The Extinct Helmeted Musk-ox (Bootherium bombifrons) and Appalachian Grassy Balds during the Pleistocene

The tops of some mountains in the southern Appalachians are relatively treeless, resembling a man with a bald spot on his head, hence the name Appalachian bald.  There are 2 types of balds–grassy and heath.  Grassy balds are open environments surrounded by forests of red spruce, yellow birch, red oak, chestnut oak, and buckeye.  Heath balds consist of shrubby plants that prefer acid soils including rhododendron, azalea, and blueberry. Peter Weigl theorizes Appalachian balds are relic communities of ancient origin.  He believes harsh climates during Ice Age glacial maximums killed the trees growing at higher elevations in the southern Appalachians.  Prolonged sub-zero temperatures, frequent ice storms, and windy conditions increased tree mortality.  In some areas grass and in others shrubs took advantage of these sunny environments and became the dominant flora.  This attracted herds of megafauna such as mammoths, bison, and horses.  Their trampling and grazing further favored the dominance of grass and shrubs over trees.  Grass can withstand heavy grazing and rapidly grow back in this cool moist climate, while the toxic leaves of heath shrubs protected them from hungry megaherbivores.  Elk and bison continued grazing on the these mountain meadows until colonial times, keeping them open even as the climate became more favorable for tree growth, and then settlers brought their livestock to the balds  inadvertently maintaining these ancient landscapes.  But since the early 20th century when farmers switched to industrial jobs and stopped using the balds for livestock grazing, trees have been encroaching on them.  Some have disappeared completely, while others are in danger of being overgrown in forest.  Ecologists have recognized balds as unique natural communities and are attempting to save them with a combination of mowing and goat grazing.

Modern ecologists are using goats to help maintain grassy balds.  The helmeted musk-ox was closely related to goats.  I believe the helmeted musk-ox was the most important species of megafauna involved in the maintenance of grassy balds during the Pleistocene.

Peter Weigl notes several lines of evidence supporting his belief that southern Appalachian balds are ancient natural communities originating early during the Pleistocene or before.  There are many endemic species of  plants growing on balds that are found nowhere else.  The process of evolution is slow and it’s unlikely so many different species would have evolved so rapidly.  There are also many disjunct species found more commonly in more northerly latitudes.  They are relics surviving in higher cooler elevations but were once more widespread in the region when overall climates were cooler.  This means these plants have been present here throughout many changes in climate.  There’s no evidence of fire in these cool moist higher elevations, and few fire-adapted species of plants live here. This rules out anthropogenic fire as a mechanism in bald creation. Balds were described before Europeans settled here, and even as late as 1790 only 80 settlers lived in the region–not enough people to have cleared so much land.  Relic and endemic species of small mammals, birds, reptiles, and amphibians are also evidence of ancient origin.  Finally, without regular mowing and grazing, these open environments are converting to forest, suggesting a dependence on megafauna presence.

The Canadian blackberry (Rubus canadensis) is native to high elevations in the southern Appalachians, but it has become a problematic plant on grassy balds.  This plant will encroach on grassy areas, completely taking over.  Trees begin to sprout in the brier patches–the first stage of succession leading to the termination of a grassy bald.  Ecologists use goats (Capra aegagrus) to knock back the Canadian blackberry because they readily feed upon the thorny plants.  Goats are closely related to the extinct helmeted musk-ox.  Upon considering this relationship and the coincidental use of goats in bald maintenance, it occurred to me that the helmeted musk-ox may have been the single most important species of megafauna responsible for the existence of Appalachian balds.  Along with caribou (Rangifer tarandus) and flat-headed peccaries (Platygonnus compressus), they were probably the first large mammals to colonize the higher elevations during the harshest climatic phases of the Ice Ages.  Caribou could survive on lichen, while musk-oxen and flat-headed peccaries could subsist on tougher vegetation.  From fossil coprolites scientists know this species of musk-oxen ate woody vegetation during long winters.  They were hardier than bison, horses, and mammoths.  I think these latter 3 species didn’t move to higher elevation balds until warmer phases of climate.  Though they aren’t picky eaters, they prefer lush grasses over thorny blackberry vines.  It was the musk-oxen and perhaps the flat-headed peccary that consumed the thorny vines which would otherwise eventually crowd out grasses here.

Life size model of the helmeted musk-ox.  This species lived as far south as Louisiana.  I’m not sure how accurate this model is.  Helmeted musk-oxen were taller and less wooly than the extant species of musk-ox. 

