Testudines

Testudines (also known as Chelonia, from the Latin testudo, "tortoise," and Greek chelone) is the order of sauropsid reptiles comprising approximately 360 extant species of marine sea turtles, freshwater terrapins and softshell turtles, and terrestrial tortoises (Testudinidae). Easily recognized by one of the most radical body-plan transformations in vertebrate evolution—a rigid bony box consisting of a dorsal carapace fused to the vertebrae and ribs and a ventral plastron derived from the interclavicle, clavicles, and abdominal gastralia—turtles lack teeth at all post-Triassic stages, cropping food instead with a sharp, keratinous beak (rhamphotheca).

For over a century, paleontologists classified turtles as the sole surviving members of "Anapsida" (basal stem-reptiles that never evolved temporal skull openings) because the modern turtle skull lacks classic open temporal fenestrae. In 2012–2015, genome-wide ultraconserved element (UCE) and whole-genome phylogenies (Crawford et al., 2012; Shaffer et al., 2013; Field et al., 2014), together with Middle Triassic stem-turtle fossils possessing unmistakable diapsid upper and lower temporal fenestrae (Pappochelys rosinae, ~240 Ma, described by Rainer R. Schoch and Hans-Dieter Sues, 2015), settled the phylogenetic position of Testudines: turtles are secondarily closed diapsids nested inside Archelosauria as the living sister clade to Archosauria (Crocodylia + Aves).

Unlike armadillos, glyptodonts, ankylosaurs, or crocodilians—whose armor shells consist of dermal osteoderms floating outside a normal ribcage—the turtle carapace is built directly from the trunk ribs and vertebrae, resulting in an adult skeleton where the shoulder blade (scapula) and pelvis sit inside the ribcage:

1. The Carapacial Ridge (CR) and Lateral Rib Redirection: As demonstrated by Shigeru Kuratani, Hiroshi Nagashima, and colleagues (2009) in embryos of the Chinese softshell turtle (Pelodiscus sinensis), a specialized ectodermal–mesenchymal signaling fold called the carapacial ridge (CR) (expressing Lef1, Wnt5a, Fgf10, and Sp5) forms along the lateral flank between the forelimb and hindlimb buds. Instead of growing ventrally into the body wall to meet a sternum (turtles have no sternum!), the ten pairs of trunk ribs are arrested at the axial–lateral border and redirected laterally into the dorsal dermis over the top of the embryonic scapula, folding the body wall inward to encapsulate the shoulder girdle inside the ribcage.
2. Composite Bony Plates and Staggered Keratinous Scutes: The mature bony carapace consists of ~50 fused elements: a midline row of neural plates fused to the thoracic vertebral neural spines, eight pairs of expanded costal (pleural) plates formed by intramembranous bone spreading between the periosteum of the ribs, and a perimeter ring of dermal peripheral, nuchal, and pygal plates. The ventral plastron is fused from nine bones: the anterior unpaired entoplastron (homologous to the interclavicle), paired epiplastra (homologous to the clavicles), and three pairs of abdominal plates (hyoplastra, hypoplastra, and xiphiplastra derived from neural-crest/gastralia progenitors). Overlying these bony plates is an outer mosaic of 38 carapacial and 12–16 plastral β-keratin epidermal scutes whose seams are deliberately staggered out of phase with the underlying bony sutures—functioning like brickwork to maximize mechanical impact resistance (reduced to leathery skin in Trionychidae softshell turtles and the pelagic leatherback sea turtle, Dermochelys coriacea).
3. Diaphragmatic / Abdominal Muscle Ventilation Inside a Rigid Shell: Because the ribs are fused immovably into the carapace, a turtle cannot expand and contract its ribcage to breathe! Instead, as shown by F. Harvey Pough and C. G. Gans, turtles ventilate their multichambered dorsal lungs using two antagonistic pairs of sheet-like abdominal muscles spanning the open limb pockets at the front and rear of the shell: contracting the transversus abdominis and pectoralis compresses the viscera upward against the lungs to drive active exhalation, while contracting the obliquus abdominis and serratus pulls the flank membranes outward to create negative intra-shell pressure for active inhalation (and during locomotion, limb protraction and retraction actively assist lung ventilation!).

