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Enameloid is a hard mineralized tissue covering chondrichthyan and actinopterygian teeth. Over the past 40 years, it has been extensively studied in various extinct and extant sharks, leading to the broad use of microstructural characters to differentiate between hybodont and neoselachian teeth. However, the chondrichthyan taxic diversity is disproportionately high compared to the number of taxa explored for enameloid microstructure, and the generalization of these few observations to the whole group is problematic. Indeed, many other groups, in particular modern rays and skates, have been completely overlooked, and almost nothing is known about their tooth histology. Furthermore, the recent discovery of typical neoselachian character in cladodontomorph sharks teeth clearly indicates that we have had an over-simplified perception of the chondrichthyan enameloid distribution, which put into question the previously proposed evolutive scenarios dealing whith this tissue. We propose a brief historical overview of the study and understanding of chondrichthyan enameloid diversity and briefly discuss preparation issues encountered when dealing with the study of chondrichthyan hypermineralized tissues. Then, the variation of enameloid microstructures encountered in ctenacanthiforms, hybodonts, selachimorphs, and batomorphs is explored, summarized, and discussed. Although the full extent of the diversity and variability of the enameloid microstructure in many of these groups and others remains to be fully determined, we are able to show that most possess a much more complex enameloid microstructure than expected, and propose a revised and more fitting chondrichthyan enameloid terminology, based on the recognition of two main units: an external Single Crystallite Enameloid (SCE) and an internal Bundled Crystallite Enameloid (BCE). Our study reveals new insights in the understanding of character distribution among batomorphs and sets a framework for tackling global chondrichthyan tooth enameloid evolution. © 2015 The Linnean Society of London

The Permian and Triassic were key time intervals in the history of life on Earth. Both periods are marked by a series of biotic crises including the most catastrophic of such events, the end-Permian mass extinction, which eventually led to a major turnover from typical Palaeozoic faunas and floras to those that are emblematic for the Mesozoic and Cenozoic. Here we review patterns in Permian-Triassic bony fishes, a group whose evolutionary dynamics are understudied. Based on data from primary literature, we analyse changes in their taxonomic diversity and body size (as a proxy for trophic position) and explore their response to Permian-Triassic events. Diversity and body size are investigated separately for different groups of Osteichthyes (Dipnoi, Actinistia, 'Palaeopterygii', 'Subholostei', Holostei, Teleosteomorpha), within the marine and freshwater realms and on a global scale (total diversity) as well as across palaeolatitudinal belts. Diversity is also measured for different palaeogeographical provinces. Our results suggest a general trend from low osteichthyan diversity in the Permian to higher levels in the Triassic. Diversity dynamics in the Permian are marked by a decline in freshwater taxa during the Cisuralian. An extinction event during the end-Guadalupian crisis is not evident from our data, but 'palaeopterygians' experienced a significant body size increase across the Guadalupian-Lopingian boundary and these fishes upheld their position as large, top predators from the Late Permian to the Late Triassic. Elevated turnover rates are documented at the Permian-Triassic boundary, and two distinct diversification events are noted in the wake of this biotic crisis, a first one during the Early Triassic (dipnoans, actinistians, 'palaeopterygians', 'subholosteans') and a second one during the Middle Triassic ('subholosteans', neopterygians). The origination of new, small taxa predominantly among these groups during the Middle Triassic event caused a significant reduction in osteichthyan body size. Neopterygii, the clade that encompasses the vast majority of extant fishes, underwent another diversification phase in the Late Triassic. The Triassic radiation of Osteichthyes, predominantly of Actinopterygii, which only occurred after severe extinctions among Chondrichthyes during the Middle-Late Permian, resulted in a profound change within global fish communities, from chondrichthyan-rich faunas of the Permo-Carboniferous to typical Mesozoic and Cenozoic associations dominated by actinopterygians. This turnover was not sudden but followed a stepwise pattern, with leaps during extinction events.

