Igor V. Novikov & Andrey G. Sennikov
Paleontological Institute of the Russian Academy of Sciences, 123 Profsoyuznaya Street, Moscow, 117647, Russia
The continental Lower Triassic of European Russia is known for a rich and diversified vertebrate fauna: dipnoans, temnospondyl amphibians, procolophonids, prolacertiforms, thecodonts, theriodonts and other groups. Five faunal sequences are distinguished in the Early Triassic evolution of the principal tetrapod group so that, unlike anywhere else in the world, the Lower Triassic section in this region may be subdivided into five horizons: Vokhmian, Rybinskian, Sludkian, Ust’-Mylian and Yarenskian.
The Lower Triassic is most completely represented in the Moscow Syncline. The vicinity of Rybinsk city in the Upper Volga River Basin is a stratotype region of the Rybinskian Horizon represented here by the Rybinsk Formation and correlated to the Lower Olenekian Substage. The Rybinsk Formation contains a rich and diversified fauna and flora, in some cases demonstrating a unique state of preservation. The most well known locality in this respect is Tikhvinskoye, situated on the right bank 12 km down the Volga River from Rybinsk.
The Tikhvinskoye locality displays the middle portion of the Rybinsk Formation, the so-called “Parshino beds”. It consists of alternating grey clays and fine-grained sandstones with marl concretions representing deposits of a big shallow brackish-water lake. Typical of the Tikhvinskoye fossils is their very good preservation exhibiting fine morphological details and bone surface structures. Yet, the vertebrates are represented mostly by separated skulls and isolated bones. Evidently the organic remains after long maceration on the bottom of the lake were buried without any substantial transfer.
Remains of vertebrates, invertebrates and plants have been recovered from Tikhvinskoye. Among the vertebrates temnospondyl amphibians prevail. These include two trematosauroids – Thoosuchus yakovlevi and Benthosuchus korobkovi – and the capitosauroid Wetlugasaurus angustifrons. Fishes are represented by dipnoans (Gnathorhiza triassica and G. lozovskii), several undescribed palaeoniscoids, actinopterygians (Saurichthys obrutchevi, S. proximus, S. tertius and S. eximius) and by the elasmobranch Hybodus. The remains of reptiles are extremely rare. Only three vertebrae of the thecodont Chasmatosuchus rossicus are known from the locality.
The invertebrates include insects (beetles, cockroaches, grylloblatids), the swordtail Limulitella volgensis, phylopods and ostracods. The insect fauna of the Tikhvinskoye invertebrate community is rather interesting because Early Triassic insect localities are not known anywhere else in the world, and the Early Triassic stage presents a gap in our knowledge of insect evolution. The plants are represented by the lycopodiform Pleuromeia rossica, the equisetum Equisetites and occasional ferns and ginkgos. Spores and pollen of these groups were also found.
Most fossil remains from the Tikhvinskoye locality belong to animals and plants of a lake community: temnospondyls, fishes, phyllopods, pleuromeyas etc., inhabiting intermediate and shallow waters and banks. Fragmentary remains of terrestrial communities (thecodonts, insects) occur occasionally.
In 1967 Tikhvinskoye was registered as a geological monument under state protection. Upon the request from the Paleontological Institute the locality was included into the UNESCO World Heritage List in 1993. This is the first geological site of such status in Russia.