Protocinctus Rahman & Zamora 2009, GEN. NOV.
Creators: name: Rahman, Imran A
name: Zamora, Samuel
Publisher: name: Zenodo
Year: 2009
Subjects: Biodiversity
Taxonomy
Animalia
Echinodermata
Homostelea
Cincta
Sucocystidae
Protocinctus

description: GENUS PROTOCINCTUS GEN. NOV.
Derivation of name: Greek, protos (first) + cinctus.
Diagnosis: Relatively elongate, oval-shaped theca composed of ten marginal plates (M5r–M4l). Single and long left marginal groove, extending from M1r to M 1l. Lintel composed of three suropercular plates. Supraoral plate boot-shaped. Anterior right embayment formed by M0, M1r, and M2r. Weakly developed ventral swellings on the lower surfaces of M0, M1r, and M2r. Marginal frame open in posterior ventral aspect, with an intercalated plate between M4l and M5r.
PROTOCINCTUS MANSILLAENSIS SP. NOV.
(FIGS 3, 4)
Derivation of name: After the Mansilla Formation, where the material was collected.
Diagnosis: As for the genus (monotypic).
Holotype: MPZ2004 /170 (Fig. 4A–G, J): a fully articulated specimen preserved as recrystallized calcite in a siltstone matrix.
Description: In the following description the orientation and plate notation follows that of Friedrich (1993), with the marginal plates abbreviated as M (Fig. 3). The anterior medial marginal plate is termed M0, and the other marginal plates are numbered sequentially around the margin (anterior to posterior) as M1l, M1r, M2l, M2r, etc., with l and r indicating their location to the left or right of M0 in dorsal aspect (Fig. 4B).
Protocinctus gen. nov. is composed of an anterior theca (body) and a posterior stele (appendage). The theca is flat (at least four times wider than thick), with a frame of ten marginal plates (the cinctus) enclosing the dorsal and ventral plated surfaces (the supracentral and infracentral integuments) (Figs 3, 4). The oval-shaped theca is longer than it is wide. In the holotype (MPZ2004 /170) it is approximately 17 mm in length, with a maximum width of 10 mm (Fig. 4A, B). Other specimens are approximately 12–14 mm in length and 7.5–8.5 mm in width, with a mean length to width ratio of 1.6: 1.
Other material: MPZ2004/171, MPZ2004/172 (Fig. 4H), MPZ2007/2476, MPZ2007/2477 (Fig. 4I), and MPZ2007/2478: nearly complete articulated specimens. Specimens are housed in the Área y Museo de Paleontología, Universidad de Zaragoza (MPZ). Virtual data sets are available on request.
Type locality: Borraca Creek, 3 km south-east of Purujosa village, in the northern part of the Iberian Chains.
Type horizon: Early Middle Cambrian (Cambrian Series 3, Stage 5), Upper Leonian, Mansilla Formation, Eccaparadoxides asturianus Zone.
Orifices: Two openings are preserved in Protocinctus gen. nov. The first is a small circular aperture in the anterior right corner of the cinctus (best preserved in MPZ2004/170, and approximately 1.2 mm in diameter) that occurs between M1r and M2r (Figs 3E, 4D). It is bounded dorsally by the boot-shaped supraoral plate (approximately 1 mm in length and 2 mm in width; MPZ2007/2477), which articulates with the inner dorsal surfaces of M1r and M2r (Figs 3E, 4A, D, I). A single left marginal groove covered by multiple sets of small plates (the labrum, Sdzuy, 1993) leads into this opening (Figs 3E, 4C, D, I).
The second opening is also anterior but is much larger, being approximately 3.8 mm in diameter in MPZ2004 /170 (Figs 3A, D, 4C, D): it is termed the porta (Sdzuy, 1993). The porta penetrates the marginal frame coincidently with the midline, is covered dorsally by a large rounded plate (the operculum; Figs 3C, E, 4A, C, D, G–I), and is bounded ventrally and laterally by M0, M 1l. and M1r, which give the opening a concave lower border (Figs 3D, E, 4C, D). The upper posterior margin of the opening is delimited by three large polygonal plates from the supracentral integument (the lintel; Figs 3C, E, 4H, I). In MPZ2004 /170 the operculum measures approximately 2.7 mm in length and 2.1 mm in maximum anterior width, filling the anterior of the porta. It is bowl-shaped, with a flat to concave dorsal surface and a convex ventral one. At its posterior margin the operculum articulates with all three lintel plates (Figs 3C, 4I).
