Abertella pirabensis
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
Abertellidae
Abertella
Abertella pirabensis

description: Abertella pirabensis (Marchesini Santos, 1958)
Figures 1, 4, 9.
1958 Karlaster pirabensis Marchesini Santos: 16–19, pl. 5: figs. 1–3.
1979 Karlaster pirabensis Santos—Brito: 736–738, pl. 3: fig. 1, pl. 4: fig. 6.
1997 Abertella pirabensis (Marchesini Santos) —Martínez & Mooi: 61.
2000 Abertella pirabensis (Marchesini Santos) (as senior synonym of Abertella complanata Brito 1986)—Mooi et al.: 266.
Diagnosis. An Abertella in which the posterior notch is deep and sharply defined, the periproct just barely between the second pair of posterior interambulacral postbasicoronals, and the length of the disjunction between the interambulacral basicoronal and first postbasicoronals much less than half the length of the corresponding basicoronal in paired interambulacra.
Type material studied. The holotype, DNPM 4493, is the only known specimen.
Description. In general, the original description by Marchesini Santos (1958) is accurate and detailed, except for the instances discussed below. Her images do not satisfy modern standards, and we provide additional images of both surfaces, detailed views of the oral surface and wall of the notch, and a reconstructed plate map (Figs. 4, 9). The following description is of the same format as for placatenellid species discussed above, to facilitate comparisons.
Holotype (Fig. 9A, B) approximately 51 mm TL (as defined in Table 1). Ratio of width to length including lobes on either side of notch 1.21. Aboral surface slightly domed, oral surface extremely flat, nearly planar and without significant sculpting or radial depressions. Highest point of test approximately 11% TL, located at apical system. Well-defined posterior notch, depth estimated to be approximately 11% TL, width approximately 10% TL at ambitus, notch widening near ambitus. Marginal indentations extremely shallow where perradial suture meets ambitus in posterior paired ambulacra, nearly absent in anterior paired ambulacra.
Apical system monobasal, star-shaped, 54% TL from ocular III to anterior edge of test, length 9% 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) longest, but only slightly so, each extending 65% of corresponding test radius, but 34% TL; anterior paired petals (II and IV) 62% of corresponding test radius, but 33% TL; anterior unpaired (III) shortest, 61% of corresponding test radius, but 32% TL. Petal V width at widest point 13% TL, interporiferous zone 4% TL; petal IV width 13% TL, interporiferous zone 5% TL; petal III width 15% TL, interporiferous zone 6% TL. Petals not obviously lyrate, but have outer edges of each pore pair column in a given petal parallel for most of its length, petals almost closed distally, with three or four trailing tube feet at distal end of each column of respiratory tube feet. Respiratory tube foot pore pairs strongly conjugated, inner pore slightly elongate or almost circular, outer pore extremely elongated, comprising about two thirds length of pore pair, apparently subdivided by stereom septae. Five or six occluded plates present at tips of petals. At ambitus, ambulacra greatly widened, forming strip-like ambital plates, curving strongly adapically to form test wall along each side of posterior notch (Figs. 4, 9C, D). 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, three or four postbasicoronal plates in each halfinterambulacrum in interambulacrum 5, four or five in the other interambulacra. Widest point of interambulacra 1 and 4 at first or second postbasicoronals, about one third of way to ambitus, narrowing distally to about two thirds that width so that paired interambulacra about 21% width of adjacent ambulacra at ambitus. In each paired interambulacrum, first postbasicoronal slightly elongated, two to three times as long as wide in posterior paired interambulacra, three to four times as long as wide in anterior paired interambulacra. Unpaired, posterior interambulacrum narrowing as it approaches ambitus inside notch (Fig. 4). All interambulacral basicoronals discontinuous, separated from first postbasicoronals by adjacent ambulacral first postbasicoronals, very widely so in interambulacrum 5 (Fig.4), but by far less than half length of a corresponding basicoronal in paired interambulacra.
