Spiders are some of the earliest animals to find shelter on land, dating to about 400 million years ago. Biologists theorized that the evolution of spiders occurred about 400 million years ago from what are the big-waisted arachnid ancestors that had just emerged from their aquatic environments. The first true spiders are the thin-waisted arachnids with segmented abdominal parts and the spinnerets that produce the silk, with good examples being the Attercopus fimbriungus. Scientists can know these from the fossil remains. This spider existed 380 million years ago during what is called the Devonian Period – around 150 million years before the first dinosaurs walked on earth.
The evolution of spiders is still ongoing from when it commenced closed to 400 million years ago. The origin is traced to the arachnid subgroup that has the characteristic feature of the book lungs. Spiders overall evolved from the water-dwelling ancestors. Almost 50,000 extant species are known to science and they have been classified into 114 families. Biologists think that there are over 120,000 species. The rates of fossil diversity explain a huge fraction than the extant diversity can suggest with almost 1,600 arachnid species. Entomologists have focused on the study of fossil and extant species.
A good number of the early segmented fossil spiders are categorized under the Mesothelae, which are a group of ancient spiders that possessed spinnerets under the center of their bellies rather than the end like we observe in spiders of today.
There is the probability that these were predators living on the land or the fern forests in the mid-late Palaeozoic. It is thought that they preyed on other ancient arthropods like slaters, cockroaches, millipedes, giant silverfish, and many others. As for the silk, it is believed to serve the purpose of protection for the eggs, a trapdoor, or even as a layer for a retreat hole.
The diversification of the insects and plants is also reflected in the use of silk by spiders. Spiders with spinnerets located at the ends of their abdominal segments were roaming the planet over 250 million years ago. The presumption is that this led to the advancement of more sophisticated webs for the capture of prey on vegetation and the ground.
By the time the dinosaurs appeared in the Jurassic Period around 180 million years ago, there were already complicated webs woven by spiders to trap various airborne insects. There has also been a transformation in the techniques employed by the spiders. But even with all of these, it must be pointed out that the spider fossil record is not very impressive. It was during the Tertiary Period that amber spider fossils were formed. These amber-coated fossils are reputable sources of tremendous information for scientists interested in studying arachnid evolution.
Mesothelid spiders with segmented bodies managed to survive in the eastern flanks of Asia right from the late Palaeozoic times to modern times. These huge spiders inhabit soil burrows complete with trapdoors in caves and forests. These segmented arachnids share many features with their ancestors, and they have not changed much.
The Biggest ‘Spider’ Ever
It is believed that this record is held by a 300-million-year-old fossil arachnid that measured almost 80 cm. it is named Megarachne servinei and it was initially given a description of a spider, but it is believed to share features with another kind of ancient arachnid. The outstanding characteristics include huge jaws, tremendous proportions, and an armor-like covering in the abdominal sections of the body.
Scientists agree that this arachnid must have depended on hunting relatively big prey like giant millipedes or cockroaches. But this has led to a more interesting question – scientists are interested in knowing why the big predator needed such an armored body. Principal developments in the evolution of spiders include secretion of silk or spinnerets development.