Pyknotylacanthus spathianus gen. et sp. nov., a new ctenacanthoid from the Early Triassic of Bear Lake (Idaho, USA)

Abstract

-A fairly completely preserved finspine recovered in association with dermal denticles from the Early Triassic of Bear Lake County (Idaho, USA) represents the youngest unequivocal record of a Bythiacanthuslike shark (Ctenacanthoidea incertae sedis). The accompanying invertebrate fauna indicates a late Early Spathian age (Early Triassic). Comparison of the finspine with sparse Late Paleozoic records allows identification of predominantly plesiomorphic, "ctenacanthoid" characters such as shape and cross-section of the finspine, the concave posterior wall, and the stellate morphology of the tubercles. One presumably derived character, the large and densely spaced tubercles with reduced ornament, distinguishes Pyknotylacanthus gen. nov. from its Carboniferous predecessors. It is suggested that "Cosmacanthus humboldtensis" from the Middle Triassic West Humboldt range (Nevada, USA) may represent an even younger member of the same lineage. The sample of associated dermal denticles is difficult to interpret but believed to belong at least in part to the same individual. The variability of denticles exceeds the range of expected individual variation, and is discussed in detail.

Marine Early Triassic Actinopterygii from Elko County (Nevada, USA): implications for the Smithian equatorial vertebrate eclipse

“…One of the longest known collecting areas for Early Triassic marine vertebrate fossils is southeast Idaho, yielding ichthyoliths (Evans, 1904; Goddard, 1907; Mutter and Rieber, 2005; Romano et al, 2012; Brayard et al, 2017), but also articulated fishes (Tanner, 1936; Romano et al, 2012), and some ichthyopterygian reptile remains (Massare and Callaway, 1994; Scheyer et al, 2014). In addition to these published specimens, new vertebrate material (isolated bones or articulated partial/complete skeletons) was recently discovered at several sites in Idaho, for instance in the middle Smithian ‘ Meekoceras beds’ (personal observation, CR, JJ, KGB, HB, 2013–2016), the late Smithian Anasibirites beds ( Saurichthys : PIMUZ A/I 4621, NMMNH P-77359; several unlabelled specimens of Actinopterygii indet.…”

The braincase of the Middle Triassic shark Acronemus tuberculatus (Bassani, 1886)

“…Conversely, some fossil shark fin spines may have an extensive ornament of tubercles but lack an anterior keel (e.g. Bythiacanthus , Pyknotylacanthus , Asteracanthus ; Maisey 1978, 1982 b ; Mutter and Rieber 2005). The fin spine ornament of Homalodontus is said to include a Nemacanthus ‐like anterior ridge, although it is ‘partly tuberculate and only fragmentarily preserved’ (Mutter et al.…”

Elasmobranchs from the Lower Triassic Sulphur Mountain Formation near Wapiti Lake (BC, Canada)

“…nov., one given specimen may possess denticles of considerable variability, and even on the same fin it is possible to find denticles that have from none to numerous ridges in variably shaped platforms. The range of individual, intraspecific and interspecific variation in the denticles is rather unsatisfactorily known in Lower Triassic sharks, and it seems impossible at present to distinguish Early Mesozoic sharks on the basis of dermal denticle morphology alone (see also Mutter & Rieber, 2005). The assignment of any specimen from the Sulphur Mountain Formation to Palaeobates sp.…”