Cretaceous Insects: Their Diversity, Evolution, and Survival

The Cretaceous period, spanning from approximately 145 to 66 million years ago, is known as the final era of the dinosaurs. This period, however, was also a transformative time for insects, which underwent a remarkable diversification, establishing many of the groups we recognize today. Studying these ancient insects provides a window into the planet’s past ecosystems, revealing a complex world of evolutionary activity. Their development was intricately linked to the changing global environment and the rise of new plant life.

The World of Cretaceous Insects: Environment and Flora

The world Cretaceous insects inhabited was vastly different from our own, characterized by a warmer global climate and higher atmospheric carbon dioxide levels. This greenhouse climate meant there were no polar ice caps, and forests could extend to the poles. The temperature difference between the equator and the poles was roughly half of what it is today, creating widespread tropical and subtropical conditions that supported a vast range of life.

Throughout the early Cretaceous, the landscape was dominated by extensive forests of gymnosperms, such as conifers, cycads, and ferns, continuing a trend from the Jurassic Period. The most significant botanical event of the era, however, was the appearance and rapid diversification of flowering plants, or angiosperms, starting around 125 million years ago. By the late Cretaceous, they had become the dominant form of plant life, creating a wealth of new ecological niches from nectar-rich flowers to new types of foliage.

A Look at Cretaceous Insect Diversity

Fossil evidence from the Cretaceous reveals a world buzzing with familiar kinds of insects, establishing ecological roles that continue to this day. Many of these ancient forms would be recognizable to us, showcasing the deep history of today’s most common insect orders.

Among the most widespread groups were the beetles (Coleoptera), which continued their long evolutionary history. The era is also notable for the rise of Hymenoptera, which includes the earliest known fossils of ants, bees, and various wasps. The first termites and aphids have been identified from Cretaceous fossils, signaling the emergence of new social structures and plant-feeding strategies.

Other major insect orders also thrived, including Lepidoptera (butterflies and moths), flies (Diptera), true bugs (Hemiptera), grasshoppers and crickets (Orthoptera), and the ancient lineage of dragonflies and damselflies (Odonata).

Insect Evolution and Coevolution in the Cretaceous

The rise of flowering plants was a principal catalyst for insect evolution, leading to some of the most classic examples of coevolution. As angiosperms diversified, they offered new food sources in the form of pollen and nectar, prompting insects to develop specialized mouthparts to access them. This reciprocal relationship, where plants evolved to attract pollinators and insects evolved to feed on them, drove the diversification of both groups, which is particularly evident in the evolution of bees and butterflies.

This period also saw a significant radiation of social insects. The first ants and termites appeared, developing complex colony structures that allowed them to exploit resources with great efficiency. Early eusocial bees also emerged, forming cooperative groups that enhanced their ability to gather food and care for their young, a strategy that proved highly successful.

Beyond pollination and sociality, insects adapted to a wide array of other ecological roles. The diversification of parasitic wasps points to the development of intricate host-parasite relationships, and new forms of herbivory appeared as insects evolved to feed on the abundant angiosperm foliage.

Uncovering Cretaceous Insects: Fossil Evidence

Our understanding of Cretaceous insects is primarily built upon two exceptional forms of fossil preservation: amber inclusions and compression fossils. Amber, which is fossilized tree resin, provides unparalleled three-dimensional snapshots of ancient life. Major deposits of Cretaceous amber from locations such as Burma (Myanmar), Lebanon, and Canada have trapped countless insects, preserving them in fine detail. These fossils can reveal an insect’s morphology and, in some cases, even its coloration.

Compression fossils, found in fine-grained sedimentary rocks, offer another view into this ancient world. While two-dimensional, they can preserve the outlines and key features of insects, often in large assemblages that tell us about the composition of ancient insect communities. These fossils are formed when an insect is buried in sediment, which later hardens into rock, leaving an imprint of its body.

This fossil evidence provides direct proof of the behaviors of Cretaceous insects. Scientists have discovered amber pieces containing parasitic wasps attacking their hosts, ants in formation, and insects carrying pollen, allowing us to reconstruct ancient food webs.

Insect Survival Through the End-Cretaceous Extinction

The end of the Cretaceous period was marked by a mass extinction event, triggered by an asteroid impact 66 million years ago. This event caused the extinction of approximately half of the world’s species, including all non-avian dinosaurs. While terrestrial plant life was significantly impacted, insects appear to have fared comparatively well due to several factors:

• Their small size meant they required fewer resources to survive.
• Diverse diets allowed them to adapt to a disrupted food web.
• Many insects are detritivores that feed on dead wood and decaying organic matter, which would have been plentiful after the event.
• Adaptable life cycles with dormant stages, such as pupae, could have helped them wait out the harshest conditions.
• Groups with aquatic larval stages found refuge in freshwater ecosystems from the devastation on land.

While some level of extinction certainly occurred, enough insect lineages survived to repopulate and rediversify in the Cenozoic Era that followed.