Fossil remains indicate the creation of the world. Living fossils Who needs prehistoric fossils buried in stone?
Most of us think that when the Earth was formed, life appeared in the seas. This is partly true, but no one knows exactly how the first life appeared. And having appeared, life immediately began to influence the surface of the planet. Without the plants that crush rocks into sediment, for example, there would not be enough materials to form tectonic plates and therefore continents. Without plants, the Earth could become just a water world.
Believe it or not, more complex life may even change the structure of global ice ages, making them less severe, with the help of " ". The discontinuous pattern of freezing and thawing goes back billions of years to a time when Earth did not have the complex web of life that exists today. Then the glaciers stretched from the poles to the equator, disrupting the entire planetary foundation.
Since then, as more and more life has filled the surface and seas, the glacial Earth has formed huge glaciers at both poles, stretching out several fingers in terms of latitudes that never reach the equator.
542 million years ago something mysterious happened on Earth
Experts call the sudden increase in the diversity and richness of Earth's fossil record, which began 542 million years ago, the "Cambrian explosion." He puzzled Charles Darwin. Why did all the ancestors of modern animals appear literally overnight, in a geological sense?
One expert opinion is that there was life before the Cambrian period, but it didn't have any hard parts. The scientists analyzed soft-bodied Precambrian fossils, some of which have no connection whatsoever with any form of modern life today, as well as young Cambrian soft-bodied fossils from Canada. It turned out that at least 50 million years before the Cambrian “explosion,” multicellular life developed. Scientists don't understand where the hard parts came from, but perhaps a genetic mutation caused a cascading effect that led to the sudden development of shells and skeletons. However, not everyone agrees with this theory. There is still no exact answer to the question of what happened to life on Earth 542 million years ago.
The first land plants may have caused a mass extinction
During the Devonian period, which was 150 million years after the Cambrian, it was good to be born a fish at the top of the food chain. Apart from a few stray plants and animals exploring the land, all life lived in the sea. After tens of millions of years, everyone came out of the sea onto land, where tall forests of ferns, mosses and mushrooms appeared.
And then the sea creatures began to die. At least 70% of all invertebrates in the sea have gradually disappeared. The Devonian extinction was one of the ten largest mass extinctions in Earth's history.
Many experts believe that land plants were to blame. They say the first forests created soil that broke rocks into minerals that eventually flowed into the ocean, causing algae blooms. This algae consumed all the oxygen, and the sea creatures suffocated. Even worse, the algae was then eaten by other organisms and became hydrogen sulfide. It turned sea waters into acid. The plants couldn't escape either. They sucked enough carbon dioxide out of the air to cause an ice age, which wiped out many of them too.
Fortunately, there are a few species left that have survived even these hellish conditions, whether at sea or on land.
Ancient life knew how to adapt
There has never been a complete extinction of species, even when the planet was hit by a massive asteroid. For example, even in the youth of the Earth, the oxygen produced was poisonous to many early life forms. While many oxygen haters died, others adapted and became more sophisticated. Extinctions have happened from time to time, but Jurassic Park's Ian Malcolm was right when he said that life will always find a way to keep going.
According to the fossil record, survival and extinction had a greater influence on demography. If a large group of species were scattered around the world, there was a chance that at least one or two individuals would survive extinction. Other conditions include environmental conditions and genetic factors that make species vulnerable or allow adaptation.
Horseshoe crabs turned out to be the best - they survived four major mass extinctions and countless smaller ones.
Finding Martian fossils changes our understanding of Earth
What is a fossil? At first glance, this is all that has been dug out of the ground, but this approach can be misleading when we are trying to understand ancient life.
At the moment, attention is focused on Mars, since besides Earth, this planet offers the friendliest planetary climate for life. Once upon a time there were even rivers and lakes. If life existed in these ancient waters, fossils may have remained. This raises an obvious question. If we're trying to understand what life was like on Earth 542 million years ago, how do we define 4 billion-year-old Martian remains?
Astrobiologists are working on this, not disdaining the help of paleontologists. Understanding what ancient fossils on Mars might be like allows scientists to sharpen their understanding of what isn't fossils on Earth.
Fossil sites
Most of the fossils we saw probably formed in water. Water is good for creating fossils. The land is not very good. In shallow waters close to the beach, for example, lots of sediment from rivers and streams quickly buries shellfish and other sea creatures, preserving them.
