We, as tetrapods, are constitute the largest group of animals in the sarcopterygian clade. Sarcopterygians include, as you may know if you have followed this blog, lungfishes, tetrapods and coelacanths. The common denominator for the animals of this clade are their lobed fins that in tetrapods have become the arms and legs.
In this group lungfishes are considered to be the closest extant relatives of all tetrapods. A common denominator of tetrapods is, as the name implies, the four paired legs used to propel the body around in the environment. This is a bit different from the coelacanths and lungfishes which still have a set of fins. However the base of their fins are more similar to our arms than you might think, as some of the bones present in their paired fins are equivalent to some of the bones in our arms. The feet is however unique to the tetrapods, believed to be a necessity to be successful in moving about on land.
Recently a paper was published by King et al. (2011) titled "Behavioral evidence for the evolution of walking and bounding before terestriality in sarcopterygian fishes". In their paper they analyzed the movement of a african lungfish (Protopterus annectens) using video recordings. Their very interesting finding was that this particular species of lungfish uses its pelvic fins as "legs" to move about on the substrate. The fins moved in a synchronized gait as you see in the gaits of tetrapods. So the lungfishes can be in a way bipedal when they move about on the bottom. They can even lift up their body with their pelvic fins, which has previously suggested to be hard without the pelvis connected to the spinal cord as all extant tetrapods.
Link to one of their videos: http://www.youtube.com/watch?v=DgqBzCOfOdE
They conclude that bottom living (benthic) sarcopterygian fishes can lift their bodies up from the substrate and possibly create tracks that later have been fossilized and that the origin of the type of gait used by tetrapods has an earlier origin already present in the water living ancestors of tetrapods.
The lungfish and the tetrapod lineage separated, according to molecular data, approximately 428 million years ago (timetree.org) which would mean that the gait that seem to be common between these two lineages is somewhat older than that. The earliest known fossil tracks was produced by a tetrapod with feets in the Eifedian (early middle devonian, about 395 million years ago) period in todays poland (Niedzwiedzki et al. 2010). This mean that feets have their origin earlier than 395 million years ago, worth noting is that the earliest known tetrapod body fossil is 18 million years younger than these tracks.
Could the gait style of tetrapods and lungfishes be even older or could it even have been reinvented in several lineages (convergent evolution)? After seeing the videos of the lungfish from the paper described above I couldn't stop thinking of several other non-sarcopterygian vertebrates that also have a similar way of moving about on the bottom substrate using their paired fins as "legs" and "feets". One very striking example is the epaulette shark (Hemiscyllium ocellatum) and its close relatives of the benthic bamboo shark group. Just as the modern lungfishes these sharks are benthic and live in shallow water. These sharks have a habit of crawling about on the bottom substrate and in the coral reef structures using their paired fins (pectoral and pelvic fins) in a similar way as a tetrapod (Goto et al. 1999). This would either suggest that the way tetrapods move have a origin way earlier than split between tetrapods and lungfishes, an origin before the bony fish lineage and the chondrichthyes about 462 million years ago (timetree.org). However since most vertebrate species are not using this type of locomotion but rather swim, just as normal fish, its possible that the jawed vertebrate body plan with its paired fins pave the way for evolving this type of gaits in benthic species that need to move efficiently over the bottom substrate, like the tetrapod ancestors, lungfish ancestors and epaulette shark ancestors. This would also make it difficult to say for sure that the gait style observed in lungfish has the same origin as the tetrapod style, even thought it is likely due to the close relationship between these lineages. It is also possible that the gait style of these two lineages evolved separately in both lineages after the divergence and that the way movement is controlled in vertebrates facilitate evolution of these very similar gait styles.
Link to video of epaulette crawling about on the substrate: http://www.youtube.com/watch?v=nHJWqYw3IIM
Niedźwiedzki G, Szrek P, Narkiewicz K, Narkiewicz M, & Ahlberg PE (2010). Tetrapod trackways from the early Middle Devonian period of Poland. Nature, 463 (7277), 43-8 PMID: 20054388
Tomoaki Goto, Kiyonori Nishida, & Kazuhiro Nakaya (1999). Internal morphology and function of paired fins in the epaulette shark, Hemiscyllium ocellatum Ichthyological Research, 46 (3)