Download BIOL 307 – Lecture 9

REPTILIA 1 (cont.)
1. Crocodilia (crocodiles, alligators, caiman, gavial = gharial)
a. Brief history
i. True crocodilians appeared in the Triassic (210-245 mya); have not changed much
morphologically since
ii. Earliest were
1. Relatively small (cat-sized), slender
2. Probably active predators on small diapsids
3. Reached peak diversity (and max body size) during Cretaceous (65-145
mya), when warm climates extended into what are now temperate regions
a. Deinosuchus had skull 2 m long
b. If body proportions same as modern crocs, would have been 12-15 m
long (about the size of T. rex)!
b. Distribution and diversity
i. Note that all are semiaquatic, limited to tropical/subtropical areas
ii. ~ 25 species in three families
1. Alligatoridae = alligators, caiman
a. About 10 species (2 alligators, 8 caiman)
b. All fresh-water
c. New World, except for Chinese alligator
2. Crocodylidae = crocodiles
a. ~ 15 species
b. Cosmopolitan distribution
c. Found in range of habitats from fresh to salt water (including
brackish waters)
d. Includes the largest modern crocodilian = Australian (saltwater)
crocodile (Crocodylus porosus); max length = 7m
3. Gavialidae = gavial = gharial
a. Single species
b. Lives in large rivers of Indian subcontinent
iii. Major differences between alligators and crocodiles
1. Alligators
a. Snout is relatively short, wide, u-shaped
b. Upper jaw wider than lower; only upper teeth visible when jaws
closed
c. Extra-renal salt glands present, weakly or not functional
d. Integumentary sense organs present on lower jaws only
2. Crocodiles
a. Snout is relatively long, slender, and pointed
b. Jaws are same width; upper and lower teeth appear to be interlocking
when jaws are closed
c. Extra-renal salt glands are present and functional
d. Integumentary sense organs are associated with all scales
c. Form and Function
i. Adaptations to semi aquatic life
1. Can breathe while body underwater (only nostrils exposed)
a. Nostrils dorsal, at tip of snout
b. Complete secondary palate
i. Shelf of bone separating nasal passages from oral cavity
(analogous to our hard and soft palates)
ii. Air can move directly into trachea even if mouth full of water
(note that can’t completely seal mouths shut)
iii. Specialized flap of tissue at base of tongue further seals off
mouth from throat to keep water out of trachea
2. Heavy, laterally compressed tail provides propulsion during swimming
(when swimming, limbs folded against body for streamlining)
3. Sensory pits on lower jaws detect small changes in water; probably used
primarily in prey detection (analogous to lateral line)
4. Specialized salt glands to excrete salt (crocodile tears)
ii. Four-chambered, nearly completely divided heart
1. Still permits shunting of blood from pulmonary to systemic circuits via
“passageway” between aortic arches
2. Can alter circulation based on activity, thermoregulatory needs, and during
diving
iii. Reproduction
1. All are oviparous
2. Most build nests – mounds or cavities
3. Many exhibit extensive parental care, including (but not limited to)
a. Helping young out of nest at hatching
b. Carrying young to water
c. Protecting young before, during, and after hatching (some up to 3
years)
4. All exhibit temperature-dependent sex determination
d. Conservation status
i. 10 species considered to be at significant risk
ii. Major causes include
1. Hunting for skins (this was especially problematic from 1930's to 1960's,
and is still a problem for some species in some places)
2. Hunting for other reasons (eggs, meat, juveniles for pet trade)
3. Habitat loss/degradation
4. Indirect losses from gill net fishing, road kill, etc.
iii. In places where species are particularly economically valuable, some successes
from commercial farming/captive breeding (including release, protection of wild
populations)
iv. Note also that crocs may play very important roles in their ecosystems: American
alligator, for example, is considered keystone species in Gulf Coast habitats
1. Keystone species = species with disproportionately large effect on
community structure/function (often because of physical effects on habitat)
2. For American alligator, effects include
a. Control of prey species populations (which, in turn, effects species
diversity within the community)
b. Formation of peat through nesting habits
c. Nest use by other species
3. Habitat alteration – create “alligator holes” where water will remain even
when other streams/ponds drying, creating refugia for other species