More Evidence against the Climate Change Model of Late Pleistocene Extinctions

Many extinct species of Pleistocene megafauna had a wide ranging geographic distribution.  Jefferson’s ground sloth, long-nosed peccary, Columbian mammoth, and mastodon occurred from coast to coast and from Florida to the glacial boundary.  These species and their similar evolutionary ancestors existed across the continent for millions of years, surviving dozens of major and minor climatic fluctuations.  They lived in a variety of environments and were capable of subsisting on many different foods.  Multiple lines of evidence show these pre-historic beasts ate a varied diet.  Mastodon coprolites (subfossil feces) contain bald cypress, buttonbush, spruce twigs, fruit, acorns, aquatic plants, and numerous other items.  Now, a new study of mastodon teeth using dental microwear texture analysis confirms that mastodons ate a wide variety of foods.

Mastodon tooth.  Scientists looked at mastodon teeth using microscope technology and determined mastodons from different regions ate different foods.

Scientists microscopically examined 65 mastodon teeth that were found in 4 different geographic locations including Florida, Missouri, Indiana, and New York.  The microwear found on mastodon teeth from Florida differed from wear on teeth from northern mastodons.  Florida mastodons primarily ate bald cypress twigs, while northern mastodons ate spruce, hemlock, pine, larch, and juniper.  The differences in tooth wear indicate mastodons could eat a variety of plant foods and were not dependent upon a single species.  The authors of this study also looked at mastodon teeth from different climatic stages in Missouri.  Mastodon teeth from a climate stage when open jack pine and prairie predominated showed little difference from teeth dated to a climate stage when spruce dominated the landscape.  The microwear on mastodon teeth resembles the microwear found on 2 living species–moose and black rhino.  Like mastodon, these 2 species subsist on woody browse.

I think this study is just more evidence against the climate change model of extinction that proposes changes in climate caused corresponding changes in plant composition, leading to megafaunal extinctions through nutritional deficit starvation.  None of the plants mastodons ate ever disappeared or even became rare in the environment. The authors of this study take a more neutral stance toward the debate.  They acknowledge the “plasticity” of mastodon diet but seem reluctant to admit their study is strong evidence against the climate change model of extinction.  Instead, they suggest future studies using dental microwear texture analysis could uncover the reason why megafauna became extinct.  In my opinion it already has.  Their data rules out the climate change model of extinction by revealing the dietary adaptability of mastodons.  Through the process of elimination, human overkill is the only plausible cause left standing.

Reference:

Green, J.; Larisa DeSantis and G. Smith

“Regional Variation in the Browsing Diet of Pleistocene Mammut americanum (Mammalia, Proboscidea) as Recorded by Dental Microwear Texture Analysis”

Paleogeography, Paleoclimatology, and Paleoecology August 2017

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Logical Flaws in Studies of Pleistocene Carnivore Tooth Wear

Van Valkenburgh co-authored a study comparing rates of broken teeth suffered by large modern carnivores with those experienced by fossil carnivores excavated from the Rancho La Brean tar pits.

http://www.cof.orst.edu/leopold/class-reading/Van%20Valkenburgh%20and%20Hertel%201993.pdf

She found that large carnivores from the La Brean tar pits suffered 3 times as many broken teeth as modern day carnivores.  The fossils she examined from the pits included saber-tooth (Smilodon fatalis), American lion (Panthera atrox), cougars (Puma concolor), bobcats (Lynx rufus), dire wolves (Canis dirus), and coyotes (Canis latrans).  The modern species that she compared them to were lions, jaguars, leopards, cheetahs, bobcats, timber wolves, hyenas, and African hunting dogs.  The specimens from the tar pits dated to between ~36,000 BP-~12,000 BP.  She concluded that competition for prey was more fierce then than it is among modern large carnivores.  She believed increased competition for food forced large carnivores to utilize more of the carcass, and this led to more broken teeth from gnawing on bone.  She dismissed the alternate explanation that the higher rates of broken teeth among large Pleistocene carnivores were the result of attempting to take down larger prey than modern carnivores usually attack.  Dr. Valkenburgh suggested the larger size of the Pleistocene carnivores compensated for the larger size of the prey.

