Turtles do not belong to a primitive group of reptiles, but to a sister group of birds, crocodiles and extinct dinosaurs, according to the International Turtle Genome Consortium's recently released report.

Based on a study of genomic information, the researchers predict that turtles must have split from this group around 250 million years ago, during the great extinction event.

"Turtles are interesting because they offer an exceptional case to understand the big evolutionary changes that occurred in vertebrate history," said Dr. Naoki Irie, from Japan's RIKEN Center for Developmental Biology, who led the study.  "We expect that this research will motivate further work to elucidate the possible causal connection between these events," said Irie. 

Using next-generation DNA sequencers, Irie and colleagues decoded the genome of the green sea turtle and Chinese soft-shell turtle and studied the expression of genetic information in the developing turtle.

Their results show that turtles are not primitive reptiles as previously thought, but are related to the group comprising birds and crocodiles, which also includes extinct dinosaurs, according to a release from the RIKEN center.

The research also unveiled that turtles follow a basic embryonic development pattern, despite their unique anatomy.

Rather than developing directly into a turtle-specific body shape with a shell, the gene sequencing shows that turtles first establish the vertebrates' basic body plan and then enter a turtle-specific development phase. During this late specialization phase, the group found traces of limb-related gene expression in the embryonic shell, which indicates that the turtle shell evolved by recruiting part of the genetic program used for the limbs. 

"The work not only provides insight into how turtles evolved, but also gives hints as to how the vertebrate developmental programs can be changed to produce major evolutionary novelties." said Irie.

The genomic sequence also showed that turtles must have an extraordinary sense of smell due to more than 1,000 olfactory receptors in the soft-shell turtle, which the center reports is one of the largest numbers to be found in a non-mammalian vertebrate.

The study is published in Nature Genetics.