Fossils challenge land evolution theory

Scientists have long believed that the earliest aquatic animals to move onto land passed through a tadpole-like stage, undergoing metamorphosis similar to modern frogs. However, new research published on Thursday in the journal Science challenges that long-standing assumption.
The study examines rare fossils from the renowned Mazon Creek fossil beds in northern Illinois, a site known for exceptionally preserved specimens dating back around 309 million years. The fossils were preserved inside iron carbonate concretions that formed in ancient swamps, shallow seas and river deltas.
Researchers analysed dozens of fossils to investigate the evolutionary transition between fish and tetrapods — the four-limbed animals that eventually gave rise to land-dwelling vertebrates.
Central to the study was a tiny fossil believed to be the juvenile form of an embolomere, a crocodile-like creature that lived primarily in water but possessed small limbs. Scientists had expected such a young specimen to display tadpole-like features, including external gills.
Instead, the fossil showed evidence of direct development. According to the researchers, the juvenile — roughly the size of a short piece of macaroni — was already built much like its adult form, lacking signs of a dramatic metamorphic stage.
'We now actually have some direct fossil record evidence', study co-lead author Jason Pardo of Chicago's Field Museum said, 'that this metamorphosis, this amphibian-like life cycle that we've for 150 years assumed was part of our history, turns out that it wasn't part of that at all'.

The findings suggest that the amphibian-style life cycle familiar today may not have been a feature of the earliest tetrapod ancestors.
Australian paleontologist John Long, who was not involved in the research, described the study as 'quite outstanding'.
'Not much was known about their early life stages', he said. 'This detailed work on a bunch of simply glorious fossils nails it that they went straight into a juvenile phase so didn't need to go through the tadpole stage'.
Jason Anderson of the University of Calgary said the paper demonstrates 'the power of fossils to address questions we thought impossible' because such developmental stages occur over brief periods and involve soft tissues that are rarely preserved.
Both Anderson and Pardo noted that the research highlights amphibians as evolutionary innovations rather than primitive holdovers.
'Our amphibians, instead of being relicts of earlier stages in the evolutionary history of tetrapods, are themselves highly evolved creatures', Anderson said.
The key fossil had sat in the Field Museum's collections for years before attracting the attention of co-author Arjan Mann. Mann and Pardo began studying it while doctoral students in Canada and spent years trying to understand its significance.
Using scanning electron microscopy at the Canadian Museum of Nature, researchers eventually confirmed it was likely a juvenile embolomere. They compared it with another young embolomere specimen and several other fossilised tetrapod relatives.
Mann said the discoveries were made possible by generations of amateur fossil collectors who searched the Mazon Creek site and uncovered many of the specimens examined in the study.
'This paper, in a way, is kind of a love letter to them', Mann said, highlighting the value of collaboration between professional researchers and the fossil-collecting community. — AFP