At least 27 species of crocodilians
Crocodylia broken into 3 families
Crocodylus 14 species and Osteolaemus – 3 species
Pretty uniform group, but vary in size – dwarf caiman to Nile crocodile and saltwater crocodile
(reputed to be 10 m long – over 30 feet, but longest reliably measured were 7 m)
Slendersnouted crocodile – narrow snout for fish eating, move jaws laterally quickly to snag fish.
Most crocodilians live in freshwater, but saltwater croc does venture into oceans, as far as 1000 km
from land in Pacific.
Crocodylus species not differentiated much genetically – found around the world in the tropics, very
similar genetically, suggesting that they have dispersed to current locations in relatively recent past,
suggesting they crossed oceans to do this
Some described from Cretaceous period (65+ million years ago), so sometimes called “living
fossil,” but turns out that idea is mistaken – closer examination of fossils indicates not closely
related to presentday crocodylus, but does indicate that crocodyluswayoflife has been around for
Where are these species coming from?
Nile Croc (C. niloticus) – enormous geographic range, entire continent of Africa, including Sahara
in oases. (partially because Sahara used to be wetter). Also found on Madagascar. Reasonably
varied species, so some described different subspecies, etc, but considered one species.
Researcher at Fordham doing genetic study of variation in niloticus – one researcher was able to
swim in Chad with crocs, very docile – collected tissue samples, sequenced DNA, found big
surprise – animals from Chad highly differentiated – deep genetic subdivision into two clades –
separation dated back to 8 million years (using molecular clock). Two clades were different in their
chromosomes, and number of chromosomes. Suggested that they’re not reproductively compatible.
Also found that North American species fit into the Nile Croc clade – suggests that nile crocs
crossed ocean into N.A. and S.A.
She had few samples from north desert area, so went to temple of Sobek, extracted little bits of
tissue from crocodile mummies. Showed that two genotypes coexisted in the past – not only
genetically differentiated, but reproductively isolated because living side by side. Turns out, that
ancient Egyptians were aware that they were two species! Egyptian priests who collected crocs
intentionally got the one that was more docile, tamer, like the ones in Chad. This species is called
Crocodylus suchus. West African Crocodile, or Sacred Crocodile.
Traditionally considered to be single genus with 1 species is actually 3 species
Highly differentiated, suggesting diverged for many millions of years
These don’t occur together, so no assessment of whether they could interbreed. (Biological species
concept not applied like it was in Nile croc – whether or not they can interbreed).
Phylogenetic species concept – they diverge greatly. Slendersnouted croc – closely related to osteolaemus – formerly a member of Crocodylus, but that
would make Crocodylus paraphyletic and systemists don’t like paraphyletic groups, so renamed the
species from Crocodylus cataphractus to Mecistops cataphractus. Turns out this one is also broken
into 2 divergent clades – now 2 species of Mecistops. Mecistops occurs in much same place that
osteolaemus, and both species split around the same time. Suggests some common environmental
event affecting both species simultaneously. Called Vicariance. (Coordinated speciation in multiple
Mecistops – 2 species!
Alligator – 2 species
American alligator and Chinese alligator (A. sinensis – only about 2 m long, much smaller). Fossil
members of alligator found throughout Eurasia and northern California, Oregon, so used to be
widespread and it’s just gone extinct in most places.
American alligators is only species that lives in places where freezing temperatures occur – it
pokes nose through ice and keeps hole to breathe through. Can get frozen into ice and can survive
short term freezing temps. (We have seen some in Massachusetts – probably pets that got loose).
Reputed to get 6 m long, but best documented are 4 m long
Caiman – 3 species
Central and South America
Most widespread is spectacled caiman – used to be common in pet trade
Dwarf caimans – live in forests, most terrestrial of crocodilians.
Black caiman – largest caiman species
Paleosuchus – 2 species
Melanosuchus – 1 species
Gavialis – 1 species
Gavial/gharial – long thin snout with lots of teeth – India, 6.5 m long, probably most aquatic of all
crocodilians. Males have a large structure on nose called ghara, males make whistling or buzzing
noise. Actual function of noises not well understood, but assumed something to be with sexual
selection, communicating either with other males or with females.
Tomistoma – slender snout with many teeth, but not as many as gavial. False Gharial – Tomistoma
schlegelii. Considered example of convergent evolution – similar morphologies, but originally
thought to be member of crocodylidae, but now molecular DNA sequencing shows that Tomistoma
and Gavialis are sister taxa.
Difference between alligator and crocodile? In general, crocodile has blunt, tapering snout, alligator
has broad snout, but this isn’t true! Some alligatorids (caimans) have narrow snouts, some crocs
have broad snouts.
One characteristic infallible: fourth tooth. In crocodiles, 4 tooth sticks out and is apparent. In
alligators, 4 tooth fits in notch in skull so can’t see it from outside.
Convergent evolution means that morphologies may be misleading when the molecular data does