MYS­TE­RI­OUS FOS­SIL

points to ori­gins of lizards and snakes

Richmond Times-Dispatch Weekend - - INSCIENCE - BY ASHER ELBEIN

Some 240 mil­lion years ago, a storm rolled over the trop­i­cal edge of the su­per­con­ti­nent of Pan­gaea and bat­tered the coastal is­lands. Rain lashed forests of horse­tails and ferns, sweep­ing plants and an­i­mals out to sea and bury­ing them in silt.

Among the vic­tims was a small rep­tile. At the time, it didn’t look like much. But re­cently in the jour­nal Na­ture, an in­ter­na­tional team of sci­en­tists iden­ti­fied the an­i­mal, called Me­gachirella,as the ear­li­est known an­ces­tor of the squa­mates, the fam­ily con­tain­ing lizards, snakes and worm­like am­phis­bae­ni­ans.

With 10,000 dis­tinct species, squa­mates com­prise one of the largest or­ders of land ver­te­brates on the planet. They’re found on six con­ti­nents, have adapted to a dizzy­ing ar­ray of habi­tats and range in size from 25-foot pythons to chameleons smaller than a pen­cil eraser. Ex­tinct mem­bers of the group in­clude mas­sive snakes like Ti­tanoboa and sea­far­ing mosasaurs, which ri­valed small whales in size.

Squa­mates “in­habit al­most ev­ery sin­gle en­vi­ron­ment,” said Ti­ago Simões, a pa­le­on­tol­o­gist at the Univer­sity of Al­berta, who co-au­thored the study. “They learned how to climb on glass, how to swim, how to glide. They can even run over wa­ter.”

But the ori­gin of squa­mates has long been an enigma. The fam­ily be­longs to a larger group called lep­i­dosaurs, a cat­e­gory that also in­cludes a sis­ter lin­eage called the rhyn­choce-phalians — rep­re­sented to­day only by the hum­ble tu­atara, a small rep­tile in New Zealand.

The two groups di­verged from a com­mon an­ces­tor in the dis­tant past, but when and how that hap­pened has been a mys­tery. Ge­netic data had long sug­gested that squa­mates orig­i­nated some­where dur­ing the Early Tri­as­sic Pe­riod, 252 mil­lion to 247 mil­lion years ago, said Adam Pritchard, a pa­le­o­bi­ol­o­gist at the Smith­so­nian Na­tional Mu­seum of Nat­u­ral His­tory.

But when it came to fos­sil ev­i­dence, the only lep­i­dosaurs any­one could find from the Tri­as­sic Pe­riod were the rhyn­cho­cephalians. The ear­li­est known squa­mate fos­sils hailed from the later Juras­sic pe­riod, leav­ing a frus­trat­ing 75 mil­lion-year gap be­tween the shared an­ces­tor and the ap­pear­ance of what are now some of the planet’s most fa­mil­iar an­i­mals.

“It’s as if we have de­fin­i­tive ev­i­dence of a fam­ily that had two sib­lings,” Pritchard said. “One sib­ling, you know a lot about their early life and how they grew up. And the other one, you have no in­for­ma­tion what­so­ever about their child­hood.”

At first, Me­gachirella didn’t ap­pear to of­fer much in­sight.

The spec­i­men was found by an am­a­teur fos­sil hunter in 1999 in an out­crop of lime­stone in the Dolomite Moun­tains of north­ern Italy. The slab, which dated to the mid­dle Tri­as­sic Pe­riod, con­tained the front half of a small, slightly squashed skele­ton with a large head and heavy fore­limbs.

The re­mains were beau­ti­fully ar­tic­u­lated, but some­what hard to make out. When sci­en­tists for­mally named it in 2003, they de­scribed it as a prim­i­tive lep­i­dosaur. Avail­able tech­nol­ogy wasn’t suf­fi­cient to probe deeper.

But in 2014, Simões — who spe­cial­izes in un­tan­gling the evo­lu­tion­ary his­tory of liv­ing and ex­tinct squa­mates — came across a pa­per on the fos­sil that caught his in­ter­est. Look­ing at the anatom­i­cal fig­ures, he knew right away that he was likely look­ing at a lizard — and a very early lizard.

He con­tacted one of the pa­per’s au­thors, Mas­simo Bernardi, a pa­le­on­tol­o­gist at the Mu­seum of Sci­ence in Trento, Italy, and sug­gested that they run the fos­sil through high-res­o­lu­tion mi­cro CT scan­ning in or­der to bet­ter see the com­pressed bones. Bernardi, who had been think­ing along sim­i­lar lines, agreed.

Un­der the high-tech gaze of the scan­ners, the sci­en­tists were able to re­con­struct dig­i­tally the small crushed bones, pick­ing out fine de­tails of anatomy in­vis­i­ble to the naked eye. They found that por­tions of the an­i­mal’s brain­case more closely re­sem­bled that of a mod­ern iguana than those of other Tri­as­sic rep­tiles; so did the ar­range­ment of teeth and the lack of per­fo­ra­tions in the lower part of Me­gachirella’s ver­te­brae.

