A Fossil Found in Utah Reveals the Origin of the Earth’s First Vertebrates

The history of life on Earth dates back more than 500 million years, when complex-structured organisms emerged during the Cambrian period. A recent discovery in the western Utah desert sheds light on this crucial period in the evolution of vertebrates, the group that includes all animals with a backbone. In a geological area known for its dinosaur fossils, a team of paleontologists from Harvard University, led by Rudy Lerosey-Aubril and Javier Ortega-Hernández, has found a key piece for understanding vertebrate origins in the fossil of a small marine creature called Nuucichthys rhynchocephalus.

This fossil, from Utah’s Marjum Formation, measures less than two centimeters and resembles a primitive fish without bones or teeth. However modest it may seem, this find holds crucial information. The study of Nuucichthys reveals the first evolutionary steps toward vertebrates and the development of complex anatomical traits.

This 500-million-year-old species represents a “stem group,” a set of organisms that share characteristics with today’s vertebrates but lack defining features such as fins. The absence of fins in Nuucichthys provides clues about its lifestyle, suggesting that these tiny animals moved slowly, undulating their bodies in the ocean’s upper layers to seek food.

The discovery of Nuucichthys takes place in a moment in life’s history known as the Cambrian Explosion. This period, beginning approximately 541 million years ago, marked a rapid diversification of marine organisms and the emergence of life forms with mineralized structures, such as bones and shells.

Nuucichthys rhynchocephalus was finless and likely lived near the sea surface, feeding on microplankton. Credit: Franz Anthony / Harvard University

During the Cambrian Explosion, the oceans underwent a radical transformation, and the first vertebrates emerged, though in much simpler forms than those known today. Despite the significant biodiversity of that time, primitive vertebrates had yet to develop bones or cartilage, making their preservation as fossils extremely rare and, therefore, of great value to scientists.

Research on Nuucichthys reveals details about the organization of its muscles, arranged in chevron-shaped blocks, a structure characteristic of modern vertebrates. Furthermore, this creature had large, complex eyes and a structured gill cavity that facilitated both respiration and food filtration. These advanced anatomical traits for its time show how evolution began experimenting with different body configurations long before the appearance of bones and other rigid structures.

Another interesting facet of the study on Nuucichthys is its comparison with similar species found in North America and Asia. In Canada, the fossil Metaspriggina, from the famous Burgess Shale, is the closest relative to Nuucichthys. While Metaspriggina does not preserve fins or body outline, fossils of Chinese species such as Myllokunmingia do.

This difference in preservation has sparked a debate among scientists: did fins not develop in some of these primitive species, or did preservation conditions simply erase these details? The discovery of Nuucichthys in Utah seems to clarify this question. In this fossil, the body outline is clearly visible and stops near the muscle blocks, with no indication of fins. This suggests that early North American vertebrates, unlike their Asian counterparts, evolved without this structure.

The Utah desert, known for its iconic dinosaur fossils, also holds one of the richest paleontological records for studying the origins of animal life. According to Lerosey-Aubril, this region is crucial for understanding the origins of vertebrates and other early life forms, as it preserves fossils from a time when seas teemed with complex organisms, including arthropods, tunicates, and other ancestors of today’s vertebrates.

The Marjum Formation, in particular, is one of the few locations in the world that allows scientists to find fossils of the quality and anatomical complexity of Nuucichthys, thanks to the unique preservation conditions in that environment.

Fossils like Nuucichthys are essential for reconstructing the evolution of vertebrates and understanding how these animals have developed an incredible diversity of forms over millions of years. The Natural History Museum of Utah houses several thousand Cambrian fossils, many of which have yet to undergo thorough investigation.

Supported by institutions like Harvard University and the Bureau of Land Management, paleontologists continue to explore Utah’s Cambrian strata, hoping to discover new species and evidence that broaden our understanding of this critical period.

SOURCES

Natural History Museum of Utah / University of Utah

Lerosey-Aubril Rudy and Ortega-Hernández Javier, A long-headed Cambrian soft-bodied vertebrate from the American Great Basin region. R. Soc. Open Sci. 11240350. doi.org/10.1098/rsos.240350