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Crawling nautiloids

Crawling nautiloids

Here is a variety of cephalopods from sediments 3𣛩25. Artinsky tier bottom Perm in the territory of the modern Southern Urals (Orenburg region and the north of Kazakhstan). These deposits formed about 290 million years ago. The photograph shows the usual spiral-coiled nautilides (3-3-3329. Nautilida 3-3-3192., Three rather large shells on the right in the upper part of the picture, with holes in the middle), 3-3-3183. ammonites 3-3-3192. , straight as sticks of orthocerides ( Orthocerida ). Four unusual shells can be seen in the lower part of the image. nautiloid 3-3r3192. - wide, low, some with a bent back end, generally similar to caps.

Cephalopods are the most active and highly organized animals among invertebrates. Such they could become, thanks to the emergence of a mechanism of neutral buoyancy among their ancestors - it allowed them to break away from the bottom and learn active swimming in the water column. The need to adapt to movement in a complex three-dimensional world full of competitors and predators led to the transformation of cephalopods into 損rimates of the sea, as the famous biologist and popularizer called them. I. A. Akimushkin . In fact, the entire evolution of the cephalopod subclass is a way to continuously improve their ability to swim actively. However, oddly enough, one group of cephalopods in the late 3𣛩41. Paleozoic 3-3-3192. moved in the opposite evolutionary direction and in the end, apparently, completely abandoned the ability to swim.

This group includes unusual shell-caps from the main photo. Such nautiloids are distinguished into two families - Scyphoceratidae and Dentoceratidae. In Scyphoceratidae (you see them at the bottom of the main photo), the shell has the shape of a short and wide bent cap with a very large angle of expansion, because of this the volume of the living chamber of adult mollusks is noticeably larger than the volume of 3-3-345. fragmentocon (part of the shell divided into partitions, filled with gas and responsible for buoyancy). The Soviet paleontologists 3𣛩47 who studied these nautiloids. V.E. Ruzhentsev and V. N. Shimansky 3-3-3107. noted 3-3-3192. that upon reaching adulthood, the scifceratids built such wide living chambers that the air chambers remained with them a kind of appendage.

Crawling nautiloids

Scyphoceras , Artinskian Stage of the Lower Permian of Kazakhstan. Photo Alexander Mironenko

This ratio of residential and air chambers is very unusual for cephalopods, because it is precisely the large volume of the fragomocon filled with gas that ensures the buoyancy of their shells. In addition, there is no ventral sinus on the sinks of scyphoceratids - a special cut on the abdominal part of the mouth, which most nautiloids have and allows the mollusk to bend funnel over the edge of the sink. The ventral sinus is needed even by the not too mobile modern 3-3-3381. Nautilus 3-3-3192. , and its disappearance in scifoceratids may be due to the disappearance or severe weakening of the funnel, the muscular tube through which the mollusk pushes water out of the mantle cavity.

But scyphoceratids are not the most unusual of the nautiloids who lived in the Artinian age on the territory of the Southern Urals. Representatives of the family Dentoceratidae that lived with them generally did not have gas-filled chambers of the fragmentocone (at least at the adult stage of development). Their adult shell consisted of only one elongated living chamber, on the top of which there was a trace from a siphon - a special tube that connected the body of the mollusk with the cameras of the fragmocone. Dentoceratids probably originated from the same scyphoceratids in which the gradual reduction of the fragmentocone led to its final disappearance. True, in youth, dentoceratids probably had a fragmentocone, but then they discarded it (this is called shell transposition; see task 3-3-3385. How to become more compact 3-3-31992.). Externally, dentoceratides are very similar to another mysterious group of mollusks (but not related to cephalopods) - 3-3-3387. Xenoconchia .

Crawling nautiloids

Dentoceratidae, in the upper part there is a trace of a siphon entering the living chamber. Photo from the book of V.E. Ruzhentsev, V.N. Shimansky, 1954. Lower Permian folded and bent nautiloids of the Southern Urals 3-3-3192.

It is interesting that residential chambers of Scyphoceratidae are sometimes found without any traces of a fragmentocon. Although it cannot be ruled out that it was separated after the death of mollusks, for other cephalopods found in the same sediments, such finds are uncharacteristic. So it is quite possible that some Scyphoceratidae at the adult stage of development completely got rid of the fragmocone.

Together, all these signs - a very large volume of the living chamber compared to the volume of the fragmocone (and even more than the loss of the fragmocon) and the absence of the ventral sinus - indicate that scyphoceratids, and especially their descendants of dentoceratide, refused to swim in the water column and moved to the bottom lifestyle - that is, they (at least adults) crawled along the bottom, like gastropods.

