NSU Scientists Have Discovered Why Cretaceous Fish Remains Are Perfectly Preserved in Transbaikalia

Taphonomic analysis of fish from the early Cretaceous period (about 124 million years ago), found in the geological section at the Turga River (Middendorf exposure in Eastern Transbaikalia), was conducted for the first time by scientists from Novosibirsk State University and the A. A. Trofimuk Institute of Petroleum Geology and Geophysics of the Siberian Branch of the Russian Academy of Sciences. Based on differences in fish preservation and the composition of orictocenoses (assemblages of fossilized remains of extinct plants and animals found in a specific geological layer or location), they reconstructed the features of the burial of fish and invertebrates in this paleolake, as a result of which the Turginsk lagerstätte was formed. The study was supported by the RSF Grant (No. 25-77-10059), its results were published in the NSU Journal «Geology and Geophysics».

Lagerstätter — a special type of fossil deposit that contains them in large quantities and often features exceptional preservation. Sometimes, due to special conditions, not only the hard parts of skeletal forms are preserved, but also imprints of soft tissues. Turginsky concentration camp (Middendorf Exposure) — a famous Early Cretaceous geological and paleontological site in Eastern Transbaikalia, first studied in the mid-19th century. Fossil findings from here have been known since 1846 following the expedition of traveler and scientist A. F. Middendorf. The location dates to the Early Cretaceous period (about 125 million years ago). The fossils found here are characterized by exceptional preservation: fossil fish retain their skeletons, all skull bones, and scales excellently, and for conchostracan crustaceans — the chitinous skeleton and egg clutches.

Identifying factors that influence the high preservation of inhabitants of specific paleowater bodies will help scientists study other Lagerstätten where these factors may also be at work. Alternatively, this research will assist in examining paleo-lakes where different factors affect the preservation of ancient flora and fauna remains. In the Turginsk Lagerstätte, excellent preservation is noted both in invertebrates and vertebrates, namely fish. The abundance of the latter’s remains has allowed scientists to use paleontological finds to reconstruct the conditions under which ancient creatures were buried.

— If some paleo-lake had good burial conditions for certain vertebrates, there is a chance that others might be preserved as well. Such as, for example, turtles, dinosaurs, pterosaurs, and others that could have entered this body of water through various means and under different circumstances. Therefore, by identifying the factors influencing the good preservation of fish, it is possible to predict that in the same place there is a likelihood of finding other representatives of the animal world from the Cretaceous period., — explained an engineer of the NSU scientific and educational center “Earth Evolution”, junior researcher of the Paleontology and Stratigraphy of the Mesozoic and Cenozoic Laboratory of the A. A. Trofimuk Institute of Oil and Gas Geology and Geophysics of the Siberian Branch of the Russian Academy of Sciences Vsevolod Yefremenko.

The scientist emphasized that taphonomic studies of lagerstätten in Russia and worldwide are conducted quite rarely. This is due to the difficulty of working with lagerstätten and the need to study a large volume of theoretical material and data on environmental factors. Such a study had not been conducted for the Turginsk lagerstätte either, despite its history of study spanning 180 years. There were only assumptions about the unique preservation of fish and invertebrate creatures, but these were not based on examining specific material and referring to experimental data. Novosibirsk scientists took a large sample of paleontological specimens, applied modern methods, compared the results with experiments on fish burial, and used their own data about this paleo-lake to reconstruct the conditions that existed in these places 125 million years ago.

— In the second half of the last century, in connection with the discovery of several fossil fish-rich sites, controlled experiments on fish remains were actively conducted in the USA and Germany, during which scientists sought to identify the influence of factors such as temperature, pressure, and water salinity on the rate of fish body decomposition. The phenomenon of postmortem fish buoyancy was also studied. It was determined under which conditions the dead fish rises to the surface and under which it sinks to the bottom. Subsequently, these data were compared with what we observe in geological strata. The data from the experiments and the huge volume of fossil fish from Mesozoic and Cenozoic sites around the world became the theoretical basis of our research., — said the scientist.

When studying the extensive collection of fossil fish from the Turginskii lagerstätte, scientists paid attention to the completeness of the skeleton and its preservation, noting the degree of articulation of the bones or their scatter on the substrate, the position of the scales, and the presence nearby of other fish, invertebrates, or other associations useful for determining the characteristics of the burial.

The size of the fish was also studied, which is important for determining the burial season and reconstructing the paleopopulation. The size of the fish was taken as the length from tail to head, regardless of the fish’s position—whether stretched out or curved. Sizes of partially preserved fish were even reconstructed. This was necessary to distinguish burials with predominantly small, medium, and large fish, their environment, and preservation. Thus, important variables for the study were fish sizes, the presence of their associations with invertebrates, and the degree of preservation.

