Early land animals skipped the tadpole phase

Starting out small

Current amphibian development may not have been typical of early land vertebrates.

For decades, biologists thought that early tetrapods, ancient vertebrates that started conquering the land over 300 million years ago, developed like modern amphibians—beginning their lives as purely aquatic tadpoles and then metamorphosing into terrestrial adults. “A lot of that comes from this old ‘scala naturae’ idea that you had fish that evolved into the next stage up, which were amphibians, and then amphibians evolved into the next stage up, which were reptiles that evolved into birds and mammals,” said Jason Pardo, a research associate at the Field Museum.

We’ve never had evidence that early tetrapods had an amphibian lifestyle; we have assumed it because it made intuitive sense. “It’s easier to make the transition from water to land if you’re already making that transition as part of your life cycle,” Pardo said. But now, a new Science study that Pardo co-authored with Arjan Mann (the Field Museum’s assistant curator of early tetrapods) shows our most basic assumptions about the first tetrapods that started living on land might be wrong.

Baby monsters

The researchers’ study focused mainly on embolomers, an extinct group of large predators that lived roughly 300 million years ago. Embolomers looked like a cross between a crocodile and an eel, with large skulls full of sharp teeth, followed by long, eel-like bodies. It had short, stocky limbs adapted mainly for paddling in water, but also capable of powering brief, clumsy excursions on land. They are thought to be one of the first vertebrates that made a partial transition from an aquatic to a terrestrial lifestyle. These animals could reach over three meters in length, but to understand the very beginning of their life cycle, scientists focused on examining some of their centimeter-scale babies.

One of the most important specimens in the study was an embolomere fossil designated FMNH PR 1082, which has been sitting in the collections at the Field Museum for decades. “I think it’s been there for about 50 or 60 years. It was originally found by a collector,” Mann said. The fossil was initially classified as a different tetrapod species due to its very small size. But armed with modern imaging techniques like electron microscopy, Pardo and Mann realized the fossil was in fact a very young embolomere that died before it could even consume its first meal. “We were able to identify features that linked it to embolomers like the shape of the vertebrae, the radial spines on the tail, and the nice little fangs,” Mann said.

The team also realized that this young embolomere looked like a miniature adult. What’s more, the individual had an abdominal yolk, a portion of the egg’s yolk sac that an embryo internalizes into its body cavity just before hatching to use as an energy reserve. (According to the researchers, this internal yolk mass suggests the ancestral tetrapod egg was relatively large and nutrient-dense, like the ones laid by reptiles and birds, as opposed to the small eggs laid by amphibians and fish.) Despite having hatched recently, the specimen lacked the external gills that tadpoles should have, and there were signs of ossification in its bones—it was a youngling, but it was no tadpole.

The lack of a tadpole phase so early in life suggested there was no metamorphosis in this animal’s life cycle, which led Pardo and Mann to hypothesize the development of early tetrapods was direct, as it is in the reptiles or mammals that appeared much later.

Then they started looking at other fossils to make sure the FMNH PR 1082 was not some kind of outlier. It turned out it wasn’t.

Skipping the tadpole

To confirm this wasn’t an isolated case, the team looked at a second, even smaller embolomere hatchling that also showed no signs of external gills. Once this was confirmed, the team started checking fossil collections in other museums across America, looking for youngsters of other ancient lineages to see whether the missing tadpole phase was a broader evolutionary trend. They found that a tiny hatchling of Phlegethontia longissima, a strange group of early tetrapods that lost their limbs entirely. These had a head dominated by large eyes and a partially ossified jaw—but again, no external gills.

Then, the researchers started looking further out along the evolutionary tree at earlier animals that hadn’t yet fully made the transition from fins to limbs. They focused on fossils that had historically been thought to be larval lungfish. Pardo and Mann’s re-examination revealed that these were young megalichthyids, early finned tetrapodomorphs that predated emobolomeres by 20 to 30 million years.

The smallest of these specimens measured just two centimeters. In a few cases, multiple individuals were preserved together in the same concretion, indicating that these animals may have remained in groups for some time after hatching. But even this species showed signs of direct development, with gradually ossifying bones rather than undergoing an abrupt, amphibian-like metamorphosis.

“We looked at a number of different species that represent different lineages in the transition from fish to tetrapods, and what we found is that none of them have anything that looks remotely like a tadpole,” Pardo said.

Without the tadpole phase and metamorphosis, though, the transition from water to land was probably way tougher than we thought.

Amphibian innovation

“We have lots of assumptions in our field that are based on relatively limited data,” 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’s lifecycle, might appear to be a simpler solution, it likely made the lives of young embolomers considerably harder.

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.

“It certainly makes it harder to not have a tadpole stage,” Pardo said.

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.

“It may be something unique to amphibians that emerged as an adaptation to their specific way of living on land,” Pardo said. “Instead of being primitive, it may actually be something new, something novel and exciting. We’ve never thought about it this way.”

Science, 2026. DOI: 10.1126/science.aeb7635

Jacek Krywko is a freelance science and technology writer who covers space exploration, artificial intelligence research, computer science, and all sorts of engineering wizardry.

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