life and death at the permo triassic boundary
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LIFE AND DEATH AT THE PERMO-TRIASSIC BOUNDARY. Pelycosaur and Therapsid Evolution.

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Pelycosaur and Therapsid Evolution

In their position on the family tree of life, the Pelycosaurs are the earliest and most primitive members of the synapsids, the group that (in the old classification) leads to or (in the new classification) includes mammals.  Thus the mammal-line split off from the rest of the reptile line (including turtles, lizards and snakes, crocodiles, and dinosaurs and birds) very early.


Pelycosaurs were abundant at low latitudes in early Permian, but disappeared by mid-Permian.

  • At low latitudes, Diapsid Reptiles became important.
  • At high latitudes, Therapsids (descendants of pelycosaurs) and Anapsid Reptiles became important.
pelycosaur vs therapsid jaw
Pelycosaur vs. Therapsid Jaw

Although Therapsids resemble pelycosaurs in some ways, they had a much larger hole in the outer skull armor, flange on lower jaw for muscle attachment, and shorter heads.  All these differences from pelycosaurs are related to more elaborate chewing.  In addition, some had a more upright posture.


Skeleton of the dinocephalian Moschops about 2,5 metres long, mounted in the American Museum, New York. The skull bones of this herbivorous reptile have undergone enormous thickening.

Ulemosaurus svijagensis -Rjabinin, 1938- skull

Therapsida: Dinocephalia: Tapinocephalidae

Locality: Isheevo, Tatarstan, eastern European Russia

Age: Late Permian, 255 million years ago

  • Dinocephalians: most were large, with massive bodies and thick skulls.  They had a differentiated tooth row (incisors, canines, flattened back teeth). Some had bizarre horns (head-butting?). Most were herbivores

Dicynodonts: wide range of sizes (from rat- to cow-sized).  They reduced all their teeth except the canines, and probably had a horny pad for shearing food.  They were very diverse herbivores, with many distinct modes of life, including some burrowers.

Reconstruction of the Late Permian South African dicynodont Diictodon

Dicynodon trautscholdi- skull, Age: Late Permian



Theriodonts: Mostly active predators. They had longer legs, saber teeth, and much more erect posture. Some were large, but other were small carnivores and insectivores.

  • The fact that these animals were able to invade high latitudes, as well as the fact that they are the group from which mammals arose, raises questions about how they regulated their body temperature. 
  • Similarly, their upright posture raises questions about patterns of activity and exercise. 
  • These issues will be discussed later in the context of dinosaur temperature regulation and activity patterns.

The Paleozoic Marine Fauna

  • Dominant animals:
  • Articulate brachiopods, bryozoans, archaic corals, crinoids, cephalopods
  • Dominant modes of life:
  • Sessile, filter feeders extremely common.
  • Mobile detritus feeders (Cambrian-style organisms) are still around.
  • Mobile carnivores and herbivores are more common.
  • Free swimming and burrowing animals still somewhat rare.
  • Range:
  • These organisms became important by end Ordovician, drop in importance after Permian.

The Paleozoic Marine Fauna cont.

  • Diversity:
  • Many more groups trying each mode of life.
  • Local communities have more species ~ 9 species in stressed zones, 18 in near shore zones, 30 in open marine regions
  • Tiering:
  • One important ecological feature of Paleozoic marine communities.
  • Non-mobile, filter-feeding organisms are intercepting particles from the water column at different heights above the sediment-water interface. 
  • They are chopping up space vertically, kind of like land plants.

The End Permian Extinction

  • The Paleozoic was marked by a series of major extinctions, including ones in the late Ordovician (takes out the Cambrian Fauna), the late Devonian (takes out armored fish) and the late Permian (takes out almost everything).
  • In this end Permian extinction, ~ 95% of marine species died.

The End Permian Extinction cont.

