Your Daily Fossil

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. 

Macrauchenia

Skeleton on display at the American Musuem of Natural History, NYC. 

Reconstruction is part of the traveling exhibit Extreme Mammals which started at the American Museum of Natural History. 

When: Late Miocene to Late Pleistocene (7 million to 20,000 years ago)

Where: South America 

What: Macrauchenia is a hoofed mammal from South America. This animal has been known to science for a very long time. The first fossils were found by none other than Charles Darwin when he was traveling on the Beagle. He gathered up the fossilized  vertebra and limb bones and brought them back to England, where they were studied by Richard Owen, who coined the name Macrauchenia (meaning ‘long neck’), and supposed the whole animal would have resembled a llama. Later fossil finds, including several almost complete specimens, confirmed that Macrauchenia  did somewhat resemble a llama, with its slender legs and long neck. However, it was very diffent in some critcal areas, such as having 3 hoofed toes per foot and a mobile trunk. How do we know this animal had a trunk from just the bones? In living mammals with long trunks (such as the elephant and the tapir) the skull is transformed for the musculature that allows such a structure to move, and the skull of Macrauchenia has many features which closely match that of these modern trunked species. 

So with this long llama-like neck and the tapir-like trunk, how does Macrauchenia fit into the mammal family tree? That is a subject of much debate, but it is certain that it is not especially closely related to either artiodactyls (llamas) or  perissodactyls (tapirs).  Macrauchenia is in the order Litopterna, a group of mammals which is only found in South America. Litopterna is assuredly an order of placental mammals, but its exact placement relative to the other major clades is uncertain at this time. It has been suggested they, and other South American ungulate groups, may fall somewhere close to Afrotheria. 

South America  was isolated from all others for millions of years, in ‘splendid isolation’, during which time the mammals upon it radiated to fill all available niches, and this resulted in dozens of cases of convergent or parallel evolution. There are ungulate fossil froms known from South America that closely resemble not only llamas but also horses, rabbits, and even elephants! Carnivorous forms got in on the act too, such as Thylacosmilus, which looked very simular to the Saber-toothed ‘tigers’ of North America. Many South American natives went extinct during the great faunal interchange, but Macrauchenia survived until the end of the last glacial period. There is hope that one day we might recover some ancient DNA of this animal, which would be very helpful in determining where it falls in the great family tree of placental mammals. 

Mammuthus exilis - The Channel Islands Mammoth or Pygmy Mammoth

When: Late Pleistocene to Early Holocene (~50,000 to 11,000 years ago)

Where: The Channel Islands off the coast of California, USA. 

What: Mammuthus exilis is a tiny mammoth.  The largest specimens found stood only 7 feet (~2 meters) tall at the shoulder and smaller full grown individuals reached only 4.5 feet (~1.4 meters). This species of mammoth has been found on several of the Channel Islands, which lie off the coast of Southern California. M. exilis is descendant from the Columbian Mammoth (Mammuthus columbii) which roamed throughout North America. 

Yesterday the giant rabbit of Minorca showed an excellent example of one side of the island effect, and this little mammoth gives us the other end of the scale. Giant herbavores become small for a two main reasons: first there is just not enough food on an island to support a population of full sized mammoths for very long, but secondly there is nothing to prey upon the mammoths if they stay small. Therefore, selection pressures only push for smaller and smaller individuals. M. exilis was not a wooly mammoth, and is not closely related to other known examples of dwarf proboscideans. Island dwarfism is something the elephant lineage has undergone time and time again in their evolutionary history. 

When the Channel Islands mammoth established a population on the islands they were not the separate land masses of today, but instead were one large island that is called Santa Rosae. At the end of the last glacial event, the waters rose and the modern Channel Islands were formed. It has been proposed that this shrinking habitat and changing vegetation on the islands stressed the pygmy elephant too much. 

Mammoth exilis is another great example of allometry; the tiny mammoth is not just a large mammoth shrunken down. It is nothing but head and tusks instead! 

Archaeotherium - The Hell Pig

Mounted specimen on display at the American Museum of Natural History, NYC

When: Oligocene (38-25 million years ago)

Where: North America 

What: Archaeotherium was one of the ferocious predators of the North American Oligocene. This nasty animal stood 4 feet (~1.2 meters) at the shoulder and weighed up to 600 lbs (~270 kilos). It was a dominate predator in its time, hunting animals much larger than itself. Numerous skeletons of rhinoceros relatives have been found with tooth marks on the bone which line up exactly with Archaeotherium dentition. Some skulls of Archaeotherium have simular markings upon them, suggesting these animals fought with one another quite often. In the reconstruction above a pack of them have successively hunted several Poebrotherium, a relative of camels. 

Archaeotherium  has been called the ‘hell pig’ or ‘giant boar’. This animal did fill an ecological niche very simular to that of wild boars today, hunting vertebrate prey and supplementing their diet with plant material. However, it is not in the pig family, but a member of the extinct group Entelodontidae. This group is within the larger clade Artiodactyla (even toed ungulates) but they are more closely related to hippos and whales than to pigs.

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. 

Epicyon haydeni - the largest dog 

When: Mid to Late Miocene (~20 to 5 million years ago) 

Where: Throughout much of  North America, excepting northern Canada. 

What: Epicyon haydeni is the largest canid known. It is estimated to have weighed in at roughly 375 lbs (~170 kg).  Even though it was the size of a bear, it still retained the relatively long legs and resulting fast speed that characterizes dogs. These dogs were not just ‘scaled up’ wolves, they were much more solidly built in general and had teeth more adapted for bone crunching. While they were top predators, and perhaps hunted in packs, they were no doubt also scavengers - able to crush bone in order to eat what had been left behind by other hunters. 

Epicyon is a genus in the clade Borophaginae. This is one of the three major subclades of the dog family. The last common ancestor of the borophagines and the modern canines lived over 30 million years ago. While this subclade is characterized by large bone crushing dogs, it also contained dogs which more more resemble living forms such as the wolf. In the reconstruction image the large dog is Epicyon haydeni and the smaller is another member of the same genus.   On the whole, borophagines were more omnivorous than their canine relatives.