A 2026 study published in Science indicates that early tetrapods, including embolomers and megalichthyids, underwent direct development without a tadpole stage. This finding challenges the hypothesis that early tetrapods developed like modern amphibians, which involves an aquatic larval stage followed by metamorphosis.

The study was co-authored by Jason Pardo, a research associate at the Field Museum, and Arjan Mann, the museum's assistant curator of early tetrapods. Researchers analyzed an embolomere fossil designated FMNH PR 1082 from the Field Museum collections. The fossil had been in the collections for approximately 50 to 60 years after being found by a collector. Initially, the specimen was misclassified as a different tetrapod species due to its small size.

Using electron microscopy, the researchers identified FMNH PR 1082 as a young embolomere that died before 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 specimen resembled a miniature adult and contained an abdominal yolk, suggesting the ancestral tetrapod egg was large and nutrient-dense, similar to those of reptiles and birds. It lacked external gills and showed signs of bone ossification, indicating a direct development process. Researchers also examined a second, smaller embolomere hatchling that similarly lacked external gills.

Embolomers were an extinct group of large predators that existed approximately 300 million years ago. They featured large skulls with sharp teeth, long eel-like bodies, and short, stocky limbs adapted for paddling and brief movement on land. These creatures could reach lengths exceeding three meters. Juvenile embolomers possessed weak, poorly developed limbs at hatching.

The researchers also re-examined fossils historically classified as larval lungfish and identified them as young megalichthyids. Megalichthyids were early finned tetrapodomorphs that predated embolomers by 20 to 30 million years. The smallest megalichthyid specimens measured two centimeters. Multiple megalichthyid individuals were found preserved together, indicating they might have remained in groups after hatching. These specimens showed signs of direct development with gradually ossifying bones rather than undergoing abrupt metamorphosis. Researchers additionally examined fossils of Phlegethontia longissima, another early tetrapod group that lacked limbs. The Phlegethontia longissima hatchling had large eyes and a partially ossified jaw, but no external gills.

"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. He stated that amphibian metamorphosis might be an evolutionary innovation that developed later in response to challenges associated 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.