Cinctus: The cinctus or marginal frame is composed of the following plates: M0, M1l, M2l, M3l, M4l, M1r, M2r, M3r, M4r, and M5r (Fig. 3A, B). These plates confer a rigid structure to the animal. Marginal plates are subtriangular in cross section: the lower surface of the frame is wide and flat or slightly convex (Fig. 4E, F), whereas the upper surface is thin and strongly convex (Fig. 4A, G). On their inner faces plates are strongly concave, and their outer faces are gently curved (Fig. 4G). Ventral swellings in the marginal plates are greatly reduced compared with those seen in other cinctans. Protocinctus gen. nov. has only one indistinct swelling, located in the ventral surfaces of M0, M1r, and M2r (Figs 3B, 4B); this is most obvious in larger individuals. Dorsal and ventral furrows run continuously along the inside of the cinctus, close to the upper and lower borders of the frame (Figs 3A, 4A, G). These furrows presumably acted as insertions for the supracentral and infracentral integuments. The inner junctions between the marginal plates were apparently flat.
At the anterior of the cinctus, M0 (Figs 3A, B, E, 4B, D) articulates with M1l and M1r, forming the floor of the porta. It is subrectangular in outline and longer than it is wide, with a length to width ratio of approximately 1.5: 1. A distinct groove is present in the anterior margin of M0, occupying almost one half of the anterior plate height: this is part of the left marginal groove.
Both M1l and M1r possess dorsal articulation facets and portions of the marginal groove. M1l is curved in shape (Figs 3A, B, 4A, B), and articulates rigidly with M0 and M2l, but more flexibly with the infracentral integument and the lintel. The length of the plate is approximately twice the width, with the width decreasing in the posterior of the plate. The marginal groove occurs on the anterior margin of the plate, but weakens and then disappears completely as the plate curves laterally (Figs 3D, E, 4D). On the dorsal inner surface of M1l, the articulation facet occurs at almost one half of the distance from the anterior margin of the plate; this was likely to have been an articulation surface with lintel plate 1 (Figs 3A, C, 4A, I).
M1r is similar to M0 in ventral aspect (Figs 3B, 4B): approximately equal in width, but greater in length (length of M0 approximately 0.8 times the length of M1r). It articulates with M2r, M0, the supraoral plate, infracentral plates, and the lintel (as in M1l, articulations are rigid with plates from the cinctus, and loose with other plates). The lateral margin of M1r is strongly concave where the marginal groove (Figs 3E, 4D) leads into the right circular aperture (located just below the right articulation facet). The articulation facet on M1r was for lintel plate 3 (Figs 3A, C, 4A, I): it occurs in the same plane as the facet on M1l and is similar in size.
The anterior right corner of the cinctus (M0, M1r, and M2r) curves at a greater angle than the left corner (approximately 60 ° on the right, and 40 ° on the left), forming the diagnostic anterior right embayment (Figs 3A, 4A). The inner anterior margin of M2r is strongly incised where the circular aperture occurs, giving the plate a concave shape similar to the lateral margin of M1r. In other respects it is similar to M2l, M3l, M3r, and M4r, which have a near identical elongate shape. Plates on the left of the frame are longer than those on the right (length to posterior width ratio approximately 3: 1 on the left, and 2.5: 1 on the right; Figs 3A, 4A).
M4l and M5r comprise the posterior of the cinctus (Figs 3A, B, 4A, B, G, J). They possess the same general shape, being less elongate than the other cinctus plates. Posterior portions of both plates decrease in width (to approximately two thirds the width of anterior parts), giving a relatively narrow posterior margin. The cinctus is open in ventral aspect, with a single intercalated plate inserted between plates M4l and M5r (Figs 3B, 4J). This plate is polygonal, with a pentagonal outline (1.3 mm in length and 1 mm in width; MPZ2004/170).
Integuments: The dorsal (supracentral) and ventral (infracentral) integuments (Figs 3A, B, 4A, B, G) are made up of numerous polygonal plates, which confer a tessellate structure to the theca. The integuments were inserted into the furrows close to the upper and lower borders of the cinctus during life (Figs 3A, 4G). The integument plates lack external ornamentation, and are typically pentagonal-shaped in marginal areas, and hexagonal in the centre. The infracentral integument is composed of approximately 75–100 plates, a much lower number than the supracentral integument (plate number uncertain). The plates are quite homogeneous in size, with the infracentral plates approximately two or three times the size of the supracentral plates. A cone-shaped depression, which narrows posteriorly, occurs in the central interior portion of the infracentral integument (Fig. 4G): this measures approximately 12 mm in length and 2.8 mm in maximum width (MPZ2004/170).
