The trachea of adult leatherback sea turtles enables deep dives via a compressible cartilaginous structure.

"Adult leatherbacks are large animals (300–500 kg), overlapping in size with marine pinniped and cetacean species. Unlike marine mammals, they start their aquatic life as 40–50 g hatchlings, so undergo a 10,000-fold increase in body mass during independent existence. Hatchlings are limited to the tropics and near-surface water. Adults, obligate predators on gelatinous plankton, encounter cold water at depth (<1280 m) or high latitude and are gigantotherms that maintain elevated core body temperatures in cold water. This study shows that there are great ontogenetic changes in tracheal structure related to diving and exposure to cold. Hatchling leatherbacks have a conventional reptilian tracheal structure with circular cartilaginous rings interspersed with extensive connective tissue. The adult trachea is an almost continuous ellipsoidal cartilaginous tube composed of interlocking plates, and will collapse easily in the upper part of the water column during dives, thus avoiding pressure-related structural and physiological problems. It is lined with an extensive, dense erectile vascular plexus that will warm and humidify cold inspired air and possibly retain heat on expiration. A sub-luminal lymphatic plexus is also present. Mammals and birds have independently evolved nasal turbinates to fulfil such a respiratory thermocontrol function; for them, turbinates are regarded as diagnostic of endothermy. This is the first demonstration of a turbinate equivalent in a living reptile. (Davenport et al. 2009:3440)


"Functionally it appears that the deep diving mammals have a cartilaginous tracheal design that will facilitate progressive collapse with increasing depth, whereas shallow divers have rigid upper portions of the tracheae that remain patent during divesthe adult leatherback trachea has a structure markedly different from that of other living sea turtles. It consists of an elliptical tube of nearcontinuous uncalcified cartilage, rather than a sequence of circular, closely packed tracheal rings. The tube is easily compressibleWe believe that the elasticity of tracheal cartilage (combined with expansion of the remaining tracheal air during ascents) will be sufficient to reinflate the trachea"  (Davenport et al. 2009:3440, 3445-6)

Journal article
Ontogenetic changes in tracheal structure facilitate deep dives and cold water foraging in adult leatherback sea turtlesJournal of Experimental BiologyOctober 20, 2016
J. Davenport, J. Fraher, E. Fitzgerald, P. McLaughlin, T. Doyle, L. Harman, T. Cuffe, P. Dockery

Leatherback Sea TurtleDermochelys coriaceaSpecies