Sauropsida

Sauropsida (coined by Thomas Henry Huxley in 1864 from the Greek sauros, "lizard," and opsis, "appearance") is one of the two primary evolutionary branches of Amniota—the sister clade to Synapsida (mammals and their extinct relatives)—from which it diverged during the Late Carboniferous (~318–312 million years ago) as documented by the earliest known sauropsid skeleton, Hylonomus lyelli, from Joggins, Nova Scotia. Encompassing more than 23,000 living species—including ~12,000 species of squamate lizards, snakes, and tuataras (Lepidosauria), ~360 species of turtles (Testudines), ~27 species of crocodilians (Crocodylia), and ~11,000 species of birds (Aves)—as well as extinct Paleozoic parareptiles (Mesosaurus, Pareiasaurus) and Mesozoic ichthyosaurs, sauropterygians (plesiosaurs), pterosaurs, and non-avian dinosaurs, Sauropsida is the monophyletic evolutionary replacement for the paraphyletic grade "Reptilia" (which historically excluded birds).

1. Corneous Beta-Proteins ("β-Keratins") in Scales, Claws, Shells, and Feathers: Whereas amphibians and synapsids (mammals) keratinize their epidermis exclusively using coiled-coil α-helical intermediate-filament α-keratins, sauropsids evolved a unique, lineage-specific family of genes within the Epidermal Differentiation Complex (EDC) encoding corneous beta-proteins (CBPs, historically termed "β-keratins"). These small (10–25 kDa) proteins fold into antiparallel β-pleated sheets that polymerize into rigid 3-nm microfibrils cross-linked by disulfide bonds, imparting high tensile stiffness and hydrophobic impermeability to lizard scales, turtle carapace scutes, crocodilian osteoderm coverings, and avian claws, beaks, and pennaceous feathers.
2. Uricotelic Nitrogen Excretion and Water Conservation: Unlike mammals—which detoxify ammonia into water-soluble urea via the hepatic ornithine–urea cycle and must possess long renal loops of Henle to excrete hyperosmotic urine at the cost of substantial water loss—sauropsids are predominantly uricotelic: they convert excess amino-acid nitrogen via the purine synthesis pathway into uric acid (and urate salts), which has an aqueous solubility of only ~0.06 g/L (~15,000 times less soluble than urea!). Precipitating as a harmless white semisolid paste inside the cloaca (where water and Na⁺ are resorbed across the coprodeum and urodeum) or inside the sealedembryonic allantois, uricotelism allowed sauropsids to lay calcified cleidoic eggs on dry land without toxic osmotic buildup inside the egg and enabled lizards, snakes, tortoises, and birds to thrive in hyper-arid deserts despite lacking long loops of Henle in most nephrons (often supplemented by nasal or orbital cranial salt glands that excrete concentrated NaCl/KCl solutions).
3. Diapsid Skull Architecture and Streptostylic Cranial Kinesis:
   • Ancestral Diapsid Condition: Soon after the basal Carboniferous sauropsid Hylonomus and Paleothyris (which lacked temporal openings), crown-group sauropsids (Diapsida, represented in the Late Carboniferous by Petrolacosaurus) evolved two pairs of temporal fenestrae behind each eye orbit: a dorsal (supratemporal) fenestra and a ventral (infratemporal) fenestra, separated by a horizontal bony bar formed by the postorbital and squamosal bones and bounded below by a lower temporal bar (jugal + quadratojugal).
   • Cranial Kinesis in Lepidosauria vs. Rigid Fusion in Archosaurs/Turtles: Whereas the tuatara (Sphenodon punctatus) and crocodilians retain both temporal bars in a classic rigid diapsid configuration, squamates (lizards and snakes) deleted the lower temporal bar (and in snakes, the upper bar as well), freeing the quadrate bone to swing forward and backward (streptostyly)—allowing snakes to unhinge and walk their independent left and right palatomaxillary arches over prey vastly wider than their own head! Conversely, turtles (Testudines) secondarily closed both temporal fenestrae into a solid secondarily anapsid skull roof (confirmed by both the Triassic stem-turtle Pappochelys rosinae, ~240 Ma, which preserves two diapsid openings, and whole-genome phylogenomics!).

One of the most important breakthroughs of 21st-century vertebrate phylogenomics (Shedlock et al., 2007; Crawford et al., 2012; Wang et al., 2013; Field et al., 2014) was resolving the long-debated evolutionary position of turtles (Testudines): rather than being basal "anapsid" parareptiles or sister to lepidosaurs, turtles are diapsids that form the sister clade to Archosauria (crocodilians + birds), uniting turtles and archosaurs into the superclade Archelosauria:

