Ventastega

Extinct genus of tetrapodomorphs

Ventastega (Venta referring to the Venta River at the Ketleri Formation where Ventastega was discovered) is an extinct genus of stem tetrapod that lived during the Upper Fammenian of the Late Devonian, approximately 372.2 to 358.9 million years ago. The group of stem tetrapods contains any extinct taxon that, according to morphological analysis, is more closely related to lissamphibians (living amphibians) and amniotes than to any other crown group.

Ventastega has helped further research on the Fish Tetrapod transition, the event during the Devonian when digit bearing tetrapods evolved from finned, fish ancestors. Research on rates of character changes in tetrapods have shown that there were high rates of character changes in the Devonian, which led to the conflicting hypotheses of either the tetrapods had few major changes that occurred during the Devonian or had many small but rapid morphology changes. The braincase of Ventastega has a mixture of fish like and tetrapod like characteristics, indicating that changes in the braincase during the fish tetrapod transition occurred through a series of many small changes instead of one large change. While no fossilized trackways from Ventastega have yet been discovered, discoveries of late Devonian trackways from closely related Ichthyostega and Acanthostega along with other early tetrapods indicate that the most stable gait for early tetrapods, including Ventastega, would a lateral sequence walk. The trackway patterns, including marks from the tetrapods dragging their tails and bellies on the ground, have both been subaerial and subaqueous, indicating that the origin of terrestrial locomotion originated in tetrapods, not in their fish ancestors.

Paleogeologists estimate that the Famennian, the global average temperature was about 19.5 °C. However, the sedimentology of the Ketleri Formation, including sands interbedded with siltstones and clays, indicates that Ventastega was deposited in a low tidal, near shore environment in saline water. Per Erik Ahlberg, Ervīns Lukševičs, and Oleg Lebedev collected Ventastega material both from the Ketleri locality where the original material was found, but also from another locality in the Ketleri formation, the Pavāri locality (. The Ketleri locality is at the Venta river near the former Ketleri hamlet, while the Pavāri locality is across the Ciecere river from the former Pavāri hamlet. The formation itself is composed of sand, sandstone, clay, and dolomitic marls, with the formation being 45m thick in Lithuania and 41 56m thick in Latvia. The formation is composed of three members, the lowermost Nīgrange member which has thick clayey carbonate deposits, the Pavāri member which consists of sand and sandstone with high levels of quartz, feldspar, and mica, and the upper Varkaļi member which is composed of fine feldspar quartz sands and sandstones with high quantities of mica. The Pavāri member had a shallow erosional channel filled with well sorted sands indicating the presence of a stream in the Devonian that flowed in the channel from NNE to SSW, which is the characteristic direction of flow across the whole Ketleri formation. Sand particles filled in the erosional channel, and then were followed by slow, calm intertidal streams depositing poorly sorted silts and clays.