Farewell to life on land - thoracic strength as a new indicator to determine paleoecology in secondary aquatic mammals

Abstract

Habitat shifts from land to water have occurred independently in several mammal lineages. However, because we do not know completely about the relationship between skeletal morphology and function, both reliable life reconstructions of each extinct taxon and the timing of those shifts in locomotor strategies are yet to be fully understood. We estimated the strengths of rib cages against vertical compression in 26 extant and four extinct mammal specimens including cetartiodactyls, paenungulates, and carnivorans, representing 11 terrestrial, six semi-aquatic, and nine obligate aquatic taxa. Our analyses of extant taxa showed that strengths were high among terrestrial/semi-aquatic mammals, whose rib cages are subjected to vertical compression during the support on land, whereas strengths were low among obligate aquatic mammals, whose rib cages are not subjected to antigravity force in the water. We therefore propose rib strength as a new index to estimate the ability of an animal to be supported on land while being supported by either the forelimbs or thoracic region. According to our analyses of extinct taxa, this ability to be supported on land was rejected for a basal cetacean (Cetartiodactyla: Ambulocetus) and two desmostylians (Paenungulata: Paleoparadoxia and Neoparadoxia). However, this ability was not rejected for one desmostylian species (Desmostylus). Further study of the ribs of extant/extinct semi-aquatic taxa may help in understanding the ecological shifts in these groups.

Keywords: aquatic; desmostylian; early cetacean; reconstruction; rib.

Figure 1

Mechanical models of thoracic ribs of mammals in terrestrial stances (a) on forelimbs and (b) on thorax; and (c) in the water. The body weights of animals (the downward solid arrows) are balanced by the upward ground reaction force (GRF) or by upward components of the buoyancy forces (BF). In (a) and (b), the thoracic ribs are subjected to compressive force between the body weight and GRF, which is transmitted to the distal end of the ribs via forelimb skeleton/sternum and the thoracic muscles, such as m. serratus ventralis and m. pectoralis. h, humerus; p, m. pectoralis; r, rib; rh, m. rhomboideus; sc, scapula; st, sternum; sv, m. serratus ventralis; v, vertebra.

Figure 2

Phylogenetic relationships in (a) Cetartiodactyla (Wang & Yang, 2013), (b) Carnivora (Flynn et al. 2005), and (c) Afrotheria (Nishihara et al. 2005). A, obligate aquatics; S, semi‐aquatics; T, terrestrial quadrupeds; X, unknown. Asterisks: The taxa which include the study taxa.

Figure 3

(a) Mechanical models of an arch formed by a vertebra and associated ribs in dorsal‐side‐up position. The centrum (cross) is fixed to avoid translations and rotations, and the distal end of each rib is subjected to a vertical load [P (N)]. The arch is vertically compressed between the upward lifting force [2P (N) in total] and the downward reaction force [−2P (N)] on the centrum. (b) Measurements taken from mounted skeletons. The width (2w) and the thickness (2t) of the transverse section of the rib, and the distances between the necks (2n), the lateral‐most points (2l), and the distal ends (2d) on the left and right ribs are shown. The moment arms L _n and L _l were calculated as equations ‘|d – n|’ and ‘|d – l|’, respectively.

Figure 4

Relationship between the body mass [BM (kg)] and the thoracic strengths [TF (N)] in study specimens on double logarithmic plots. Equations of the regression lines are as follows: TF = 228.44 × BM ^0.8489 in terrestrial quadrupeds; TF = 632.48 × BM ^0.7306 in semi‐aquatics; and TF = 259.36 × BM ^0.6577 in obligate‐aquatics. The oblique coordinate system between the dashed lines and the vertical line indicates the relationship between the body mass (kg) and the safety factors (N/N) of the thoraces, which is defined as the thoracic strengths (TF) standardized by the body weight [BM×g, where g is a standard gravitational acceleration (9.8065 ms⁻²)]. The horizontal bar for each plot indicates the range from minimum to maximum body mass (Nowak, 1999). See Table 1 for the identification numbers (IN) of the study specimens.