L18 - Archosauria: Crocodilians and the Ruling-Reptile Radiation

archosaurs are the last common ancestor of

crocodilians + birds, and all of its descendant

• everything outside that node is a stem archosaur, however “archosaur-like” it looks

archosaurs origins

permian-triassic

they have a fast start after the

end-permian extinction

fast origin -

crown archosaurs appear less than 5 mar after the end-permian extinction

minimum age -

fossils set a floor for the crocodile-line/bird split; no verified molecular date yet

cause unproven

extinction as the trigger of archosaur suc

< 5 Myr from

the end-permian extinction to the first date crown archosaurs

the archosauriform stem

late permian

archosaurian characters

• antoribital fenestra; serrated teeth in deep sockets

• mandibular fenestra; pronounced 4th trochanter

• pubic boot; metatarsal II > IV

• advances mesotarsal (hinge) ankle

mapping archosaurian characters on the tree common error

socketed teeth and ant orbital fenestra “define" archosaurs

ankles: ancestral is not

diagnostic

ankles - crocodiles (normal)

• rotation between astragals and calcaneum (peg-and-socket)

• allows sprawling or an erect “high walk”

ankle - advanced mesotarsal

• simple hinge between proximal and distal tarsal

• limits the limb to an erect, fore-and-aft (parasagittal) swing

  • most rornithodirans

ankles - ancestral for the crown

teleocrater, and early bird-line archosaur, still has a crocodile-normal ankle

• so it cannot define the crocodile line

triassic peusdosuchians out-diversified

early dinosaurs

triassic pseudosuchians - higher disparity

late triassic crocodile-line archosaurs spanned more body plans than early dinosaurs

triassic pseudosuchians - same pace

rates of character evolution were statically indistinguishable

triassic pseudosuchians - implication

the dinosaurs’ dire reflects which lineages survived the end-triassic extinction, not “superiority”

the end-triassic filter - one survivor

every crocodile-line clade except crocodylomorphs ended at or before the boundary

crocodylomorph radiations - early jurassic

• rebound after the end-triassic extinction

• mostly small, slender, land-dwelling predators

crocodylomorph radiations - jurassic seas

marine thalattosuchaisn” teleosauroids and metriorhynchoids

crocodylomorph radiations - cretaceous land

• neosuchaisn ( the lineage including crocodylians) and the land-dwelling notosuchains

  • jaw shapes diversify sharply

crocodylomorph radiations - drivers

warm clime raised speciation on land and at sea (not in fresh water_; high sea level raised extinction

• neither biotic nor abiotic factor dominated

Alligatoroidea =

alligators, caimans

Crocodyloidea =

true crocodiles

gavials =

gharial

tomistoma =

false gharial

living crocodylians =

25 species in three families

living crocodylians = recent splits

Crocodylus suchus, Mecistops leptorhynchus, Osteolaemus osborni.

living crocodylians - field marks

crocodiles show the 4th lower tooth with the mouth closed; in alligators and caimans it sits hidden in an upper-jaw pit; gavials have long slender snouts

archosaur biology - heart and lungs

• divided ventricle - level I’

  • one-way lung airflow - level I’; alligator has it without air sacs

archosaur biology - metabolism

• likely secondary ectotherms - descend from high-metabolism ancestors

• evidence - heart and lungs; bone histology; bone chemistry

archosaur biology - bracket check

air sacs, hollow bones, and endothermy in non-avian dinosaurs are level II’ (birds only)

temperature-dependent sex determination distribution

all crocodilians; birds are ZW

• no bracket onto dinosaurs

cardiac shunt distribution

crocodylians

• no bracket into dinosaurs

jaw sensory organs distribution

crocodyliforms

ectothermy distribution

crocodylians (probably secondary)

crocodilian specializations - not “living” fossils

living crocodilians are derived specialists

temperature-dependent sex determination - males

in the middle (31.7-35.9)

temperature-dependent sex determination - females

ends (<31.7, >35.9)

