A remarkably preserved fossil from Mongolia’s Gobi Desert is reshaping scientists’ understanding of mammal evolution during the age of dinosaurs. The new species, described today in the journal Nature by a team of scientists from the American Museum of Natural History, Stony Brook University, University of Arizona, and Arcadia University, reveals that a group of extinct mammals known as zhelestids were not close relatives of modern placental mammals as many researchers thought for decades. Instead, they belonged to an entirely different branch of early mammals, overturning a longstanding interpretation based primarily on fossil teeth.

“This discovery illustrates why fieldwork remains indispensable to understanding life’s history,” said the study’s lead author Andres Giallombardo, who found the specimen as a graduate student on a Museum-sponsored expedition in 2004. “It was the thrill of a career to find a new species so completely preserved that also solves a longstanding scientific problem, and a reminder that the Gobi Desert, which is well known for fossils, continues to change science.”

Zhelestids have been known from isolated teeth and fragmentary fossils for nearly 40 years. Their distinctive teeth, which are more specialized for eating plants than the sharp, insect-eating teeth found in many Cretaceous mammals, led paleontologists to suggest that zhelestids represented an unknown group of hoofed mammals. Researchers have debated whether these animals were examples of Cretaceous placental mammals—the group that today includes humans and most living mammals—or a separate lineage that evolved similar features independently. The idea that placental mammals existed during the Cretaceous is supported by molecular clock studies, which suggest that placentals originated long before the end of the Cretaceous.

The fossil described in the new study provides evidence to the contrary.

“Molecular models based on living animals can predict the past, but fossils provide the evidence needed to test those predictions,” said study coauthor Paul Velazco, a comparative biologist at Arcadia University.

Discovered in the eastern Gobi Desert, Tamirkhan balcarceli is the most complete zhelestid found yet and solves the mystery of what this group looked like. Although Tamirkhan possesses the low, rounded teeth characteristic of zhelestids, it also has long, ever-growing incisors and a suite of distinctive skull features found in a group of small, shrew-like insectivore mammals called zalambdalestoids. In addition, “Tamirkhan’s hind legs are long and slender with distinctive ankles, traits that are unmistakably zalambdalestoid,” said study author Shawn Zack, a paleontologist at the University of Arizona.

Based on these findings, the researchers conclude that zhelestids were not placental mammals or particularly placental-like, but a subset of the zalambdalestoid group. The discovery also shows that the mammal species in the Cretaceous were not as anatomically diverse as might have been anticipated based on the teeth alone. Their distinctive rounded teeth—common to plant-eating mammals—is likely a case of convergent evolution, where unrelated animals evolve similar traits because they adapt to similar lifestyles.

“More than 200 years ago, French naturalist Georges Cuvier famously argued that a single tooth could allow scientists to predict the anatomy of an entire animal,” said study coauthor Maureen O’Leary, a paleontologist at Stony Brook University and research associate at the Museum. “While teeth remain among the most informative fossils available, this work demonstrates that teeth cannot always tell us how the whole animal looked.”

Measuring between 6-7 inches from head to tail, Tamirkhan had elongated hind limbs that gave it an almost rabbit-like appearance. Paleontologists have sometimes informally referred to zhelestids as “Cretaceous rabbits,” because they were rabbit mimics despite their distant relationship to modern rabbits.

The specimen itself illustrates the extraordinary scientific importance of the Gobi Desert, which, in addition to Kazakhstan, Kyrgyzstan, Uzbekistan is one of the few fossil locations to preserve zhelestids.

“The Gobi is one of the only places where we routinely recover such remarkably complete Cretaceous mammals,” said coauthor Michael Novacek, a curator in the Museum’s Division of Paleontology who has co-led annual expeditions to the Gobi since 1990 in partnership with the Mongolian Academy of Sciences. “These extraordinary fossils continue to transform our understanding of mammalian evolution.”

The study’s evolutionary conclusions were made possible through a comprehensive mammalian phylogenetic research platform called MorphoBank. First published in 2013, the database has expanded over more than a decade to include an ever-growing number of key fossil species, providing the most comprehensive framework yet assembled for analyzing early mammal relationships.

Other authors on this study include Eva Hoffman from Yale University.

The Gobi Desert field and laboratory research for this study was supported in part by the Margaret and Will Hearst Paleontological Research Fund and the Frick Laboratory Endowment at the Museum. This research was also supported by the U.S. National Science Foundation, grant numbers MRI-R2 0959384, EAR 2506729, and EAR 2506727. 

Study DOI: 10.1038/s41586-026-10861-5

ABOUT THE AMERICAN MUSEUM OF NATURAL HISTORY (AMNH)

The American Museum of Natural History in New York City, founded in 1869 with a dual mission of scientific research and science education, is one of the world’s preeminent scientific, educational, and cultural institutions. The Museum encompasses more than 40 permanent exhibition halls, galleries for temporary exhibitions, the Rose Center for Earth and Space including the Hayden Planetarium, and the Richard Gilder Center for Science, Education, and Innovation. The Museum’s scientists draw on a world-class permanent collection of more than 30 million specimens and objects, some of which are billions of years old, and on one of the largest natural history libraries in the world. Through its Richard Gilder Graduate School, the Museum offers two of the only free-standing, degree-granting programs of their kind at any U.S. museum: the Ph.D. program in Comparative Biology and the Master of Arts in Teaching (MAT) Earth Science residency program. Visit amnh.org for more information.

