History: (Nickles & Bassler, 1900)
- 1883 Amplexopora robusta Ulrich, Jour. Cincinnati Soc. Nat. Hist., VI, p. 82, pl. i, 1-1b.
- 1889 Amplexopora robusta Miller, North American Geol. Pal., fig. 450 (p. 292).
- 1890 Amplexopora robusta Ulrich, Geol. Surv. Illinois, VIII, fig. 7d (p. 318).
- Cincinnati (Lorraine): Cincinnati, Ohio; Boyle County, Kentucky.
Map point data provided by iDigBio.
- C4 Sequence (Arnheim: Sunset)
- C3 Sequence (Mt. Auburn)
- C2 Sequence (Bellevue)
Identification in Hand Sample
- Zoarium Morphology: Dendroid, of smooth cylindrical stems; 10-12 mm in diameter branches
- Zoecia: Polygonal, thin-walled; 7-8 in 2 mm
- Mesozooids: Absent
- Monticules: None
- Maculae: Conspicuous at intervals of 3-4mm. Composed of larger apertures.
Amplexopora robusta from the Arnheim Formation of Brookville, Indiana (OUIP 1026)
Steven Holland (UGA Strat Lab 2013):
- Ramose growth form
Ernst & Key (2007):
- Description. – Ramose colony with oval cross-section, 6– 9 mm in diameter. Exozone 0.9–1.4 mm wide, endozone 4.2– 6.2 mm wide. Secondary overgrowths common. Autozooecia growing for a long distance in endozone, then bending sharply and intersecting colony surface at angles of 80–90◦. Autozooecial apertures polygonal, 5–6 per 2 mm in intermacular areas and 4–4.5 in macular areas. Diaphragms in endozone thin, straight, spaced 2–3 per 1 mm length; in exozone thin to thick, straight and inclined, 5–10 spaced per 1 mm autozooecial length. Mesozooecia rare, polygonal to rounded in cross-section, short, small. Acanthostyles rare, more common in maculae, having distinct hyaline cores and wide, dark sheaths, often inflecting autozooecia, originating in outer part of endozone. Autozooecial walls 0.005 mm thick, granular in endozone; displaying reverse V-shaped lamination with a dark median lining, 0.05–0.10 mm thick in exozone. Maculae consisting of megazooecia, 1.7–2.1 mm in diameter.
- Zoarium ramose, consisting of cylindrical, oftener of flattened branches, dividing dichotomously at rather frequent but irregular intervals, and usually varying in diameter from .4 inch to .7 inch. A very large specimen in my cabinet has a length of 4.3 inches; the central stem is flattened and varies in diameter from 1.1 inches to 1.7 inches. The bases of two branches are on one side, and three on the other, the mean diameter of which is about .6 inch. Monticules are not developed. The cells are moderately thin-walled, polygonal, and consist of one kind only (i.e., the interstitial cells are wanting); their arrangement is quite regular, and, when well preserved, shoe at the angles of junction the elevated points of the spiniform tubuli. At intervals of about .15 inch, the surface exhibits conspicuous clusters of cells larger than the average, with a mean diameter of 1/60th of an inch. The smaller or ordinary cells have a diameter varying from 1/100th to 1/90th of an inch.
- Tangential sections show that the cell-walls are comparatively thin and polygonal and that their cavities are only occasionally rounded by a secondary deposit of sclerenchyma; and further that the original line of separation between adjoining cells is always more or less distinctly marked. The spiniform tubuli do no constitute a conspicuous feature in sections of this species, and unless carefully examined might be overlooked. With an occasional exception, they always occupy the angles of junction of the cells. (Their appearance is very well represented by the figures.) Lastly, in many sections, some of the cell-cavities inclose a small circular ring, that is due to the peculiar funnel-shaped diaphragms seen in longitudinal sections.
- In longitudinal sections, the tubes in the ‘immature’ region are thin-walled and crossed by straight diaphragms from two to four tube-diameters apart. The nearly equal curve of the tubes, from the axis of the branch to the peripheral portion, constitutes a characteristic feature of the species. As they enter the peripheral or ‘mature’ region their walls are considerably thickened, and the diaphragms become much more numerous, being from less than one-half to one tube-diameter distant from each other. The funnel-shaped diaphragms noticed in the preceding species (A. cingulata) are much more numerous in this species. Not infrequently two or three open into each other in such a manner that by the coalescence of the contracted parts of the superimposed funnels, a smaller irregular tube is found within the proper tube-cavity. These diaphragms in their normal condition are represented in the section by two thin converging lines, spring from the walls of the tubes, and nearly meeting near the center of the tube cavity. Frequently, however, one of these lines is missing. In this case, the diaphragm extends from one wall nearly across the tube toward the opposite wall.
- Superficially, the species above described resembles the type of the genus, though not nearly enough to be confounded with it by one experienced in the determination of this group of fossils. The cell walls are thinner, and the groups of larger cells more conspicuous in A. robusta than in A. cingulata. Internally, the comparatively thin cell-walls and numerous funnel-shaped diaphragms, and the small number of spiniform tubuli of A. robusta will further distinguish it from that species. Care must be taken in separating the species from Monotrypella aequalis, Ulrich, which the smaller specimens of A. robusta strongly resemble. The former, however, is restricted to the lower 150 feet of the strata exposed at Cincinnati, O., while the latter is limited to the few feet of strata at least 225 feet higher in the series.