Astragalar trochlea asymmetry, functional significance in eutherian mammals

The symmetry of the astragalar trochlea is considered an important morphofunctional and phylogenetic character in palaeontology. Here, we seek simpler methods to measure and evaluate it. We use a dataset of 89 genera from 56 families of Eutheria, including extinct Folivora, Litopterna, and Notoungulata. Several studies considered asymmetrical and shallow trochleas as characteristic of plantigrades, following the assumption that it would allow plantigrades greater eversion and inversion. However, here we observed that the eutherian upper ankle joint is a hinge joint restricted to a single principal movement. The symmetry index on eutherian plantigrades and digitigrades has considerable overlap, with plantigrades being more variable. The depth of the astragalar trochlea has more statistical significance than symmetry. Our results suggest plantigrady allows considerable morphological variation in the astragalar trochlea, while unguligrady and digitigrady are more restrictive. Eutherian mammals restraining movement of the foot to the parasagittal plane modify the transverse tarsal joint primarily, this joint being responsible for inversion-eversion movements. Smutsia and pilosans with a pivot-type transverse tarsal joint have the most asymmetrical trochleas. The trochlea in Pleistocene ground sloths has been described as an odontoid-discoid morphology. In our analysis, this highly asymmetrical trochlea tali shares the morphospace with the silky anteater and the giant pangolin, plantigrades ecologically very different from giant ground sloths. In the pangolin, the large lateral condyle is more related to the shape of the fibula than to the tibia. Our results question the use of the asymmetry of the astragalar trochlea to differentiate plantigrady and digitigrady. The plantigrade-digitigrade-unguligrade model may fall short in representing the relationship between feet and substrate, particularly when fossil diversity is included. A more nuanced approach, considering the complex interplay of various factors, is necessary to avoid premature conclusions.