Rare endemic species such as the Gray’s lily are evidence that Appalachian grassy balds are of ancient origin.

Fossil evidence of the helmeted musk-ox has yet to be found near the present day location of Appalachian balds.  They may never be found here because the most common fossil sites in this region are caves, and musk-oxen probably didn’t venture into caves where they could get ambushed by predators.  However, bones of this species have been dredged from 2 sites off the coast of North Carolina.  Individual musk-oxen bones have turned up on a North Carolina beach and in a stone quarry near the coast as well.  Skeletal evidence of musk-ox as far south as Louisiana and Mississippi has also been reported.  A line drawn on a map between these far separate locations goes right through the southern Appalachians, so it’s a safe assumption their range included this region, and I’m convinced they played an important role in the origin of grassy balds.

Reference:

Weigl, Peter; and Travis Knowles

“Temperate Mountain Grasslands: A Climate-Herbivore Hypothesis for Origins and Persistence”

Biological Reviews 89 (2) 2013

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New Trout Cave, West Virginia

No scientist has written a comprehensive paleoecological review of the fossil remains excavated from New Trout Cave, West Virginia.  Bones of mammals, birds, reptiles, amphibians, and fish were found in well stratified deposits here dating from 13,000 BP-over 50,000 BP.  The composition of species represent different climatic stages, making this a valuable site for ecologists interested in how faunal composition changed over time.  This blog entry is my layman’s review based on information I gathered from the paleobiology database and individual papers written about the vampire bat and pika remains dug from the cave.  A paper written about the reptiles and amphibians found here was published in an obscure journal I can’t obtain with convenience.  The bird and fish remains are completely undescribed in the scientific literature.  My review is entirely based on the ~50 species of mammal remains from the cave.

 

 

New Trout Cave is located in Pendleton County, West Virginia.

Many of the species excavated from the cave were typical of those preyed upon by owls.  These smaller species prefer specific environments, so they are a better indicator of nearby paleohabitats than larger species.  The full glacial environment of the West Virginia/Virginia border seems to have been a mix of cold arid grassland, especially at higher elevations; and boreal forests in the valleys.  I counted 9 species with a definite preference for spruce forests–snowshoe hare, least chipmunk, red squirrel, northern flying squirrel, porcupine, boreal bog lemming,  heather vole, rock vole, and pine marten.  All but the least chipmunk, boreal bog lemming, and taiga vole still live in West Virginia, a state that still hosts some spruce forests.

The highest elevations in this region must have included tundra-like habitat.  The Labrador collared lemming (Dicrostonyx hudsonicus) occurred here during the Ice Age when its present day range was covered by uninhabitable glacier.  The taiga vole (Microtus xanthognatus) is another rodent that prefers tundra habitat.  Caribou (Rangifer tarandus) were also comfortable in these tundra-like conditions but likely wandered down to lower elevations.  Pikas (Ochotona sp.), a relative of the rabbit, formerly lived at higher elevations in the Appalachians.  They are known from 9 Pleistocene-aged sites in the northeast, including New Trout Cave.  These adorable animals like to take refuge under boulders where they store the alpine vegetation they eat.  Grassy rocky balds formerly provided excellent habitat in the Appalachian Mountains, but pikas became extirpated in the east following the end of the Ice Age.  Pikas still live in the Rocky Mountains where they were able to adjust to warming climate by moving to higher elevations.

Taiga vole.  They no longer occur anywhere near West Virginia.  The presence of this species in West Virginia 29,000 years ago is evidence winters were harsher here then.

Present day taiga vole range map.

Caribou and white-tailed deer were the 2 species of deer that lived in West Virginia 29,000 years ago.

Present day range map of the Labrador collared lemming.

The American pika, known today only from high elevations in the Rocky Mountains, lived in West Virginia during the late Pleistocene.

The short-faced skunk (Bracyhprotoma obtusata) is an extinct species known from just 6 sites, including New Trout Cave.  This small skunk was probably a denizen of spruce forests.  It never expanded its range north when the glaciers receded.  Instead, it died out, and no scientist has a good explanation for its demise.

The grassy cold hilltops that provided habitat for collared lemmings and pikas also attracted western prairie fauna, including badgers, 13-lined ground squirrels, plains pocket gophers, and prairie voles.  None of these species occur in West Virginia today.  Horses (Equus sp.), flat-headed peccaries (Platygonnus compressus), and helmeted musk-ox (Bootherium bombifrons) were among the larger mammals that preferred the open spaces, though horses are a semi-generalist animal that can live in woods, if patches of grass are available.  Remains of lions (Panthera atrox), bison (Bison antiquus), and woolly mammoths (Mammuthus primegenius)  were not found in New Trout cave but I strongly suspect they were a part of the faunal mix here during the Last Glacial Maximum.