1. Ancestral Archosaurian Unidirectional Lung Ventilation: For over a century, the unidirectional flow-through lung of birds (Aves)—in which air flows in a continuous one-way loop through rigid dorsobronchi and parabronchi during both inhalation and exhalation via aerodynamic aerodynamic valves rather than dead-end mammalian alveoli—was thought to be a unique avian adaptation evolved specifically for powered flight. However, in 2010–2013, C. G. Farmer, Kent Sanders, and Emma R. Schachner demonstrated using endoscopic flow-probe measurements and CT imaging that American alligators (Alligator mississippiensis), Nile crocodiles, and even monitor lizards (Varanus exanthematicus) also exhibit unidirectional airflow through their multichambered lungs! This discovery proved that aerodynamic valve-driven unidirectional respiration evolved in the Early Triassic ancestor of Archosauria (or Diapsida)—helping archosaurs outcompete synapsids in the hypoxic atmosphere (~12–16% O₂) following the Permian–Triassic extinction.
2. Van der Waals Dry Adhesion in Gecko Setae (Gekko gecko): As demonstrated by Kellar Autumn and colleagues (2000, 2002), the subdigital toe pads of geckos (and convergently in Anolis lizards) adhere to vertical polished glass or ceilings without glue, suction, or capillary meniscus forces. Instead, each toe pad bears ~500,000 β-keratin hairs (setae, ~100 μm long), and each seta branches at its tip into 100 to 1,000 nanoscale spatula-shaped pads (spatulae, ~200 nm wide). When the gecko uncurls its toes and pulls slightly parallel to the wall, billions of flexible β-keratin spatulae conform within <1 nm of the surface molecules, generating a collective intermolecular van der Waals dispersion force capable of supporting >40 kg on a single gecko's four feet while detaching effortlessly in 15 milliseconds when peeled above a critical 30° angle!
3. Thermosensory TRPA1 Pit Organs in Pit Vipers, Pythons, and Boas: Crotaline pit vipers (Crotalus rattlesnakes, Bothrops) and boas / pythons independently evolved facial pit organs that form a thermal infrared pinhole camera capable of detecting a warm-blooded mouse in complete darkness from a temperature contrast of 0.001 °C (1 millikelvin). As discovered by Elena O. Gracheva, David Julius, and colleagues (2010), the thin (~15 μm), vascularized pit membrane suspended over an air-filled cavity is innervated by trigeminal nerve fibers expressing a custom-tuned orthologue of the Transient Receptor Potential A1 (TRPA1) ion channel (the "wasabi receptor" in mammals), which opens directly in response to far-infrared thermal heating (λ ≈ 5–30 μm) above ~28 °C and projects to the optic tectum to superimpose a real-time thermal image onto the snake's visual field!

How did the embryonic development of the turtle shell (carapace and plastron) evolve from an un-shelled diapsid ancestor?

Unlike armadillos, ankylosaurs, or crocodilians—whose armor shells are built outside the ribcage from dermal osteoderms—the turtle dorsal carapace is built from the ribcage itself! During turtle embryonic development (Shigeru Kuratani, Nagashima et al., 2009), a unique lateral ectodermal signaling fold along the body wall—the carapacial ridge (CR), expressing Fgf10, Wnt5a, and Lef1—arrests the normal ventral growth of the ribs into the body wall and redirects the ten pairs of trunk ribs laterally into the dorsal dermis above the shoulder blade (scapula), folding the body wall inward so that in adult turtles the shoulder girdle sits inside the ribcage. This developmental sequence matches the Triassic fossil transition step-for-step: from Permian Eunotosaurus africanus (~260 Ma, broadened T-shaped ribs) and Middle Triassic Pappochelys (~240 Ma, broadened ribs + gastralia) to Late Triassic Odontochelys semitestacea (~220 Ma, full ventral plastron + broadened dorsal ribs + marginal teeth) and Proganochelys (~210 Ma, complete carapace + toothless keratinous beak).

What separates Genetic Sex Determination (XX/XY and ZZ/ZW) from Temperature-Dependent Sex Determination (TSD) across Sauropsida?

Unlike eutherian mammals (which are fixed for male-heterogametic XX/XY sex chromosomes governed by SRY) and birds (which are fixed for female-heterogametic ZZ/ZW sex chromosomes governed by DMRT1 dosage), non-avian sauropsids exhibit extraordinary evolutionary lability between genotypic sex determination (GSD: both XX/XY and ZZ/ZW) and temperature-dependent sex determination (TSD)—in which the incubation temperature of the egg during the thermosensitive middle third of development determines gonad fate in all crocodilians, the tuatara, most turtles, and many lizards (Eublepharis, Pogona). As demonstrated by Clare Holleley, Arthur Georges, Blanche Capel, and colleagues (2015, 2018) in the red-eared slider turtle (Trachemys scripta) and Australian bearded dragon (Pogona vitticeps), temperature directly regulates the calcium-influx / STAT3 phosphorylation and histone demethylase KDM6B (JMJD3) pathway (alongside temperature-sensitive alternative splicing and polycomb JARID2 recruitment), which turns on the ovary-determining aromatase gene (CYP19A1) or the testis-determining gene (DMRT1).

1. Autumn, K., Sitti, M., Liang, Y. A., Peattie, A. M., Hansen, W. R., Sponberg, S., ... & Full, R. J. (2002). Evidence for van der Waals adhesion in gecko setae. Proceedings of the National Academy of Sciences, 99(19), 12252–12256. doi:10.1073/pnas.192252799
2. 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
3. Farmer, C. G., & Sanders, K. (2010). Unidirectional airflow in the lungs of alligators. Science, 327(5963), 338–340. doi:10.1126/science.1180219
4. Gracheva, E. O., Ingolia, N. T., Kelly, Y. M., Cordero-Morales, J. F., Hollopeter, G., Chesler, A. T., ... & Julius, D. (2010). Molecular basis of infrared detection by snakes. Nature, 464(7291), 1006–1011. doi:10.1038/nature08943
5. Crawford, N. G., Faircloth, B. C., McCormack, J. E., Brumfield, R. T., Winker, K., & Glenn, T. C. (2012). More than 1000 ultraconserved elements provide evidence that turtles are the sister group of archosaurs. Biology Letters, 8(5), 783–786. doi:10.1098/rsbl.2012.0331
6. Ge, C., Ye, J., Weber, C., Sun, W., Zhang, H., Zhou, Y., ... & Qian, G. (2018). The histone demethylase KDM6B regulates temperature-dependent sex determination in a turtle species. Science, 360(6389), 645–648. doi:10.1126/science.aap8328