There has been many greatly preserved Stenopterygius fossils found but there has not been much research for this creature because it is very much like the Ichthyosaurus. Stenopterygius just mainly did not get the same publicity that the Ichthyosaurus did. Both prehistoric Mammals were from the same group. The many Stenopterygius fossils were found in Germany, England, and South America, and the only main difference between Ichthyosaurus and it was just this "mammal" had a more narrow skull and flippers. The mouth was somewhat like today's common Dolphin armed with a lot of small piercing teeth which seemed made for eating slippery fish. The paddles of the mammal had sub-divided finger rows.

Many fossil discoveries were found with baby species together with it showing that the Stenopterygius must've had trouble giving birth and died doing so. Unlike many prehistoric animals the Stenopterygius gave birth to live babies and not eggs. Through fossil discoveries the baby came out tail first to give the offspring time to avoid drowning during the process.