Over 27 species recovered from the cave were temperate or generalist species such as woodchuck, southern flying squirrel, gray squirrel, eastern chipmunk, jumping mice, beaver, muskrat, southern bog lemming, pine vole, meadow vole, wood rat, cottontail rabbit, white tail deer, red fox, coyote, dire wolf, raccoon, black bear, weasels, Jefferson’s ground sloth, shrews, and bats.   I can’t determine from the information available whether they co-occurred with the boreal/tundra/grassland species or represent a fauna from a different climatic phase.  It’s likely relic temperate habitat persisted in valleys, even during full glacial climate phases.  Some beech, oak, and hickory probably grew among the spruce in protected moist coves.  I’m sure beavers were able to survive the harsher winters wherever waterways existed.  Black bears live as far north as Alaska, so they remained in West Virginia during the glacial maximum.  Jefferson’s ground sloths ranged as far north as Alaska as well and undoubtedly were capable of surviving harsh winters.  White tailed deer are at home in Canada today.  They may have roamed with caribou in mixed herds here then.  Following the end of the Ice Age, most of these temperate species increased in abundance here, while the boreal/tundra species retreated north or perished.

Some of the fossil remains found in New Trout Cave represent a warmer climate phase than enjoyed by residents of the region today.  The Pleistocene vampire bat (Desmodus stocki) and the Florida muskrat (Neofiber alleni) required mild winter temperatures.  The remains of the vampire bat were found at a level dated to about 30,000 BP, a time of a weak interstadial before the Last Glacial Maximum.  But they are probably older than this.  The scientists who studied these remains noted the reddish color of the vampire bat bones.  This color matched those of bones that were from much older sediment, including the Florida muskrat remains. The younger remains from the colder climate phase were lighter in color, and apparently, the older vampire bat bones “intruded” into this younger layer.

The Florida muskrat lived as far north as West Virginia during the late Pleistocene.  Fossils of this species were found in a deeper older level and were associated with vampire bats and an herpetofauna that suggests warmer winters than occur presently.

Present day range map for the Florida muskrat. 

Florida muskrats need standing water with year round green vegetation.  Modern day vampire bats can’t survive subfreezing temperatures, and it can be assumed Pleistocene vampire bats were also not well adapted to freezing temperatures, though because of their larger size, I believe they likely could withstand light frost.  During the Sangamonian Interglacial Pleistocene vampire bats were widespread across North America, living as far north along a line from northern California to West Virginia.  Even during this warm phase, some light frosts during winter must have occurred. ( See: https://markgelbart.wordpress.com/2011/10/24/the-pleistocene-vampire-bat-desmodus-stocki/)  They also may not have been year round residents in the northern parts of their range.  I believe they may have followed migrating herds of mammoths and mastodons north when those behemoths sought summer foraging grounds.

Incredibly, the presence of the Florida muskrat and vampire bat suggests West Virginia experienced winters as mild as present day south Georgia or north Florida during this ancient warm climate phase.  The remains probably date to the Sangamonian Interglacial (~132,000 BP-~118,000 BP) when climate was warmer than modern day temperatures.  We don’t know the exact date because carbon dating can’t be used to date anything older than 50,000 BP, and other types of radiometric dating may not be possible here.  West Virginia suffered a stunning climate reversal following this warm climate phase when arctic tundra species invaded the once forested hilltops, while vampire bats retreated south.

Reference:

Grady, Fred; et. al.

“Northernmost Occurrence of Pleistocene Vampire (Chiroptera: Phillostomidae: Desmodus stocki) in Eastern North America)

Tributes to the Career of Clayton Ray: Series Publications of the Smithsonian Institution

Mead, Jim; and Fred Grady

“Ochotona Lagomorphs from Late Quaternary Cave Deposits in eastern North America”

Quaternary Research 1996

Musk-oxen are More Closely Related to Goats than They are to Cows

In one of the first articles I ever wrote for this blog, I speculated there were 3 species of Pleistocene bovines roaming the area known today as Georgia.  Bison antiquus, a late Pleistocene species of bison, is thought to have evolved from Bison latifrons about 24,000 years BP, yet  a specimen of the latter found in Clark Quarry near Brunswick, Georgia suggested there was a temporal overlap between these 2 species. This specimen  was originally dated to 14,000 BP.  Since I wrote that article, scientists have resubmitted this specimen to radiocarbon dating. and this time it produced a date of 24,000 BP–within the accepted time span this species is thought to have existed.  It’s still one of the more recent specimens of this species, but it is not evidence of temporal  overlap with B. antiquus. There was never more than 1 species of bison living in the region at the same time.

