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

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Type Species: Helopora tenuis (James, 1878)
History: (Nickles & Bassler, 1900)

  • 1882 Pachydictya Ulrich, Jour. Cincinnati. Soc. Nat. Hist., V, p. 152.
  • 1887 Pachydictya Foerste, Bull. Sci. Lab. Denison Univ., II, p. 162.
  • 1889 Pachydictya Miller, North American Geol. Pal, p. 313.
  • 1890 Pachydictya Ulrich, Geol. Sur. Illinois, VIII, pp. 390, 522.
  • 1893 Pachydictya Ulrich, Geol. Minnesota, III, p. 145.
  • 1897 Pachydictya Simpson, Fourteenth Ann. Rep. State Geologist New York for the year 1894, p. 530.

Species found in the Cincinnatian, USA

  • Pachydictya bifurcata (Hall, 1883)
  • Pachydictya cyclostomoides (Eichwald, 1855)
  • Pachydictya everetti (Ulrich, 1890)
  • Pachydictya firma (Ulrich, 1890)
  • Pachydictya kuckersensis (Bekker, 1919)
  • Pachydictya magnipora (Ulrich, 1889)
  • Pachydictya splendens (Ulrich, 1890)
  • Pachydictya uralica (Astrova, 1965)

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Geologic Range
Middle Ordovician – Early Silurian

Stratigraphic Occurrences

Geographic Occurrences

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Sequences (Formations)

  • C5 Sequence (Whitewater)

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Identification in Hand Sample:

  • Zoarium branching, commonly bearing wide margins
  • Zooecia subelliptical to subcircular zooecia
  • Lacking continuous longitudinal walls
  • Monticules common, flat or raised, locally with scattered zooecia

Pachydictya from Wilmington, Illinois (CMC 35358)

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Steve Holland (2013, UGA strat lab):

  • Zooaria branching, commonly bearing wide margins. Subelliptical to subcircular zooecia.

Treatise on Invertebrate Paleontology. Part G (1983):

  • Zoarium branched; branches commonly with wide margins. Mesothecae generally straight. Median rods subcircular in cross section, commonly with diameter greater than width of median granular zone. In endozones, autozooecia in indistinct ranges, subelliptical to subcicular in cross section, partly contiguous, partly separated by extrazooecial stereom, and lacking continuous longitudinal walls. In exozones, autozooecia in relatively distinct ranges, broadly elliptical in cross section, partly contiguous or separated by extrazooecial steroem and lacking continuous longitudinal walls. Autozooecial walls locally vesicular in inner exozones. Mural styles common, mostly in autozooecial boundaries, also scattered in walls and laminar stereom. Living chambers subelliptical to subcircular in cross section. Chamber lining common, relatively thick. Other lateral chamber structures absent. Basal diaphragms straight to curved, generally common. Monticules common, flat or raised, locally with scattered zooecia. Monticules commonly vesicular in inner exozones, with mural styles singly or in indistinct rows in outer stereom. Extrazooecial skeleton common; vesicles localized in endozones and inner exozones, stereom scattered throughout exozones.

Ross (1961)

  • Emended definition.- Colonies are bifoliate and have ribbon-shaped or explanate forms of growth. Zooecia at the surface of the colonies are aligned in longitudinal ranges which are not clearly demarcated by raised zooecial walls of the lateral interspaces. Zooecia at the surface are oval in cross section and the zooecial tubes are outlined by circular or oval rims. Acanthopores are numerous in the zooecial walls. Mesopores are absent. Zooecia grow from the mesotheca at a low angle and form an inner zone having thin zooecial walls. The gradual deflection of the zooecia away from the mesotheca where the tabulate interspaces generally first appear marks the beginning of the outer zone of thickened zooecial walls. The inner zooecial walls lining the zooecial tubes are composed of exceptionally long laminae aligned parallel to the direction of zooecial growth. They curve into the broadly arched intertonguing laminae of the outer walls. The numerous acanthopores which multiply in the outer zone commonly obscure the laminate structure of the outer zooecial walls in which no zooecial boundaries are apparent. Hemisepta are absent in the zooecial walls. The tabulate interspaces first appear in the outer part of the inner zone and commonly extend for some distance into the outer zone. The simple mesotheca contains numerous median tubuli.
  • Remarks. – The tabulate interspaces which develop near the base of the outer region of the zooecia and which extend for some distance before they are closed by thickened zooecial walls are a distinctive feature of the genus Pachydictya. As the walls of these tabulate interspaces are for the greater part narrow they occasionally appear as if they are a series of overlapping cystiphragms along the zooecial walls.

Bassler (1911):

  • All of the four species of Pachydictya found in the Baltic strata prove to belong to that section of the genus in which the zoarium is narrow with subparallel margins and the longitudinal arrangement of the zooecia predominates. The type species of the genus has a broad, palmate zoarium, but its internal structure is essentially the same as in the narrow, branched forms. The zooecial apertures in Pachydictya are oval and have well-marked, ring-like walls, this character alone being so marked that it serves as a distinction from other genera of Rhinidictyonidae.
  • Genotype.—Pachydictya robusta Ulrich. Middle Ordovician (Ottosee) shales of east Tennessee.

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