Science News - August 1, 2009

In the News

A turtle’s shell
isn’t so terribly
bizarre after all
A critical fold may separate
the reptile from its brethren
STORY ONE

Key
s Scapula
s Ribs
s Muscle plate

Different body plans

A fold (orange arrow, left) in a turtle’s muscle plate that forms in the embryonic stage leads to an unusual body plan with the scapulae, or shoulder blades, inside the ribs. The muscle plate in humans and other land-dwelling vertebrates doesn’t have the deep fold (orange arrow, right), and the scapulae sit outside the ribs.

By Susan Milius

Turtles may be weird, but accord- ing to new research, they’re not hat weird. Their funny arrange- ment of shell and shoulder is just the same old land-dweller vertebrate stuff — with a little fold.

SHIGERU KURATANI, H. NAGASHIMA

Early on, a turtle embryo grows much like a chicken or mouse. But then the developing body wall makes a critical fold, and the usual body plan starts to become an unusual turtle, Hiroshi Nagashima of the RIKEN Center for Developmental Biology in Kobe, Japan, and his colleagues report in the July 10 Science.

Nothing else has a body plan like a turtle. Its ribs don’t grow inside its chest as a cage but instead fuse in the developing

skin layer on its back to create one bony armored covering.

“It is not just that turtles ‘grew a shell,’” says paleontologist Ben Kear of La Trobe University in Melbourne, Australia. In the evolution of that shell, bones and muscles had to shift around relative to other reptiles, birds and mammals, and turtle shoulders ended up inside the rib cage. “In essence this means that the turtle skeleton is inside out,” he says.

Odd as they are, turtles clearly belong to the lineage of amniotes, which includes mammals, birds and reptiles. Turtles, which have existed for at least 200 million years, “have survived all kinds of stuff — we’re talking extinction of the dinosaurs and myriad climate changes,” Kear says. Yet there’s scant fossil evidence of turtles-in-the-making to explain how the shell-and-shoulder arrangement arose as turtles split off from birds and crocodiles.

Knowing how a basic amniote embryo ends up developing into something so radically different could shed light on turtle

history, says paleontologist Michael Lee of the South Australian Museum at the University of Adelaide. “Some intermediate stages in this process might resemble real intermediate — fossil — stages in evolution,” he says.

To sort out how turtles develop, Nagashima and his colleagues worked with eggs of Chinese soft-shelled turtles (Pelodiscus sinensis) bought from farms. The researchers used tissue-specific stains as well as substances that detect activity of particular genes to figure out which bits of the tiny embryos were on their way to becoming the bones and muscles of the adult.