Durhamicystis americana, Zamora & Sprinkle & Sumrall, 2020
|
publication ID |
https://doi.org/ 10.4202/app.00718.2019 |
|
persistent identifier |
https://treatment.plazi.org/id/63120F46-FFFB-D676-A5E3-FAF4D5D0FCE1 |
|
treatment provided by |
Felipe |
|
scientific name |
Durhamicystis americana |
| status |
sp. nov. |
Durhamicystis americana sp. nov.
Figs. 1–3 View Fig View Fig View Fig , 6D View Fig .
Zoobank LSID: urn:lsid:zoobank.org:act:E7B7523F-14C8-4112-946F-A37EC172B26C
Etymology: In reference to North America, the continent where the type species was found.
Type material: Holotype: USNM 642510 View Materials , a complete specimen preserving the theca, brachioles, and proximal stem ( Fig. 1A View Fig ) . Paratypes: USNM 642512 View Materials , another complete specimen ( Fig. 1B View Fig ) , USNM 642511 View Materials , a small slab containing two well preserved overlapping specimens ( Fig. 2 View Fig ); all from type locality and horizon .
Type locality: Schemeompf’s old Quarry, top of hill, west of Conococheague Creek , about 2.1 miles (3.4 km) NNE and 0.5 miles (0.8 km) east of Wilson , Washington Co., Maryland.
Type horizon: Chambersburg Formation , Sandbian, lower Upper Ordovician .
Material.— USNM 642513–642515 View Materials , fragments and isolated plates; all from type locality and horizon .
Diagnosis.— Rhipidocystid with large theca composed of ten marginal plates with only two basals on the posterior side. Two ambulacra corresponding to BC and DE unsplit rays. Seven large oral plates with brachioles attached either on orals or smaller flooring plates. Brachioles between 22 and 26, biserial, long, slender, and showing a cryptic suture between paired brachiolars. Anal pyramid lateral aligned with the BC ray, hydropore between O1–O7, and gonopore within O1.
Description.— Rhipidocystid with large theca ( Fig. 1A 1, B 1 View Fig ) composed of ten marginal plates and orals closing the marginal framework (see below). Three apertures are recognized that allow orientation of the material. Peristome appears in the central anterior part, bounded by seven oral plates ( Fig. 1A View Fig 2 View Fig , B 1 View Fig ). Two elongate ambulacra corresponding with BC and DE rays. Anal pyramid ( Fig. 1B View Fig 2 View Fig ) covering the periproctal opening ( Figs. 1A View Fig 3 View Fig , 2A View Fig 4 View Fig ) appears in the left anterior side of the marginal ring. This is a large aperture of ca. 2 mm ( Fig. 1A View Fig 3 View Fig ) that is covered by a domed anal pyramid composed of about eight?, thin, subtriangular plates ( Fig. 1B View Fig 2 View Fig ). A single small pore (ca. 300 μm) within the plate O1 probably corresponds to the gonopore ( Fig. 3B View Fig ). The suture between O1 and O7 has a slit corresponding with the hydropore ( Fig. 3B View Fig ).
NineU-shapedmarginalplatescontributetothemarginal framework on the “ventral” surface closing it ( Fig. 1A 1 View Fig ), and an extra small plate with a vertical suture lies above the periproct on the “dorsal surface” ( Fig. 1A View Fig 3 View Fig ); thus, most of the marginal framework is singly plated. This framework is closed posteriorly with two large marginal plates (M1, M2), usually considered as basals ( Fig. 1A 1 View Fig ). Those plates are very large and abut with large sutures closing the marginal frame. They are both trapezoidal in shape and articulate anteriorly with plates M3 and M4, and posteriorly form the stem facet. They both have large, thin projections on the ventral side that contribute to the central part of the theca. Marginal plates M3–M5, and M6 are longer than wide and also have large, thin projections contributing to the central theca. Marginals M4 and M6 are the longest marginals ( Fig. 3A View Fig 2 View Fig ). Marginal plate M7 articulates posteriorly with M5, forms the posterior edge of the periproct, and is overlain by M9 and M10 separated by a vertical suture ( Fig. 1A View Fig 3 View Fig ) at the left anterior corner of the theca, followed anteriorly by oral plates. M8–M10 contribute to the peristomial opening; M8 contributes to the marginal framework in both dorsal and ventral aspects, while M10 and M9 are only visible in dorsal and ventral views, respectively. M9 also articulates anteriorly with an ambulacral flooring plate.
Ventral central plates are large, polygonal, and very thin in cross section ( Fig. 3 View Fig ). They are probably four in number and form a tessellate pavement that covers the ventral surface of the theca. Dorsal central plates are more disrupted and only visible in one specimen ( Figs. 2A View Fig 3, 3B View Fig ); they are apparently smaller than ventral centrals, polygonal, and also very thin in section. They form a tessellate integument that covers the dorsal surface and connects with marginal plates and anterior oral plates.
