Mesoamerican Burrowing Toads: Rhinophrynidae

With its round and flat body and tiny, pointy-snouted head, the Mesoamerican burrowing toad is an odd-looking creature. It is so unusual, in fact, that some people might not even guess it is a type of frog until they see its hind legs and webbed feet sticking barely out from the saggy skin of its back. The skin droops along the sides of the body and up by the front legs, too. Its legs are short and pudgy, but very strong. The back legs are longer than the front legs, but the front legs are quite thick, as if the frog were a bodybuilder. The edges of the feet also have large, shovel-like bumps that help the frog dig. The toes of its front feet are much shorter than its very long back toes. It typically sits with its hind legs bent up against the body, so they almost disappear. In all, it looks like a dark, round blob with four feet poking out the sides. The head is little more than a cone-shaped lump on the front of the frog. It is small and has no obvious neck supporting it. The reason that the head looks so small is that the frog's shoulders are much closer to the head than they are in other types of frogs. The shoulder blades even wrap around the back of the head, which makes the frog look as if it is always shrugging up its shoulders.

The frog has two dark, small, round, beady eyes, two even smaller nostrils sitting just in front of the eyes on the top of the short snout, and thick lips. Most of the skin on this frog is smooth, but the skin on the snout is tough because it is covered with hard bumps, called spicules (SPIK-yuhlz), that can only be seen with a microscope. The spicules are rounded on the bottom of the snout, but pointy on top. In many other frog species, two eardrums are easily seen on the sides of the head, but the tiny head peeking out from the shoulders in the Mesoamerican burrowing toad leaves no room for any eardrums to show.

Its tongue is different, too. The Mesoamerican burrowing toad does not have a long tongue to flip far out of its mouth as many other frogs do. Instead, it has a sticky, triangular-shaped tongue that it can stick straight out just a little way. Its mouth is toothless.

The frog also has an unusual pattern on its thick back skin that helps to tell it apart from other species. Its back is dark grayish brown, sometimes nearly black, with a single, thin, stripe of yellow, orange, or reddish orange running down the middle. This stripe may be broken here and there, but it is still obvious. It also has yellow and/or orange spots or blotches on either side of the stripe that continue down its sides. Its legs may have a few spots, but the limbs are mostly just grayish brown, and the webbing is typically a lighter gray or bluish color. Its head is also a little lighter in color than the back and may be light gray to light brown. Its underside is dark brown, gray, or bluish gray and has none of the speckles, spots, or stripes of the back.

The name "toad" in this species can be confusing, because it is not actually a toad at all. The only true toads are in the family Bufonidae. They have warty skin and short legs. The Mesoamerican burrowing toad has smooth skin and quite large hind legs. Even though its common name includes the word "toad," scientists consider it to be a frog and not a true toad. This is a small- to medium-sized frog, and the adults grow to 1.8 to 2.6 inches (4.5 to 6.5 centimeters) long from the tip of the snout to the end of the rump.

The tadpole of the Mesoamerican burrowing toad also looks a bit different from the average tadpole. It has the typical head and long tail, but the head is wider and flatter than other tadpoles, and it has unusual, tiny, fleshy "whiskers" poking from the front of the mouth. These bits of flesh are called barbels (BAR-bulls). Unlike most other frog species, these tadpoles do not have horny beaks on their mouths. Other tadpoles use these beaks to scrape algae (AL-jee), or tiny plantlike organisms that live in water, off rocks or plants for their meals. The Mexican burrowing toad tadpole eats instead by sifting out little bits of floating algae from the water. The only other tadpoles that look like the burrowing toad tadpoles are those of the Pipidae family, which include the clawed frogs and Surinam toads that live mostly in South America and Africa. Like the Mesoamerican burrowing toad, the clawed frogs and Surinam toads also have a very unusual look that includes an oddly flat head. Scientists believe that these two families of frogs are very closely related because the tadpoles are so much alike and because of some details in their skeletons. Paleontologists (PAY-li-un-TA-luh-jists), or scientists who study fossils, at the Carnegie Museum of Natural History are also now studying a new fossil discovered at Dinosaur National Monument, which is located in Colorado and Utah. This fossil may be a newly discovered relative of the two families.