Heterotrypa subfrondosa

• Zoarium Morphology: Frondescent to ramose (sometimes encrusting); generally thick, broad, fan-shaped colonies
• Zoecia: Acanthopores small
• Mesozooids: Abundant (generally)
• Monticules: Broad, low, and rounded

• Generally appears in thick, broad, fan-shaped colonies; covered with broadly rounded monticules

Brown & Daly (1985):

• Diagnosis: Zoaria frondescent, with small thin-walled zooecia, abundant diaphragms, numerous small mesozooecia, and abundant acanthopores.
• Description: Zooecia in endozone polygonal, arising from slender, pointed proximal tips and bending sharply into exozone. Intermonticular zooecia in exozone generally ovate to subpolygonal, less commonly polygonal, with straight to curved sides, small (about 9 in 2 mm, 0.20 mm MZD; table 15), and generally at right angles to surface.
  Zooecial walls thin in endozone, generally irregular, uncommonly flexuous locally, thickening slowly into early exozone, and generally thin (0.03 mm ZWT) in exozone, though commonly varying somewhat in thickness. Walls consisting of both dark and light curved laminae, typically appearing translucent in tangential section with darker and occasionally poorly laminated outer zone. Translucent wall lining rare.
  Diaphragms in endozone thin, generally straight, and typically spaced one to three zooecial tube diameters apart. Diaphragms in exozone much more numerous, commonly spaced one-fourth to one tube diameter apart, and typically thin, straight, and horizontal, though some inclined or, rarely, cystose. Diaphragms in exozone bending into zooecial walls as short diaphragm-wall units.
  Mesozooecia angular to polygonal in shape, variable in size, and commonly abundant (15 M1M), though may be sparse locally, generally poorly beaded, typically developed in curved zone and early exozone, and generally of short length. Tangential sections showing considerable range in distribution of mesozooecia from numerous and almost completely isolating zooecia (these areas tending to have oval or subpolygonal zooecia) to sparse or rare (these areas having almost regularly polygonal zooecia). The numbers of mesozooecia likely related to depth of section.
  Acanthopores variable in size (0.04 mm MAD, though ranging from 0.01 to 0.12 mm) and numbers (24+ A1M, with a range of 11 to 49), typically conspicuous at wall angles and inflecting in generally thin walls. Endacanthopores large, arising in outer endozone and early exozone, and occasionally oblique. Exacanthopores developing within exozone, small, generally of short length, and commonly in between wall angles where many are offset into zooecia. Some specimens showing considerable development of acanthopores in zones parallel to surface. Tangential sections of differing depths, therefore, showing considerable variation in numbers, depending on levels being cut; some sections showing numerous acanthopores in one place and fewer elsewhere. Those areas with counts of 11 to 35 A1M varying only slightly, but those areas with counts from 36 to 49 showing great variation. All acanthopores composed of sharply conical laminae with prominent central cores.
  Monticules nearly flush to low mounds, generally large, with numerous ovate to subpolygonal megazooecia, faintly laminated walls, numerous angular mesozooecia, and some acanthopores.
• Discussion: This is a common species in the middle and upper parts of the Dillsboro Formation, ranging to the base of the overlying Whitewater Formation. It is typically frondescent, as are many of the Cincinnatian species of Heterotrypa, and similar in some respects to many of these species. Cumings (1908, p.822) said that “this species seems to me to quite completely break down the line between Heterotrypa and Dekayia and Dekayella as formerly understood. In the form of the zoarium it certainly would be considered as a specimen of D[ekayia] frondosa. The presence of two well-marked sets of acanthopores would throw it into the genus Dekayella, and the absence of mesospores in some regions of the zoarium would suggest Dekayia.”
  The figured and unfigured types of H. subfrondosa (IU 9024-6, 9024-7, 9025-23, 9027-2, 9153-3, 9153-4, 9028-8 [figured in 1908], and 9030-11 [figured in 1908]) were examined and do not differ substantially from the Dillsboro specimens.
  Recent revisions (Boardman and Utgaard, 1966) have eliminated much of this difficulty by clarifying the nature of the differences between the three cited genera and especially the two sets of acanthopores. Species previously assigned to Dekayella are reassigned to Dekayia or Heterotrypa.
  Few Dillsboro species of Heterotrypa have the same general characters as H. subfrondosa; these include endozone diaphragms, thin zooecial walls, and numerous acanthopores and mesozooecia. H. microstigma has more acanthopores than H. subfrondosa, and H. cystata has slightly fewer. H. frondosa has similar quantitative data for ZTM, Z1M, MAD, and M1M, but it also has considerably thicker zooecial walls and slightly smaller zooecia and lacks diaphragms in the endozone. H. solitaria has few mesozooecia.

