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EVOLUTION AND PHYLOGENETIC CLASSIFICATION OF THE DIPLOGRAPTACEA

Abstract of the paper:
Mitchell, Ch. E.. 1987. Evolution and phylogenetic classification of the Diplograptacea. - Palaeontology 30, 2, 353-405
The complex astogenetic patterns produced by the specialized, first few thecae (primordial thecae) of
graptoloid rhabdosomes were conserved during evolution and provide a reliable guide to 'propinquity
of descent' among diplograptids. Using this principle one can redefine their taxonomy, establish
phylogenetically meaningful higher taxa, and obtain an improved understanding of diplograptacean
phylogeny. The Diplograptacea comprises four major subclades: 1, Orthograptidae: archaic
orthograptids (species of the '
Glyptograptus' teretiusculus species group), Orthograptus and
Amplexograptus, together with archiretiolitids, lasiograptids, and 'Climacograptus' typicalis - 'G.'
lorrainensis groups; 2, Dicranograptidae: dicranograptids plus nemagraptids; 3, Diplograptidae:
pseudoclimacograptids and
Climacograptus s.s., together with Diplograptus s.s. and offshoots; 4,
Monograptidae: '
G' dentatus and descendants including Undulograptus paradoxus, G. euglyphus,
'
C.' normalis, and all the Silurian diplograptids including the uniserial monograptines. The first three
families dominated Ordovician faunas. Taxa with complex proximal end structures were succeeded  
during the Llandeilo and Caradoc by taxa with simpler astogenies. Following extinction of the
dominant Ordovician taxa, monograptids (sensu lato) underwent explosive evolution in the
Llandovery. New generic group taxa:
Archiclimacograptus, Arnheimograptus, Diplacanthograptus,
Eoglyptograptus, Geniculograptus, Hustedograptus, Oelandograptus, Pseudoamplexograptus, and
Urbanekograptus. New family group taxa: Eoglyptograptinae and Orthograptidae.
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