Amniota

While early tetrapods and living amphibians (Lissamphibia: frogs, salamanders, and caecilians) conquered terrestrial shorelines during the Devonian and Carboniferous, their reproduction remained tied to fresh water or saturated soil by anamniote eggs encased only in water-permeable gelatinous envelopes and by aquatic, gill-breathing larvae (tadpoles). During the Late Carboniferous (Pennsylvanian, ~318–312 million years ago)—preserved inside hollow lycopod tree stumps (Sigillaria) at Joggins, Nova Scotia, in the skeletons of Hylonomus lyelli (the earliest known sauropsid reptile) and Archaeothyris florensis (an early synapsid)—a monophyletic lineage of anthracosaur-grade tetrapods evolved a suite of embryonic, respiratory, and integumentary innovations that severed the reproductive link to standing water forever: the clade Amniota.

Immediately after its origin, crown-group Amniota split into two primary evolutionary branches that have evolved independently for more than 315 million years:

1. Synapsida (The Mammal Line): Encompassing Carboniferous–Permian "pelycosaurs" (Dimetrodon, Edaphosaurus), Permian–Triassic therapsids (gorgonopsians, dicynodonts, and cynodonts), and crown-group Mammalia; and
2. Sauropsida (The Reptile and Bird Line): Encompassing Lepidosauria (tuataras, lizards, and snakes) and Archelosauria (Testudines [turtles] + Archosauria [crocodilians, pterosaurs, non-avian dinosaurs, and Aves]).

The defining synapomorphy of Amniota is the cleidoic ("closed" or semi-enclosed) amniotic egg, which functions as an autonomous life-support capsule on dry land by surrounding the developing embryo with four specialized extraembryonic membranes (three of which—amnion, chorion, and allantois—are evolutionary inventions of Amniota, while the yolk sac is inherited from fish ancestors):

1. The Amnion (Ectoderm + Somatic Mesoderm): Folds upward over the embryo to enclose a sealed, fluid-filled amniotic cavity—creating an artificial "private pond" that cushions the embryo mechanically and prevents desiccation on dry land.
2. The Allantois (Endoderm + Splanchnic Mesoderm): Buds outward as a vascularized sac from the embryonic hindgut to serve two vital physiological functions: (1) storing nitrogenous metabolic waste (insoluble uric acid in sauropsids or soluble urea transferred across the placenta in therian mammals); and (2) fusing with the outer chorion to form the richly vascularized chorioallantoic membrane (CAM), which presses directly against the porous inner shell membrane to execute respiratory O₂ uptake and CO₂ elimination across the eggshell.
3. The Chorion (Serosa — Ectoderm + Somatic Mesoderm): The outermost extraembryonic membrane enclosing the embryo, amnion, allantois, and yolk sac; in therian mammals, the chorion forms the fetal contribution (trophoblast) of the placenta.
4. The Yolk Sac (Endoderm + Splanchnic Mesoderm): Surrounds the lipid- and lipovitellin-rich vitellogenin yolk mass and develops the first embryonic blood islands (vitelline circulation) to digest and transport nutrients to the embryo.

Surrounding these membranes is a fibrous proteinaceous shell membrane impregnated with calcium carbonate (CaCO₃, as aragonite in turtles or calcite columns in archosaurs and monotremes) perforated by thousands of microscopic gas-diffusion pores, along with an aqueous albumen (egg white) reservoir rich in antimicrobial lysozyme and ovotransferrin.

Beyond the egg, living and fossil amniotes are united by four post-cranial and physiological synapomorphies that allow paleobiologists to identify an amniote skeleton even when eggs are not preserved:

• Costal (Rib-Driven) Aspiration Breathing vs. Amphibian Buccal Pumping: Lissamphibians lack movable ribs and inflate their lungs by positive-pressure buccal pumping (gulping air with the floor of the mouth and forcing it down the throat), which restricts amphibians to broad, flat skulls and relies heavily on moist skin for cutaneous CO₂ release. Amniotes evolved mobile intercostal muscles attached to elongated thoracic ribs joining a ventral sternum, generating negative intrathoracic pressure (costal aspiration) to ventilate high-surface-area lungs while freeing the skull and jaws for high-bite-force mechanical feeding, chewing, and keratinized skin waterproofing.
• Waterproof Keratinized Epidermis (Stratum Corneum): Amniote skin is sealed by a dead outer layer of cornified keratinocytes packed with α-keratins (in both synapsids and sauropsids, giving rise to mammalian hair, claws, and nails) and sauropsid-specific corneous beta-proteins (β-keratins) (encoded by the Epidermal Differentiation Complex, EDC, which builds tough reptile scales, turtle scutes, and avian feathers), laminated with extracellular ceramide and cholesterol lipids.
• The Astragalus Ankle Bone and Internal Fertilization: In the tarsus (ankle), amniotes fused three ancestral amphibian proximal tarsal bones (tibiale, intermedium, and proximal centrale) into a single load-bearing bone—the astragalus (talus)—forming a stable ankle hinge with the calcaneum. Simultaneously, because a shelled cleidoic egg must be fertilized before the shell is secreted in the oviduct, all amniotes evolved obligatory internal fertilization.

