Fibularia kieri ( Tandon & Srivastava, 1980 )

Mooi, Rich, Kroh, Andreas & Srivastava, Dinesh K., 2014, Phylogenetic re-evaluation of fossil and extant micro-echinoids with revision of Tridium, Cyamidia, and Lenicyamidia (Echinoidea: Clypeasteroida), Zootaxa 3857 (4), pp. 501-526 : 517-520

Fibularia kieri ( Tandon & Srivastava, 1980) (new combination)

Figures 1–2 , 4 , 12

1980 Tridium kieri Tandon & Srivastava : 1–3, pl. 1: figs. 1–6

Type material. Department of Geology , University of Lucknow, K 651 (holotype), K 652–K 653 (paratypes) .

Material studied. DGUL: 10 specimens (K 654–K 663) , and numerous specimens without repository numbers; CASG: 4 specimens (72990, 72991) ; NHMW: 5 specimens ( NHMW 2011 /0420/0001–0005), all from type locality .

Type locality. Near village of Guvar, Kachchh, India (23°38′10″ N, 68°32′30″ E) .

Type stratum. Nummulites beaumonti Zone of Tandon (1976), Middle Eocene.

Other occurrences. Jhadwa (23°30′30″ N, 68°36′30″ E), Panandro (23°41′ N, 68°45′05″ E) villages and Ratchelo nala, 3.2 km south of Baranda village (23°34′20″ N, 68°43′10″ E), Kachchh, India.

Emended diagnosis. Very small, almost spherical Fibularia unique among laganiforms in having only 3 gonopores, lacking gonopore in interambulacrum 2 position; infundibulum completely lacking, with peristomial region everted outwards until even with surrounding oral surface; buccal pores facing directly downwards, not into peristome; distinct circumoral ring proximal to buccal pores; periproct surrounded by slight, spout-like ridge; density of spine tubercles greatly reduced, only 2 to 4 primary tubercles found in tight grouping near centre of ambital plates; sutures of ambital plates smooth, lacking external structures such as spine or pedicellaria tubercles or glassy tubercles.

ZooBank LSID. urn:lsid:zoobank.org:act:8AC353E9-96E0-4274-BC94-561CC0AA8130

Description. Size and shape —Corona small, largest specimens not exceeding 5 mm TL; corona globular, almost spherical, with barely subequal length, width and height; aboral outline nearly circular, very slightly acuminate posteriorly; profile of corona likewise circular, very slightly flattened adorally in peristomial region.

Internal buttressing —Absent.

Apical system —Central in aboral view; monobasal, with three gonopores, lacking gonopore in interambulacral position 2; gonopores well within madreporic plate; single, central hydropore, opening flush with plate surface, not in pit or groove; ocular pores small and indistinct, almost directly between most proximal pores in each petal.

Ambulacra —Ambulacral plating simple; petals short, consisting of 4 to 6 large, faintly conjugate respiratory pore pairs in each ambulacrum; pore pairs lying strongly oblique, crossing ambulacral plates; distal pore pairs becoming even more strongly oblique with distance between pores in each pair slightly decreasing; width of interporiferous zones remaining largely constant in adapical two thirds of each petal, but decreasing slightly towards distal end; distally, obliqueness of pore pairs increases so that most distal pore pairs are oriented almost parallel to radius; in some adapical pore pairs, outer pore pierces interambulacral plate rather than ambulacral; petaloid region large, extending almost 70% of TL in aboral view; buccal pores large, facing downwards rather than into the peristome; accessory pores small, barely larger than openings in stereom meshwork; usually crowded in patches around large tubercles in middle of plates; except for very well-preserved specimens, pores only visible close to peristome; food grooves absent.

Interambulacra —Adapically, two unpaired plates lie in tandem adjacent to apical system; four, occasionally five post-basicoronal interambulacrals in each column visible in oral view; interambulacrum 5 not expanding in region accommodating periproct, but continuously divergent in oral view; a few scattered accessory pores visible in some interambulacrals; at ambitus, interambulacra at least as wide as ambulacra; each column comprising large, hexagonal plates of equal height and width.

Tuberculation —Patchy and sparse, with 1 to 4 tubercles concentrated in small clusters separated from tubercles in adjacent plates by areas of unornamented stereom; areoles wide, approximately three times diameter of boss, slightly sunken; miliary tubercles indistinct, although small tubercles about 20% diameter of the primary tubercles likely represent miliaries, or perhaps attachment points for pedicellariae; glassy tubercles absent.

Peristome —Small, approximately 17% TL; circular; facing directly downwards almost centrally on oral surface but slightly displaced approximately 1–2% TL posteriorly; opening not sunken, lacking infundibulum; rimmed by sharp, raised ridge of relatively dense stereom forming circumoral ring; framed by basicoronal circlet in which ambulacral plates are approximately same length as adjacent interambulacrals; 4 to 6 enlarged primary tubercles, areoles abutting, in each interambulacral area adjacent to peristome; in each ambulacrum, slightly elevated ridge or bar of stereom just distal to buccal pores, extending only slightly laterally from perradial bulge containing sphaeridium.

Periproct —Small, about 13% TL; subcircular to subpentagonal in outline, sometimes slightly anteroposteriorly elongated; facing downwards halfway between peristome and posterior margin; bounded by first (5.a.2, 5.b.2) and second (5.a.3, 5.b.3) pairs of post-basicoronal plates; margin of periproct very slightly raised into slight "spout".

Perignathic girdle —Consisting of five small processes (auricles), one on the internal surface of each interambulacral basicoronal.

Sphaeridia —One per ambulacrum; fully enclosed; situated beneath low transverse bar just distal to buccal pores.

Spines, pedicellariae, lantern —Unknown.

