Lower Aeronian triangulate monograptids of the genus Demirastrites Eisel, 1912: biostratigraphy, palaeobiogeography, anagenetic changes and speciation

 

Authors: Štorch P, Melchin MJ

Published in: Bulletin of Geosciences, volume 93, issue 4; pages: 513 - 537; Received 31 October 2018; Accepted in revised form 3 December 2018; Online 20 December 2018

Keywords: graptolite, Silurian, Aeronian, zonal index species, stratigraphy, taxonomy, evolution, intraspecific variability, Prague Synform, Czech Republic,

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Abstract

Bed-by-bed sampling of the lower Aeronian black shale succession exposed at the Hlásná Třebaň section in the Prague Synform of the Czech Republic has provided a rich and continuous fossil record of biostratigraphically important “triangulate monograptid” graptolites referred to the genus Demirastrites Eisel, whose diagnosis is revised. Eight morphological forms assigned to four species, including the biozonal index taxa Demirastrites triangulatus (Harkness) and D. pectinatus (Richter), as well as D. major (Elles & Wood) and D. campograptoides sp. nov., are distinguished and described. Also documented are patterns of morphological change within species that we interpret to be anagenetic changes, as well as apparent patterns of morphological divergence that we interpret to represent speciation events. The proposed phylogeny suggests that the D. triangulatus lineage underwent significant anagenetic changes throughout its range before it split at the base of the pectinatus Biozone and gave rise to D. pectinatus, which then underwent further anagenetic changes between its early and late forms. At a slightly later stage, in the lower pectinatus Biozone, the D. triangulatus stem lineage underwent further significant changes, giving rise to D. major, which then saw further significant anagenetic changes before it became extinct in the middle pectinatus Biozone, well below the highest occurrence of D. pectinatus. D. campograptoides is a rare and apparently short-ranging species, thus far recorded only from Bohemia, derived from either early D. triangulatus or its direct ancestor. Reconsideration of the species’ records worldwide suggest that none of these lower Aeronian forms was truly cosmopolitan.

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