Paleobiology!University of Wisconsin – Madison Andrew A. Zaffos April 18, 2016

Triassic-Jurassic Terrestrial Transition

Competitive wedge P

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rtion

Time

The over arching framework

The timeline of events

Early Triassic n  Primitive proto-mammals (Synapsids)

n  Dicynodont (Two-Dog Tooth) n  Herbivores characterized by two tusks.

n  Cynodonts (Dog Teeth) n  Carnivorous and herbivorous n  Ancestors of modern mammals

n  Primitive proto-dinosaurs (Diapsids) n  Thecodonts (Socket Teeth)

n  Teeth are were set in sockets in the jawbone, but mammals also have this feature, hence why the term is now obsolete.

n  Now, we just say Archosaurids. Basically crocodiles.

Middle Triassic n  Primitive proto-mammals (Synapsids)

n  Dicynodont (Two-Dog Tooth) n  Herbivores characterized by two tusks.

n  Cynodonts (Dog Teeth) n  Carnivorous and herbivorous n  Ancestors of modern mammals

n  Primitive proto-dinosaurs (Diapsids) n  Thecodonts (Socket Teeth)

n  Teeth are were set in sockets in the jawbone, but mammals also have this feature, hence why the term is now obsolete.

n  Now, we just say Archosaurs (ruling reptiles). Basically crocodiles. n  Rhynchosaurs (Beak Lizards)

n  Broad skulls indicating powerful jaw muscles. n  Had a scissor like bite. n  Described as swine-like herbivores

Gomphodont (Bolted Tooth)

Late Triassic n  Archosaurs split into two major lineages

n  Ornithodira (“bird necks”) / Avematatarsalia (“bird feet”) n  Dinosaurs, Birds, and Pterosaurs

Late Triassic n  Archosaurs split into two major lineages

n  Crurotarsi (“cross ankles”) n  More crocodile like stuff n  Phytosaurs (“Plant Reptile”)

n  Elongated snout with front nostrils n  “Rausichians”

n  The apex predators of the Triassic

What makes a dinosaur n  Elongated deltopectoral crest

n  Open acetabulum

n  Temporal musculature extends anteriorly onto skull roof

n  Epipophyses on the cervical vertebrae

n  Articulation facet for fibula occupying less than 30% of the transverse width of the astragalus

n  Femoral fourth trochanter asymmetrical, with distal margin forming a steeper angle to the shaft.

n  Posterior process of the jugal bifurcated to articulate with quadratojugal

The old view n  Polyphyletic

n  Dinosaurs arose several times through convergent evolution, as a result of similar competitive pressures.

n  Timing n  The origin of dinosaurs was a long drawn-out affair, involving long-term

competition from the Middle Triassic though to the Jurassic.

n  Competition n  Dinosaurs had superior mobility n  Dinosaurs were warm blooded n  Dinosaurs grew faster

n  Evolutionary Progress n  Species become superior through time.

Erect Dinosaurs

Erect Dinosaurs

Dinosaur growth

Arguments for endothermy n  Agility and Endothermy

n  Dinosaur morphology indicates great agility, generally metabolism is proportional to locomotion.

n  Competition n  Endotherms generally “outcompete” ectotherms in modern ecosystems, so the

Dinosaurs had to have been endotherms to outcompete mammals.

n  Blood pressure n  Erect posture of large dinosaurs requires high blood pressure, like a giraffe.

This means that a four-chambered heart is required.

n  Bone structure n  Dinosaur bone is more similar to mammal and avian bone in cross section than

it is to ectothermic bone.

Counters to the old view n  Abrupt Transitions

n  Dinosaurs take-over at major boundaries, rather than gradually replacing the non-dinosaurs

n  Early Adaptation n  The earliest dinosaurs (Middle Triassic) were also ectothermic and erect, but

they did not radiate until much later.

n  Shared Adaptation n  Some Archosaurs also had erect or semi-erect gait.

n  Coevoltuion n  Other taxa go extinct in tandem with the “Thecodonts” and “Synapsids”, most

importantly the transition from seed ferns to conifers.

Actual evidence?

Counters to the Counter view n  Abrupt Transitions (What is abrupt?)

n  Dinosaurs take-over at major boundaries, rather than gradually replacing the non-dinosaurs

n  Early Adaptation (Tempo and Mode?) n  The earliest dinosaurs (Middle Triassic) were also ectothermic and erect, but

they did not radiate until much later.

n  Shared Adaptation (So what?) n  Some Archosaurs also had erect or semi-erect gait.

n  Other extinctions (Opposite direction?) n  Other taxa go extinct in tandem with the “Thecodonts” and “Synapsids”, most

importantly the transition from seed ferns to conifers.

Red Queen and Court Jester

Red Queen and Court Jester

True Red Queen n  The probability of extinction for a subtaxon within a given group (i.e., a

species within a family) remains constant over time.

n  There is no correlation between the age of a taxon and its probability of extinction (relative to other members of the same group).

n  What this means, if true, is that we can assume the evolutionary properties of species are essentially “equivalent”, which means we can model them as interchangeable evolutionary entities.

True Red Queen

True Red Queen

Alternative testing methods Phanerozoic trends in the geographic range selectivity of genus survivorship.

True Red Queen

Coevolution