At some point in the Asthatian period, most likely during the late Agonian stage, Mawsoniid coelacanths of the order Lagotoichthyiiformes (bunnyfish) began adapting to a more benthic lifestyle.
But, instead of becoming dorsoventrally flattened as most fish would, these already laterally flattened Lagotoichtyiiforms would instead evolve for life on the seafloor in a different way. Akin to the Pleuronectiformes of old Earth, these bunnyfish went through a cranial metamorphosis that saw one of their eyes migrate to the other side. This allowed them to skillfully lay flat on their sides on the sediment, becoming "flattacanths".
These "flattacanths" became the order Planosolincolae, and rapidly spread around the world as they faced very little competition in this new niche they created for themselves.
Initially they kept to shallow coastal waters and stuck to their niche. But soon they would expand into deeper waters and would even start leaving the seafloor to pursue more mobile prey.
Eventually, Planosolincoles would venture into freshwater ecosystems and start exploring habitats outside the water. This would give rise to the most diverse group of flattacanths today: the terrestrial Anisospondyli.
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