1. Month-Long Winter Survival Without Oxygen (Brain Anoxia Tolerance and Shell Buffering): Whereas the human brain suffers irreversible ischemic death after 3 to 5 minutes without O₂ (as ATP depletion halts Na⁺/K⁺-ATPase pumps, triggering glutamate excitotoxicity and Ca²⁺ overload), North American painted turtles (Chrysemys picta) and snapping turtles (Chelydra serpentina) overwinter buried in ice-covered, completely anoxic (0% O₂) pond mud at 3 °C for 3 to 5 consecutive months! As elucidated by Donald C. Jackson, Kenneth B. Storey, and Göran E. Nilsson:
   • The turtle depresses its whole-body metabolic rate by 90% to 95% (channel arrest and spike arrest, releasing inhibitory GABA and adenosine in the brain while downregulating NMDA glutamate receptors to prevent Ca²⁺ influx);
   • Simultaneously, to neutralize the massive buildup of anaerobic lactic acid in the blood (up to 150–200 mmol/L—ten times the fatal level in mammals!), the turtle mobilizes calcium and magnesium carbonate (CaCO₃ / MgCO₃) out of its own bony shell into the bloodstream and sequesters calcium lactate back into the bone matrix, turning its skeleton into a giant rechargeable acid–base buffer!
2. Bimodal Aquatic Cloacal and Pharyngeal Respiration: Several aquatic turtles—most famously the Australian Fitzroy River turtle (Rheodytes leukops) and Mary River turtle (Elusor macrurus)—pump oxygenated water in and out of two giant, richly vascularized, papillae-lined pouches branching off the cloaca (cloacal bursae), extracting up to 70% of their total O₂ uptake through their cloaca so they can remain submerged for days at a time; meanwhile, softshell turtles (Pelodiscus, Apalone) extract O₂ underwater via vascularized villiform projections in their throat (buccopharyngeal respiration).

Why did the turtle shell originally evolve: for armor defense or for underground digging?

While the completed shell of Proganochelys (~210 Ma) clearly functioned as anti-predator armor, the earliest stem-turtles—such as the Permian Eunotosaurus africanus (~260 Ma)—possessed only broadened, overlapping T-shaped ribs that locked the torso rigid and slowed down breathing and lateral undulation long before they formed a protective carapace. In 2016, micro-CT histology and forelimb biomechanics by Tyler R. Lyson, Beverley J. Pobiner, Gabe S. Bever, and colleagues revealed that Eunotosaurus had spade-like claws, massive humeral muscle attachments, and high-acuity burrowing eyes: its broadened ribs evolved first as a rigid thoracic anchor tobrace the trunk against recoil forces during scratch-digging of underground burrows in arid Karoo drought environments, and were only later co-opted into a complete dermal armor shell!

How do sea turtles navigate thousands of kilometers across featureless ocean back to their exact natal beach?

Hatchling loggerhead (Caretta caretta) and green sea turtles (Chelonia mydas) emerge from tropical sand beaches, swim into open-ocean gyres for 10–30 years, and return as adults to nest on the exact coastline where they hatched (natal homing). As demonstrated in classic coil-tank experiments by Kenneth J. Lohmann and Catherine M. F. Lohmann (1996–2018), sea turtles possess a bicoordinate geomagnetic map: they measure both the inclination angle of Earth's magnetic field lines relative to the surface and the total magnetic field intensity (which together form a unique two-coordinate signature for different latitudes and longitudes along a coastline), updating their Natal beach's magnetic signature even as secular geomagnetic drift shifts the isolines over decades.

1. Jackson, D. C. (2002). Hibernating without oxygen: Physiological adaptations of the painted turtle. The Journal of Physiology, 543(3), 731–737. doi:10.1113/jphysiol.2002.024729
2. Li, C., Wu, X.-C., Rieppel, O., Wang, L.-T., & Zhao, L.-J. (2008). An ancestral turtle from the Late Triassic of southwestern China. Nature, 456(7221), 497–501. doi:10.1038/nature07533
3. Nagashima, H., Sugahara, F., Takechi, M., Ericsson, R., Kawashima-Ohya, Y., Narita, Y., & Kuratani, S. (2009). Evolution of the turtle body plan by the folding and creation of new muscle connections. Science, 325(5937), 193–196. doi:10.1126/science.1173826
4. Schoch, R. R., & Sues, H.-D. (2015). A Middle Triassic stem-turtle and the evolution of the turtle body plan. Nature, 523(7562), 584–587. doi:10.1038/nature14472
5. Lyson, T. R., Rubidge, B. S., Scheyer, T. M., de Queiroz, K., Schachner, E. R., Smith, R. M. H., ... & Bever, G. S. (2016). Fossorial origin of the turtle shell. Current Biology, 26(14), 1887–1894. doi:10.1016/j.cub.2016.05.020
6. Brothers, J. R., & Lohmann, K. J. (2018). Evidence that magnetic navigation and geomagnetic imprinting shape genetic population structure in sea turtles. Current Biology, 28(8), 1325–1329. doi:10.1016/j.cub.2018.03.022