Rhipidocystids are a small group of Palaeozoic Blastozoan Echinoderms, exclusively found in the Ordovician, which had a flattened theca, long brachioles, and reduced stems. The family Rhipidocystidae was erected by Otto Jaekel in 1901 to accommodate the genus Rhipidocystis, then considered a member of the Carpoidea because it bore a flattened theca. A second genus of Rhipidocystids, Batherocystis was described in 1950, with the genus Petalocystites being erected in 1973 and Mandalacystis in 1987; this being one of the better-known taxa within the family, including complete specimens that preserve the stem, brachioles, and the oral area. Some specimens of Rhipidocystis baltica preserved the oral area and show a pentaradial pattern with a BC-DE flattening direction. In 1989 the genus Neorhipidocystis was created to accomodate some material originally described as Rhipidocystis, which included bilateral forms with only the two shared ambulacra. Ambulacral homologies in flattened forms with two ambulacra remain unresolved, and this results in part, from a lack of information from the oral area, which is critical for the understanding of homologies in early pentaradial Echinoderms.
Zamora
et al. describe four specimens from a historic collection housed at the
Smithsonian Institution (
National Museum of Natural History), Washington DC, USA. These specimens are fully articulated in a dark grey highly lithified limestone. The specimens were apparently hand prepared by the original collectors. They are preserved in calcite that appears to be softer than the surrounding matrix.
Information on the original labels indicates that the material was 'brought in by amateurs' and shown to John Wyatt Durham of the
University of California, Berkeley, who was visiting the Smithsonian, on June 6, 1966. This small collection of Rhipidocystid Echinoderms was collected from the 'Edinburg Formation', in western Maryland. Specimens were collected in an old quarry, west of Conococheague Road, east of Wilson, Maryland. The first record of these specimens being deposited at the Smithsonian is from June 24, 1969, indicating that at least 'two specimens of
Rhipidocystis sp. and one specimen of disarticulated Rhipidocystid plates' were received by Thomas Phelan, who was Curator Porter Kier’s Assistant at that time, who placed them in the
Springer Echinoderm Collection. Since then, nothing has been done with these specimens, with the exception of James Sprinkle who, while visiting Durham at the Smithsonian in August, 1967, made a drawing of one of the Rhipidocystid brachioles to use in his dissertation. While reviewing the collections at the National Museum of Natural History in 2017, Samuel Zamora rediscovered these specimens in a museum drawer, but there is no historical record about what happened with these fossils between 1966 and 2017.
The original label indicates that specimens derive from what was formerly called the 'Edinburg Formation', and were collected from the Schemeompf’s old Quarry, located at the top of a hill, west of Conococheague Creek, about 3.4 km north and 0.8 km east of Wilson, Washington County, Maryland. Coordinates recorded on the label place the quarry at 77º between 49’ + 50’’ W; 39º approximate 41’ N. Based on
Google Earth, there is now a mostly filled-in quarry north of Cresspond Road, North Wilson, northwest of Hagerstown, Maryland, that almost certainly is the Schemeompf’s old Quarry.
The Edinburg Formation is now only used in the Shenandoah Valley in western and northern Virginia. The equivalent unit in western Maryland (Hagerstown Valley) and southern Pennsylvania is now called the Chambersburg Formation. The Chambersburg Formation belongs to the Amorphognathus tvaerensis Conodont Zone, in the lower and middle Turinian (Laurentian) Stage, belonging to the lower Mohawkian Series, Sandbian (Upper Ordovician). The Chambersburg Formation in Maryland is about 150 m thick and was deposited in a shallow outer ramp with a small amount of deep ramp deposits. The nodular-bedded outer-ramp facies of the lower Chambersburg Formation contains Echinosphaerites beds, showing that the Chambersburg Formation is fossiliferous and favorable for Echinoderm preservation.
The new species is named Durhamicystis americana, where 'Durhamicystis' honours the palaeontological specialist on Echinoids and Cambrian Echinoderms, John Wyatt Durham, and 'americana' refers to to North America, the continent where the type species was found.
Durhamicystis americana is a Rhipidocystid with large theca composed of ten marginal plates and orals closing the marginal framework. Three apertures are recognised that allow orientation of the material. The peristome appears in the central anterior part, bounded by seven oral plates. Two elongate ambulacra correspond with the BC and DE rays. An anal pyramid covering the periproctal opening appears in the left anterior side of the marginal ring. This is a large aperture of about 2 mm that is covered by a domed anal pyramid composed of about eight?, thin, subtriangular plates. A single small pore (roughly 300 μm) within the plate O1 probably corresponds to the gonopore. The suture between O1 and O7 has a slit corresponding with the hydropore.
Rhipidocystid Echinoderm Durhamicystis americana from the Upper Ordovician Chambersburg Formation of Maryland, USA. (A) Holotype USNM 642510 showing the theca, proximal stem, and brachioles; general view of the complete specimen in ventral view (A₁), detail of oral area and proximal part of the brachioles (A₂), specimen in lateral view showing the periproct surrounded by three plates (A₃). (B) Paratype USNM 642513; general view of the complete specimen in ventral view (B₁), note central plates in thecal interior, and single columnal attached to basals; detail of the anal pyramid on left edge (B₂). Specimens whitened with ammonium chloride sublimate. Zamora et al. (2020).
