Late Cretaceous to ?Paleocene freshwater, brackish-water and marine molluscs from Al-Khodh, Oman

Abstract

brackish-water molluscs is currently a topic of great inter est, mostly due to the importance of these taxa for conservation biology (e.g. Lopes-Lima et al. 2016, 2018; Do et al. 2018; Zieritz et al. 2018). Most freshwater mollusc clades show a high degree of diversification, ende mism and environmental restrictedness, and, as a result, species have a high risk of extinction (e.g. Cuttelod et al. 2011). Brackish-water taxa, although often more wide spread, show similar patterns and face similar problems, as suitable habitats are often confined to river deltas, which are restricted in extent and usually densely populated by humans (e.g. Barnes 1999, Szabo et al. 2016). Consequently, combined molecular and morphologic studies of freshwater and brackish-water molluscs are en vogue, and for several groups, comprehensive phylogenies at global scale are gradually becoming available. In contrast, the fossil component of freshwater and brackish-water mollusc evolution is significantly understudied and comparatively poorly understood. There are several reasons for this. Most importantly, the fossil record of these taxa is scattered and discontinuous, since the preservation potential of the strata enclosing them is low (e.g. Dunhill et al. 2014). As a result, many species or genera are only known from their type locality (e.g. Henderson 1935, p. 4) and phylogenetic relationships remain enigmatic. Additionally, the level of convergence in many of the higher taxa of freshwater and brackishwater molluscs is an issue, as new and surprising cases are continuously revealed by molecular studies (e.g. Bolotov et al. 2017, 2018). Last, but not least, many freshwater and brackish-water species are highly variable and shells react plastically to changes in habitat or water chem istry. Whoever is not put off by these caveats and attempts to apply rigorous taxonomy and systematics to fossil freshwater and brackish-water shells is, however, often rewarded with interesting results concerning evolutionary relationships and pathways, palaeobiogeography or palaeoecology. Moreover, fossils supply the component of time to molecular phylogenetics and are thus essential for the calibration of evolution. Such a case, exemplified by a restricted, low diversity fauna from the latest Cretaceous of northern Oman, is presented herein.