Even more in­trigu­ing were the an­i­mal’s wrist bones, shoul­ders and arms, more char­ac­ter­is­tic of those seen in liv­ing lizards. But Me­gachirella had some ves­ti­gial bits of anatomy that are ab­sent in mod­ern lizards and snakes: gas­tralia, belly-ribs still found in tu­ataras and croc­o­diles, and a small cheek bone called the quadra­to­ju­gal.

Both Me­gachirella’s ap­pear­ance in the mid­dle Tri­as­sic and the later re­mains of rhyn­cho­cephalian fos­sils make a strong case that the ear­li­est squa­mates emerged to­ward the end of the Per­mian, a pe­riod 270 mil­lion years ago that was abruptly shat­tered by one of the worst known mass ex­tinc­tions. At the dawn of the fol­low­ing Tri­as­sic Pe­riod, over 90 per­cent of life on the planet had been wiped out.

The sur­viv­ing an­i­mal groups rapidly di­ver­si­fied through­out the emp­tied world, said Michelle Stocker, a pa­le­o­bi­ol­o­gist at Vir­ginia Tech. The trees filled with chameleon-like drepanosaurs and even­tu­ally with the first pterosaurs; the oceans soon gave rise to the first ma­rine rep­tiles; ar­chosaurs mor­phed into bipedal croc­o­diles and skulk­ing di­nosaurs.

Lep­i­dosaurs scur­ried along in the shad­ows, un­der­go­ing their own ex­plo­sion in di­ver­sity. And while rhyn­cho­cephalians fared well for tens of mil­lions of years, Pritchard said, it now seems that squa­mates gained on them, be­com­ing even more com­mon and di­verse.

Some­where in the Juras­sic, 75 mil­lion years af­ter Me­gachirella, the first rec­og­niz­able lizards emerged, still bear­ing some of the anatom­i­cal mark­ers of their dis­tant an­ces­tor.

“The Per­mian ex­tinc­tion opened the way for the ori­gin of these lin­eages, and opened up op­por­tu­ni­ties for col­o­niz­ing land and sea,” Simões said.

“A lot of the mod­ern fauna in the world to­day — in terms of rep­tiles es­pe­cially — orig­i­nated at some point in the Tri­as­sic,” he added. “The first mam­mals are from the Tri­as­sic, the first di­nosaurs are from the Tri­as­sic, and now we know the first lizards are from the Tri­as­sic, too.”

Me­gachirella’s mix­ture of ar­chaic and mod­ern anatomy is also a clue to which of to­day’s lizard fam­i­lies were the first to emerge, Simões said. This is a con­tentious topic. Since 2014, DNA-based fam­ily trees have sug­gested that geckos were the ear­li­est group of mod­ern lizards to evolve.

But sci­en­tists who looked pri­mar­ily at fos­sils cham­pi­oned the large and di­verse igua­nian fam­ily — a group that in­cludes chameleons, igua­nas and aga­mas — as the ear­li­est mod­ern lizards. Simões in­cor­po­rated data from Me­gachirella’s anatomy into a larger data set he had as­sem­bled by study­ing squa­mate species from 50 mu­seum col­lec­tions in 17 coun­tries. The re­sult­ing tree sug­gested that geckos were in­deed the first of the sur­viv­ing lizard fam­i­lies to emerge, in the Juras­sic pe­riod (though there’s no ev­i­dence they ac­quired their dis­tinc­tive toe-pads un­til the Cre­ta­ceous pe­riod).

Igua­nian fos­sils show up in the Cre­ta­ceous, and the ge­netic data group them closely with mon­i­tor lizards and snakes, Simões said. By the end of the Cre­ta­ceous, the rhyn­cho­cephalians had all but van­ished, leav­ing only the tu­atara. But the squa­mates con­tin­ued to di­ver­sify into their present ar­ray of forms.

“The amaz­ing thing is that those few sur­viv­ing lin­eages be­came very di­verse,” Simões said. “Imag­ine how di­verse the world must have been in the Me­so­zoic, with all the un­cut branches of the rep­tile fam­ily tree still around.”

For Stocker, part of the value of Me­gachirella lies in the fact that so much in­for­ma­tion was pulled from its bones long af­ter the fos­sil was dis­cov­ered and de­scribed — a tes­ta­ment to the value of the dusty spec­i­mens ly­ing in mu­seum col­lec­tions world­wide.

“Even the in­for­ma­tion we can get out of a fos­sil to­day, it might pale in com­par­i­son to what tech­nol­ogy might let us un­der­stand about that ex­tinct an­i­mal in the fu­ture,” Stocker said. “So it’s im­por­tant that we take care of these fos­sils now for fu­ture re­searchers.”

DA­VIDE BONADONNA

A ren­der­ing shows Me­gachirella, iden­ti­fied as the ear­li­est known an­ces­tor of the fam­ily con­tain­ing lizards, snakes and am­phis­bae­ni­ans.

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