Ammonites and Septarian Nodules


At the end of September 2018, an open-air paleontological exhibition «Late Cretaceous Ammonites of Sakhalin» opened in the Sakhalin Regional Museum. 19 large fossils are exhibited on the territory adjacent to the Museum building including the fossilized shells of extinct cephalopod mollusks — ammonoids (ammonites) of the family PACHYDISCIDAE and a nodule with a «tortoise shell». All fossils have been found by museum specialists during fieldwork expedition in Sakhalin, in the field of development of Late Cretaceous sediments of the Campanian Stage. The approximate age of deposits containing fossils is estimated at 8472 million years.

Location of the new exposition

General view of the complex

Concretions with «tortoise shell» structure, in the foreground

How are the ammonites formed?
The exhibition presents ammonites fossils precisely the shells of mollusks filled with marine sediment when they were buried in the sediments of the Mesozoic Sea. When they were alive, the ammonoid cephalopods had external planispiral shells with several spirals coiled in a single horizontal plane and the diameter increases away from the axis of coiling adjoining or overlapping each other to some degree. Such shells are called monomorphic. The ammonoid shell was divided into a series of several air chambers, the chamber closest to the mouth is called living chamber. The length of the living chamber can be from 0.5 to 2 spirals. Judging by the close relatives of the ammonoids like the modern nautilus, most of the chambers were filled with gas (probably nitrogen) and several with liquid (hydrostatic chambers). By pumping in or out water from the chambers through a siphon (a tube with porous walls connecting the chambers), these mollusks could float or sink into the water column like a bathyscaph.

Internal structure of ammonite shell

The septa between the ammonite chambers has a complex frilled edge, with which it joins to the inner wall of the shell — the suture line. Its structure is one of the main systematic features of ammonites, according to which ammonite can be attributed to one or another taxon. A complex suture line shows that ammonites adopted to move vertically in the water column, since a complex suture line allows more surface area for septa attachment between septal joints, and, therefore, strengthens the shell better.

How does ammonite differ from ammonoidea?
Ammonoids are extinct Mesozoic marine organisms, cephalopods that lived in shells. They had large eyes, a mouth with a rattle — radula and many short tentacles. Most of the ammonoids species were predators and planktonophagous, some led the benthic lifestyle, eating organic debris — detritus.
Ammonites are fossil ammonoid shells, partially retaining their lifetime structure. Sometimes shells can be completely or partially dissolved, but retain their original shape as fossil (fossilized) sediment filled the internal cavity of the shell. Often, they are re-mineralized with calcite, pyrite, or chalcedony while being in the rock mass.

What size ammonites are?
Adult ammonoids could have a wide variety of sizes, depending on the biological type of animal, and sometimes on the sex. The female form of an ammonite could be larger (macroconch) than males of the same species (microconch). The micro- and macroconchs species in some groups of ammonites could evolve at very different speeds. There is nothing surprising in the different rates of evolution of males and females of ammonites: animals, the size of which differed substantially, were affected by various environmental factors differently.
The smallest adult ammonoid species could have a shell just a few millimeters in diameter. The largest known ammonite shells reached 3 meters in diameter. The Late Cretaceous sediments in Sakhalin are distinguished by the content of large ammonites, therefore it is quite possible to find such a giant in our island.
So far the life span of ammonoids has been assessed by some researchers ambiguously, that is from 12 years to 1520 years — by analogy with modern relatives — nautilus.

Reconstruction of the ammonoid

Scientific importance
Ammonites are an extremely important group of marine fossils for geology. The ammonoids species developed great variability over time and their rapid wide dispersal from the region of origin has determined the fact that ammonites are considered to be extremely important guiding forms stratotypes for determining the age of layers of sediments.
Ammonite shell is considered to be a symbol of infinity in the culture of many nations of the world.
In Ireland they were called «petrified snakes», in Germany — «golden snails», in Japan — «stone chrysanthemums». Shamans and fetish-men used ammonites to communicate with the «other» world and to enhance foresight. The Greeks, like the Egyptians, put the ammonite shell at the head at night and believed that they would see a good dream. Pliny the Elder in the 1st century AD named the fossils of these organisms as ammonis cornua («the horns of Ammon») — in honor of the ancient Egyptian sun god Amun of Thebes, depicted with twisted horns of a ram, which resembles an ammonite shell.
Sakhalin archaeologists have repeatedly found ammonite shells at the sites of Neolithic people. Obviously, even then they attracted attention because of their unusualness and mystery.

