Science News - August 1, 2009

SN Today at www.sciencenews.org

turtle embryos veered off on their own path. The developing muscle tissue that would lie along adult ribs in a standard amniote began to fold underneath itself in the turtle. This tissue tucked inward, bending up to lie below the developing ribs. On this kinked-under section, the shoulder blades, or scapulae, formed.

If this fold could be straightened out, the scapulae would lie outside the rib cage, as they do in chickens, mice and people. For turtles then, “the position of the scapula is not a novelty,” Nagashima says. Essentially, “turtles have the same body plan as other amniotes.”

That critical fold in the tissue maps out a line that becomes an important embryological feature of turtle embryos called the carapacial ridge. Earlier research has shown that this ridge drives the development of the bony back of the animal. The fold also allows developing muscles to form connections in ways that they

don’t in the mouse and the chicken.

The researchers also note that the turtle ribs stop short in comparison with mice and chickens. Turtle ribs grow out only along the sides of what will become the backbone instead of curving into the

Back Story | TURTLES’ ALTERNATIVE ANATOMY

Growing a shell of fused ribs means that skeletons and muscles are connected in different ways in turtle bodies than in bird, mammal and other reptile bodies. Here the turtle is compared with a chicken.

ld

lsr

as sc

Cross-section view
Bones
sc – Scapula
r – Ribs

Muscles
as–Serratus anterior
lsr–Levator scapulae
and rhomboid complex
ld – Latissimus dorsi
p – Pectoralis

ld

r

sc as

lsr

p

Chicken

Turtle

ld

r

p

Side view
Bones
r – Ribs
nu – Nuchal
(in turtle only)

Muscles
ld – Latissimus dorsi
p – Pectoralis

nuld

p

6 | SCIENCE NEWS | August 1, 2009

In a new study, researchers tracked developing embryos from Chinese soft-shelled turtles (grown turtle, left) and found that, compared with chickens, ribs in turtles are shorter. The ribs stop at a ridge of tissue (arrows in embryo, below) and fuse to form the shell.

body wall to form the whole rib cage. Those short turtle ribs mingle with the skin tissue, creating the fused bony shell on the turtle’s back.

“Very, very sophisticated work,” says reptile paleontologist Olivier Rieppel of the Field Museum in Chicago in describing the extensive detective work required to trace all the tissues and muscles.

He has studied the oldest known fossil of an ancestral turtle, and he says the new interpretation of turtle embryology may fit well with the fossil record.

Last year he and colleagues described Odontochelys semitestacea from a fossil collected in 220-million-year-old marine sediments in southwestern China. The turtle had a standard armored underside but not a full shell on its back. Its ribs widened, but its shoulder blades still lay forward of the ribs instead of inside them.

Nagashima speculates that the embryonic fold was evolving a bit at a time and maybe hadn’t reached as far around the body in this ancestral turtle as it does today. Clever suggestion, Rieppel says.

To understand turtle history, paleontologists really need more fossils, says Robert Reisz of the University of Toronto’s Mississauga campus in Canada. In the meantime, the new Japanese paper “clarifies a unique evolutionary event, one that gave rise to a really neat group of animals, our beloved turtles.” s