Crocodylia

Crocodylia (or Crocodilia, from the Ancient Greek krokodeilos) is the crown order of large, predatory, semi-aquatic archosaurian reptiles (Sauropsida) comprising 27 recognized extant species distributed across the tropics and warm subtropics in three families: Alligatoridae (alligators and caimans), Crocodylidae (true crocodiles and the African dwarf crocodile Osteolaemus), and Gavialidae (the narrow-snouted Indian gharial Gavialis gangeticus and the Malayan false gharial Tomistoma schlegelii). Together with birds (Aves), crocodilians are the only living survivors of Archosauria—making an American alligator (Alligator mississippiensis) or Nile crocodile (Crocodylus niloticus) more closely related to a sparrow or hummingbird than to any lizard, snake, or turtle.

Although modern crocodilians are often colloquially called "living fossils," paleontological and physiological evidence (Seymour et al., 2004; Stubbs et al., 2021) demonstrates that their Triassic and Jurassic pseudosuchian and early crocodylomorph ancestors (Sphenosuchus, Terrestrisuchus, Protosuchus) were small, long-legged, active terrestrial cursorial predators (alongside Mesozoic marine pelagic thalattosuchians† with tail flukes and terrestrial herbivorous notosuchians† such as Simosuchus with multicuspid mammal-like chewing teeth!).

Several seemingly "advanced" physiological traits of modern crocodilians—including their complete four-chambered heart, unidirectional air-flow lungs, and hepatic-piston diaphragm—are evolutionary relics of an ancestral endothermic (warm-blooded), cursorial Triassic archosaur that secondarily readapted to a sit-and-wait ectothermic aquatic ambush lifestyle (Seymour et al., 2004):

1. The Four-Chambered Heart, Foramen of Panizza, and Cog-Tooth Pulmonary Valve: Unlike squamates and turtles (which have a three-chambered heart with an incompletely divided ventricle), crocodilians possess a complete interventricular septum separating the heart into four distinct chambers (left and right atria, left and right ventricles), just like birds and mammals. However, unlike birds and mammals, crocodilians retain two systemic aortic arches: the right systemic aorta exits the left ventricle (carrying oxygenated blood to the brain and body), while the left systemic aorta exits the right ventricle alongside the pulmonary artery (and connects to the right aorta via a narrow aperture at the base of the semilunar valves called the foramen of Panizza, discovered by Bartolomeo Panizza in 1833):
   • During Air-Breathing on Land: High pressure in the left ventricle forces oxygenated blood through the foramen of Panizza into both the right and left systemic aortae while keeping the left aortic valve shut, so systemic and pulmonary circulations remain 100% separated;
   • During Prolonged Underwater Diving (Right-to-Left Vascular Shunt): Under vagal parasympathetic stimulation, interdigitating connective-tissue nodules inside the right ventricular outflow tract—the cog-tooth valve—actively constrict to increase resistance into the pulmonary artery. Right ventricular pressure rises to match left ventricular pressure, opening the left aortic valve and shunting deoxygenated venous blood directly into the left systemic aorta (which anastomoses directly with the celiac artery supplying the stomach!), bypassing the lungs to conserve O₂ for the brain and heart while delivering CO₂-rich venous blood to the stomach.
2. Alkaline Tide, CO₂ Shunting, and Bone-Dissolving Gastric Acid: Why does the crocodilian left aorta deliver CO₂-enriched venous blood straight to the stomach after a meal? As demonstrated by C. G. Farmer, Thomas J. Uriona, and colleagues (2008), swallowing a whole deer, wild boar, or turtle shell requires secreting massive volumes of concentrated hydrochloric acid (HCl, pH 1.2–1.5) via gastric parietal-cell carbonic anhydrase (CO₂ + H₂O → H⁺ + HCO₃⁻). Surgically occluding the left aorta in juvenile alligators reduces their rate of gastric acid secretion and bone dissolution, proving that the right-to-left cardiac shunt diverts metabolic CO₂ away from the lungs to fuel gastric acid synthesis while buffering the massive postprandial "alkaline tide" (HCO₃⁻ surge) in the blood!
3. The Hepatic-Piston Muscular Diaphragm and Unidirectional Airflow: Crocodilians ventilate their multichambered lungs using a unique hepatic piston: paired diaphragmaticus muscles running from the pelvis (pubis) to the posterior capsule of the liver contract during inhalation to pull the liver and viscera posteriorly like a syringe plunger, expanding the pleural cavity, while contraction of the abdominal muscles pushes the liver anteriorly to expel air. As discovered by C. G. Farmer and Kent Sanders (2010), airflow inside the crocodilian lung is unidirectional (caudo-cranial) through the dorsobronchi and parabronchi during both inhalation and exhalation—homologous to the avian lung.
4. Secondary Bony Palate, Gular Valve, and Integumentary Sensory Organs (ISOs):
   • Fusion of the premaxillae, maxillae, palatines, and pterygoids creates a complete secondary bony palate spanning the roof of the mouth, shifting the internal nostrils (choanae) to the back of the throat behind a fleshy palatal (gular) velum. This allows a crocodile to float with only its nostrils and eyes above water or thrash and drown struggling prey underwater with its jaws wide open without inhaling water.
   • Each scale on the upper and lower jaws is dotted with a microscopic pigmented dome—an integumentary sensory organ (ISO) / dome pressure receptor innervated by trigeminal mechanoreceptors (Di-Poï & Milinkovitch, 2013)—capable of localizing a single drop of water disturbing the surface from meters away in total darkness!