Tropical forest rain can be as heavy and rich as shallow sea shelf water, but it won't produce many fossils. Plants and animals that die in it will quickly decompose due to the moisture. In addition, predators will quickly carry away the corpses, and the rest will be destroyed by wind and rain.
Standing water in low-lying areas such as swamps and lagoons is also suitable because it does not contain much oxygen and does not support many decomposing organisms. In addition, there is also a shift in fossils toward bodies with hard parts, as well as groups of animals and plants that are large, long-lived, and dispersed over a wide geographic area. Time also affects. Geological processes like mountain building and plate subduction tend to wear away fossils, which is why the oldest ones are so hard to find.
Fossils rarely resemble living things
The physical processes after a plant or animal dies are complex and messy. There is a separate field of science that studies these processes. While it certainly helps in many ways, it does not provide a perfect map of the original living being. Some solid fossils, such as insects and carnivorous plants, trapped in amber are the exception, but they are all relatively young. For the most part, only a small part of the organism is preserved. And as far as we know, fossilization only occurs in the hard, tough parts of a plant or animal, so experts must reconstruct animals from a couple of teeth and, if they're lucky, a few bones.
Paleoartists use fossil evidence to reconstruct ancient living things, but they fill in the gaps with details taken from modern descendants of a plant or animal. Often new discoveries confirm reconstructions. Sometimes - more often in the case of feathered dinosaurs - the first reconstructions turn out to be inaccurate.
Not all fossils are petrified
Scientists love to stick to words. A paleontologist describing a 200-million-year-old tree that has turned to stone might call it "mineralized" or "replaced" rather than petrified.
Mineralization occurs because there are empty cavities in the wood. Let's say a tree falls into a lake that contains a lot of dissolved minerals from a nearby volcano that released its ash stuff into the water. These minerals, especially silicates, enter the wood and fill the pores and other cavities, so parts of the wood become encased in the stone and are preserved.
The tree can also be replaced. This is a longer process. Suppose our tree did not fall into the lake when it fell, but went into the soil. Groundwater began to seep in and after a certain geological time, minerals replaced the entire tree, all woody parts, molecule by molecule. All "petrified" trees are good, but paleontologists extract more information from a tree that has undergone molecular replacement than from a mineralized tree.
It turns out that the saber-toothed “tiger” was not the only ancient creature with long teeth. Sabretooths are an example of convergent evolution, where unrelated species independently evolve the same useful function. Sabretooths were useful for all types of predators that had to hunt animals larger than themselves.
There are many other examples of convergent evolution. Modern giraffes, for example, are not related to dinosaurs, but have the same long necks as brachiosaurs and other dinosaurs. The long-extinct mammal Castorocauda looked and behaved similar to the modern beaver, although the two species are unrelated.
One of the strangest cases of convergent evolution involves us. Koalas have fingerprints that look just like ours, even though they are marsupials (they have pouches on their bellies) and we are placentals (our unborn young feed through the placenta). Scientists believe koalas may have developed tiny curls on their toes to make it easier for them to climb trees, just as we did in the past.
Ancient Animals Live and Thrive Today
It often happens that some strange species of animal or plant, which everyone already thought had disappeared, turns out to be alive and well. We think of them as relics, not suspecting that there are still many ancient organisms on Earth that have undergone virtually no changes.
As we have already noted, horseshoe crabs have survived many mass extinctions. But they are not the only ones. The same cyanobacteria that once killed off much life on Earth by starving them of oxygen billions of years ago are also alive and well. also show themselves perfectly as ancient life. For example, rove beetles date back to the Triassic period (more than 200 million years ago). Today, this family of beetles probably contains the largest number of living organisms in the world. And their ancestors were probably familiar with Triassic water bugs, like those that sometimes appear in ponds and scare people.
Most amazingly, some species of sulfur-producing anaerobic bacteria, which were among the first living organisms on Earth, live with us today. Moreover, these are one of those microbes that inhabit our digestive tract. Fortunately for us, the Earth's atmosphere has been improving significantly over the years. Or most of them, at least.
If someone is lucky enough to find fossilized shells on the beach, it is not difficult to recognize them. But there are also many fossils, looking at which it is difficult to guess what they were. Compounding the problem, many of the fossils are incomplete or poorly preserved. Sometimes even scientists have doubts. Our review of 10 fossils that went unrecognized for many decades.
