Revision of the genus Anasibirites Mojsisovics (Ammonoidea): an iconic and cosmopolitan taxon of the late Smithian (Early Triassic) extinction

Abstract

The family Prionitidae Hyatt represents a major component of ammonoid faunas during the Smithian (Early Triassic), and the genus Anasibirites Mojsisovics is the most emblematic taxon of this family. Its stratigraphical range is restricted to the beginning of the late Smithian (Wasatchites distractus Zone). The genus is also characterized by an unusual cosmopolitan distribution, thus contrasting with most earlier Smithian ammonoid distributions that were typically restricted by latitude. Because the late Smithian witnessed an extinction of the nekton (e.g. ammonoids, conodonts) whose amplitude is equal to or larger than that of the end‐Permian crisis, the number of valid species that should be included in the genus Anasibirites becomes a highly relevant question when addressing this extinction at the highest possible taxonomic resolution. Based on a new extensive collection from Timor, the composition of the genus Anasibirites is herein revised with respect to its intraspecific and ontogenetic variations. Comprehensive morphological and biometric studies (c. 950 measured specimens) indicate that, of the c. 60 available species names, only two are valid, namely A. kingianus (Waagen) and A. multiformis Welter. Continuous ranges of intraspecific variation enable us to synonymize A. nevolini Zakharov, and A. angulosus (Waagen) with A. kingianus. The contribution of Anasibirites to species diversity during the late Smithian extinction is thus significantly less than previously estimated, therefore accentuating the severity of this event.

Learning from beautiful monsters: phylogenetic and morphogenetic implications of left-right asymmetry in ammonoid shells

“…Usually, faint ribs are associated with compressed apertures and involute coiling, while strong ribs are associated with depressed apertures and evolute coiling. These patterns of intraspecific variation have been observed in Triassic [68][69][70][71][72][73][74][75][76][77], in Jurassic [66] and in Cretaceous [78] ammonoids. As these patterns of covariation have been observed in phylogenetically distant ammonoids at several different time periods, they represent evolutionary convergences that stem from the developmental constraints imposed by accretionary growth [19,[79][80][81][82][83].…”

New late Smithian (Early Triassic) ammonoids from the Lusitandiadalan Member, Vikinghøgda Formation, Svalbard

“…This is not the first late Smithian occurrence of taxa known from middle Smithian strata. Jattiot et al (2015) recorded two specimens of Galfettites omani Brühwiler & Bucher (2012a) in a late Smithian fauna from Noe Tobe, Timor, representing the first and youngest known occurrence of this taxon. Another arctoceratid, Churkites Okuneva, 1990, occurs in the latest middle Smithian in the western USA, but also in the latest Smithian in South Primorye (Shigeta & Kumagae, 2015).…”

“…As discussed in the recent revision by Jattiot et al (2015), this genus can be identified by its distinctive megastriae, which are ridge-like structures generated by discontinuities in secretion of the shell. Megastriae are asymmetric, superficial features, while ribs on the contrary are more symmetric and their formation also involves the deeper mantle layers.…”

Early Triassic environmental dynamics and microbial development during the Smithian–Spathian transition (Lower Weber Canyon, Utah, USA)

“…Hofmann et al, 2014). A robust middle to late Smithian biostratigraphic framework based on ammonoids is also available for most parts of the basin (Brayard et al, 2009(Brayard et al, , 2013Jattiot et al, 2016Jattiot et al, , 2017. Various Smithian and Spathian microbial deposits have already been reported within the SFB (Schubert and Bottjer, 1992;Pruss et al, 2004;Bottjer, 2009, 2011;Vennin et al, 2015;Olivier et al, 2016), but to our knowledge, no Smithian MISS.…”