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Description
Reconstruction of the microcleidid plesiosaur Seeleyosaurus guilelmiimperatoris from the Early Jurassic of Germany.

Seeleyosaurus was first described as a species of the genus Plesiosaurus in 1895. This taxon was among the many plesiosaurs assigned to this genus, which became a "wastebasket". In the future, most of them were separated into separate genera. Seeleyosaurus received its current name in 1940. It was a relatively small plesiosaur, 2.9–3.6 m long. It is known from two samples. The holotype was discovered in Württemberg. Despite the dismembered state, he was quite complete. He even has soft tissue impressions, including a caudal fin, possibly rhomboidal in shape. The second specimen, also very complete, was attributed to a new species, S. holzmadensis, which, however, was later recognized as a synonym of the type species.

As I have already mentioned, the Seeleyosaurus holotype has preserved a partial caudal fin. However, its position on the tail is unknown: whether it was vertical or horizontal. Many paleontologists have leaned towards the vertical fin, but the opposite point has also appeared. Fraas (1910) and Wegener (1914) argued that the fin was horizontal because the tails of plesiosaurs are short and wide, without the strong bending of the terminal part observed in ichthyosaurs, metriorhynchids and mosasaurs. A. G. Sennikov adheres to the same point of view in the publication from 2018: www.researchgate.net/publicati… The idea that plesiosaurs had a horizontal caudal fin is well consistent with their manner of movement, in which the main role is played by flippers, and the tail makes vertical oscillations. Unfortunately, it is difficult to understand the true position of the fin from the skeleton with sost tissue impressions, so new specimens will be needed to confirm this or that point of view. In this reconstruction, Seeleyosaurus has a horizontal fin.

Black ballpoint and gel ink pens and black colored pencil, 2023.

Description
Reconstruction of the microcleidid plesiosaur Seeleyosaurus guilelmiimperatoris from the Early Jurassic of Germany.

Seeleyosaurus was first described as a species of the genus Plesiosaurus in 1895. This taxon was among the many plesiosaurs assigned to this genus, which became a "wastebasket". In the future, most of them were separated into separate genera. Seeleyosaurus received its current name in 1940. It was a relatively small plesiosaur, 2.9–3.6 m long. It is known from two samples. The holotype was discovered in Württemberg. Despite the dismembered state, he was quite complete. He even has soft tissue impressions, including a caudal fin, possibly rhomboidal in shape. The second specimen, also very complete, was attributed to a new species, S. holzmadensis, which, however, was later recognized as a synonym of the type species.

As I have already mentioned, the Seeleyosaurus holotype has preserved a partial caudal fin. However, its position on the tail is unknown: whether it was vertical or horizontal. Many paleontologists have leaned towards the vertical fin, but the opposite point has also appeared. Fraas (1910) and Wegener (1914) argued that the fin was horizontal because the tails of plesiosaurs are short and wide, without the strong bending of the terminal part observed in ichthyosaurs, metriorhynchids and mosasaurs. A. G. Sennikov adheres to the same point of view in the publication from 2018: www.researchgate.net/publicati… The idea that plesiosaurs had a horizontal caudal fin is well consistent with their manner of movement, in which the main role is played by flippers, and the tail makes vertical oscillations. Unfortunately, it is difficult to understand the true position of the fin from the skeleton with sost tissue impressions, so new specimens will be needed to confirm this or that point of view. In this reconstruction, Seeleyosaurus has a horizontal fin.

Black ballpoint and gel ink pens and black colored pencil, 2023.