Ctenodactyloids are pivotal players in rodent history. They diversified widely across eastern and central Asia from the early Eocene to the early Miocene, and they are regarded as ancestral members of the mouse-related clade that gave rise to a large share of modern rodent diversity. Today, however, the group is a shadow of its former self, restricted to just four genera and five species confined to North and East Africa, where they inhabit rocky crevices in arid regions. That contrast between a rich fossil past and a narrow extant present makes ctenodactyloids an ideal natural experiment for studying how brains adapt to changing environments.

The research team, working at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, scanned five well-preserved skulls with a GE v|tome|x m micro-CT scanner at consistent parameters of 140 kV and 110 μA, yielding more than a thousand CT images per specimen at interpixel distances near 13 μm. The fossils included the early Eocene Exmus mini from the Yuhuangding Formation in Hubei Province, China, and two skulls of the early Oligocene Bounomys ulantatalensis from the Ulantatal Formation in Nei Mongol. Virtual endocasts were reconstructed in VGStudio Max and compared with two extant species, Ctenodactylus sp. from the American Museum of Natural History and Pectinator spekei from the University Museum of Zoology in Cambridge, whose CT files were obtained from MorphoSource.

The endocasts reveal a striking trajectory of change. From the Eocene to the present, the olfactory bulbs shrank in relative size, the degree of neocorticalization increased, the temporal lobes expanded laterally, the petrosal lobules of the cerebellum diminished, and overall encephalization rose. The cerebrum itself changed shape, evolving from an ovoid form in the early Eocene to a trapezoidal profile in the Oligocene and finally to a distinctive teardrop shape in living species, narrow at the front and broad at the back, driven by the lateral expansion of the temporal lobes.

The olfactory story is particularly clear. In Exmus mini, the olfactory bulbs extend anteriorly above the fourth premolar and cover roughly two-thirds of the orbital length, with a combined width-to-length ratio of 99.5 percent. The olfactory bulbs-to-total endocast length ratio is 24.94 percent in the Eocene species, substantially larger than the 14.73 to 16.82 percent seen in Bounomys and the 13.56 to 16.61 percent in extant taxa. Olfactory bulb volume relative to total endocast volume drops from 5.73 percent in Exmus to 3.61 percent in Bounomys and 2.66 to 3.13 percent in living species. Crucially, when olfactory bulb volume is measured against estimated body mass, the ratio falls from 0.08 percent in the Eocene to 0.025 to 0.035 percent in the Oligocene, a decline the authors interpret as a genuine reduction in olfactory acuity during the Eocene–Oligocene interval rather than a mere proportional effect of other brain regions growing.

The cerebrum tells a complementary tale. The cerebrum-to-total length ratio, a potential indicator of occipital lobe development and visual cortex position, rises only modestly from 51.39 percent in Exmus to 52.4 to 56.1 percent in Bounomys and 53.03 to 61.48 percent in extant species. The authors argue that this slight increase does not reflect a significant posterior expansion of the occipital lobe, unlike the more pronounced trend documented in sciurid rodents. Instead, the dominant cerebral change is lateral. The cerebellar width to maximum cerebral width ratio falls from 74.37 percent in Exmus to 61.01 to 66.18 percent in Bounomys and 52.63 to 60.52 percent in living species, quantitative support for progressive lateral expansion of the temporal lobes over time.

Because the temporal lobe is predominantly composed of auditory cortex, its sustained lateral expansion suggests enhanced auditory processing in ctenodactyloids as they adapted to open environments. A similar pattern of pronounced temporal lobe expansion has been documented independently in fossil and extant caviomorph rodents, and the combination of teardrop-shaped cerebral hemispheres with prominent temporal lobes is also seen in multiple extant rodent species such as Chinchilla, Cavia, and Trinomys yonenagae that inhabit open habitats. The morphological evolution of the ctenodactyloid cerebrum is therefore interpreted as an adaptive response to open woodland and grassland ecosystems, during which lateral temporal expansion likely contributed to improved auditory function.

The petrosal lobules, cerebellar structures associated with stabilizing eye movements and smooth pursuit, show a continuous decline in relative size across ctenodactyloid evolution. The ratio of petrosal lobule volume to total endocast volume falls from 2.13 percent in Exmus to 1.37 percent in Bounomys and 0.88 to 0.93 percent in extant taxa. When measured against body mass, the ratio drops from 0.03 percent in the Eocene to 0.01 percent in the Oligocene and remains stable thereafter. Taken together, these analyses indicate a reduction in neural investment related to smooth pursuit eye movement control during the Eocene to Oligocene transition, followed by functional stabilization in later lineages.

Encephalization quotients, calculated with a rodent-specific equation from Pilleri and colleagues, reveal a nuanced pattern. The EQ of Exmus mini is 0.65 and that of Bounomys ulantatalensis is 0.48, both lower than the 0.85 and 0.69 of extant Ctenodactylus and Pectinator, suggesting a general increase in encephalization toward the present. However, a marked decrease in EQ occurred between the Eocene and Oligocene, driven primarily by the fact that body mass in Bounomys more than doubled relative to Exmus while brain size did not increase proportionally. This disparity between body and brain growth likely reflects a significant ecological adaptation, indicating that the Oligocene taxon occupied a distinct niche from its Eocene predecessor in response to environmental changes across the Eocene–Oligocene Transition roughly 34 million years ago.

The global cooling event at the Eocene–Oligocene Transition reshaped ctenodactyloid skeletons as well as brains. Skeletal studies indicate a shift in zygomatic morphology from protrogomorphous to hystricomorphous, associated with processing tougher vegetation, alongside the development of higher-crowned cheek teeth, interpreted as direct responses to climatic cooling, increased aridification, and the expansion of open forest-grassland ecosystems. The new endocast data now show that these same pressures left their mark on the brain, reducing olfactory acuity and smooth pursuit capacity while boosting neocorticalization and encephalization. After the Oligocene, features associated with olfaction and eye movement control remained stable in size, while encephalization quotients and the degree of neocorticalization continued to rise, completing a 50-million-year record of how a lineage of early rodents rewired its brain to survive a changing world.

Subject of Research: Virtual endocasts of the Paleogene Ctenodactyloidea and brain evolution in ctenodactyloid rodents

Article Title: Virtual endocasts of the Paleogene Ctenodactyloidea and brain evolution in ctenodactyloid rodents

Article References: Li, Q., Bi, A., & Li, Q. (2026). Virtual endocasts of the Paleogene Ctenodactyloidea and brain evolution in ctenodactyloid rodents. Swiss Journal of Palaeontology, 145, 77-95. https://doi.org/10.3897/sjp.145.181868

Image Credits: AI Generated

DOI: 10.3897/sjp.145.181868

Keywords: Virtual, endocasts, Paleogene, Ctenodactyloidea, brain, evolution, ctenodactyloid, rodents, scientific research