The Early Cretaceous period (145 to 100.5 million years ago), positioned between the Jurassic and Late Cretaceous, was a transformative chapter in Earth’s history. This epoch saw significant shifts in global geology and dinosaur evolution as landmasses reshaped, influencing the diversification of ancient inhabitants. This era set the stage for many iconic dinosaur forms of the Mesozoic Era.
A World in Transition: The Early Cretaceous Environment
During the Early Cretaceous, the supercontinent Pangea continued fragmenting, a process begun in the Jurassic. This led to increasing isolation of continental landmasses, such as Gondwana separating into Africa, South America, India, Antarctica, and Australia. The widening Atlantic Ocean created new coastlines and greater geographic isolation, fostering distinct, regional dinosaur faunas.
Alongside these geological shifts, a botanical transformation was underway. Angiosperms, or flowering plants, appeared around 125 million years ago and radiated significantly by 100 million years ago. This emergence created new food sources and exerted evolutionary pressures on herbivorous dinosaurs, changing terrestrial ecosystems.
The Age of Iguanodonts and Armored Plant-Eaters
Early Cretaceous ecosystems saw herbivorous dinosaurs thrive, particularly Iguanodontians. Iguanodon bernissartensis, a prominent example, lived approximately 126 to 122 million years ago in parts of Europe, including Belgium, Germany, England, and Spain. These large, bulky herbivores could move on all fours or rise onto two legs to browse on vegetation. Their hands featured a specialized thumb spike, potentially used for defense or foraging, along with a flexible fifth digit for manipulating food.
Iguanodon possessed a robust chewing mechanism, a transverse power stroke, allowing for efficient grinding of plant matter. Their dental structure, with a thick enamel layer on the outer surface of the crown and a robust primary ridge, enabled effective breakdown of diverse vegetation, including gymnosperms and newly evolving angiosperms.
Armored dinosaurs, such as Nodosaurids, also thrived during this period. Unlike their later Ankylosaurid relatives, Nodosaurids lacked a tail club but were heavily protected by large bony plates and spines covering their backs, shoulders, and necks. Sauropelta, an Early Cretaceous Nodosaurid, exemplifies this group with its significant shoulder spikes that diminished in size down its body. Known from North America, Europe, China, and Antarctica, these heavily armored forms relied on their defensive plating against predators.
Early Ceratopsians, represented by genera like Psittacosaurus, emerged in the Early Cretaceous, particularly across Asia between 125 and 105 million years ago. While smaller and bipedal, lacking the large frills and prominent facial horns of their later descendants like Triceratops, Psittacosaurus possessed a distinct parrot-like beak and flared cheekbones. Abundant and exceptionally preserved Psittacosaurus fossils, including soft tissue impressions and gut contents, provide insights into the primitive features and early evolution of horned dinosaurs.
A New Cast of Carnivores
The Early Cretaceous also saw the rise of large predatory theropods, shifting the apex predator roles previously held by groups like Allosaurs in the Jurassic. Carcharodontosaurids became prominent, with species found across Gondwana, North America, Europe, and potentially Asia. These carnivores included some of the largest land predators, such as Acrocanthosaurus from North America, which could rival the size of later Tyrannosaurus. Their large, ornamented skulls were armed with long, fang-like teeth adapted for tearing flesh. These predators filled the top predatory niche in their ecosystems.
Spinosaurids, with their semi-aquatic adaptations, also diversified during this time. Baryonyx walkeri, discovered in England and dating to approximately 130-125 million years ago, is a prime example. Baryonyx possessed an elongated, narrow, crocodile-like snout with numerous conical teeth, well-suited for catching fish. Evidence from stomach contents, including fish scales, and oxygen isotope analysis of their bones, resembling modern semi-aquatic reptiles, supports their piscivorous diet and preference for fluvial and lacustrine environments. Their robust forelimbs and a large claw on the first finger suggest adaptations for grasping prey, allowing them to exploit both aquatic and terrestrial food sources.
Dromaeosaurids, popularly known as “raptors,” also made a significant appearance in the Early Cretaceous. Utahraptor ostrommaysi, from Utah, United States, lived around 139 to 135 million years ago and was one of the largest members of this family. Measuring 6–7 meters (20–23 feet) long and weighing around 500 kilograms (1,100 pounds), Utahraptor was a heavily built, bipedal carnivore. A distinctive feature was the large, retractable sickle-shaped claw on its second toe, which could reach up to 24 centimeters (9.4 inches) in length, used for immobilizing and dismembering prey. Its discovery challenged earlier perceptions that dromaeosaurids were exclusively small and lived only in the Late Cretaceous.
The Feathered Revolution
The Early Cretaceous saw a shift in understanding dinosaur appearance, due to fossil discoveries from sites like China’s Yixian Formation (129.7 to 122.1 million years ago). This formation is known for its well-preserved fossils, including numerous feathered dinosaurs. Its preservation conditions, often involving fine lacustrine sediments and volcanic ash, captured soft tissues and integumentary structures in detail.
These finds revealed that feathers were not exclusive to bird-like theropods but were widespread across various dinosaur groups. For instance, Sinosauropteryx, a compsognathid, was among the earliest non-avian dinosaurs found with feather-like filaments, which covered its body in a fur-like coat. Even large tyrannosauroids like Yutyrannus, reaching up to 9 meters (30 feet) in length and weighing 1.4 metric tons (1.5 short tons), were discovered with extensive feathery coverings, suggesting insulation against a temperate climate with seasonal chills. These discoveries from the Early Cretaceous altered scientific understanding of dinosaur appearance and solidified the evolutionary link to modern birds.