eutherian

3

Basilosaurus

Mounted skeleton on display at the Smithsonian.  

When: Late Eocene (40-34 million years ago)

Where: North American and the MIddle East. 

What: Basilosaurus is an extinct whale. The word ‘saurus’, which means lizard, is in its name as the first fossils found in the early 1800s were isolated vertebra which were misidentified as reptilian. Soon thereafter additional material was found which easily diagnosed Basilosaurus as a form of whale.  However, due to the rules of nomenclature, the first name that is applied is the name of the taxon for all time. The first Basilosaurus material was found in Louisiana, USA and specimens were commonly found throughout the southern United States. At one point vertebra were turning up so often they were used as furniture by the locals. 

Basilosaurus is not closely related to any modern whales, diverging from the cetacean lineage prior to the odontocete (toothed whale) mysticete (baleen whales) spilt. Though it is not the longest whale ever known, at 'only' 72 feet (22 meters) long , it reached this length in a manner unlike modern cetaceans. All of this elongation comes from duplications of vertebra past the ribs whereas modern whales of this length are just overall bigger in all aspects - Basilosaurus was extremely narrow for its body length. Basilosaurus did not swim like modern whales, which move the whole tail as one unit, instead it is thought it swam much more like an eel, with the movement undulating though the long body - though in a vertical fashion rather than horizontal. Another interesting aspect of Basilosaurus is it retained extremely small hind limbs, which were useless in locomotion. These tiny appendages were most likely used as copulatory guides, such as seen in some snakes, to make sure that the proper bits of a mating pair lined up. 

2

Leptictidium

When: Eocene (~48-40 million years ago)

Where: Found at the Messel fossil site in Germany

What: Leptictidium is one of the more common mammals found in the Messel fossil pit in Germany. The adults ranged from about two (~60 cm) to three (~90 cm) feet in length, with most of this length being in the long tail. Leptictidium had extremely short forelimbs relative to the length of its legs, and has been reconstructed as the first bipedal mammal. There is debate as to its precise mode of locomotion, with some researchers proposing that the animal was a fast runner and others suggesting it was saltatorial (hopping). More recent studies have supported a hopping and leaping mode of locomotion. Thanks to the extraordinary preservation of fossils from Messel, we know the tail was bald for much of its length, that Leptictidium had a short ‘trunk’, like the elephant shrews of the modern day, and that this animal ate insects and small vertebrates. Contemporaries of Leptictidium include the tiny horse Propalaeotheirum and the predatory giant flightless bird Gastornis

Leptictida is the larger clade that includes Leptictidium and its kin. The first members of this group appear in the latest Cretaceous of western North America and the order quickly spreads throughout the northern continents, lasting until the early Oligocene about 30 million years ago, when the forests worldwide started to give way to grasslands. Previously leptictids were thought to be related to either the living lipotyphla (hedgehogs, shrews, and moles) or elephant shrews, but recent studies of the relationships of mammals have placed them outside of placental mammals entirely, making them stem eutherians and not members of Placentalia.