Science, Tech, Math › Science A Fossil Picture Gallery Share Flipboard Email Print Alice Cahill / Getty Images Science Geology Types Of Rocks Landforms and Geologic Features Geologic Processes Plate Tectonics Chemistry Biology Physics Astronomy Weather & Climate By Andrew Alden Geology Expert B.A., Earth Sciences, University of New Hampshire Andrew Alden is a geologist based in Oakland, California. He works as a research guide for the U.S. Geological Survey. our editorial process Andrew Alden Updated January 21, 2020 Fossils, in the geological sense, are ancient, mineralized plants, animals, and features that are the remains of an earlier geological time period. They may have been petrified but are still recognizable, as you can tell from this gallery of fossil pictures. Ammonoids Ammonoids ruled the seas for 300 million years. ThoughtCo / Andrew Alden Ammonoids were a very successful order of sea creatures (Ammonoidea) among the cephalopods, related to the octopuses, squids, and nautilus. Paleontologists are careful to distinguish the ammonoids from the ammonites. Ammonoids lived from Early Devonian times until the end of the Cretaceous Period, or from about 400 million to 66 million years ago. Ammonites were a suborder of ammonoid with heavy, ornamented shells that thrived starting in the Jurassic Period, between 200 and 150 million years ago. Ammonoids have a coiled, chambered shell that lies flat, unlike gastropod shells. The animal lived at the end of the shell in the largest chamber. The ammonites grew as large as over three feet across. In the wide, warm seas of the Jurassic and Cretaceous, ammonites diversified into many different species, largely distinguished by the intricate shapes of the suture between their shell chambers. It is suggested that this ornamentation served as an aid to mating with the right species. That would not help the organism survive, but by ensuring reproduction it would keep the species alive. All the ammonoids died at the end of the Cretaceous in the same mass extinction that killed off the dinosaurs. Bivalves The classic shellfish dates from Cambrian times. ThoughtCo / Andrew Alden Bivalves, classified among the mollusks, are common fossils in all rocks of Phanerozoic age. Bivalves belong in the class Bivalvia in the phylum Mollusca. "Valve" refers to the shell, thus bivalves have two shells, but so do some other mollusks. In bivalves, the two shells are right-handed and left-handed, mirrors of each other, and each shell is asymmetrical. (The other two-shelled mollusks, the brachiopods, have two unmatching valves, each one symmetrical.) Bivalves are among the oldest hard fossils, showing up in Early Cambrian times more than 500 million years ago. It is believed that a permanent change in the ocean or atmospheric chemistry made it possible for organisms to secrete hard shells of calcium carbonate. This fossil clam is young, from the Pliocene or Pleistocene rocks of central California. Still, it looks just like its oldest ancestors. For lots more detail on the bivalves, see this lab exercise from SUNY Cortland. Brachiopods They look like bivalves but are quite different. ThoughtCo / Andrew Alden Brachiopods (BRACK-yo-pods) are an ancient line of shellfish, first appearing in the earliest Cambrian rocks, that once ruled the seafloors. After the Permian extinction nearly wiped out the brachiopods 250 million years ago, the bivalves gained supremacy, and today the brachiopods are restricted to cold and deep places. Brachiopod shells are quite different from bivalve shells, and the living creatures within are very different. Both shells can be cut into two identical halves that mirror each other. Whereas the mirror plane in bivalves cuts between the two shells, the plane in brachiopods cuts each shell in half—it's vertical in these pictures. A different way to look at it is that bivalves have left and right shells while brachiopods have top and bottom shells. Another important difference is that the living brachiopod typically is attached to a fleshy stalk or pedicle coming out of the hinge end, whereas bivalves have a siphon or a foot (or both) coming out the sides. The strongly crimped shape of this specimen, which is 1.6 inches wide, marks it as a spiriferidine brachiopod. The groove in the middle of the one shell is called a sulcus and the matching ridge on the other is called a fold. Learn about brachiopods in this lab exercise from SUNY Cortland. Cold Seep ThoughtCo / Andrew Alden A cold seep is a place on the seafloor where organic-rich fluids leak from the sediments below. Cold seeps nurture specialized microorganisms that live on sulfides and hydrocarbons in the anaerobic environment, and other species