Remarks. In the original description Tandon & Srivastava (1980: fig. 2) showed the third pair of postbasicoronal plates forming part of the periproctal margin, but this was not in agreement with their own description in the text. We were able to determine that the third pair of post-basicoronals plays no part in the construction of the periproctal margin. In addition, Tandon & Srivastava (1980: fig. 2) depicted a small plate just distal to each ambulacral basicoronal, which in turn suggested that the ambulacral basicoronals were significantly shorter than the interambulacral basicoronals, causing the ambulacral basicoronals to be isolated from the adjacent interambulacral first post-basicoronals in every case. However, these small, additional plates are not apparent from our observations. In fact, the ambulacral basicoronals are nearly equal in length to the interambulacral basicoronals. The ambulacral basicoronals are in contact with the adjacent interambulacral first post-basicoronals in nearly every radius, as in other fibulariids. The illustration in Tandon & Srivastava (1980: fig. 1) also shows the outer pores of the petals not piercing the interambulacra, but we have identified several outer pores proximal to the apical system that emerge through the interambulacrals.

Tandon & Srivastava (1980) and Smith & Kroh (2011) identified the presence of just three gonopores as the sole autapomorphy of the genus, thereby distinguishing it from Fibularia . We have identified several other features that are autapomorpic in T. kieri (see diagnosis above), but these unique, phylogeneticially uninformative features do not serve to distinguish T. kieri from Fibularia in a cladistic sense, and we here consider Tridium a junior synonym of Fibularia . Smith & Kroh (2011) additionally questioned whether three gonopores were consistent in larger populations. Genital pore 2, however, is missing in all known specimens (N> 350) and it is unlikely that this is a case of abnormal development. Except in rare cases of specimens obviously affected by sublethal predation or other growth deformations, gonopore development is usually consistent within clypeasteroid species and not prone to variation.

Curiously, Tandon & Srivastava (1980: p. 2) indicated that "no female was found". Sexual dimorphism in clypeasteroids, and even in micro-echinoids such as Fibularia plateia , F. cribellum , and F. japonica , is known from differences in gonopore diameter and sometimes even from the presence of brood pouches, as in F. nutriens . However, Tandon & Srivastava (1980) made no such measurements. It is most likely that this species exhibits no readily detectable sexual dimorphism than the rather low likelihood that there were no females represented in the over 350 specimens collected.

Re-examination of Fibularia kieri shows that the missing gonopore is not the sole autapomorphy of that taxon. The patchy tuberculation, with concomitant lack of ornamentation in proximity to the plate sutures, and especially features of the peristomial region are very different from what is usually observed in fibulariids or in other echinoids in general. The most adoral tubercles usually support spines that are internally directed towards the mouth in the centre of the peristomial membrane, more or less parallel or at low angles to the surface of that membrane. Likewise, the buccal podia (and the pores supporting them) are directed towards the mouth in most clypeasteroids, so that the podia can more easily reach into it ( Mooi 1986).

In F. kieri , the situation is quite different. Here the adoral tubercles and buccal pores face downwards rather than towards the peristome. The tubercles would have supported spines that extended at right angles to the test and to the plane of the peristomial membrane. Unless they were significantly bent inwards, they would not have formed a grill across the membrane as seen in other clypeasteroids. In addition, the buccal podia would have been well situated to reach downwards at right angles to the test, but not into the mouth region. A peristomial infundibulum, observed in almost all other clypeasteroids, is lacking, giving the peristomial region a flat aspect flush with the surrounding corona. The entire organization has the appearance of having been everted, or “turned outwards” ( Fig. 4 ).

The function of the inner circumoral ring is unclear. In comparison with taxa in which the infundibulum is not everted, the position of the ring would correspond to the line of insertion of the peristomial membrane. No similar structure has been observed in other clypeasteroids. A superficially similar ring is developed in Leniechinus and Cyamidia , but this lies more distally, aboral to the buccal pores. In these taxa, this outer ring is formed by lateral expansion of the ambulacral bars overlying the sphaeridia . In Tridium these are slightly expanded laterally as well, but fail to meet interradially, being separated by wide gaps across the interambulacra.

Figures

FIGURE 1. SEM images of the test of two specimens of Tridium (= Fibularia) kieri. A. NHMW 2011/0420/0001, aboral view. B. NHMW 2011/0420/0001, oral view. C. NHMW 2011/0420/0001, left lateral view. D. NHMW 2011/0420/0002, aboral view. E. NHMW 2011/0420/0002, oral view. F. NHMW 2011/0420/0002, right lateral view.

FIGURE 2. SEM images showing details on test surface of Tridium (= Fibularia) kieri, NHMW 2011/0420/0001. A. Apical region, pores of each respiratory tube foot linked by dotted lines, other apertures labelled: H = hydropore; G1, G3, G4 = gonopores of interambulacra 1, 3, and 4 respectively; ocular pores indicated by arrows. B. Surface features of test just above ambitus in interambulacrum 2. C. Peristomial region showing circumoral ring, buccal pores, and proximal part of periproct (bottom of image). All scale bars 100 µm long.

FIGURE 4. Morphology of the peristomial region in six species of fibulariids, including Cyamidia, Tridium (= Fibularia), Lenicyamidia, and Leniechinus. On the left for each species is the oral view depicting tuberculation, sculpturing, and positions of buccal pores and sphaeridial chambers, with accompanying scale bar. On the right for each species is a sagittal section through the test (hatched) around the peristome, anterior to the left, with accompanying scale bar. In all cases, scale bar is 1.0 mm long.

FIGURE 12. Plate architecture of Tridium (= Fibularia) kieri, CASG 72990. Labelling conventions in Fig. 6.