Metrarabdotos elongatum, Ramalho, Tavora. & Zagorsek, 2017

Ramalho, Laís V., Távora, Vladimir A. & Zagorsek, Kamil, 2017, New records of the bryozoan Metrarabdotos from the Pirabas Formation (Lower Miocene), Pará State, Brazil, Palaeontologia Electronica (Weinheim an der Bergstrasse, Germany) 10 (15), pp. 1-11 : 5-7

Metrarabdotos elongatum sp. nov.

Figure 3.1 -6

zoobank.org/ 2CE3BC56-8BBB-4D10-888C-E032CAE9F443

Referred specimens. Holotype: MG-6303-I - i, B-17 Mine, fertile branch fragment, 10 mm long x 4 mm wide; Paratype: Atalaia Beach (Salinópolis city): MG-6303-I - h, fertile branch fragment, 4 mm long x 3 mm wide; MG-6303-I - l, infertile branch fragment, 4 mm long x 3 mm wide; MG-6303-I - m, fertile branch fragment, 8 mm x 5 mm; MG-6303-I - n, infertile branch fragment, 5 mm long x 5 mm wide; MG-6303-I - o, fertile branch fragment, 4 mm long x 5 mm wide; MG-6303-I - p, infertile branch fragment, 41 mm long x 12 mm wide; MG-6303-I - q, fertile branch fragment, 5 mm long x 4 mm wide; MG-6303-I - r, fertile branch fragment, 11 mm long x 5 mm wide.

B-17 Mine (Capanema city): MG-6303-I - j, infertile branch fragment, 2 mm long x 3 mm wide; MG-6303-I - k: fertile branch fragment, 6 mm long x 4 mm wide; MG-6303-I - s, fertile branch fragment, 8 mm long x 5 mm wide.

Etymology. The name elongatum is the latinized word for “elongate,” referring to the shape of the rostrum of the avicularium.

Diagnosis. Colony erect, bilamilate; orifice circular to ellipsoidal, tridenticulate. Ordinary avicularia often paired, pre-mandibular portion located above or level to distal margin of the orifice, rarely midline or proximally; rostrum elongated, straight or with one side curved, ending in a curved triangular or quadrate-spatulate tip. Special avicularia large on zooecia adjacent to ooecia and slightly smaller on common zooecia; pre-mandibular portion placed very distally to secondary orifice; mandibular portion elongate, narrower in the middle part and ending in a spatulate or rounded tip. Ooecium longer than wide, the surface moderately flat and perforated by pores between weak ribs.

Description. Colony erect, bilamilate with 14-21 zooecia disposed alternately in longitudinal series ( Figure 3.1 ).

Ordinary autozooids rectangular, longer than wide (409–733 [578] µm long x 144–277 [208] µm wide). Frontal wall finely tuberculate, flat; 15-23 sunken areolare around the lateral and proximal border ( Figure 3.2 ). Primary orifice hiding by the peristome; secondary orifice almost circular to ellipsoidal, almost as long as wide (100–167 [134] µm long x 100–180 [131] µm wide) with a broad Ushaped sinus bearing a prominent median denticle ( Figure 3.2-3 ) and a pair of lateral denticles (orifice tridenticulate) ( Figure 3.4 ).

Ordinary avicularia (67–229 [130] µm long) paired in most zooecia, including the maternal ones almost equal in size and position ( Figure 3.2- 3 ), infrequently single; pre-mandibular portion frequently placed above or level to the distal margin of the secondary orifice, rarely midline or more proximally; mandibular portion elongate, the tip touching lining the lateral margin of the orifice; rostrum straight to curved and narrowed but ending in a curved triangular or quadrate-spatulate tip, directed proximally and sometimes slightly inward; crossbar complete ( Figure 3.2-3 ). Special avicularia variable in size (122–356 µm long), longer on zooecia adjacent to ooecia, and slightly smaller on common zooecia. Avicularia on the marginal zooecia usually comparable to those of the central zooecia, sometimes slightly longer ( Figure 3.1, 3.5 - 6). Special avicularia with pre-mandibular region placed very distally in relation to the orifice, and their proximal portion is in the midline of the orifice; mandibular portion elongate, ending in a spatulate or rounded tip; rostrum straight or with one or both sides curved and tip directed (lining) to orifice (proximally and medially). Maternal zooecia elongated (704–1267 [938] µm long) with crescent-like orifices (180–400 [242] µm wide) ( Figure 3.1, 3.6 ), distal lip poorly developed, upturned forming the narrow and straight proximal border of the ooecium, proximal lip not protruded, smooth; one pair of ordinary avicularia located near the proximal corners, unequal in size (76–114 [100] µm long), similar in shape to those on the zooecia; directed proximally and inward ( Figure 3.6 ). Ooecium longer than wide (360–533 [470] µm long x 322–467 [367] µm wide), with a sub-triangular to arcuate distal end; surface moderatelly flat perforated by pores between weak ribs, and bordered by larger areolar pores, mostly along the distal end ( Figure 3.1, 3.6 ).

Remarks. The elongate shape, size, and position of the ordinary avicularia distinguish this species from others described in this study and from those previously known. Metrarabdotos colligatum Canu and Bassler, 1919 has similar special avicularia but it can be distinguished for the secondary orifice with two or rarely three denticles, the smaller ordinary avicularium that is triangular and placed level to the orifice; the absence of avicularia associated to ooecia.

Range. Lower Miocene.

Occurrence. Pirabas Formation: Atalaia Beach, Salinópolis city, and B-17 Mine, Capanema city, Pará state.

Figures

FIGURE 3. Metrarabdotos elongatum sp. nov. 1. General view of a fragment showing some fertile and infertile zooecia. 2. Detail of some zooecia showing orifice with median denticles, paired avicularia, and areolar pores. 3. Detail of the secondary orifices showing a median denticle and a pair of laterally ordinary avicularia. 4. Internal view of some zooecia showing tridenticulate orifices. 5. Detail of the branch edge showing zooecia with large ordinary avicularia. 6. A maternal zooid with complete ooecium and paired ordinary avicularia surrounded by zooecia, each with a special avicularium paired with an ordinary one, and by infertile zooecia with paired ordinary avicularia. Scale bars equal 1: 500 µm; 2, 4-5: 200 µm; 3: 20 µm; 6: 100 µm. 1, 6: MG-6303-I-i; 2: MG-6303-I-p; 3: MG-6303-I-s; 4: MG-6303-I-l; 5: MG- 6303-I-h.