make a living with their help. Cold seeps make up part of a global network of seafloor oases along with black smokers and whale falls. Cold seeps have only recently been recognized in the fossil record. California's Panoche Hills has the largest set of fossil cold seeps found in the world so far. These lumps of carbonates and sulfides have probably been seen and ignored by geologic mappers in many areas of sedimentary rocks. This fossil cold seep is of early Paleocene age, about 65 million years old. It has an outer shell of gypsum, visible around the left base. Its core is a jumbled mass of carbonate rock containing fossils of tubeworms, bivalves, and gastropods. Modern cold seeps are very much the same. Concretions NNehring / Getty Images Concretions are the most common false fossils. They arise from the mineralization of sediment, although some may have fossils inside. Coral (Colonial) ThoughtCo / Andrew Alden Coral is a mineral framework built by immobile sea animals. Colonial coral fossils can resemble reptile skin. Colonial coral fossils are found in most Phanerozoic (541 million years ago) rocks. Coral (Solitary or Rugose) ThoughtCo / Andrew Alden Rugose or solitary corals were abundant in the Paleozoic Era but are now extinct. They're also called horn corals. Corals are a very old group of organisms, originating in the Cambrian Period more than 500 million years ago. The rugose corals are common in rocks from Ordovician through Permian age. These particular horn corals come from the Middle Devonian (397 to 385 million years ago) limestones of the Skaneateles Formation, in the classic geologic sections of the Finger Lakes country of upstate New York. These horn corals were collected at Skaneateles Lake, near Syracuse, early in the 20th century by Lily Buchholz. She lived to the age of 100, but these are some 3 million times older than she was. Crinoids ThoughtCo / Andrew Alden Crinoids are stalked animals that resemble flowers, hence their common name of sea lily. Stem segments like these are especially common in late Paleozoic rocks. Crinoids date from the earliest Ordovician, about 500 million years ago, and a few species still inhabit today's oceans and are cultivated in aquaria by advanced hobbyists. The heyday of crinoids was Carboniferous and Permian times (the Mississippian subperiod of the Carboniferous is sometimes called the Age of Crinoids), and whole beds of limestone may be composed of their fossils. But the great Permian-Triassic extinction nearly wiped them out. Dinosaur Bone ThoughtCo / Andrew Alden Dinosaur bone was just like the bones of reptiles and birds: a hard shell around a spongy, rigid marrow. This polished slab of dinosaur bone, shown about three times life-size, exposes the marrow segment, called trabecular or cancellous bone. Where it came from is uncertain. Bones have a lot of fat inside them and a lot of phosphorus too—today whale skeletons on the seafloor attract lively communities of organisms that persist for decades. Presumably, marine dinosaurs held this same role during their heyday. Dinosaur bones are known to attract uranium minerals. Dinosaur Eggs ThoughtCo / Andrew Alden Dinosaur eggs are known from about 200 sites around the world, the majority in Asia and mostly in terrestrial (nonmarine) rocks of Cretaceous age. Technically speaking, dinosaur eggs are trace fossils, the category that also includes fossil footprints. Very rarely, fossil embryos are preserved inside dinosaur eggs. Another piece of information derived from dinosaur eggs is their arrangement in nests—sometimes they are laid out in spirals, sometimes in heaps, sometimes they are found alone. We don't always know what species of dinosaur an egg belongs to. Dinosaur eggs are assigned to paraspecies, similar to the classifications of animal tracks, pollen grains or phytoliths. This gives us a convenient way to talk about them without trying to assign them to a particular "parent" animal. These dinosaur eggs, like most on the market today, come from China, where thousands have been excavated. It may be that dinosaur eggs date from the Cretaceous because thick calcite eggshells evolved during the Cretaceous (145 to 66 million years ago). Most dinosaur eggs have one of two forms of eggshell that are distinct from the shells of related modern animal groups, such as turtles or birds. However, some dinosaur eggs closely resemble bird eggs, particularly the type of eggshells in ostrich eggs. A good technical introduction to the subject is presented on the University of Bristol "Palaeofiles" site. Dung Fossils ThoughtCo / Andrew Alden Animal dung, like this mammoth turd, is an important trace fossil that yields information about diets in ancient