IO9 had a charming profile today on some real alien life forms, which happen to have existed right here on Earth.  Ediacarans ruled the planet between 575 and 542 million years ago, during a time period named after them, the Ediacaran.  They blossomed into being suddenly, with a global distribution, ruled briefly, then vanished just as suddenly.

The most recognizable Ediacaran looks like a giant feather stuck in the sand of the sea bed.  Visually, it looks like the modern sea pen.  But the physical resemblance is the only similarity.  Some Ediacarans looked like plants - like ferns, fronds, or in the case of Bradgatia, "a squashed cabbage."  Other Ediacarans looked like "sacks of mud, disks, hubcaps, and mattresses."  Blobs, in other words.  For about 33 million years, the entire planet was ruled by blobs.

In fact, Ediacarans were such strange looking creatures that there still remains some controversy as to whether or not they were animals, plants, or fungi.  (One of the difficulties lies in defining the difference between those three categories - a difference which can be hard to pin down, even today, with creatures like the slime molds.)

Ediacarans entirely predate bilaterian animals, like ourselves (and everyone else around these parts now).  They had no mouth, digestive tract, or anus.  Most scientists believe that they fed by osmosis, absorbing the nutrients they needed from the ocean around them.  

They may also have preyed upon each other in the same way that amoebas do today, and those which lived in shallower waters might have used chlorophyll to metabolize sunlight directly.  The most iconic Ediacaran, Charnia, lived deep in the ocean, well below the reach of light, and was anchored firmly to the sea bed.  Since it seems to lack any other means of making a living, most scientists assume that it obtained nutrients through osmosis.

There are a few contemporary life forms which predate the Ediacarans, such as sponges, jellyfish, and certain forms of algae.  If they remember the days of the Ediacarans, they aren't talking.

The Ediacarans ruled the planet right up until the Cambrian age began, with its explosion of underwater life.  These mobile predators, armed with innovations like light sensing organs, mouths, and feeding palps, made short work of the non-moving Ediacarans.  Upstart new life forms like the trilobites made short work both of the Ediacarans, and of the microbial mats which surrounded them, and which may have helped to provide them with food.

The IO9 article was sparked by the original article which appeared in Astrobiology Magazine.  The timely appearance of this article coincides with the news that there are more habitable planets in the universe than we originally thought.  

The Ediacaran explosion, right after the Earth thawed out enough to allow life, violates what many early evolutionary biologists thought would be a very long time span required for evolution.  Instead, their rapid appearance (and disappearance) in the fossil record definitely supports the theory of "punctuated equilibrium."  Creatures like the Ediacarans could easily be what we find when we voyage to other planets.