Arthropods make up 80% of living species and have a 600 million year history. Extinct arthropods have been discovered as fossils, including the famous trilobite and sea scorpions. Arthropods were the first terrestrial animals to leave fossils and evolved alongside plants. Larger arthropods evolved during the Carboniferous period, including dragonflies and millipedes. Their size is limited by the amount of oxygen they can absorb.
Arthropods are a huge animal phylum that includes crustaceans, arachnids, myriapods (millipedes and centipedes), insects and their close relatives. With over a million species described, arthropods make up 80% of all living species. If aliens visited Earth and were required to take a sample of a “typical land animal,” they would take an arthropod.
While there are millions of arthropod species alive today, there are many millions more of extinct arthropods that went extinct during the 600 million year history of multicellular life. Paleontologists have unearthed a few tens of thousands of these extinct arthropods as fossils and have done their best to analyze exactly what they looked like and how they lived. A tremendous amount of knowledge has been discovered and we now know of many extinct arthropod species that have not been alive for hundreds of millions of years.
Arthropods are one of many phyla that emerged around the Cambrian-Precambrian boundary, 542 million years ago. Indeed, a growing body of evidence shows that arthropods may have emerged even earlier, in the Ediacaran Period, 555 million years ago or more. Diplichnites and Parvancorina are two early arthropod candidates. Arthropods are thought to have detached from velvet worms (onychophorans) in the Late Precambrian. Some of these early segmented forms of extinct arthropods appear as primitive trilobites.
The most famous of all extinct arthropods is surely the trilobite, the shield-shaped arthropod that was very successful in the Cambrian before going into deep decline. Trilobites may have been surpassed by early fish, which were smarter and had faster metabolisms. Trilobite fossils are so numerous that you can buy them for a few dollars on eBay and other fossil sources.
Another early arthropod were the eurypterids, sea scorpions that emerged during the Cambrian and continued to exist for the rest of the Paleozoic, giving rise to land scorpions before becoming extinct. Some of these sea scorpions, such as Jaekelopterus, reached 2.5m in length, larger than a man.
Arthropods were the first purely terrestrial land animals to leave behind fossils, as evidenced by the discovery of a 428-million-year-old fossil millipede, Pneumodesmus newmani. Within 10 million years, millipedes also appear in the fossil record. These animals likely evolved from aquatic versions that ate sponges and small crustaceans.
When plants colonized the earth, arthropods went with them. More than 50 million years passed before fish developed legs and started walking on land, becoming amphibians and preying on arthropods. Around the same time, about 375 million years ago, larger arthropods began to evolve, whose bodies were made possible by the high levels of oxygen in Earth’s atmosphere. Since arthropods lack a closed circulatory system, their size is limited by the oxygen they can passively absorb.
During this period, called the Carboniferous because of all the carbon deposits left by the forests of the time, there were relatives of dragonflies, such as Meganeura with a wingspan of two feet, and a relative of millipedes, Arthropleura, which reached about 2.5 m (8 feet) in length, tying with ancient sea scorpions for the title of largest known arthropod.
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