About a month ago, a reader also alerted me to an error I made by classifying the helmeted musk-ox (Bootherium bombifrons) as a bovine.  I wrongly assumed because of its appearance, and the common name ox, that it was closely related to cows. An oxen is a word used for a castrated cow. I should have paid closer attention to my mammalogy books.  Musk-oxen are more closely related to goats and sheep than they are to cattle.  Many laymen likely share my misconception, so I’ve reviewed the literature and will now sort out the bovids.

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Woolly musk-oxen are like a really large goat and are not close kin to cattle.  A species of musk-oxen adapted to temperate climates  lived as far south as Louisiana during the Iice Age.  It was taller, had shorter hair, and a different horn structure than the species  still extant.

Mountain goats are in the same subfamily as muskoxen.

Saiga antelope are also in the same subfamily as musk-oxen.  During the Ice Age saiga antelope lived as far east as Alaska.

The Bovidae family is divided into 2 subfamilies: the caprinae and the bovinae.  Bovids originated in Africa and most species are adapted to warmer climates, but during the Pleistocene a number of species became adapted to the cooler climates of the Ice Ages, and they spread throughout northern Eurasia and across the Bering Landbridge to North America.  None ever made it to South America.

Pleistocene and modern American species in the caprinae subfamily include saiga antelope (Saiga tatarica), shrub oxen (Eucatherium sp.), mountain goats (Oreamnos americanus), Harrington’s mountain goat (Oreamnos harringtoni), bighorn sheep (Ovis Canadensis), dall sheep (Ovis dalli), woolly musk-oxen (Ovibos moschantus), and the helmeted musk-oxen (Bootherium bombifrons)..  The  bovinae species that colonized North America were bison (Bison sp.), and the yak (Bos grunniens), the latter having been restricted to mountains in Alaska during some climate stages of the Pleistocene.

Saiga antelopes still occur on central Asian steppes, but they expanded their range to the grassy mammoth steppe of Alaska during the Last Glacial Maximum.  Shrub oxen were probably the first bovids to colonize North America early during the Pleistocene, and they didn’t become extinct til about 11,000 BP.  Shrub oxen are not known to have occurred east of Iowa.  Harrington’s mountain goat was a sister species of the modern day mountain goat and is also now extinct.

The only species of caprinae that ever colonized southeastern North America was the helmeted musk-ox.  Fossils of this species have been found in Texas, Lousiana, Mississippi, western Tennessee, and Virginia; but notably not in the abundant fossil deposits of Florida. (See https://markgelbart.wordpress.com/2011/07/27/the-south-central-salient-of-the-helmeted-musk-ox-ovibos-cavifrons-or-bootherium-bombifrons/ )  This suggests the southeasternmost range limit was in north or central Georgia or possibly South Carolina’s piedmont.  The helmeted musk-ox expanded its range during the Last Glacial Maximum when grassy desert scrub habitat became a widespread type of environment throughout North America.  Fossil coprolites, originally excreted by helmeted musk-oxen, show these tough animals fed upon green vegetation during summer but could subsist on dry twigs during the long cold Ice Age winters.  Like their goat cousins, they could absorb nutritional value from dead plant material, reminding one of the cartoon stereotype of goats eating the wrappers off tin cans. This amazing animal was able to extract nutrition from cellulose.

Because helmeted musk-oxen were able to survive on dry twigs, I do not believe climate change could have caused their extinction.  There has never been a shortage of dry twigs in North America during any climate phase.  I do believe the replacement of their favored habitat of desert scrub and grassland with forests and woodlands following the end of the Ice Age did cause the range of this species to contract.  This range contraction made them more vulnerable to human overhunting.  Like wooly musk-oxen, the helmeted musk-ox formed defensive circles when confronted with predators.  This was effective against wolves or saber-tooth cats but disastrous against spear-wielding humans who could slaughter an entire herd at once from a safe distance.  R. Dale Guthrie speculated competition with bison caused the extinction of helmeted  musk-oxen, but I reject this hypothesis because these 2 species co-existed for 300,000 years.  I believe bison avoided extinction because they run away from people and migrate long distances and perchance found regions where the population of humans was too low to eat them faster than they could reproduce.

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