Marginal grooves: Protocinctus gen. nov. possesses a single left marginal groove (Figs 3D, E, 4D). It is horizontally oriented, running from the anterior right opening along the exterior of the cinctus, and terminating around halfway along the length of M 1l. The groove is at its maximum size where it enters the anterior right opening (approximately 0.9 mm in height; MPZ2004 /170). It decreases in size left of this, and maintains a relatively constant height of approximately 0.75 mm (MPZ2004 /170) across M0 and the anterior of M1l, before it ends abruptly. The total length of the groove is approximately 5 mm. As briefly mentioned above, the groove is covered by numerous small plates that collectively form a structure termed the labrum (Fig. 4A, C, I). The plates that comprise the labrum are small (approximately 0.4–0.6-mm wide; MPZ2004 /170) and polygonal. The organization of individual plates comprising this structure is uncertain.
Appendage (stele): The stele is most completely known in the holotype (MPZ2004 /170), where it measures approximately 6.5 mm in length, and forms an angle of approximately 145 ° with M4l and M5r (Figs 3A, B, 4A, B). The ratio of stele length to theca length is approximately 1: 2.5. The stele is composed of lateral pairs of polygonal-shaped sphenoids (Figs 3A, B, 4J), with diamond-shaped mesosphenoids inserted centrally in alternate pairs of sphenoids. The size of the stele plates decreases posteriorly. On the ventral surface, the cinctus articulates with two large, elongate sphenoids, and a single intercalated plate (Figs 3B, 4J). The preservation of the dorsal stele is incomplete (Figs 3A, 4A, G), and it cannot be described in detail; the dorsal articulation between the cinctus and the stele probably included two mesosphenoids (Fig. 3A). In cross section the stele is triangular, with a flat ventral surface; this shape is constant along the length of the stele. The ventral part of the stele is oriented so that it projects below the ventral cinctus (Fig. 4E, F).
Remarks: The characters displayed by Protocinctus gen. nov. are all known from other cinctans, but the character combination is unique; consequently, assigning Protocinctus gen. nov. to an existing cinctan family is problematic. The presence of a single left marginal groove that reaches M1l is a trait shared with some species of Sucocystis Cabibel, Termier & Termier, 1959; however, Protocinctus gen. nov. differs from these forms in other respects (the shape and height of the theca, the number of marginal plates, and the development of ventral swellings). The elongate thecal shape of Protocinctus gen. nov. is similar to that of Gyrocystis Jaekel, 1918, Progyrocystis Friedrich, 1993, or Elliptocinctus Termier & Termier, 1973. The arrangement of three plates in the lintel is similar to Gyrocystis, Sucocystis, and Elliptocinctus, and the absence of prominent ventral swellings is a feature shared with Trochocystoides parvus Jaekel, 1918, T. planus, S. ubaghsi, A. jaekeli, and Gyrocystis platessa Jaekel, 1918. We tentatively assign Protocinctus gen. nov. to the family Sucocystidae, based on the presence of a single long left marginal groove and the anterior continuity of the ventral swellings. Protocinctus gen. nov. is probably basal within the family, because it possesses an elongate theca and relatively reduced ventral swellings. This assignment is preliminary, and will be tested in a cladistic analysis to be published by the second author (Smith & Zamora, in press).
In one specimen (MPZ2007/2477), a small marginal plate is inserted between M1r and M2r (Fig. 4I). The number of marginal plates can vary within a single species of cinctan (Friedrich, 1993), but in no known specimen does an additional plate occur in the anterior frame. Although it is uncertain how cinctans added plates during growth, it is doubtful that additions could have occurred in the anterior of the frame; here, distinctive processes occur on M1l and M1r that are never transposed to other plates. Instead, because of the similarity of the marginal plates and proximal sphenoids, it is more likely that cinctans added new plates to the cinctus from the stele. As the presence of a small plate between M1r and M2r is not seen in any other cinctans, it probably resulted from abnormal plate growth. Similar abnormalities occur in stylophoran carpoids (Ware & Lefebvre, 2007).
description: Published as part of Rahman, Imran A & Zamora, Samuel, 2009, The oldest cinctan carpoid (stem-group Echinodermata), and the evolution of the water vascular system, pp. 420-432 in Zoological Journal of the Linnean Society 157 (2) on pages 422-427, DOI: 10.1111/j.1096-3642.2008.00517.x, http://zenodo.org/record/5443690