Locomotor pattern fails to predict foramen magnum angle in rodents, strepsirrhine primates, and marsupials

Abstract

Foramen magnum position has traditionally been used as an indicator of bipedality because it has been thought to favor a more "balanced" skull position. Here, we analyzed foramen magnum angle (FMA) in relation to locomotion in three mammalian orders that include bipedal or orthograde species in addition to quadrupedal or pronograde species. In marsupials and strepsirrhine primates, we found that there is no relationship between locomotor pattern and FMA. In rodents, we found that there is a significant difference in FMA between bipedal and quadrupedal rodents. However, when these species are analyzed in the context of enlarged auditory bullae, this relationship is no longer significant. Additionally, we find a significant relationship between relative brain size and FMA in strepsirrhine primates. Taken together, these data indicate that several developmental modules of the cranium influence FMA, but that locomotion does not. We caution that basicranial evolution is a complex phenomenon that must be explored in the context of each taxon's unique evolutionary and developmental history.

Response of the Axial Skeleton to Bipedal Loading Behaviors in an Experimental Animal Model

“…Russo and Kirk () responded to these critiques by Ruth et al . () using an expanded data set, and demonstrated that in fact brain size (i.e., encephalization quotient) had no discernible effect on either FMP or FMO, and that the relationship between FMP and FMO was weak or nonexistent within the mammalian clades they (and Ruth et al, ) examined (rodents, marsupials, primates).…”

“…Many researchers accept that the relatively anterior position of the foramen magnum in humans (compared to other extant apes) is likely an adaptation for maintaining balance of the head atop a vertical neck during bipedalism and/or the assumption of orthograde trunk postures (Topinard, ; Dart, ; White et al, ; Kimbel and Rak, ; Russo and Kirk, , ; Kimbel et al, ). However, that the position as well as orientation of the foramen magnum may be linked to other structural and/or functional systems in the body (e.g., brain expansion and/or reorganization, facial size) has contributed to ongoing debate concerning the functional anatomy of this region (Biegert, , ; Kimbel and Rak, ; Russo and Kirk, , ; Ruth et al, ; Villamil, ). For example, Ruth et al .…”

“…() recently rejected the use of Russo and Kirk's () metrics for assessing FMP, asserting these metrics instead captured changes in other regions of the skull across mammals (e.g., brain size in strepsirrhine primates, masticatory apparatus size in marsupials). They further contended that both measures of FMP and FMO could be captured by a single metric (“foramen magnum angle”) that they devised (Ruth et al, ). Russo and Kirk () responded to these critiques by Ruth et al .…”

“…Metrics have already been devised for quantifying the position and inclination of the foramen magnum (as basion: the anteriormost border of the foramen magnum in the midsagittal plane) in morphologically and taxonomically diverse mammalian samples that include representatives of Muridae (the family to which the rodent species R. norvegicus used in this experiment belongs) (Russo and Kirk, , ; Ruth et al, ) (Fig. ).…”

Etho-Eco-Morphological Mismatches, an Overlooked Phenomenon in Ecology, Evolution and Evo-Devo That Supports ONCE (Organic Nonoptimal Constrained Evolution) and the Key Evolutionary Role of Organismal Behavior

“…In another recent study that also comprised strepsirrhines-i.e., the sister-group of all other extant primates, which includes lemurs and lorisoids such as lorises and galagos -, as well as rodents and marsupials, Ruth et al (2016) tested the usually accepted assumption that the foramen magnum position is an indicator of bipedalism because it is supposed to favor a more "balanced" position of the skull. They examined the connection between the angle of this foramen and locomotion in these three clades including bipedal or orthograde species and quadrupedal or pronograde species.…”