Fossils dating back 309 million years have cast doubt on the ‘frog’ model of evolution

A study of rare fossilised juveniles of ancient relatives of tetrapods — a group comprising all four-legged vertebrates, including amphibians, reptiles, birds and mammals — has been published in the journal *Science*. The authors concluded that some early forms developed directly: they did not go through a ‘tadpole’ stage, but grew as smaller versions of the adult animals.

Details

The study is based on fossils from the Maison Creek site in northern Illinois, near Chicago. This is one of the most famous palaeontological sites in the US: around 309 million years ago, the area consisted of swamps, shallow seas and river deltas, and animal remains were sometimes preserved in iron-rich concretions along with soft tissues.

It was precisely the preservation of soft tissues that was critically important. If ancient young animals had indeed gone through a stage similar to the larval stage of modern amphibians, scientists might have expected signs such as external gills or a dramatic restructuring of the body during maturation. However, no such signs were found in the specimens studied.

The main specimen in the study is a probable juvenile of an embolomer, an ancient semi-aquatic predator which is often compared in popular descriptions to a crocodile-like animal. It was tiny: the authors and museum records describe it as a very small specimen, almost ‘child-like’ in scale, yet already exhibiting signs of adult-type development.

Embolomerids had limbs, but they led a predominantly aquatic lifestyle. It was therefore previously expected that their juvenile stages would resemble the larval forms of amphibians. However, the juveniles studied did not resemble ‘tadpoles’, which subsequently undergo a radical restructuring of their bodies. Rather, they appeared to grow directly — from a small form to a larger one.

What is the ‘frog model’?

By the ‘frog model’ here, we do not mean the frog itself, but a type of development: first an aquatic larva with gills, followed by metamorphosis, during which the organism undergoes a drastic change. This is clearly evident in modern frogs: a tadpole does not simply grow larger, but completely restructures its body, organs and way of life.

For a long time, this pattern was often used as a convenient analogy for the early history of tetrapods. The logic was clear: if modern amphibians develop through an aquatic larval stage, it is possible that early vertebrates at the interface between water and land followed a similar path.

New research suggests that this analogy may be flawed. According to the authors, in stem tetrapods, on both sides of the transition from fin to limb, there are signs of direct development rather than a mandatory larval stage involving true amphibian metamorphosis.

Why this matters

The emergence of vertebrates onto land is one of the key transitions in the history of life. Without it, there would be no terrestrial amphibians, reptiles, birds, mammals or humans. It is therefore important for scientists to understand not only how fins evolved into limbs, but also how the young of these early animals developed.

Until now, there has been very little direct fossil evidence of early development. Juveniles are poorly preserved, they are easily confused with other small animals, and soft features such as gills almost never survive the passage of hundreds of millions of years. This is why the finds from Maison Creek are particularly valuable: they allow us to investigate questions that previously seemed almost beyond the reach of palaeontology.

The research also changes our understanding of modern amphibians. Frogs, salamanders and newts are not necessarily ‘living models’ of the first tetrapods. They themselves have undergone a long evolutionary history, and their life cycle should not automatically be extrapolated to the most ancient relatives of terrestrial vertebrates.

What the scientists found

The researchers studied several rare fossils of juvenile early tetrapods and their close relatives. The key specimen had long been held in the Field Museum’s collection and was subsequently examined using scanning electron microscopy, which helped to confirm its relationship to the embolomerans.

The scientists were looking for features one would expect to see in an amphibian-like larva: external gills, a distinct larval body plan, and evidence of a drastic restructuring during maturation. Instead, they observed a pattern of direct development: the young specimens were already organised, on the whole, according to the adult plan.

This is particularly significant because it is not a matter of a single isolated specimen. According to the Field Museum, a similar pattern was observed in another, smaller embolomer, as well as in other fossilised juveniles of tetrapod relatives.

Background

Tetrapods are vertebrates with four limbs or their descendants. This group includes amphibians, reptiles, birds and mammals. Their distant ancestors evolved from fish-like forms, and the transition from fins to limbs was one of the most significant events in the evolution of vertebrates.

Trunk tetrapods are ancient forms close to the base of this evolutionary lineage. They are not modern amphibians, but they help us understand what animals looked like and how they evolved on the path from aquatic vertebrates to terrestrial ones.

Maison Creek is significant precisely because it preserves not only the large bones of adult animals, but also tiny, rare, and sometimes soft-tissue details. Without such finds, the question of whether early tetrapods had ‘tadpole’ stages would remain almost entirely theoretical.

Source

Study: Jason D. Pardo, Arjan Mann and colleagues, “Direct development of stem tetrapods across the fin-to-limb transition”, Science, 2026.