Fig. 5.

A possible evolutionary scenario consistent with our results. Having invaded shallow waters, where the down-welling component of sunlight is significant, better visual range is obtained with eye sockets moved to the top of the skull, providing upward vision (Fig. 4A1) as shown here for Panderichthys. Possibly driven by low oxygen, animals surfaced near shore to breathe through the spiracles that had also dorsalized to just behind the eyes in the elpistostegalians, as shown here for Tiktaalik. Without correction for the differing refractive index of air, they initially saw blurry outlines of invertebrate fauna (33) that had already been living on land for 50 My. With continued surfacing and selection of the slight changes to lens and cornea to enable a focused image of their quarry, in a small fraction (34) of the 12-My transition from finned to digited tetrapod eye sizes, the full power of long-range vision would have emerged. The strong derivative of visual volume with respect to eye size would have facilitated the observed selection for larger eye size. Simultaneously, selective advantages of limbs with digits over limbs with fins made animals like Acanthostega better suited for longer forays onto land, culminating in more terrestrial forms, such as Pederpes, 30 My after Tiktaalik. The colored portion of the simplified tree marks an evolutionary phase with substantial body plan modifications. Shown in green in Left are the spiracles (what becomes the Eustachian tube) likely used for breathing at the water surface while using aerial vision. Total animal lengths are between 50 cm and 1.5 m and are not drawn to scale. Age spans from 385 My for Eusthenopteron to 355 My for Pederpes.