Placatenella complanata
Creators: name: Mooi, Rich
name: Martínez, Sergio A.
name: Del Río, Claudia J.
name: Ramos, Maria Inês Feijó
Publisher: name: Zenodo
Year: 2018
Subjects: Biodiversity
Taxonomy
Animalia
Echinodermata
Echinoidea
Clypeasteroida
Eoscutellidae
Placatenella
Placatenella complanata

description: Placatenella complanata (Brito, 1981)
Figures 1–4.
1981 Abertella complanata Brito: 3–4, figs 1–2.
2000 Abertella pirabensis Marchesini Santos—Mooi et al.: 266, in part.
Diagnosis. As for the genus, which is emended from Brito's (1981: 3) original diagnosis or description (it is not stated which). Brito's information does not diagnose the species because it does not distinguish it from other members of the genus Abertella to which it was originally assigned (translated from the Portuguese): "Test flattened, flat on the oral and convex on the aboral surface, with a semicircular outline, and a marked anal indentation, a small but sharp indentation in the anterior ambulacrum, and only a sinuosity in the paired ambulacra. Petals closed, measuring approximately two-thirds of the test radius with the poriferous zones slightly narrower than the interporiferous. Central peristome, from which extend five food grooves that soon fork. Periproct ventral in the posterior indentation."
Type and other material studied. The holotype, MNRJ 5460 -I, is the only originally described specimen, and is housed at the Museu Nacional, Rio de Janeiro, Brazil. We also examined other specimens assigned to this species by Brito (1986), including MNRJ 5536-I, MPEG-886-I, MPEG 1753-I, MPEG 2405-I, and DNPM 6217. These are generally of higher quality than the holotype, revealing new information concerning the species.
Description. Brito (1986: 2) expanded upon the original description/diagnosis of the species based on new material, adding that there were five distinct genital pores. This is evidently an error, as Brito's (1986) own photos and original description indicate four gonopores. Our emended description is as follows.
Holotype (Fig. 2A, B) approximately 25 mm TL (measured as described in Table 1, from junction of perradial suture of ambulacrum III with anterior edge of test to junction of interradial suture of interambulacrum 5 with posterior edge of test [i.e. inside notch]). Largest known specimen (Brito 1986: Fig. 1) approximately 45 mm TL, almost 55 mm in test width at widest point. Best preserved specimen (MNRJ 5536-I, Fig. 2C, D) approximately 38 mm TL (Fig. 2C, D), 57 mm test width. Ratio of width to length including lobes on either side of notch 1.17. All ensuing percentage calculations for species are from MNRJ 5536-I (Fig, 2C, D). Aboral surface slightly domed, oral surface flat. Highest point of test approximately 17% TL, located at apical system. Very sharply defined, parallel-sided, narrow posterior notch in largest specimens, depth just over 25% TL, width 10% TL, notch slightly widening near ambitus in holotype (Fig. 2A, B). Broad but shallow marginal indentations present where perradial suture meets ambitus in each ambulacrum, particularly in posterior paired ambulacra.
Apical system monobasal, pentagonal (not star-shaped), 49% TL from ocular III to anterior edge of test, length 11% TL, numerous hydropores scattered over madreporic plate. Four gonopores, one in each of paired interambulacra and located at suture between madreporic plate and first adapical plates of interambulacral column. Ambulacra petaloid adapically. Posterior paired petals (I and V) noticeably longest, each extending 67% of corresponding test radius, but 39% TL; anterior paired petals (II and IV) 59% of corresponding test radius, but 33% TL; anterior unpaired (III) shortest, 68% of corresponding test radius, but 31% TL. Petal V width at widest point 16% TL, interporiferous zone 7% TL; petal IV width 18% TL, interporiferous zone 10% TL; petal III width 16% TL, interporiferous zone 9% TL. Petals lyrate, almost closed distally, with four or five trailing tube feet (sensu Mooi 1989) at distal end of each column of respiratory tube feet (Fig. 3A). Respiratory tube foot pore pairs strongly conjugated, inner pore slightly elongate or almost circular, outer pore extremely elongated, comprising about half length of pore pair, apparently subdivided by stereom septae. Four or five occluded plates present at tips of petals. At ambitus, ambulacra strongly widened, forming strip-like ambital plates that follow contour of shallow indentation at each perradius, and curving strongly adapically to form test wall along each side of posterior notch (Fig. 4). Ambulacra all in agreement with Lovén’s Rule (sensu David et al. 1996). Ambulacral basicoronal plates all similar, narrow and straight with almost parallel radial sutures on each side (Fig. 4).
