|
A
Computed Tomography rendering of a snapping turtle
(Chelydra serpentina) showing the skeleton (white), lungs
(blue),
and abdominal muscles (red and pink) used to ventilate
the lungs. Because
turtles have locked their ribs up into the
iconic turtle shell, they can no
longer use their ribs to breathe as
in most other animals and instead have
developed a
unique abdominal muscle based system.
Photo credit: Emma R.
Schachner.
|
Through
the careful study of modern and early fossil tortoise, researchers now have a
better understanding of how tortoises breathe and the evolutionary processes
that helped shape their unique breathing apparatus and tortoise shell. The
findings published in a paper, titled: Origin of the unique ventilatory
apparatus of turtles, in the scientific journal, Nature Communications, on
Friday, 7 November 2014, help determine when and how the unique breathing
apparatus of tortoises evolved.
Lead
author Dr Tyler Lyson of Wits University's Evolutionary Studies Institute, the
Smithsonian Institution and the Denver Museum of Nature and Science said:
"Tortoises have a bizarre body plan and one of the more puzzling aspects
to this body plan is the fact that tortoises have locked their ribs up into the
iconic tortoise shell. No other animal does this and the likely reason is that
ribs play such an important role in breathing in most animals including
mammals, birds, crocodilians, and lizards."
Instead
tortoises have developed a unique abdominal muscular sling that wraps around
their lungs and organs to help them breathe. When and how this mechanism
evolved has been unknown.
"It
seemed pretty clear that the tortoise shell and breathing mechanism evolved in
tandem, but which happened first? It's a bit of the chicken or the egg
causality dilemma," Lyson said. By studying the anatomy and thin sections
(also known as histology), Lyson and his colleagues have shown that the modern
tortoise breathing apparatus was already in place in the earliest fossil
tortoise, an animal known as Eunotosaurus
africanus.
This
animal lived in South Africa 260 million years ago and shares many unique
features with modern day tortoises, but lacked a shell. A recognizable tortoise
shell does not appear for another 50 million years.
Lyson
said Eunotosaurus bridges
the morphological gap between the early reptile body plan and the highly
modified body plan of living tortoises, making it the Archaeopteryx of turtles.
"Named
in 1892, Eunotosaurus is
one of the earliest tortoise ancestors and is known from early rocks near
Beaufort West," said Professor Bruce Rubidge, Director of the Evolutionary
Studies Institute at Wits University and co-author of the paper.
"There
are some 50 specimen of Eunotosaurus.
The rocks of the Karoo are remarkable in the diversity of fossils of early
tortoises they have produced. The fact that we find Eunotosaurus at the base of the Karoo succession strongly
suggest that there are more ancestral forms of tortoises still to be discovered
in the Karoo," Rubidge added.
The
study suggests that early in the evolution of the tortoise body plan a gradual
increase in body wall rigidity produced a division of function between the ribs
and abdominal respiratory muscles. As the ribs broadened and stiffened the
torso, they became less effective for breathing which caused the abdominal
muscles to become specialized for breathing, which in turn freed up the ribs to
eventually -- approximately 50 million years later -- to become fully
integrated into the characteristic tortoise shell.
Lyson
and his colleagues now plan to investigate reasons why the ribs of early
tortoises starting to broaden in the first place. "Broadened ribs are the
first step in the general increase in body wall rigidity of early basal
tortoises, which ultimately leads to both the evolution of the tortoise shell
and this unique way of breathing. We plan to study this key aspect to get a
better understanding why the ribs started to broaden."
Citation
Lyson
TR, Schachner ER, Botha-Brink J, Scheyer
TM, Lambertz M, Bever GS, Rubidge, BS de Queiroz K. Origin of the unique
ventilatory apparatus of turtles. Nature Communications, 2014; 5: 5211 DOI: 10.1038/ncomms6211