Calloway Creek limestone and Ashlock and Drakes Formations (Upper Ordovician) in south-central Kentucky

Arthrorhachis tarda (Barrande, 1846) and Sphaeragnostus cingulatus (Olin, 1906) are described from the Upper Ordovician (Harjuan Series) of Sweden and Bornholm, Denmark. S. cingulatus is extremely rare, while A. tarda is generally abundant in middle Ashgillian strata and known from a great number of localities in southern and south-central Scandinavia. Outside Scandinavia, A. tarda is frequently encoun- tered in the pre-Himantian Ashgill of Poland, Bohemia, various parts of the British Isles, and Kazakhstan. In most of these areas, the species is generally confined to mudstone sequences, and it is usually associated with faunas of Mediterranean type. The generic concept of Sphaeragnostus is revised.

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The morphology and ultrastructure ofBrevigraptus quadrithecatusn. gen., n. sp. (Diplograptacea), and Its convergence uponDicaulograptus hystrix(Bulman)

Journal of Paleontology ◽

10.1017/s0022336000059242 ◽

1988 ◽

Vol 62 (3) ◽

pp. 448-463 ◽

Cited By ~ 7

Author(s):

Charles E. Mitchell

Keyword(s):

Striking Similarity ◽

New Taxon ◽

Cortical Tissue ◽

Upper Ordovician ◽

The Third ◽

Growth Increments ◽

South Central

Rhabdosomes ofBrevigraptus quadrithecatusn. gen., n. sp. (Lasiograptinae, Diplograptacea), from the Upper Ordovician Viola Springs Formation of south-central Oklahoma, comprise four fully developed thecae. The sicula and the first two thecae are fully sclerotized. The ultrastructure of the fusellum is unusually dense but is overlaid by a typical diplograptacean bandaged cortex. The third and fourth thecae consist of clathria covered by a cortical sheet. Lacinia are absent. The cortical sheet comprises bandages deposited in a support dominated pattern that matches expectations of the pterobranch model of peridermal secretion. Lists are fusellar derivatives and exhibit traces of fuselli-like growth increments but no continuous fusellum is present. Lists are strongly thickened with cortical tissue. The fabricational pattern employed in list construction reveals the operation of strong historical constraints during the evolutionary reduction of the fusellum.The thecal form and list architecture ofBrevigraptus quadrithecatusare nearly identical to those ofPipiograptus hesperusWhittington.Brevigraptus quadrithecatuspossesses a Pattern G astogeny and exhibits several derived astogenetic features that it shares withP. hesperusandOrthoretiolites hamiWhittington. 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 ofB. quadrithecatusexhibit striking similarity withDicaulograptus 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. hystrixis probably not closely allied to the Lasiograptinae.

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