My office is located in a loft above the main room of my house. The ceiling is basically the same shape as the roof and if I reach up from my chair I can touch the wood that separates me from the outside. As I sit here in the early morning hours, typing at the keys so that I can form the sentences you will read today, the rhythm of the keys is joined by the gentle patter of rain on the roof above my head. My imagination starts to wander outside, searching for pools of rainwater in the fields and forests that will be the central focus of the breeding season for so many of our region’s amphibians.

The great, great ancestors of modern amphibians evolved during the Geologic Period known as the Devonian. This period is known as the “Age of Fishes,” but one enterprising group of fish was experimenting with the idea of not being fish any more. The freshwater streams that they inhabited were dangerous places filled with all sorts of terrible predators and it was somehow determined that leaving the water might be a good idea. So, the first steps of a vertebrate animal onto dry land were made by a fish that just wanted to find some peace and quiet. This happened about 375 million years ago.

Clearly, this presented major problems for a fish because fish are adapted to living under water. Gills allowed them to exchange gasses in water, but without that constant supply of water there was a serious problem that needed to be addressed. How do you breathe air? Well, time is the magic ingredient that often provides solutions to these sorts of questions. If you combine time with the variations brought on by mutations and natural selection, then solutions can gradually be found.

Millions of years went by and certain descendants of these special fish adapted to spending more and more time out of the water. Gills gradually moved deeper inside their bodies and eventually the lung was born. This was a really big deal because lungs opened the door for the rise of amphibians and the eventual spread of vertebrates across the face of the Earth. The oldest amphibian fossils date back to about 368 million years ago. To put this in perspective, all of this happened about 45 million years before the first reptile appeared, and 282 million years before the Tyrannosaurus Rex had a firm grip on the world.

The word Amphibian, taken from the Greek words “amphi,” which means “double,” and “bios,” which means “life.” This is a reference to a life spent both in water and on land. Today’s amphibians are represented by the “Anura” (the frogs and toads) and the “Caudata” (the salamanders). Going “deep nerd” here: the word “caudata” is a reference to the fact that salamanders have tails. The tail fin of a fish is called a “caudal fin.” OK, exiting nerd mode.

Most amphibians have very thin skins that must be kept moist and as a result they cannot stay out in the sun for too long without drying out and dying. Toads are an exception, but they still require moist conditions. This thin skin is capable of exchanging gasses with the atmosphere and there are actually a couple species of very small salamanders in our area that have no lungs at all. They don’t live in water, but they are completely reliant on moist conditions for their survival.

One of the most satisfying aspects of amphibians is their life cycle because it sort of captures the evolutionary transition from aquatic to terrestrial organisms. The majority of amphibians must lay their eggs in water and their young emerge from the eggs with gills. In this regard the young are very “fishy.” If you look at the embryos of salamanders, frogs and toads they actually look just like little fish, but this doesn’t last long.

Salamander embryos remain long and linear, but frogs and toads show an accumulation of mass in the front of the body. In frogs and toads this is known as the “pollywog” stage. Then, very slowly, the legs begin to develop. The hind legs are visible, but the front legs are shielded from view by the skin that covers the gills. Salamanders are a little different and their front legs are visible because their gills are not encapsulated. Despite these slight differences, the aquatic larval forms of frogs, toads and salamanders eventually replace their gills with lungs and they are then no longer able to stay underwater forever. I should also note that salamanders keep their tails, while the frogs and toads absorb their tails once they mature.

Although our native salamanders are silent creatures, our frogs and toads have evolved to be quite vocal and this is purely in the pursuit of breeding. Animals that can leave the water must be called back to it for breeding and males sing wonderful songs throughout the spring and summer. Sometimes the songs come from ponds, sometimes from small pools and sometimes they come from mere puddles in the forest. Most of this singing is done at night, when it is safe, and the male frogs are advertising good sources of water where the females can lay eggs. But these songs work on people just as well as they work on female frogs.

So, if you find yourself itching to go for a walk, consider walking in the direction of any frog songs that you may have heard. If you are lucky you might find a collection of beautiful eggs in the cool water and then you can return again and again to see how the little offspring are developing. Use your imagination and you can see yourself as a time traveler; popping in every few million years to see how the creatures evolve from aquatic to terrestrial organisms.

Bill Danielson has been a professional writer and nature photographer for 25 years. He has worked for the National Park Service, the U.S. Forest Service, the Nature Conservancy and the Massachusetts State Parks and he currently teaches high school biology and physics. For more in formation visit his website at www.speakingofnature.com, or head over to Speaking of Nature on Facebook.]]>