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Frogs' skulls are more bizarre (and beautiful) than you ever imagined

Hemiphractus scutatus, a South American horned tree frog, has a wide skull with a large gape that enables it to prey on vertebrates.
Hemiphractus scutatus, a South American horned tree frog, has a wide skull with a large gape that enables it to prey on vertebrates.
(Image: © Florida Museum/Image by Edward Stanley)

Frogs' heads may look smooth and rounded on their surfaces, but peek under the skin of some species and you'll find skulls that resemble the heads of mythical dragons, studded with spikes, spines and other bony structures.

Scientists recently highlighted the diversity of frog skulls in a series of incredible images, part of a new study investigating skull evolution and function in armored frogs.

In these frogs, skulls can be shield-shaped or exceptionally wide; they may be pocked by grooves or adorned with pointy bits that may provide extra protection against being eaten, the researchers reported.

Related: In photos: Cute and colorful frogs

Artificial color in the images indicates variations in bone density in different skull parts, said lead study author Daniel Paluh, a doctoral candidate in the Department of Biology at the University of Florida. In the image of the horned frog Hemiphractus scutatus, "blue parts of the skull, such as the braincase, are lower density than the green regions, including the jaws," Paluh told Live Science in an email.

There are approximately 7,000 known frog species. For the study, the scientists collected data from 158 species representing all of the major frog families. They found that not only was there a lot of variety in skull shapes; some of those variations appeared across different lineages, separated by millions of years of evolution.

"For example, large, fortified skulls with intricate patterns of pits and grooves have independently evolved in the African bullfrog, South American horned frog and the Solomon Island leaf frog," Paluh said. "And all of these species are ambush predators that will eat other vertebrates."

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Bony spikes on the skull of Anotheca spinosa, a tree frog from Central America, may protect it against predators.

Bony spikes on the skull of Anotheca spinosa, a tree frog from Central America, may protect it against predators. (Image credit: Florida Museum/Image by Edward Stanley)
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Diaglena spatulata, a shovel-headed tree frog from Mexico, uses its spiked skull as a shield.

(Image credit: Florida Museum/Image by Daniel Paluh)
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The aquatic frog Barbourula busuangensis is also known as the Philippine flat-headed frog.

The aquatic frog Barbourula busuangensis is also known as the Philippine flat-headed frog. (Image credit: Florida Museum/Image by Daniel Paluh)
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Ceratophrys aurita (Brazilian horned frog, Brazil). Several frogs, including this Brazilian horned frog, are known to hunt other vertebrates (including other amphibians, mammals, birds, and reptiles) and have fortified skulls covered in intricate patterns of grooves, ridges and pits formed by extra layers of bone. This trait, which is called hyperossification, likely allows these species to more effectively eat larger, harder prey.

Ceratophrys aurita (Brazilian horned frog, Brazil). Several frogs, including this Brazilian horned frog, are known to hunt other vertebrates (including other amphibians, mammals, birds, and reptiles) and have fortified skulls covered in intricate patterns of grooves, ridges and pits formed by extra layers of bone. This trait, which is called hyperossification, likely allows these species to more effectively eat larger, harder prey. (Image credit: Image courtesy of Daniel J. Paluh)
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Pyxicephalus adspersus (African bullfrog, sub-Saharan Africa). Some of these voracious predators, including this African bullfrog, have a formidable bite due to large, bony fangs on the lower jaw (highlighted in orange).

Pyxicephalus adspersus (African bullfrog; sub-Saharan Africa). Some of these voracious predators, including this African bullfrog, have a formidable bite due to large, bony fangs on the lower jaw (highlighted in orange). (Image credit: Image courtesy of Daniel J. Paluh)
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Aparasphenodon brunoi (Bruno's casque-headed frog, Brazil). Three species of hyperossified frog, including Bruno's casque-headed frog, were recently discovered to be venomous with enlarged poison glands that are associated with the spines of the skull. When a predator rams the head of one these frogs, specialized spikes pierce through the glands just under the skin as a defense.

Aparasphenodon brunoi (Bruno's casque-headed frog, Brazil). Three species of hyperossified frog, including Bruno's casque-headed frog, were recently discovered to be venomous with enlarged poison glands that are associated with the spines of the skull. When a predator rams the head of one these frogs, specialized spikes pierce through the glands just under the skin as a defense. (Image credit: Image courtesy of Daniel J. Paluh)
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Brachycephalus ephippium (Pumpkin toadlet, Brazil). It was once thought that hyperossification may be linked to very small body sizes (miniaturization) in frogs, as the trait is present in the miniscule pumpkin toadlet, which is less than an inch long. But our research demonstrates that the trait is present across the entire spectrum of frog body sizes.

Brachycephalus ephippium (Pumpkin toadlet, Brazil). It was once thought that hyperossification may be linked to very small body sizes (miniaturization) in frogs, as the trait is present in the miniscule pumpkin toadlet, which is less than an inch long. But our research demonstrates that the trait is present across the entire spectrum of frog body sizes. (Image credit: Image courtesy of Daniel J. Paluh)

Shovel-headed tree frogs, whose flattened skulls resemble gardening tools, use their heads to block entry to the cracks and holes where they live. Their skulls also have spines, ridges and grooves, "in addition to very wide skull roof bones that provide protection from predators," Paluh explained. 

"Because all frogs look so similar, there has been limited interest in studying the evolution of their anatomy," Paluh said. "Our study demonstrates there is still much to learn about the evolution, ecology and anatomy of these amazing animals."

The findings were published online today (March 27) in the journal Proceedings of the National Academy of Sciences.

Originally published on Live Science.

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