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Pardo and Mann 2026 describe baby embolomere fossils

Friday, June 19, 2026 14:09

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With headlines like,
“Baby crocodile-like fossils just blew up a long-held evolution theory“

and opening paragraphs like,
“New evidence published today in the journal Science upends decades of evolutionary theory about when animals first walked on land. The new findings suggest these first animals were not, as you might have learned in biology class, anything like modern amphibians.”

I sense a little bombast here. And some misdirection.

I have never heard that the first animals to walk on land were anything like modern metamorphosing amphibians.

Note: This paper is behind a paywall at present.
So the present data comes from the publicity and suppdata files.

The publicity is peppered with
images of near limbless baby embolomeres (described and defined below, Fig 1).

The Canadian Museum of Nature in Ottawa,
“confirmed that the fossil was a baby embolomere using electron microscopy. But that only presented more questions because the fossil didn’t show any classic amphibian tadpole traits. While the tiny fossils didn’t have limbs (those would develop as they grew), they also didn’t have external gills. And without gills, it became clear to Mann and Pardo that these baby embolomeres wouldn’t have gone through a classic amphibian metamorphosis from tadpole to adult.”

“We looked at a number of different species that represent different lineages in the transition from fish to tetrapods [the branch of the fish family that developed legs and crawled onto land], and what we found is that none of them have anything that looks remotely like a tadpole. And if you don’t have a tadpole, then you don’t have a metamorphosis,” said Pardo.

Description
“Embolomeres are characterized by their vertebral centra, which are formed by two cylindrical segments, the pleurocentrum at the rear and intercentrum at the front. These segments are equal in size. Most other tetrapods have pleurocentra and intercentra which are drastically different in size and shape.”

Figure 1. *Proterogyrinus* had a substantial neck and embolomeri-type vertebrae.

Definition
Michel Laurin (1998) formally defined Embolomeri as “the last common ancestor of Proterogyrinus and Archeria and all of its descendants. This definition excludes Eoherpeton, which is almost always considered a close ally of the group.“

Archeria has not yet been added to the LRT.

In the large reptile tree (LRT, 2340 taxa) Proterogyrinus (Fig 1) and Eoherpeton are both members of the Stegocephalia, derived from the likes of Ichthyostega and Acanthostega and these ultimately were derived from flattened Early Carboniferous, Greerepeton (Fig 2). This taxon had tiny limbs a little larger than the tiny limbs of not-flattened outgroup taxa like Colosteus (Fig 4) and Pholidogaster (Fig. 2).

These were not the earliest tetrapods, but they were the most primitive in the LRT.

Figure 2. Classic Collosteidae include Collosteus, Pholidogaster, Greererpeton and Deltaherpeton all to scale.

From the Pardo and Mann abstract:
“Exceptionally preserved stem tetrapod hatchlings show that a transient larval stage was absent in tetrapods both before and after the fin-to-limb transition; instead we identified soft- and hard-tissue evidence of direct development, falsifying hypotheses of an ancestral origin of metamorphosis or of a gradual larval-postlarval transition serving as a template for lissamphibian metamorphosis. We argue that a transient larval period culminating in metamorphosis originated near or within the tetrapod crown group as part of a broader suite of traits associated with terrestrialization.”

Not so, according to the LRT. Instead, phylogenetic miniaturization and neoteny were present at the origin of frogs and salamanders (= Lissamphibia) from larger ancestors. This had nothing to do with the transition from fins to fingers (Fig 4) in the LRT.

Figure 3. Late Carboniferous Amphibamus reconstructed. DGS colors added here. Also from the Mazon Creek Formation, note the developing ossification in the limbs.

The basal reptilomorph Amphibamus
(Fig 3) was also part of this study because it is also found in the Mazon Creek Formation. Pardo and Mann report in the “Early Juvenile State: larger specimens. The skull is nearly fully ossified, as are the dermal pectoral girdle, humerus, ribs, neural arches, and proximal hind limb elements. Prominent external gill filaments extend from posterior to the temporal region of the skull.”

That last trait, “gill filaments” came as a surprise. The figures (Fig 3) do not show filaments.

Pardo and Mann looked a LOT of Mazon Creek specimens.
And they are not easy to interpret. Again, I have not read the text of the paper. Even so, the LRT seems to indicate the authors might not have been looking at the correct transitional taxa at the fin-to-finger transition (Fig 4).

Figure 4. Tinirau to scale with Koilops and Colosteus to scale. Fingers of Colosteus in yellow enlarged at right. This is the fin-to-finger transition according to the LRT. Note the phylogenetic miniaturization at this transition as in so many other transitions.

In the LRT
the large, flat rhipidistian, Tinirau (Fig 4), from the Middle Devonian of Nevada, is the large, flat, finned taxon that (through phylogenetic miniaturization) evolved into Koilops (a skull-only taxon, but basal to Tiktaalik and other flat and finned taxa) and then into Colosteus, which had four tiny flexing fingers. None of these are Mazon Creek taxa. None seem to have been studied for this paper by Pardo and Mann.

Let me know if this is an error by sending a PDF if you have it.

References
Pardo JD and Mann A 2026. Direct development of stem tetrapods across the fin-to-limb transition. Science 392(6804): https://www.science.org/doi/10.1126/science.aeb7635

wiki/Embolomeri

Source: https://pterosaurheresies.wordpress.com/2026/06/20/pardo-and-mann-2026-describe-baby-embolomere-fossils/

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