The South Central Salient of the Helmeted Musk-ox (Ovibos cavifrons or Bootherium bombifrons)

Portrait of the woodland or helmeted musk ox from the Illinois Museum of Natural History

The way scientists assign scientific names to extant and extinct species can be confusing.  The rule is simple–the scientific name given by the scientist who first describes the first specimen ever discovered (also known as the “type specimen”) is the one that supercedes all other later names.  Complications arise when other scientists discover new fossil material of the same species and mistakenly believe they’ve discovered a new species, and they assign a completely different scientific name to the specimen.  Decades often pass before other scientists compare the fossils and determine they’re one and the same species.  The extinct helmeted musk-ox provides a case study of this confusion.  Scientists discovering fossils of this wide-ranging species from all over the continent have assigned many different scientific names to this species including Bos bombifrons, Bootherium bombifrons, Symbos cavifrons, Ovibos cavifrons, Bison appalachicolus, Liops zuniensis, Symbos australis, Bootherium nivicolum,  Symbos converifrons, Ovibos giganteus, and Bootherium brazosis.  This sounds like a joke, but it’s not.  It created a real mess for later scientists to straighten out.

Fossil hunters first found a helmeted musk-ox fossil in 1804 at the Big Bone Lick Fossil site in Kentucky.  The first scientist to look at it put it in the Bos genus which is the same one domesticated cattle belong to.  It wasn’t until 50 years later that it was recognized as a type of musk-ox.  Extreme sexual dimorphism caused the proliferation of species names for just one animal.

The horns on a male helmeted musk-ox were much larger and robust than those on the female.  Scientists mistakenly thought they were from two different species.  Note the difference between the male and the female horns (f & e) in this diagram.  Diagram from the below referenced paper written by R. Dale Guthrie.

Male helmeted musk-oxen are so much larger than females that for a long time scientists believed they were seeing different species: males were classified as Symbos cavifrons, and females were classified as Bootherium bombifrons.  Finally, in a paper written in 1989 Jerry McDonald and Clayton Ray thoroughly studied all the fossil material and determined they were one and the same species.  Because Bootherium bombifrons was the older name, that is now considered the official scientific name for the species.  I mistakenly thought Ovibos cavifrons was the official name, and it makes sense because its close living relative, the woolly musk-ox goes by the scientific name of Ovibos moschatus. It seems the woolly and helmeted musk-ox should be in the same genus, but they’re not.

I’ve noticed a curious geographical distribution of helmeted musk-ox fossils.

Distribution map for helmeted musk-ox.  This map includes just the skulls, so I added a dot for a musk-ox fossil found in Nashville, Tennessee, and drew the line to illustrate the salient.  Helmeted musk-ox must have been an adaptable animal to range this far.  Map from the below referenced paper written by McDonald and Ray.

Helmeted musk-oxen fossils come from Bayou Sara in south central Louisiana, Texas, and Mississippi north to Alaska and west to northern California and east to the continental shelf off what today is New Jersey.   Their range seem to have extended south along the Mississippi River.  A south central salient of its range is notable.  Fossil sites in the piedmont region of the southeast are so rare there is no telling where its southeasternmost range occurred.  None have been found in Florida’s abundant sites, indicating its probable absence there.  Fossil sites in Nashville, Tennessee and Saltville, Virginia yield helmeted musk-oxen specimens, but it may have been absent from the southeastern coastal plain.  Looking at the line I drew on the above map, it’s likely helmeted musk-oxen did range over northern Alabama and the ridge and valley region of northwestern Georgia. 

If they could survive the warmer climates of Mississippi, Louisiana, and Texas, it couldn’t have been climate that prevented them from colonizing the southeastern coastal plain.  Perhaps, because long-horned bision were already abundant here and occupied a similar niche, helmeted musk-oxen never could become established.  Maybe, it’s just chance they never migrated into the region.  Or maybe they were present but in such low numbers as to be invisible in the fossil record. 

The extinct helmeted musk-ox differed from the living woolly musk-ox in many characteristics.  As evident from its much wider range, the helmeted musk-ox was much more adapatable.  It had longer legs allowing it to run greater distances and with more speed.  It was taller and more slender, suggesting it had more endurance.  Scientists found fur on one specimen.  The hair was shorter and finer–evidence that it did not have a heavy woolen coat like its living cousin.  The two species did coexist in parts of Alaska and unglaciated Canada, but Dr. Guthrie sees evidence of habitat parturation.  Helmeted musk-oxen lived in arid grassy steppes in this region whereas woolly musk-oxen inhabited more mesic tundra.  Dr. Guthrie favors the common name helmeted musk-ox over woodland musk-ox because the species was just as likely to be found on grassy steppes and savannahs as open parkland woods.  I agree with him and from now on will refer to it on this blog as the helmeted musk-ox.