  • In the oceans:
  • Trilobites and Archaic corals gone for good.
  • Brachiopods, bryozoans, crinoids, ammonoids (shelled cephalopods) hit very hard.
  • Snails, clams, nautiloids (another type of shelled cephalopod) do ok.
  • Absolutely no reefs known for the next 15 million years
  • Stromatolites spread into "normal" environments for the first time since the Ordovician.
  • On land:
  • J ust 2 groups of therapsids survive.
  • ~ 67% of the amphibians (including all reptile-like amphibians) go extinct.
  • ~ 30% extinction of the orders of insects go extinct.

What happened?

  • Asteroid or Comet Impact
  • Volcanism
  • Drop in sea Level
  • Stagnant Ocean Hypotheses
  • Drop in oxygen content of surface ocean water
  • Belch of carbon dioxide from deep ocean

After the fall: the Triassic Recovery

  • Recall that prior to the extinction, in the late Permian, land communities at high latitudes were dominated by a type of Synapsid Amniote, the Therapsids. 
  • After the extinction, it is a different story. 
  • Diapsid reptiles become increasingly important at all latitudes.

There are two major groups of diapsids:

  • 1. Lepidosaurs:
  • Scaly Reptiles: first appear in the late Permian.
  • Their shared novelties include a forked tongue and a forked penis.
  • Smallish lizard-like animals.
  • Active insectivores and carnivores.
  • Ancestors of modern snakes and lizards.6000 living lepidosaur species.
lepidosaurs scaly reptiles
Lepidosaurs - Scaly Reptiles

Tuatara from New Zealand


2. Archosaurs:

  • Ruling Reptile: also first appear in the Permian.
  • Their shared novelties include one hole in the head armor in front of the eye, and teeth that are in separate sockets, rather than a long groove.
  • When they first appear, they were also small lizard-like animals. 
  • In the Triassic, they diversify in spectacular fashion, in a series of pulses or waves
archosaurs ruling reptiles
Archosaurs - Ruling Reptiles

Early Archosaur (?Dinosaur), Euparkeria


The Three Waves of Triassic Archosaurs

1. Early Triassic: "Primitive" archosaurs

2. Middle Triassic: Crocodile-like archosaurs

- Phytosaurids:

- Crocodiles:

- Aetosaurs

- Rauisuchians:

3. Late Triassic: Dinosaurs and Pterosaurs

rhynchosaurs beaked lizard
Rhynchosaurs: ”Beaked lizard"

Cistecephalus - late Permian period

Abundant pig-sized herbivores. Triangular head, large area for jaw muscles, beak, 2 tooth rows above, 1 row below, jaws close like a pocket knife, digging claws, sprawled gait.

Primitive carnivorous archosaurs: medium-to-large (up to 2m), sprawling to semi-erect gait with limbs swung out to side. Aquatic and terrestrial forms.

phytosauridae plant lizard
Phytosauridae - "Plant lizard":


The Phytosaurs (this unfortunate name means "plant lizards", because it was originally mistakenly believed that petrified.mud fillings in the jaw of the first specimen found were herbivore teeth) were crocodile-like semi-aquatic thecodonts that suddenly appeared and became very abundant during the latter part of the Triassic period.


Neoaetosauroides engaeus from the Upper Triassic

Aetosaurs were sizeable reptiles that grew to be one to five meters long, the average being about three meters (10 feet). Most aetosaurs possessed possessed a rather narrow crocodile-like body, although some had a broad turtle-like midsection, probably expanded to contain a large fermenting gut. The animal was protected throughout by an armor covering over the neck and the upper and under surfaces of the trunk and tail. Some species, such as Desmatosuchus, also had heavy spikes along the shoulders and flanks. Based on their fossils and the fact that they were herbivores, aetosaurs probably relied on their armor and large size rather than speed to protect them from predators.


Saurosuchus is not a dinosaur, but it shares Dinosaur World with other non-dinosaurs and dinosaurs. With teeth like these, Saurosuchus is a meat-eater with a lot to smile about.  Notice that these teeth are in various sizes and degrees of wear.  Throughout their lives meat-eaters replaced old, worn teeth, probably broken from biting into bone.  Old meat-eaters most likely did not die of old age but of starvation, when their last set of teeth wore out.

SAUROSUCHUS"Lizard crocodile"