Three Pleistocene Lions: Were they Distinct Species or the Same Animal?

Some time ago, I wrote a blog entry detailing an excellent 2009 study by Per Christiansen that suggested Panthera atrox was a unique distinct species and not a subspecies of lion, as previously believed (https://markgelbart.wordpress.com/2010/07/28/panthera-atrox-what-kind-of-cat-was-it/).  Dr. Christiansen measured every part of the skulls from numerous specimens of all 4 extant species in the Panthera genus, plus some from the extinct Panthera atrox.  He statistically analyzed the measurements and found that the measurements from  Panthera atrox skulls clustered distinctly from those of lions, tigers, jaguars, and leopards.  He concluded this meant they were a unique species of panthera that was neither lion, jaguar, nor tiger.  Although the skulls of  Panthera atrox greatly resembled those of extant lions, they aren’t an exact match.  Moreover, their jaws are a closer match to those of jaguars.   So he assumed Panthera atrox shared an immediate common ancestor with the jaguar.  He believed they were isolated from the Eurasian parent population by an Ice Age glacier, and diverged into 2 different species–Panthera atrox and jaguars.

Panthera atrox vs. Smilodon

Cave painting of Eurasian cave lion, Panthera spelaea.  I chose this one over other illustrations on the web because it was drawn by artists who actually saw the beast.

Three months after Dr. Christiansen published his paper, another group of scientists led by Ross Barnett published their studies of panthera mtDNA.  Evidence from this study of genetics suggests Panthera atrox diverged from the Eurasian cave lion about 340,000 years ago.  Not coincidentally, this is the same time period bison, a major prey item, colonized North America.  This precludes the possibility that Panthera atrox shared a close evolutionary relationship with the jaguar because the latter occupied North America long before this date.  Moreover, there is a wide genetic variance between those species, but there is a marked similarity between Panthera atrox and lion DNA.

The results of the DNA study do support Dr. Christiansen’s findings that Panthera atrox was a distinct species.  The authors of the DNA study noticed a lack of gene flow between the 3 types of Pleistocene lions, even though there was no known geographical barriers between the African and Eurasian lions and the Eurasian lions and American lions for millenia.

Range map of Pleistocene lions from the below referenced study.  The Beringian lion (color coded red) is considered a subspecies of the Eurasian cave lion.

An African species of the panthera genus appeared first in the fossil record from a site in Tanzania that dates to ~3.5 million years BP.  One of its descendents, the African lion (Panthera leo), ranged from southern Africa to southern Eurasia, including India and Greece.  The Eurasian cave lion (Panthera spelaea) appeared in the fossil record as suddenly as the African lion.  Its oldest dated remains come from a fossil site in Italy dating to ~700,000 years BP.  The genetic evidence suggests that Panthera spelaea was derived from the African lion.  Their ranges came into contact in parts of the Near East, yet it’s apparent the 2 types of lions stopped interbreeding–almost certain evidence they evolved into 2 separate species.  Some Russian scientists have also suggested that Panthera spelaea was a distinct species from the African lion based on notable differences in morphology.  The Eurasian cave lion colonized Alaska when sea levels  lowered during Ice Ages, thus creating the Bering land bridge.  It was this population that gave rise to Panthera atrox ~340,000 years BP.  A glacier separated the 2 populations initially, and the ones south of the ice evolved into atrox.  However, the glacier completely dissipated at least twice for tens of thousands of years at a time, so for much of this ancient era, there was no geographical barrier between the 2 populations.  Yet here too, there was no gene flow–again almost certain evidence they’d evolved into separate species, despite the genetic similarity.

Panthera atrox was on average 25% larger than the African lion, and it had a significantly larger brain.  Considering the consistent size difference, the lack of gene flow, and Dr. Christiansen’s statistical analysis of skull differences, I’m convinced Panthera atrox was indeed a unique and now extinct species.  There is one caveat concerning the genetic study.  Although they used plenty of samples of Panthera leo and Panthera spelaea, they only used a small sample size of Panthera atrox specimens.  They had 2 specimens from Consolidated Pit 48 in Edmonton, Canada, and 2 from Natural Trap Cave in Wyoming.  Panthera atrox lived all across North America as far east as South Carolina (1 fossil from Edisto Beach) and Florida, and it ranged into South America at least as far south as Peru.  It would be interesting to learn the genetic variance from populations of Panthera atrox that were farther removed geographically from their ancestral population.

Incidentally, there’s evidence from a cave in Idaho that humans cooked and ate one particular Panthera atrox.  The youngest carbon date of this species is about 13,000 calender years from the Edmonton site.

The study of lion DNA found another interesting tidbit of ecological mystery.  About 50,000 BP the genetic diversity of Eurasian cave lions greatly declined from 8 haplogroups to 1.  The genetic diversity of wooly mammoths, bison, and horses also declined during this time period.  This puzzles scientists, but I think I have a viable explanation.  The decline in genetic diversity coincides with the timing of a severe stadial.  A more varied environment of forests, scrubs, and lightly wooded savannah gave way to an expansion of arid frigid grasslands known as the Mammoth Steppe.  Only the populations of those species best adapted to this type of environment survived whereas those better adapted to woodland, partially wooded savannahs, and scrub habitat died out.

Reference:

Barnett, Ross; et. al.

“Phylogeography of lions (Panthera leo ssp) Reveals Three Distinct Taxa and a Late Pleistocene Reduction in Genetic Diversity”

Molecular Ecology 2009