AboutZoos, Since 2008


True grit’ erodes assump­tions about evolution

pub­lished 07 March 2013 | mod­i­fied 07 March 2013
Gran-Barranca-ArgentinaDin­ing on field grasses would be ruinous to human teeth, but mam­mals such as horses, rhi­nos and gazelles evolved long, strong teeth that are up to the task.

New research led by the Uni­ver­sity of Wash­ing­ton chal­lenges the 140-​year-​old assump­tion that find­ing fos­silised remains of pre­his­toric ani­mals with such teeth meant the ani­mals were liv­ing in grass­lands and savan­nahs. Instead it appears cer­tain South Amer­i­can mam­mals evolved the teeth in response to the gritty dust and vol­canic ash they encoun­tered while feed­ing in an ancient trop­i­cal for­est. The find­ings have been pub­lished in an arti­cle on Feb­ru­ary 12 in the jour­nal Nature Com­mu­ni­ca­tions.

The new work was con­ducted in Argentina where sci­en­tists had thought Earth’s first grass­lands emerged 38 mil­lion years ago, an assump­tion based on fos­sils of these spe­cialised teeth. But the grass­lands didn’t exist. Instead there were trop­i­cal forests rich with palms, bam­boos and gin­gers, accord­ing to Car­o­line Ström­berg, UW assis­tant pro­fes­sor of biol­ogy and lead author:

The assump­tion about grass­lands and the evo­lu­tion of these teeth was based on ani­mal fos­sils. No one had looked in detail at evi­dence from the plant record before. Our find­ings show that you shouldn’t assume adap­ta­tions always came about in the same way, that the trig­ger is the same envi­ron­ment every time.

To han­dle a life­time of rough abra­sion, the spe­cialised teeth — called high-​crowned cheek teeth — are espe­cially long and mostly up in the ani­mals’ gums when they are young. As chew­ing sur­faces of the teeth wear away, more of the tooth emerges from the gums until the crowns are used up. In each tooth, bone-​like dentin and tough enamel are com­plexly folded and lay­ered to cre­ate strong ridged sur­faces for chew­ing. Human teeth have short crowns and enamel only on the out­side of each tooth.

cau­tion is required when using this func­tional trait for habi­tat reconstruction
In Argentina, mam­mals appar­ently devel­oped spe­cialised teeth 20 mil­lion years or more before grass­lands appeared, Ström­berg said. This was dif­fer­ent from her pre­vi­ous work in North Amer­ica and west­ern Eura­sia where she found the emer­gence of grass­lands coin­cided with the early ances­tors of horses and other ani­mals evolv­ing spe­cialised teeth. The cause and effect, how­ever, took 4 mil­lion years, con­sid­er­ably more lag time than pre­vi­ously thought.

The idea that spe­cialised teeth could have evolved in response to eat­ing dust and grit on plants and the ground is not new. In the case of Argen­tine mam­mals, Ström­berg and her co-​authors hypoth­e­size that the teeth adapted to han­dle vol­canic ash because so much is present at the study site. For exam­ple, some lay­ers of vol­canic ash are as thick as 20 feet (six meters). In other lay­ers, soils and roots were just start­ing to develop when they were smoth­ered with more ash.

Chew­ing grasses is abra­sive because grasses take up more sil­ica from soils than most other plants. Sil­ica forms minute par­ti­cles inside many plants called phy­toliths that, among other things, help some plants stand upright and form part of the pro­tec­tive coat­ing on seeds.

Phy­toliths vary in appear­ance under a micro­scope depend­ing on the kind of plant. When plants die and decay, the phy­toliths remain as part of the soil layer. In work funded by the National Sci­ence Foun­da­tion, Ström­berg and her col­leagues col­lected sam­ples from Argentina’s Gran Bar­ranca, lit­er­ally “Great Cliff,” that offers access to lay­ers of soil, ash and sand going back mil­lions of years.

The phy­toliths they found in 38-​million-​year-​old lay­ers — when ancient mam­mals in that part of the world devel­oped spe­cialised teeth — were over­whelm­ingly from trop­i­cal forests, Ström­berg said.

In mod­ern grass­lands and savan­nahs you’d expect at least 35 to 40 per­cent — more likely well over 50 per­cent — of grass phy­toliths. The fact we have so lit­tle evi­dence of grasses is very diag­nos­tic of a forested habi­tat,” she said.

The emer­gence of grass­lands and the evo­lu­tion of spe­cialised teeth in mam­mals are regarded as a clas­sic exam­ple of co-​evolution, one that has occurred in var­i­ous places around the world. How­ever, as the new work shows, “cau­tion is required when using this func­tional trait for habi­tat recon­struc­tion,” the co-​authors write.

(Source: Uni­ver­sity of Wash­ing­ton news release, 04.03.2013)
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