In a new study released by the Florida Museum of Natural History on Monday, scientists report spiky skulls evolved at least 25 separate times in different kinds of frogs. The survey was reportedly to be published in Proceedings of the National Academy of Sciences.
Lead author Daniel Paluh, a Ph.D. candidate at the University of Florida, said, "Superficially, frogs may look similar, but when you look at their skulls, you see drastic differences [...] Some of the weirdest skulls are found in frogs that eat birds and mammals, use their heads as a shield, or in a few rare cases, are venomous. Their skulls show how strange and diverse frogs can be."
Some scientists had previously suggested smaller frogs would tend to develop spiky skulls, but that is not what this study reports.  Here, researchers evaluated CT scans on 158 different species. They found frogs that had similar ecological niches — meaning they either acquired food in the same way or defended themselves from predators in the same way — tended to develop skulls with similar grooves, spikes, or specialized jaw joints regardless of whether they were ancestrally related to each other.
According to the study, skulls with spikes, grooves or ridges — called hyperossification — often accompanied frogs eating very large prey relative to their own body size.  The strong, spacious skulls would allow the frogs to have very big mouths with which to catch birds, rodents, reptiles and other frogs.  Spikes sometimes coincided with venomous frogs. The researchers speculated the spikes make it dangerous for predators to hit the frog's head, as the spikes would break venom sacs under the frog's skin.
Other bone formations included projections resembling fangs or lower teeth  that different species of frogs use for catching prey and fighting. Very few frogs have true lower teeth.
One of the study's authors, herpetologist David Blackburn, told Discover Magazine the frequency of spiky skulls evolving in otherwise unrelated frogs suggested some deep pattern frogs fell into as circumstances arose: "Somehow, these frogs are turning on some ancient developmental machinery in their DNA."
Since 1973, when frogs skulls were last comprehensively surveyed, scientists have documented enough new species to double the number known to humans.  Also, modern techniques exploit CT scans, of which Paluh and his team made liberal use.  The research was part of the oVert project, funded by the National Science Foundation, meant to comprehensively CT-scan over 20,000 vertebrate specimens from United States museums. 

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Florida frog skull survey shows spikes, say scientists