Figure 13.26: A northern water snake shedding its skin. ( 2 )Alligators and Crocodiles
Crocodilia, containing both alligators and crocodiles, is an order of large reptiles. Reptiles
belonging to Crocodilia are the closest living relatives of birds, as the two groups are the only
known living descendants of the Archosauria, a subclass of reptiles, including the dinosaurs.
The basic crocodilian body plan (Figure13.27) is a very successful one that has changed
little over time; modern species closely resemble their Cretaceous ancestors of 84 million
years ago. Crocodilians have a flexible semi-erect (semi-sprawled) posture. They can walk
in low, sprawled “belly walk,” or hold their legs more directly underneath them to perform
the “high walk.” Most other reptiles can only walk in a sprawled position.
All crocodilians have, like humans,thecodont dentition, (teeth set in bony sockets), but
unlike mammals, they replace their teeth throughout life. Crocodilians also have a secondary
bony palate that enables them to breathe when partially submerged, even if the mouth is
full of water. Their internal nostrils open in the back of their throat, where a special part
of the tongue called the “palatal valve” closes off their respiratory system when they are
underwater, allowing them to breathe when submerged.
Crocodiles and gharials (large crocodilians having elongated jaws) have modified salivary
glands on their tongue (salt glands), which are used for excreting excess salt ions from their
bodies. Crocodilians are often seen lying with their mouths open, a behavior called gaping.
One of its functions is probably to cool them down, but it may also have a social function.
Like mammals and birds and unlike other reptiles, crocodiles have a four-chambered heart;
however, unlike mammals, oxygenated and deoxygenated blood can be mixed. Crocodilians