Nine U-shaped marginal plates contribute to the marginal framework on the 'ventral' surface closing it, and an extra small plate with a vertical suture lies above the periproct on the 'dorsal surface'; 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. 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. 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 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.
Rhipidocystid Echinoderm Durhamicystis americana from the Upper Ordovician Chambersburg Formation of Maryland, USA. (A) Paratype USNM 642511a, b; general view showing two nearly complete superimposed specimens (A₁); detail of the oral area, proximal brachioles attached to marginal, oral, and floor plates, and three apertures (periproct, hydropore slit, and tiny gonopore) (A₂); specimen in left lateral view showing the thin central plates from both integuments (A₃); specimen in right lateral view showing the position of the periproct (A₄); detail of facets between brachiolar plates and cryptic vertical sutures indicated by arrows (A₅); detail showing thin internal projections of weathered, U-shaped, marginal plates (A₆). Specimens whitened with ammonium chloride sublimate. Zamora et al. (2020).
Ventral central plates are large, polygonal, and very thin in cross section. 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; 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.
Rhipidocystid Echinoderm Durhamicystis americana from the Upper Ordovician Chambersburg Formation of Maryland, USA. (A) USNM 642510. (B) USNM 642511a, b. Photographs of specimens submerged in water to increase contrast of plates and plate boundaries (A₁, B₁). Camera lucida drawings indicating plate arrangements (A₂, B₂). Abbreviations: FP, flooring plate (green); G, gonopore; H, hydropore; M1–M9, marginal plates (yellow); O1–O7, oral plates (red); Pe, periproct. Central plates colored in blue, stem in purple, and brachioles in brown. Zamora et al. (2020).
At least seven oral plates are recognized in the anterior part of the theca. 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. 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 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. Brachiolar plates appear to be adjacent (not alternating), and have cryptic sutures. 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, 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. The adjacent brachiolars here may be partly fused, but still show different crystallographic orientations.
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. 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.
Understanding the orientation of Echinoderms that have undergone ambulacral reduction is sometimes problematic and Rhipidocystids are a clear example. This is exacerbated because flattening in Eocrinoids is polyplyletic, and any attempt to recognise possible orientation requires recognition of oral plates, ambulacra, and in some cases thecal openings. Many clades of Echinoderms have independently developed flattened theca in response to living in close proximity to the substrate or as adaptations to high energy environments with oscillatory currents. Flattening in Echinoderms is recognized to occur in three planes: lateral where the sides of the theca area are compressed along the BC-DE axis, anterior-posterior where the sides of the theca are compressed along the A-CD axis, and dorsal-ventral where the theca is compressed along the polar axis.
Thecae showing lateral flattening have the BC and DE sides of the theca compressed parallel to the polar axis of the theca. This results in a lens-shaped to discoidal theca that is flattened parallel to the A-CD and polar axes, and perpendicular to the BC-DE axis. This occurs only in Rhipidocystis. Anterior-posterior flattening occurs where flattening is along the A-CD axis. This results in a lens-shaped to discoidal theca that is flattened parallel to the BC-DE and polar axes and perpendicular to the A-CD axis. This is seen in Pleurocystitid Glyptocystitoids, the Eocrinids Lingulocystis, Cardiocystites, and Haimacystis, and many Rhipidocystids such as Neorhipidocystis, Petalocystites, and Durhamicystis. Dorsal-ventral flattening occurs where flattening is along to the polar axis of the theca. This is seen in many Echinoderms such as Asteroids, some Echinoids, Edrioasteroids, and Cyclocystoids.
As with other rhipidocystids, Durhamicystis has a flattened theca and large U-shaped marginal plates. The position of inferred oral plates and openings allows comparison with other pentaradial Echinoderms, suggesting that ambulacra correspond to the shared BC and DE rays.
Rhipidocystids have been argued to be related to Paracrinoids based on similar peristomial plate arrangement, three distinctive basal plates, position of main thecal apertures (gonopore, hydropore, and periproct), and proximal stem morphology. Most highly flattened Rhipidocystids with U-shaped marginals, and most lens-shaped paracrinoids with 2–3 ambulacra, appear to be flattened into the anterior-posterior plane (through the A-CD), although there is considerable variability in where the periproct is located and what ambulacra are retained (usually BC and DE). At least three taxa, Petalocystites, Neorhipidocystis, and Durhamicystis agree with this configuration, and thecal plates can be homologized across the three taxa. Rhipidocystis, that gives the name to the family Rhipidocystidae, is apparently flattened in the opposite direction (see discussion above). If confirmed this probably responds to different flattening planes in Rhipidocystidae, and thus the family, as presently described, should be reconsidered.
Rhipidocystids include some of the most enigmatic Echinoderm taxa from the lower Palaeozoic. Their fattened body and aberrant morphology, with little morphological information from the oral area, limited in part the understanding of homologies across different taxa. The discovery and description of Durhamicystis americana from the Upper Ordovician Chambersburg Formation in western Maryland, preserving both thecal sides and the oral area, and major apertures including peristome, periproct, gonopore, and hydropore, allows orientation of the flattening this species and proper comparison across all Rhipidocystids. This questioned previous hypothesis about homology within Rhipidocystids, and suggests that two different groups, one including Rhipidocystis and the other comprising Petalocystites, Neorhipidocystis, and Durhamicystis, show flattening in opposite directions.
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