Peristome circular, relatively small, about 4% TL, with distinct perradial process in each ambulacrum extending into peristome beyond slight bulge containing sphaeridium. Anterior edge of peristome 54% TL from anterior edge of test. Periproct small, about 4% TL, situated 90% TL from anterior edge of test, just barely between second pair of postbasicoronals, with slight apparent contact with one of the first pair of postbasicoronals (Fig. 4).
Aboral tuberculation homogeneous, oral tuberculation nearly so, without discernible enlargement of tubercles in oral interambulacral regions. Tube foot pores visible in food grooves (Fig. 9C).
Food grooves well developed (Figs. 4, 9B, C), 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 or non-existent. No significant depressions along perradial sutures on oral surface, no evidence of pressure drainage channels.
Occurrence. A. pirabensis is known only from the early Miocene Pirabas Formation, Ponta de Pirabas, Ilha de Fortaleza, State of Pará, Brazil.
Remarks. We suggest that all previous attempts to allocate this taxon to a genus were incorrect to some degree. We were able to confirm the original suggestion, indicated by the figures of Marchesini Santos (1958) and Brito (1979), that all interambulacra are discontinuous. This alone suggests that the attempts to place A. pirabensis in the Monophorasteridae Lahille, 1896 are misguided because all the interambulacra of all species in that family are broadly continuous, both postbasicoronal plates being in contact with the basicoronal, with a minor exception in a specimen of M. telfordi (see Mooi et al. 2016). Martínez and Mooi (1997) and Mooi et al. (2000) compared A. pirabensis with what was then known as Abertella complanata (here placed in the new genus Placatenella), and became convinced that A. complanata was a junior synonym of A. pirabensis. However, this was before the oral surface plate pattern of A. pirabensis could be confirmed to be of the Abertella configuration. Moreover, the hitherto unknown plate architecture of the oral surface of what we here recognize as Placatenella complanata has turned out to be unique among all taxa with a posterior notch because all the oral interambulacra are continuous. This is clearly unlike any Abertella, and undermines any relation between A. pirabensis and P. complanata.
The conclusion is that the only known specimen of A. pirabensis is the holotype, described as the type specimen of a new genus, Karlaster, by Marchesini Santos (1958). Karlaster Marchesini Santos (1958) is not recognized by the present revision, and in this respect, we follow the work of Mooi et al. (2000) in regarding Karlaster a junior synonym of Abertella.
We have been unable to support the assertion by Marchesini Santos (1958) that A. pirabensis possessed an anal lunule. Breakage along the posterior edge of the only known specimen would have rendered it impossible to make the determination that the distal parts of the lobes meet again at the ambitus. However, the curvature of the ambulacral plate sutures on either side of the posterior indentation is not consistent with that observed in all other scutelliforms that possess an anal lunule. The anal lunule of all mellitids and monophorasterids possesses walls constructed of interambulacral plates in the cross-linked pattern (sensu Seilacher 1979: Fig. 8B, Mooi et al. 2000: Fig. 4.4), a situation than can be considered diagnostic for the anal lunule. In A. pirabensis, there are no traces of the cross-linked pattern, leading us to reconstruct A. pirabensis with a notch of moderate depth through extrapolation of the aforementioned ambulacral plate curvatures (Fig. 4).
Examination of the anterior wall of the notch reveals no evidence for the existence of an opening that could be interpreted as a periproct (Fig. 9D). Marchesini Santos (1958) appears to have been correct in interpreting the aperture on the oral surface as the periproct. It is neither a trace of an anal lunule, nor damage to the fossil. However, there does appear to be some damage to the test around the edges of the aperture, causing enlargement that could explain why the periproct appears to be in slight contact with the first interambulacral postbasicoronals (Fig. 9C). The periproct of A. pirabensis is between the second pair of interambulacral postbasicoronals, not solely enclosed within the first pair as in all monophorasterids. Therefore, the periproct position of A. pirabensis is like that of abertellids, but not of monophorasterids. The unequal development of the interambulacral basicoronals is also very similar to the condition seen in other Abertella, notably A. miskellyi, further undermining placement of A. pirabensis in the monophorasterids.
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 318-321, DOI: 10.11646/zootaxa.4369.3.1, http://zenodo.org/record/1135772