Late Jurassic Epoch: The Age of Giant Dinosaurs and Shifting Continents
The Late Jurassic (also known as the Upper Jurassic) represents the final epoch of the Jurassic period. Spanning from approximately 161.5 ± 1.0 to 143.1 ± 0.6 million years ago (Ma), this era was a pivotal time in Earth's history, characterized by the fragmentation of supercontinents and the rise of some of the most iconic creatures to ever walk the planet.
In the field of European lithostratigraphy—the study of rock layers—rocks from this period are often referred to as "Malm." While this term was once used to describe the time period itself, modern geologists now use "Malm" specifically for the rock units to maintain a clear distinction between the physical strata and the chronological time scale.
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Key Facts
- Time Span: 161.5 ± 1.0 to 143.1 ± 0.6 million years ago.
- Major Landmasses: Pangaea had split into Laurasia (North) and Gondwana (South).
- Climate: Generally warmer than today, with ice-free polar summers.
- Defining Life: The peak of giant sauropods and the appearance of the first birds.
- Subdivisions: Divided into the Oxfordian, Kimmeridgian, and Tithonian ages.
Chronology and Subdivisions
The Late Jurassic is formally recognized by the International Commission on Stratigraphy (ICS). It is divided into three distinct ages, which are identified by the faunal stages found within the rock layers.
| Age | Lower Boundary (Ma) |
|---|---|
| Oxfordian | 161.5 ± 1.0 |
| Kimmeridgian | 154.8 ± 0.8 |
| Tithonian | 149.2 ± 0.7 |
Defining the exact boundaries of this epoch is a complex task. For the lower boundary, scientists look to candidates such as the horizon of the ammonite Cardioceras redcliffense in Dorset, UK, or sites in Provence, France. The upper boundary is suggested by markers like the base of Chron M18r (a magnetic reversal) or the first appearance of the ammonite Berriasella jacobi.
Paleogeography and Climate
During this epoch, the supercontinent Pangaea continued its dramatic breakup. It had already split into Laurasia (comprising Eurasia and North America) and Gondwana (comprising Africa, Arabia, South America, Antarctica, Australia, India, and Madagascar), separated by the vast Tethys Ocean.
As the North Atlantic Ocean widened and North America drifted northward, moisture increased in the previously arid subtropical interiors of the landmasses. Additionally, the Wrangellia terrane became accreted (attached) to Western North America.
The global climate was significantly warmer than it is today. Deep ocean basins covered the poles, which prevented the formation of polar ice caps and ensured that polar summers remained ice-free. However, this warmth was not uniform; Europe, for instance, became progressively more arid as the epoch progressed.
Life Forms of the Late Jurassic
The Late Jurassic is perhaps most famous for its diverse and massive fauna. This era saw the dominance of several dinosaur groups, including the long-necked sauropods, the predatory theropods, the armored thyreophorans, and the bird-hipped ornithopods. It also witnessed the emergence of the first birds and various crocodylomorphs.
Notable Land Animals
- Theropods: Predatory dinosaurs like Allosaurus (common in North America and Europe) and the giant Saurophaganax.
- Sauropods: Massive herbivores including Brachiosaurus, Diplodocus, and the potentially record-breaking Supersaurus and Maraapunisaurus.
- Thyreophorans: Armored dinosaurs such as Stegosaurus and Kentrosaurus.
- Early Mammals: Basal mammals like Juramaia appeared in Asia.
Air and Sea Creatures
The skies were ruled by pterosaurs, ranging from the small Anurognathus to the long-tailed Rhamphorhynchus. This period also produced Archaeopteryx, a critical bird-like maniraptoran. In the oceans, marine reptiles flourished, including the pliosaur Liopleurodon and the common ichthyosaur Ophthalmosaurus, alongside ammonites like Perisphinctes.
Frequently Asked Questions
What is the difference between the Late Jurassic and the Upper Jurassic?
The terms refer to the same time period but are used in different contexts. "Late Jurassic" is the geochronological name (referring to time), while "Upper Jurassic" is the chronostratigraphic name (referring to the physical rock layers).
What happened to Pangaea during the Late Jurassic?
Pangaea continued to break apart into two larger landmasses, Laurasia in the north and Gondwana in the south, with the Tethys Ocean separating them and the North Atlantic Ocean beginning to widen.
Were there ice caps at the poles during this time?
No. Deep ocean basins covered the poles and ocean currents distributed heat from low to high latitudes, inhibiting the formation of polar ice caps and keeping polar summers ice-free.
Which dinosaurs were most prominent in the Late Jurassic?
The epoch was dominated by giant sauropods (like Brachiosaurus and Diplodocus), large theropods (like Allosaurus), and armored thyreophorans (like Stegosaurus).
What is the "Malm" in geological terms?
"Malm" is a term used in European lithostratigraphy to describe the rock strata that date back to the Late Jurassic epoch.