What is the Cretaceous Period?
The Cretaceous Period marks the final of the three geological periods of the Mesozoic Era, beginning approximately 145 million years ago and concluding around 66 million years ago. It is notable for being the longest period in the history of life, extending over 79 million years, and is characterized by a significant mass extinction event that led to the extinction of numerous species at its end.
The term “Cretaceous” is derived from the Latin word “creta,” meaning chalk. Chalk refers to a type of soft, fine-grained limestone predominantly composed of microscopic plate-like shells of algae, known as coccoliths, which thrived during the late Cretaceous.
It’s worth noting that not all rock formations classified as chalk are exclusive to this period; rather, the majority of chalk deposits were formed during the Cretaceous. These rocks are particularly significant, as they provide clear details about this era, which remain largely untouched and relatively close to the Earth’s surface, facilitating easier access for geological study.
The Cretaceous is pivotal in bridging the gap between ancient and modern forms of life that currently dominate the Earth. During this period, dinosaurs were the prevailing land animals, especially the duck-billed dinosaurs. Additionally, marine reptiles were prevalent in the oceans, while flying reptiles ruled the skies.
Flowering plants began to appear around the beginning of the Cretaceous and flourished as time progressed, culminating in a significant period of marine productivity by the end of this era. Scientists have made these discoveries by studying the thick layers of chalk deposits and the fossils found within them, remnants of the great mass extinction events that marked the conclusion of the period.
Cretaceous Fossils and Their Significance
In the early part of the Cretaceous, the positions of continents and oceans were markedly different from today’s arrangement. Distant continents moved closer together, while neighboring ones drifted apart, resulting in some continents becoming divided. This shifting also led to changes in seas and oceans, with some landmass becoming submerged due to tectonic activities. These geological changes were inferred through fossil studies across various regions.
During this time, the North and South Atlantic Oceans remained largely closed, although the central section began to expand in the late Jurassic Period, which, along with the Cretaceous, formed what is known as the Mesozoic Era. As the Cretaceous progressed, sea levels rose significantly, indicated by the thickness of sediment layers that contained marine fossils. Much of the land we recognize today was submerged underwater.
By the end of the Cretaceous, continents began resembling their modern configurations. South America and Africa took on distinct forms, though India had yet to position itself alongside Asia, and Australia remained part of Antarctica. Below are some notable fossils from this period:
Enantiornithine Birds
The Enantiornithine birds were once the dominant avian species of their time, exhibiting traits reminiscent of dinosaurs. Their name translates to “opposite birds,” reflecting the reversed shoulder bone structure compared to modern birds. The “Protopteryx” is considered the most primitive member of this subgroup, featuring three different types of feathers: soft down covering the head and body, typical flight feathers, and a peculiar central tail feather. The long tail feathers appear to contain fossils of birds from the age of dinosaurs.
All modern birds are believed to have evolved from feathered dinosaurs, leading to a clear taxonomic distinction between birds and dinosaurs, with birds surviving the great mass extinctions of the Cretaceous, while dinosaurs did not.
Fossilized Fish
The unique fossil fish known as “Aipichthys velifer” belongs to the order Beryciformes, within the family Dinopterigidae. It is thought to have been a predator, preying on smaller fish, and became extinct by the early Cretaceous, leaving no descendants. Additionally, there exists a fossil shrimp called “Carpopenaeus septemspinatus,” which derives its name from its seven spike-like structures.
Moreover, a pair of fossils representing sea stars, known as “Geocoma libanotica,” along with other species, illustrate the limited biodiversity of the Cretaceous ecosystem in shallow waters, the area now recognized as modern-day Lebanon, which will undergo significant geological changes over the next 95 million years.
Ammonites
Ammonites, deriving from the genus “Audoliceras” found in Cretaceous deposits in Russia, are characterized by their spiral shells that maintain a consistent shape throughout growth. A major group of ammonites emerged in the late Jurassic, featuring non-coiled shells often referred to as “heteromorphic.” Although much remains unknown about their lifestyle, the structure of their shells suggests they may have relied on gentle movements in the water, feeding on prey within reach. Like their heteromorphic counterparts, ammonites became extinct by the end of the Cretaceous.