Pelvic spur

Pelvic spurs, also called vestigial legs, are small bumps found near the cloaca in some types of snakes. These bumps are made from a tiny bone called the femur, which is covered by a hard, claw-like layer. The femur bone comes from the legs that ancient snakes and their relatives once had.

Even though these spurs can't help the snake move, they still have important jobs. In some snakes, males use their spurs to hold and stimulate females when they are getting ready to have babies. The spurs can also be used by males to fight with each other.

These structures show us how snakes evolved from animals that had legs to snakes that slither today. They are a reminder of the past, even though they don't work like legs anymore.

Fossil record

The fossil record of snakes is not very large, but it does show how snakes lost their legs over time. One important fossil snake is Najash rionegrina, first described in 2006. Its fossils were found in the Patagonia region of Argentina and date back to the Upper Cretaceous period. This snake had a sacrum, a pelvic girdle, and strong hind limbs outside its ribcage, showing that these limbs were used for movement.

This discovery is important because, at the same time, there were three other known legged snake species, Pachyrhachis problematicus, Haasiophis terrasanctus, and Eupodophis descouensi. However, these were likely sea snakes and did not have the sacrum region that N. rionegrina had. Scientists believe N. rionegrina is the oldest known snake, and the other three species are more closely related to modern snakes in the group Alethinophidia.

Recent studies of many fossils support these ideas and show how pelvic and hind limb structures got smaller in these snake families. We can also see evidence of these structures in some living snakes, like Candoia carinata or Eunectes murinus, which have a triradiate pelvis. This can also be seen in the skeletons of modern lizards.

Distribution

Pelvic spurs are not found in most snake groups. They are mainly seen in some early groups of snakes, such as Booidea, Pythonoidea, Amerophidia, and one member of Uropeltoidea, the Cylindrophiidae. These groups are often called primitive snakes because they are some of the earliest types of snakes.

In Booidea and Pythonoidea, these spurs are well-known and can be used by male snakes during courtship and competition. In 2019, scientists found similar structures in a small snake species called Liotyphlops beui, which belongs to a group of snakes that is not very well understood.

Importance to social behavior

Scientists have studied how pelvic spurs are used by male snakes in certain groups. They found that these spurs are usually bigger in males than in females.

Research shows that males use pelvic spurs in shows of strength. In Indian pythons, males were seen using their spurs while also biting each other to decide who is stronger. In another type of snake called Madagascan boas, males hold each other tightly and point their spurs outward instead of biting.

Males also use pelvic spurs when mating with females. In Burmese pythons and diamond pythons, males move their spurs to help position the female better for mating. In red-tailed boas, males change the position of their spurs to help the female during mating.