Larson, Neal L. (1997). Ammonites and the Other Cephalopods of the Pierre Seaway: Identification Guide. Geoscience Press. ISBN 978-0-945005-25-4. The chambered part of the ammonite shell is called a phragmocone. It contains a series of progressively larger chambers, called camerae (sing. camera) that are divided by thin walls called septa (sing. septum). Only the last and largest chamber, the body chamber, was occupied by the living animal at any given moment. As it grew, it added newer and larger chambers to the open end of the coil. Where the outer whorl of an ammonite shell largely covers the preceding whorls, the specimen is said to be involute (e.g., Anahoplites). Where it does not cover those preceding, the specimen is said to be evolute (e.g., Dactylioceras). Crinoids were thought to be extinct until 2021. NOAA Ocean Exploration and Research How do living fossils form?

Brachiopods are shelly marine animals with long, fleshy stalks that live in burrows on the seafloor. They act as reef-dwelling organisms, filter-feeding from the water around them. Brachiopods living today, such as Lingula, look more or less the same as their Cambrian counterparts from about 500 million years ago! They are considered the oldest known animal (genus) that still contains living representatives. Eldridge, Niles; Stanley, Steven M., eds. (1984). "Tragulids as Living Fossils". Living Fossils. Casebooks in Earth Sciences. pp.87–94. doi: 10.1007/978-1-4613-8271-3_9.However, it is estimated that over 10,000 species of ammonite - possibly even over 20,000- have been discovered. Preserved remains become fossils if they reach an age of about 10,000 years. Fossils can come from the Archaeaean Eon (which began almost four billion years ago) all the way up to the Holocene Epoch (which continues today). The fossilized teeth of wooly mammoths are some of our most "recent" fossils. Some of the oldest fossils are those of ancient algae that lived in the ocean more than three billion years ago.

So far this year, 42 new dinosaur species have been discovered, according to the University of Maryland’s Tom Holtz, who maintains a database of new dinosaur finds. What has sustained this pace? For one, Holtz says, “it’s more people doing the work: more eyes on the ground, more teams, more parts of the world being investigated.” Dinosaur paleontology is a more diverse and more global discipline than ever before—with huge benefits to science. This one takes the atavistic cake. After an extraordinary proliferation in earlier seas, totaling some 10,000 species throughout history, only seven species of nautilus made it into the modern era. But they made it with a basic blueprint that has weathered well over 500 million years and five mass extinctions. Their logarithmically spiraling shell is a magnificent instance of the Fibonacci sequence in nature . Beyond that, it holds the key to their fascinating mode of transportation.There are many forms of aptychus, varying in shape and the sculpture of the inner and outer surfaces, but because they are so rarely found in position within the shell of the ammonite it is often unclear to which species of ammonite one kind of aptychus belongs. A number of aptychi have been given their own genus and even species names independent of their unknown owners' genus and species, pending future discovery of verified occurrences within ammonite shells. That fossil, described in April, is the second dinosaur from Japan that lived during the Maastricthian age, which lasted from 72 million until 66 million years ago—right up until the asteroid-driven extinction event at the end of the Cretaceous period. The dinosaur, Yamatosaurus izanagii, is named for an ancient term for part of the Japanese archipelago, as well as Izanagi, a deity in Japanese mythology. See also: List of ammonite genera An ammonite shell viewed in section, revealing the internal chambers and septa. Large polished examples are prized for both their aesthetic and scientific value.