times. Fecal fossils may be petrified, like the Mesozoic dinosaur coprolites found in any rock shop, or merely ancient specimens recovered from caves or permafrost. We may be able to deduce an animal's diet from its teeth and jaws and relatives, but if we want direct evidence, only actual samples from the animal's guts can furnish it. Fish ThoughtCo / Andrew Alden Fishes of the modern type, with bony skeletons, date from about 415 million years ago. These Eocene (approximately 50 million years ago) specimens are from the Green River Formation. These fossils of the fish species Knightia are common items at any rock show or mineral shop. Fish like these, and other species like insects and plant leaves, are preserved by the millions in the creamy shale of the Green River Formation in Wyoming, Utah, and Colorado. This rock unit consists of deposits that once lay at the bottom of three large, warm lakes during the Eocene Epoch (56 to 34 million years ago). Most of the northernmost lake beds, from the former Fossil Lake, are preserved in Fossil Butte National Monument, but private quarries exist where you can dig your own. Localities like the Green River Formation, where fossils are preserved in extraordinary numbers and detail, are known as lagerstätten. The study of how organic remains become fossils is known as taphonomy. Foraminifers Comstock Images / Getty Images Foraminifers are the tiny one-celled version of mollusks. Geologists tend to call them "forams" to save time. Foraminifers (fora-MIN-ifers) are protists belonging to the order Foraminiferida, in the Alveolate lineage of the eukaryotes (cells with nuclei). Forams make skeletons for themselves, either external shells or internal tests, out of various materials (organic material, foreign particles or calcium carbonate). Some forams live floating in the water (planktonic) and others live on the bottom sediment (benthic). This particular species, Elphidium granti, is a benthic foram (and this is the type specimen of the species). To give you an idea of its size, the scale bar at the bottom of this electron micrograph is one-tenth of a millimeter. Forams are a very important group of indicator fossils because they occupy rocks from Cambrian age to the modern environment, covering more than 500 million years of geologic time. And because the various foram species live in very particular environments, fossil forams are strong clues to the environments of ancient times—deep or shallow waters, warm or cold places, and so on. Oil drilling operations typically have a paleontologist nearby, ready to look at the forams under the microscope. That's how important they are for dating and characterizing rocks. Gastropods ThoughtCo / Andrew Alden Gastropod fossils are known from Early Cambrian rocks more than 500 million years old, like most other orders of shelled animals. Gastropods are the most successful class of mollusks if you go by a number of species. Gastropod shells consist of one piece that grows in a coiled pattern, the organism moving into larger chambers in the shell as it becomes larger. Land snails are also gastropods. These tiny freshwater snail shells occur in the recent Shavers Well Formation in southern California. Horse Tooth Fossil ThoughtCo / Andrew Alden Horse teeth are hard to recognize if you've never looked a horse in the mouth. But rock-shop specimens like this are clearly labeled. This tooth, about twice life-size, is from a hypsodont horse that once galloped over grassy plains in what is now South Carolina on the American east coast during Miocene times (25 to 5 million years ago). Hypsodont teeth grow continuously for several years, as the horse grazes on tough grasses that wear its teeth down. As a consequence, they can be a record of environmental conditions over the course of their existence, much like tree rings. New research is capitalizing on that to learn more about the seasonal climate of the Miocene Epoch. Insect in Amber ThoughtCo / Andrew Alden Insects are so perishable that they are rarely fossilized, but tree sap, another perishable substance, is known for capturing them. Amber is fossilized tree resin, known in rocks from recent times back to the Carboniferous Period more than 300 million years ago. However, most amber is found in rocks younger than Jurassic (about 140 million years old). Major deposits occur on the southern and eastern shores of the Baltic Sea and the Dominican Republic, and this is where most rock-shop and jewelry specimens come from. Many other places have amber, including New Jersey and Arkansas, northern Russia, Lebanon, Sicily, Myanmar, and Colombia. Exciting fossils are being reported in Cambay amber from western India. Amber is considered