One of the best-documented transitional sequences in the entire fossil record—predicted embryologically by Karl Bogislaus Reichert (1837) and Ernst Gaupp (1913) and confirmed across hundreds of Permian–Jurassic synapsid fossils (Dimetrodon → Thrinaxodon → Probainognathus → Morganucodon → Yanoconodon → Liaoconodon)—is the transformation of the ancestral reptile/synapsid posterior lower-jaw bones into the three-ossicle mammalian middle ear:

1. Ancestral Non-Mammalian Synapsids (Dimetrodon, Thrinaxodon): Like reptiles, early synapsids hinged their jaw between the articular bone (at the back of the lower jaw, derived from the ventral end of Meckel's cartilage) and the quadrate bone (at the back of the upper skull, derived from the palatoquadrate), while the tooth-bearing dentary bone occupied the front of the lower jaw and the angular bone supported a mandibular eardrum for low-frequency ground vibrations.
2. Double-Jaw-Joint Transitional Mammals (Morganucodon, Diarthrognathus, Late Triassic ~205 Ma): As cynodonts evolved high-metabolism endothermy, a secondary bony palate (separating breathing from chewing), and precise tribosphenic molar occlusion, the tooth-bearing dentary expanded backward until its condyle contacted the squamosal bone of the cranium—creating a simultaneous double jaw joint (lateral dentary–squamosal + medial articular–quadrate) side-by-side!
3. Detachment from Meckel's Cartilage into the Middle Ear (Liaoconodon, Crown Mammalia): Once the robust dentary–squamosal (temporomandibular) joint took over 100% of chewing forces, the miniaturized post-dentary bones were freed from mechanical bite loads and detached from the lower jaw through programmed breakdown of Meckel's cartilage (preserved ossified in Cretaceous Yanoconodon and Liaoconodon):
   • The Articular became the Malleus (hammer);
   • The Quadrate became the Incus (anvil);
   • The Angular became the Ectotympanic ring (holding the eardrum); and
   • They linked in series to the existing Stapes (stirrup)—forming an impedance-matching three-lever mechanical amplifier that extended mammalian high-frequency airborne hearing above 20 to 100 kHz!

Where do turtles (Testudines) sit on the Reptile Tree of Life?

Because living turtles have a solid skull roof lacking temporal openings (anapsid condition), classical paleontology placed them at the very base of Sauropsida ("Parareptilia"). However, every genome-wide ultraconserved element (UCE), microRNA, and coding-sequence phylogeny (Crawford et al., 2012; Wang et al., 2013; Field et al., 2014), together with the discovery of the 240-million-year-old Middle Triassic stem-turtle Pappochelys rosinae (Schoch & Sues, 2015, which possessed two open diapsid temporal fenestrae alongside broadened T-shaped belly ribs!), proved that turtles are derived diapsid reptiles that sit as the sister group to Archosauria (crocodilians + birds/dinosaurs) inside the clade Archelosauria.

Why did mammals lose two of their three ancestral Vitellogenin (VTG) egg-yolk genes?

As demonstrated by David Brawand, Walter Wahli, and Henrik Kaessmann (2008), birds, reptiles, and amphibians possess three functional vitellogenin genes (VTG1, VTG2, VTG3) that load egg yolk with lipoprotein nutrients. Egg-laying monotreme mammals (the platypus Ornithorhynchus anatinus and echidnas, which lay leathery-shelled cleidoic eggs incubated for ~10 days before nursing hatchlings with milk rich in casein proteins) retain one functional VTG gene alongside two VTG pseudogenes. In contrast, viviparous marsupial and eutherian (placental) mammals inactivated all three VTG1–3 genes into decaying pseudogenes between 170 and 70 million years ago as the chorioallantoic/yolk-sac placenta and lactation completely replaced egg yolk!

1. Reisz, R. R. (1997). The origin and early evolutionary history of amniotes. Trends in Ecology & Evolution, 12(6), 218–222. doi:10.1016/S0169-5347(97)01060-401060-4)
2. Luo, Z. X. (2007). Transformation and diversification in early mammal evolution. Nature, 450(7172), 1011–1019. doi:10.1038/nature06277
3. Brawand, D., Wahli, W., & Kaessmann, H. (2008). Loss of egg yolk genes in mammals and the origin of lactation and placentation. PLoS Biology, 6(3), e63. doi:10.1371/journal.pbio.0060063
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. Simões, T. R., Kammerer, C. F., Caldwell, M. W., & Pierce, S. E. (2022). Successive climate crises in the deep past drove the early evolution and radiation of reptiles. Science Advances, 8(33), eabq1898. doi:10.1126/sciadv.abq1898