Climacograptus pungensRuedemann, 1904 and the definition of the Darriwilian (Ordovician) graptolite genusArchiclimacograptusMitchell, 1987

Abstract

The genus Archiclimacograptus represents one of the earliest taxa of axonophoran graptolites with geniculate thecae, but its current status and evolutionary relationships are difficult to access even after a number of cladistic analyses have been executed. Archiclimacograptus? pungens (Ruedemann), a poorly known little species from the lower Darriwilian of eastern North America is interpreted as a possible member of the genus. The species belongs to a group of axonophorans with highly asymmetrical proximal end and pattern C astogeny, characterized by the lack of an apertural spine on th1 2 , ranging from the Levisograptus dentatus Biozone (Darriwilian) to the Nemagraptus gracilis Biozone (Sandbian). Specimens are common in North America and the Argentinian Precordillera, but have not been reported from any Atlantic Faunal Realm localities. The distribution of the group indicates a restriction to the Pacific Faunal Realm, providing the earliest indication of a developing biogeographic differentiation of axonophoran graptolites after their origin in the oceanic biofacies during the Upper Dapingian to lower Darriwilian time interval. Archiclimacograptus? ambiguus is described as a new species.Résumé : Si le genre Archiclimacograptus représente un des taxons les plus anciens de graptolites axonophores dotés de thèques géniculées, son statut actuel et ses liens évolutionnaires sont difficiles à évaluer et ce, même après diverses analyses cladistiques. Archiclimacograptus? pungens (Ruedemann), une petite espèce méconnue du Darriwilien inférieur de l'est de l'Amérique du Nord, est interprété faisant possiblement partie de ce genre. L'espèce appartient à un groupe d'axonophores présentant une extrémité proximale très asymétrique et une astogénie de type C caractérisée par l'absence d'une épine aperturale sur th12, dont la distribution va de la biozone à Levisograptus dentatus (Darriwilien) à la biozone à Nemagraptus gracilis (Sandbien). Des spécimens sont répandus en Amérique du Nord et dans la précordillère argentine, mais leur présence dans des localités du domaine faunique atlantique n'a jamais été signalée. La distribution du groupe indique qu'il se limite au domaine faunique pacifique, constituant l'indice le plus ancien d'une différentiation biogéographique des graptolites axonophores après leur apparition dans le faciès océanique durant l'intervalle allant du Dapingien supérieur au Darriwilien infé-rieur. Archiclimacograptus? ambiguus est décrit comme étant une nouvelle espèce.[Traduit par la Rédaction]

The classification of the Pterobranchia (Cephalodiscida and Graptolithina)

“…Fig. 21) and uncertain as is seen from the interpretation in Maletz (2011b, fig. 3) deriving Pseudoclimacograptus and its descendants from an ancestor such as Haddingograptus, a genus without the typical proximal spines of Archiclimacograptus and derived taxa.…”

“…Maletz (2011b, figs 2, 3) without re-analyzing the climacograptids, provided two alternatives for the interpretation of early climacograptids (archiclimacograptids), but was unable to resolve the problems of early climacograptid evolution. His alternate interpretation (Maletz 2011b, fig. 3) suggested an early differentiation of the climacograptids from Undulograptus and a separate and independent evolution of the archiclimacograptids.…”

“…(Štorch et al 2011, p. 368) provided a cladistic definition for the Neograptina, even though they called it a diagnosis. Undulograptus is here referred to the Climacograptidae, but its precise phylogenetic relationship at the roots of Climacograptidae and Normalograptidae is uncertain (see Maletz 2011b). The origin and early evolution of proximally spineless (except for the virgella) axonophorans is completely unresolved and Maletz (2011b) suggested an origin of the climacograptids (Pseudoclimacograptus and descendants) from a proximally spineless ancestor (Fig.…”

“…23B), but this still has a number of characters similar to those of a pattern C astogeny and its evolutionary origin is uncertain (Maletz 2011c). As Maletz (2011b) provided an alternative for the evolutionary relationships of the early Climacograptidae (Fig. 21), the origin of Normalograptidae may have to be re-evaluated and it is preferred here to base them on the origin of a pattern H astogeny for the moment, instead of including taxa with a pattern C astogeny and without proximal spines except for the virgella (e.g.…”

“…The biserialuniserial rhabdosome shape of Dicranograptus appears to be a secondary development, originating through a di- Figure 21. Origin of Climacograptidae, alternative interpretation of Maletz (2011b, fig. 3).…”