Small and simple was the key to mammalian evolution

The ancestors of modern mammals developed simplified skulls over time, allowing them to take advantage of new food sources and helping mammals to become one of the most successful animal lineages to evolve.
Small and simple was the key to mammalian evolution
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Over 300 million years ago, the earliest ancestors of modern mammals possessed skulls and jaws comprised of numerous bones. This was a characteristic seen across many vertebrate (backboned) groups at this time, including fish and reptiles. However, as mammals evolved, the number of bones decreased and a new jaw joint developed, leading to simplification of the skull approximately 100 to 150 million years ago. 

While this simplification is seen in fossils, the purpose of skull simplification is still under debate. Some studies have suggested that reducing the number of bones in the jaw may have helped to increase bite force, or that fusing bones in the skull improved skull strength. We used computer simulations and stress analyses to investigate potential causes and responses to skull simplification further.

Digital reconstructions were generated from CT scans of six fossil skulls belonging to precursors of modern mammals - non-mammalian cynodonts and mammaliaforms (Thrinaxodon liorhinus, Diademodon tetragonus, Chiniquodon sanjuanensis, Probainognathus sp., Morganucodon oehleri and Hadrocodium wui). Skull models were also generated for three living mammals - Monodelphis domestica (Gray short-tailed opossum), Dasyurus hallucatus (Northern quoll) and Petropseudes dahli (Rock ring tail possum) as modern comparative counterparts.

For the study, the jaw musculature was reconstructed to assist biomechanical modelling and the skull was divided into six anatomical regions: narial, frontal, skull roof, zygoma, palate and braincase. Finite element analysis (an engineering technique to simulate the behaviour of objects during loading conditions) was used to quantify stresses that propagate through these six skull regions. As it is not always known whether an extinct organism bit down predominantly with its canines or posterior teeth, both scenarios were tested for all taxa. Deformation of the skull associated with bite forces was then modelled to visualise where stress is distributed throughout the skull under different biting scenarios.

Digitally reconstructed cynodont and mammaliaform skulls with suggested musculature.

Digitally reconstructed skulls and jaw musculature for studied taxa. (a) Thrinaxodon liorhinus, (b) Diademodon tetragonus, (c) Probelesodon sanjuanensis, (d) cf. Probainognathus sp., (e) Morganucodon oehleri, (f) Hadrocodium wui, (g) Monodelphis domestica, (h) Dasyurus hallucatus, (i) Petropseudes dahli. Credit: Dr Stephan Lautenschlager.

Our new study, published in Communications Biology, reveals that reducing the number of skull bones did not lead to higher bite forces or increased skull strength. Instead, we found that the skull shape of early mammals redirected stresses during feeding in a more efficient way. In cynodonts, compressive stresses were concentrated in the skull roof and orbital region, whereas stress shifted to the zygoma (cheekbones) in mammaliaforms (except for Hadrocodium wui, which shows very high stresses across the majority of the skull in all bite scenarios). Additionally, the rearrangement of jaw muscles did not lead to a more efficient transfer of muscle force to bite force, but may have allowed a greater range of movement in the jaw when feeding.

Stress distribution modelled across the skulls of Thrinaxodon and Morganucodon. The shift of stress from the braincase to the zygomatic arches is clearly highlighted by warmer tones.

Stress distribution modelled across the skulls of Thrinaxodon and Morganucodon. The shift of stress from the braincase to the zygomatic arches is clearly highlighted by warmer tones. Credit: Dr Stephan Lautenschlager.

The transfer of stress to the zygomatic arches is consistent with their changing shape during mammalian evolution. Most cynodonts had high and expanded arches that represented 20 to 30% of the skull length, whereas taxa from closer to the cynodont-mammaliaform transition had smaller arches (5 to 10% of skull length). It is worth noting that cynodonts were predominantly carnivorous or herbivorous whereas mammaliaforms were predominantly insectivorous. We therefore suggest that adaptation to insectivory could have reduced the need for a strong cranial structure and powerful jaw muscles. 

Reorganisation of the cranial skeleton during the transition from cynodonts to mammals appears to have reduced the stresses experienced by the braincase and the skull roof during feeding. We propose that the reduction in stress experienced by these parts of the skull may have allowed for the brain size to increase along the mammalian evolutionary lineage. At the same time, early mammals experienced miniaturisation of body size, which would have restricted feeding options to mostly insects. The combination of these characteristics may have helped mammal ancestors to continue to thrive in the shadow of dinosaurs before diversifying significantly following the end Cretaceous extinction 66 million years ago.

Read more about our research here

Artistic reconstruction of early mammal ancestors (Hadrocodium wui) hunting insect prey.

Artistic reconstruction of early mammal ancestors (Hadrocodium wui) hunting insect prey. Credit: Dr Stephan Lautenschlager.

Charlotte Bird and Dr Stephan Lautenschlager are researchers at the University of Birmingham, UK. Charlotte is also a freelance Research Communities Content Manager at Springer Nature, sharing research stories via social media.

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