1. Ammonites
Fossilized ammonites are still quite common today, but for thousands of years they were mistaken for anything other than mollusks. The ancient Greeks believed that these were ram's horns, and named the ammonites in honor of the Egyptian god Amun, who was depicted with approximately the same horns. The ancient Chinese called them horn stones for a similar reason. In Nepal, fossilized ammonites were considered a shrine left by the god Vishnu. The Vikings considered them the sacred fossilized offspring of the world serpent Jormungard.
In the Middle Ages, ammonites were known in Europe as snake stones because they were believed to be the petrified bodies of coiled snakes that were petrified by Christian saints. Today it has become known that ammonites are just the fossilized shells of creatures that became extinct about four hundred million years ago.
2. Fish teeth
Fossil fish teeth were considered different objects in different centuries. Some ancient fish species had flat molars for crushing shellfish. In Greece, and later in much of Europe, the fossilized remains of such teeth were considered magical stones, and were often called toad stones. Such teeth were used in jewelry, and it was also believed that they could be used to cure epilepsy and poisoning. In Japan, the fossilized flat and sharp teeth of sharks were considered the claws of the terrible monster tengu, in Europe the teeth were considered the tongue of the devil.
3. Trees
Lepidodendron is an ancient tree whose bark was covered with large, flat scales, like a pine cone. The leaves of this tree themselves were like stems, so lepidodendron is considered more of a grass than a tree. Most of the coal deposits in Europe are the remains of these ancient plants. Previously, entire fossilized trunks of lepidodendrons were often found; the length of such a trunk could be up to thirty meters, and the thickness - about a meter. In the 19th century, they were passed off as the bodies of snakes and dragons.
4. Foraminifera
Quite unusual grains of sand can be found on the Pacific beaches in southern Japan. Many of them are shaped like tiny stars, less than a millimeter in diameter. Local legends claim that these are the remains of unfortunate children from the heavenly union of two stars. These star children died either from falling to the ground or were killed by a monstrous serpent living in the sea near the Japanese island of Okinawa. In fact, these tiny stars are the remains of the spiny shells of another life form: amoeba-like creatures called foraminifera.
5. Protoceratops
Dinosaurs called Protoceratops were relatives of the more famous Triceratops. They walked on four legs and were about the size of a large dog, although much heavier. Most protoceratopsians had a large skull with a bird-like beak and a bony frill growing from the back of the skull. To people unfamiliar with dinosaurs, the preserved skeletons of Protoceratops resembled fantastic and bizarre creatures. Because of their size, these dinosaurs were thought to be small lions with a hooked beak like an eagle. It is possible that Protoceratops is the prototype of the mythical griffins.
6. Belemnites
Belemnites were ancient animals that resembled squids. Unlike squids, they had a skeleton, and all ten of their tentacles were the same length, and they were covered with tiny hooks. Belemnites lived at the same time as dinosaurs, inhabiting the seas. The most common fossilized parts of belemnite skeletons are those that look like long bullets. In Europe, people thought that these fossils were the thunder arrows of the gods that fell to the earth. Other people thought that belemnites belonged to the elves rather than the gods, believing them to be the fingers of the elves, fairy candles, or the arrows of the elves.
7. Anchisaurs
Anchisaurs were one of the earliest dinosaur species. They were herbivores with long necks and tails, and were early relatives of the more famous brontosaurus and diplodocus. Only, unlike them, the size of the anchisaurs was only 2 m. Paradoxically, the bones of these dinosaurs were initially mistaken for the bones of a primitive human ancestor.
8. Mastodons and mammoths
Just a few thousand years ago, giant mammoths and mastodons roamed the icy land. They resembled hairy elephants with huge tusks. Like modern elephants, these animals had very developed strong trunks, which is why the skeletal structure of these animals suggested a large hole in the skull. People who had never seen an elephant assumed that these huge fossilized skulls with a giant hole in the front belonged to Cyclopes, the mythical giant one-eyed anthropoids.
9. Sea urchins
Sea urchins are spiny, spherical creatures that are commonly found along seashores. Sea urchins have existed for hundreds of millions of years, and their ancient ancestors left behind many fossils. In England, such fossils were mistaken for supernatural crowns, loaves of bread, or magical snake eggs. In Denmark they were considered thunder stones because they supposedly released moisture before strong storms.
