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Mas­sive geo­graphic change may have trig­gered Explo­sion of Ani­mal Life

pub­lished 07 Novem­ber 2014 | mod­i­fied 22 April 2019

A paper by Ian Dalziel of The Uni­ver­sity of Texas at Austin’s Jack­son School of Geo­sciences, sug­gests a major tec­tonic event may have trig­gered the rise in sea level and other envi­ron­men­tal changes that accom­pa­nied the appar­ent burst of life.

Cambrian explosionThe Cam­brian explo­sion is one of the most sig­nif­i­cant events in Earth’s 4.5-billion-year his­tory. The surge of evo­lu­tion led to the sud­den appear­ance of almost all mod­ern ani­mal groups. Fos­sils from the Cam­brian explo­sion doc­u­ment the rapid evo­lu­tion of life on Earth, but its cause has been a mystery.

The sud­den burst of new life is also called “Darwin’s dilemma” because it appears to con­tra­dict Charles Darwin’s hypoth­e­sis of grad­ual evo­lu­tion by nat­ural selection.

At the bound­ary between the Pre­cam­brian and Cam­brian peri­ods, some­thing big hap­pened tec­ton­i­cally that trig­gered the spread­ing of shal­low ocean water across the con­ti­nents, which is clearly tied in time and space to the sud­den explo­sion of mul­ti­cel­lu­lar, hard-​shelled life on the planet,” said Dalziel, a research pro­fes­sor at the Insti­tute for Geo­physics and a pro­fes­sor in the Depart­ment of Geo­log­i­cal Sciences.

Beyond the sea level rise itself, the ancient geo­logic and geo­graphic changes prob­a­bly led to a buildup of oxy­gen in the atmos­phere and a change in ocean chem­istry, allow­ing more com­plex life-​forms to evolve, he said.

The paper, first pub­lished online on 26 Sep­tem­ber in the jour­nal Geol­ogy, is the first to inte­grate geo­log­i­cal evi­dence from five present-​day con­ti­nents – North Amer­ica, South Amer­ica, Africa, Aus­tralia and Antarc­tica – in address­ing pale­o­geog­ra­phy at that crit­i­cal time.

It appears ancient North Amer­ica was ini­tially attached to Antarc­tica and part of South Amer­ica, not to Europe and Africa, as has been widely believed

Ian Dalziel, Insti­tute for Geo­physics, Jack­son School of Geo­sciences, The Uni­ver­sity of Texas at Austin, USA »

Dalziel pro­poses that present-​day North Amer­ica was still attached to the south­ern con­ti­nents until some­time into the Cam­brian period. Cur­rent recon­struc­tions of the globe’s geog­ra­phy dur­ing the early Cam­brian show the ancient con­ti­nent of Lau­ren­tia – the ances­tral core of North Amer­ica – as already hav­ing sep­a­rated from the super­con­ti­nent Gond­wana­land.

In con­trast, Dalziel sug­gests the devel­op­ment of a deep oceanic gate­way between the Pacific and Iape­tus (ances­tral Atlantic) oceans iso­lated Lau­ren­tia in the early Cam­brian, a geo­graphic makeover that imme­di­ately pre­ceded the global sea level rise and appar­ent explo­sion of life.

The rea­son peo­ple didn’t make this con­nec­tion before was because they hadn’t looked at all the rock records on the dif­fer­ent present-​day con­ti­nents,” he said.

The rock record in Antarc­tica, for exam­ple, comes from the very remote Ellsworth Mountains.

Peo­ple have won­dered for a long time what rifted off there, and I think it was prob­a­bly North Amer­ica, open­ing up this deep sea­way,” Dalziel said. “It appears ancient North Amer­ica was ini­tially attached to Antarc­tica and part of South Amer­ica, not to Europe and Africa, as has been widely believed.”

Although the new analy­sis adds to evi­dence sug­gest­ing a mas­sive tec­tonic shift caused the seas to rise more than half a bil­lion years ago, Dalziel said more research is needed to deter­mine whether this new chain of pale­o­geo­graphic events can truly explain the sud­den rise of mul­ti­cel­lu­lar life in the fos­sil record.

I’m not claim­ing this is the ulti­mate expla­na­tion of the Cam­brian explo­sion,” Dalziel said. “But it may help to explain what was hap­pen­ing at that time.”

Another expla­na­tion for sud­den shifts in evo­lu­tion­ary records has been sug­gested by Stu­art A. New­man with his alter­na­tive model in propos­ing that the orig­i­na­tion of the struc­tural motifs of ani­mal form were actu­ally pre­dictable and rel­a­tively sud­den, with abrupt mor­pho­log­i­cal trans­for­ma­tions favoured dur­ing the early period of ani­mal evolution.

(Source: The Uni­ver­sity of Texas news release, 03.11.2014; Wikipedia)

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