Unit 18 {2} Crocodilia

Describe the phylogeny of Archosauria:

contains crocodilians, dinosauria, and aves

sister to testudines, make up the archelosauria

sauropsid

2 internal clades: pseudosuchia & avemetatarsalia

What are the features of Archosauria?

diapsid with antorbital & mandibular fenestrae, antorbital secondary lost in crocodilians & fenestrae hard to see in birds

thecodont teeth, or set into deep sockets, if teeth present

4th trocantur on femur (little bump)

lost vomeronasal/Jacobson’s organ for additional sense of smell

Describe features of Pseudosuchia

false but true crocodilians

distinguished from sister Avemetatarsalia due to ankle: their astragalus/ankle bone is sutured to tibia & ankle rotates due to a process of astragalus fitting with calcaneus

Describe the stem group Phytosauria:

sister to modern crocodilians, called plant lizards thought for herbivory, no secondary palate & heavy bony armor, serrated teeth (not cones), nostrils near eyes but not on snout

Describe Crocodilia:

the crown crocodilians appearing in late Cretaceous

includes 28 extant species of alligators, caimans, gharials, & crocodiles

8 species endangered or critically endangered

What are the characteristic features of Crocodilia?

complete secondary palate

thecodont dentition & conical teeth

non-pneumatic/air sacked skeleton

scutes: strongly ossified dermal osteoderms

gular flaps: open mouth underwater w/o flooding lungs

What is the Crocodilian clade Longirostres?

contains gharials & crocodiles

Describe the clade in Crocodilia’s Longirostres, “Galvialidae”

gharials & false gharials

thin snouts for piscivory/fish eating, false gharials more generalist

both species endangered due to habitat loss

gharial native to india, false gharials malaysia & borneo

y-shaped snouts

Describe the clade in Crocodilia’s Longirostres, “Crocodylidae”

true crocodiles, 18 species & V-shaped snout, when mouth closed both upper & lower teeth visible, semiaquatic ambush predators

Describe the clade in Crocodilia “Alligatoridae”

includes alligators & caimans

Describe the clade in Crocodilia’s Alligatoridae, “Alligatorinae”

2 living species (american apex predator & keystone, chinese endangered), u shaped snout, lower teeth not visible when mouth closed, split from caimans 60 mil year ago

Describe the clade in Crocodilia’s Alligatoridae, “Caimaninae”

6 species, lacks bony septum/divider in nostrils, teeth longer & sharper than alligators, ventral scutes develop in 2 parts & are sutured together, lower teeth not visible in closed mouth, u-shaped snout

What is the left-to-right shunt that crocodilians have?

when diving or after meal, valve closes off flow from right ventricle to pulmonary arteries

blood instead flows to left systemic aorta (which is homologous to mammalian aorta)

appears to aid in digestion, w/o it means less gastric acid & slower digestion

not related to co2 content of blood, but exact role in digestion still being studied