphibious. Specimens this complete are virtual ‘Rosetta stones’ providing insight into functional capabilities and life history of extinct animals that cannot be gained any other way.

BackgroundThe best European locality for complete Eocene mammal skeletons is Grube Messel, near Darmstadt, Germany. Although the site was surrounded by a para-tropical rain forest in the Eocene, primates are remarkably rare there, and only eight fragmentary specimens were known until now. Messel has now yielded a full primate skeleton. The specimen has an unusual history: it was privately collected and sold in two parts, with only the lesser part previously known. The second part, which has just come to light, shows the skeleton to be the most complete primate known in the fossil record.Methodology/Principal FindingsWe describe the morphology and investigate the paleobiology of the skeleton. The specimen is described as Darwinius masillae n.gen. n.sp. belonging to the Cercamoniinae. Because the skeleton is lightly crushed and bones cannot be handled individually, imaging studies are of particular importance. Skull radiography shows a host of teeth developing within the juvenile face. Investigation of growth and proportion suggest that the individual was a weaned and independent-feeding female that died in her first year of life, and might have attained a body weight of 650–900 g had she lived to adulthood. She was an agile, nail-bearing, generalized arboreal quadruped living above the floor of the Messel rain forest.Conclusions/Significance Darwinius masillae represents the most complete fossil primate ever found, including both skeleton, soft body outline and contents of the digestive tract. Study of all these features allows a fairly complete reconstruction of life history, locomotion, and diet. Any future study of Eocene-Oligocene primates should benefit from information preserved in the Darwinius holotype. Of particular importance to phylogenetic studies, the absence of a toilet claw and a toothcomb demonstrates that Darwinius masillae is not simply a fossil lemur, but part of a larger group of primates, Adapoidea, representative of the early haplorhine diversification.

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Worn enamel surfaces of the cheek teeth in fossil and recent rhinoceroses are characterized by fine, parallel ridges aligned perpendicular to the enamel-dentin interface. We show that these ridges result from an unusual enamel ultrastructure in which a primitively horizontal layering of the prisms has become vertical. The new structure apparently appeared between early and middle Eocene, at the time when the superfamilies of perissodactyls were rapidly diverging. Similar modifications of the enamel structure occurred in certain parts of the cheek teeth in tapiroids, chalicotherioids and brontotherioids, but hardly at all in the equids. The modified enamel structure, where it occurs in groups other than rhinocerotoids, is associated with lophs but not cusps. Experimental evidence shows that the modified enamel is more resistant to wear than the unmodified enamel. The consistent association with thin lophs rather than cusps suggests that the modified enamel evolved to prolong the life of the lophs, where occlusal pressures are highest and attrition greatest. The dominance of modified enamel in rhinocerotoids correlates with the higher degree of compression of the cusps and extreme lophodonty in this group. The absence of modified enamel structure in the equids, even in the ectoloph, correlates with the lesser importance of the ectoloph in equids relative to brontotherioids, chalicotherioids and rhinocerotoids.

Hunter-Schreger bands (HSB) are seen in teeth that are composed of crossed sets of enamel prisms. They are present in the teeth of man and many other mammals, but absent in most insectivores and multituberculates. It has been suggested that the presence of HSB makes the tooth enamel less likely to split and is associated with chewing ability. We have traced the occurrence of HSB back to the arctonic condylarths of the early Palaeocene (Puercan) age; this must be close to the first appearance of the bands in placental mammals. Our data indicate that the teeth of almost all large mammals since the early Palaeocene have contained these bands, in an orientation that is optimal for limiting the propagation of vertical fractures. The appearance of the bands is associated with the differentiation of herbivores and carnivores from insectivores and our data indicate that their development was critical to the diversification of mammals because it allowed the use of new types of foods.