Long bone scaling in Captorhinidae: do limb bones scale according to elastic similarity in sprawling basal amniotes?

Abstract

Captorhinids are a speciose clade of sauropsids that are crucial to understand several aspects of basal amniote general biology. Members of the Captorhinidae explored different diets and, amongst basal amniotes, were one of the first groups to demonstrate high‐fibre herbivory. Several papers have been published on the cranial anatomy of captorhinids, but there are relatively few studies which focus on the post‐cranium, especially on the appendicular skeleton and long bones. This contribution presents the first quantitative long bone scaling in Captorhinidae performed through morphometric analyses. From classical biomechanical research, it is well‐established that to accommodate an increase in size, gravity will result in elastic deformation of long bones. This outcome is especially significant in terrestrial tetrapods with a sprawling limb posture such as captorhinids, where great torsional stresses are applied to long bones, both during locomotion and in the resting phase. In this paper, we test whether the consistent evolutionary size increase in captorhinids led to major re‐patterning in long bone structure as theoretically expected, based on the theory of elastic similarity. Morphometric analysis shows that, apart from a small positive allometry in the humerus, captorhinid long bones scale geometrically as body size increases. Thus, the predicted elastic similarity to maintain similar levels in peak stress with an increase in dimensions does not seem not to apply to long bone evolution in captorhinids. We propose that, as already observed experimentally in larger‐bodied varanid lizards, large captorhinids could also mitigate size‐related increases in stress by reducing femur rotation and increasing the percentage of the stride cycle during which the right hindfoot was on the ground (i.e. the duty factor). In this way, large captorhinids could avoid reaching peak stress thresholds by sacrificing speed during locomotion and without a substantial long bone re‐patterning or postural change.

Body mass estimate of Anancus arvernensis (Croizet and Jobert 1828): comparison of the regression and volumetric methods

“…By contrast, overestimations of more than 200% have been obtained using formulas based on the lengths of the condyles in the humerus and femur, the results of which are therefore of little value for the estimation of the BM, at least for the genus Anancus. Similar results have also been recently obtained for the circumference of the ulna and femur in Mammuthus meridionalis (Romano et al, 2022b), returning plausible values quite close to the volumetric ones; however, in that case the circumference of the humerus returned an overestimated BM value of about 23%, probably due to the very robust and 'overbuilt' structure (sensu Romano, 2017;Romano and Rubidge, 2019a) of the anterior stylopodium in the taxon.…”

“…These inconsistent results are probably due to the general structure of long bones in extinct tetrapods, which exhibit proportions that are sometimes significantly different from the conditions characterising extant taxa used to derive regression formulas. In particular, previous studies have shown that many of these bones appear to be ‘overbuilt’ ( sensu Romano, 2017; Romano and Rubidge, 2019a), meaning they are more robust and have larger circumferences than would be expected to support a given body weight. In taxa with a primitive sprawling posture and large body masses, this more robust structure may be explained by greater torsional forces compared with a posture with columnar limbs.…”

“…Volumetric methods provide BM values that are more realistic and closer to actual conditions, and they also offer much narrower estimation ranges (Romano et al, 2022b and references therein). In several cases, the regression estimates based on limb‐bone measurements resulted in very consistent overestimations of the BM, related to the fact that in several extinct tetrapods long bones are very robust (‘overbuilt’ sensu Romano, 2017; Romano and Rubidge, 2019a), in comparison to the condition in the extant taxa used to derive the regression formulas.…”

A captorhinid-dominated assemblage from the palaeoequatorial Permian of Menorca (Balearic Islands, western Mediterranean)

“…more robust (e.g., Modesto et al 2019), but not as much as theoretically expected (Romano & Rubidge 2019). In order to reduce the mechanical stress on the limb bones during locomotion, those herbivorous large forms probably moved more slowly (Romano & Rubidge 2019), which did not compromise their ability to obtain plant food, contrarily to smaller captorhinids, which needed to catch their prey.…”

Volumetric Body Mass Estimate and in vivo Reconstruction of the Russian Pareiasaur Scutosaurus karpinskii

“…As stressed in the text we prefer the volumetric method for body mass estimate, since the classic regression formulas based on long bone dimensions can lead to substantial under-or overestimation of the body weight of extinct tetrapods (see Sellers et al, 2012;Bates et al, 2015;Brassey et al, 2015;Larramendi, 2016;Romano and Manucci, 2019;Romano and Rubidge, 2019a;Romano et al, 2021). It has been empirically shown that the discrepancy between estimates obtained from regression formulas are greater when applied to clades phylogenetically distant from those used to construct the dataset (Brassey, 2016;Romano and Manucci, 2019;Romano and Rubidge, 2019a), and in particular to taxa with a "primitive" sprawling posture characterized by excessively "overbuilt" long bones (sensu Romano, 2017b;Romano and Rubidge, 2019b). The latter applies directly to Scutosaurus which has extremely strong and stocky long bones in proportion to their length, most likely to withstand the great torsional efforts caused by the sprawling posture.…”