Mesozoic Evolution

The Mesozoic era is dominated by reptiles, specifically the dinosaurs. The Triassic saw devastated ecosystems that took over 30 million years to fully re-emerge after the Permian Mass Extinction.  The first appearance of many modern groups of animals that would later flourish occurred at this time.  This includes frogs (amphibians), turtles (reptiles), marine ichthyosaurs and plesiosaurs (marine reptiles), mammals, and archosaurs. The archosaurs (“ruling reptiles”) include ancestral groups that went extinct at the end of the Triassic, the flying pterosaurs, crocodilians, and the dinosaurs. Archosaurs, like the placental mammals after them, occupied all major environments: terrestrial (dinosaurs), in the air (pterosaurs), aquatic (crocodilians), and even fully marine habitats (marine crocodiles). The pterosaurs, the first vertebrate group to take flight, like the dinosaurs and mammals, start small in the Triassic.

At the end of the Triassic, another mass extinction event occurred, the fourth major mass extinction in the geologic record. This was perhaps caused by the Central Atlantic Magmatic Province flood basalt. The end-Triassic extinction made certain lineages go extinct and helped spur the evolution of survivors like mammals, pterosaurs (flying reptiles), ichthyosaurs/plesiosaurs/mosasaurs (marine reptiles), and dinosaurs.

Mammals, as previously mentioned, got their start from a reptilian synapsid ancestor, possibly in the late Paleozoic. Mammals stayed small, in mainly nocturnal niches, with insects being their largest prey. The development of warm-blooded circulation and fur may have responded to this lifestyle.

In the Jurassic, previously common species flourished due to a warmer and more tropical climate. The dinosaurs were relatively small animals in the Triassic period of the Mesozoic but became truly massive in the Jurassic. Dinosaurs are split into two groups based on their hip structure, i.e., the orientation of the pubis and ischium bones in relationship to each other. These orientations are referred to as the “reptile-hipped” saurischians and the “bird-hipped” ornithischians. However, this idea has recently been questioned by a new idea for dinosaur lineage.

The bird-hipped dinosaurs include many herbivorous species, such as the armored Stegosaurus, Ankylosaurus, the horned Triceratops, and the duck-billed Hadrosaurs.

The lizard-hipped dinosaurs were divided into two groups – the herbivorous sauropods, such as the 100-ton, long-necked brachiosaurus and apatosaurus, and the more vicious and carnivorous theropods, such as Tyrannosaurus Rex and Velociraptor. Strangely enough, modern birds have evolved from theropods, with the “lizard-hip” rather than dinosaurs that possessed the “bird-hip.”

The major adaptive advantage dinosaurs had was changes in the hip and ankle bones, tucking the legs under the body for improved locomotion as opposed to the semi-erect gait of crocodiles or the sprawling posture of reptiles.

There is a paucity of dinosaur fossils from the Early and Middle Jurassic, but by the Late Jurassic, they were dominating the planet.

The pterosaurs grew and diversified in the Jurassic, and another notable aerial organism developed and thrived in the Jurassic: birds. When Archaeopteryx was found in the Solnhofen Lagerstätte of Germany, a seeming dinosaur-bird hybrid, it started the conversation on the origin of birds. The idea that birds evolved from dinosaurs occurred very early in the history of research into evolution, only a few years after Darwin’s On the Origin of Species. This study used a remarkable fossil of  Archeopteryx, which had visible feathers preserved, from a transitional animal between dinosaurs and birds. Small meat-eating theropod dinosaurs were likely the branch that became birds due to their similar features. A significant debate still exists over how and when powered flight evolved. Some have stated a running-start model, while others have favored a tree-leaping gliding model or even a semi-combination: flapping to aid climbing.

The Cretaceous saw a further diversification, specialization, and domination of the dinosaurs and other fauna. One of the biggest changes on land was the transition to angiosperm-dominated flora. Angiosperms, plants with flowers and seeds, originated in the Cretaceous, switching many plains to grasslands by the end of the Mesozoic. By the end of the period, they had replaced gymnosperms (evergreen trees) and ferns as the dominant plant in the world’s forests. Haplodiploid eusocial insects (bees and ants) are descendants of Jurassic wasp-like ancestors that co-evolved with the flowering plants during this period.

The breakup of Pangea not only shaped our modern world’s geography but biodiversity at the time as well. Throughout the Mesozoic, animals on the isolated, now separated island continents (formerly parts of Pangea) took strange evolutionary turns. This includes giant titanosaurian sauropods (Argentinosaurus) and theropods (Giganotosaurus) from South America.