Researchers affiliated with the Field Museum of Natural History have documented fossilized evidence of juvenile embolomeres, reptilian-like predators that inhabited primeval waterways and marshlands during the period spanning from 350 to 280 million years ago. These ancient fauna belonged to the earliest terrestrial vertebrates, and contrary to prevailing scientific assumptions, their young did not exhibit tadpole-like morphology.
New fossil evidence suggests that embolomeres did not undergo a metamorphosis the way that modern amphibians do when growing up, which challenges a long-standing scientific belief that amphibians, reptiles, and mammals evolved from animals that had a tadpole stage. Image credit: Berit Godring.
“Many of us were presented with a simplified evolutionary narrative during our formative educational years, suggesting a progression from fish to amphibians, followed by reptiles, and ultimately mammals,” remarked Jason Pardo, a paleontologist at the Field Museum.
“Our investigation indicates that this fundamental premise, namely that the initial four-limbed vertebrates developed akin to amphibians, is inaccurate.”
In their recent scientific endeavor, Dr. Pardo and his collaborator, Dr. Arjan Mann, meticulously examined tetrapod fossils, many of which contained remarkably preserved soft tissues, originating from the Mazon Creek Lagerstätte located in Illinois.
“It stands as one of the preeminent fossil sites globally, particularly renowned for its exceptional preservation of soft tissues and delicate fossil specimens,” stated Dr. Mann.
“The fossils from Mazon Creek serve as invaluable temporal snapshots, capturing otherwise unattainable details.”
As mature specimens, embolomeres could attain lengths exceeding 3 meters (10 feet), functioning as formidable apex predators that inhabited fluvial, lacustrine, and palustrine environments from 350 million years ago (the Carboniferous period) until 280 million years ago (the Permian period).
The specimens unearthed at Mazon Creek present a starkly contrasting perspective: juveniles measuring only a few centimeters in length, yet conveying sufficient information to challenge a century of established scientific conjecture.
According to the study’s authors, the juvenile embolomeres lacked critical amphibian tadpole characteristics, such as external feathery gills.
Even in instances where larval stages underwent significant transformations en route to adulthood, there were no discernible indications of genuine amphibian metamorphosis.
Consequently, the ontogenetic trajectories of these ancestral tetrapods appear to bear a closer resemblance to our own developmental patterns—or those of fishes—than to those of amphibians.
“We analyzed numerous species representing diverse lineages during the transition from fish to tetrapods, and our findings revealed a complete absence of any morphology remotely resembling a tadpole,” Dr. Pardo elaborated.
“Without a tadpole stage, there is no metamorphosis.”
“Therefore, if creatures like the embolomere did not possess a tadpole form or undergo true amphibian metamorphosis, it invalidates the widely championed hypothesis that reptiles and mammals originated from amphibian-like ancestors.”
“The prevailing narrative posited that metamorphosis was the mechanism facilitating the terrestrial transition for ancestral animals. This narrative is no longer tenable; it has dissolved into irrelevance.”
The discoveries have been published in the esteemed journal Science.
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Jason D. Pardo & Arjan Mann. 2026. Direct development of stem tetrapods across the fin-to-limb transition. Science 392 (6804): 1292-1296; doi: 10.1126/science.aeb7635