a sign of ancient tropical forests. Like a miniature version of the tar pits of La Brea, resin traps various creatures and objects in it before becoming amber. This piece of amber contains a fairly complete fossil insect. Despite what you saw in the movie "Jurassic Park," extracting DNA from amber fossils is not routinely, or even occasionally successful. So although amber specimens contain some amazing fossils, they are not good examples of pristine preservation. Insects were the first creatures to take to the air, and their rare fossils date back to the Devonian, about 400 million years ago. The first winged insects arose with the first forests, which would make their association with amber even more intimate. Mammoth ThoughtCo / Andrew Alden The woolly mammoth (Mammuthus primigenius) until recently lived throughout the tundra regions of Eurasia and North America. Woolly mammoths followed the advances and retreats of the late Ice Age glaciers, thus their fossils are found over quite a large area and are commonly found in excavations. Early human artists depicted living mammoths on their cave walls and presumably elsewhere. Woolly mammoths were as large as the modern elephant, with the addition of thick fur and a layer of fat that helped them endure the cold. The skull held four massive molar teeth, one on each side of the upper and lower jaw. With these, the woolly mammoth could chew the dry grasses of the periglacial plains, and its huge, curving tusks were useful in clearing snow off the vegetation. Woolly mammoths had few natural enemies—humans were one of them—but those combined with rapid climate change drove the species to extinction just at the end of the Pleistocene Epoch, about 10,000 years ago. Recently a dwarf species of mammoth was found to have survived on Wrangel Island, off the Siberian coast, until less than 4,000 years ago. Mastodons are a slightly more ancient type of animal related to mammoths. They were adapted to life in shrublands and forests, like the modern elephant. Packrat Midden drferry / Getty Images Packrats, sloths and other species have left their ancient nests in sheltered desert places. These ancient remains are valuable in paleoclimate research. Various species of packrats live in the world's deserts, relying on plant matter for their entire intake of water as well as food. They gather vegetation in their dens, sprinkling the stack with their thick, concentrated urine. Over the centuries these packrat middens accumulate into rock-hard blocks, and when the climate changes the site is abandoned. Ground sloths and other mammals are also known to create middens. Like dung fossils, middens are trace fossils. Packrat middens are found in the Great Basin, of Nevada and adjoining states, that are tens of thousands of years old. They are examples of pristine preservation, precious records of everything that local packrats found interesting in the late Pleistocene, which in turn tells us much about the climate and ecosystem in places where little else remains from those times. Because every bit of the packrat midden is derived from plant matter, isotopic analyses of urine crystals can read the record of ancient rainwater. In particular, the isotope chlorine-36 in rain and snow is produced in the upper atmosphere by cosmic radiation; thus packrat urine reveals conditions far above the weather. Petrified Wood and Fossil Trees ThoughtCo / Andrew Alden Woody tissue is a great invention of the plant kingdom, and from its origin almost 400 million years ago to today, it has a familiar look. This fossil stump at Gilboa, New York, of Devonian age, testifies to the world's first forest. Just like the phosphate-based bone tissue of vertebrate animals, durable wood made modern life and ecosystems possible. Wood has endured through the fossil record to today. It can be found in terrestrial rocks where forests grew or in marine rocks, in which floating logs can be preserved. Root Casts Fossils of grass roots show the direction of up. ThoughtCo / Andrew Alden Fossil root casts show where sedimentation paused and plant life took root. The sediments of this terrestrial sandstone were laid down by the swift waters of the ancient Tuolumne River in central California. Sometimes the river laid down thick sandy beds; other times it eroded into earlier deposits. Sometimes the sediment was left alone for a year or more. The dark streaks cutting across the bedding direction are where grasses or other vegetation took root in the river sand. The organic matter in the roots remained behind or attracted iron minerals to leave the dark root casts. The actual soil surfaces above them, however, were eroded away. The direction of root casts is a