February 25, 2009

Fossils Reveal Nature Of Sex Among Fish During Prehistoric Times

New research published Wednesday shows that sex has been around for much longer than many scientists had previously believed, with internal fertilization prevalent among prehistoric fish living on tropical reefs during the Devonian period 380 million years ago.

The study reveals new insight on the reproductive history of all jawed vertebrates, including humans.

"It shifts how we think about how reproduction evolved. You're a jawed vertebrate and I'm a jawed vertebrate, so this is our own history," paleontologist Zerina Johanson of the Natural History Museum in London told Reuters.

Johanson and colleagues unearthed fossils in Australia, and concluded that copulation was widespread among armored placoderms, an extinct shark-like species, after discovering embryos inside Austroptyctodus, Materpiscis, and Incisoscutum placoderms.

Further supporting evidence was discovered in a modification in the pelvic fin on the stomach of adult fish, something scientists believe was used by male fish to grip females during mating, as happens with modern sharks.

Placoderms are believed to be among the oldest jawed vertebrates, the largest of which were as big as a great white shark.  They were formidable predators with bony armor across their heads forming the biting surfaces of their jaws, which acted like self-sharpening scissors.

Fossil evidence of reproduction is rare.  Indeed, experts initially failed to notice signs in one specimen, where a tiny embryo was first thought to be a final meal.

Scientists had previously believed such ancient fish would exhibit a more primitive type of reproduction, with eggs and sperm combining in the water externally as with many modern fish.


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