Episode 146 – The Jurassic-Cretaceous Transition

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Some geologic boundaries are marked by mass extinction and radical planetary transformations. This episode, discuss one that’s marked mostly by mystery: the Jurassic-Cretaceous Transition.

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A Mysterious Border

Geologists divide the history of Earth into segments (Eras, Periods, Epochs, etc.). Each slice of time has its own distinguishing features in the geologic record, and neighboring time periods are separated by geologic boundaries, which are often marked by significant changes in surface environments and ecosystems.

The shape of the Earth’s surface toward the end of the Jurassic Period.
Map by C. R. Scotese, PALEOMAP Project.

The boundary between the Jurassic Period and Cretaceous Period, around 145 million years old, is an unusual one. For one thing, it is the only Period boundary of the Mesozoic Era that doesn’t feature one of the “Big Five” mass extinctions (the others being the end-Permian, end-Triassic, and end-Cretaceous). For another, it’s a strangely undefined boundary. For every other boundary between geologic Periods in the Phanerozoic Eon (the last 550 million years or so), there is at least one official global indicator (usually a particular fossil species or geochemical marker) that geologists use to identify the precise boundary. But there isn’t one for the Jurassic-Cretaceous. At least not yet.

Partly, this uncertainty has to do with the fact that rock formations from this time period tend to be very distinct from each other, not sharing as many universal features as we might like to see. This makes it difficult to assess exactly what was going on during this time period. For a long time, paleontologists suspected that a mass extinction separated Jurassic from Cretaceous, but as more research has been done, it seems things aren’t quite so straightforward.

A Tumultuous Time

To be clear, there is a lot of extinction around this boundary. During the Late Jurassic and Early Cretaceous, many groups of animals on land and in the water suffer major losses, including certain groups of dinosaurs, pterosaurs, marine reptiles, and marine invertebrates. But there’s more than just extinction happening. Some groups are on the rise at this time, diversifying into many new species and even evolving new ecological lifestyles, such as birds and certain insects. Some groups experience gains and losses that largely balance each other out. And still some others sail by just fine. Whereas certain geologic boundaries, like the end-Cretaceous, feature huge amounts of apocalyptic loss, this one seems instead to be a long period of dramatic but complex ecological changes.

Artist’s reconstruction of a Jurassic ecosystem, by Lucas Attwell, CC BY-SA 3.0
Many of the important reptile groups of the Jurassic, such as megalosaurs, allosaurs, stegosaurs, early sauropods, and small pterosaurs, were replaced in the Cretaceous by animals with similar lifestyles.

Not surprisingly, there was a lot of changing going on in Earth’s environments at this time, too. From the Late Jurassic to the Early Cretaceous, the continuing breakup of the Pangaean supercontinent led to a drop in sea level, which shrunk continental seaways and separated shallow marine habitats. There was also a global climate shift from dry to wet. A number of craters provide evidence for major asteroid impacts during this time, and multiple large igneous provinces were formed by massive pulses of volcanic activity. Exactly how any or all of these events might have impacted life at the time is hard to say for sure, but it’s clear that this was a time of dramatic change.

A map of the world at the Jurassic-Cretaceous transition, showing the shape of coastlines and the positions of various craters and volcanic provinces. Image from Tennant et al. 2016.

The Jurassic-Cretaceous transition seems to have been a time of complex environmental change and cascading ecological disturbances, a drawn-out series of events that dramatically, if gradually, transformed the world of the Jurassic into that of the Cretceous.

Learn More

2016 review of the Late Jurassic-Early Cretaceous transition (technical, open access)

How sea level impacted animal diversity at the Jurassic-Cretaceous transition (semi-technical)

Troubles with identifying the Jurassic-Cretaceous boundary (technical, open access)

The case of Machimosaurus rex, the marine croc we mentioned in this episode, part of a group of crocs previously thought to have gone extinct at the end-Jurassic

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