This Ancient 'Mass Death' Offers a Glimpse Into Early Tetrapod Life on Land

The history of life on Earth is filled with tales of intrepid organisms that broke into new territories and niches to secure an evolutionary edge.

One of the most dramatic examples is the colonization of land by aquatic tetrapods, a giant leap that enabled the emergence of countless species, including humans. Given that we owe our very existence to this bold move, scientists have long been fascinated by how our tetrapod ancestors pushed out of the sea to become "part of that world," to channel The Little Mermaid.

Research published Wednesday in Nature sheds new light on this pivotal transition. A team led by Sophie Sanchez, an evolutionary biologist based at Uppsala University in Sweden, studied the remains of at least 20 individuals from the early Acanthostega tetrapod clan, preserved in what scientists called a "mass-death deposit," discovered in Greenland.

Dating back 365 million years to the Devonian epoch, this group was likely swept away by a sudden flooding event that left them stranded the in isolated pools. These temporary safe havens eventually dried up, killing the exposed animals en masse.

Sanchez and her colleagues used a synchrotron (a type of particle accelerator) to scan four humeri—or upper arm bones—from this mass-death deposit. The result is "the first life history of a Devonian tetrapod species," according to the authors.

The biggest revelation was that all the Acanthostega individuals in the deposit were juveniles, despite their differing sizes (Acanthostega is thought to have measured about two feet, or 60 centimeters, in adulthood).

The synchrotron data showed that the arm bones had not fully ossified into mature adult skeletons. Because their juvenile cartilaginous frames would have been hard-pressed to bear weight on land, these Acanthostega tetrapods likely spent their younger years as an entirely aquatic species, they concluded.

Based on the lines of growth within the fossils, the team also inferred that these transitional animals spent upwards of six years as water-bound juveniles that schooled together, with few adults around.

"Acanthostega does not seem to be the only stem tetrapod with a long juvenile stage," Sanchez told me. "It grew almost to full recorded size before the onset of limb ossification. This aligns it with a range of lobe finned fishes and tetrapods […] suggesting that a long juvenile stage could be primitive for tetrapods."

Reconstructing the specific life stages of our distant ancestors is essential for understanding how they were able to slowly venture out into the terrestrial environment we humans now so casually occupy. Sanchez hopes to build on this research by conducting synchrotron analysis of other rare fossilized tetrapods from this period in time.

"I strongly think that knowing more about the life-history traits of other stem tetrapods will help us build up a more accurate theory on the tetrapod move to land," she said.

It seems somewhat ironic that this group of young Acanthostega died due to exposure to land, while the larger evolutionary surge they represented went on to successfully colonize that very sphere. Then again, if they hadn't been imprisoned in ever-shrinking pools—like some Devonian riff on a Bond villain trap—we would not have been able to piece together details about their lifestyles and behavior today.

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