Taxonomy, evolution, and biostratigraphy of theOrthograptus quadrimucronatusspecies group (Ordovician, Graptolithina)

Abstract

TheOrthograptus quadrimucronatusspecies group currently comprises 22 taxa, which, with the exception ofO. ruedemanni, are distinguished from other orthograptids by the presence of paired apertural spines on their thecae. A detailed morphometric analysis of this group indicates that these taxa comprise four basic morphotypes that actually represent only six distinct species:O. pageanus, O. quadrimucronatus, O. spinigerus, O. ruedemanni, O. eucharis, andO. rivai.Specimens ofOrthograptus pageanushave a rapidly widening rhabdosome with elongated spines on the first two or four thecae.Orthograptus pageanusis divided into two chronological subspecies, an older, smaller form,O. pageanus pageanus, and a younger, larger form,O. pageanus maximusn. subsp. Specimens ofO. spinigerushave elongated spines on one or two thecal pairs between the sixth and thirteenth thecal pairs, a very narrow proximal end, and a rapidly widening rhabdosome. Specimens ofO. quadrimucronatushave a more parallel-sided shape and no unusually long spines. The subspeciesO. quadrimucronatus richmondensisis differentiated by the absence of both a median septum and a th 12spine.Orthograptus ruedemanni, O. rivai, andO. eucharisare all dwarf forms.Within a chronostratigraphic framework based on K-bentonite bed correlations the pattern and process of evolutionary change in the members of theO. quadrimucronatusspecies group is examined. Morphometrically, each species remains a coherent entity through time, occupying its own distinct morphospace. Each speciation event probably occurred rapidly, precluding the preservation of intermediates, and the new species retained their basic morphology throughout their existence. Within these lineages, phyletic changes were either absent or encompassed only changes in size, even when accumulated for much longer intervals than would be expected for speciation events. These patterns appear to be consistent with the evolutionary model of punctuated equilibrium.