The star-shaped fossil Evactinopora is distinctive and attractive, making it a desirable addition to any fossil collection. Specimens collected from shale matrix can often be prepared to remove much of the matrix, revealing a range of form that extends from simple star shapes to elongate vaned colonies with rounded bases. Attempts at species identification of these varied forms quickly reveals that there is little recorded knowledge of this bryozoan and considerable confusion about species categories. There is almost no understanding of the growth or life habits of this bryozoan, apart from the fact that it was free-living (unattached) on the seafloor. Attempts to obtain reliable species identifications led us into making a careful study of this fossil.
The quest for species determination
The bryozoan genus Evactinopora was first described by Meek and Worthen in 1865, with E. radiata as type species, from the Lower Carboniferous (=Mississippian) Keokuk Limestone. The type colony for E. radiata is an 8-rayed star. Subsequently, Meek and Worthen (1868) named two more species: E. sexradiata, a 6-rayed colony, and E. grandis, a large colony with 4 vanes. Another species was named as E. quinqueradiata by Ulrich in 1890, for 5-rayed specimens. Ulrich accepted the view that the number of vanes in a colony is an important character for separating species, but this view has not been tested. The nomenclature of the Evactinopora species has not been evaluated since that early work.
We have noticed much variation in the number of vanes on colonies of Evactinopora and have become skeptical of the concept that the number of vanes in a colony can be used to identify species in the genus. Colonies with different number of vanes can be identical in other characters. We think that the number of vanes is not a species-defining character and will be using Evactinopora radiata as a species name for the specimens in this study. In addition to being the first name applied to these fossils, it does not specify the number of rays and thus is a good "generic" specific name (pun intended).
Sample collection
Fern Glen Limestone quarry
Mississippian - Osagean Series
Jefferson County, Missouri
A large number of Evactinopora radiata was collected from the Fern Glen Formation (Mississippian) in east-central Missouri to determine their morphological characteristics and variability. The Fern Glen Formation is composed of shale and limestone with two dominant colors: "reddish" and "white". Although both color variants are fossiliferous, most people collect from the reddish material because Fern Glen fossils are white and are more noticable on reddish matrix.
Methods
Fern Glen Formation - Evactinopora radiata
Several buckets of reddish shale were washed and collected in 1mm sieves to
remove the fines. The washed shale was hand picked to collect all fossils
and the Evactinopora species were separated by the number of rays, and stored
in transparent coin folders.
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Example1
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Example 2
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