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Described in 1858,  Trematospondylus macrocephalus  is one of the earliest established plesiosaur taxa. However, despite being historically significant, the taxon disappeared from the literature shortly after its initial description and has not been mentioned for over a century. Thus, it has never been properly assessed. The holotype comprises seven vertebrae, including a supposed sacral vertebra and caudal vertebrae. The specimens were collected from the Lochen mountain massif near Balingen in southwestern Germany and derive from the Dentalienton Formation. As such, they are most likely early Bathonian (Middle Jurassic) in age. Here, we provide a thorough redescription of the material, illustrate it, and assess its phylogenetic affinities through a parsimony analysis. Our results show that  T. macrocephalus  shares characters with members of the Rhomaleosauridae, including, for example, the length-height ratio of the caudal centra as well as the placement of caudal ribs and hemapophyseal facets. Rhomaleosaurid affinities of the material are likewise supported by our phylogenetic analysis. Comparisons further indicate that the taxon belonged to large-bodied members of the clade, with a body length similar to that of  Rhomaleosaurus . Even though the material does not enable  Trematospondylus  to be properly diagnosed, thus making it a nomen dubium, it still adds to the rare Middle Jurassic record of rhomaleosaurid plesiosaurs. At the same time, it enhances our knowledge of the plesiosaur taxic diversity during the pre-Callovian Middle Jurassic, which has been very limited.

A ‘long-forgotten’ plesiosaur provides evidence of large-bodied rhomaleosaurids in the Middle Jurassic of Germany