The Late Jurassic period was a significant chapter in Earth’s history, marked by the continued reign of dinosaurs and substantial global environmental changes. This era witnessed a diverse array of life forms thriving across evolving landscapes. Its unique conditions led to the preservation of an extensive fossil record, offering scientists invaluable insights into ancient ecosystems.
Defining the Late Jurassic
The Late Jurassic epoch spans approximately 161.5 to 145 million years ago, marking the final stage of the Jurassic Period within the broader Mesozoic Era. During this interval, the supercontinent Pangea continued its fragmentation.
This breakup separated Pangea into two primary landmasses: Laurasia in the north (comprising what would become North America and Eurasia) and Gondwana in the south (including future South America, Africa, Antarctica, Australia, and India). The widening Tethys Ocean divided these landmasses, creating new coastlines and influencing global geography.
Life of the Late Jurassic
The Late Jurassic was dominated by dinosaurs. Gigantic long-necked sauropods, such as Brachiosaurus, Diplodocus, and Apatosaurus, were widespread and reached immense sizes, becoming the largest land animals to ever live. These herbivores often moved in herds. Armored stegosaurs, notably Stegosaurus with its distinctive back plates and tail spikes, were also prominent herbivores, found across regions like North America and Europe.
Predatory theropods, like Allosaurus, were apex carnivores, hunting large sauropods and other dinosaurs. These bipedal hunters could reach over 9 meters long. Early birds began to emerge; Archaeopteryx, a feathered dinosaur with both reptilian and avian features, lived during this time. Pterosaurs, flying reptiles distinct from birds, diversified significantly, with both long-tailed rhamphorhynchids and the emerging short-tailed pterodactyloids.
Marine environments included large marine reptiles such as ichthyosaurs and plesiosaurs, alongside diverse fish and invertebrate populations. The flora of the Late Jurassic was characterized by the dominance of gymnosperms, including conifers, cycads, and ferns, which formed the primary food source for many herbivorous dinosaurs. Flowering plants had not yet evolved.
Global Landscape and Climate
The Late Jurassic period was characterized by a generally warm and humid climate. There were no polar ice caps, and global temperatures were higher than present-day levels, likely due to elevated atmospheric carbon dioxide.
The continued breakup of Pangea influenced ocean currents, leading to the formation of new, more diverse marine environments. Narrow ocean basins emerged as continents drifted apart, bringing increased moisture to formerly arid interior regions. The topography included floodplains, shallow seas, and nascent mountain ranges, shaping varied habitats for the period’s diverse life forms.
Key Paleontological Insights
Our understanding of the Late Jurassic period comes from significant fossil discoveries made at renowned sites worldwide. The Morrison Formation in the western United States is a particularly fertile source, yielding thousands of dinosaur specimens including Stegosaurus, Allosaurus, Apatosaurus, and Diplodocus. This formation, composed of mudstone, sandstone, siltstone, and limestone, preserves remnants of ancient rivers and floodplains. Volcanic ash within the Morrison Formation’s bentonite clays allows for radiometric dating of these fossils.
Another crucial site is the Solnhofen Limestone in Bavaria, Germany, famous for its exceptionally preserved fossils, including soft-bodied organisms and the iconic Archaeopteryx. The fine-grained nature of the limestone, formed in stagnant, oxygen-depleted lagoons, allowed for the exquisite preservation of details like feathers and even internal organs. Paleontologists analyze these fossilized bones, footprints, and impressions to reconstruct ancient ecosystems, infer animal behaviors, and understand the appearance of extinct creatures.
Transition to the Cretaceous
The Late Jurassic epoch gradually transitioned into the Early Cretaceous period, marked by ongoing geological and biological shifts rather than an abrupt event. Continental drift continued, reshaping the global map. This ongoing tectonic activity influenced ocean circulation and climate patterns, setting the stage for the environmental conditions of the subsequent period.
Biologically, the end of the Late Jurassic saw the diversification of certain life forms, building upon the established dinosaurian dominance. While the major groups of dinosaurs continued to thrive, the early stages of the Cretaceous would bring further evolutionary developments and the appearance of new species. The fossil record reflects a continuous evolutionary trajectory, with no major extinction event defining the boundary between the Late Jurassic and Early Cretaceous.