Lower Homerian (Silurian) Pristiograptus from the Zdanów section, Bardo Mountains (Sudetes, Poland) and their palaeobiogeographical implications

Abstract

recorded from the Llandovery to the upper Přídolí, but is most common in the Wenlock–Ludlow interval and is one of the longest ranging monograptid genera. The genus is characterized by cylindrical thecae without distinct apertural processes and a slightly curved or straight, simple rhabdosome (after Urbanek 1958). This conservative monograptid group survived all of the Wenlock–Ludlow biotic crises and radiated and prospered while populations of other monograptids decreased. After those biotic crises new genera of monograptids developed from Pristiograptus via speciation and adaptive radiation. The extinction of the Pristiograptus lineage was the penultimate “nail in the coffin” of monograptid evolution. Jaekel (1889) erected the genus Pristiograptus. According to him, the rhabdosome of Pristiograptus is unbranched, is straight or a little curved, the thecae are cylindrical, inclined to rhabdosome axis, the thecal mouth opening free (the mouths of the thecae are not covered by any lid or hood) and, if present, apertural processes stand as spines at the lower edge of the mouth. Today, some of Jaekel’s Pristiograptus taxa are assigned to other genera [e.g. Testograptus testis (Barrande)]. Přibyl (1943) revised Pristiograptus based on material from Czechia and lit­ e rature data, producing the first catalogue of the genus. Münch (1952) assembled his own summary of known graptolites, including Pristiograptus, and presented short descriptions and illustrations of all known taxa. This was later followed up by phylogenetic studies of Pristiograptus and related genera presented by Rickards & Wright (2003), Radzevičius (2003), Radzevičius et al. (2008), Urbanek et al. (2012) and Whittingham et al. (2020). In short, the taxa of Pristiograptus genera are well known and very important for graptolite evolution and palaeogeography. Only one problematical species, Pristiograptus fre­ quens Jaekel (described as having thecae of colonus type, as opposed to the unornamented thecae typical of P. frequens), has been recorded from the Zdanów outcrop (Malinowska 1955), although graptolites are common there. Surprisingly, there have been very few graptolite studies based on material from the Zdanów outcrop, despite graptolites from the lower Silurian to the lower Devonian in this section being very common in this section. The Zdanów outcrop is thus very important in the Sudetes region because of its considerable stratigraphical extent. In this work two species of Pristiograptus are docu­ mented for the first time from the Sudetes Mountains. We