The Carboniferous Period, spanning roughly 359 to 299 million years ago, was a time when life on Earth thrived in warm, humid conditions. Giant insects roamed the dense forests, and early forms of spiders also appeared. Many people wonder just how large spiders were during this ancient era. In this topic, we’ll explore the size of spiders in the Carboniferous Period, what types of arachnids existed, and why they grew to such impressive sizes.
The Carboniferous Environment
The environment during the Carboniferous Period was perfect for life to flourish. Thick forests covered much of the land, creating a humid, oxygen-rich atmosphere. Oxygen levels were higher than today, reaching up to 35%. This abundance of oxygen not only supported enormous insects but also allowed arachnids to grow larger than their modern counterparts.
Early Spiders and Arachnids
Not True Spiders Yet
Technically, the creatures that lived during the Carboniferous were not modern spiders as we know them today. Instead, they were early relatives of spiders, including species like Attercopus fimbriunguis. These ancient arachnids shared many characteristics with today’s spiders but also retained features from their more primitive ancestors.
Characteristics of Early Arachnids
Most early arachnids from the Carboniferous had segmented bodies, simple eyes, and could spin silk. However, unlike modern spiders, they did not create webs to catch prey. Silk was likely used for lining burrows or wrapping eggs, offering protection in a dense, competitive environment.
How Big Were Carboniferous Spiders?
Size Estimates
Fossil evidence suggests that early spider-like creatures were relatively large compared to today’s common house spiders. Many species measured around 2 to 4 centimeters (0.8 to 1.6 inches) in body length. With their legs fully extended, they could reach up to 10 centimeters (about 4 inches) across.
While these sizes may not seem gigantic compared to some insects of the time, they were quite large for arachnids. Some Carboniferous arachnids, like Palaeotarbus jerami, were among the largest known and closely resembled modern spiders in shape.
Comparison with Modern Spiders
Today’s largest spiders, such as the Goliath birdeater, have body lengths of up to 12 centimeters (about 5 inches) and leg spans reaching 30 centimeters (about 12 inches). Carboniferous spiders were smaller than these modern giants but still impressive for their time.
Why Weren’t Spiders as Big as Insects?
Several reasons explain why early spiders did not reach the gigantic sizes of Carboniferous insects
1. Different Respiratory Systems
Insects use a network of tubes called tracheae to deliver oxygen directly to their tissues, and higher oxygen levels allowed them to grow larger. Spiders, however, use book lungs or tracheae less efficiently. This difference in respiratory systems may have limited their maximum size.
2. Predation and Competition
The Carboniferous forests were home to many predators, including large amphibians and other arthropods. Being too large could have made spiders more visible and vulnerable to predators, creating natural limits on their growth.
3. Physical Limitations
The exoskeleton of spiders must support their body weight. As size increases, the weight grows faster than the strength of the exoskeleton, placing physical limits on how large they could become.
Other Giant Arachnids of the Carboniferous
Mesothelae The Primitive Spiders
One group of early spiders, the Mesothelae, appeared in the Carboniferous and still exists today in a more primitive form. These spiders had segmented abdomens and lived in burrows. Fossils show that early Mesothelae could grow to notable sizes for spiders of their time.
Giant Scorpions
While true spiders were modest in size, other arachnids like scorpions achieved much greater sizes. Pulmonoscorpius kirktonensis, a giant scorpion from the Carboniferous, could grow up to 70 centimeters (about 28 inches) long, dwarfing the early spiders.
The Role of High Oxygen Levels
High atmospheric oxygen was crucial for the size increase of many arthropods during the Carboniferous. Oxygen helped power the metabolisms of these ancient creatures, enabling more activity and growth. However, the specific breathing systems of spiders meant they could not take full advantage of the abundant oxygen like insects did.
Evolution Toward Modern Spiders
Over millions of years, spiders evolved new adaptations that made them successful predators. The ability to produce stronger silk, create webs, and develop venom glands allowed spiders to diversify and spread across different environments. Though they remained relatively small compared to some other ancient arthropods, their survival strategy proved extremely effective.
Spiders during the Carboniferous Period were larger than most modern spiders but not as enormous as some of the insects and other arthropods that lived alongside them. Fossil evidence suggests body lengths of up to 4 centimeters and leg spans around 10 centimeters. Factors such as respiratory limitations, predation pressure, and physical constraints kept their size in check.
Despite their modest size, early spiders were significant players in their ecosystems. They laid the foundation for the incredible diversity of spider species we see today. Understanding their size and role in the Carboniferous forests gives us a glimpse into the fascinating evolutionary journey of arachnids.