Skull com­pletes the pic­ture

A Gra­ham­stown sci­en­tist has helped pro­vide the link with the last ma­jor ver­te­brate group to the tree of life. Steven Lang re­ports

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oost­huizeni around the world.

They are strange-look­ing fish with names that re­flect their pe­cu­liar ap­pear­ances: rat­fish, rab­bit fish, ghost sharks, St Joseph sharks or ele­phant sharks.

These deep-wa­ter-dwelling fish have large eyes which help them for­age in dimly lit wa­ters, sug­gest­ing that they may have adapted to lightlim­ited con­di­tions.

The new ev­i­dence sug­gests that this adap­ta­tion oc­curred ear­lier than pre­vi­ously thought. In­stead of teeth, chi­maeras have tooth plates adapted for grind­ing their meals. Dwykaselachus oost­huizeni is named af­ter Roy Oost- huizen, an ama­teur palaeon­tol­o­gist who found the fos­sil on his farm near Prince Al­bert in the 1980s. Sus­pect­ing that a smooth stone in the veld was a nod­ule (the hard case around a fos­sil, that tends to weather out as a peb­ble) he cracked it open.

The ex­posed part of the bro­ken rock re­vealed to sci­en­tists that Oosthuizen had found a fos­sil skull be­long­ing to an ex­tinct shark-like car­ti­lagi­nous fish. It was named in 1986.

The new study, that re­veals the whole skull for the first time, al­lows the au­thors to iden­tify it as com­ing from an ex­tinct or­der of ‘sharks’, the Sym­mori­idae.

Anal­y­sis of the scans also re­vealed tell­tale struc­tures of the brain, ma­jor cra­nial nerves, nos­trils and in­ner ear show­ing that in­ter­nally Dwykaselachus is very sim­i­lar to mod­ern-day chi­maeras. This com­bi­na­tion of shark-like and chi­maeroid fea­tures has al­lowed the re­searchers to solve the rid­dle of where chi­maeroids branched from sharks.

Lead au­thor of the study, Pro­fes­sor Michael Coates of the Uni­ver­sity of Chicago, said that chi­maeras are un­usual through­out the long span of their fos­sil record.

“Be­cause of this, it’s been dif­fi­cult to un­der­stand how they got to be the way they are in the first place. This dis­cov­ery sheds new light not only on the early evo­lu­tion of shark-like fishes, but also on jawed ver­te­brates as a whole.”

Chi­maeras rarely fos­silise be­cause, like sharks, their skele­tons are made of car­ti­lage and not bone. Un­til now, the chi­maeroid evo­lu­tion­ary record con­sisted mostly of iso­lated spec­i­mens of their hy­per-min­er­alised tooth plates.

*Dr Robert Gess of the Albany Mu­seum is a South African Cen­tre of Ex­cel­lence in Palaeon­tol­ogy part­ner who was based at the Rhodes Ge­ol­ogy De­part­ment at the time.

Photo: Kris­ten Ti­et­jen

Re­con­struc­tion of a sym­morid shark, with brain case.

Photo: Coates, et al, Na­ture Photo: Uni­ver­sity of Chicago

Right: Rock nod­ule con­tain­ing the Dwyka fos­sil.

3D printed mod­els of the in­te­rior and ex­te­rior of the Dwykaselachus brain case.

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