Palaeodiversity 7: 23–38; Stuttgart 30 December 2014. 23 Humerus morphology and histology of a new marine reptile (Diapsida) from the Muschelkalk-Keuper-Grenzbonebed (Middle Triassic, Ladinian) of Southwest Germany NICOLE KLEIN & HANS HAGDORN 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 Grenz- bonebed is a tempestitic condensation horizon at the base of the Keuper that contains prefossilized vertebrate fos- sils 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 Heno- dus. 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 plexi- form 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 Grenz- bonebed, Cyamodus humeri are not pachyostotic, thus suggesting different modes of locomotion in a similar shal- low 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. K e y w o r d s : Germanic Basin, environment, bone histology, pachyosteosclerosis, radiating fibro-lamellar bone, high vascular density. 1. Introduction Marine reptiles, especially Sauropterygia, were diverse and abundant faunal elements in the Middle Triassic of the western and eastern Tethys and in various epiconti- nental Peri-Tethys basins. Although the fossil sites have been intensively studied for more than 150 years in the Central European Basin and for some 50 years in South China (summarized in RIEPPEL 2000; LI 2006) the last dec- ade has produced a number of new marine reptile taxa, which mainly originated from the Middle Triassic of the eastern Tethys (e.g., LI & RIEPPEL 2002; LI et al. 2011; LIU et al. 2011; JIANG et al. 2008a, b; ZHAO et al. 2008; SHANG et al. 2011; CHENG et al. 2012a, b; LI et al. 2013). However, also in the early Anisian Muschelkalk of Winterswijk (The Netherlands) three new taxa were recently discov- ered (KLEIN & SCHEYER 2013; NEENAN et al. 2013; KLEIN & SICHELSCHMIDT 2014), indicating that our knowledge of Tri- assic marine reptile diversity is still incomplete. The black shale deposits of the Alpine and Chinese Triassic produced a high number of articulated skele- tons, whereas the Germanic Basin yielded mostly isolated bones, many of them originating from condensation hori- zons, the classical Muschelkalk and Lower Keuper bone beds. Thus, alpha taxonomy of Middle Triassic Saurop- terygia from the Germanic Basin is mostly based on skull morphology and postcranial material is reliably assigned only for a few taxa (summarized in RIEPPEL 2000a; KLEIN 2012). Taxonomic assignment is additionally hampered because many taxa (i.e., Eosauropterygia) have a simi- lar postcranial morphology, because of morphological changes during ontogeny, and because of an assumed high intraspecific variability. However, if adequate samples are available, long bone histology can contribute to clarify the taxonomic status of sauropterygian postcranial material (e.g., KLEIN 2010, 2012; SANDER et al. 2013). The Muschelkalkmuseum Ingelfingen (MHI) holds five humeri collected in quarries near Crailsheim and Vellberg along the Jagst and Bühler rivers (Baden-Würt- temberg, Southwest Germany) exposing the Muschel- kalk-Keuper boundary bone bed (“Grenzbonebed”). The humeri exhibit a very characteristic morphology, which differs from all previously known taxa. It is the aim of the current paper to describe and compare their morphology and bone histology.