New Primate Genus Provides Evidence of Land Bridges between Old World and New

Texas, that vast state in the southern part of the United States may be synonymous with westerns, cattle and stetsons but forty-three million years ago the area was a tropical paradise, one of the last places in North America to suffer from global cooling as the world got drier and colder.  Flourishing in this verdant, tree covered world was a primate, an animal similar to a small, modern lemur in appearance.

The fossil evidence was found in the Devil’s Graveyard area of the Badlands of western Texas, although not complete, remains of the jaw bone have been discovered providing an insight into the animal’s facial characteristics and diet.  The animal has been formally named Mescalerolemur horneri.

M. horneri lived during the Eocene Epoch about 43 million years ago.  It is a member of an extinct primate group –- the Adapiforms, that were found throughout the Northern Hemisphere in the Eocene.  However, just like Mahgarita stevensi, a younger fossil primate found in the same area in 1973, Mescalerolemur is more closely related to Eurasian and African Adapiforms than those from North America.

Chris Kirk, Associate Professor in the Dept. of Anthropology at the University of Texas commented:

“These Texas primates are unlike any other Eocene primate community that has ever been found in terms of the species that are represented.  The presence of both Mescalerolemur and Mahgarita, which are only found in the Big Bend region of Texas, comes after the more common Adapiforms from the Eocene of North America had already become extinct.  This is significant because it provides further evidence of faunal interchange between North America and East Asia during the Middle Eocene.”

By the end of the Eocene, primates and other tropically adapted species had all but disappeared from North America due to climatic cooling, so Kirk is sampling the last burst of diversity in North American primates.  With its lower latitudes and more equable climate, West Texas offered warm-adapted species a greater chance of survival after the cooling began, a sort of safe haven against the rigours of climate change.

Associate Professor Kirk stated that Marie Butcher, a then undergraduate who graduated with degrees in anthropology and biology from The University of Texas at Austin, found the first isolated tooth of Mescalerolemur in 2005.  Since that time, many more primate fossils have been recovered by Kirk and his team at a locality called “Purple Bench.”  This fossil locality is three to four million years older than the Devil’s Graveyard sediments that had previously produced Mahgarita stevensi.

Kirk stated:
“I initially thought that we had found a new, smaller species of Mahgarita.”

However, as more specimens were prepared at the Texas Memorial Museum’s Vertebrate Paleontology Lab, Kirk realised he had discovered not just a new species, but a new genus that was previously unknown to science.

Fossils of Mescalerolemur reveal it was a small primate, weighing only about 370 grams, less than an IPad. This body weight is similar to that of the living greater dwarf lemur.  Mescalerolemur’s dental anatomy reveals a close evolutionary relationship with Adapiform primates from Eurasia and Africa, including Darwinius masillae, a German fossil primate previously claimed (in a controversial paper) to be a human ancestor.

To read more about Darwinius: Debate over “Ida” Rumbles On

However, the discovery of Mescalerolemur provides further evidence that Adapiform primates like Darwinius are more closely related to living lemurs and bush babies than they are to humans.

For example, the right and left halves of Mescalerolemur’s lower jaws were two separate bones with a joint along the mid-line, a common trait for lemurs and bush babies.  Mahgarita stevensi, the closest fossil relative of Mescalerolemur, had a completely fused jaw joint like that of humans.

The Fossil Jaws of Mescalerolemur horneri

Picture Credit: University of Texas at Austin (USA)

The picture shows Mescalerolemur horneri’s partial upper jaw (in two pieces, at left) and partial lower jaw (at right) (scales = 2 mm). Explaining the significance of the difference in jawbones, Kirk said:

“Because Mescalerolemur and Mahgarita are close relatives, fusion of the lower jaws in Mahgarita must have occurred independently from that observed in humans and their relatives, the monkeys and apes.”

The new genus is named after the Mescalero Apache, native Indians, who inhabited the Big Bend region of Texas from about 1700-1880.  The species name, horneri, honours Norman Horner, an entomologist and professor emeritus at Midwestern State University (MSU) in Wichita Falls, Texas.  Horner helped to establish MSU’s Dalquest Desert Research Site, where the new primate fossils were found.  The taxonomic relationship between the various types of Old World and New World Adapiforms suggest that faunal exchange was taking place over much of the Eocene Epoch, perhaps land bridges existed that permitted this exchange of genera.

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