Chondrichthyans are newly reported from the autochthonous Wordian Khuff Formation (middle Permian), cropping out in well-exposed, low-palaeolatitude sections in the interior Haushi-Huqf area of Oman. The shark remains comprise isolated teeth, dermal denticles and fin spines and have been recovered by processing limestone in buffered acetic acid from bulk rock samples. The fauna consists of mainly ctenacanthiform and hybodontiform taxa, identified as Glikmanius cf. myachkovensis, Glikmanius culmenis sp. nov., Omanoselache hendersoni gen. et sp. nov., Omanoselache angiolinii gen. et sp. nov., cf. Omanoselache sp., Reesodus underwoodi gen et sp. nov., Teresodus amplexus gen. et sp. nov., Gunnellodus bellistriatus, Khuffia lenis gen. et sp. nov., Khuffia prolixa gen. et sp. nov. and Euselachii sp. indet. Additional specimens include rare teeth of the lonchidiid cf. ‘Palaeozoic Genus 1’ sp., of the neoselachian Cooleyella cf. fordi and a further indeterminate neoselachian, of an indeterminate petalodont and of the holocephalan Deltodus aff. mercurei and Solenodus cf. crenulatus. Fin spines add a further two taxa, Nemacanthus sp. and Amelacanthus cf. sulcatus, which have neoselachian affinities and therefore an unclear relationship to the recovered teeth. The occurrence of Nemacanthus within this Wordian fauna represents the oldest record of this taxon and its only known occurrence in the Palaeozoic. Of the remaining genera, Glikmanius has previously been recorded from the Wordian, whereas for all the others, this study represents their youngest known stratigraphic occurrence and first occurrence in Guadalupian (middle Permian) strata. This adds significantly to our knowledge of the global diversity of chondrichthyans preceding the end-Guadalupian biotic crisis. Palaeogeographically, for all taxa, this study represents the first record from the western fringe of the marine Neotethyan basin, and only Cooleyella was previously known from the southern (Gondwanan) part of the Pangaean continental margin.

Elasmobranchs are reported for the first time from Lower Triassic deposits in Oman. The well-preserved remains consist of isolated teeth, dermal denticles and fin spines, recovered from conodont residues. The low-palaeolatitude sections consist of Lopingian–Olenekian shallow and pelagic carbonates in exotics, olistoliths and breccia blocks that have been redeposited in younger allochthonous strata of the Hawasina Basin throughout the Oman Mountains at Jabal Safra (olistoliths within the Jurassic Guwayza Formation, Olenekian), as well as at Wadi Alwa (exotic Alwa Formation, Lopingian–Olenekian) and Wadi Wasit Block (slope breccia in the Al Jil Formation, Induan), both of which occur in the Ba'id region. The recovered fauna contains a small number of pre-existing genera, but is mainly composed of new hybodont and neoselachian taxa. They are identified as: Omanoselache halli Koot & Cuny sp. nov., cf. Omanoselache sp., Safrodus tozeri Koot & Cuny gen. et sp. nov. and Polyfaciodus pandus Koot & Cuny gen. et sp. nov., based on the majority of the recovered dental remains. Spine fragments are identified as cf. Amelacanthus sp. This fauna represents the second published record of neoselachian teeth from the Induan and the most extensive record from the Lower Triassic in terms of abundance and diversity. The fauna is dominated by Neoselachii, whereas other Early Triassic faunas are hybodont-dominated, and histological study of the neoselachian enameloid significantly adds to our knowledge of the early stages of their evolution. All described taxa are new to the Oman fossil record and that of western Neotethys, apart from Omanoselache and Amelacanthus, which have been recognized from Wordian deposits, and Omanoselache is the second genus from Oman known to have survived the late Permian mass extinction. The level of faunal diversity recognized here is comparable to other Early Triassic faunas but is much reduced compared to the Wordian pre-extinctions fauna.http://zoobank.org/urn:lsid:zoobank.org:pub:E82E8BC2-1743-4883-B27B-3564DAAF2DF1