Interambulacra narrow and straight on oral surface, narrowing towards ambitus, but containing paired, zig-zag plates right up to madreporic plate. On oral surface, two postbasicoronal plates in each half-interambulacrum in interambulacrum 5, about four in interambulacra 1 and 4, and about five in interambulacra 2 and 3. Widest point of each interambulacrum about two thirds of way from basicoronal to ambitus, narrowing distally to about half that width so that paired interambulacra only about 14% width of adjacent ambulacra at ambitus. In each paired interambulacrum, first postbasicoronal greatly elongated, nearly five times as long as wide, about twice length of corresponding second postbasicoronal. Unpaired posterior interambulacrum 5 very narrow near basicoronal, widening distally, then narrowing as it approaches ambitus inside notch (Fig. 4). All interambulacral basicoronals broadly in contact with both corresponding first postbasicoronals.
Peristome circular, relatively small, about 4% TL, with distinct perradial process in each ambulacrum extending into peristome beyond slight bulge containing sphaeridium (Fig. 3B). Anterior edge of peristome 60% TL from anterior edge of test. Periproct small, about 4% TL, situated at ambitus in anterior wall of posterior notch, between second and third pair of postbasicoronals.
Aboral tuberculation homogeneous. Very slight enlargement of tubercles in oral interambulacral regions relative to those in ambulacral regions. In specimens with best preservation of surface detail, distinct tube foot pores visible in food grooves.
Food grooves well developed (Figs. 2, 3, 4), restricted to oral surface, with primary bifurcation near adapical ends of ambulacral basicoronal plates. After this branch point, food grooves continuously diverging as they approach ambitus. Secondary branching faint in all specimens, apparently poorly developed. Slight depressions along perradial sutures on oral surface forming extremely shallow channels reminiscent of rudimentary pressure drainage channels.
Occurrence. Known from the holotype collected from the early Miocene Pirabas Formation in the town of Castelo on the island of Fortaleza, State of Pará, Brazil, and from several additional specimens from the same formation, 5 km north of Capanema, near Colônia Pedro Teixeira, Pará, Brazil.
Remarks. See the description of the family for etymology of the genus name. None of Brito's (1981, 1986) descriptions illustrates oral plate patterns, but these are crucial to interpreting the taxonomic placement of Placatenella complanata. Our investigations indicate that Placatenella is very different in many respects from Abertella pirabensis. Although these two taxa are found near each other in similar stratigraphic circumstances, it is clear that Mooi et al. 2005 were incorrect in assuming that P. complanata and A. pirabensis were conspecific, and that examination of Brito's (1986) additional material of the former was not going to shed light on the morphology of the latter. It was only through re-examination of the type material of both taxa that the major differences became apparent, particularly in oral view. For example, the periproct is on the oral surface in A. pirabensis (see below), not marginal as in Placatenella.
The interambulacra of A. pirabensis are all discontinuous (Fig. 4), as is typical for all adult specimens of any species of Abertella. In all specimens of Placatenella in which plate patterns can be discerned either completely (Fig. 4), or in part (including the holotype), all interambulacra are continuous. Unusual among scutelliforms, and perhaps unlike any known species of abertellid, P. complanata typically has more oral postbasicoronal plates in the anterior paired interambulacra than in the posterior paired interambulacra. In addition, the oral tuberculation of Placatenella is more strongly differentiated in the interambulacral and ambulacral regions, and the regions between the branches of the food grooves are more depressed along the perradial suture, than in Abertella. In A. pirabensis, tuberculation of the oral surface is very uniform, and the surface itself is nearly planar, without significant perradial depressions. Placatanella also differs from A. pirabensis in petal shape. In the former, the petals are more lyrate, whereas in the latter, the two columns of pore pairs are more parallel, and remain so for a great portion of the petals' lengths. In addition, the petals of A. pirabensis vary less in length, are distinctly narrower relative to test length, and have narrower interporiferous zones than in Placatenella.
The posterior notch of Placatenella is remarkable for its depth and narrowness. This condition is unmatched in other South American non-lunulates, including the abertellids and the other genus of placatenellid, Camachoaster. The only species in the Americas that has a posterior notch similar to that of P. complanata is Abertella palmeri Durham, 1957, which stands out in this respect even among the abertellids. However, A. palmeri is easily assigned to the genus Abertella, having all interambulacra markedly discontinuous, and the periproct on the oral surface. The test outline of A. palmeri is also much more strongly alate (sensu Mooi et al. 2000) than in P. complanata. There can be little question that these two forms are not closely related.
description: Published as part of Mooi, Rich, Martínez, Sergio A., Del Río, Claudia J. & Ramos, Maria Inês Feijó, 2018, Late Oligocene - Miocene non-lunulate sand dollars of South America: Revision of abertellid taxa and descriptions of two new families, two new genera, and a new species, pp. 301-326 in Zootaxa 4369 (3) on pages 306-310, DOI: 10.11646/zootaxa.4369.3.1, http://zenodo.org/record/1135772