Watching a snake swallow its prey whole is an awesome experience. Snakes feed like no other animal, thanks to their amazingly stretchable jaws. But how this flexible jaw evolved from the inflexible jaws of snakes? lizard ancestors has posed a puzzle up to now. Now an intermediate stage in the evolution of a stretchable jaw has at last been identified in the extinct mosasaurs, fossil lizard cousins of present-day snakes, by Michael S.Y. Lee from the University of Queensland and colleagues.

The mosasaurs were formidable predatory marine lizards, 3 to 10 metres long, that hunted sea turtles and other marine reptiles in the Late Cretaceous seas around 100 million years ago. As Lee?s team describe in Nature[12 August], unlike all other known lizards, fossil or living, they had highly flexible lower jaws like snakes and long teeth on the palate for gripping their prey. But they retained the immobile upper jaw typical of a lizard.

Snakes evolved from lizard-like ancestors, losing their legs in the process and acquiring many more vertebrae in their backbone. They also acquired a quite new way of feeding, developing highly mobile and extensible jaws that can spread right round an item of prey and draw it down into the throat.

In snakes, the individual bones of the jaws are not fused to each other as they are in lizards and other vertebrates, but can all move independently, enabling the snake to stretch its mouth around an animal many times the size of the snake?s head.

Mosasaurs share a common ancestor with the snakes, but their evolution took another path. They had a long streamlined body, a deep tail and paddle-shaped limbs. Their strong, snout-like jaws were lined with sharp teeth for gripping their prey.

But the lower jaw of the mosasaur resembles that of a snake far more than that of a lizard. It has a loose joint between the front and back halves of the lower jaw, which immediately makes it able to stretch round a larger object than the jaw of a lizard of the same size. Mosasaurs still have an inflexible upper jaw like that of lizards, but, like snakes, have long gripping teeth at the back of the palate.

Lee?s group has thus been able to trace the anatomical changes that accompanied the evolution of snakes? unique way of feeding. Mobility in the lower jaw must have developed first, in some shared ancestor of mosasaurs and snakes. Once the snake line had split off from that of the mosasurs, the jaws acquired their other unique characteristics.

The most primitive living snakes have flexible joints between the bones of the upper jaw as well as additional gripping teeth on the palate. The most advanced living snakes have, in addition, acquired a joint at the front of the lower jaw that enables the two halves of the lower jaw to move apart.