Around 260-mil­lion years ago, the earth was dom­i­nat­ed by mam­mal-like rep­tiles called ther­a­p­sids. The largest of these ther­a­p­sids were the dinocephalians, a genus com­posed of sev­er­al her­biv­o­rous and car­niv­o­rous species.

Then some­thing enor­mous hap­pened: a mass extinc­tion event killed off between 75% and 80% of all the crea­tures that lived on land around the world. Many ocean crea­tures were also ren­dered extinct. The dinocephalians were wiped out.

Sev­er­al hypothe­ses have been offered about what could have pro­voked this mass extinc­tion. For instance, many sci­en­tists have favoured the notion that a vol­canic erup­tion was the trig­ger. It has been demon­strat­ed that at the time of the extinc­tion inter­val a gigan­tic vol­canic erup­tion occurred at Emeis­han in the south of Chi­na that last­ed for almost two mil­lion years. It released around 300 000 km³ of lava.

My col­leagues and I won­dered whether a change in cli­mate might have caused or con­tributed to the mass extinc­tion in South Africa. So we exam­ined envi­ron­men­tal change dur­ing the extinc­tion event in what’s today the west­ern and north­ern Cape of South Africa. We stud­ied the fos­sil teeth of Diictodon feli­ceps, a small her­biv­o­rous ther­a­p­sid which lived before the extinc­tion event and sur­vived through it (as with much else from this peri­od, we don’t know how the species sur­vived the catastrophe).

Our find­ings sug­gest that a local event rather than a glob­al shift in cli­mate was to blame for the mass extinc­tion in South Africa. Specif­i­cal­ly, we pro­pose that defor­ma­tions in the Cape Fold Moun­tains meant there was less water avail­able; species in the region were decimated.

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The Drak­en­stein Moun­tains are part of the Cape Fold Belt. (Image: Flickr)

This marks the first time that data has shown a cor­re­la­tion between the mass extinc­tion event and an event of arid­i­fi­ca­tion – the process that occurs when a region becomes increas­ing­ly dry. It’s a long-term shift in cli­mate rather than a sea­son­al vari­a­tion. Our find­ings are sup­port­ed by ear­li­er work con­duct­ed on sed­i­ment which showed a decrease in riv­er and stream out­put in the basin of the Cape Fold Moun­tains dur­ing that time.

It seems from this study that while the Emeis­han erup­tion may have trig­gered the mass extinc­tion event, oth­er more local events might have ampli­fied it.

Teeth reveal cli­mate secrets

Fos­sil teeth con­tain dif­fer­ent atoms, like oxy­gen or car­bon, which are rep­re­sent­ed with sev­er­al forms”, called iso­topes. Exam­in­ing their ratio allows sci­en­tists to inter­pret how the humid­i­ty and tem­per­a­ture were chang­ing when the ani­mal in ques­tion was alive.

The teeth we stud­ied showed that an arid­i­fi­ca­tion – a decrease in humid­i­ty as pro­duced by water sup­ply or rain­fall – occurred at the time of the extinc­tion. But there was no change in tem­per­a­ture. This sug­gest­ed that glob­al cli­mate change didn’t lead to the mass extinc­tion, since in the case of glob­al cli­mate shifts humid­i­ty and tem­per­a­ture changes go together.

Our focus was on the sta­ble oxy­gen and car­bon iso­tope com­po­si­tions in the fos­sil teeth.

Sev­er­al fac­tors can affect the ratio between the dif­fer­ent iso­tope of these ele­ments. For instance, the ratio of oxy­gen iso­topes is cor­re­lat­ed with the air tem­per­a­ture of the envi­ron­ment; for car­bon, the iso­tope ratio depends on the water avail­abil­i­ty. This makes it a good proxy to esti­mate an environment’s aridity.

Mea­sur­ing the oxy­gen and car­bon iso­tope com­po­si­tion in the teeth helped us esti­mate the tem­per­a­ture and the arid­i­ty of the envi­ron­ment where the ani­mal lived.

Future learning

The Conversation

This kind of work is valu­able because it shows how local cli­mate events like arid­i­fi­ca­tion can affect a region’s ecosys­tems. It might also help cli­mate change researchers to bet­ter esti­mate how cer­tain parts of the plan­et might react as water becomes scarcer.

This arti­cle was orig­i­nal­ly pub­lished on The Con­ver­sa­tion. Read the orig­i­nal arti­cle.