Questions and Comments
As has been discussed in the Nomenclature page, there are a variety of species names that refer to the number of rays in the Evactinopora colony. The specimens collected at the Fern Glen formation ranged from four to eight rays; those collected by Weller (1909) ranged from four to nine rays. In both collections, colonies with five, six and seven vanes are very common and together account for 80 - 85% of the colonies. There are a few specimens with four vanes and a few with eight or nine vanes, but they are not notably different than the more common colonies except in the number of vanes. It is reasonable to conclude that in a population of Evactinopora radiata the number of vanes varies and that species cannot be defined on the exact number of vanes in a colony. This implies that species defined on the vane number such as E. quinqueradiata and E. sexradiata are not valid species.
Should the number of rays be the defining characteristic of the species of Evactinopora, or should there be only one species with the number of rays being variants of that species?
How does the animal decide the number of rays to produce in its colony? The colony does not grow by the addition of rays but by enlarging the existing ray number. Colony growth appears to be accretion of the matrix from the base out, with the number of zooids on the vanes of the star increasing as the vanes elongate.
Is the morphology of the colony under genetic control or is this an environmentally controlled character where the colony is forced by stronger water currents to provide a greater base of support with more vanes than less?
The presence of a depression/pit at the center of the base of many colonies is a puzzle. This feature is most common on small diameter colonies, but some large colonies have well developed pits.
Could the depression be a detachment scar associated with change from attached to free-living condition? Another puzzling aspect is that on some specimens the depression developed into a deep pit with a circular margin and extending deep along the central axis of the colony. This appears to have grown larger and deeper as the colony aged, but in some of the small eight-veined colonies, the pit appears to be extremely deep compared with the rest of the colony.
Meek and Worthen (1865, 1868) state that the chambers housing zooids are arranged in a quinqunx pattern (Quinqunx: regular units in a back row positioned between and behind the units in a front row -- eg. the pattern of the 5 spots on a pair of dice). This is partly evident in these samples, but the arrangement is not as regular as their statement implies. Better material is needed to see if quinqunx is typical of the Fern Glen Evactinopora.
Richard Gottfried and Barry Sutton
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