Lecture 12
Therapsids
Super continent of Pangea formed in the Permian
Overall global climatic drying trend
Therapsids synapsid reptiles
Evolved from sphenacodont pelycosaurs
Inhabited mid- to high-latitudes, not the tropics
Seasonal and dry climates with abundant vegetation, inclding the
famous leaf fossil Glossopteris
Seasonal temps means that sophisticated thermoregulation existed
besides just behavioral
Mammals evolved from therapsids in the Late Triassic
Different from modern African carnivores in that they were larger
and more numerous
Therapsid Evolution
Theriodonts large canines, some approaching sabor-toothed
condition
Gorgonopsians dominant carnivore of the Late Permian
Cynodonts evolved into mammals in the Late Triassic
Dicynodonts large-bodied herbivores and carnivores
Interlocking canines and incissors
Dinocephalians dominant worldwide herbivores of the Late
Permian
Make-up 90% of known therapsid fossils
Lost nearly all teeth except for upper cannines
Weak jaw moved forward and back in shearing action
Rat-sized to cow-sized
Diapsid Reptiles
Significant evolution in diaspids as well as synapsids in the Permian and Triassic
In Triassic, diaspids replaced synapsids as dominant land vertebrates
Diapsids evolved from Petrolacosuarus
Diapsid groups
1. Turtles lost the two holes in the skull during shell
adapatation
2. Lepidosauromorphs small, lizard-like ectothermic,
terrestrial
New Zealand Sphenodon
3. Archosauromorphs (Archosauria) consist of 4 groups
a. thecodonts med to large aquatic, terrestrial, and amphibious
carnivores
b. crocodylia crocodiles and aligators
c. dinosaurian prosauropods, sauropods, ornithischians,
theropods) bird ancestors
d. pterosauria
Early diapsids with poorly understood phylogeny
1. weigeltisaurs includes Coelursauravus (glider) and mesosoaurs
(aquatic)
2. ichthyosaurs
Hovasaurus, aquatic carnivore, is the best preserved
Late Permian diapsid
Shares traits with thecodonts
Triassic Diapsid Takeover
Early Permian forests were dominated by large pelycosaurs
Middle Permian therapsids replaced pelycosaurs as dominant land reptiles
Late Permian dominated by therapsids, began invading cooler climates
Early Triassic therapsid diversity dropped, but widespread and numerous
Middle Triassic therapsids were the dominant herbivores,
improved running
Abundant carnivores
Late Triassic archosaurs (dinosaurs) replace thecodonts
Dominance continued through Jurassic and Cretaceous
Respiration and Locomotion
Carrier's Constraint - Sprawling vertebrates cannot run and breath at the same time due to twisting trunk
Anaerobic glycolysis helps, but detrimental effects of oxygen debt and lactic acid
Stops in running are for breathing
Amphibians and reptiles have a 3 chambered heart, mammals and birds have 4 chambers
Pelycosaurs had a synapsid mitigation of Courier's Constraint stiffened background, thus not so much twisting, and "wheel barrow" propulsion
erect stance also helped, evolved diaphram, maybe required
for distance running
Triassic diapsids the first to make this breakthrough?
Therapsids evolved in cooler climates, diapsids in warmer climates
Erect gait did not evolve until the Early Jurassic in synapsids
Thecodonts
Stem group of archosaurs
Mid Late Triassic largest carnivores were thecodonts
(rauisuchians and ornithosuchians)
Up to 6 or 7 m long and 2 m high
First Dinosaurs
The first dinosaurs appeared in the Late Triassic ~ 225 MY as small, agile, bipedal carnivores