A Living Fossil: The Lungfish

Watch this video first, it will give you a good introduction to the lungfish.

The relevant stuff can be seen at beginning-1:37 and 4:30-end, but I highly suggest watching the whole video, you get to see the Thorny Devil, another awesomely cool lizard, whose adorably grumpy face your looking at right now.

First things first, sorry about this but I love phylogeny trees. I think they are one of the greatest things ever invented and are super fun to look through. If you don't know what that is or if you've never seen one, this is a phylogenetic tree for lungfish, it can also be called an evolutionary tree if you prefer that.

,--Family Diabolichthyidae (extinct)
| ,--Family Uranolophidae (extinct)
| |  __,--Family Speonesydrionidae (extinct)
'-|-|  '--Family Dipnorhynchidae (extinct)
    |     ,--Family Stomiahykidae (extinct)
    '----|___ ,--Family Chirodipteridae (extinct)
          |      '-|--Family Holodontidae (extinct)
          |------Family Dipteridae (extinct)
          |  __,--Family Fleurantiidae (extinct)
          '-|  '--Family Rhynchodipteridae (extinct)
              '--Family Phaneropleuridae (extinct)
                     | ,--Family Ctenodontidae (extinct)
                       '-| ,--Family Sagenodontidae (extinct)
                          '-|--Family Gnathorhizidae (extinct)
                             '--Order Ceratodontiformes
                                  |--family Asiatoceratodontidae (extinct)
                                  |--Family Ptychoceratodontidae (extinct)
                                  |--Family Ceratodontidae
                                  |  '--Genus Ceratodus (extinct)
                                  |  '--Genus Metaceratodus (extinct)
                                   '--Family Neoceratodontidae
                                        | '--Genus Mioceratodus (extinct)
                                        | '--Genus Neoceratodus - Queensland lungfish
                                        '--Order Lepidosireniformes
                                               '--Family Lepidosirenidae - South American                                                                                    lungfish 
                                               '--Family Protopteridae - African lungfish

As can be seen from the phylogenetic tree, the Queensland, or Australian, lungfish is the only surviving member of its order and family, while the South American lungfish and the African lungfish are of the same order and therefore more closely related. There are only six surviving species of lungfish; one Australian, one South American and four African.

From the fossil records it can be assumed that some form of lungfish has been around for at least 380 million years, since before the formation and subsequent breakup of the supercontinent Pangaea.

The supercontinent Pangaea, 300 million years ago

Lungfish fossils have been found in Australia that are nearly identical to the current Queensland lungfish species which means that this genus of lungfish has remained virtually unchanged for over 100 million years. The Queensland lungfish is considered to be a living fossil and is one of the oldest living vertebrate genera in the world. 

Australian lungfish, or Queensland lungfish
Note the flipper-like fins

As the video said lungfish have the ability to breath air by using a modified swim bladder, it's not exactly a lung as we know it but it's more complex than the swim bladders used by other fish. The Australian lungfish can survive out of water for a few days and only if their skin stays moist. These fish have one "lung" and are facultative air breathers, meaning they only breath air when they need to, if the water is low in oxygen for example, and rely mostly on their gills. They also have a rather elaborate mating ritual that I will not go into here because I'm only just getting to the good part.

The six species of lungfish come in various sizes, the smallest species being about 2 feet long and the largest reaching 6.6 feet. They are also long-lived animals, a Queensland lungfish in a Chicago aquarium is at least 80 years old and has been there since 1933. 

A South American lungfish
Note the string-like fins rather the Queensland's flipper-like fins

As the South American and African lungfish share the same order they are more similar to each other than they are to the Queensland lungfish. The video featured one of the the African species but the South American species can burrow underground and become dormant as well. Both of these families of fish build and guard nests for their young. The young lungfish use external gills until they start breathing air at about seven weeks old. The gills on adults are so reduced that they are basically nonfunctional, this makes them obligate air breathers, which means they have to breath air periodically or they will suffocate and drown.

The fins on all lungfish are also special, unlike most fish whose fins have a limited range of motion the joints that attach these fins are similar to a tetrapod's joints. Tetrapods being four-limbed terrestrial animals. 

A Marbled lungfish, one of the four African species
These fins are long and flat but not really flipper-like

One last thing I will mention about these fish because it is rather amazing, the Marbled lungfish, as seen above, has the largest known genome of any vertebrate on the planet at 130 billion base pairs.