By the Campanian, snakes had already taken divergent ecological paths

More than 4,200 species of snakes are alive today, occupying habitats ranging from deep underground and forest floors to tree canopies and aquatic environments. Yet the ecological setting in which snakes first evolved their highly specialized body plan has long been debated. Some hypotheses propose a marine origin, others suggest that early snakes were fossorial, and still others favour a terrestrial origin. A major obstacle is the extremely sparse Mesozoic fossil record: fewer than ten articulated snake fossils are currently known, and three-dimensionally preserved specimens are rarer still. A Cretaceous snake from Brazil, Tametara mirim, now suggests that early snakes had already diversified into markedly different ecological lifestyles, accompanied by equally divergent brain morphologies.

Reconstruction of Tametara mirim（Courtesy of Gabriel Ugueto ）

The fossil was discovered in 2020 in the Adamantina Formation of São Paulo State, Brazil, and dates to approximately 85–75 million years ago, spanning the late Santonian to early Campanian of the Late Cretaceous. The specimen preserves the posterior half of the skull, the mandibles and the anterior portion of the body, including 103 precloacal vertebrae, with a total preserved length of about 41.9 cm. Based on skull proportions, the researchers estimated a complete skull length of approximately 3.3 cm. Despite its small size, the skull is exceptionally well preserved in three dimensions, allowing high-resolution micro-computed tomography (micro-CT) to digitally separate individual bones from the surrounding rock and reconstruct the endocranial cavity, cranial nerves and inner ear.

Holotype specimen (MPM 420) of Tametara mirim. (a) Dorsal view of the specimen, showing the articulated skull and postcranial skeleton; (b) lateral view of the skull; (c) dorsal view of the skull; (d) three-dimensional reconstruction of a cervical vertebra in lateral view; (e) the same cervical vertebra in anterolateral view; (f) three-dimensional reconstruction of a dorsal vertebra in dorsal view; (g) the same dorsal vertebra in left lateral view; (h) three-dimensional reconstruction of the skull in right lateral view; (i) right lateral view of the skull with the brain endocast exposed; (j) dorsal view of the skull with the brain endocast exposed. Abbreviations: Ant, anterior; Bl, block number; Ce.V., cervical vertebrae; Do.V., dorsal vertebrae; Post, posterior; Sk, skull; CB, compound bone; F, frontal; Oto, otoccipital; P, parietal; PFr, postfrontal; Pro, prootic; Ptg, pterygoid; Q, quadrate; Soc, supraoccipital; St, supratemporal; Mes., mesencephalon; Rho., rhombencephalon; Tel., telencephalon（Image source：Simões TR et al. (2026)， CC BY-NC-ND 4.0 ）

Phylogenetic analyses place Tametara near the base of snake evolution. Most analyses recover it as one of the earliest-diverging stem snakes, close to Najash rionegrina from Argentina. Stem snakes are early members of the snake lineage that fall outside the crown group containing the common ancestor of all living snakes and its descendants. The skull of Tametara retains several plesiomorphic features. For example, the left and right otoccipitals do not meet along the midline, and parts of the braincase retain conditions more similar to those of non-ophidian squamates. Based on the articulation of the specimen, its preservation and the micro-CT scans, the forelimbs are interpreted as absent. Whether hindlimbs were present cannot be determined because the preserved body does not extend to the cloacal region.

Phylogenetic position of Tametara mirim（Image source：Simões TR et al. (2026)， CC BY-NC-ND 4.0 ）

The shape of the endocranial cavity can provide clues to brain morphology. In Tametara, the olfactory bulbs are relatively short and transition posteriorly into mildly enlarged cerebral hemispheres, while the optic tectum of the mesencephalon is poorly defined. This combination resembles the condition seen in many highly fossorial squamates. The inner ear provides an additional clue: Tametara has an enlarged vestibular region and a relatively simplified semicircular canal system, both features commonly associated with burrowing snakes.

The researchers did not rely on a few anatomical characters alone to infer its lifestyle. They compared telencephalon shape in 58 living squamate species and two fossil snakes, Tametara and Dinilysia patagonica, using three-dimensional geometric morphometrics. Tametara falls well within the region of morphospace occupied by fossorial taxa. A linear discriminant model classified it as fossorial with a probability of 99.2%. The model's overall classification accuracy across all habitat categories was only 65%, however, so this percentage should not be treated as definitive on its own. The stronger case comes from several independent lines of evidence pointing in the same direction: telencephalon shape, inner-ear anatomy and cranial bone microstructure all support the interpretation that Tametara was adapted to a fossorial lifestyle.

Comparison of telencephalon shape across species in morphospace. Different colours represent different habitat preferences: fossorial, aquatic, arboreal and terrestrial（Image source：Simões TR et al. (2026)， CC BY-NC-ND 4.0 ）

Its brain morphology, however, does not completely match the typical pattern seen in living burrowing snakes. Some regions of the Tametara brain carry strong fossorial signals, whereas others do not. Its elongated mid-hindbrain and enlarged pituitary region, for example, are more commonly found among extant aquatic, surface-dwelling or semi-fossorial taxa. This mosaic pattern of brain evolution means that the ecology of the entire animal cannot reliably be reconstructed from any single brain region.

The researchers also compared skull-roof microstructure in 106 living squamate species and the two fossil snakes. Their analyses included bone compactness, relative thickness, the degree of overlap between roofing bones, cranial elongation and other traits. Fossorial squamates generally possess more compact and thicker skull bones together with more elongated skulls. In this analysis, Tametara again falls within the morphospace occupied by fossorial snakes.

Comparison of skull-roof bone microstructure across species in morphospace（Image source：Simões TR et al. (2026)， CC BY-NC-ND 4.0 ）

Dinilysia, however, tells a very different story. Its telencephalon occupies a region of morphospace far removed from that of Tametara, and the morphological distance between the two fossils exceeds that between most pairs of living snakes included in the dataset. The discriminant model assigned Dinilysia a 56.7% probability of being terrestrial and a 25.7% probability of being fossorial. Because its morphological affinity with every habitat category remained relatively low, the researchers conservatively interpreted Dinilysia as non-fossorial without assigning it to a more specific lifestyle.

This contrast makes the ecological origin of snakes difficult to reduce to a single habitat. Alongside the fossorial Tametara and the non-fossorial Dinilysia, the Cretaceous also included snake lineages adapted to marine environments. Stem snakes had therefore already evolved highly distinct ecological specializations, none of which necessarily represents the ancestral condition of crown snakes. This is where the importance of Tametara lies: during the long early history of snake evolution, fossoriality was only one of several ecological pathways. Different lineages had already moved underground, remained on the surface, or entered marine environments long before the diversification of the snake groups that dominate ecosystems today.

Author: Shui-Ye You

Reference:

Simões TR et al. (2026). Exceptional brain and ecological diversity in the earliest snakes. Nature.