Do mudskippers and lungfishes elucidate the early evolution of four-limbed vertebrates?

Abstract

Background: Recently, the origin of four-limbed vertebrates (tetrapods) was re-assessed, based on footprints and trackways that were made by early land animals almost 400 million years (my) ago. That is 18 my earlier than the oldest known tetrapod body fossils, such as Acanthostega and Ichthyostega from Greenland, and 10 my older than the earliest 'tetrapodomorph fishes' (Panderichthys, Tiktaalik etc.). These and other facts suggest that the first tetrapods may have thrived in the marine-influenced intertidal and/or lagoon zone as well as in brackish and freshwater environments associated with land vegetation, as previously thought. Results: Here, we discuss the controversial question whether or not extant air-breathing fishes, such as mudskippers, amphibious gobioids that inhabit mangrove swamps, can be interpreted as living model organisms, with reference to the earliest land plants. In addition, recent developmental and behavioural studies on lungfishes (Dipnoi) are summarized and evaluated. Conclusions: We conclude that mudskipping 'walking fishes' (Periophthalmus sp.) and Dipnoi (Protopterus sp.) shed light on the gradual evolutionary transition of ancient fishes to early tetrapods that occurred during the Devonian in muddy, salty waters. However, they are not the ancestors of tetrapods, because extant organisms cannot be progenitors of other living beings.