This is Mazon Monday post #327.  What’s your favorite Mazon Creek fossil?  Tell us at email:esconi.info@gmail.com.

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A significant paper on tetrapod development was published last week, and Mazon Creek fossils played a central role. “Direct Development of Stem Tetrapods Across the Fin-to-Limb Transition” by Jason Pardo and Arjan Mann of the Field Museum appeared in the journal Science. Lauer Foundation for Paleontology, Science, and Education also contributed to the study.

The paper presents an important new perspective on the life history of the earliest tetrapods, which are the vertebrates that eventually gave rise to amphibians, reptiles, birds, and mammals. Using exceptionally preserved fossils from the Mazon Creek, the authors conclude that these early tetrapods did not pass through an amphibian-like larval stage followed by metamorphosis. Instead, they developed directly from hatchlings into juvenile and adult forms.

The researchers examined numerous specimens previously assigned to Esconichthys apopyris, long thought to represent larval lungfish, along with other juvenile tetrapod fossils. Their work identified early post-hatching stages of three stem tetrapod groups: a megalichthyid, an aïstopod, and an embolomere. They also recognized early developmental stages of several stem lissamphibians, including temnospondyls.

Together, these fossils represent the most phylogenetically diverse collection of early stem tetrapod developmental stages yet assembled. The material provides the clearest evidence to date of hatchling anatomy and early life history in stem tetrapods, offering new insights into one of the most important transitions in vertebrate evolution.

The fossils used in the study came from major museum collections as well as specimens collected and donated by avocational paleontologists and ESCONI members. Notably, the acknowledgements specifically recognize the contributions of collectors including Richard Rock, David Douglass, Paul Demkovich, Tom Testa, Jack Wittry, Ben Riegler, and assistance from Andrew Young, Rich Holm, Cal So, and Nate Cookson.

This is yet another example of how Mazon Creek fossils continue to reshape our understanding of major evolutionary transitions more than 300 million years after these animals lived.

Editor’s summary

The earliest tetrapod colonizers of land have been considered to have life histories somewhat similar to modern amphibians, including the presence of an aquatic larval stage, and it has been hypothesized that this transition may have facilitated the broader transition from water to land. Pardo et al. looked at many fossils of stem tetrapods from the Mazon Creek Lagerstatte and found no evidence of such larval morphological change. Instead, both before and after the fin-to-limb transition, hatchling to adult growth proceeded through a direct development model. —Sacha Vignieri

Abstract

Modern amphibians are characterized by an aquatic larval stage that ends abruptly with a period of widespread tissue remodeling (metamorphosis) upon transition to terrestrial adulthood. A transient larval stage ending in gradual metamorphosis is often assumed for the earliest digited tetrapods, but direct evidence of this larval stage is lacking. 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.

Smithsonian Magazine had a very nice article which adds the human element behind the science. Long-time ESCONI member Rich Rock donated a key piece to the puzzle.

Richard Rock—a Vietnam War veteran, a Master Gardener and an avid fossil collector—has been picking up rocks for 66 years. His favored site is Mazon Creek, a prolific fossil bed located about 70 miles southwest of Chicago. It’s renowned not only for its fossils but also for the dedicated community of amateur scientists who brave the heat, poison ivy and Lyme-bearing ticks to collect and catalog artifacts from one of the world’s great paleontological reservoirs.

In 2023, fellow fossil enthusiast Andrew Young asked Rock if he could photograph his collections. Rock “had display cases throughout the house and an overloaded storage area in the garage,” Young recalls. “I went into his study, looked at the glass shelves, began to take specimens down, and I saw one that had a small, laminated label that said ‘baby lamprey.’ And I thought to myself, ‘This is not a lamprey.’”

The fossil, it turned out, was something far more important. In a study published today in Science, Field Museum researchers Arjan Mann and Jason Pardo describe the specimen as a hatchling tetrapod, a member of the four-limbed lineage that gave rise to all living amphibians, reptiles, birds and mammals. Paired with a similar fossil already in the museum’s archives and analyses of dozens of fossilized relatives, the hatchling upends the widely held idea that metamorphosis was essential for helping bring the Earth’s first land-dwelling vertebrates out of the water.