By Nick Carne
Most frogs look pretty benign, with round and friendly faces, but don’t be fooled. Below the surface are some sports fake teeth, helmet-like fastenings and even gift-giving spikes.
Now US researchers have a better idea of how they developed and why.
A team at the Florida Museum of Natural History used 3D data to study the skull shape of 158 species that represent all living frog families, and found that radically shaped skulls are often covered with intricate patterns of grooves, ridges, and pits that are supplemented by additional ones Bone layers are formed.
This characteristic, known as hyperossification, has evolved more than 25 times in frogs, and species with the same eating habits or defense mechanisms have tended to develop similar skulls, even if they have been separated by millions of years of evolution.
“On the surface, frogs may look similar, but when you look at their skulls, there are drastic differences. Their skulls show how strange and diverse frogs can be,” says Daniel Paluh, lead author of a paper in the magazine National Academy of Sciences procedure.
The work is long overdue. According to Paluh, the last comprehensive study of frog skulls was published in 1973. Since then, scientists have doubled the number of frog species described and updated our understanding of their evolutionary relationships.
They found that frogs that eat other vertebrates such as birds, reptile mice and other frogs often have huge, spacious skulls with a jaw joint near the back, which gives them a larger gap with which to take in their prey. Their skulls are littered with tiny pits that Paluh suspects could offer additional strength and bite power.
Almost all frogs lack teeth on the lower jaw, but some, such as Budgetts frogshave developed lower catch-like structures that allow them to inflict stab wounds on their prey. A kind of, Günther’s marsupial frog, has real teeth on both jaws and can eat more than half his body length.
Other frogs clog their caves with their heads as protection from predators. These species tend to have cavernous skulls covered with small spines. A few like Bruno’s cascaded frogwere recently discovered to be toxic.
The question is, which came first – hyperossification or imaginative skull shape? Did frogs start eating big prey and then develop stronger skulls or vice versa?
According to co-author David Blackburn, the common ancestor of today’s 7000 species of frogs had no decorated skull, but heavily fortified skulls occur in even older frog ancestors.
“While the ancestor of all frogs had no hyperossified skull, the skulls of fairly old amphibian ancestors were built this way,” he says. “These frogs could use old development paths to create traits that characterized their ancestors deep in the past.”
Previous studies have shown that frogs have developed hyperossification to prevent water loss in dry environments. However, the new study found that habitat and hyperossification are not necessarily related.
However, the habitat affects the shape of the skull. For example, water frogs tend to have long, flat skulls, while excavation types often have short skulls with pointed snouts and can use their mouths like chopsticks.