Evidence from fossilized feces and plant fragments in fossil teeth show that helmeted musk-oxen ate a wide variety of plant foods including sedges, grasses, blueberry bushes, and willow twigs.  In fact one sample of fossilized dung consisted of nothing but willow twigs.  Some times their feces was pelletized.  Dr. Guthrie states that this is evidence of a capability to eat a lot of dried plant foods without having to eat snow for water.  It’s an adaptation of moisture conservation, enabling the species to survive frigid arid environments. 

Helmeted musk-oxen were able to live in temperate climates where human populations eventually became high.  I believe this is the reason they became extinct while woolly musk-oxen avoided human overhunting in remote regions of the arctic.  It’s ironic that an animal that was more adapatable died out because of this adaptability.  I disagree with scientists who think natural environmental changes caused their extinction.  This highly adaptable animal enjoyed a wide range during the Pleistocene as the above map shows.  This means they inhabited a great variety of habitats from Alaskan grassy steppe to prairies to open forests interspersed with meadows.  Suitable environments  for helmeted musk-oxen, especially in the midwest and upper south, never completely disappeared.  The foods they ate–grasses, sedges, blueberry bushes, willow trees–always remained abundant throughout much of the continent.  Moreover, their fossils are known from deposits as old as 300,000 years BP, meaning they survived previous transitions from glacial to interglacial.  I see no change in the environment that would cause their extinction, other than the increase in the number of people hunting them.  Dr. Guthrie suggest bison outcompeted musk-oxen on the prairies, but this makes little sense–musk-oxen and bison coexisted for hundreds of thousands of years.

Woolly musk-oxen survived extinction because they inhabit remote regions of the arctic.  Even here, human hunting reduced their numbers.  Human hunters wiped out woolly musk-oxen  in Alaska before the introduction of firearms, though the beasts have been reintroduced here.  Still, there are large areas of suitable habitat in Alaska where woolly musk-oxen are absent due to human hunting.   During the Ice Age woolly musk-oxen were more widespread living as far south as Spain.  Undoubtedly, climate change and the resulting contraction of barren tundra has played a role in diminishing habitat for this species, but suitable habitat for helmeted musk-oxen expanded following the retreat of the glaciers, especially in the midwest.  Suitable habitat in the upper south remained the same. 

I hypothesize men exploited the musk-ox’s habit of standing in a defensive circle to face down predators.  It may prove effective against wolves or big cats as long as they don’t panic and begin running (as seen in this video http://www.youtube.com/watch?v=pb6Rke7jiTc ) but it was catastrophically fatal for helmeted musk-oxen when a gang of humans confronted them.  Humans could use projectile weapons to slaughter entire herds standing stationary in one place.

Some scientists claim the scant evidence that humans hunted helmeted musk-oxen suggests they played no role in their demise.  This argument frustrates me.  There isn’t a single skeleton of a human from the Clovis era in North America.  Why should we expect to find remains of their meals?  Nevertheless, there is some evidence of humans exploiting helmeted musk-oxen.  Clovis age arrowheads associated with musk-oxen bones and with bovine blood on them were found at the St. Mary’s reservoir in Alberta, Canada.  And a human “modified” musk-oxen bone turned up at the fossil site in Saltville, Virginia (a pre-clovis site).  Scientists who argue against the human overkill model of extinction never say how much evidence would be enough to convince them.

References:

Guthrie, R. Dale

“New Paleoecological and Paleoethological Information on the Extinct Helmeted Musk-oxen from Alaska”

Ann. Zool. Fenninci 28: 175-186 Feb. 1992

McDonald, Jerry; and Clayton Ray

“The Autochthonous North American Musk-Oxen Bootherium, Symbos, Gidleyea (Mammalia: Artiodactyl: Bovidae)”

Smithsonian Contributions to Paleobiology #66 1989

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Tags: Bayou Sara Louisiana, Bootherium bombifons, helmeted musk ox, Jerry McDonald, Ovibos cavifrons, Ovibos moschatus, R. Dale Guthrie, woodland musk-ox