Fluvionectes

The holotype specimen of Fluvionectes is a partial skeleton preserving an osteologically mature, likely a young adult individual that would have reached 5–5.2 m (16–17 ft) long and weighed 392 kilograms (864 lb).^[1][2] A more mature, larger, but more fragmentary specimen (TMP 2009.037.0007) is also known, consisting of a partial rib and gastralium, and left humerus, indicating that this taxon may have reached 7 m (23 ft) in maximum body length. A number of other fragmentary specimens are also known.^[1]

The holotype skeleton had 76 gastroliths, largely disc-shaped stones. All were composed of black chert and grey quartzite, the largest of which weighed 15.3 grams.^[1][2]

The describers placed Fluvionectes in Elasmosauridae, in a clade with Albertonectes, Nakonanectes, Styxosaurus, and Terminonatator, which by definition places it in the Elasmosaurinae subfamily.^[1]

Fluvionectes appears to have been a freshwater and brackish water animal based on its discovery from a non-marine to paralic sedimentary unit. Both the holotype and the largest specimen (TMP 2009.037.0007) were found in brackish estuarine deposits, but a number of other specimens were found in nearby freshwater fluvial deposits. This is significantly different in contrast to most elasmosaurs which were oceanic.^[1]

Other fossils associated with the holotype specimen included the turtle Kimurachelys slobodae and the rhinobatoid ray Myledaphus. Three dinoflagellates were also found, suggesting a marine influenced environment, although their low abundance and diversity suggests that it was not an open-marine environment.^[1]

The holotype was discovered alongside many pieces of coalified wood, which is interpreted as the carcass having been caught in a log jam.^[2]