Phylum: Bryozoa
Class: Stenolaemata
Order: Cryptostomata
Genus: Arthrostylus Ulrich, 1882

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Type Species: Helopora tenuis (James, 1878)
Species found in the Cincinnatian, USA

  • Arthrostylus curtus (Ulrich, 1882)
  • Arthostylus obliquus (Ulrich, 1890)
  • Arthrostylus tenuis (Ulrich, 1890)

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Geologic Range
Middle Ordovician (Blackriv.) – Late Silurian (Wenlock. or ?Ludlov.)

Stratigraphic Occurrences


Geographic Occurrences

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Sequences (Formations)

  • C2 Sequence (Mount Hope)
  • C1 Sequence (McMicken, Southgate, Economy, Fulton)

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Identification in Hand Sample:

  • Branching zoarium (but only at ends of segments); slightly curving, polygonal segments ~3mm wide
  • Zooecia in 2-4 rows; on one side of segment; separated by prominent longitudinal ridges
  • Zooecia rounded to subpolygonal in cross section
  • Nonlaminated material locally forming endozonal wall

Arthrostylus from the Kope Formation of Cincinnati, Ohio (CMC 70521)

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Steve Holland (2013, UGA strat lab):

  • Zooarium branching, but only at ends of segments. Straight to slightly curving, polygonal segments, approximately 3 mm wide. Apertures in 2 to 4 rows on one side of segment, separated by prominent longitudinal ridges, with prominent longitudinal ridges on opposite side of segment

Treatise on Invertebrate Paleontology. Part G (1983):

  • Zoarium dendroid, jointed, branching at ends of segments only. Individual segments straight or slightly curved, segment diameters approximately 0.3 mm, diameters usually constant between joints except for terminal enlargement at joint surfaces. Segment cross section polygonal. In segments from single populations, apertures in 2 to 4 longitudinal rows on obverse surfaces, offset in adjacent rows. Prominent longitudinal ridges developed on reverse surfaces, between rows of apertures, and between successive chambers; peristomes present. Lateral zooecia budded from walls of reverse surfaces; medial zooecia, where developed, budded from walls of lateral zooecia. Zooecial bases inflated. Zooecia recumbent in endozone, diverging from reverse surface at zooecial bend; rounded to subpolygonal in cross section. Zooecial bends abrupt, living chambers in exozones about 90* to segment surfaces. Zooecial length from 4 to 6 times diameter; arrangement of zooecia regular. One or two thin diaphragms common near base of zooecia. Exozonal width between ridges approximately one-quarter zooecial diameter. Zooecial boundaries well defined, nonlaminated wall locally well defined , nonlaminated wall locally well developed between zooidal wall along reverse surface and extrazooidal wall, and along zooecial boundaries in endozone. Nonlaminated material locally forming endozonal wall; zooecial boundaries elsewhere irregular, or locally not visible. Lamellar profiles rounded over longitudial ridges, between laterally adjacent zooecia; flattened between longitudinally successive zooecia. Paurostyles present, weakly developed.
  • Arthrostylus is distinguished by budding patterns, zooecial shape and orientation, development of nonlaminated wall, and paurostyle development of nonlaminated wall, and paurostyle development. Although distinctive in growth habit and development of nonlaminated walls, it resembles some other arthrostylid genera in jointed pattern and stylet development. Simpson (1897) suggested separation of Arthrostylus from other arthrostylid genera at the family level. Budding patterns and wall development suggest affinities between Arthrostylus and the Phylloporinidae

Bassler (1911):

  • This interesting member of the Arthrostylidae is represented in the Russian collections by a few individuals which can be referred to the two American Black River species, A. conjunctus and A. obliquus of Ulrich. These specimens consist of free segments of the zoarium, pointed at one end for articulation with the preceding segment, and with a socket-like hollow at the other end. The complete zoarium in Arthrostylus consists of numerous very small subquadrate segments united by terminal articulation and branching dichotomously until a small pinnate or bushy colony is the result; each of these segments has one of its faces longitudinally striated and each of the other faces, of which there are commonly three, bears a linear series of apertures between longitudinal ridges.
  • Genotype.—Helopora tenuis James. Earliest Silurian (Richmond) of the United States.

Nickles & Bassler (1900):

  • Zoarium bushy, dichotomously branching, the whole consisting of numerous exceedingly slender, equal, subquadrate segments, united by terminal articulation; one face longitudinally striated, on each of the other, commonly three faces a linear series of apertures between longitudinal ridges.

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