Without the tadpole phase and metamorphosis, though, the transition from water to land was probably way tougher than we thought.<\/p>\n
Amphibian innovation<\/p>\n
\u201cWe have lots of assumptions in our field that are based on relatively limited data,\u201d Pardo said. One of those assumptions was that a distinct aquatic larval stage made the water-to-land transition easier for early tetrapods. While direct development, without undergoing a radical metamorphosis early in the animal\u2019s lifecycle, might appear to be a simpler solution, it likely made the lives of young embolomers considerably harder.<\/p>\n
The first challenge they faced was being tied to the same environment throughout their entire lifespan. Unlike amphibian tadpoles, they lived in the same ecological niche as larger juveniles and three-meter-long adults and had to compete with them for resources. Then there was the problem of supporting their body mass on land. The team noted in the paper that juvenile embolomers had weak, poorly developed limbs at hatching, which probably left them unable to move long distances across land. They were likely stuck wherever they hatched.<\/p>\n
\u201cIt certainly makes it harder to not have a tadpole stage,\u201d Pardo said.<\/p>\n
He suggested that this is evidence that amphibian metamorphosis is not an ancient evolutionary stepping stone that enabled the first animals to expand out of water and conquer the land, which was then preserved in modern frogs or toads. Instead, it might be an evolutionary innovation that amphibians developed much later in response to challenges coming with the water-to-land transition.<\/p>\n
\u201cIt may be something unique to amphibians that emerged as an adaptation to their specific way of living on land,\u201d Pardo said. \u201cInstead of being primitive, it may actually be something new, something novel and exciting. We\u2019ve never thought about it this way.\u201d<\/p>\n