I never paid much attention to this study because using rates of durophagy as a proxy for carnivore competition seemed like a dubious assumption.  (Durophagy is just a fancy word for bone-eating.)  I think different species either eat more bone than other species because they have different nutritional needs, or they eat more for some other unknown reason that has nothing to do with competition.

Last year, a new study was published that completely contradicts Dr. Valkenburgh’s study.

http://www.plosone.org/article/info:doi/10.1371/journal.pone.0052453

Several scientists looked at teeth from Rancho La Brean predators and extant carnivores using Dental Microwear Textural Analysis (DMTA).  DMTA requires a lot of fancy equipment and procedures including a white light confocal profilometer, a scale sensitive fractal analysis, and a scanning electron microscope.  The scientists are able to see the teeth on a computer screen in 3D.  This eliminates observer measurement error.  Carnivores that avoid bone, such as cheetahs and lions, have different types of microwear on their teeth than those that eat a lot of bone, such as African hunting dogs and hyenas.

This is the equipment used to look at tooth wear in animals.

These scientists found little difference in rates of durophagy between the extinct and extant carnivores, meaning the competition between predators in the late Pleistocene in this region was no more or less fierce than that of today’s Africa.  Of course, I don’t buy the whole proxy assumption in the first place.  Most of the broken teeth were canines rather than molars.  Canines are more likely to break when taking down prey; molars are more likely to break when chewing bones, therefore they conclude the larger size of prey was the factor that explains the higher incidence of broken teeth among large Pleistocene carnivores.

One of the conclusions of this more recent study has a logical flaw.  The scientists authoring this 2nd study used DMTA to look at the teeth of Smilodon and Panthera atrox over time.  Different tar pits hold fossils of different spans of time ranging from ~36,000 BP-~12,000 BP, so they looked at 5 specimens of each species from pits of chronologically different ages.  They found no difference in tooth wear between the older specimens and the young specimens that dated closer to the time of extinction.  Many scientists think large Pleistocene carnivores became extinct because the animals they preyed upon became extinct.   They expected evidence of increased durophagy among the most recent saber-tooths and lions as they were forced to utilize more of the carcass.  But they found no evidence of this.  I think this doesn’t disprove the likelihood that carnivores did die out because their prey disappeared.  The final sentence in the abstract seems to imply that it does though.  It states : “The difference in DMTA attributes from older to younger deposits offers little evidence that declining prey resources were a primary cause of extinction for these large cats.”  Brian Switek, who writes an online blog for National Geographic, discussed this study and even goes so far as to ask what caused the extinctions of large Pleistocene carnivores, as if this study somehow disproves declining populations of suitable prey was the cause.  In my opinion this is flawed thinking.  Besides the dubious assumption that certain species of carnivores utilize more bone if prey is scarce, it seems unreasonable to expect to find the last nutritionally-stressed members of a population to be represented in the fossil record.  The odds of an animal becoming fossilized is so rare that it’s extremely unlikely that a member of the last remnant of a species headed for extinction would become fossilized.  The sample size–just 5–is also way too insignificant to detect whether or not this was occurring.  Moreover, big cats that eat mostly meat and organs and avoid bone are more likely to die of starvation before they chew on many bones.

I contacted the main author of this study, Dr. Larisa Desantis, and pointed out the logical flaws of this conclusion but she never responded.  I tried to comment on Brian Switek’s blog but he wouldn’t even allow my comment to be published.

I believe competition with humans drove saber-tooths and American lions to extinction.  Humans directly hunted them and overhunted their prey, and it was this combination that made it impossible for these species to survive.  To expect to find evidence of this by looking at a handful of fossil teeth under a microscope is ridiculous.