Scientists have revealed a spectacular insight into turtle evolution – how the unique animals get their shells.
A Japanese team studied the development of turtle embryos to find out why their ribs grow outward and fuse together to form a tough, external carapace.
Reporting in the journal Science, the researchers compared turtle embryos with those of chicks and mice.
They found that, as turtles developed, part of their body wall folded in on itself forcing the ribs outward.
The team of researchers from the Riken Center for Developmental Biology in Kobe, Japan, described the turtle shell as an “evolutionary novelty”.
It represents such a leap from the soft-bodied ancestors that turtles share with mammals and birds, that scientists have long puzzled over how exactly it came about.
“Other groups have looked into why the shoulder blade in turtles is encased inside the rib cage,” said Olivier Rieppel from Field Museum in Chicago, an expert in reptile evolution who was not involved in this study.
“That makes them unique.”
… Dr Kuratani explained that some of the connections between developing bones and muscles were the same as in birds and mammals, but there were some, including the pectoral muscles, that “showed entirely unique (types of) connectivity in turtles”.
The discovery helps define a position in evolutionary history for a 220-million-year-old turtle fossil discovered last year in China, which had an incomplete shell that only covered its underside.