Some modern octopuses and cuttlefish also able to move along the bottom surface, they walk on it with the help of tentacles. This allows them to save energy and stealthily approach production. It is possible that scyphoceratids and dentoceratides also moved along the bottom with the help of tentacles. However, bottom octopuses, if necessary, can tear off the bottom at any time and quickly swim away, pushing water out of 3-3-3121. mantle cavity 3-3-3192. through the funnel. And the representatives of Scyphoceratidae, judging by their small fragmentocon volume, and Dentoceratidae, who did not have a neutral buoyancy apparatus at all, were deprived of such an opportunity. The maximum that they could do was get pulled into the sink and press their mouth to the bottom.

The question arises - why did these nautiloideans abandon the swimming ability, most important for cephalopods, and return to the bottom? To answer it, you need to look at family ties and the origin of these unusual nautiloids. Both Scyphoceratidae and Dentoceratidae belong to the suborder Rutocerina (Rutoceratida), which has long been included in the Nautilida order of the subclass Nautiloidea. Modern nautiluses belong to the nautilids, and this is the only nautiloid detachment that has survived to this day. Although modern nautiluses cannot compete in speed and maneuverability with cephalopods having an internal shell, such as squid or octopus, mobility and active swimming have always been very important for representatives of Nautilida. And the appearance among them of forms crawling or walking along the bottom with the help of tentacles, which actually abandoned the neutral buoyancy apparatus, looked extremely strange.

Crawling nautiloids

Residential chambers of Scyphoceratidae from sediments of the Artinsky tier of Kazakhstan. They were found without fragments, they were either discarded during life or lost after the death of mollusks, but before the burial of their shells. Photo Alexander Mironenko

However, in recent decades, 3-3-3149. it turned out that rutoceratins have nothing to do with nautilides. They are the descendants of a completely different detachment from the same subclass of nautiloids - 3-3-3151. Oncocerida . True, paleontologists used to look for the ancestors of nautilids precisely among oncocerids, rutoceratins were considered an intermediate link between these units, 3-3-33199.

but now it became clear that Oncocerida and Nautilida are not related to each other at all. Moreover, representatives of Oncocerida and another detachment close to them. Discosorida always been connected to the bottom. Occasionally, forms that lived in the water column appeared among them, but the vast majority of oncocerids have always been near-bottom inhabitants. Their shells were located vertically in the water, often had a backward bend (so that the cap-like shape was quite usual for them) and swam near the bottom. Some are practiced 3-3-3192. shell translation - its rear end was discarded, either in order to make it less noticeable, or to reduce its buoyancy. There were practically no speed swimmers among them. Thus, it turns out that the transition of Scyphoceratidae and Dentoceratidae to the bottom mode of life is not a strange deviation from the main development path, but only an extreme form of specialization of the already inactive benthic oncocerids. True, in the Early Paleozoic oncocerids the mouth of the adult shell was, as a rule, very narrow, closed (for details, see the picture of the day 3-3-331. Masks of nautiloid 3-3-3192.), And the mouth of the Permian rutoceratin is very wide. This fact, apparently, is associated with some changes either in the lifestyle of these nautiloids, or in their environment.

Crawling nautiloids

The Silurian Oncocerida also had shells with a short fragmentation window and a wide living chamber, though with a narrow mouth. Image from the book of S. Stridsberg, 1985. Silurian oncocerid cephalopods from Gotland

In the first half and in the middle of the Paleozoic, oncocerides, although living near the bottom, retained a small but clearly functional buoyancy apparatus. In Perm, something made them abandon him. What was the reason for such unusual changes? Apparently, the matter was in the development of other cephalopods and in colossal competition in the water column. Numerous 3𣛩183 shells are found in locales of Artinian age in the Southern Urals along with scyphoceratides and dentoceratides. ammonoid 3-3-3192. (more than a dozen different genera), direct-shell Nautiloids from detachments 3-3-3185. Bactritida , Orthocerida and Pseudorthocerida and spiral-rolled nautilide shells. There are numerous remains of bones and teeth of fish. Apparently, the water in these places was boiling with life and sedentary oncocerides and it was more profitable and safer to snuggle to the bottom and completely abandon competition with actively swimming distant relatives - ammonoids and other nautiloids, which, moreover, were predators and could not only compete, but also hunt for sedentary oncocerides. At the same time, the shells of ordinary bottom dwellers - gastropod mollusks - are rare in these deposits - either conditions did not suit them in some way, or the bottom oncocerids replaced them.

To date, Scyphoceratidae and Dentoceratidae are the latest representatives of oncocerids (although due to the confusion with the systematic origin of rutocerin, in many sources you can find information about the extinction of oncocerides back in the middle of the Carboniferous ). Unfortunately, the final transition of oncocerides to bottom lifestyles in the Permian period turned out, like many other extreme forms of specialization, to be a 搊ne-way ticket - representatives of this detachment are no longer found in younger deposits.

Photo Alexander Mironenko.

Alexander Mironenko

11 棹睃 2020 /
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