Mammal origins have been pushed farther back in time by new fossil evidence, suggesting mammals emerged much earlier than previously thought. A study centered on Brasilodon quadrangle, a small mammal-like creature about 20 centimeters in length, dated to roughly 225 million years ago, provides a pivotal glimpse into early mammalian history. This discovery adds a crucial chapter to the story of how mammal characteristics first appeared in the fossil record and challenges the long-held timeline of mammalian evolution.
Researchers analyzed the growth patterns of teeth across the fossil specimens. By studying the developmental stages encoded in the dentition, the team concluded that these remains represent true mammals rather than reptilian ancestors. The careful examination of tooth formation and replacement supported the idea that Brasilodon possessed mammalian dental characteristics, reinforcing its placement in the mammal lineage.
Previously, the oldest widely accepted mammal record dated to about 205 million years ago. The new findings push back this benchmark by several million years, suggesting a much earlier origin for the mammalian class and prompting a reevaluation of how and when key mammalian traits first appeared in the fossil record.
According to Moya Meredith Smith, a contributing author and professor of Evolution and Development of Dentoskeletal Anatomy at King’s College London, the analysis of tooth construction and development is essential for establishing brasilodonts as mammals. The study demonstrates that their place in mammalian ancestry is rooted in deep time, predating previous estimates and enriching our understanding of early mammal evolution.
While mammary glands and milk production are defining features of modern mammals, those soft-tissue structures do not fossilize. Consequently, scientists rely on hard tissues such as bones and teeth to infer mammalian traits. In the case of Brasilodon quadrangle, the preserved dental and skeletal evidence serves as a reliable proxy for tracing mammalian ancestry, offering insights into the evolution of feeding, growth, and life history in early mammals.
By examining the dentition of Brasilodon quadrangle from southern Brazil, dated to around 225 million years ago in the Upper Triassic period, researchers identified evidence of a single, preserved set of permanent teeth. This dentition pattern aligns with diphyodonty, a hallmark of mammals, where a first set of teeth develops during embryonic stages and a second, permanent set emerges after birth. Such a dental scheme contrasts with reptilian dentitions, which typically display polyphodontia, a pattern involving repeated tooth replacement throughout life.
Diphyodontia is more than just a tooth count. It reflects a suite of developmental changes that shape skull anatomy and dental replacement over time, including the evolution of a secondary palate. The secondary palate enables a young animal to suckle and breathe simultaneously, a feature associated with a broader trajectory of mammalian adaptation and versatility in feeding strategies.
These findings contribute to a broader narrative about the pace and nature of early mammal evolution. The work highlights how tooth development, jaw structure, and skull remodeling collectively inform the emergence of mammalian features long before other lineages show similar traits. The discovery adds texture to the timeline of mammalian origins and prompts ongoing questions about how widespread diphyodont dental patterns were among early mammaliforms and their close relatives.
Reference work: Journal of Anatomy, 2023 edition, detailing the dental and skeletal analyses that underpin these conclusions. The study is part of a growing effort to redefine the origins of mammals using fossil data and comparative anatomy across Triassic vertebrates.