A 500-million-year-old fossil is a crucial clue to a prehistoric mystery: the origin of bone-bearing animals, or vertebrates.
The fossil is a three-quarter-inch, long-headed fish-like fossil with no bones or teeth.
The new fossil species from Utah’s Marjum Formation in the West Desert, Nuucichthys rhynchocephalus, offers a unique glimpse into the evolution of animal life in the Cambrian, a pivotal period in the history of Earth when marine life diversified rapidly and animals with mineralized bones and shells appeared.
The genus name Nuucichthys comes from Núu-ci, the name Ute Indians give to themselves, and ichthys, the Greek word for fish. The creature is one of the oldest vertebrate representatives known to science, dating back half a billion years to the Cambrian Period.
“This time interval saw the emergence of animals; that’s really the moment when these organisms became prominent components of marine ecosystems,” explains Rudy Lerosey-Aubril, a research scientist with Harvard’s Museum of Comparative Zoology at a Natural History Museum of Utah affiliate.
Once an inconspicuous life form, animals rapidly diversified during this period, colonizing most marine habitats. Known as the Cambrian Explosion, this event set the stage for the vast diversity of life we see today, and irreversibly revolutionized the functioning of oceans. However, early vertebrates lacked bones or cartilage, making their fossilization and eventual discovery a highly unlikely and thus valuable event.
Through the study of this ancient species, scientists can reconstruct the evolutionary steps that led to the complex body plan characterizing the extant representatives of the group. Nuucichthys exhibits surprisingly advanced anatomical features for a 500-million-year-old fish, which highlights the rapid pace of evolutionary innovation during the Cambrian Explosion.
“Utah is home to an incredible paleontological archive,” Lerosey-Aubril says. “The Beehive State is renowned for its spectacular dinosaurs, but fewer people know that it is also one of the world’s most important regions for studying the origins of animal life.”
Despite its simple appearance, Nuucichthys exhibits several characteristics that betray its vertebrate affinities.
“It is what we call a ‘stem group’ species, which means that it already possessed some characteristics of modern vertebrates, but not all,” Lerosey-Aubril notes.
One of the most informative features of this ancient species is its muscle organization. Its arrangement into distinct chevron-shaped muscle blocks is a hallmark of vertebrates and their relatives. Nuucichthys also had large, complex eyes and a structured branchial cavity. This cavity housed early gill structures and served a dual role in breathing and filtering food.
The preservation of such complex features in a Cambrian animal is remarkable, offering insights into the anatomical sophistication that characterizes the vertebrate lineage. Yet, Nuucichthys would stand out in an aquarium today, as it noticeably lacks a key characteristic of fish—fins.
The first discovery of a stem-group vertebrate in the Western United States improved our understanding of the early history of these animals. Only four similar species were hitherto known: one in Canada, one in the Eastern US, and two in China.
“The jury was still out concerning the presence of fins in some of these Cambrian species,” Lerosey-Aubril explains. Fins were clearly visible in early fish fossils from China, including Myllokunmingia, but they were conspicuously absent in the North American species.
“The fossils of Metaspriggina—a species from the Burgess Shale in Canada and the closest relative of Nuucichthys—do not preserve fins, but they do not preserve the body outline (i.e., the skin) either.”
So, the question remained: were fins originally absent in this species, or were they lost during the geological history of the fossils?
Most scientists favored the second hypothesis (poor preservation).
“People thought, ‘Okay, since the skin isn’t visible either, maybe the external features were particularly prone to decay and disappeared before fossilization,'” Lerosey-Aubril elaborates. “But in the new fossil from Utah, the outline of the body is clearly visible under the microscope and stops very close to the muscle blocks. Beyond this line, there is nothing else.”
This genuine absence of fins in Nuucichthys and its closest relatives in North America influences our understanding of these organisms’ lifestyles. These small early fish were clearly not cruiser-type swimmers, rather they inhabited the upper layers of the sea, occasionally using body undulation to reach plankton-rich areas.
This distance to the seafloor, along with the delicate constitution of their bodies, may explain why their remains are only found in a few Cambrian fossil sites renowned for their exquisite preservation—a select group that includes Utah’s Marjum Formation.
Nuucichthys is just one of several thousand Cambrian fossils in the collections of the Natural History Museum of Utah. Harvard-led investigations are also ongoing in the fossil-rich Cambrian layers that yielded this new species. Both sources hold the promise of exciting discoveries from a time long past when ancestors of vertebrates, arthropods, comb jellies, tunicates, and a myriad of other animals were transforming the oceans into vibrant ecosystems teeming with life.
The researchers describe the fossil in the journal Royal Society Open Science,
The curation of this specimen was supported by the Bureau of Land Management.
This story is reposted from the Natural History Museum of Utah’s Science Stories blog.
Source: University of Utah