Journal

Nature

Funder

U.S. National Science Foundation

DOI

10.1038/s41586-026-10861-5

Biofluorescence, the phenomenon through which organisms absorb light at one wavelength and re-emit it at another, more vivid color, has been documented across various marine animals. Yet, its evolutionary roots and the precise breadth of its spectral emissions have remained elusive. The studies, conducted under the leadership of Emily Carr, a Ph.D. candidate at the Museum’s Richard Gilder Graduate School, employed cutting-edge phylogenetic analysis combined with sophisticated fluorescence imaging to map the historical emergence and diversification of biofluorescence in teleost fishes—bony fishes constituting the largest group of vertebrates alive today.

One of the landmark revelations is the independent evolution of biofluorescence over 100 times within marine teleost lineages. Data suggest these fluorescent traits first appeared approximately 112 million years ago, with eels representing some of the earliest instances. This extensive convergence underscores the adaptive significance of biofluorescence, particularly in the complex visual environments such as coral reefs, where the majority of biofluorescent fish species are found. Coral reef habitats, rich in both biodiversity and intricate light environments, appear to have acted as evolutionary crucibles driving the proliferation and diversification of fluorescence in marine fish.

The emergence of biofluorescence also parallels significant paleontological events, notably the Cretaceous-Paleogene (K-Pg) extinction around 66 million years ago, which saw the demise of all non-avian dinosaurs. Following this mass extinction, the rise of modern coral-dominated reefs provided new ecological niches that fostered rapid evolutionary radiation among reef-associated teleosts. Biofluorescence seems to have tracked this diversification closely, evolving at approximately ten times the rate in reef-dwelling fishes compared to their non-reef counterparts. This pattern suggests a strong ecological and perhaps behavioral linkage between fluorescence and reef environments.

The second study delved into the fine-scale characterization of fluorescence emission spectra across a variety of teleost families. Utilizing a novel photographic setup with ultraviolet (UV) and blue excitation light sources combined with emission filters, researchers examined specimens collected from diverse biogeographic regions including the Solomon Islands, Greenland, and Thailand. This methodological approach uncovered an extraordinary range of fluorescent emissions, spanning multiple wavelengths in the green, yellow, orange, and red spectra. Remarkably, some fish families exhibited at least six distinct fluorescent peaks, indicating highly complex and varied fluorescent pigmentation.

The phenotypic variability revealed raises compelling questions about the functional roles of biofluorescence. The researchers speculate that species-specific fluorescent emission patterns could serve as intricate signaling systems employed in camouflage, mate attraction, species recognition, or predation tactics. Such signaling mechanisms would be particularly advantageous in coral reef environments, where visual communication is pivotal amidst the vibrant and dynamic backdrop of reef habitats. Understanding the ecological significance of these fluorescent displays could transform our comprehension of sensory biology and communication in marine organisms.

In addition to ecological implications, the expanded diversity of fluorescent molecules discovered could have profound biomedical and biotechnological applications. Fluorescent proteins and molecules are invaluable in medical diagnostics, fluorescence-guided surgery, and cellular imaging due to their ability to emit light under controlled illumination. The discovery of novel biofluorescent molecules within teleost fishes opens potential avenues for the development of new fluorescent markers with unique spectral properties, possibly enhancing imaging clarity and specificity in clinical and research settings.

The collaboration on these studies spanned multiple institutions and disciplines, combining expertise in ichthyology, molecular phylogenetics, marine biology, and imaging technology. Among the co-authors are Rene Martin, Mason Thurman, Karly Cohen, Jonathan Huie, David Gruber, and Tate Sparks, all contributing to a comprehensive understanding of biofluorescence biology. The support for this research came from a range of sources including the National Science Foundation, Dalio Foundation, Stavros Niarchos Foundation, and institutional grants from the American Museum of Natural History.

Beyond the scientific discoveries, this research embodies the importance of long-term specimen collection, sophisticated imaging technology, and integrative phylogenetic approaches in uncovering evolutionary history and biodiversity. The specimens analyzed, many collected over the past fifteen years, illustrate the value of museum archives in enabling modern scientific breakthroughs. Such collections provide an irreplaceable resource for examining traits like biofluorescence that can only be understood in the context of broad comparative and temporal analyses.

As biofluorescence continues to intrigue scientists, this body of work sets a foundation for future research aimed at deciphering the molecular basis of fluorescence, its ecological and behavioral functions, and its evolutionary drivers across marine taxa. It challenges researchers to explore not just the visible expressions of biofluorescence but its underlying genetics and biochemistry, fostering interdisciplinary studies that integrate ecology, evolution, and molecular biology.

In conclusion, the elucidation of biofluorescence’s ancient origins and the identification of its extraordinary color diversity revolutionize our understanding of this captivating biological phenomenon. It underscores the adaptive ingenuity of marine fishes and highlights coral reefs as epicenters of evolutionary innovation. These insights broaden the horizon for new scientific inquiries into marine biodiversity, sensory biology, and the translational potential of biofluorescence in medicine and technology. The research spearheaded by Emily Carr and colleagues exemplifies how combining phylogenetics with advanced imaging can unlock nature’s luminous secrets and inspire future exploration of life’s hidden colors.

Subject of Research: Evolutionary origins and color diversity of biofluorescence in marine teleost fishes.

Article Title: Not explicitly stated in the provided content.

News Publication Date: Not specified.

Web References:

• Nature Communications study: https://www.nature.com/articles/s41467-025-59843-7
• PLOS One study: https://doi.org/10.1371/journal.pone.0316789

References:

• Nature Communications DOI: 10.1038/s41467-025-59843-7
• PLOS One DOI: 10.1371/journal.pone.0316789

Image Credits: © John Sparks and David Gruber

Keywords: Fish, Animal science, Marine biology, Ichthyology, Natural history