10. Hominids
The ancestors of modern humans left behind many fossils throughout the earth. Because of their apparent inconsistency with human bones, such fossils were often considered evidence of various anthropoid mythical creatures mentioned in the Bible, such as giants and demons. In other cultures, the discovered skeletons of Neanderthals gave rise to legends about the Yeti and other hominid creatures.
Cretaceous deposits in our region appeared thanks to the Riga glacier, which brought them from their original locations thousands of years ago. Residents of Grodno have been using chalk for a very long time, mainly in construction and in crafts: it was added to mortar, used to whiten the walls of buildings, etched animal skins, and added to the mixture during glass melting.
Nowadays, chalk is an integral material in the construction industry. In the Grodno region there are several places with large chalk deposits where open-pit mining takes place, but the largest are quarries in the village of Krasnoselsky and quarries near Grodno.
Chalk - sedimentary rock of organic (zoogenic) origin. This means that chalk was formed millions of years ago from the compressed remains of marine organisms.Often in chalk there are largelarge Cretaceous fossils (the last period of the Mesozoic era,145 - 66 million years ago), which we will talk about.
Fossilized remains of a bivalve shell. Photo by the author (Mieczyslaw Supron)
The most extensive and, accordingly, richest in finds are the Krasnoselsky quarries.
View of the main quarry in Krasnoselsk. Photo: Alexander Belyay
At different times, a large number of fossil remains of the Cretaceous period were found here. Perhaps the most important find is the remains of a fossilized fish. The exhibit is currently stored in the museum at the KrasnoselskStoymaterials enterprise.
Fossilized fish in the Krasnoselsk museum.Photo: Alexander Belyay
Near Grodno, chalk is currently being mined in a quarry near the village of Zarechanka. The quarries between Pyshki and Lososno have been mined out and abandoned for several years now. Quarry No. 1 in Pyshki. Currently flooded:
On the banks of this quarry, an interesting specimen of belemnite rostrum from the genus Belemnitella, frozen into silicon, was found:
Belemnite in silicon. Photo by the author
Quarry No. 2 in Pyshki is also flooded. However, we found fossil remains here too.
You just need to look more closely at the cliffs of the chalk “rocks”...
And voila! A beautiful piece of belemnite sticks out right above the cliff.
In addition, here we found the remains of a fish, unfortunately poorly preserved:
The bones and shells of fish are clearly visible against the chalk background. Photo by the author.
The most famous Grodno quarries are “Sinka” and “Zelyonka” - now lakes that got their names because of the color of the water.
"Blue". Photo by the author.
In the process of searching. Nikita Pinchuk's lens captured the author of these lines at "work"
Over the course of several years, during the forays of a group of amateurs, many fossilized remains of animals from the Cretaceous period were found. Perhaps the most interesting finds are shark teeth.
Shark teeth from the Cretaceous period from Morocco (1-5 on the left), a shark tooth found on Sinka (2nd on the right) and a shark vertebra found in Krasnoselsk (1st on the right). Photo from the collection of Alexander Belay
The most common find at Sinka is belemnites.
Belemnite. Photo by the author
The belemnite shell is covered with bites. This belemnite was probably eaten by a shark (belemnites were the favorite delicacy of these sea predators). Photo from the collection of Alexander Belay.
After belemnites, the most common fossilized shells in the shape of a cylindrical tube that can be found here are baculites, the closest relatives of ammonites. On "Sinka" there are baculites of gigantic sizes - up to 1 m (!). Sometimes in chalk rock these shells are preserved in mother-of-pearl, then the shell shimmers in the light with all the colors of the rainbow.
Baculitis. Photo from the collection of Nikita Pinchuk
If you arrange the baculite into chambers, you will get this interesting mural. Photo from the author's collection
Double shell. Photo from the author's collection
Fossilized shells.Photo from the author's collection
Double shell.Photo from the collection of Alexander Belay
Sinks. Photo from the author's collection
Brachiopods.Photo from the author's collection
Shell valve.Photo from the author's collection
Sea urchins are not a common find. Sometimes you come across the spines of sea urchins, but separately from the shells.