At least seven oral plates are recognized in the anterior part of the theca ( Figs. 1A View Fig 2, 2A 2 View Fig , 3 View Fig ). Four of them are large plates that articulate aborally with marginal plates M8–M10 and have facets for feeding appendages. O1 and O6 are on the dorsal side of the oral area and have an indented aboral margin to accommodate a small pentagonal O7 plate ( Figs. 2A 2 View Fig , 3B View Fig 2 View Fig ). O3 and O4 are large, relatively thick, subpentagonal elements that appear on the anterior side of the oral area. O2? and O5? are questionably recognized (aborally from the central orals) at the ends of the ambulacra (where branches BC and DE would normally split ontogenetically). They are smaller elements bearing facets for feeding appendages and are visible on both sides of the theca.
Flooring plates appear aborally from the large central oral plates and also have facets for feeding appendages ( Figs. 1A View Fig 2, 2A 2 View Fig , 3 View Fig ). They are difficult to distinguish in size and morphology from oral plates O2? and O5?, except that they do not wrap around the ends of the ambulacra.
Brachioles range from 22 and 26 in number, are biserial, long, and slender ( Fig. 2A 2 View Fig ). Brachiolar plates appear to be adjacent (not alternating), and have cryptic sutures ( Fig. 2A View Fig 5 View Fig ). They attach either in groups of two (rarely three) on both oral plates and flooring plates. Proximally, the three or four lowest segments of the feeding appendages are wider than long and have deep facets composed of a transverse prominent ridge and three fossa ( Fig. 2A View Fig 4 View Fig , A 5 View Fig ), a large posterior one and two small anterior ones. These create wedgeshaped spaces for ligament attachment, and indicate some mobility. Higher segments are longer than high, tightly sutured, and have a thin, vertical suture visible on the aboral side accentuated by weathering ( Fig. 2A View Fig 4 View Fig , A 5 View Fig ). The adjacent brachiolars here may be partly fused, but still show different crystallographic orientations (see Sprinkle 1973: fig. 5C).
The stem is incomplete in all specimens. Proximally, there is a single large, tapering, subtriangular columnal with a distally positioned facet for one or more additional columnals ( Fig. 1B 1 View Fig ). Only one? columnal is present in the best preserved specimens. Some disarticulated material preserved in slab USNM 642514 suggests the presence of a normal stem with cylindrical columnals showing crenulae with symplexy articulations in the columnal facets, but this columnal material is not closely associated with any of the complete rhipidocystid specimens.
Remarks.— Durhamicystis americana shows many features that are similar to other members of the family, except for: (i) having only two basals. None of these specimens show M0, or it does not extend out to the margin to form part of the stem facet. This is a difference from the other rhipidocystid genera, with only Neorhipidocystis and few specimens of R. baltica (Sergei Rozhnov, personal communication 2020) also having two basals; (ii) the lack of an elongate M4, only documented in two other genera ( Petalocystites and Mandalacystis ); (iii) orals (that otherwise look like adoral marginals) supporting the brachioles (usually two of each); (iv) a relatively small stem facet, possibly with normal, thin columnals attached, unlike Batherocystis , which has only a few, elongate, inflated columnals; (v) about 22–26 brachioles present on two elongate ambulacra, probably combined BC and DE; this number of brachioles is intermediate between Mandalacystis (38–44 brachioles) and Petalocystis (16–18 brachioles); the 4 other rhipidocystids all have fewer brachioles; and (vi) 10 marginals total, including a split marginal above the periproct, instead of all marginals being thick, with a U-shaped cross section, and extending onto both sides; Neorhipidocystis is the only other genus that shows a split marginal like this. Also, the brachioles appear to be uniserial, with wedges near their attachment, and with a faint suture line down the back side, indicating that the brachiolars are composites with different crystallography (noted in Sprinkle 1973: fig. 5C). Both the thecal plates and brachioles also show faint to moderate pustules, another typical rhipidocystid feature.
O2? and O5? are questionably recognized in the current material. These elements seem to show on both sides of the theca, but this is difficult to prove based on the current material. Favoring such an interpretation is that aboral (distal) elements from O2? and O5? bearing facets for feeding appendages are single elements on both sides.
Stratigraphic and geographic range.—Chambersburg Formation, Sandbian, lower Upper Ordovician; Maryland, USA.
No known copyright restrictions apply. See Agosti, D., Egloff, W., 2009. Taxonomic information exchange and copyright: the Plazi approach. BMC Research Notes 2009, 2:53 for further explanation.