Middle Ordovician – Late Ordovician
Ectenocrinus is an extinct genus of stationary intermediate-level epifaunal suspension feeders
Identification in Hand Sample:
- Possesses unusual trimeric columnals which make it instantly recognizable
- Cup cylindrical
- Basals short
- Primibrachs normally free above radials, but first primibrachs may be fixed in cup
- Two stout rami to a ray, composed of wedge-shaped brachials that support long, slender ramules on alternate sides of every other, or every third or second, brachial
- Stem transversely round, columnals composed of trimeres
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- The crinoid Ectenocrinus has unusual trimeric columnals which make it instantly recognizable.
- At many exposures in the Mohawk and Black River Valleys (Figures 1, 2, 3), distinctive trimeric columnals (Figures 4, 5) of the crinoid genus Ectenocrinus can be seen through a sequence of nearly 100 m of the lower, middle, and upper Trenton Limestone. These columnals display both temporal and spatial patterns of variation. When they first appear the columnals are generally triangular in shape with nearly triangular lumina (Figures 4.1-4.13, 5.1-5.6). In overlying strata the columnals are progressively more rounded with gradually more pentagonal lumina (Figures 4.16-4.25, 5.7-5.10). The form seems to have evolved. There is, however, also a geographic, or clinal, pattern to this variation. The triangular columnals are found most abundantly in the deeper water facies, which are to the east, while the round columnals dominate in the shallower water facies, which prevail in the west. Thus, these patterns of variation, when viewed temporally, can be interpreted to indicate gradualistic evolution but, when viewed spatially, seem to represent facies-controlled, clinal variation.
- Ectenocrinus has a stem which can be reliably identified and is not xenomorphic. The trimeric columnal of this genus is immediately recognizable (Meek, 1873; Ulrich, 1924; Warn and Strimple, 1977; Donovan, 1986). Trimerism is extremely rare among Ordovician crinoids (Donovan, 1986) and it can be safely assumed that no other trimeric forms occur in the Trenton Group. In fact, only Ectenocrinus calyxes have been found attached to trimeric stems.
- The proximal portion of the stem of Ectenocrinus tapers rapidly and consists of equal-sized columnals (Warn and Strimple, 1977). Distal to this, equal-sized columnals continue for a short length. Below this the stem is heteromorphic but simple. It consists of a long sequence of alternately wide and narrow columnals (Warn and Strimple, 1977). Columnal and lumen shape are virtually unchanged throughout the length of the stem. Consequently, any Ectenocrinus columnal can be regarded as representative of the whole stem from which it was derived. Spatial change in such columnals can be regarded as facies related; temporal changes are evolutionary.
Treatise on Invertebrate Paleontology, Part T, Vol. 2(2) (1978):
- Cup cylindrical; basals short. Anal X small, pentagonal, barely in top of cup, supporting long, slender anal sac. Primibrachs normally free above radials, but first primibrachs may be fixed in cup; primibrach 2 axillary in all rays. Two stout rami to a ray, composed of wedge-shaped brachials that support long, slender ramules on alternate sides of every other, or every third or second, brachial. Stem transversely round, columnals composed of trimeres.
- Crown of moderate size. Dorsal cup pyriform, rotund, round or slightly oval in top and bottom views, wider than high, sides confluent with proximal column and expanding uniformly upward reaching the greatest width at a point near the radial and fixed brachial sutures, sides rounded, distal constriction at level of fixed brachials. All plates thick, smooth, unornamented and flush along sutures. Sutures along inferradials and anal X somewhat arcuate. Basal circlet equal to about 20 percent of dorsal cup height, forming a small conical base with an expansion angle of 500 to 550, deep invagination on dorsal surface at juncture with proximal columnals. Five basals, wider than high, irregularly pentagonal, vertical sutures highest in A and D ray position.
- Radial circlet equal to about 40 percent of dorsal cup height. Radials simple in D ray and apparently in A ray, wider than high. D ray radial octagonal (obscured by arcuate sutures), sharing sutures with two basals, two superradials, two inferradials, anal X and one fixed brachial. The nature of the plates in the A ray cannot be distinguished clearly but there is some indication that the radial was simple and probably heptagonal in outline, sharing sutures with two basals, two inferradials, two superradials and one fixed brachial. Compound radials in B,C, and E rays, compound radials together somewhat higher than the simple radials. Inferradial with stroingly arcuate sutures on lower edge., approximately three times wider than high, C ray inferradial sharing sutures with five adjacent plates; B and E inferradials probably sharing six and five sutures respectively. C,B, and E ray superradial hexagonal, laterally in contact with both plates of B ray compound radial and with D ray radial, supporting anal X on slanted edge of upper left corner and one fixed brachial directly above. E and B ray superradials probably rectangular; both supporting a single fixed brachial.
- Anal X deeply set within dorsal cup, resting in small notch between the D and C ray radials and superradial respectively, flanked by fixed brachials, upper edge level with the distal edge of fixed brachials. Anal sac not observed. Only proximal 15mm of column known; column round, enlarged at base of dorsal cup. dimated of colu,m decreasing to less than half at a distance of seven mm from base of dorsal cup. Axial canal round with diameter equalling about 15 percent of columnal diameter. Proximal-most columnals very thin, cryptosymlectic articulation; columnal becoming thicker distally with well developed symplectic articulation, outer crenularium occupying approximately 40 percent of columnal diameter; columnals apparently not tripartite.
- General form very elongate; calyx small, subcylindrical, moderately expanding; basals 5, unequal; radials irregular, four plates in three series, before the bifurcation of the free arms, and three in each of the other two series; arms 10, long; pinnules strong; azygous plates 3, following each other, but not in a direct line; vault unknown; column very long, round, tripartite, and attaching by an expanded base.