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 84Ц72 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 ammoniteТs 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 1Ц2 years to 15Ц20 years by analogy with modern relatives nautilus.