Panderichthys

Panderichthys is a genus of extinct sarcopterygian (lobe-finned fish) from the late Devonian period, about 380 Mya. Possible tetrapod tracks dating back to before the appearance of "Panderichthys" in the fossil record were reported in 2010, which suggests that "Panderichthys" is not a direct ancestor of tetrapods, but nonetheless shows the traits that evolved during the fish-tetrapod evolution. 
Due to the fact that oxygen is much less soluble in water than air, the decreased oxygen in the atmosphere would have caused the oxygen concentrations in any type of water to decrease substantially. This in turn would have caused any aquatic animal that could breathe air to have an advantage and be more likely to thrive. 
In addition to its ability to move in shallow water, "Panderichthys" could also breathe air. Its strong pectoral fins in theory could allow it to prop up its head in shallow water and take a breath. The enlargement of the spiracular chamber itself as well as its opening to the outside suggests that "Panderichthys" was part of a transition to an increased capacity for air breathing that was completed in tetrapods. 
Description.
"Panderichthys" is a 1.5–2 m (4 ft 11 in – 6 ft 7 in) long fish with a large tetrapod-like head that is flattened, narrow at the snout and wide in the back. The intracranial joint, which is characteristic of most lobe-finned fishes, has been lost from the external elements of the skull, but is still present in the braincase. The patterns of external bones in the skull roof and cheeks are more similar to those of early tetrapods than those of other lobe-fins. The transitional qualities of "Panderichthys" are also evident in the rest of the body.
Classification.
In January 2010, "Nature" reported well-preserved and "securely dated" tetrapod tracks from Polish marine tidal flat sediments approximately 397 million years old. These fossil tracks suggest that a group of two meter long tetrapods lived in the fully marine intertidal or lagoonal areas on the south coast of Laurussia during the time elpistostegids were living. This implies that "Panderichthys" is not a transitional fossil and represents its own adaptive morphology. Therefore, "Panderichthys" can only be a "late-surviving relic", showing traits that evolved during the transition from fish-like creatures to tetrapods, but whose date does not reflect that transition. The tracks "force a radical reassessment of the timing, ecology and environmental setting of the fish–tetrapod transition, as well as the completeness of the body fossil record."
Paleobiology.
Air-breathing structures.
As an intermediate in the fish-tetrapod evolution, "Panderichthys" had the capacity to breathe air. The trend from the early sarcopterygians to the first tetrapods was an increase in the size of the spiracular chamber and its opening to the outside. Compared to "Eusthenopteron", the spiracular chamber of "Panderichthys" is greatly expanded and the hyomandibula is shorter compared to those in fish. The opercular series was also shorter compared to other osteolepiforms. "Panderichthys" also has a single external nasal opening and a palatal choana. In contrast to earlier osteolepiforms, the palatal choana is elongated and the nariochoanal lamina is narrow. Along with the spiracular chamber, this feature in "Panderichthys" can be considered transitional during the evolution from fish to tetrapods. Sarcopterygians such as "Panderichthys" can be considered at least facultative air breathers and demonstrate an intermediate form as air breathing was becoming more abundant.
External links.
"" on English Wikipedia