Cynodont cranial evolution

Lautenschlager et al 2023 report,
“Here, we combine digital reconstruction and biomechanical modelling to show that there is no evidence for an increase in cranial strength and biomechanical performance. Our analyses demonstrate the selective functional reorganisation of the cranial skeleton, leading to reduced stresses in the braincase and the skull roof but increased stresses in the zygomatic region through this transition. This cranial functional reorganisation, reduction in mechanical advantage, and overall miniaturisation in body size are linked with a dietary specialisation to insectivory, permitting the subsequent morphological and ecological diversification of the mammalian lineage.”

There are more functional analyses nowadays, perhaps to keep students busy, as the list of new, largely complete fossil taxa appear to be dwindling likely due to reaching a saturation point. At least this is so in the LRT where there are no large gaps left to fill.

Figure 1. Gorgonopsids, therocephalians and cynodonts to scale.

The authors cite Kemp 2005 when they report
“It has been argued that the selection pressure for stronger skulls could represent one possible driver for the morphological transformation across the cynodont–mammaliaform transition. However, this proposition has not been tested quantitatively.”

“It is further unclear how, on the one hand, there existed a trend towards strengthening of the skull, while, on the other, reductions of the bones supporting the musculature could potentially weaken cranial strength.”

Traditionally it was thought the skulls at the pelycosaur > cynodont > mammal transition points are developing more muscle tissue to drive chewing while losing unnecessary bone in the process.

In practice, the change from pre-cynodont to post-cynodont (Fig 1) is not that great. The lateral temporal fenestra does rotate to a dorsal position (which must be remembered when scoring). The cheek bar thins and thickens (Fig 1) with no directional trend.

“Here, we test the hypothesis that the cranial skeleton did indeed become stronger across the cynodont–mammaliaform transition, using a combination of digital reconstruction methods and biomechanical analysis techniques.”

The authors are testing bone alone, not supported by other tissues, primarily muscles, tendons and ligaments found in the living organism.
Plus, there is no reason to assume the skull should become stronger phylogenetically. The external food sources, whether insects, meat or plants, are not changing. Ingestion must continue unabated and efficiently in every generation… or that lineage.

Phylogenetic miniaturization attended the origin of mammals (= Megazostrodon (Figs 1, 2) and Hadrocodium, Fig 2) which means smaller skull, smaller prey, easier to bite and chew.
Even so, the differences in skull morphology between Pachgenelus and Monodelphis (Fig 2) focus more on the reduction and migration of auditory bones and the adoption of more molar cusps with single tooth replacement, rather than changing the shape of rest of the skull.

Figure 2. Basal mammals and their proximal ancestors. Here taxa below Megazostrodon are mammals. Those above are not. Hadrocodium is uniquely reduced, but this occurs within the Mammalia.  The dual jaw joint was tentatively present in Pachygenelus.

According to the authors,
they were looking for “stresses on the skull roof” and “stress hotspots”.

The authors summarized their findings,
“A comparison of bite forces across the studied taxa shows that absolute bite forces decrease from the cynodonts towards the mammaliaforms and increase again in the extant taxa.
This is unsurprising considering the considerable size differences between the taxa.“

“Quantified against dietary regimes, insectivorous taxa have both the lowest absolute bite forces and mechanical advantage. Herbivorous taxa, conversely, have the highest absolute and relative bite forces, but it should be noted that our sample includes only one fossil and one modern herbivorous species.”

That makes sense. More chewing.

Unfortunately, taxon exclusion strikes again.
The authors’ cladogram starts off echoing the LRT in the cynodont grade, but falls into a bit of a mess at the base of the Mammalia. For instance, the late-surviving cynodont, Repenomamus (Fig 1), is incorrectly nested within Mammalia (= Megazostrodon and all its descendants). Several other taxa are likewise incorrectly nested at the transitional grade at the base of the Mammalia. Adding taxa resolves this problem.

Build your own LRT
so you’ll have a concrete foundation for subsequent hypotheses. Don’t cherry-pick taxa and expect to understand evolution in detail. Let a wide gamut taxon list tell you where clades begin and end. Ideally we should all be working from the same topology at every node.

References
Kemp TS 2005. The Origin and Evolution of Mammals (Oxford University Press,
2005).
Lautenschlager S et al (5 co-authors) 2023. Functional reorganisation of the cranial skeleton during the cynodont–mammaliaform transition. Nature communications biology https://doi.org/10.1038/s42003-023-04742-0