Where can ammonites and other fossil organisms be found?
Ammonites, like many other fossil of extinct organisms, are usually found contained in concretions or in a form altered by the forces of nature such as gravity, water, air and sun.

Concretion containing ammonoid

Concretions in Cretaceous outcrops

Concretions are sometimes found in sedimentary layers exposed along river banks and the sea, on steep slopes or in quarries. Often, tracks of fossil organisms can be found outside of concretions, in fine-grained rocks, where they are beautifully preserved.
The geological age of the host rocks also matters. For example, ammonites are no longer found in the Cenozoic rocks since the ammonoids finally became extinct at roughly the same time as the dinosaurs and many other organisms disappeared that is on the boundary between the Mesozoic and Cenozoic.

Concretion containing ammonite

How do concretions appear?
Concretions (septarian nodule, nodules) are mineral formations occurring around inclusions (growth seed centers) in fine-grained porous sedimentary rocks. Growth seed can be any mineral or organic object that has fallen into the sedimentary deposits of a reservoir, in the same way as a speck of dust which initiates a violent crystallization around itself when it falls in a supersaturated solution. Usually concretions are found in the layers of marine sediments.

Scattered nodules up to 50 cm in diameter. Ishigualasto Natural Park, Argentina

The formation of sedimentary rock (sandstone, siltstone) from sand or silt involves several stages: from suspension in water, through loose watery mass, which loses water when submerged to a depth under the newly formed layers of sediment, to a gel («jelly») being under pressure, which, upon reaching a certain physicochemical state, is cemented (lithified) into solid rock. All this time, the processes of dissolution and redistribution of substances are taking place in the sediment.
When substance reaches a colloidal state around organic or mineral inclusion-seed, the new mineral microcrystalline aggregates begins to form, more often it is calcium carbonate. A colloidal proto-concretion is formed around the inclusion, followed by crystallization. At the same time both the substance of sediment and the substance of inclusion-seed are used for the growth of new minerals. Therefore, often the fossil organisms enclosed in the nodule, are partially or even completely dissolved. And the latter is more common in nature, because over millions of years the sedimentary sequence undergoes many more transformations. As a result, when we encounter a concretion on the surface, most often it is empty, that is, the initial center, the seed of concretion growth, has already disappeared. This is especially evident in septarian nodule («tortoise-shaped concretion»).

«Turtle septarian nodule»

Such concretions, although they are exotic, are quite common on Sakhalin. These concretions are characterized by the presence of numerous «shrinkage» cracks, which are formed under certain conditions when the volume of the concretion decreases at the stage of its transformation from a semi-liquid (gel-like) state to a solid (consolidated) state. Cracks destroy the inclusion (for example, a shell or a bone), fresh solutions coming from the surrounding sediment, finally dissolve the residue of inclusion. But from these carbonate or siliceous solutions that fill the cracks, the minerals eventually crystallize such as aragonite or chalcedony, sometimes pyrite. As a result, if a concretion is opened (cut) properly, an amazing mineral formations can be observed

Cut and polished septarian nodule

How do concretions appear? In Sakhalin, concretions can be found up to five and more meters in size and diameter.

You can learn more about ammonites, concretions and other geological phenomena by joining a guided tour around the exhibition «History of the Geological Development of Sakhalin and the Kuril Islands».

For information, call: 727555.

Prepared by: A.V. Solovyov, Researcher



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Yuzhno-Sakhalinsk, Russia, 693010
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