Exoskeletons are external skeletons used for support and defense by many animal species, including arthropods and mollusks. They first appeared in the fossil record 550 million years ago with Cloudina, and their evolution allowed for larger body sizes and protection against predators. Exoskeletons are more common than endoskeletons, with millions of species having them compared to only a few thousand with endoskeletons.
In biology, an exoskeleton refers to the chitinous or calcified external skeleton used by a number of animal taxa for structural support and defense against predators. Exoskeletons can be contrasted with the endoskeletons (internal skeletons) that humans and other vertebrates have. In the animal world, exoskeletons are much more common than endoskeletons: millions of species have exoskeletons, while only a few thousand have endoskeletons. Eighteen lineages are thought to have evolved from calcified exoskeletons alone, with others evolving chitinous and other types of exoskeletons. Exoskeletons are especially popular among arthropods and molluscs, two of the largest animal phyla in existence.
The exoskeleton first appears in the fossil record very early, about 550 million years ago, when small tube-shaped animals called Cloudina appear in the fossil record. Paleontologists don’t quite agree on what Cloudina actually was, but the current popular hypothesis is that it was a polychaete, a marine annelid. Cloudina is the first of the small shell fauna, numerous carbonate-shelled animals that evolved at the very beginning of the Cambrian 545 million years ago. The appearance of the small shell fauna marks the beginning of the Cambrian period.
An exoskeleton has several benefits for the organism or lineage that evolves it: First, it provides protection. It is apparently one of the easiest defense mechanisms to evolve and probably emerged in the early days of animal predation. Even the earliest exoskeletons in the fossil record appear to have holes, indicative of predation. Many of the first animals to develop an exoskeleton were apparently molluscs. Kimberella, a mollusk-like creature that existed a whopping 555 million years ago, had a hard shell, but it wasn’t mineralized, making it not a true exoskeleton. The mineralized exoskeletons would appear in large numbers shortly thereafter.
In addition to protecting against predation, an exoskeleton provides structural support for an animal. In some cases, this allows them a larger maximum size than they would otherwise be able to achieve. For example, Dunkelosteus, a 6 m (20 ft) fish considered one of the most fearsome marine hunters of all time, was a placoderm, an animal whose large size was partially made possible by the heavy armor that covered its head. Although vertebrates are generally larger than invertebrates (in part because they dominate the land), medium-sized invertebrates with exoskeletons are generally better off than those without, as evidenced by the success of the largest phyla in the animal kingdom, the arthropods.
Protect your devices with Threat Protection by NordVPN
