And the era of radiant

The most diverse and prosperous group of vertebrates on our planet is radiant fish (Actinoptygii). The vast majority of fish species belong to this subclass. According to molecular phylogeny and paleontology, modern radiant fish (bony, Teleostei) appeared about 200 million years ago. They were small and evolved quite slowly. The surge of their number and diversity occurred 100-50 million years ago. Mass extinction of species also accounted for this period, caused by the fall of the asteroid on the Yukatan peninsula 66 million years ago, which deprived us of dinosaurs and divided the chalking period and Paleogen. It would be logical to assume that it was this event that caused the expansion of the radiant, but there was no evidence.

The evidence was discovered by the professor of the Institute of Oceanography of the Skripps University at San Diego Richard Norris and his graduate student Elizabeth Sibert. Paleontologists were interested in how the community of Pelagic fish - the inhabitants of the open sea survived great extinction. After the meteorite falls, the shallow species of fish disappeared almost everything, most of the sharks and slopes survived, and little was known about pelagic bony fish. (Sharks and slopes of the skeleton of Khryashchev, and they belong to the class of cartilaginous fish, Chondrichthyes.)

Researchers analyzed ichthyolites-microscopic fish teeth and mineralized skin scales (dentikules) of sharks, which extracted from the cores of deep-water sedimentary breeds of five areas of the Pacific and Atlantic oceans, as well as from the lime outcrops in the south of Italy-once in this place was the bottom of the Ocean of Attis. Ichthyolites are well preserved in sedimentary rocks and are found in large quantities, more often than other elements of the skeleton. Their multiplicity allows you to investigate not individual individuals, but of fish communities, to follow a change in their number and biodiversity.

Scientists worked with ichthyolites in the size of more than 106 microns. In each sample, they determined the ratio of teeth to denticulas and analyzed a change in the size and morphology of the teeth. Sakuli teeth make up less than 1% of ichthyolites, they are easy to distinguish from the teeth of radiant fish: they are flat, triangular, often have toothed edges. Therefore, the teeth were considered the belonging of radiant fish, and dentikules with a shark attribute. If the ratio of teeth to denticulas in a certain volume of the breed was less than one, this, according to scientists, testified to the predominance of sharks, with a ratio there was more than a unit above the number of radiants.

In all Kernes, the boundary of the Cretaceous period and paleogene is clearly defined by a layer with an abnormal content of iridium, as well as by the presence of textites - vitreous formations that arose as a result of a meteorite strike on the solid surface of the planet. At the end of the chalk period, the shark, of course, dominated the radiant fish. Their great extinction practically did not affect: the absolute number of shark scales remains relatively constant in each sample and before and after this event. However, the ratio of the number of teeth of radiant fish to the number of dentikuls after extinction changes. At the end of the Cretaceous period, it was less than one, however, in 500 thousand years after extinction, the number of teeth and dentikul was almost equal. Their ratio continued to grow steadily and after 24 million years reached eight times the difference.

Not only the number of radiant fish was rapidly increasing in all oceans, but also their diversity. In the Cretaceous period, the radiant teeth were small, but at the beginning of Paleogen, shortly after extinction, quite a lot of radiant with large teeth appeared. The dimensions of the teeth in Pelagic fish cannot serve as an indicator of the size of the body: in some large fish, the teeth are small, and in small fish they are longer than in other large species. Researchers believe that the radiant began to actively occupy new environmental niches, as a result of which large fish appeared. They welfare of 6-7 million years, and then they were somewhat supplied with fish with medium -sized teeth, but the Bolshez were not disappeared anywhere (Fig. 1). The shape of the teeth of radiant has also become varied (Fig. 2).

Richard Norris and Elizabeth Siber emphasize that the community of pelagic vertebrates of late chalk was extremely stable both in terms of the ratio of ichthyolites of radiant and sharks, and in the dimensional structure of the teeth. Researchers found no trends in slow changes. Consequently, the changes that occurred at the beginning of the Paleogene occurred as a result of a sudden event, most likely the fall of a meteorite and great extinction. Predators disappeared: large sea reptiles and ammonites - an ancient group of cephalopods. Some types of ammonites feeding on plankton competed with fish for food. Having got rid of the oppression of predators and competitors, the radiants rushed to master the liberated ecological niches, previously inaccessible to them. Sharks did not follow their example and only supported the previous number.

The rapid evolution of radiant fish resembles the history of the success of mammals, which at the same time unfolded on land. Ten millions of years, mammals existed in the shadow of dinosaurs, could not even afford to grow, but after a great extinction they reached an amazing variety of sizes and forms.

Researchers note that, if there were no great extinction, which changed the incredibly stable ecosystems of pelagic vertebrates, the radiant fish would not have a chance to conquer the ocean, all the more so quickly. It was extinction that became a turning point in the evolution of radiant fish and the reason for their dominance in the modern seas. The current era can be called the era of radiant.

Sibert EC, Norris RD New Age of Fishes Initiated by the Cretaceous - Paleogene Mass Extination // Procedings of the National Academy of Sciences. 2015. DOI: 10.1073/pnas.1504985112