Adaptation to graviportality in Rhinocerotoidea? An investigation through the long bone shape variation in their hindlimb

Abstract

Weight support is a strong functional constraint modelling limb bones in heavy quadrupeds. However, the complex relations between bone shape, mass, size and body proportions have been poorly explored. Rhinocerotoidea is one of the groups showing the highest body mass reached by terrestrial mammals through time. Here, we explore the evolutionary variation of shape in hindlimb stylopod and zeugopod bones and its relationship with mass, size and gracility in this superfamily. Our results show that bones undergo a general increase in robustness towards high masses, associated with reinforcements of the main muscle insertions. The shape of the femur, carrying a marked phylogenetic signal, varies conjointly with mass, size and gracility, whereas that of the tibia appears related to gracility and mass only. The shape of the fibula does not vary according to that of the tibia. Moreover, congruent variation of shape between the distal part of the femur and the complete tibia underlines the potentially strong covariation of the elements constituting the knee joint. These results, coupled with those previously obtained from forelimb study, allow a better comprehension of the relationship between bone shape and mass among Rhinocerotoidea, and a refining of the concept of ‘graviportality’ in this superfamily.

Functional morphology of the hindlimb of fossilized pygmy hippopotamus from Ayia Napa (Cyprus)

“…This asymmetry probably acted as an effective knee-locking mechanism, by which the patella and the medial patellar ligament passively locked at the proximal edge of the medial trochlea, reducing the muscle energy consumption during knee extension. The same suggestion has been proposed for perissodactyls (Hermanson & MacFadden, 1996;Mallet et al, 2022).…”

“…The more vertically oriented sustentaculum tali may suggest a more horizontal placement of the calcaneus in P. minor, compared with that in the other insular and extant hippos, thus increasing the moment arm of the common calcaneal tendon as a hock extensor. This suggests that the more distal attachment of the common calcaneal tendon in the Cypriot species provided a powerful but not fast motion to the joint (Barr, 2014;Christiansen;Curran, 2015;Etienne et al, 2021b;Mallet et al, 2019).…”

“…As it occurs in the carpus, the tarsal elements also displayed a limitation in the transversal movements, mainly explained by the presence of prominent medial malleolus of the tibia, which is articulated with a well-marked facet at the medial side of the astragalus (Boekschoten & Sondaar, 1966;Caloi & Palombo, 1996;Mallet et al, 2019). Such a restriction of movements in the transversal plane has been also noted in many other fossil endemic herbivores of Mediterranean islands (Ambrosetti, 1968;Bover et al, 2005;Caloi & Palombo, 1994, 1996Spaan, 1996;Theodorou, 1983;van der Geer, 1999).…”

“…The hippopotamuses are animals with short limb bones relative to their body mass and, hence, they are categorized as graviportal mammals (Alexander & Pond, 1992;Etienne et al, 2021b;Mallet et al, 2019), losing cursorial capacities. Despite this, it seems that the reduced length of the metatarsals of P. minor did affect its speed compared with that of its large-sized ancestor.…”

How can research on modern and fossil bones help us build more resistant columns?

“…Indeed, the giant rhinocerotoid Paraceratherium was much heavier than modern elephants, with a mass estimated between 10 and 17 tonnes [55,56]. This taxon shows relatively long and straight limbs, slender than in modern rhinos although not columnar like in elephants [57]. We might thus expect this taxon to possibly show a radius inner structure very well adapted to heavy weight support and that could be more efficient than Ceratotherium's.…”