Brain morphology of the Early Triassic cynodont Thrinaxodon liorhinus and the impact of variation in digital endocranial reconstructions on understanding mammalian brain evolution

Bird, Charlotte (2024). Brain morphology of the Early Triassic cynodont Thrinaxodon liorhinus and the impact of variation in digital endocranial reconstructions on understanding mammalian brain evolution. University of Birmingham. Ph.D.

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Abstract

The mammalian brain has an evolutionary history which can be traced back to the late Permian, when basal cynodonts show the first glimmers of mammal-like characteristics. One such early cynodont, Thrinaxodon liorhinus, has been widely studied with regards to cranial and postcranial anatomy, yet the morphology of its brain has remained elusive. Given Thrinaxodon can provide insights into the baseline brain shape from which mammalian features developed, it is important to peer inside digitally-rendered (‘digitised’) skulls to reconstruct the soft tissues that may have once been present. Presented here are the first digital reconstructions of Thrinaxodon liorhinus’ endocranial anatomy for both juvenile and adult specimens, with the main mammalian brain features present (olfactory bulbs, cerebral hemispheres and cerebellum). Yet, between the growth stages there is variation in brain size and shape which has shed light on the possible ontogenetic growth trajectory the brain may have experienced. Comparisons with contemporary cynodonts and the extant opossum, Monodelphis domestica, allowed conclusions to be drawn about the cognitive, sensory and behavioural capabilities that may have been influenced by the changing size and shape of the brain. Studies have suggested that basal cynodonts may have been nocturnal, fossorial and could have cared for young. Reconstructed morphological changes to specific brain regions appear to support these statements, particularly when considering the environmental stressors persisting during cynodont diversification following the Permian-Triassic mass extinction. However, shifting endocranial morphology was also considered in the context of variation and the impact the model maker has on endocranial reconstructions. Through experiment, it was found that nineteen modellers produced noticeably different endocasts for a single skull and that neither education level nor segmentation experience significantly improved the consistency of outputs. As such, improvements to the segmentation workflow have been proposed to help minimise inter-modeller variation. Digital reconstructions of Thrinaxodon’s brain have provided a piece of the evolutionary puzzle for mammalian brain development and shown that cynodonts were important stepping stones within the advancement of the mammalian lineage.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):