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201316Nov10:07

Old­est fos­sil skull ever of a big cat found in Tibet may reshape feline fam­ily tree

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pub­lished 16 Novem­ber 2013 | mod­i­fied 03 Novem­ber 2014
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In the sum­mer of 2010, husband-​and-​wife pale­o­bi­ol­o­gist team Z. Jack Tseng and Juan Liu trav­elled to the Zanda Basin in west­ern Tibet with a group of col­leagues. The remote area, a week’s drive from Bei­jing and near the bor­der of Pak­istan and China, is “basi­cally bad­lands every­where, with deeply cut val­leys through­out,” Tseng says.

Panthera-blytheae-skull-and-lifeTo explore the val­leys, the team drove up dirt trail after dirt trail before com­ing upon a dense patch of fos­sils stick­ing out of the ground halfway up a hill. “In the lit­tle con­cen­tra­tion of fos­sils, there were lots of limb bones from antelopes and horses obscur­ing every­thing else,” says Tseng, who was then a grad­u­ate stu­dent at USC and is now with the Amer­i­can Museum of Nat­ural His­tory. “It wasn’t until we started lift­ing things up, one by one, that we saw the top of a skull, and we thought, from the shape, that it looked some­thing like a cat.”

It’s not a huge cat, like a lion or a tiger, but closer to a leopard
« Z. Jack Tseng, pale­o­bi­ol­o­gist, Amer­i­can Museum of Nat­ural History

After a few years of analy­sis, Tseng’s team has dis­cov­ered that the skull doesn’t belong to any old cat. As they’ve doc­u­mented in a study pub­lished on 13 Novem­ber in Pro­ceed­ings of the Royal Soci­ety B, the skull and six asso­ci­ated fos­silized jaw­bone frag­ments are the first evi­dence of a newly dis­cov­ered species, which they’ve called Pan­thera blytheae. The dis­cov­ery rep­re­sents the old­est “big cat” (a group that includes large preda­tory cats like lions, jaguars, tigers and leop­ards) ever found by a wide margin.

Zanda basin-TibetThe sed­i­ments that make up the Zanda basin as a whole range from 6 mil­lion to 400,000 years in age, so the group dated the fos­sil by analysing the age of the par­tic­u­lar rock lay­ers it was buried in. This involved using tech­niques of mag­ne­tostratig­ra­phy, in which sci­en­tists analyse the mag­netic ori­en­ta­tion of the rocks and com­pare it to known rever­sals of the Earth’s mag­netic field. This method can only pro­vide rough esti­mates for an item’s age, but it revealed that the skull is between 4.10 and 5.95 mil­lion years old. Pre­vi­ously, the old­est known big cat fos­sils — a num­ber of tooth frag­ments found in Tan­za­nia — were 3.6 mil­lion years old.

The new find fills a gap in the evo­lu­tion­ary record of big cats. By analysing the DNA of liv­ing species, sci­en­tists had pre­vi­ously esti­mated that big cats had split from the Feli­nae a sub­fam­ily of the Fel­i­dae fam­ily — which includes smaller wild cats, like cougars, lynxes, along with domes­tic cats — about 6.37 mil­lion years ago. The very exis­tence of P. blytheae con­firms that the split hap­pened prior to when this big cat roamed.

But how much ear­lier? The find could sug­gest, Tsang says, that big cats branched off from smaller cats much far­ther back than thought. By com­par­ing the skull’s char­ac­ter­is­tics with fos­sils from other extinct big cats, the anatomy of liv­ing cat species, and DNA sam­ples taken from both liv­ing cats and a few recently extinct, Ice Age-​era species (known as cave lions), the researchers assem­bled a new evo­lu­tion­ary fam­ily tree for all big cats. Using known rates of anatom­i­cal changes over time and the observed anatomy of P. blytheae, they pro­jected back­wards, and esti­mated that the ear­li­est big cats likely branched off from the Feli­nae sub­fam­ily between 10 and 11 mil­lion years ago.

The new fos­sil also solves a geo­log­i­cal mys­tery. Pre­vi­ously, using DNA analy­sis of all liv­ing big cats and map­ping the the fos­sils exca­vated from var­i­ous sites around the world, researchers had deter­mined it was most likely that their com­mon ances­tor had lived in Asia. The old­est known spec­i­mens, how­ever, were found in Africa. The new species pro­vides the first direct evi­dence that cen­tral Asia was indeed the big cats’ ances­tral home, at least as far back as the cur­rent fos­sil record cur­rently goes.

From the frag­mented fos­sils, it’s hard to know much about the extinct species’ behav­iour and lifestyle, but the researchers were able to make some basic extrap­o­la­tions from the skull’s anatomy. “It’s not a huge cat, like a lion or a tiger, but closer to a leop­ard,” Tsang says. The creature’s habi­tat was likely sim­i­lar to the cur­rent Tibetan plateau, so Tseng spec­u­lates that, like the snow leop­ards that cur­rently live in the area, this species did not hunt on the open plains, but rather cliffs and val­leys. Tooth wear pat­terns also sug­gest sim­i­lar­i­ties with cur­rent snow leop­ards — the rear teeth, likely used for cut­ting soft tis­sue, remain sharp, whereas the front teeth are heav­ily worn, per­haps reflect­ing their use in pry­ing open car­casses and pick­ing meat off bones.

Tseng says that he and col­leagues plan to return to the area to search for more fos­sils that could help enlighten us on the evo­lu­tion­ary his­tory of big cats. “The gap still isn’t com­pletely filled yet,” he says. “We need to find older big cats to put the pic­ture together.”

As you might expect there is some doubt amongst researchers about the reli­a­bil­ity of the claim that P. blytheae is a closely related species to the snow leop­ard. The rela­tion­ship is deter­mined based on a just few fos­sils and a lim­ited num­ber of skull, jaw and teeth fea­tures, says William Mur­phy, a mol­e­c­u­lar geneti­cist who co-​authored a study on evo­lu­tion­ary his­tory of pan­ther­ines pub­lished in 2006. But he agrees that the only way to clear up these rela­tion­ships is to dig up more com­plete fos­sils of sim­i­lar ‘old’ big cats.



(Source: Smith­son­ian mag­a­zine Sur­pris­ing Sci­ence, 12.11.2013; Sci­ence mag­a­zine news, 12.11.2013)


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