Cumings (1902):

• Zoarium growing upward from an expanded cylindrical basal attachment into flat fronts of a thickness of 10 mm. to 15 mm. and a breadth of as much as 60 mm. A specimen nearly complete, except the cylindrical base, has a height of 110 mm. The frond has a tendency to give off compressed branches along the free edges. Entire surface covered with small rather abruptly elevated monticules with an average diameter of 1.5 mm. From 12 to 13 occupy one square centimeter. At the apices of the monticules, the cells are smaller than the average. Cells mostly of one kind, 0.25 mm. in diameter, 40 cells to the cm.
• The internal structure of this species as seen in tangential sections is highly instructive. In tangential sections cutting the mature region, the cells are seen to be rather thin walled, the walls of adjacent zooecia being apparently amalgamated. That this is not the case is well shown in fig. 8, Pl. X, where the section cuts a portion of the zoarium that has been fractured and infilled with calcite along the fracture. The zooecia are spread apart, the wall formerly apparently common to two zooecia being now half on one side, half on the other side of the calcite seam. Where an acanthopore is present the zooecial wall separates from it cleanly. Indeed the acanthopore is sometimes left completely isolated in the calcite, showing that these structures belong to neither zooecial wall. The attention of those who deny the duplex character of the inter zooecial wall should be called to this phenomenon.
• Only a moderate number of small tubes are seen throughout the main part of ordinary tangential sections, Fig. 8, Pl. IX, shows a cluster of small tubes in a portion of a section in which the walls are also thicker than is usual. Tangential sections of the branchlets, however, present almost identically the same appearance as sections of D. ulrichi robusta (pl. IX, fig. 4).
• Acanthopores are numerous and conspicuously of two sizes, as is best shown in fig. 7, Pl. X. They are not confined to the angles of the zooecia but frequently indent their walls.
• Longitudinal sections (fig. 3, Pl. X) show that the mature region is very deep, the thickness of the zooecial walls varying but little from where the tubes bend outward, to the surface. The large acanthopores are conspicuous features of such sections (a, fig. 3, Pl. X). The walls present the beaded appearance characteristic of the genus. This I believe is in some cases due to the fact that the section cuts in and out of the side of an acanthopore. The large acanthopores traverse the entire mature region and are sometimes present even in the axial region. Diaphragms are abundantly developed, horizontal or, rarely, curved, or infundibular, from one-third to two tube diameters apart in the zooecia, and closer set in the mesopores. The walls of the latter are constricted where the diaphragms join them.
• In the crowded diaphragms, beaded walls, constricted mesopores, and two sizes of acanthopores, this species is a typical Dekayella, very similar to D. ulrichi-robusta (pl. X, fig. 2, pl. IX, fig. 4). In the thinness of the walls and fewness of the mesopores it is a typical Dekayia (cf. figs. 7 and 10, pl. IX; figs. 7 and 10, pl. X), and may be compared with such a form as D. multispinosa Ulr. In the shape of the zoarium, frequency, and expression of the monticules and tabulation of the zooecia Dekayia subfrondosa is very similar to D. perfrondosa (v. fig. I, pl. XI; fig. I, pl. X).

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