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The Carboniferous Period: When Giant Insects Ruled the Land and Sky

Lisa has studied natural history for over 15 years and is a fossil collector as well. She loves sharing her knowledge with others.

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The Carboniferous Period

Over 70 million years before the first dinosaurs and pterosaurs appeared on Earth, a far different group of animals reigned supreme. Insects reached their largest sizes during the Carboniferous Period (359-299 million years ago), with some flying species growing as large as a present-day hawk and one type of flightless arthropod growing even larger than an average-sized adult human!

The air was hot and humid during this period, and vast swampy rainforests covered the land, making it the perfect environment for these animals. They were able to diversify greatly, competing with amphibians that were forming their own distinct groups. Despite the strong presence of amphibians at this time, arthropods were able to colonize the land in many successful forms, from cat-sized scorpions to large proto-cockroaches.

This article will teach you about some of the largest known arthropods of the Carboniferous Period, including:

  • Meganeura monyi
  • Anthropleura armata
  • Pulmonoscorpius kirktonensis
  • Mazothairos enormus

Meganeura Monyi

Meganeura monyi is known as one of the largest flying insects of the Carboniferous Period. It had a wingspan of up to 75 centimeters (about 2.5 feet) and lived between 305 and 299 million years ago in what is now Europe. Meganeura monyi closely resembled a present-day dragonfly in appearance and was a distant relative. It was best suited to open habitats rather than heavily forested areas. Similar to today's dragonflies, this insect had excellent visual acuity with large compound eyes that were adapted for long-distance sight.

Scientists have deduced that the insect was a predator which fed on other insects and likely amphibians as well. Meganeura monyi's legs had strong spines that would have allowed it to snatch up prey in the air and hold them in place as it fed. Based on fossil evidence of its very close relative Meganeurites gracilipes, it can be assumed that Meganeura monyi's mouth also possessed powerful mandibles with large, sharp teeth for crushing and slicing.

Meganeura monyi

Meganeura monyi

Fossil specimen of Meganeura monyi

Fossil specimen of Meganeura monyi

Arthropleura Armata

Arthropleura armata was the largest land invertebrate to have ever lived on Earth. It was a species of giant millipede that lived in Scotland and North America around 320 million years ago. While not a true insect, it was part of the arthropod phylum along with Meganeura. Arthropleura armata was known to reach an impressive 2.5 meters (over 8 feet) in length and 50 centimeters (over 1.5 feet) in width, but some incomplete fossils that have been discovered suggest that it may have grown even larger. It had a flat, armor-plated body consisting of about 30 jointed segments and 60 legs (one pair of legs per body segment).

Scientists initially believed that this arthropod was a predator with strong mouthparts designed for cutting into prey. However, more fossil evidence that has been collected over the years actually points to Arthropleura armata being a vegetarian instead, feeding on plant matter as it scurried across the forest floor. Although many fossils of the species have been discovered, not a single specimen had a preserved mouth, leading scientists to conclude that it lacked strong mouthparts and was not a carnivore. In addition, fossils of this animal's gut and feces have been found to contain plant material such as fern spores and lycopod fragments.

Arthropleura armata

Arthropleura armata

Size comparison between an average adult Arthropleura and a 6.5 ft tall human

Size comparison between an average adult Arthropleura and a 6.5 ft tall human

Pulmonoscorpius Kirktonensis

Pulmonoscorpius kirktonensis was a large scorpion species that lived between 336 and 326 million years ago. Its fossils have been discovered at the East Kirkton Quarry in Scotland and its body design was similar to that of a present-day scorpion. Some complete specimens were 28 centimeters (close to 1 foot) in length, but a large fragmentary specimen is believed to have been a frightening 71 centimeters (over 2 feet) in length when alive. Pulmonoscorpius kirktonensis had four eyes, a pair of median eyes which were likely used for visual acuity and spatial differentiation, and a pair of lateral eyes which probably served mainly as light detectors.

Scientists believe that this arthropod was carnivorous, feeding on smaller arthropods, amphibians, and early reptiles. It likely used its front claws and tail in a manner similar to today's scorpions, clamping onto prey with its claws and injecting them with venom from the stinger on its tail before feeding on them.

