Evolutionary Morphology of Early Cenozoic Mammals

In the wake of the end-Cretaceous mass extinction, placental mammals radiated into vacant ecological niches, exhibiting a remarkable array of skeletal and neuroanatomical adaptations during the Paleogene. This period saw the emergence of novel locomotor repertoires—from arboreal quadrupeds to specialised fossorial diggers—and concomitant modifications in cranial architecture, brain size and sensory regions. Comparative studies reveal that shifts in habitat and behaviour drove mosaic evolution of the skull and brain: expansions of olfactory bulbs in insectivorous lineages, enlargement of neocortical regions in arboreal taxa, and diversification of the petrosal lobules associated with gaze stabilisation. Limb proportions and joint morphologies document the early divergence of hoofed, rodent and primate baupläne, while endocasts preserve evidence for increases in cerebral complexity and gyrification. These advances not only inform reconstructions of ancestral mammalian morphology but also underpin modern debates on the tempo and mode of morphological innovation across deep time.

Research from Nature Portfolio

Recent studies have shown that cerebral reorganisation and increases in cortical folding predated overall brain enlargement in certain anthropoid and cercopithecoid primates, indicating independent pathways to encephalisation in apes and Old World monkeys. Virtual endocasts of early primate relatives reveal that frontal-lobe sulcation patterns emerged before volumetric brain expansions, suggesting a decoupling of neural complexity and absolute brain size. Parallel work on early rodents demonstrates that locomotor ecology exerted a strong influence on brain morphology: species with arboreal habits exhibit relatively larger neocortices and petrosal lobules than their fossorial or terrestrial counterparts. These findings underscore the role of ecological pressures in shaping neural investment and highlight convergent trends across Mammalia during the early Cenozoic.

Evolutionary Morphology of Early Cenozoic Mammals publication trend

The graph below shows the total number of articles in evolutionary morphology of early cenozoic mammals across all publications each year (not limited to Nature Index journals).

Technical terms

Endocast: A cast or digital reconstruction of the internal cranial cavity, reflecting outer brain morphology.

Encephalization quotient: A ratio comparing observed brain size to the expected brain size for an animal of a given body mass.

Neocortex: The cortex of the mammalian forebrain involved in higher-order sensory processing and cognition.

Petrosal lobules: Cerebellar structures involved in vestibular processing and eye-movement coordination.

References

1. Cerebral complexity preceded enlarged brain size and reduced olfactory bulbs in Old World monkeys. Nature Communications (2015).
2. The impact of locomotion on the brain evolution of squirrels and close relatives. Communications Biology (2021).
3. Small within the largest: brain size and anatomy of the extinct Neoepiblema acreensis, a giant rodent from the Neotropics. Biology Letters (2020).
4. Cranial endocast of the stem lagomorph Megalagus and brain structure of basal Euarchontoglires. Proceedings of the Royal Society B (2020).
5. CT-Informed Skull Osteology of Palaeolagus haydeni (Mammalia: Lagomorpha) and Its Bearing on the Reconstruction of the Early Lagomorph Body Plan. Frontiers in Ecology and Evolution (2021).