Your Daily Fossil

Toxodon

Mounted specimen on display at Harvard Museum of Natural History

Reconstruction by Roman Uchytel

When: Pleistocene (~2.6 million to 16,000 years ago) 

Where: South America

What: Toxodon is another one of the large herbivorous animals that roamed over South America. Charles Darwin purchased the skull of the first Toxodon known to the Old World during his journey on the Beagle. This skull was sent back to England were Sir Richard Own described it and named the animal Toxodon - ‘bow teeth’ based on the curving nature of its gigantic molars. Soon complete skeletons of this amazing animal were known. The first interpratations reconstructed Toxodon as a semi-aquatic animal, much like the modern hippo, but later studies of the limbs and teeth of speciemens show this was incorrect. Toxodon  was more the analogue of today’s rhinos than a hippo, a fully terrestrial animal with teeth well adapted for grinding tough plants in somewhat arid environments. Some Toxodon specimens have been found associated with arrowheads, showing that the first people to emigrate into South America had contact with these animals, and appear to have hunted them. 

Where does Toxodon fit into the tree of life? Like its contemporary Macrauchenia (which you can see in the background of the reconstruction), its relationship to living mammals is uncertain. It falls into the larger clade of Notoungulata, literally Southern Ungulates, but the placment of this group within placental mammals is highly uncertain. They maybe have a close relationship with animals in the group Afrotheria but research in mammalian systematics is only beginning to be able to evaluate that, and other hypotheses.  So what is Toxodon? We just don’t know. 

Pachycrocuta - The Giant Hyena 

Mounted specimen from the Zhoukoudian Museum, Beijing. 

Reconstruction by Mauricio Antón

When: Pliocene to Pleistocene (~ 5 million to .5 million years ago)

Where: Europe, Asia, and Africa

What:Pachycrocutais a prehistoric member of the Hyaenidae. Today hyenas are restricted to Africa and western Asia, but their fossil record has revealed they were once much more wide spread. Pachycrocuta has been found in Africa and Asia, but most specimens have been found in Europe, with many localities in the Iberian Peninsula. The largest species was Pachycrocuta brevirostris, which stood over 3 and a half feet (~100 cm) at the shoulder and is estimated to have weighed over 400 lbs (190 kg). This makes it about the size of a modern lioness! Cave deposits in both Spain and China have revealed multiple almost complete skeletons, suggesting that these animals lived in packs and utilized these caves as their dens. 

As Pachycrocuta is even more heavyset and adapted for bone crunching than the living bone-crunching hyenas, it has been suggested that this fossil form was even more dependent on scavenging kills than living species. But there really is not much evidence to pack this up other than thought-experiments. As it appears that some large cat species were displaced when Pachycrocuta moved into their ranges, it is more likely it was a direct hunter that would take advantage of pre-killed remains when it could drive away other predators. Like 99% of carnivores today. The predator/scavenger divide is really not a fast or hard line at all. Even more evidence of hunting comes from the remains of interactions between Pachycrocuta and Homo errectus. These two species overlapped and bones of our poor relative have been found in Pachycrocuta dens in China! 

Procoptodon - The giant short faced kangaroo 

Mounted skeleton on display at Victoria Fossil Cave, Naracoorte Caves National Park, South Australia

Reconstruction by Peter Trusler. 

When: Pleistocene (~ 2 million to 15,000 years ago)

Where: Throughout Australia

What: Procoptodon is a giant fossil kangaroo. Exactly how ‘giant’ it is has been a bit exaggerated, heights of up to 10 feet (~3 meters) have been reported, but this would have been its maximum height when it reared up fully on its hind legs, with its arms reaching up for high branches. Procoptodon was capable of this posture, but (like living kangaroos) it did not stand fully upright most of the time. In its normal feeding (and most everything else) poster it would have stood about 6.5 feet (~ 2 meters) tall; about the same height as the largest of the modern red kangaroos. Procoptodon was not the same size as these animals though, it was much more massive and would have been over twice the weight of a red kangaroo of equivalent height. 

Procoptodon was very well adapted for the semiarid conditions that characterized much of Australia during the Pleistocene, but fossil remains have also been found in the more hospitable regions of prehistorical Australia.  The marsupials of Australia are well known for their convergence evolution upon forms from other continents (such as the tasmanian tiger and the marsupial mole), but the kangaroo does not look like any placental mammal known. However, in terms of its lifestyle, the ecological niche that it inhabits, the group is convergent upon hoofed animals, such as deers!  Procoptodon overlapped with human habitation of Australia, and it is thought some Aboriginal folktales are about this massive kangaroo. 

Procoptodon is a member of the group Sthenurinae - the shortfaced kangaroos. As you probably guessed these kangaroos had much shorter snouts than the modern species of kangaroos. This group is completely extinct. It is one of the subgroups of the Macropodidae, the clade of marsupials that contains all kangaroos and wallabies, as well as a few other  groups. It has been proposed that within the Macropodidae the closest living relative of Procoptodon is the Banded hare-wallaby, though this is not universally accepted. 

In the prehistoric outback Procoptodon would have co-exsited with the largest marsupial of all time Diprodoton and was a hunted by the marsupial lion Thylacoleo. And the second link you can see this marsupial predator hunting a close relative of Procoptodon!  

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! 

Gigantopithecus blacki 

Note: There are 3 species within the genus Gigantopithecus, but this write up will be only on G. blacki.

When:Pleistocene (~1 million to 300,000 years ago)

Where: China, India, and Vietnam. 

