The Oldest Known Fossil Evidence of Mustelinae

A fossil made up of only a few teeth and part of a lower jaw can sometimes be enough to rewrite the evolutionary record of an entire animal group. This is precisely the case with a small mustelid fossil discovered at the Las Casiones fossil locality in Spain. The animal has been named Galanthis baskini, and it is regarded as the oldest known member of the subfamily Mustelinae. Until now, most palaeontologists had considered the earliest true weasels to come from the Pliocene of central Europe, dating to roughly 4.0 to 3.5 million years ago. Galanthis baskini, however, comes from the Messinian stage of the Miocene, approximately 6.56 to 6.26 million years ago, pushing the record of Mustelinae back by more than two million years.

Reconstruction of Galanthis baskini (AI-generated and manually verified)

Today, weasels and their relatives are distributed across the Northern Hemisphere and South America. They are typically small carnivores with elongated bodies, relatively short limbs, stout skulls and sharp dentition, well suited for hunting small vertebrates, especially rodents and lagomorphs. Their slender bodies allow them to enter burrows and narrow spaces, their short legs keep them close to the ground, and their carnassial teeth are adapted for processing meat. Yet these same animals are not easily captured in the fossil record. Their small size, low population densities and preference for forested or concealed habitats mean that small mustelid fossils are rarely preserved or recovered. Many early forms may have existed long before they were discovered, but their remains were either never found or were too fragmentary to reveal their precise evolutionary position.

Las Casiones lies near Teruel in Spain and belongs to the Late Miocene. The fossil-bearing deposits are associated with lacustrine environments and consist mainly of green and black clays, with occasional layers of fine sand. The Galanthis baskini material recovered from this locality is incomplete. The holotype, KS-9a, is a fragmentary right hemimandible preserving the fourth lower premolar and the first lower molar. A second specimen, KS-9b, consists of a fragmentary right fourth upper premolar. At first glance, this may seem like very limited material. For palaeontologists studying fossil carnivorans, however, the morphology of premolars and molars is highly diagnostic. These teeth can reveal diet, phylogenetic relationships and evolutionary stage.

Jaw fragment of Galanthis baskini above, compared with the more complete jaw of the living least weasel (Mustela nivalis) below（Courtesy of Alberto Valenciano ）

The teeth of Galanthis baskini are extremely small. Based on mathematical estimation, its body mass was about 135 grams, close to the average size of the smallest living weasels. Its fourth lower premolar is relatively short and low, while the first lower molar is slender. The arrangement of the cusps indicates a highly carnivorous dentition. This means that, by the Late Miocene, early members of Mustelinae had already occupied a clear ecological role.

The research team also used micro-computed tomography to examine the roots of the teeth and the details of the jaw. They then performed a phylogenetic analysis combining cranial and dental morphology, molecular data and fossil ages. The results place Galanthis baskini at the base of Mustelinae, close to the initial divergence of the subfamily and earlier than the split between the two major lineages, Mustela and Neogale. This is especially important because molecular-clock studies had already estimated the origin of Mustelinae at around 8.69 to 6.2 million years ago, and the age of Galanthis baskini falls directly within that range.

The study also re-examined another Late Miocene mustelid fossil from China. The specimen previously known as Proputorius minimus was reassigned to a new genus, Zdanskyictis, becoming Zdanskyictis minimus. However, it was not placed within Mustelinae. Instead, the authors argue that it is more closely related to Lutrinae and Ictonychinae. Together, these fossils suggest that different mustelid lineages were already diversifying across Eurasia during the Late Miocene.

Body-mass reconstruction on the phylogenetic tree suggests that the ancestral musteline may have been three to four times heavier than Galanthis baskini. This implies that the early miniaturization of Mustelinae may have occurred quite rapidly. At the same time, Las Casiones appears to have experienced increasing aridity after a long humid interval, producing a heterogeneous landscape of forested areas, open habitats and diverse microhabitats. Fossils of talpids, rodents and lagomorphs have also been found in this environment, providing the kinds of prey that could have shaped the body plan of early mustelines. With their tiny but agile bodies, these animals would later move quietly into underground burrows, forest margins and grassland spaces, beginning an evolutionary journey that eventually carried their descendants across the Northern Hemisphere and the Americas.

Author: Shui-Ye You

Reference:

Valenciano A et al. (2026). Oldest evidence of a weasel reveals a Miocene origin of the Mustelinae (Mammalia, Carnivora). Palaeontology.