Postsacral Vertebral Morphology in Relation to Tail Length Among Primates and Other Mammals

Abstract

Tail reduction/loss independently evolved in a number of mammalian lineages, including hominoid primates. One prerequisite to appropriately contextualizing its occurrence and understanding its significance is the ability to track evolutionary changes in tail length throughout the fossil record. However, to date, the bony correlates of tail length variation among living taxa have not been comprehensively examined. This study quantifies postsacral vertebral morphology among living primates and other mammals known to differ in relative tail length (RTL). Linear and angular measurements with known biomechanical significance were collected on the first, mid-, and transition proximal postsacral vertebrae, and their relationship with RTL was assessed using phylogenetic generalized leastsquares regression methods. Compared to shorter-tailed primates, longertailed primates possess a greater number of postsacral vertebral features associated with increased proximal tail flexibility (e.g., craniocaudally longer vertebral bodies), increased intervertebral body joint range of motion (e.g., more circularly shaped cranial articular surfaces), and increased leverage of tail musculature (e.g., longer spinous processes). These observations are corroborated by the comparative mammalian sample, which shows that distantly related short-tailed (e.g., Phascolarctos, Lynx) and long-tailed (e.g., Dendrolagus, Acinonyx) nonprimate mammals morphologically converge with short-tailed (e.g., Macaca tonkeana) and long-tailed (e.g., Macaca fascicularis) primates, respectively. Multivariate models demonstrate that the variables examined account for 70% (all mammals) to 94% (only primates) of the variance in RTL. Results of this study may be used to infer the tail lengths of extinct primates and other mammals, thereby improving our understanding about the evolution of tail reduction/ loss. Anat Rec, 298:354-375, 2015. V C 2014 Wiley Periodicals, Inc.

Evolution of the hominoid vertebral column: The long and the short of it

“…E) and Nacholapithecus (Fig. F) have more elliptically shaped (mediolaterally wide, dorsoventrally narrow) and more caudally tapered bodies that bear shallow dorsal grooves (instead of neural arches), as well as reduced transverse processes; they also lack zygapophyseal articular processes . In contrast, first postsacral vertebrae in primates and mammals with long tails exhibit more circularly shaped articular surfaces, well‐developed transverse processes, neural arches bearing dorsally projecting spinous processes, and prezygapophyses that articulate with the postzygapophyses of the distal sacrum to form a mobile sacrocaudal joint (Fig.…”

“…In contrast, first postsacral vertebrae in primates and mammals with long tails exhibit more circularly shaped articular surfaces, well‐developed transverse processes, neural arches bearing dorsally projecting spinous processes, and prezygapophyses that articulate with the postzygapophyses of the distal sacrum to form a mobile sacrocaudal joint (Fig. D) …”

“…She is particularly interested in questions concerning tail function and loss, and adaptations to transverse processes; they also lack zygapophyseal articular processes. 2,4,[9][10][11] In contrast, first postsacral vertebrae in primates and mammals with long tails exhibit more circularly shaped articular surfaces, well-developed transverse processes, neural arches bearing dorsally projecting spinous processes, and prezygapophyses that articulate with the postzygapophyses of the distal sacrum to form a mobile sacrocaudal joint (Fig. 1D).…”

“…1D). 4,11 Tail loss is also reflected in the morphology of presacral vertebrae. Primates and other mammals with long tails bear well-developed anapophyses (styloid processes) that project caudally from the inferior pedicle ventrolateral to the postzygapophyses of lower thoracic and upper lumbar vertebrae, anchoring longissimus and a common tendon for extensor caudae lateralis (a basal tail extensor muscle) and intertransversarius ( Fig.…”

Tail function during arboreal quadrupedalism in squirrel monkeys (Saimiri boliviensis) and tamarins (Saguinus oedipus)

“…Our results also have the potential to inform our functional understanding of tail‐length variation in primates. Tail loss or tail reduction has independently occurred in several primate radiations, including lorises, indriid lemurs, some Old World monkeys, and apes (Russo, ). Within these clades, evolutionary changes in relative tail length are often interpreted with reference to locomotor biomechanics and ecology, signaling an allometric change in inertial righting mechanisms (i.e., indriids: Preuschoft et al, ), a relaxation of selection on tail function associated with increased terrestriality (i.e., cercopithecoids: Wilson, ; Rodman, ), or a transition to less agile, more deliberate forms of arboreal quadrupedalism (i.e., lorisids and hominoids: Cartmill and Milton, ; Kelley, ).…”