strong indicator of up and down in this rock: clearly, it was built up in the rightward direction. The amount and distribution of fossil root casts are clues to the ancient riverbed environment. The roots may have formed during a relatively dry period, or perhaps the river channel wandered away for a while in the process called avulsion. Compiling clues like these over a wide region allows a geologist to study paleoenvironments. Shark Teeth ThoughtCo / Andrew Alden Shark teeth, like sharks, have been around for more than 400 million years. Their teeth are almost the only fossils they leave behind. Shark skeletons are made of cartilage, the same stuff that stiffens your nose and ears, rather than bone. But their teeth are made of the harder phosphate compound that makes up our own teeth and bones. Sharks leave a lot of teeth because unlike most other animals they grow new ones throughout their lives. The teeth on the left are modern specimens from the beaches of South Carolina. The teeth on the right are fossils collected in Maryland, laid down at a time when sea level was higher and much of the eastern seaboard was underwater. Geologically speaking they're very young, perhaps from the Pleistocene or Pliocene. Even in the short time since they were preserved, the mix of species has changed. Note that the fossil teeth are not petrified. They're unchanged from the time the sharks dropped them. An object doesn't need to be petrified to be considered a fossil, merely preserved. In petrified fossils, the substance from the living thing is replaced, sometimes molecule for molecule, by mineral matter such as calcite, pyrite, silica, or clay. Stromatolite ThoughtCo / Andrew Alden Stromatolites are structures built by cyanobacteria (blue-green algae) in quiet waters. Stromatolites in real life are mounds. During high tides or storms, they become covered with sediment, then grow a new layer of bacteria on top. When stromatolites are fossilized, erosion uncovers them in a flat cross-section like this. Stromatolites are rather rare today, but at various ages, in the past, they were very common. This stromatolite is part of a classic exposure of Late Cambrian-age rocks (the Hoyt Limestone) near Saratoga Springs in upstate New York, approximately 500 million years old. The locality is called Lester Park and is administered by the state museum. Just down the road is another exposure on private land, formerly an attraction called Petrified Sea Gardens. Stromatolites were first noted at this locality in 1825 and formally described by James Hall in 1847. It may be misleading to think of stromatolites as organisms. Geologists actually refer to them as a sedimentary structure. Trilobite Danita Delimont / Getty Images Trilobites lived throughout the Paleozoic Era (550 to 250 million years ago) and inhabited every continent. A primitive member of the arthropod family, trilobites went extinct in the great Permian-Triassic mass extinction. Most of them lived on the sea floor, grazing in the mud or hunting smaller creatures there. Trilobites are named for their three-lobed body form, consisting of a central or axial lobe and symmetrical pleural lobes on either side. In this trilobite, the front end is on the right, where its head or cephalon ("SEF-a-lon") is. The segmented middle part is called the thorax, and the rounded tailpiece is the pygidium ("pih-JID-ium"). They had many small legs underneath, like the modern sowbug or pillbug (which is an isopod). They were the first animal to evolve eyes, which looks superficially like the compound eyes of modern insects. Tubeworm ThoughtCo / Andrew Alden A Cretaceous tubeworm fossil looks just like its modern counterpart and attests to the same environment. Tubeworms are primitive animals that live in the mud, absorbing sulfides through their flower-shaped heads that are converted into food by colonies of chemical-eating bacteria inside them. The tube is the only hard part that survives to become a fossil. It's a tough shell of chitin, the same material that makes up crab shells and the outer skeletons of insects. On the right is a modern tubeworm tube; the fossil tubeworm on the left is embedded in shale that was once seafloor mud. The fossil is of latest Cretaceous age, about 66 million years old. Tubeworms today are found in and near seafloor vents of both the hot and cold variety, where dissolved hydrogen sulfide and carbon dioxide supply the worm's chemotrophic bacteria with the raw material they need for life. The fossil is a sign that a similar environment existed during the Cretaceous. In fact, it is one of many bits of evidence that a large field of cold seeps was in the sea where California's Panoche Hills is today.