Pyrisinella meniscacantha ( Taylor & McKinney, 2006 ) Taylor & McKinney, 2006

Martino, Emanuela Di & Taylor, Paul D, 2012, Pyrisinellidae, a new family of anascan cheilostome bryozoans, Zootaxa 3534, pp. 1-20 : 4-6

Pyrisinella meniscacantha ( Taylor & McKinney, 2006) n. comb.

( Figs 1−8 ; Table 1)

Setosinella meniscacantha Taylor & McKinney, 2006: 110 −112, pls. 75−76.

Material examined. Holotype: NHM BZ 4796(1a), Maastrichtian, Prairie Bluff Chalk, Livingston, Sumter County, Alabama, USA. Paratypes: BZ 4796(1b), encrusting same shell as holotype; BZ 4810(2), BZ 5042, Maastrichtian, Ripley Formation, Jefferson, Marengo County, Alabama, USA; BZ 5182, Maastrichtian, Peedee Formation, Ideal Quarry, New Hanover County, North Carolina, USA.

Description. Colony encrusting, multiserial, unilaminar, usually irregular in outline shape. Growing edge stepped, implying intrazooidal budding, some incomplete buds present comprising basal wall and stumps of vertical walls ( Figs. 7, 8 ). Distal pore chamber larger than distolateral pore chambers; pore windows oval (mean L = 33 µm, mean W = 16 µm) surrounded by a mural ring. Ancestrula having the appearance of a small astogenetically mature autozooid (mean L = 165 µm, mean W = 140 µm) with an indeterminate number of spine bases; budding a single periancestrular zooid distally ( Fig. 2 ). Autozooids in zone of primary astogenetic change transitional in size between ancestrula and later autozooids. Autozooids small, rhomboidal, longer than broad (mean L/W = 1.20), separated by deep furrows and arranged in well-defined rows. Gymnocyst narrow, usually broadest proximally. Cryptocyst depressed, forming an extensive flat shelf, finely and densely granular, with granules immediately proximally of orifice tending to be aligned in weak radial rows. Mural rim salient, together with distal rim of opesia forming pear-shaped ridge enclosing cryptocyst and opesia. Opesia trifoliate, the larger, distal semielliptical part divided from the shallower proximal part by an indentation in the mural rim ( Fig. 5 ). Six to eight oral spine bases, large and closely spaced, arranged in a prominent crescentic arch around distal rim of orifice; distalmost pair of oral spines hidden by ovicell in ovicellate zooids. Ovicell hyperstomial, globose, prominent, with a dumbbell-shaped opening, resting on proximal gymnocyst of distal zooid and indenting its mural rim ( Fig. 6 ). Intramural buds observed especially in damaged zooids. Closure plates depressed beneath level of cryptocyst, with a sunken pore located almost centrally. Avicularia interzooidal or more commonly adventitious, confined to furrows between autozooids and budded onto autozooidal gymnocyst, most orientated obliquely distally; gymnocyst variably developed, more extensive in interzooidal than adventitious avicularia; cryptocyst granular and broadest proximally; opesia a uniformly tapering teardrop shape; rostrum acuminate, longer than wide; pivotal bar calcified.

Remarks. Taylor & McKinney (2006) originally assigned this species from the Late Cretaceous of the Atlantic and Gulf Coast plains of the USA to Setosinella Canu & Bassler, 1933, having questioned the presence of opesiules in the type species of this genus, S. prolifica . However, new SEM studies have shown that opesiules are present in S. prolifica (see below), whereas they are clearly lacking in Pyrisinella .

Small, fan-like outgrowths observed in some of the larger colonies of Pyrisinella meniscacantha from the Prairie Bluff Chalk were considered by Taylor & McKinney (2006) to be peripheral subcolonies budded from the larger colony but new SEM observations show this not to be the case. Instead, they represent new colonies founded by larvae that settled close to the edge of a dormant or dead colony, in some instances within a partly formed zooid at the growing edge.

Distribution. Maastrichtian (Upper Cretaceous) of Alabama, Mississippi and North Carolina, USA.

N (colonies, zooids) Mean SD Range Zooid length 3, 30 361 29 295–405 Zooid width 3, 30 304 33 243–374 Orifice length 3, 30 83 17 47–123 Orifice width 3, 30 98 17 57–132 Ovicell length 3, 25 133 21 78–167 Ovicell width 3, 25 154 19 126–191 Avicularia length 3, 12 139 28 87–133 Avicularia width 3, 12 77 12 50–90

N, Number of colonies and number of zooids measured; SD, standard deviation.

Figures

FIGURES 1 – 8. Pyrisinella meniscacantha (Taylor & McKinney, 2006) n. comb, Cretaceous, Maastrichtian, Prairie Bluff Chalk, Livingston, Alabama, USA 1. view of colony, NHM BZ 4796 (1 a), holotype, scale bar = 500 µm; 2. same, ancestrula and early astogeny showing overgrowth of some early zooids at centre, scale bar = 100 µm; 3. same, close-up of a nonovicellate autozooid, scale bar = 100 µm; 4. same, ovicellate autozooids and avicularia, scale bar = 100 µm; 5. close-up of two autozooids and interzooidal avicularium, showing the trifoliate opesia and six orificial spine bases, NHM BZ 4796 (1 b), paratype, scale bar = 100 µm; 6. close-up of ovicell and avicularium, NHM BZ 4796 (1 a), holotype, scale bar = 100 µm; 7. same, stepped edge of colony with some incomplete buds, pore windows and closure plates visible along the margin, scale bar = 200 µm; 8. same, intramural bud, scale bar = 100 µm.