Phylum: Bryozoa
Class: Stenolaemata
Order: Cystoporata
Genera: Nicholsonella Ulrich, 1890

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Type species: Nicholsonella ponderosa (Ulrich, 1890)Species in Cincinnatian of Ohio, Indiana, Kentucky (

  • Nicholsonella ponderosa (Ulrich, 1890)
  • Nicholsonella peculiaris (Cumings & Galloway )
  • Nicholsonella vaupeli (Ulrich, 1883)

Taxonomic History (Nickles & Bassler, 1900)

  • 1890 Nicholsonella Ulrich, Geol. Sur. Illinois, VIII, pp. 374, 421.
  • 1889 Nicholsonella (Ulrich, in press), Miller, North American Geol. Pal., p. 313.
  • 1893 Nicholsonella Ulrich, Geol. Minnesota, III, p. 313.
  • 1896 Nicholsonella Ulrich, Zittel’s Textb. Pal. (Engl. ed.), p. 276.
  • 1897 Nicholsonella Simpson, Fourteenth Ann. Rep. State Geologist New York for the year 1894, p. 590.

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Stratigraphic Occurrences


Geologic Range

Geographic Occurrences

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Sequences (Formations)

  • C5 Sequence (Lower Whitewater, Liberty, Waynesville)
  • C4 Sequence (Arnheim)
  • C3 Sequence (Mount Auburn)
  • C2 Sequence (Bellevue)

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Identification in Hand Sample:

  • Zoarium Morphology: Ramose to frondose
  • Zoecia: Thick-walled; styles common
  • Mesozooids: Large, polyongal; filled with calcareous deposits; separate zooecia
  • Monticules: None
  • Maculae: None

Species Differentiation

Nicholsonella from Cincinnati, Ohio (CMC 57833)

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Holland (UGA Strat Lab, 2013):

  • Nicholsonella is ramose to frondose. Zooecia are thick-walled and separated by large polygonal mesozooids filled with calcareous deposits. Diaphragms and styles are common

Bassler (1911):

  • Species of Nicholsonella are readily recognized in thin sections by the presence of a granular, calcareous deposit in the outer part of the mature zone, filling the interzooecial spaces and obscuring the walls of the mesopores. The walls of both zooecia and mesopores are traversed longitudinally by minute tubuli, which, in tangential sections and at the surface appear as granular, acanthopore-like structures. Somewhat similar granular structures are present in Constellaria with which Nicholsonella is believed to be allied, although the absence of such distinct, star-shaped maculae in the latter is an easy means of separation. The following well-marked species has characters quite different from any known American form of Nicholsonella but there seems to be no reason for not referring it to the genus.
  • Genotype.—Nicholsonella ponderosa Ulrich. Middle Ordovician (Black River) of Illinois and Minnesota.

Bassler (1906):

  • The following two species are apparently the last representatives of this genus, which is identified for the first time in the Silurian system. As might be expected, both differ in some respects from the typical Ordovician species, but the general aspect of each is such that Nicholsonella seems the only genus to which they can be referred with some certainty. Both develop the peculiar character of Nicholsonella, namely, that with age the walls of the mesopores become obscured by a calcareous deposit filling the interzooecial spaces. Indeed, the walls of the zooecia and mesopores have an indefinite structure throughout the mature region that is in marked contrast with the clear-cut walls of Liodema, to which genus both species had formerly been referred.
  • Zoarium an explanate expansion varying from 3 to 10 mm. in thickness and sometimes attaining a width of 50 mm., although the specimens are usually seldom over 25 mm. wide. Base marked with a wrinkles epitheca. Celluliferous surfaces without monticules, even, the maculae being inconspicuous and composed of groups of zooecia of the normal size but separated by interspace slightly wider than usual. Zooecia large, rounded, two to three in 2 mm., and on an average slightly less than 0.5 mm. in diameter. Apertures floriform, the acanthopores inflecting the zooecial walls which encroach upon the zooecial cavity. Interzooecial spaces open in young specimens and showing one or two rows of mesopores separating adjoining zooecia, but in old specimens closed with a solid, granulose deposit. Acanthopores large, numerous, averaging four or five to each zooecium, and inflecting the walls so as to give the floriform or petaloid appearance to the zooecial aperture.
  • Internal characters. – Carefully prepared vertical sections show a very short, immature region in which the zooecia are prostrate, and although their walls are of a clearer structure than that obtaining in the mature zone, even here their indistinct nature is clearly shown. With the inception of the mature region, the bend to which is quite abrupt, mesopores and acanthopores are developed. Here the wall structure already noted becomes still less distinct, and, finally, in the older parts of the zoarium, the walls of the zooecia, and especially of the mesopores, become obliterated by a deposit of granular tissue. Diaphragms are inserted in the zooecial tubes at an average distance apart of a trifle less than their own diameter. The diaphragms of the mesopores are at a like distance from each other, but because of their narrower diameter appear more crowded.
  • As is usual in most Trepostomata, the diaphragms are more abundant in the later than in the earlier parts of the mature region. The acanthopores of this and of the following species are quite different from the normal form of this structure, as they are seen in vertical sections to be simple tubes with a more or less definite wall enclosing a central clear space. No trace of the usual laminated tissue arranged in cone-like layers and pierced vertically by a central canal can be observed.
  • Tangential sections present various appearances, according to the portion of the zoarium cut by them. At the beginning of the peripheral regions the walls of the zooecia and mesopores are thin and although hazy in structure are still fairly clear. Here the acanthopores are small and inconspicuous. Higher up in the mature region the walls are considerably thickened and the acanthopores have increased in size and number. Finally, in the most mature or aged condition, the interzooecial space is occupied by a granular tissue obscuring the mesopores and through which the acanthopore pierce.
  • The large, rounded, isolated zooecia, floriform apertures, closed interspaces, numerous acanthopores, and explanate growth, are characters which make the identification of this species quite easy. The closely related form N. ringuebergi n. sp. is distinguished by the presence of much smaller zooecia. Liodema explanatum has a similar method of growth but the smaller zooecia and open mesopores are points of difference distinguished by the unaided eye. Internally the two are also quite different as their respective generic references indicate.

Nickles & Bassler (1900):

  • Zoarium a laminar expansion, sometimes giving off flattened intertwining branches or fronds; interspaces often granose, interzooecial spaces wide, filled with numerous mesopores, which have thicker and more numerous diaphragms than the zooecial tubes; with age, the spaces become filled up with a calcareous deposit, rendering the walls of the mesopores unrecognizable.

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