To reconstruct the climate in the area of this paleo-lake, scientists used previously obtained data from spore-pollen analysis and sedimentology, indicating how sediment accumulation occurred—whether quickly or slowly, in areas far from and close to the shore. They also applied data from organic geochemistry, having previously studied the organic matter preserved in rocks from the Mesozoic era. Using these data, scientists determined what kind of life inhabited this paleo-lake, the oxygen content in the water, and whether stratification of the water column was present.

— There is a dependence between the rate of fish decomposition and the water temperature of the paleo-lake. If it is warm, the dead fish float “belly up” due to the accumulation of gases that are products of bacterial decomposition. In the suspended state within the water column, it begins to decompose. The process starts at the head, and the fish first loses the skull bones. When the decomposition reaches the abdominal area, the fish sinks to the bottom. In this case, it has very little chance to be preserved in the fossil record. Surprisingly, in our sample of paleontological material, there were no fish that showed prolonged decomposition in the water column or prolonged decomposition at all. This means that the dead fish did not float up but immediately fell to the bottom and lay on the substrate for some time. We found specimens with several fish of varying degrees of preservation within a single millimeter-thick layer. This means they fell to the bottom sequentially and rapid burial did not occur. They simply lay on the bottom for some time and, most likely, decomposed slowly. This indicates that there was some factor that could have prevented rapid decomposition of fish in this paleo-lake.According to experimental data, these factors are well known: temperature, water salinity, its pH level, and the depth of the paleo-lake, — explained Vsevolod Efremenko.

Based on the fauna composition, namely insect larvae (mayflies) and crustaceans (phyllopods), scientists concluded that the paleo-lake was shallow, since these animals inhabited only relatively shallow waters. This hypothesis was supported by a large number of small fish, the presence of microfauna, sedimentology data, and organic geochemistry. Most likely, the Turginskoye paleo-lake was extensive in area and its depth did not exceed 15 meters. Therefore, depth was not a factor influencing the postmortem sinking of fish to the bottom.

Spore-pollen analysis data indicate a moderate warm climate, transitioning to subtropical. In this climate, summer temperatures easily reach levels sufficient to decompose fish in the water column (16-18°), but scientists find fish burials together with layers filled with crustacean shells. Such shell burials can only occur during the summer period because it is precisely at this time of year that these invertebrates experience a population peak. The large number of fry found in the burials also supports this particular season.

The burials from the Turginsky paleo-lake are unique precisely because they contain fish alongside crustaceans. Fish are absent in the paleo-lakes of China. In this case, they are present in large quantities and are well preserved. This means that their preservation was not influenced by temperature factors. Scientists concluded that this factor is the alkalinity of the water — that is, the pH level. It might seem that alkaline lakes are uncomfortable for life and have poor biodiversity. In the Turginsky paleo-lake, remains of fish from only two taxa (genera Lycoptera and Stichopterus) were found, and one genus clearly predominates in the sample — for every hundred specimens of one genus, there is only one specimen of the other.

— It has been experimentally shown that the weathering of volcanic rocks can produce alkalis, which then enter the paleobasin and affect the chemistry of its waters. The base of the Turginsky paleolake, or more precisely the depression in which it was situated, is composed of alkaline basalts. Probably, as these basalt rocks weathered, the lake became filled with alkalis, creating a specific geochemical environment in which fish could be buried with a high degree of preservation even during the summer. Surprisingly, this factor was previously not considered in taphonomic models for similar paleolakes. In our study, we encountered a contradiction — fish in layers with crustaceans should not have been preserved at all, but in our case, they somehow were. Moreover, the water pH level aligns well with data on the geological structure of the Turginsky paleolake depression and the results of taphonomic experiments, which showed that at a water pH level of 8.6, a dead fish does not float but remains on the bottom for over a month, decomposing very little.

An indirect indication that the pH level in the Turgin paleo-lake was high is evidenced by the very large number of crustaceans — phyllopods, which reach their maximum numbers specifically in alkaline water. In fresh or acidic water, their numbers decrease to the point of disappearance.

As a result of the study, scientists identified summer burials containing fish, crustaceans with remains of fry and insects, as well as, likely, winter burials consisting predominantly of large fish of very good preservation, devoid of accompanying invertebrates and insects.

— We have determined that the main factors responsible for the high degree of preservation of Cretaceous fauna representatives in the Turginsky paleo-lake are the absence of hydrodynamic influence, oxygen deficiency in the bottom layer, and the alkalinity of the water. An important factor in preservation is the lack of prolonged post-mortem presence of fish bodies in the water column. In the ancient reservoir we studied, in winter, the low water temperature prevented the floating up of dead fish carcasses, and in warm seasons their post-mortem floating was prevented by increased pH levels. The bedding surfaces containing shell pavements from conchostracans Eosestheria middendorfii accumulated in the summer period. We find numerous remains of small fish in them. Larger fish were buried during the autumn and winter seasons. No invertebrates are observed near them. — said Vsevolod Efremenko.