Horaffia kugleri: a new Ladinian Triassic Marine Reptile With Unknown Affinities

Humerus morphology and histology of a new marine reptile (Diapsida) from the Muschelkalk-Keuper-Grenzbonebed (Middle Triassic, Ladinian)

Authors:

Klein et al

Abstract:

A survey in the collection of the Muschelkalkmuseum Ingelfingen revealed a growth series of five humeri from the Grenzbonebed collected from localities in northeastern Baden-Württemberg (Southwest Germany). The Grenzbonebed is a tempestitic condensation horizon at the base of the Keuper that contains prefossilized vertebrate fossils reworked during a 100 ka minor sedimentary cycle. The humeri share a unique but simplified morphology due to adaptation to aquatic life. They are very robust and pachyostotic and have a dorsopreaxially elongated margin, a massive and ventrally protruding triangular proximal head as well as a preaxially slanted asymmetrical distal end. A detailed morphological comparison with humeri of other Triassic marine reptiles showed that they differ from humeri of all known taxa, including those of placodonts such as Paraplacodus, Placodus, Cyamodus, and Henodus. However, an analysis of the bone microstructure and histology revealed a close relationship with the humeri of Cyamodus, despite of distinct morphological differences. The five humeri and humeri of Cyamodus share a plexiform radiating fibro-lamellar bone tissue with moderate to high vascular density in the middle and outer cortex and a poorly developed medullary region, indicating osteosclerosis. Contrary to the pachyostotic humeri from the Grenzbonebed, Cyamodus humeri are not pachyostotic, thus suggesting different modes of locomotion in a similar shallow marine environment. The histological features of the pachyostotic humeri from the Grenzbonebed, which could point to placodont or pistosauroid affinities, respectively, are no reliable phylogenetic markers because convergent evolution of bone tissue is common among aquatic vertebrates sharing a similar environment and life style. Due to the unique humerus morphology the establishment of a new taxon, Horaffia kugleri gen. et sp. nov., is justified, although the remains are too poor to include them into a phylogenetic analysis or to assign them to a certain group.