13.5: Mesozoic Mass Extinctions

Introduction

There are three mass extinctions associated with the Mesozoic Era. The first occurred 252 million years ago. It marked the start of the Mesozoic Era and was addressed in the previous chapter. The second occurred at the end of the Triassic Period, and the third occurred at the end of the Mesozoic Era. These two are explained below.

End of Triassic Extinction

Figure \(\PageIndex{1}\): A drawing of the early plesiosaur Agustasaurus from the Triassic of Nevada.

At the end of the Triassic Period, 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.

Figure \(\PageIndex{2}\): Reconstruction of the small (<5″) Megazostrodon, one of the first animals considered to be a true mammal.

Effects on Mammals

Mammals, as previously mentioned, got their start from a reptile-like synapsid ancestor possibly in the late Paleozoic. True mammals did not evolve until the Triassic, and after the extinction, mammals generally stayed small, in mainly nocturnal niches, with insects being their largest prey. The development of warm-blooded circulation and fur may have been a response to this lifestyle.

Figure \(\PageIndex{3}\): Closed structure of a ornithischian hip, which is similar to a birds.

Effects on Dinosaurs

In the Jurassic, species that were previously common, 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. orientation of the pubis and ischium bones in relationship to each other. This is referred to as the “reptile hipped” saurischians and the “bird hipped” ornithischians. This has recently been brought into question by a new idea for dinosaur lineage.

Figure \(\PageIndex{4}\): Open structure of a saurischian hip, which is similar to a lizards.

Most of the dinosaurs of the Triassic were saurischians, but all of them were bipedal. 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. In the Jurassic, limbs (or a lack thereof) were also important to another group of reptiles, leading to the evolution of Eophis, the oldest snake.

Figure \(\PageIndex{5}\): Therizinosaurs, like Beipiaosaurus (shown in this restoration), are known for their enormous hand claws.

There is a paucity of dinosaur fossils from the Early and Middle Jurassic, but by the Late Jurassic they were dominating the planet. The saurischians diversified into the giant herbivorous (plant-eating) long-necked sauropods weighing up to 100 tons and bipedal carnivorous theropods, with the possible exception of the Therizinosaurs. All of the ornithischians (e.g Stegosaurus, Iguanodon, Triceratops, Ankylosaurus, Pachycephhlosaurus) were herbivorous with a strong tendency to have a “turtle-like” beak at the tips of their mouths.

Figure \(\PageIndex{6}\): Iconic “Berlin specimen” Archaeopteryx lithographica fossil from Germany.

Effects on Birds

The pterosaurs grew and diversified in the Jurassic, and another notable arial organism developed and thrived in the Jurassic: birds. When Archeopteryx 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 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 in climbing.

Figure \(\PageIndex{7}\): Reconstructed skeleton of Argentinosaurus, from Naturmuseum Senckenberg in Germany.

Effects on Plants

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, which are plants with flowers and seeds, had 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 from Jurassic wasp-like ancestors that co-evolved with the flowering plants during this time 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.

K-Pg- End of Mesozoic Mass Extinction

Figure \(\PageIndex{8}\): Graph of the rate of extinctions. Note the large spike at the end of the Cretaceous (labeled as K).

Similar to the end of the Paleozoic era, the Cretaceous Period and Mesozoic Era ended 66 million years ago with the K–Pg mass extinction event, also known as the K–T extinction. This event marks the end of the Cretaceous Period (K) and the beginning of the first period of the Cenozoic Period, the Paleogene (Pg).

Cause of Mass Extinction

Extraterrestrial Impact

This extinction event was likely caused by a large extraterrestrial impactor such as an asteroid, meteoroid, or comet that collided with earth. Ninety percent of plankton species, 75% of plant species, and all the dinosaurs went extinct at this time.

Figure \(\PageIndex{9}\): Artist’s depiction of an impact event

One of the strongest pieces of evidence comes from the element iridium. Quite rare on Earth, and more common in meteorites, it has been found all over the world in higher concentrations at a particular layer of rock that formed at the time of the K-T boundary. Soon other scientists started to find evidence to back up the claim. Melted rock spheres, a special type of “shocked” quartz called stishovite, that only is found at impact sites, was found in many places around the world. The huge impact created a strong thermal pulse that could be responsible for global forest fires, strong acid rains, a corresponding abundance of ferns, the first colonizing plants after a forest fire, enough debris thrown into the air to significantly cool temperatures afterward, and a 2-km high tsunami inferred from deposits found from Texas to Alabama.

Figure \(\PageIndex{10}\): The land expression of the Chicxulub crater. The other side of the crater is within the Gulf of México.

Still, with all this evidence, one large piece remained missing: the crater where the bolide impacted. It was not until 1991 that the crater was confirmed using petroleum company geophysical data. Even though it is the third largest confirmed crater on Earth at roughly 180 km wide, the Chicxulub Crater was hard to find due to being partially underwater and partially obscured by the dense forest canopy of the Yucatan Peninsula. Coring of the center of the impact called the peak ring contained granite, indicating the impact was so powerful that it lifted basement sediment from the crust several miles toward the surface. In 2010, an international team of scientists reviewed 20 years of research and blamed the impact for the extinction.

Alternate Explanations

With all of this information, it seems like the case would be closed. However, there are other events at this time which could have partially aided the demise of so many organisms. For example, sea levels are known to be slowly decreasing at the time of the K-Pg event, which is tied to marine extinctions, though any study on gradual vs. sudden changes in the fossil record is flawed due to the incomplete nature of the fossil record. Another big event at this time was the Deccan Traps flood basalt volcanism in India. At over 1.3 million cubic kilometers of material, it was certainly a large source of material hazardous to ecosystems at the time, and it has been suggested as at least partially responsible for the extinction. Some have found the impact and eruptions too much of a coincidence, and have even linked the two together.

Figure \(\PageIndex{11}\): Geology of India, showing purple as Deccan Traps-related rocks.

It seems that by the end of the Mesozoic, life on Earth was in a fragile state, and at the same time, a meteorite larger than Mount Everest was hurtling towards Earth.

Video: The Day the Dinosaurs Died – Minute by Minute

The events that led to the extinction of the dinosaurs are dramatically summarized in the video "The Day the Dinosaurs Died – Minute by Minute", by Kurzgesagt – In a Nutshell.

Video: How the Dinosaurs Actually Died

The argument for volcanic activity causing the extinction of the dinosaurs is well explained in the video "How the Dinosaurs Actually Died", by Kurzgesagt – In a Nutshell.

Key Terms

• Chicxulub Crater - a ~180‑kilometer‑wide impact structure buried beneath the Yucatán Peninsula in Mexico formed about 66 million years ago by the collision of a large asteroid, meteoroid or comet
• Deccan Traps - one of Earth’s largest continental flood basalt provinces, located primarily in western India and erupted in multiple phases around 66 million years ago
• K–Pg mass extinction - also referred to as the K-T Extinction, this extinction occurred at 66 million years and eliminated roughly 75% of Earth’s species