Sea urchin.Photo from the author's collection
Sea urchins from the collection of Alexander Belay
The shell of a sea urchin consists of plates. Photo from the author's collection
N orca annelid on the shell of a hedgehog. Photo from the author's collection
Sink.Photo from the author's collection
Shells of nautiluses (cephalopods) are rare. Often their shells crumble in the hands, so only a few are able to take them with them:
Nautilus shells.Photo from the author's collection
Attention! Please do not try again to find fossils in the quarries mentioned above, as this may endanger your health and even your life. The walls of the quarries are extremely unstable, in many places they threaten to collapse, and steep cliffs create a high risk of collapse.
Everyone knows from childhood or youth, or rather heard and remembers, that life on Earth originated 3.5 billion years ago. Huge number isn't it? I don’t know about you, but to me it is perceived almost the same as the infinity of space. Yes, yes, I don’t perceive values close to infinity :). Even in my youth, I tried to imagine the infinity of the universe, and in order to understand and realize something, I definitely need to imagine it, and so, since then, my consciousness has refused to fully comprehend the “billionth” and other dubiously constant things. And every time I hear 285, or 400 million years ago, my consciousness generalizes this to a long time ago in ancient times. All this pile of zeros is not perceived at all, and you don’t even think about them, clinging only to the first three digits, or even just passing by as an unnecessary fact. And yet there are times when you wonder about it all. What's all this for? Of course, many of you know, Samarians certainly know, that the Zhiguli, I mean the Zhiguli Mountains, are made of limestone rocks. They were formed millions of years ago, at the bottom of ancient seas, from marine sediments, during the Carboniferous and Permian periods of the Paleozoic era. And the phrase that you read above seems like a dry fact about the past of our planet until you come across such an artifact.
And then all this information that you once heard or read and until that moment, somewhere dormant in the labyrinths of memory, suddenly gathers into a single bundle and, as if acquiring energy, rolls over you in a wave. And the lack of information forces you to rummage through articles in search of answers to emerging questions. And the Zhiguli Mountains themselves become interesting for you not only for their relief, natural beauty, gorgeous views, but also for the information that the layers of rock from which they are composed carry, page after page revealing their history to you, taking you millions of years into the past, telling a story about a world that not a single representative of the human race has ever seen.
It's hard to imagine now. But 300 million years ago, the waters of an ancient sea roared here, filling the trough of the East European Platform; it was connected in the north with the Arctic and the Tethys Ocean in the south. What we see now was formed over millions of years and owes its appearance to living organisms that lived in ancient seas; countless shells of dead mollusks, corals, and bryozoans formed colossal deposits of limestone. Of course, all of them are not completely preserved, but are fragmented and changed by subsequent processes. But sometimes you can find quite clearly preserved forms. For example, in the limestones of the Zhiguli Mountains, fossils of fusulinids are often found, as if fossilized grains scattered by someone, they protrude from the rock.
Fusulinids, an order of extinct foraminifera whose fusiform shells, from which they get their name (fusus - spindle), are twisted in a spiral and divided into chambers by partitions. Fusulinids are bottom dwellers found only in sediments of the Carboniferous and Permian periods of the Paleozoic era.
It’s not always easy to spot a fossil in a rock; sometimes you have to look closely and only then do you see an alien from the past frozen in stone, like this four-rayed rugosa coral.
Rugosa are single polyps with an external limestone skeleton; their remains are very often found here in the Zhiguli and Sokoli mountains. They had a horn-like shape, some had a lid that closed the mouth in case of danger. Having increased requirements for temperature and water transparency, they lived in shallow water, usually in the shelf zone of the sea, attaching themselves to the seabed with the sharp end of the cone.
Along with the fusulinids, they became extinct at the end of the Permian period, during the most massive extinction event in the entire history of the Earth. Then, 96% of marine species of organisms and 70% of terrestrial vertebrates died, and this was the only known mass extinction of insects (about 57% of genera and 83% of species of the entire class), after which it took about 30 million years to restore the biosphere.
Here's another copy of my fossil photo collection. This is a cross section of a sea lily stem.
Despite its name, the sea lily is not a plant; it is an animal with a sedentary lifestyle, feeding on plankton - foraminifera, small crustaceans, and invertebrate larvae. Fossil crinoids are known from the Lower Ordovician; they reached their greatest prosperity in the Middle Paleozoic, when there were over 5,000 species, most of which became extinct, but some species exist to this day. The body of the animal resembles a cup, standing on a stem-leg in the center of which there is a mouth, and “arms” grow from the cup in different directions, externally resembling a flower.