What: Gigantopithecus blacki is the largest ape known, though it is not know by very much.  The vast majority of fossil evidence we have is isolated teeth and the most complete fossil we have is a partial lower jaw. While we can glean significant information from this material, there is also much about the morphology and life habits of Gigantopithecus that is extremely difficult, if not impossible, to determine. This does not deter researchers from trying, however! 

First for the conclusions that can be at least somewhat confidently drawn from the preserved material. Gigantopithecus is a primate, and within primates is an ape. Within apes its closet living relative is the orangutan, contra earlier research which thought this gigantic ape could be more closely related to the human lineage. The anatomy of its teeth show that it was predominantly, if not entirely, a plant eater, with bamboo as its major food source. It would have had to consume tremendous amounts of bamboo everyday, like the living giant panda. 

While the teeth and jaw material can tell us this animal was much larger than any living ape, it is uncertain exactly how much larger. The most famous size estimate of Gigantopithecus is the largest, which puts it at almost 10 feet (3 meters) tall for the largest specimens known. However, this is all based off the dental evidence, and just because the teeth have grown very large, does not mean the rest of the animal is equally enlarged. This can be seen if the teeth of a giant panda are compared with that of other bears. The skulls can be roughly the same size, but the panda teeth are enormous, to allow it to process the bamboo that it eats. If this is taken into account for Gigantopithecus then a size estimate of only 6 feet (1.8 meters) tall on average is produced, with adult males large than this. 

Thylacoleo - The marsupial lion 

Mounted skeleton located in the Victoria Fossil Cave, Naracoorte Caves National Park, South Australia

Close up reconstruction by Jeanette Muirhead.

Reconstruction of Thylacoleo hunting short faced Kangaroos by Mauricio Anton. 

When: Pleistocene (2 million to 46,000 years ago) 

Where: Australia

What: Thylacoleo is another example of an extinct australian megafauna. It was the largest marsupial predator Australia has ever seen, weighing in at 250 lbs (~115kg) on average, with individuals half again as big occurring with some regularity. The common name of ‘Marsupial Lion’ comes from its large size, shortened face, and retractable claws - the latter making it unique among marsupials. The forearms of this predator were very robust and had a semi-opposable thumbs, allowing them to drag down their prey. This interpretation is supported by the morphology of the hindlimbs and pelvis, which suggests Thylacoleo habitually reared up on its haunches. It had a formidable set of teeth as well, the extremely large sets of shearing teeth gave it the most powerful bite force of any known mammal. A find of eight skeletons in a cave in southern Australia suggests these animals lived in packs. 

Thylacoleo is in the clade Diprotodontia, which contains living koalas, kangaroos, and wombats, but is not closely related to any of these forms. Rather it is in the totally extinct subclade Thylacoleonidae; all the members of this clade were carnivorous, but some were only as small as a house-cat. Like many other large endemic australian mammals, Thylacoleo vanished just under 50,000 years ago. It is thought some aboriginal cave art depicts this lost predator. The genus was named based on material shipped back to the English scientist Sir Richard Owen in the mid 1800s. 

Diprotodon - The Giant Wombat

Mounted specimen on display at the Melbourne Museum, Australia 

Reconstruction by Peter Trusler. 

When: Pleistocene (1.6 million to 46,000 years ago)

Where: Australia

What: Diprotodon is the biggest marsupial to have ever lived. The largest specimens found were roughly the size of an extant hippopotomus; 10 feet (3 meters) long, 6.5 feet (2 meters) tall at the shoulder, and with a weight estimate of over 6,000 lbs (over 2,500 kgs). They inhabited forests and grasslands in Australia, and were herbivores that had an extremely varied diet. There was not much that their large grinding cheek teeth could not process. There are multiple ‘bone-bed’ deposits containing almost nothing but Diprotodon skeletons, offering strong support that they also traveled in herds. Many of these deposits are reconstructed as deaths due to droughts; it took a lot of plant material to sustain a Diprotodon. They occupied  simular niches as large ungulate herds today on other continents. 

The closest living relatives of Diprotodon are koalas and wombats. This was the largest member of the apt named Australian mega-fauna. This giant animal and many other Australian mega-fauna went extinct shortly after the arrival of humans on the continent, in a mirror of the extinction of the North American mega-fauna 10,000 years ago. In both extinction events this colonization was accompanied by climate changes, leading to much debate as to how influential human habitation was on the loss of these forms. It is thought that Diprotodon and its close relatives may be the basis for the bunyip of aboriginal folklore.  

Megalonyx- Jefferson’s ground sloth 

When: Late Miocene to end Pleistocene (~10 million years to 10,000 years ago)

Where: Throughout North America 

What: Megalonyx is a giant ground sloth, that grew to roughly 8-10 feet (~2.5 to 3.0 meters) long.  They are the genus of giant ground sloth most closely related to the living two-toed sloth Choloepus. Sloths originated, and most of them diversified in South America, moving northward during the great American interchange, but Megalonyx is a major exception. Its ancestors reached North America millions of years prior to the massive migrations of other South America taxa; via island hopping. This relatively early arrival allowed it to spread throughout the northern continent. Megalonyx is the only species of sloth to have reached as far north as Alaska and the Yukon. It was common in many of the lower 48 states. Like many ground sloths, Megalonyx went extinct at the end of the last glacial period. 

A more recent historical note about Megalonyx; this genus was the first fossil from the Americas to be described, and the person who did so was none other than Thomas Jefferson. He proposed the name Megalonyx for the genus, based on first material recovered - the gigantic claws. Later this genus name was formalized and a species named in his honor:  Megalonyx jeffersoni (this species is the state fossil of West Virginia). Jefferson was very hopeful that living Megalonyx would be found in the uncharted west, he told Lewis and Clark to be sure to be on the look out for this beast and report back when it was discovered.