Sheffield and Sumrall—Generic revision of North American Holocystitidae
765
clearly diagnosable from other plates in summit area, slightly elevated to form a moderately high spout-like protuberant summit. Oral plates relatively large. Theca narrows into constricted distal holdfast (Fig. 9.2). Diplopores present on thecal plates.
Remarks.—Triamara appears to bear most aspects of the ste- reotypical holocystitid peristomial border plating pattern. Unfortunately, heavy taphonomic disarticulation has affected the oral areas of most curated specimens, and details concerning the number and placement of the facetals cannot be determined at this time. Orals 1–6 surround the ovate peristome. Orals 1, O6, and O7 are within the CD interray, with O1 and O6 pre- cluding O7 from the peristome. Facetals D and E are in the expected position, but the position of the facetal in association with shared B and C ambulacrum is unclear due to taphonomic overprinting (i.e., disarticulation and breakage of plates in the oral area, along with noticeable thecal compaction). In some specimens of Triamara (e.g., Triamara ventricosa Paul, 1971), a plate appears to separate O7 from the periproct, unlike holo- cystitids (Fig. 9.3). It is unclear whether this is common to Triamara or unique to T. ventricosa. Both oral plates and facetal plates are extremely large, even when considering that Triamara is relatively larger than most holocystitids. Theca plates are relatively large and appear to have two generations of plates, primary and secondary (Fig. 9.4). Diplopores, as opposed to humatipores, are densely and evenly spread across the thecal plates.
The peristomial border plating pattern of Triamara bears
normal holocystitid oral area, with O1–O7 in the standard configuration. However, the lateral non-facet-bearing facetal plates are absent, leaving only facetals A–E. Gonopore visible on O7; hydropore slit straddles the suture between O1 and O6. (2) Side view. Theca elongate. Plates numerous and flat without depressed sutures. Theca narrows into holdfast. (3) Triamara ventricosa (UC5997). Oral area potentially shows a peristomial border-plating pattern similar to that of holocystitids, but due to poor preservation, this cannot be confirmed. A ambulacrum absent; shared ambulacrum BC present, as well as D and E. D ambulacrum food groove bifurcates distally and terminates in two separate facets on top of facetal bearing plates. The position and number of the facets cannot be determined from this specimen. A crinoid holdfast is growing around the border of the periproct, which is separated from O7 by a thecal plate. (4) Side view. Theca proportionally large and elongate. Plates numerous and appear to represent two generations. Plates flat without impressed sutures. Theca narrows considerably into holdfast. Scale bar =1 cm.
Figure 9. (1) Pustulocystis pentax (MUMG-T 266). Oral area shows the
genera within Aristocystitidae are placed within subfamilies, except for Triamara, due to uncertainty about its placement (Paul, 1971).
Genus Triamara Tillman, 1967 Figure 9.3, 9.4
Type species.—Triamara cutleri Tillman, 1967
Emended diagnosis.—Three ambulacra, likely shared BC, D, and E because of their relative positions with respect to oral plates, extend from peristome to facet scars that lie on top of a facetal plate (Fig. 9.1). The D ambulacrum bifurcates and leads to two distinct facets on the D facetal plates. Facetal plates not
strong similarities to Holocystitidae, as does the makeup of the theca. However, due to dissimilarities in the sizing of the oral and facetal plates, the presence of diplopores, the different positioning of the periproct with respect to O7, and the poor preservational detail of the oral area that pervades curated specimens, it is unclear whether Triamara is a member of the holocystitid clade. As such, it will not be reassigned until new data can be collected.
Conclusions
Previous studies of the Holocystites Fauna were problematic because of oversplitting of poorly preserved species and a mis- interpretation of the peristomial border plate system resulting in a complicated and unparsimonious evolutionary history. Care- ful analysis of numerous well-preserved specimens shows that the peristomial border plate system among holocystitid taxa is much more conservative than previously described. Many of the ascribed differences proposed by previous authors were based on taphonomic differences or a lack of understanding of the homologous elements of the oral plating system. Consequently, Osgoodicystis Frest and Strimple, 2011 in Frest et al., 2011 is a junior synonym of Pentacystis, Paul, 1971. The systematic placement of Triamara Tillman, 1967 is unclear due to poor preservation. While it bears many peristomial border similarities to holocystitids, there are also many differences in the size and placement of these oral area plates. Numerous examples of blastozoan respiratory structures re-evolving suggests that delineating higher-level systematics solely on the basis of the presence of humatipores or diplopores is likely not valid.
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