Pulmonoscorpius kirktonensis

Pulmonoscorpius kirktonensis

Mazothairos Enormis

Mazothairos enormis was a giant flying insect that lived 309 million years ago in what is now North America. It was close in size to Meganeura monyi, with a wingspan estimated at 56 centimeters (almost 2 feet) based on fragmentary fossil remains. Fragments of this species have been found at the Mazon Creek fossil beds located in Illinois.

Mazothairos enormis is believed to have been an herbivore that fed on plant juices. This can be assumed due to the fact that its closest relatives possessed beak-like mouthparts with long, sharp stylets and possibly a pump-like organ for piercing through plants and sucking out the juices. Scientists also believe that Mazothairos enormis had a pair of small winglets located in front of its larger pair of wings, another characteristic of its closest relatives.

Mazothairos enormis

Mazothairos enormis

Other Notable Arthropods

Flying Insects

Earth was home to other large arthropods during the Carboniferous Period, including the flying insects Bojophlebia prokopi and Bohemiatupus elegans. Bojophlebia prokopi was a large species of mayfly with a wingspan of 45 centimeters (almost 1.5 feet). Its fossil fragments have been found in late Carboniferous rocks from Moravia, Czech Republic. Bohemiatupus elegans was a species of griffinfly from the same family as Meganeura monyi. It had a wingspan of over 50 centimeters (over 1.5 feet) and was similar in appearance to Meganeura monyi. Its fossils were discovered near the village of Radnice in western Bohemia (Czech Republic).

Proto-Cockroaches

The Carboniferous Period is also known for its Blattoptera. Blattoptera were cockroach-like insects that first appeared during this period and were capable of growing twice the size of some modern cockroaches. They had a similar build to today's roaches, possessing wings, a flat body, long antennae, a large, disc-like pronotum ("shield") that covered most of the head, and legs that were designed for running. It is likely that they shared habits similar to our roaches, probably scurrying across the forest floor in search of dead plant and animal matter.

The largest complete fossil of a proto-cockroach measures 8.9 centimeters (3.5 inches) in length and was found in a coal mine in eastern Ohio. Although this may seem small, it is still significantly larger than an adult American cockroach, which only averages between 3.6 and 4.1 centimeters (1.4 and 1.6 inches) in length. The fossil has not been assigned to a specific species within the Blattoptera group yet, but it is well preserved and scientists have been able to confirm that it is indeed classified correctly as a proto-cockroach.

Largest complete fossil of a proto-cockroach, measuring 3.5 inches in length

Largest complete fossil of a proto-cockroach, measuring 3.5 inches in length

Another fossil proto-cockroach, this one having preserved legs and antennae

Another fossil proto-cockroach, this one having preserved legs and antennae

What Caused These Animals to Grow so Large?

Scientists are not completely certain as to how and why insects grew to such large sizes during the Carboniferous Period. Below are some possible theories, and it is likely that a combination of different factors resulted in their large sizes.

1. Significantly high oxygen levels coupled with a dense atmosphere

Rainforests covered much of the Earth's landscape during the Carboniferous Period. These forests consisted of huge 30 meter (almost 100 feet) tall scale trees called Lepidodendrales and a wide variety of other plants, including Cycadophyta (cycads), Filicales (ferns), Equisetales (horse-tails), and Lycopodiales (club mosses). The large number of trees and plants released so much oxygen into the air that it increased to 30%, which is significantly higher than today's atmospheric oxygen amount of 21%. Many scientists believe that the high amount of oxygen enabled insects and other arthropods to grow very large. Unlike land vertebrates such as ourselves, insects lack a pair of lungs and instead must utilize a series of openings across their bodies that connect to the tissues in need of oxygen. This type of respiratory system puts an upper limit on insect body size. However, high atmospheric oxygen levels would have easily helped meet the metabolic needs of larger insects.

2. Larvae grew large to avoid oxygen poisoning

Some scientists believe that aquatic insect larvae grew large in order to protect themselves from the high amount of oxygen. Higher amounts of atmospheric oxygen would also lead to higher amounts of dissolved oxygen in the water. The aquatic larvae of today's insects have little to no control over how much oxygen they absorb due to the fact that they absorb it directly through their skin. Ancient larvae likely functioned the same way as modern larvae and probably were not capable of controlling their oxygen intake well either. These ancient larvae may have grown large to avoid oxygen poisoning, as larger larvae would absorb lower amounts of oxygen relative to their body sizes.

