Anatomy and function of expanded ribs in certain edentates and primates.

TLDR: Anteroposteriorly expanded ribs occur in all three species of myrme-cophagids (Cyclopes didactylus, Tamandua tetradactyla, and Myrmecophaga tridACTyla) and Functionally, expanded ribs may increase the stability of the thorax, which, in turn, increases the stability in the vertebral column.

Abstract: Anteroposteriorly expanded ribs occur in all three species of myrme-cophagids (Cyclopes didactylus, Tamandua tetradactyla, and Myrmecophaga tridactyla). Similar rib expansion occurs in some armadillos (for example, Dasypus novemcinctus, Priodontes giganteas, Euphractus sexcinctus) and in some primates (Arctocebus calabarensis, Perodicticus potto, and Loris tardigradus). In C. didactylus and A. calabarensis rib expansion is so extensive that adjacent ribs imbricate. the epaxial and hypaxial musculature related to the expanded ribs in all three myrmecophagid species is described. the intercostal spaces in T. tetradactyla and M. tridactyla are approximately 50 per cent narrower than in various other mammals with unexpanded ribs; the intercostal musculature of these two species is approximately twice as thick as that in other mammals with unexpanded ribs. the proportion of intercostal musculature in C. didactylus is comparable to that in other mammals. Functionally, expanded ribs may increase the stability of the thorax, which, in turn, increases the stability of the vertebral column. This specialization in T. tetradactyla and M. tridactyla probably relates to their fossorial habitus; the trunk is stabilized during fossorial activity by xenarthrous processes in the lumbar region and by expanded ribs and robust intercostal muscles in the thorax. in strictly arboreal forms such as C. didactylus and A. calabarensis, lumbo-thoracic stability may be employed in methodical arboreal locomotion. C. didactylus, at least, is able to anchor itself with hind feet and tail and extend its trunk and forelimbs to reach an adjacent branch—a feat requiring unusual lumbothoracic stability.