In 2012, Gregory M. Erickson, Kent A. Vliet, and colleagues measured the direct in vivo bite forces of all 23 recognized crocodilian species using piezoelectric force transducers:

• A 4.5-meter saltwater crocodile (Crocodylus porosus) generated a molariform crushing bite force of 16,414 Newtons (3,690 pounds-force)—the highest bite force ever directly measured in any living animal (powered by massive pterygoideus and adductor mandibulae muscles filling the posterior skull and inserting under the lower jaw), while extrapolated 6.5-meter individuals reach 27,000–34,000 N.
• Unlike mammals (which replace their teeth only once in life, diphyodonty), a crocodile replaces each of its 60–80 socketed (thecodont) conical teeth up to 45 to 50 times over a 70-year lifespan (polyphyodonty), driven by a permanent niche of Sox2⁺ / Wnt-responsive dental lamina stem cells (Wu et al., 2013) nested lingually beside every functional tooth root.

Why is the false gharial (Tomistoma schlegelii) a classic case of morphological vs. molecular phylogenetic conflict?

For over a century, anatomists placed the Southeast Asian false gharial (Tomistoma schlegelii) inside Crocodylidae (treating its long, narrow fish-catching snout as convergent with the Indian gharial, Gavialis gangeticus, which skeletal cladistics placed as the most basal split of all living crocodilians diverging in the Late Cretaceous). Every molecular dataset—from immunological albumin and mitochondrial DNA to whole-genome sequencing (Willis et al., 2007; Oaks, 2011; Lee & Yates, 2018)—proved unequivocally that Tomistoma and Gavialis are close sister species within a single family (Gavialidae) that diverged from Crocodylidae in the Eocene–Oligocene (~38–30 Ma), demonstrating how strong biomechanical selection on skull shape in long-snouted piscivores had misled osteological parsimony trees.

How does incubation temperature determine whether crocodilian eggs hatch as males or females?

All 27 living species of Crocodylia lack sex chromosomes (no X, Y, Z, or W chromosomes) and instead exhibit Type II (Female–Male–Female) Temperature-Dependent Sex Determination (TSD). During the thermosensitive period in the middle third of embryonic development (days 30–45), clutches incubated at cool (<31 °C) or hot (>34 °C) nest temperatures develop as females, whereas clutches incubated in the intermediate window (32.0–33.5 °C) activate the transient receptor potential channel TRPV4, histone demethylase KDM6B, and SOX9 / AMH cascade to develop testes and hatch as males.

1. Seymour, R. S., Bennett-Stamper, C. L., Johnston, S. D., Carrier, D. R., & Grigg, G. C. (2004). Evidence for endothermic ancestors of crocodiles at the stem of archosaurs. Physiological and Biochemical Zoology, 77(6), 1051–1067. doi:10.1086/422766
2. Farmer, C. G., Uriona, T. J., Olsen, D. B., Steenblik, M., & Sanders, K. (2008). The right-to-left shunt of crocodilians serves digestion. Physiological and Biochemical Zoology, 81(2), 125–137. doi:10.1086/524150
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. Erickson, G. M., Gignac, P. M., Steppan, S. J., Lappin, A. K., Vliet, K. A., Brueggen, J. D., ... & Webb, G. J. W. (2012). Insights into the ecology and evolutionary success of crocodilians revealed through bite-force and tooth-pressure experimentation. PLOS ONE, 7(3), e31781. doi:10.1371/journal.pone.0031781
5. Di-Poï, N., & Milinkovitch, M. C. (2013). Crocodylians evolved scattered multi-sensory micro-organs from ancestral regionalized epidermal appendages. EvoDevo, 4, 19. doi:10.1186/2041-9139-4-19
6. Grigg, G., & Kirshner, D. (2015). Biology and Evolution of Crocodylians. Cornell University Press / CSIRO Publishing. doi:10.1071/9781486300679