The morphology and ultrastructure of Brevigraptus quadrithecatus n. gen., n. sp. ( Diplograptacea), and its convergence upon Dicaulograptus hystrix (Bulman).

Rhabdosomes of Brevigraptus quadrithecatus n.gen.n.sp. (Lasiograptinae, Diplograptacea), from the Upper Ordovician Viola Springs Formation of S-central Oklahoma, comprise 4 fully developed thecae. The sicula and the 1st 2 thecae are fully sclerotized. The thecal form and list architecture of Brevigraptus quadrithecatus are nearly identical to those of Pipiograptus hesperus Whittington. Brevigraptus quadrithecatus possesses a Pattern G astogeny and exhibits several derived astogenetic features that it shares with P. hesperus and Orthoretiolites hami Whittington. Both thecal and astogenetic similarities suggest the new taxon is a member of the Lasiograptinae, and is closely allied to the aforementioned species. The thecae of B. quadrithecatus exhibit striking similarity with Dicaulograptus hystrix (Bulman). However, both the details of thecal construction and primordial astogeny differ markedly between these species. The thecal similarities appear to be convergent. Accordingly, D. hystrix is probably not closely allied to the Lasiograptinae. -from Author