Crocodylia - crocodiles, alligators, and gharials

Diversity and Lower Taxonomy:

The 22 living members of the order Crocodylia are divided into three families:

• Gavialidae - containing a single species, the gharial.
• Crocodylidae - the crocodiles, comprising 14 species in 3 genera.
• Alligatoridae - 7 species of alligators and caimans in 4 genera.

Distribution and Habitat:

• Species present on every continent except Europe and Antarctica.

Features:

• The presence of a secondary palate, which acts to separate the oral cavity into two defined chambers:

1. The nasal passage, travelling from the nostrils to the internal nares at the back of the throat, for breathing.

2. The mouth, from the mouth opening to the oesophagus, for the mastication of food.

This is an archosaurian derived feature (which convergently evolved in mammals) that allows crocodylians to inhale and exhale solely through their nostrils, and thus eat and breathe simultaneously - a feature usually only seen in endotherms.

• Nostrils at the tip of a long snout, connected to the internal nares via the nasal passage created by the secondary palate. This feature acts like a snorkel, meaning crocodylians can lead a semi-aquatic lifestyle - the mouth can be fully submerged yet have no effect on air passing through the nasal passage.
• Semi-erect gait: they can pull their bodies upwards, straightening their legs slightly, resulting in a stance halfway between that of a horse and a lizard. This allows more efficient locomotion, as each stride acts more in the direction of travel and can therefore be longer, compared with the semicircular arc of a lizard's sprawling stride. A semi-erect gait also clears the trunk further from the ground, affording increasingly efficient breathing by permitting greater expansion of the lung cavity, as well as a more even compression of the lungs during each stride.

HINT: You will find that the evolution of locomotory efficiency often goes hand in hand with the evolution of respiratory efficiency - this coevolution is due to the fact that without the ability of an animal to gain enough oxygen to meet the energetic demands of a fast moving body, adaptations for locomotory efficiency and performance become redundant, as they are physiologically too advanced.

• The presence of two openings in the skull and lower jaw: the antorbital fenestra and mandibular foramen, respectively. These are both defining characthers, or synapomorphies, of the archosaurs, which includes crocodylians, dinosaurs, and birds. The antorbital fenestra can be located on the snout between the orbit and the nostril, while the mandibular foramen can be seen as a small hole in the lower jaw.
• Laterally flattened, serrated teeth.
• Crocodiles can be discriminated from alligators by differences in the shape of their mandibles, specifically the positioning of the large fourth tooth in the lower jaw:
  
  
  • In crocodiles, this tooth sits outside the upper jaw when the mouth is closed (notice the recess in the exterior of the upper jaw).
  • In alligators, it is contained within the oral cavity, slotting into a complimentary groove in the palate of the upper jaw. Therefore, all the teeth of an alligator's upper jaw bite outside those of the lower jaw.