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category: Paleontology

Dawn of the Deed, Part 1: Down and Dirty in the Devonian

Post: October 2, 2012 9:27 am
Author: John Long         Source: ETVOL Exclusive

If I were ever to appear on a TV quiz show, my bizarre specialty would be given as "Fishes of the Devonian Period (359-416 million years ago)." I became fascinated by ancient armor-plated fishes called "placoderms" just over 30 years ago when I collected my first placoderm fossils as an undergraduate working in eastern Victoria, Australia.

In my postgraduate days I led a field trip each year, taking first-year geology students into the remote wilds of the Victorian bush to examine outcrops of limestone and study their fossils. It was the first taste of the excitement of working in the wilderness for many of them. Some days, we had to delicately step around large venomous black snakes sunning themselves by the creek. On one occasion, we got lost and ended up swimming across a river with our geology hammers in our mouths like golden retrievers. And every year, I put a standing offer on the table for the students: a dozen bottles of beer for a complete placoderm skull.

One year, a student found the treasure, and I dutifully coughed up the beer. I used weak acetic acid to prepare the skull out of the limey rock, revealing a stunning 3-D specimen that became an entire chapter in my doctorate thesis for the new information it revealed. I've been hooked on placoderms ever since. Yet little did I know then that a lifetime of studying placoderms would one day draw me into the seedy world of Devonian fish sex!

In late 2007, I was preparing a superb 380-million-year-old placoderm that we collected from the Gogo site in Western Australia, using the same acid technique that I’d used back in my student days. Peering down the microscop, I noticed a tiny cluster of bones within its body cavity. My colleague Kate Trinajstic also examined it and we concluded that we had discovered an unborn embryo of a placoderm, in fact the first one ever found. Around it was a twisted white mineralized "rope" that later proved to be a fossilized umbilical cord. The discovery pushed the evidence for live birth in our distant vertebrate ancestors back by some 180 million years and was later listed in the 2010 Guinness Book of World Records.


embryo sm
embryo label sm
The tiny remains of the 380-million-year-old placoderm fish Materpiscis attenboroughi showing the delicate embryonic bones with white mineralized umbilical cord wrapped around them inside the tail section of the adult mother fish.


The next week, I woke up in a cold sweat one morning thinking that I’d seen similar tiny embryonic bones somewhere before. I scoured my computer for old images until I found another Gogo placoderm that I’d found in 1986. There, staring right at me, were 3 complete tiny embryos! I’d found a mother with triplets.

A month or so later, we were thinking about the implications of our discoveries. Previous finds of bony claspers (extensions on the pelvic fins) in ptyctodontid placoderms suggested the ability for sexual reproduction by copulation, but without embryos, there was no hard evidence to prove this. Our find now proved that these ancient armored fishes reproduced by internal fertilization. We had in fact discovered the oldest fossil evidence for the intimate act of vertebrate sex using copulation.

(More to come in the next installment.)

Reference
Long, J.A., Trinajstic, K., Young, G.C & Senden, T. 2008. Live birth in the Devonian Period. Nature, 453: 650-652.


John A. Long is Vice President of Research and Collections at the Natural History Museum of Los Angeles County. His most recent book is Dawn of the Deed: The Prehistoric Origins of Sex, published by the University of Chicago Press.