LIFE AND DEATH AT THE PERMO-TRIASSIC BOUNDARY. 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.
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
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 End Permian Extinction cont Permian, but disappeared by mid-Permian. .
There are two major groups of diapsids: Permian, but disappeared by mid-Permian.
Tuatara from New Zealand
Early Archosaur (?Dinosaur), Euparkeria
The Three Waves of Triassic Archosaurs Permian, but disappeared by mid-Permian.
1. Early Triassic: "Primitive" archosaurs
2. Middle Triassic: Crocodile-like archosaurs
3. Late Triassic: Dinosaurs and Pterosaurs
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.
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.