Another photo trap for me was this fragment of an ammonite shell. Unfortunately, I was not able to find a whole shell.
These cephalopods, distant relatives of modern nautiluses, squids and octopuses, lived in almost all seas and today the fossilized shells of these mollusks can be found in almost any area of the globe. Ammonites ended their existence approximately 65-70 million years ago.
They disappeared along with the dinosaurs, although they appeared much earlier than them.
Well, similar bivalves exist to this day in the seas and rivers.
The sea level changed, the temperature and salinity of the water changed, all this influenced the biosphere of the sea and now this is clearly reflected in the section of sediment layers.
The East European platform rose, and the sea retreated; the last sea, the waters of which rose to our latitudes, was the Akchagyl Sea. It came from the direction of the current Caspian Sea; the Zhiguli Mountains already existed then and rose like an island above the raging waters.
Looking at it layer by layer, as if leafing through the pages of a book, you involuntarily think about how fragile this whole world around us is.
How fragile life itself is and how great is the desire of all living things for life.
For a long time now I have had several pebbles of limestone-shell rock with fossilized imprints of ancient organisms. They were picked up at different times and in different places, I can’t remember now. Some were probably found in a limestone quarry, some were brought to me from the Atarskaya Luki, some, perhaps, brought from the Crimea.
I’ve had them for a long time, I just haven’t gotten around to photographing and describing them. Today the planned walk in the forest was cancelled, I had some free time and I took a few pictures. This is what one of the pebbles looks like. It is small in size, a little more than 3 cm.
What it consists of used to be the remains of living organisms of warm shallow seas that fell onto the muddy bottom. Here you can see pieces of shells of ancient mollusks, imprints of bryozoans and pieces of the stem of crinoids (sea lilies). Let's figure out which one is which.
Bryozoans, especially the order Gymnolaemata is easily recognized by its reticulate structure. These are colonies of marine invertebrate organisms, known since the Ordovician period, and still existing in seas of varying salinity. As the name suggests, the colonies of some bryozoans resemble a continuous cover of moss. Some bryozoans form colonies in the form of crusts and clumps on hard surfaces (rocks, shells, etc.), others have a fan-shaped or bush-like appearance. Modern bryozoans, for example, look like this:
They form the bulk of recognizable fragments on the stone. But don’t forget, bryozoans are not plants, although they look like them, they are full-fledged animals that feed on various microorganisms and diatoms.
Let's look at another stone:
Here, in the same way, the bulk of the fossils are reticulate fragments of bryozoans.
At the bottom in the middle you can see a round piece with notches and a hole in the center (the same “gear” can be found on the right side in the first photo). This is one of the stalk segments sea lily(or crinoids, lat. Crinoidea). These are bottom-dwelling animals with a sedentary lifestyle, belonging to the phylum echinoderms. They are even more similar in appearance to plants - their body consists of a stem, a calyx and brachioles - arms.
Most species of modern crinoids have lost this stalk. During the life of the animal, the stalk consisted of round segments connected by muscles; in the fossil state they often fall apart. Fossilized segments of crinoids are called trochites. Due to their similarity to gears, theories about alien contact millions of years ago constantly arise, and attempts are made to present trochites as ancient parts of alien mechanisms. And they have been known since ancient times; the first written mentions date back to the 17th century. The British called the star-shaped polygonal segments of crinoids “stone stars” and made various assumptions about their connection with celestial bodies. On the Northumberland coast these fossils are called "St Cuthbert's rosary". Whole sea lily prints look like this:
Crinoids (photo by user galamish from Yandex.photos)
Of course, the stone contains a large number of fragments and impressions of shells of various mollusks:
Moreover, they have a completely recognizable shape, characteristic of modern seashells. For example, the shell at the top center of the bottom photo, next to the trochite, is quite similar to a modern scallop.
It’s hard for me to say what kind of long fossil is in the photo below. Maybe a piece of stem, maybe something else.
And just a couple more pictures, try to identify something in them yourself:
Also known and common fossils that you can find, for example, on the banks of rivers are belemnites(popularly called the “devil’s finger”), which are the remains of the fossilized internal shell of ancient mollusks that resemble squid in appearance. Well-preserved mother-of-pearl shells or simply imprints of cephalopod shells are also widely known. ammonites. Their spiral-twisted ribbed shells can range from 1-2 centimeters to 2 meters in diameter.