A collection of exceptionally well-preserved 300‑million‑year‑old fossils indicates that early tetrapods did not experience a metamorphic transformation from juvenile to adult, overturning traditional assumptions about terrestrial vertebrate evolution.
“We have for a very long time assumed that these animals were broadly amphibian-like, and that this life cycle would have bridged the gap between life in the water and life on land,” says Jason Pardo at the Field Museum of Natural History in Chicago.
Modern reptiles, birds, mammals, and amphibians belong to the tetrapod clade, which descended from lobe‑finned fish roughly 390 million years ago. However, the early developmental biology of these ancestral fish remains poorly understood, according to John Long of Flinders University in Adelaide, Australia.
Pardo and his collaborator Arjan Mann, also affiliated with the Field Museum, studied a series of fossils recovered from the Mazon Creek site, located southwest of Chicago, between the 1960s and 1990s. These specimens date to the Carboniferous Period, approximately 307–309 million years ago.
Embolomeres, reaching up to two metres in length as adults, were among the largest Carboniferous tetrapods and occupied a top‑predator niche. Although predominantly aquatic, they possessed modest limbs capable of limited terrestrial locomotion.
The assemblage includes two baby embolomeres measuring only two centimetres, whose outstanding preservation reveals soft tissues and even residual yolk material.
In tadpoles, the yolk sac stays within the body for several days after hatching to provide energy, whereas the embryonic embolomeres displayed an external yolk sac, a condition reminiscent of certain fish larvae such as lungfish.
Unlike amphibian larvae, which possess external gills for aquatic respiration, the embryonic embolomeres lacked external gills throughout development, a key indicator of their distinct life‑history strategy, Pardo notes.
According to Pardo, the skull and post‑cranial skeleton exhibit all the characteristic features of mature embolomeres, indicating that these organisms exhibited little morphological change from hatchling to adult.
Human development follows a pattern in which growth occurs gradually and proportions shift slowly, unlike the abrupt metamorphosis seen in frogs or salamanders. Pardo emphasizes that the fossil record demonstrates this developmental mode was typical of the earliest land‑colonising vertebrates.
Although embolomeres were aquatic, Pardo contends that the available evidence indicates early terrestrial vertebrates did not possess a tadpole‑like developmental stage. The research team also examined fossil material of two additional early tetrapod species that co‑existed with embolomeres in the same environment.
‘None of these specimens exhibit any trace of a tadpole‑like stage,’ Pardo adds. ‘Similarly, early lungfishes and coelacanths lack such a stage. While it cannot be entirely ruled out that a tadpole phase arose and later disappeared, the current dataset renders this scenario highly improbable.’
Long notes that this research addresses a critical gap in understanding. It demonstrates that early tetrapod‑like fishes inhabiting the Earth around 308 million years ago could colonise land without requiring a tadpole‑like developmental phase, contrary to earlier hypotheses.
Illustration of young embolomeres Berit Godring
The skull and skeleton have “all the important parts seen in an adult embolomere”, says Pardo. The fossils show that embolomeres remained more or less the same from the time they hatched from their eggs until they reached adulthood.
Human bodies basically work the same way from birth through adulthood, but we get bigger and our proportions change, but we don’t undergo the sort of fast, rapid change you see in a frog or salamander,” says Pardo. “Our fossils show that this sort of life cycle was the norm for our earliest terrestrial ancestors, too.”
Although embolomeres were aquatic, Pardo argues that the evidence available suggests our earliest terrestrial ancestors did not have a tadpole-like stage either. The team also studied the fossil remains of two other early tetrapod species that were alive at the same time and in the same place as the embolomeres.
“None of these show any evidence of a tadpole-like stage,” says Pardo. “Neither do other fishy tetrapod relatives such as early lungfishes and coelacanths. So is it impossible that a tadpole stage showed up somewhere and was subsequently lost? Maybe, but it seems vanishingly unlikely with the data we have.”
This study fills in a much-needed knowledge gap, says Long. “It shows how early tetrapod-like fishes living about 308 million years ago did not need to develop a tadpole phase in order to invade land, as was previously thought by some scientists.”
