Millions of years ago and half a world apart, crocodilians were evolving handy new tricks like eyeballs on the top of their skulls. New fossil evidence indicates this was not a case of some advanced individual mutating this feature and spreading it around the world. Instead, gavialoids on two continents independently developed the same useful – albeit disturbing – attribute.
Gavialoids are a group of reptiles that form part of the same order as crocodiles and alligators. Their surviving representatives are the Indian gharial (Gavialis gangeticus) and controversially the false gharial (Tomistoma schlegelii), but there were once many other members of the family, although paleontologists have had trouble deciding whether some extinct crocodylian species should be included.
The gharial's eyes are widely separated and stick up from its head, an efficient system for hunting their prey. Zoologists describe them as “telescoped,” and the same feature is visible in extinct species from South America.
Now 13-million-year-old specimens of a new species, named Gryposuchus pachakamue, has been discovered in Peru, the oldest gavialoid known from the Amazon Basin. The finders named the species after Pachakamue, a South American god known as the “storyteller” for revealing the origins of life to the native peoples of the area.
A Gryposuchus pachakamue mandible being uncovered in the field D. De Francesci
In PLOS ONE a team of paleontologists report that pachakamue had only partially telescoped eyes. This is exactly what would be expected if the useful property evolved gradually, and we were lucky enough to find transitional fossils (the sort of thing creationists claim has never been found).
The authors note that pachakamue's ancestors appear to have been marine species for whom telescoped eyes were apparently less useful. The feature was an adaptation to moving into rivers and swamps, an environment in which they appear to have thrived, based on the frequency of fossils. Pachakamue specimens are common in deposits laid down when the Amazon River flowed westward, ending in a huge lake, surrounded by wetlands.
The gavialoids are of interest to zoologists, the paper claims, because they have taken the elongation of the front of their skull further than pretty much any other species. “Understanding their patterns of evolution is fundamental to recognizing cranial rearrangements and reconstructing adaptive pathways” associated with such an evolutionary path.
Indian gavialoids, cut off from their South American cousins by the break up of the Gondwanan supercontinent tens of millions of years previously, also have found the value in telescoped eyes when living in swampy conditions. Pachakamue, however, indicates that this is not a case of a common ancestor – the last of which lived approximately 40 million years ago – having developed this feature and passed it on to both lines before Gondwana broke up.
Instead, the Indian and South American species experienced what is known as “parallel evolution,” where similar environmental pressures produced a near identical outcome, a story worthy of such a fearsome teller.
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