temperature-dependent sex determination - in all crocodylians

dominant pattern FMF; and MF pattern has never been observed

temperature-dependent sex determination - sensitive window

roughly the middle third of incubation

parental care - embryos call

nile crocodile call inside the egg; calls synchronize hatching and prompt the mother to open the nest

parental care - level I’

care occurs in both brackets, so attentive is likely ancestral; its forms is not inferable

parental care - dinosaurs on nests

oviraptorids and troodontids on clutches; parental care proposed, challenged

conservation seven crododylains

are critical endangered

• about half threatened

• asia hit hardest

pterosauromorpha

pterosaur-like braincase and inner ear

pterosaur

wing on a highly elongated 4th finger

pterodactyloidea

short tail; long neck and skull

azdarchoidea

toothless jaws

pterosaurs - powered

flight, evolved separately

pterosaurs - range

228-66 Mal gone at the K-Pg

pterosaurs and avialans

evolved flight separately twice within ortinodira line

dinosaur =

the last common ancestor of Triceratops and birds, and all its descendants

• birds are dinosaurs, “non-avian dinosaurs” is a paraphyletic grade

dinosauria stable 3 clades

ornithischians (Triceratops, Stegosaurus), sauropodomorphs

(Brachiosaurus), theropods (Tyrannosaurus, birds).

ornithischians

Triceratops, Stegosaurus

sauropodomorphs

(Brachiosaurus

theropods

Tyrannosaurus, birds

Ornithischia =

beaked plant-eaters

Ornithischia shared traits

• predentary → extra bone at the lower-jaw tip, sheathed in a beak

• backward-pointing pubis → more room for a plant-fermentating gut

• leaf-shaped teeth - and likely cheeks

Ornithischian specializations

• armor

• domes and frills

• dental batteries

• nesting

Ornithischian specializations - armor

osteoderms in thyreophorans; stegosaur plate f unction debated

Ornithischian specializations - dental batteries

hadrosaurs": interlocked teeth form one grinding surface

Ornithischian specializations - domes and frills

thick skull roofs; frills and a new beak bone

Ornithischian specializations - nesting

maiasaura nests with worn-toothed nestlings: parents likely fed young (inference)

saurop

two legs to four - stance mapped on the

sauropodomorph tree

plateosaurus

two-legged, late triassic

massospondylus

two-legged, early jurassic

mussarus

shifted with age/either

• young on four legs, adults on two

ledunahavdi

four-legged, crouched forelimbs

sauropods -

four legs, columnar limbs

two legs to four - more than once

four-legged walking arose separately in the ledmahadi lineage (crouched arms) in sauropods (pillar limbs)

two legs to four - size first

giant size came before the sauropod limb design

how sauropodomorphs went from two legs to four

• limb bones

• step by step

• growth

• trackways

how sauropodomorphs went from two legs to four - limb bones

ledumahadi: thick, crouched limbs - a separate route to four legs

how sauropodomorphs went from two legs to four - step by step

sauropod ankle and arm traits added gradually, not all at once

how sauropodomorphs went from two legs to four - growth

mussaurus center of mass moved toward the hip as it grew

how sauropodomorphs went from two legs to four - trackway

two-legged and four-legged sauropodomorph track occurs in late triassic age strata

how sauropodomorphs went from two legs to four - not “stay juvenile”

in mussarus, the shift came from a growing tail, so sauropods probably did not turn quadrupedal by keeping baby proportions

theropods are the

bipedal predators and birds

theropods shared traits

blade-like serrated teeth, large grasping hands, a flexible joint within the lower jaw’ always two-legged

theropods diets - not all

meat-eaters

• split into fish-eaters, bone-crushers, and beaked plant- and omnivore-eaters

theropods diets - spinosaurids

long snouts, conical teeth; ate much aquatic food

theropods diets - tyrannosaurus

• huge skulls, deep-rooted teeth

• bone-crushing bite; tiny two-fingered arms

theropods diets - Ornithomimosaurs

• toothless beaks, long legs

• omnivores or herbivores

feathers on the theropod tree

evolved in the middle of the tree

maniraptoran traits

• larger brain, long arms

• half-moon wrist bone lets the hand fold

• vaned feathers