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A Vietnam Veteran Collected Fossils for 66 Years. One, Mislabeled ‘Baby Lamprey,’ Made Paleontologists Reconsider How Vertebrates Moved From Water to Land
The fossil turned out to be a hatchling of a crocodile-like creature, and it suggests, according to a new study, that early animals did not use metamorphosis to evolve to dwell on land
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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.
Did you know? The earliest tetrapods
- Vertebrate tetrapods first transitioned to living on land around 390 million years ago. Some invertebrates, like the ancestors of millipedes, were already living on land at the time.
- Today, even legless reptiles such as snakes are considered tetrapods, because they descend from the four-limbed tetrapod common ancestor.
You probably saw pictures of tetrapods in high school biology. They were ancient and vaguely lizard-like creatures, initiating vertebrates’ initial transition from water to land. For decades, scientists widely assumed that tetrapods managed this shift through metamorphosis—beginning as waterborne tadpoles, much like modern frogs, then reorganizing their entire body plan to become partially land-dwelling adults.
This hypothesis emerged in part because the modern animals that look the most like early tetrapods are salamanders that experience metamorphosis. But Florian Witzmann, the curator of the fossil fish and amphibian collection at Berlin’s Natural History Museum who was not involved in the new research, argues that the analogy was always flawed. It makes sense, he says, that metamorphosis might have evolved later.
Before Rock’s fossil turned up, Mann and Pardo had spent several years analyzing another specimen from the Field Museum collections. While working late in the lab, Mann noticed that the mysterious fossil had a tiny, budding limb. To figure out what creature they were looking at, the researchers, as Mann puts it, “went through a list of candidate animals from the time based on the morphology, slowly eliminated those based on anatomical features, which excluded those groups, until we landed on our diagnosis.”
Ultimately, imagery with a scanning electron microscope allowed the scientists to identify the animal as an embolomere: a crocodile-like predator that dominated ecosystems between 280 million and 350 million years ago. Rock’s fossil appeared to be the same species. His “specimen shows more of the outside of the body at a younger state,” Mann says. “So it augmented the story significantly.”
Importantly, these two hatchlings did not show signs of being in a tadpole-like life phase. The embolomere preserved by Rock’s fossil has miniature legs, for instance, and it lacks external gills—the feathery appendages that salamander larvae use to breathe underwater before reabsorbing them at metamorphosis. These two fossils “are basically miniaturized versions” of the adults, Mann explains. “They just progressively get bigger and bigger and bigger until you get a giant, alligator-sized animal.”
That is the signature of direct development—the same growth strategy used by mammals, birds, reptiles and even many amphibians today. The study suggests that early tetrapods followed this plan rather than metamorphosis, so the water-to-land transition must have initially taken a different path. According to Nadia Fröbisch, an evolutionary biologist at Berlin’s Natural History Museum who was not involved in the research, some tetrapod experts had been expecting a hatchling fossil with the characteristics of direct development to appear. “We kind of all have been waiting for it,” she says. “We never had direct evidence, and now Arjan Mann and Jason Pardo found the direct evidence.”
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Still, these two fossils alone did not rule out metamorphosis across tetrapods. Perhaps the embolomeres were a branch that just happened to evolve toward direct development. But Mann and Pardo examined thousands of juvenile fossils from other early tetrapod relatives, and they found no evidence in any of them of metamorphosis. “It’s not one species,” Pardo says. “We can go to every stem group tetrapod, every animal that’s part of this fin-to-limb transition—every animal that we find in the Mazon Creek assemblage that falls into that category lacks anything that would point to metamorphosis.”
Fröbisch thinks that there is little to debate about Mann and Pardo’s conclusion. “I cannot think of anybody who would say this … does not fit into the evolutionary picture at all,” she says. Witzmann agrees. “The fossils are very beautiful,” he says. “You have so many soft parts preserved—cartilage, bone, skin. I would make the same interpretation.”
This pristine preservation is a rare feature of the fossils found in Mazon Creek. In most other locations, finding specimens of hatchling animals is difficult or impossible. Babies are small, and their skeletons are partly made of cartilage, which degrades rapidly after death. It takes a specific set of conditions to preserve the minuscule packages of soft tissue left behind by juveniles.
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Fortunately, Mazon Creek was up to the task. Roughly 309 million years ago, the region was a vast river delta, swept by seasonal floods that dragged thick, iron-rich mud into the sediment and buried many unlucky plants and animals beneath it. In other locations, bacteria would have broken down these organisms completely. There, however, the dissolved iron in the mud surrounding the carcasses reacted with the carbon dioxide released by bacteria. This turned the mud into siderite, an iron carbonate mineral. Within days or weeks of an organism’s death, it would be encased in a hard mineral nodule, sometimes before soft tissue had time to rot away.
This geochemical process left behind millions of concretions: “little egg-shaped stones,” as Young describes them, that collectors can smash or freeze-thaw to reveal the fossils within. Usually, collectors would have had to dig into the ground to recover these orbs. But in the mid-20th century, the coal mining industry began to strip-mine Mazon Creek, which made the concretions readily accessible in the exposed shale.
Over the past eight decades, Mazon Creek has garnered attention from collectors and citizen scientists. “You have a coal mining site near a major population during a time period when people socialized in rock swaps and had a penchant for collecting stuff,” says Young, who serves as a bridge between the Mazon Creek citizen-scientist community and professional researchers at the Field Museum. Today, some of these enthusiasts visit Mazon Creek multiple times each week to collect more concretions. “These are such smart and educated people,” Pardo says. “They know what they’re looking at better than I do sometimes.”
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Since 1960, Rock has been a pre-eminent member of this community of citizen scientists. Inspired by fossil ferns at a science fair, he went to Mazon Creek for the first time in middle school, with his dad. He “showed me what a good concretion was and showed me that you can smash it with a hammer,” Rock says. “So I started collecting.”
A few years later, his dad decided to count how many fossils Rock had brought home. “I had to go someplace,” Rock recalls. “I came back four or five hours later, and my dad was sitting at my desk, and I said, ‘Well, did you get done?’ And he said, ‘No … I quit at 50,000.’ And he said he had created a monster.”
Rock doesn’t remember picking up the baby tetrapod. But once he’d heard Mann and Pardo’s thoughts on the specimen, he knew he needed to donate it to the museum. “I realized very quickly that it was an extremely important fossil,” Rock says. In the museum’s collections rather than his own, he adds, the object could “help us learn something about the past that we don’t know.”
If metamorphosis did not form the evolutionary bridge from water to land, then some other set of traits must have done the job. For now, no one knows the complete story. “Sometimes you have to destroy in order to create,” Mann says. “We’ve just torn down a longstanding hypothesis. Now we have to start from scratch.”
As for Rock, he’s pleased that his 66 years of collecting could contribute something to the enterprise. The fossil he found is “the size of a dime,” he recalls. “My wife said, ‘How could you ever even see to pick it up?’ And as I told her, I pick everything up—doesn’t matter what it is—because you don’t know what it is until you get some time to look.”
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