[D66] New fossil finds filling in history of tetrapods

[Henk Elegeert]

New fossil finds filling in history of
tetrapods<http://arstechnica.com/science/news/2012/03/new-fossil-finds-filling-in-history-of-tetrapods.ars>
By Scott K. Johnson <http://arstechnica.com/author/scott-johnson/> |
Published about 5 hours ago
[image: New fossil finds filling in history of tetrapods]
An amphibious tetrapod from the Carboniferous period

Back in 2004, a fossil tetrapod from the Canadian Arctic achieved
near-celebrity status, at least for mineralized skulls. The newly
discovered specimen, named *Tiktaalik roseae*<http://tiktaalik.uchicago.edu/>,
made appearances on the Colbert Report and PBS NewsHour. (The scientists
involved got to share a bit of the spotlight, as well.) The find was
noteworthy because it supplied a long sought-after link between fish and
their four-legged descendants. We had plenty of fossil fish that fill the
bill as ancestor and a number of early tetrapods, but we were missing some
of the details about how our ancestors lost their "fishiness."

The old joke about finding a new fossil is that it simply creates two new
gaps in the record, one on either side. But a bit after *Tiktaalik*, there
was an actual gap: Romer's
Gap<http://arstechnica.com/science/news/2006/11/6076.ars>,
named for the scarcity of fossils that covered the first appearance of
four-limbed animals on land. Several new finds in Scotland are now filling
in Romer's gap.

In the 1950s, paleontologist A.S. Romer pointed out a 30 million year long
break in the tetrapod fossil record, a blank space that picked up his name.
Later work whittled the gap down to 15 million years, but the transition
was still pretty stark.

Prior to the gap, tetrapods were amphibious, using their four limbs to
navigate shallow water and riverine environments. On the other side of the
gap, many were already fully terrestrial. The early, amphibious tetrapods
still had fish-like tail fin structures and appendages with more than five
digits. Their terrestrial descendants had fewer digits and their skulls
were smaller and less flattened.

Romer’s Gap begins with an extinction event at the end of the Devonian,
about 359 million years ago. Geologists have found evidence that the
extinction was caused by an abrupt glacial period accompanied by large
swings in sea level. Tetrapods weren’t the only group to come out of the
catastrophe looking a bit different than they did on the way in, but it’s
not entirely clear how the glacial period drove so many terrestrial and
marine animals to extinction.

In a new study published in the *Proceedings of the National Academy of
Sciences*, researchers from Cambridge and the University of Southampton
describe the discovery of a number of new fossils that will help fill in
Romer’s Gap and allow us to test some ideas about that extinction event.

The fossils come from four sites in Scotland, just east of Edinburgh. All
the fossils were found in the same layer of mudstone, which was deposited
towards the end of the 15 million year gap. The mudstone yielded a number
of tetrapods as well as a host of other organisms; the tetrapods
represented include both terrestrial and aquatic species.

The finds suggest that Romer’s Gap is mainly a gap in collection. Rocks of
the right age simply haven’t been sought out and worked thoroughly enough.
Rather than a scenario where tetrapods changed dramatically, either right
after the end-Devonian extinction or at the end of Romer’s Gap, the
researchers see evidence of a gradual transition.

Other hypotheses about this time period will also be testable at these new
sites. In 2006, a group of paleontologists proposed that atmospheric oxygen
may have dropped sharply, stressing the early tetrapods’ newly-adapted
respiratory systems and sending them back to the water. Such a change would
also impact terrestrial arthropods, which were also uncovered at the new
sites in Scotland. A close look at those arthropods may allow researchers
to rule out the effect of atmospheric oxygen.

Gaps like Romer's are exciting for paleontologists, because where there’s
an unknown, there’s something fascinating waiting to be discovered. These
finds in Scotland promise to supply plenty of material for paleontologists
to learn about what was going on during Romer’s Gap, which may soon be a
misnomer.

This should also be a lesson for young scientists—don’t be too flattered if
your name becomes attached to an unexplained phenomenon. It won’t last.

*PNAS*, 2012. DOI:
10.1073/pnas.1117332109<http://dx.doi.org/10.1073/pnas.1117332109>
  (About DOIs<http://arstechnica.com/science/news/2010/03/dois-and-their-discontents-1.ars>
).
Image courtesy of Wikimedia
Commons<http://upload.wikimedia.org/wikipedia/commons/5/5e/LoxommaB2.jpg>
`
Een heel intelligent idee. :)
Henk Elegeert
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