A 165-million-year-old fossil discovered in Inner Mongolia, China, reveals critical mammalian traits in an ancient semi-aquatic creature, challenging previous assumptions about the evolution of swallowing, chewing, and suckling.
The fossil of Megacauda sungei, a chihuahua-sized mammaliaform, provides some of the oldest evidence of chewing, swallowing, and suckling—key traits defining mammals. Discovered in Inner Mongolia, the specimen, dated to the Middle Jurassic, features a hyoid bone structure and pharyngeal anatomy akin to modern mammals, enabling it to swallow against gravity and nurse. It's all there in one remarkable fossil,
said Zhe-Xi Luo, a University of Chicago paleontologist, whose team described the find in Nature.
Key Findings from the Bone Study
The study, published in Nature on October 7, 2026, with DOI 10.1038/s41586-026-11107-0, builds on earlier work by Luo and colleagues. The fossil’s hyoid bones and pharyngeal structure, detailed in the study, align with findings from a 2019 Science paper by Zhou et al. on mammal-like hyoid bones in Jurassic mammals. These discoveries collectively underscore the early diversification of mammalian traits.
Dr. Martin, a University of Bonn paleontologist, emphasized the fossil’s significance as the earliest evidence of an aquatic lifestyle in mammals.
The specimen’s wide, flattened tail and short hindlegs, as described in the study, suggest adaptations for swimming, akin to modern otters. The fossil’s teeth, capable of shearing action, indicate a diet of fish, salamanders, and invertebrates, as noted in the Nature paper. This semi-aquatic niche, combined with terrestrial burrowing, reflects the ecological versatility of early mammaliaforms.
Characteristics of a Semi-Aquatic Lifestyle
Analysis of Megacauda sungei’s anatomy suggests a semi-aquatic lifestyle, with a wide, flattened tail and short hindlegs, similar to modern otters. The creature likely foraged in water for fish and invertebrates while sheltering in burrows. This adaptation made brain development, eventually leading to humans, possible,
said Martin, linking the fossil’s features to later mammalian diversification. The discovery also expands the known range of docodonts, an extinct group of early mammaliaforms, showing they occupied diverse niches long before modern mammals emerged.
The study, co-authored by researchers from the University of Chicago and the University of Bonn, was published in Nature on October 7, 2026, and corroborates earlier findings about the early diversification of mammalian traits.
The fossil’s discovery challenges the notion that complex mammalian features evolved later. Instead, it shows that key adaptations—like the hyoid apparatus and semi-aquatic habits—were present in the Middle Jurassic, offering a window into the survival strategies of early mammals amid dinosaur dominance. Mammals, they're never dominant in the age of dinosaurs,
Luo said, but nonetheless, they hold out relatively well compared to all other vertebrates.
The specimen, measuring approximately 0.5 meters (1.6 feet) long and weighing 1–2 kilograms, is the largest Jurassic mammaliaform known. Most contemporaries of the time weighed only a few grams, as noted in the Nature study. Its size, combined with the presence of a soft palate and hyoid apparatus, suggests advanced feeding mechanisms. Dr. Martin highlighted that many species of animal do not have a soft palate and only have a simple hyoid bone apparatus without a swallowing mechanism,
unlike Megacauda sungei, which exhibits traits seen in living therian mammals.
April I. Neander contributed a life restoration of Megacauda sungei, depicting its otter-like features and semi-aquatic habits. The fossil’s pharynx and throat bones, exceptionally well-preserved, provide direct evidence of its swallowing capabilities. These findings align with Luo’s assertion that the anatomy for mammal-like swallowing, drinking and suckling was already present in the common ancestors of docodonts and mammals, long before modern mammals appeared.
The study’s implications extend to understanding the evolutionary origins of mammalian diversity. By demonstrating that early mammaliaforms like Megacauda sungei occupied specialized niches, the research challenges the perception of early mammals as small, generalized creatures. As Luo noted, these evolutionary innovations— and their presence among our ancient mammalian relatives—extend deep into the past to at least the middle Jurassic.
The fossil’s traits, including its hyoid structure and semi-aquatic adaptations, reveal a lineage of mammals that thrived through specialized feeding and locomotion strategies.
Megacauda sungei adds to this legacy, illustrating the adaptability and complexity of early mammals.