Metrarabdotos aquaeguttum, 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 : 3-5

Metrarabdotos aquaeguttum sp. nov.

Figure 2.1-4

zoobank.org/ 943B93BE-B715-4836-B69D-904BEFFF17AA

Examined specimens. Holotype: MG-6303-I-a: fertile fragment; 9 mm long x 4 mm wide; Paratype: Atalaia Beach (Salinópolis city): MG-6303-I - b, fertile branch fragment, 5 mm long x 4 mm wide; MG-6303-I - c, fertile branch fragment, 5 mm long x 3 mm wide; MG-6303-I - d, fertile branch fragment, 6 mm long x 4 mm wide; MG-6303-I - e, infertile branch fragment, 3 mm long x 4 mm wide; B- 17 mMine (Capanema city) MG-6303-I - f, fertile branch fragment, 10 mm long x 4 mm wide; MG-6303-I - g, fertile branch fragment, 3 mm long x 4 mm wide.

Etymology. The name aquaeguttum is the latinization for “water drop,” referring to the shape of the ordinary avicularia in this species.

Diagnosis. Colony erect forming rigid, flat branches, each branch with 12-20 longitudinal zooidal series. Orifice tridenticulate. Ordinary avicularia paired, rarely single, very small, drop-like, located alongside latero-distal margins of the orifice, directed proximally and slightly curved inwards; special avicularia approximately three times longer than ordinary ones, elongated with a rounded spatulate tip, located distally to the orifice. Ooecium elongated, semi elliptical to triangular, with about 15 narrow ribs, usually separated each other by two-to-three rows of pores.

Description. Colony erect forming rigid, flat branches, each branch with 12-20 longitudinal zooidal series ( Figure 2.1 ).

Autozoecia elongate (400–833 [average 647] µm length x 125–300 [233] µm width), frontal wall flat, smooth, or slightly granular with 17-23 areolar pores around lateral and proximal margins ( Figure 2.2 ). Peristome slightly elevated, hiding the primary orifice. Secondary orifice longitudinally elongate, oval (100–200 [155] µm long x 100–143 [118] µm wide) with a broad, irregular sinus, one median denticle (frequently broken) ( Figure 2.2-4 ), and a pair of lateral denticles.

Ordinary avicularia on autozoecia as well as in ovicellate zooecia, frequently paired, rarely single, almost equal in size (56–89 [68] µm long), smaller than regular areolar pore, drop-like shaped: premandibular portion rounded, located level to the distal edge of peristome, rarely at its mid length; mandibular portion triangular with pointed rostrum; directed proximally and slightly curved inwards ( Figure 2.2-4 ). Special avicularia approximately three times (in average) longer than the ordinary ones usually developed on zooecia adjacent to ooecia ( Figure 2.3-4 ) and on marginal zooecia ( Figure 2.2 ), usually paired with an ordinary avicularium; mandibular portion elongate, rostrum with slightly curved sides, and rounded spatulate tip; size can be very variable in the same colony (122–400 [254] µm length); pivotal bar frequently broken. Premandibular and part of mandibular portion located above the distal margin of the peristome, mandibular portion directed proximally, often slightly towards midline.

Maternal zooid long (800–1150 [964] µm including the ooecium) with very narrow, transversally elongated orifice, proximal lip moderately developed, not thickened, and distal lip lightly upturned medially to form a curved proximal border of the ooecium; a pair of ordinary avicularia in maternal zooecia have comparable shape and size (56–89 [73] µm long) than those found on autozoecia and are placed at the lateral ends of the orifice, pointing proximally and slightly medially ( Figure 2.4 ); ooecium longer than wide (400–571 [492] µm long and x 300–500 [412] µm wide), semi elliptical to triangular, surface moderately convex, sculptured by 15 thin diverging and slightly arcuated ribs, separated from each other by two-to-three rows of pores ( Figure 2.1, 2.4 ). Areolar pores uniform around the margin, except at the distal region where they are slightly larger ( Figure 2.1, 2.4 ).

Remarks. This species shows a great variability in size and shape of the special avicularia, but the other characters (ordinary avicularia, orifice and ooecium) are more stable.

Metrarabdotos aquaeguttum is distinguished from the other species described here principally by the shape, size, and location of the ordinary avicularium. M. elongatum sp. nov. has larger ordinary avicularia, rostrum elongate, straight or with one side curved ending in a rounded to quadrate tip; avicularia of maternal zooecia are larger and more inwardly directed. Metrarabdotos tuberosum Canu and Bassler, 1928 described previously to Brazil and M. auriculatum Canu and Bassler, 1923 , which has a similar ordinary avicularia, differ from M. aquaeguttum as they have a bidenticulate orifice (lacking the median denticle). The next two species have ordinary avicularia with similar shape and occupying almost the same position than M. aquaeguttum but they can be distinguished as M. floridanum ( Cheetham, 1968) has larger autozoecia, a bifid median denticle, smaller (81–185 µm length) special avicularia with straight rostrum, maternal zooid with distal lip moderately to strongly upturned and proximal lip thickened by coarser tubercles, ooecium larger (609 µm long x 585 µm wide) with strong and granulated ribs; and M. micropora ( Gabb and Horn, 1862) has secondary orifice with short, blunt lateral denticles commonly visible in frontal view; some ordinary avicularia placed midline and proximally, mandibular portion sub-triangular, rostrum straight and slightly rounded tip; special avicularia smaller in size (special: 83–156 µm long), with rostrum elongate, straight and with rounded tip; ooecium slightly wider than long (531 µm long x 583 µm wide) with strong, coarsely granulated ribs.

Range. Lower Miocene.

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

Figures

FIGURE 2. Metrarabdotos aquaeguttum sp. nov. 1. General view of the colony fragment with infertile and fertile zoecia. 2. Detail of a branch edge showing special and ordinary avicularia. 3. Detail of zooids adjacent to a maternal one showing orifice, special avicularium paired with an ordinary one, and part of the ooecium. 4. Detail of maternal zooid with a complete ooecium, ordinary avicularia, and a zooid adjacent to maternal one, with a special avicularium. Scale bars equal 1: 500 µm; 2: 100 µm; 3: 40 µm; 4: 200 µm. 1, 4: MG-6303-I-b; 2: MG-6303-I-c; 3: MG-6303-I-a.

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.

FIGURE 4. Metrarabdotos capanemensis sp. nov. 1. General view of a colony showing fertile and infertile zooecia. 2. Detail showing two maternal zooecia bearing complete ooecia, flat ordinary zooecia and a large special avicularium below the maternal zooid. 3. Some zooecia with paired, ordinary proximally located avicularia and one elongated special avicularium. Note the frontal shield with elliptical areolar pores and a median longitudinal keel. Scale bars equal 1: 1000 µm; 2-3: 200 µm. 1-3: MG-6303-I-t. Arrows indicate special avicularia.