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GEOL 104 Dinosaurs: A Natural History

Fall Semester 2021
Triassic Transitions & the Age of Reptiles


Detail of "Postosuchus and Placerias" (1986) by Douglas Henderson

Key Points:
•The greatest mass extinction of all time--the Permo-Triassic Extinction of 252 million years ago--ended the dominance of the therapsids. In the Triassic Period the diapsid reptiles, and especially the archosaurs, become most successful.
•The Pseudosuchia (the crocodile lineage) was the most successful group of archosaurs in the Triassic.
•The Triassic also ends with a major mass extinction--the Triassic/Jurassic Extinction--triggered by the eruption of the Central Atlantic Magmatic Province and the breakup of Pangaea. This event wiped out most of the dinosaur's rivals, and allowed them to inherit the Earth

I. The Triassic Reptile Radiations

At the end of the Late Permian, the greatest extinction in the history of life clobbered ecosystems on land and sea. This event totally changed the make-up of the diversity of life, and forms the boundary between the Permian Period of the Paleozoic Era and the Triassic Period of the Mesozoic Era. Consequently, it is called the Permo-Triassic Extinction. Perhaps 95% of all species died out.

The cause seems to ultimately have been the Siberian Traps, a monumentally huge series of volcanic eruptions. After a pulse of world-chilling sulfate aerosols plunged the Earth's surface into near-Ice Age cold, the massive amount of greenhouse gases raised global temperatures through extremes of greenhouse gasses; this further triggered additional greenhouse gasses being released from the sea floor. Together, extremes of temperature and of carbon dioxide and very low levels of oxygen on land and sea (and quite possibly extreme acid rain, the loss of the ultraviolet-shielding ozone layer, and more) caused mass deaths.

Regardless of precise scenario, extinction reorganizes the world. In the immediate aftermath, there was a great reduction in size and diversity of the animals present.

Lost were many of the primitive therapsids and many of the primitive reptiles.

During the Early Triassic diversity started off very low in land and sea, and recovered over the next ten million years or so. On land, derived cynodonts (advanced carnivorous therapsids) were the dominant predators, and piglet to ox-sized herbivorous dicynodont therapsids were common. But the sauropsids (and especially diapsids) began to radiate into the number of different forms:

These new predators were the precursors of Archosauria ("ruling reptiles"), the dominant group of Mesozoic sauropsids.

Archosaurs and their closest relatives are distinguished from other diapsids by:

It is not certain if these early archosauriforms had the behavioral traits found in both the living groups of Archosauria (that is, crocodylians and birds). Because both crocodylians and birds share the following derived traits, however, it is fairly certain that at least the concestor of all Archosauria had them, and passed them on to its descendants:

Middle and Late Triassic sauropsids, and diapsids in particular, radiate into many different forms:

A great diversity among the Middle and Late Triassic reptiles, though, were archosaurs. During the Middle and Late Triassic, the archosaurs displaced the therapsids as the dominant group of large terrestrial amniotes.

Archosaurs are divided into two main branches:

It is the pseudosuchians which dominated the Middle and Late Triassic. This group radiated into a number of forms:

The pseudosuchians include some of the first terrestrial animals to exceed the size of oxen and hippos. Most of them could stand with a semi-erect posture of the limbs, and a few had the fully-erect (that is, parasagittal gait).

Although pseudosuchians may have been the dominant group, plenty of other forms abounded in the Middle and Late Triassic. These included the last and largest dicynodont therapsid herbivores; more advanced, but generally small therapsids (including the oldest mammals: more about them later); many non-archosaurian diapsids (see above); and...

The First DINOSAURS!

II. The Carnian Pluvial Event & the Triassic/Jurassic Mass Extinction: Dinosaurs Inherit the Earth
Although dinosaurs appeared in the early part of the Late Triassic ecosystems, they were just a minor part. For example, other animals were far more common in the Ischigualasto Formation than Eoraptor, Herrerasaurus, and Panphagia. Theropods were not the top predators in any known Late Triassic assemblage: that distinction goes to big pseudosuchians. By the end of the Late Triassic, sauropodomorphs HAD become the largest and most common herbivores in some ecosystems (away from the equator).

Recent work shows the earliest dinosaurs and the earliest members of most dinosaur lineages showing up in the southern continents (especially South America and southern Africa) first. New discoveries from the lower part of the Chinle Formation of the American Southwest shows that diverse non-dinosaurian dinosauromorphs were still a major part of the assemblage well into the later part of the Late Triassic.

So most of the the Late Triassic is the time WHEN DINOSAURS SHARED THE EARTH!. Pseudosuchians remained ecologically diverse and important, and (especially in the early Late Triassic) non-archosaurian archosauromorphs and therapsids were also significant. In the later Late Triassic (in Gondwana and Europe, at least) things shift: pseudosuchians remain important (especially in the form of paracrocodylomorphs, aetosaurs, and phytosaurs) as did dicynodont therapsids, but sauropodomorphs expand in diversity (and size!). In recent years it has been suggested that a major environmental shift was the cause of this. Sedimentological analysis shows that the world got considerably wetter from 234-232 Ma. Its particular trigger isn't well understood, but it affected life on both land and sea. This event occurred in the Carnian Stage, and so is called the Carnian Pluvial Event ("pluvial" = "of or concerning rain"). One of the changes on land is the diversification and spread of groups of tree-sized plants (especially conifers and the now-extinct cycadeoids). It has been argued that the accumulation of this new leafy biomass may have promoted the expansion of the sauropodomorphs, and thus of Dinosauria.

(That said, the phenomenon is NOT apparent in North America! Both the excellent body fossil record of the Southwest and the trackway record of the East show no spread of sauropodomorphs, or dinosaurs in general, in North America in the later Carnian and subsequent Norian and Rhaetian stages.)

At the end of the Late Triassic, though, there was a mass extinction that affected the entire planet: the Triassic-Jurassic Extinction. The primary trigger seems to have been a period of tremendous volcanism in the middle of Pangaea. 201 Ma saw the beginnings of the rupture of the supercontinent of Pangaea, between the landmasses that are today North America, Europe, and Asia (on the north) and South America, Africa, and the other continents (to the south). This produced the Atlantic Ocean in the long term, but for the inhabitants of the Earth at the time suffered.

A vast amount of greenhouse gasses (both carbon dioxide and methane) were released, producing catastrophic global warming. But this was preceded by a burst of sulfur dioxide, which produced a rapid phase of cooling. There is additional evidence that poisons like mercury were released into the environment.

These massive pulses of environmental change were too much for many groups of organisms to survive. This is one of the five greatest mass extinctions known, with many terrestrial and marine taxa wiped out. Among terrestrial vertebrates, some of the main victims were:

With no serious competition remaining, dinosaurs radiated into numerous forms. From the beginning of the Early Jurassic onward, DINOSAURS RULED THE EARTH.


Some relevant videos:


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Last modified: 30 July 2021

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Detail of painting of the Triassic archosaurs of the Newark Supergroup in eastern New York (2002) by Douglas Henderson, with coelophysid theropod dinosaurs in the background, phytosaurs in the middle, and a rauisuchian and an aetosaur up front.