3. Lack of predators

During the Carboniferous Period, insects were the only animals that were capable of flight. Birds had not yet evolved and the reptile species that existed at the time were primitive and land-bound. This lack of agile aerial predators meant that flying insects could grow large without becoming easy targets. There also may have been no need for them to be small and elusive since terrestrial vertebrates like early amphibians were fairly slow and likely easy to avoid.

4. Size race between predators and prey

It is possible that insects and other arthropods grew large as a means of competing with one another as well. Predatory insects such as Meganeura may have grown large in order to take down large prey, and herbivores such as Mazothairos may have grown large in an attempt to ward off carnivores like Meganeura. Some flightless arthropods possibly evolved into giants to avoid becoming food for the numerous amphibians and reptiles that inhabited the land. Carnivorous arthropods may have also grown large to become apex predators and have an advantage over their competition.

A depiction of the Earth's environment during the Carboniferous Period

A depiction of the Earth's environment during the Carboniferous Period

The Carboniferous Rainforest Collapse

About 305 million years ago, Earth experienced a minor extinction event involving the collapse of the tropical rainforests. Atmospheric carbon dioxide levels began decreasing significantly during the Carboniferous Period. Cooling during the middle Carboniferous (around 320 million years ago) caused average global temperatures to drop from 20 °C (68 °F) to around 12 °C (54 °F). As a result, glaciation started to increase and sea levels dropped.

The rainforests did not fare well in cooler, drier conditions. They began to shrink into isolated patches that grew further and further apart from one another. Later, a period of global warming occurred and wiped out the remaining rainforests, which were unable to adapt to the rapidly changing conditions. This rainforest collapse left behind extensive desert regions within the continent's interior. The death of the rainforests also meant death for many of their inhabitants, such as the giant terrestrial arthropods and amphibians.

Many scientists believe these giant arthropods died out because of a decrease in atmospheric oxygen from the rainforest collapse. There was no longer a sufficient amount of oxygen to power their large bodies, so they began to shrink in size. One problem with the oxygen theory is that a giant griffinfly named Meganeuropsis permiana thrived during the Permian Period (299-252 million years ago) when oxygen levels were lower. This insect was almost as large as its close relative Meganeura monyi, possessing a wingspan of 69 centimeters (over 2 feet). However, griffinflies such as Meganeuropsis permiana and Meganeura monyi had very thin, light-weight bodies that may not have required as much oxygen as other large arthropods. This could explain why giant griffinflies were able to survive into the next period while the rest of the arthropods shrank.

Whatever the reason may be for the disappearance of large land arthropods, scientists can at least deduce that the rainforest collapse was a contributing factor in their demise. With the disappearance of the giant arthropods and many amphibians came the emergence of larger, more advanced reptile species that were well adapted to drier conditions. At no other point in Earth's history have giant arthropods populated the landscape, and many researchers believe it is highly unlikely they ever will again.

Quiz Time! Let's See What You've Learned!

For each question, choose the best answer. The answer key is below.

  1. What kind of animal was Arthropleura?
    • A spider
    • A griffinfly
    • A millipede
    • A scorpion
  2. What is a possible reason for super-sized insects during the Carboniferous Period?
    • High levels of oxygen in the atmosphere
    • Nitrates in the soil
    • Large concentrations of carbon dioxide in the air
    • Holes in the insects' bodies that were used for respiration
  3. How long was the largest Pulmonoscorpius specimen estimated to have been when alive?
    • Over 8.5 feet
    • 4 feet
    • 12 feet
    • Over 2 feet
  4. Which arthropod was from the same family as Meganeura monyi?
    • Megarachne
    • Mazothairos
    • Bohemiatupus
    • Bojophlebia
  5. Which of the following is known as the world's largest land invertebrate of all time?
    • Arthropleura
    • Bohemiatupus
    • Pulmonoscorpius
    • Meganeura

Answer Key

  1. A millipede
  2. High levels of oxygen in the atmosphere
  3. Over 2 feet
  4. Bohemiatupus
  5. Arthropleura

© 2021 Lisa Pizzoferrato

Comments

Lisa Pizzoferrato (author) from Columbus, OH on May 27, 2021:

Thanks Jodah, I'm glad you enjoyed reading! Yes, I think Pulmonoscorpius would scare me the most!

John Hansen from Gondwana Land on May 24, 2021:

Great article, very interesting. I am glad I wasn't around in the Carboniferous Period as I am not fond of most insects. Thanks for sharing this information.

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