Vertebral osteology and swimming style in living and fossil whales (Order: Cetacea)

Abstract

Morphological modi®cation of the cetacean vertebral column for axial locomotion was one of many changes that occurred during and after the invasion of aquatic habitats by the ancestors of living whales. These modi®cations resulted in a column that can be divided into neck, chest, torso, and¯uke structural and functional units. Variations in these units allow subdivision of living whales into three morphological groups that re¯ect differences in regional¯exibility of the column and of locomotor style. Both odontocete and mysticete cetaceans exhibit trends toward reduction of relative centrum length in the neck and torso, restricting the¯exibility of the column with increasing adaptation to aquatic lifestyles. Mysticetes and physeterid odontocetes retain the archaeocete pattern of constant vertebral shape along the torso, which acts as an undulatory unit. The location of secondary sites of reduced centrum length in non-physeterid odontocetes subdivides them into subgroups, one with reduced posterior, and the other with reduced anterior, torso¯exibility. The relationships between particular osteological patterns and swimming styles in living taxa are used to support predictions of swimming style in fossil species after adoption of a¯uke in the late Eocene.

Description of the skeleton of the fossil beaked whale <i>Messapicetus gregarius</i>: searching potential proxies for deep-diving abilities

“…Even though extant cetaceans may display intraspecific variation regarding the degree of fusion of their cervical vertebrae (Buchholtz, 2001), we argue that the low degree of fusion of the cervical vertebrae in stem ziphiids is consistent for three reasons. First, we assessed the ontogenetic stages of the studied specimen of Messapicetus gregarius as being an adult, although the degree of fusion generally increases with age in cetaceans (Buchholtz, 2001). Second, extant ziphiids generally possess at least three fused cervical vertebrae at adult age, thus contrasting with the condition in the analysed stem ziphiids .…”

“…First, extant ziphiids travel from the surface in a vertical position with a slow fluke rate at which the animal is gliding between each fluking period . Furthermore, their shortened and fused neck stabilizes their head to maintain a streamlined body during the descent phase (Buchholtz, 2001;Lambert et al, 2013). Likewise, their reduced flipper tucks in an indentation along the body wall to decrease drag forces (Mead et al, 1982).…”

An experimental and morphometric test of the relationship between vertebral morphology and joint stiffness in Nile crocodiles (Crocodylus niloticus)

“…In pinnipeds, relative centrum length and width, transverse process width and inter-zygapophyseal length were correlated with swimming style (pelvic versus pectoral oscillation) (Pierce et al, 2011), and in whales centrum length and width were correlated with degree of dorsoventral flexion and undulatory wavelength during swimming (Buchholtz, 2001). In a range of mammals, relationships have been found between habitual spinal loading patterns (dorsoventral shear, mediolateral shear, torsion, axial compression) and centrum width, mediolateral spacing of zygapophyses and zygapophyseal angle (Boszczyk et al, 2001), and between arboreal locomotor habits and neural spine height and angle, transverse process width, lamina width and prezygapophyseal angle (Shapiro, 2007).…”

“…The principles drawn from living animals also have been applied to reconstruct locomotor behaviour in extinct relatives (e.g. Buchholtz, 2001;Buchholtz, 2007;Finch and Freedman, 1986;Hua, 2003;Piechowski and Dzik, 2010;Pierce et al, 2011;Pittman et al, 2013;Shapiro et al, 2005).…”

Vertebral bending mechanics and xenarthrous morphology in the nine-banded armadillo (Dasypus novemcinctus)

“…Boszczyk et al, 2001;Buchholtz, 2001;Cullinane and Bertram, 2000;Filler, 2007;Gál, 1993a,b;Gaudin and Biewener, 1992;Hebrank et al, 1990;Hildebrand, 1959;Jones and German, 2014;Jones and Pierce, 2016;Molnar et al, 2014;Pierce et al, 2011;Rockwell et al, 1938;Schilling, 2011;Shapiro, 1995;Slijper, 1946;Ward and Mehta, 2014). Vertebral evolution is facilitated by regionalization, the Hox-mediated division of the column into morphological and functional units (Head and Polly, 2015;Schilling, 2011;Wellik, 2007).…”