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Ancestor Of The “living Fossil” Sheds New Light

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Ancestor of the “living fossil” sheds new light


Aug. 1, 2007

Special to World Science


Zo­ol­o­gists called it the find of the cen­tu­ry when in 1938, fish­er­men hoisted ashore a fish thought to have been ex­tinct since the di­no­saurs roamed. Called a coe­la­canth, it was a rel­a­tive of some of the first land-walk­ing crea­tures.


Now, sci­en­tists are re­port­ing a fos­sil find that helps com­plete the sto­ry: the front fin of an early coe­la­canth, which is quite dif­fer­ent from that of to­day’s coe­la­canths.


It clarifies the ev­o­lu­tion of this cru­cial struc­ture, they say, which in fish de­scen­dants evolved in­to walk­ing limbs and then arms.


The fos­sil re­veals con­nec­tions to even more prim­itve fish, and shows that the fin bones still had to evolve a fair amount be­fore the first walk­ing crea­tures arose, ac­cord­ing to the sci­en­tists. They de­scribed the find­ing in a pa­per in the Ju­ly/Au­gust is­sue of the re­search jour­nal Ev­o­lu­tion & De­vel­op­ment.


Peo­ple often see coe­la­canths as “liv­ing fos­sils,” but that’s not quite ac­cu­rate, said Matt Fried­man, a grad­u­ate stu­dent at the Un­ivers­ity of Chi­ca­go and lead au­thor of the pa­per. “If you look deep in the fos­sil rec­ord to the first mem­bers of that group, they are really dif­fer­ent and very di­verse.” Same goes for some oth­er so-called liv­ing fos­sils, he added.


The 400 mil­lion-year-old coe­la­canth fos­sil is the first known of its kind, and fills a shrink­ing ev­o­lu­tion­ary gap be­tween fins and limbs, the re­search­ers said. Sci­en­tists are in­ter­est­ed in early coe­la­canths be­cause they’re close rel­a­tives of the first so-called fleshy-finned fish­es. This is the line­age that, with their meaty fins, gave rise to limbed ver­te­brates that took the first steps on­to land.


Yet the fos­sil fin is­n’t as si­m­i­lar to mod­ern fleshy-finned fish as it is to some prim­i­tive mem­bers of the oth­er great line­age of bony fish­es—the ray-finned fish­es, Fried­man and col­leagues said. These are the larg­est class of fish, and those whose fins are webs of skin sup­ported by spines. Some liv­ing ray-finned fish­es such as paddlefish­es and stur­geons have a branch­ing ar­range­ment of bones si­m­i­lar to that found in the coe­la­canth fos­sil, Fried­man and col­leagues said.


“This ends in­tense de­bate about the prim­i­tive pat­tern for lobed fins, which in­volves the an­ces­try of all limbs, in­clud­ing our own,” said the un­ivers­ity’s Mi­chael Coates, one the re­search­ers. “To un­der­stand the de­vel­op­men­tal ev­o­lu­tion of the limbs of tetrapods [four-limbed ver­te­brates], we should­n’t be look­ing at the fins of our near­est liv­ing fish rel­a­tives—lungfish­es and coe­la­canths—be­cause they’re far too spe­cial­ized.”


Sci­en­tists be­lieve anoth­er re­cently dis­cov­ered fos­sil is a true mis­sing link be­tween fish and tetrapods. It was a fierce pred­a­tor dubbed Tik­taa­lik roseae, which lived 385 mil­lion years ago.


The early coe­la­canth fin fos­sil shows that as far as limbs go, the key dif­fer­ence sep­a­rat­ing early fleshy-finned fish­es and Tik­taa­lik was in fin bones called ra­di­als, Fried­man and col­leagues wrote. These are widely thought to have evolved in­to fin­gers.


The fos­sil coe­la­canth is named Sho­sho­nia arc­topteryx af­ter the Sho­sho­ni peo­ple of Wy­o­ming and the Sho­sho­ne Na­tional For­est in north­ern Wy­o­ming, where the spec­i­men was found. “It was as­ton­ish­ing luck,” Fried­man said, adding that the fos­sil had fall­en off a cliff about 200 feet high on­to some rocks. The four-inch (10 cm) long spec­i­men de­tails the fin of the an­i­mal, which the sci­en­tists ap­prox­i­mate would have been about 18 to 24 inches (45 to 60 cm) long.

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