A new hypsocormine pachycormid from the Upper Jurassic Nusplingen Plattenkalk (Kimmeridgian, Germany) provides new insights into body plan evolution and scale reduction in Pachycormidae (Actinopterygii)

Pachycormidae is a group of pelagic Mesozoic teleosteomorph fishes characterized by, among other characters, a reduction of the body scales, both in size and also in degree of mineralization. However, a detailed understanding of the timing and mechanism underlying scale reduction remains elusive, and histological sections of the body scales of a pachycormid fish have never previously been figured in the literature or considered in a phylogenetic context. Here, we describe a new early-diverging member of the hypsocormine subclade, Simocormus seyboldi sp. nov., from the Kimmeridgian (Upper Jurassic) of Baden-Württemberg, Germany, which provides new detailed information on postcranial anatomy and body plan evolution in the clade, but also on the primitive scale morphology in Hypsocorminae. Si. seyboldi shows unambiguously lepisosteoid-type scales, including a thin layer of ganoine on the body scales and robust peg-and-socket articulation between scales within a row. This differs from early representatives of the asthenocormine lineage, and suggests that scale reduction occurred in parallel between the two sub-clades. Interestingly, the external ganoine layer in Simocormus seyboldi did not fluoresce noticeably under UV light, nor was it obvious under the microscope. The layer was present on both scales preserved facing the sediment and water column, although in some scales facing the latter it was somewhat degraded, suggesting taphonomic factors might influence interpretation. These results imply that caution should be employed when assessing the presence of hypermineralized tissues based solely on macroscopic observation, especially in cases where these might be reduced in thickness.