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Earliest-​known arbo­real and sub­ter­ranean ances­tral mam­mals discovered

pub­lished 14 Feb­ru­ary 2015 | mod­i­fied 14 Feb­ru­ary 2015

Agilodocodon and Docofossor fossilsThe fos­sils of two inter­re­lated ances­tral mam­mals, newly dis­cov­ered in China, sug­gest that the wide-​ranging eco­log­i­cal diver­sity of mod­ern mam­mals had a prece­dent more than 160 mil­lion years ago.

With claws for climb­ing and teeth adapted for a tree sap diet, Agilodocodon scan­so­rius is the earliest-​known tree-​dwelling mam­mali­aform (long-​extinct rel­a­tives of mod­ern mam­mals). The other fos­sil, Doco­fos­sor brachy­dacty­lus, is the earliest-​known sub­ter­ranean mam­mali­aform, pos­sess­ing mul­ti­ple adap­ta­tions sim­i­lar to African golden moles such as shovel-​like paws. Doco­fos­sor also has dis­tinct skele­tal fea­tures that resem­ble pat­terns shaped by genes iden­ti­fied in liv­ing mam­mals, sug­gest­ing these genetic mech­a­nisms oper­ated long before the rise of mod­ern mammals.

These dis­cov­er­ies are reported by inter­na­tional teams of sci­en­tists from the Uni­ver­sity of Chicago and Bei­jing Museum of Nat­ural His­tory in two sep­a­rate papers (1, 2) pub­lished on 13 Feb­ru­ary in the jour­nal Science.

These new fos­sils help demon­strate that early mam­mals did indeed have a wide range of eco­log­i­cal diver­sity. It appears dinosaurs did not dom­i­nate the Meso­zoic land­scape as much as pre­vi­ously thought
Zhe-​Xi Luo, lead author of one and co-​author of the other paper, pro­fes­sor of organ­is­mal biol­ogy and anatomy, Uni­ver­sity of Chicago »

We con­sis­tently find with every new fos­sil that the ear­li­est mam­mals were just as diverse in both feed­ing and loco­mo­tor adap­ta­tions as mod­ern mam­mals,” said Zhe-​Xi Luo. “The ground­work for mam­malian suc­cess today appears to have been laid long ago.”

Agilodocodon and Doco­fos­sor pro­vide strong evi­dence that arbo­real and sub­ter­ranean lifestyles evolved early in mam­malian evo­lu­tion, con­ver­gent to those of true mam­mals. These two shrew-​sized crea­tures — mem­bers of the mam­mali­aform order Docodonta — have unique adap­ta­tions tai­lored for their respec­tive eco­log­i­cal habitats.

Using state-​of-​the-​art CT scan­ning, sci­en­tists from the Uni­ver­sity of Chicago have recon­structed the skele­tons of two fos­sils that rep­re­sent the earliest-​known arbo­real and sub­ter­ranean mam­mals. This short video gives a brief glimpse at how the jaws of these fas­ci­nat­ing crea­tures were recon­structed:

(Source: The Uni­ver­sity of Chicago Med­i­cine YouTube channel)

AgilodocoAgildocodon and Docofossor illustrationdon, which lived roughly 165 mil­lion years ago, had hands and feet with curved horny claws and limb pro­por­tions that are typ­i­cal for mam­mals that live in trees or bushes. It is adapted for feed­ing on the gum or sap of trees, with spade-​like front teeth to gnaw into bark. This adap­ta­tion is sim­i­lar to the teeth of some mod­ern New World mon­keys, and is the earliest-​known evi­dence of gum­niv­o­rous feed­ing in mam­mali­aforms. Agilodocodon also had well-​developed, flex­i­ble elbows and wrist and ankle joints that allowed for much greater mobil­ity, all char­ac­ter­is­tics of climb­ing mammals.

The fin­ger and limb bone dimen­sions of Agilodocodon match up with those of mod­ern tree-​dwellers, and its incisors are evi­dence it fed on plant sap,” said study co-​author David Gross­nickle, grad­u­ate stu­dent at the Uni­ver­sity of Chicago. “It’s amaz­ing that these arbo­real adap­tions occurred so early in the his­tory of mam­mals and shows that at least some extinct mam­malian rel­a­tives exploited evo­lu­tion­ar­ily sig­nif­i­cant her­biv­o­rous niches, long before true mammals.”

Doco­fossor, which lived around 160 mil­lion years ago, had a skele­tal struc­ture and body pro­por­tions strik­ingly sim­i­lar to the mod­ern day African golden mole. It had shovel-​like fin­gers for dig­ging, short and wide upper molars typ­i­cal of mam­mals that for­age under­ground, and a sprawl­ing pos­ture indica­tive of sub­ter­ranean movement.

Doco­fos­sor had reduced bone seg­ments in its fin­gers, lead­ing to short­ened but wide dig­its. African golden moles pos­sess almost the exact same adap­ta­tion, which pro­vides an evo­lu­tion­ary advan­tage for dig­ging mam­mals. This char­ac­ter­is­tic is due to the fusion of bone joints dur­ing devel­op­ment — a genet­i­cally influ­enced process. Because of the many anatom­i­cal sim­i­lar­i­ties, the researchers hypoth­e­size that this genetic mech­a­nism may have played a com­pa­ra­ble role in early mam­mal evo­lu­tion, as in the case of Doco­fos­sor.

The spines and ribs of both Agilodocodon and Doco­fos­sor also show evi­dence for the influ­ence of genes seen in mod­ern mam­mals. Agilodocodon has a sharp bound­ary between the tho­racic ribcage to lum­bar ver­te­brae that have no ribs. How­ever, Doco­fos­sor shows a grad­ual tho­racic to lum­ber tran­si­tion. These shift­ing pat­terns of thoracic-​lumbar tran­si­tion have been seen in mod­ern mam­mals and are known to be reg­u­lated by spe­cific genes. That these ancient mam­mali­aforms had sim­i­lar devel­op­men­tal pat­terns is an evi­dence that these gene net­works could have func­tioned in a sim­i­lar way long before true mam­mals evolved.

We believe the short­ened dig­its of Doco­fos­sor, which is a dead ringer for mod­ern golden moles, could very well have been caused by BMP and GDF [two spe­cific genes],” Luo said. “We can now pro­vide fos­sil evi­dence that gene pat­tern­ing that causes vari­a­tion in mod­ern mam­malian skele­tal devel­op­ment also oper­ated in basal mam­mals all the way back in the Jurassic.”

Early mam­mals were once thought to have lim­ited eco­log­i­cal oppor­tu­ni­ties to diver­sify dur­ing the dinosaur-​dominated Meso­zoic era. How­ever, Agilodocodon, Doco­fos­sor and numer­ous other fos­sils — includ­ing Cas­toro­cauda, a swim­ming, fish-​eating mam­mali­aform described by Luo and col­leagues in 2006 — pro­vide strong evi­dence that ances­tral mam­mals adapted to wide-​ranging envi­ron­ments despite com­pe­ti­tion from dinosaurs.

We know that mod­ern mam­mals are spec­tac­u­larly diverse, but it was unknown whether early mam­mals man­aged to diver­sify in the same way,” Luo said. “These new fos­sils help demon­strate that early mam­mals did indeed have a wide range of eco­log­i­cal diver­sity. It appears dinosaurs did not dom­i­nate the Meso­zoic land­scape as much as pre­vi­ously thought.”

(Source: Uni­ver­sity of Chicago Med­i­cine press release, 12.02.2015)

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