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Taxonomy and biochronology of Early Triassic conodonts

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... The Spiti Subdistrict of Himachal Pradesh is famous for its well-exposed Early Triassic ammonoid-and conodont-rich sections (e.g. Diener 1897, 1912; Hayden 1904; Krafft and Diener 1909; Goel 1977; Krystyn et al. 2004; Goudemand 2010; Brühwiler et al. 2010, 2012; Ware et al. in production a), but the occurrence of fish macrofossils within these strata was unknown until 2009, when two articulated specimens were discovered near the village of Mud (cf. Fig. 1) in sediments of Dienerian age (late Induan, Early Triassic; we herein use the Early Triassic subdivision of Tozer 1965). ...
... There, sections near the villages of Mud (sometimes spelled Muth or Mudh) and Gulling have been intensively exploited for ammonoids and conodonts (e.g. Diener 1897, 1908; Krafft and Diener 1909; Krystyn and Orchard 1996; Bhargava et al. 2004; Krystyn et al. 2004, 2007a, b; Brühwiler et al. 2010, 2012; Goudemand 2010, 2014; Ware et al. in production a). Early Triassic ammonoids from Spiti are known since the late 19th century (Diener 1897). ...
... Brühwiler et al. (2010) demonstrated the occurrence of typically Smithian ammonoid faunas lower than previously described in a section near Mud, which has recently been proposed as a GSSP for the base of the Olenekian stage by Krystyn et al. (2007a, b). We herein follow the definition of the Dienerian-Smithian substage boundary of Brühwiler et al. (2010), which is also supported by conodont data (Goudemand 2010, 2014). Within the Dienerian of Spiti, Ware et al. (in production a) recognised ten ammonoid zones, confirming the results obtained for faunas from the Salt Range in Pakistan (Ware et al. in production b) and the subdivision of the Dienerian into three parts (early, middle, and late; Ware et al. 2015). ...
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A new, marine osteichthyan (bony fish) fauna from the Early Triassic of northern India is presented. The material was collected in situ at localities within Pin Valley (Lahaul and Spiti District, Himachal Pradesh, India) and is dated as middle-late Dienerian (one specimen possibly earliest Smithian). The new ichthyofauna includes a lower jaw of the predatory basal ray-finned fish Saurichthys, a nearly complete specimen of a parasemionotid neopterygian (cf. Watsonulus cf. eugnathoides), as well as further articulated and disarticulated remains (Actinopterygii indet., Actinistia indet.), and thus comprises the most complete Triassic fish fossils known from the Indian subcontinent. Saurichthys is known from many Triassic localities and reached a global distribution rapidly after the Late Permian mass extinction event. Parasemionotidae, a species-rich family restricted to the Early Triassic, also achieved widespread distribution during this epoch. Comparison of the Spiti material with other parasemionotid species reveals similarities with Watsonulus eugnathoides from Madagascar. However, taxonomic ambiguities within Parasemionotidae prevent a specific attribution of the Spiti specimen. The new material also includes an isolated actinistian urohyal exhibiting morphology distinct from any previously described urohyal. Marine Dienerian black shale deposited on continental shelves are common not only in the Himalayas but also in other geographic regions. Anoxic depositional settings provide ideal preservational conditions for vertebrate fossils, suggesting that additional ichthyofaunas could still be discovered in marine Dienerian strata of other localities. The study of Early Triassic fish assemblages, including the presented one, is fundamental for our understanding of the great osteichthyan diversification after the Late Permian mass extinction event.
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Taxonomy is the very first step of most biodiversity studies, but how confident can we be in the taxa delineation? One may hypothesize that the more abundant the material, the more accurate the description of morphological variability and hence the better the taxonomic delineation. Yet, as we shall see, in the case of numerous transitional forms, this hypothesis may prove wrong. Similarly to rarefaction curves that assess the degree of knowledge on taxonomic diversity through sampling effort, we aim to test the impact of sampling effort on species delineation by subsampling a given assemblage. To do so, we use an abundant and morphologically diverse conodont fossil assemblage from the Smithian of Oman. We first recognize four well established morphospecies but about 80% of the specimens are transitional forms. We quantify the diagnostic characters in a sample of 159 P1 elements using geometric morphometrics and assess, via gradually subsampling the assemblage, the number of morphometric groups (i.e. morphospecies) using ordination and clustering analyses. Four morphospecies were detected when less than 20% of the specimens were considered. The number of detected clusters dropped to two when including more than 30% of the specimens. Such influence of sampling effort on species delineation highlights the complexity of taxonomic work, especially when transitional forms are more abundant than typical specimens. These results should encourage researchers to extensively illustrate, measure and quantitatively compare their material to better constrain the morphological variability and delineation of taxa.
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