Nautiloids are a subclass of cephalopod molluscs with a few modern species, but an important fossil group dating back 515 million years. They were the first cephalopods and top predators during the Paleozoic era, evolving a variety of shell shapes. Nautiloids have shells with numerous internal chambers, and use their tentacles and sharp inner beak to grab and consume food. They have unique lensless eyes that helped scientists understand the evolution of complex organs.
Nautiloids are a subclass of cephalopod molluscs, related to other cephalopods such as squid and octopuses. There are only a few modern species of nautiloids, such as the Chambered Nautilus, but the nautiloids are an extremely important fossil group, with specimens dating back 515 million years, during the Late Cambrian. Nautiloids were the first cephalopods and were one of the first “advanced” animals to appear in the Cambrian, with relatively large brains and nervous systems.
The heyday of nautiloids occurred during the Paleozoic era, about 515 to 251 million years ago. During this time, they were the top predators of the seas and evolved a wide variety of shell shapes. Nautiloids were the first cephalopods, evolving from simpler molluscs and occupying the predatory niche. In just about 50 million years, they grew from a few millimeters in length to 8-meter (26-foot) giants like Cameroceras, thought to be the largest animal on the planet at the time it lived, the Ordovician Period, about 450 million Years ago.
Like most molluscs, nautiloids all have shells, which evolved initially to be straight, then curved. This shell consists of numerous internal chambers, or camerae, separated by walls called septa. These distinct chambers are produced by the animal as it grows. The nautilus always occupies the last of the chambers, called the living chamber, and has a fleshy strand of tissue that passes through the chambers causing a siphon. The siphon helps the nautilus remove water from the chambers of its shell, which allows it to achieve buoyancy by closely matching its internal density to that of the surrounding seawater.
Like other cephalopods, nautiloids grab food using their tentacles and consume it with their sharp inner beak. They see the ocean around them using two lensless eyes that work using principles similar to those of pinhole cameras. These unique eyes are important evolutionary intermediaries between the light-sensitive patches found in animals such as planarians and the more sophisticated lens-based eyes of other animals such as fish. They helped scientists understand how a complex organ like the eye can evolve incrementally, with each intermediate step having an immediate adaptive benefit
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