Monticuliporidae Nicholson, 1881

Buttler, Caroline J., Cherns, Lesley & Mccobb, Lucy M. E., 2022, Trepostome bryozoans encrusting Silurian gastropods: A taphonomic window and its implications for biodiversity, Acta Palaeontologica Polonica 67 (3), pp. 569-577 : 572-574

Family Monticuliporidae Nicholson, 1881 Genus Homotrypa Ulrich, 1882

Type species: Homotrypa curvata Ulrich, 1882 ; Cincinnatian (Upper Ordovician), North America.

Emended diagnosis.—Colonies generally frondose, less commonly dendroid, may be encrusting or massive. Maculae typically formed of clusters of mesozooecia with thick diaphragms or solid stereom surrounded irregularly with large macrozooecia. Endozones uniform in cross-sectional size and autozooecial pattern, autozooecia in disordered pattern throughout, polygonal in cross-section. Diaphragms moderate in spacing and evenly spaced to few and scattered. Autozooecial walls in endozones typically irregularly undulating, some crenulated, polygonal in cross-section. In exozones autozooecial boundaries polygonal in cross-section, serrated in longitudinal sections. Some colonies with thin continuous cingulum throughout, or with cingulum irregularly distributed. Cystiphragms located in exozone. Diaphragms in exozone primary attached to cortex or secondary attached to cystiphragms or cingulum. Mesozooecia rare to abundant. Zooecial and endozonal styles may be present, some oblique to autozooecia. Small exozonal styles common.

Stratigraphic and geographic range.—Middle Ordovician– Silurian; North America, Europe, Australia, Siberia, China.

Homotrypa cochlea sp. nov.

Figs. 3 , 4 , 5 ; Table 1.

Zoobank LSID: urn:lsid:zoobank.org:act:D1C4A37E-3C72-4AE5-B6E5-A444A3623B09

Etymology: From Latin cochlea , snail; named because all specimens are found encrusting gastropods.

Type material: Holotype: NMW 2019.21 G.2, whole specimen cut into sections and including prepared thin sections . Paratypes: NMW 2019.21 G.1–10, 12, 19, 35, 36, whole specimens cut into sections and including prepared thin sections, all from the type locality .

Type locality: Delbury Quarry , Shropshire (SO 494 862, Lat/Long 52.47140087 –2.74484452)

Type horizon: Upper Leintwardine Formation , Ludlow, Silurian .

Diagnosis.— Colonies typically encrusting but can develop ramose form. Endozonal autozooecia recumbent, exozonal budding irregular. Autozooecial apertures irregularly oval to circular, basal diaphragms closely spaced and localised cystiphragms. Mesozooecia common, fairly large with thick walls, develop as closed cysts in early exozone, with abundant diaphragms, in cross-section chambers circular to ovoid. Exozonal styles present.

Description.—Colonies typically encrusting, single or multi laminar, up to five layers recognised. One colony with single erect branch ( Fig. 4 : 6.5 mm diameter, 10 mm high) developed after encrusting phase, maculae unknown. In endozones of encrusting form autozooecia restricted to recumbent buds grown from exterior basal colony walls. In ramose form autozooecia with disordered pattern in endozones. Autozooecial walls thin, smoothly curved to irregularly undulating in profile. Diaphragms typically widely spaced in endozone of erect branch. In exozones of the erect form the budding is irregular, some autozooecia budded from mesozooecia and mesozooecia budded from autozooecia, both intrazooidally. Autozooecia and mesozooecia disordered in arrangement. Autozooecial boundaries both merged and serrated in longitudinal sections, cortex walls irregular in thickness with some small patches of cingulum. In tangential sections autozooecial apertures irregularly oval to circular, mean average autozooecial diameter maximum 0.19 mm and minimum 0.16 mm, some irregularly shaped. Autozooecial basal diaphragms primary and closely spaced (0.11 mm mean average), some diaphragms inclined or attached to cystiphragms. Mesozooecia abundant and quite large (0.12 mm mean average in diameter), with thick walls, in cross-section chambers circular to ovoid. Mesozooecia develop as closed cysts in early exozone ( Fig. 5A 2 ), thick diaphragms and walls combine longitudinal views to compartmentalize chambers, followed by moderate to extremely thick annular mesozooecia walls connected by diaphragms to continue cystose shape of earlier growth. Some mesozooecial diaphragms extremely thick. Some autozooecia contain abundant cystiphragms in the exozone. Exozonal styles present ( Fig. 5B ), irregularly arranged, some on autozooecial boundaries and some inflecting autozooecial walls.

Remarks.— Homotrypa cochlea sp. nov. is characterised by the cystose or beaded shape of the mesozooecia which is unusual in species of this genus. It differs from Homotrypa oweni ( Ross, 1965) from the Caradoc (upper Ordovician) of Shropshire by having more abundant diaphragms and the nature of the mesozooecia. In morphology it is similar to Asperopora ludlovensis ( Owen, 1962) , known from the Ludlow (upper Silurian) of England and Sweden, which also forms encrusting colonies with ramose branches, but the latter species has very abundant styles and cystiphragms are absent.

Stratigraphic and geographic range.—Upper Leintwardine Formation, Ludlow Series, Silurian; Shropshire, England.

Figures

Fig. 3. Thin sections of trepostome bryozoan Homotrypa cochlea sp. nov. from Upper Leintwardine Formation; Ludfordian, upper Ludlow, Silurian; Delbury Quarry, Shropshire, UK. A. NMW 2019.21G.5.2, encrusting colonies comprising multiple layers caused by self-overgrowths (A1), apex of gastropod shell encrusted with bryozoan colony (A2), colony does not grow over the gastropod shell aperture (A3, A4). B. NMW 2019.21G.6.2, colony encrusting apertural side of shell thinner than on the abapertural side, arrow indicates Trypanites boring in bryozoan colony. C. NMW 2019.21G.7.3, colony can be seen inside the aperture of gastropod shell.

Fig. 4. Thin section of trepostome bryozoan Homotrypa cochlea sp. nov. from Upper Leintwardine Formation, Ludfordian, upper Ludlow; Delbury Quarry, Shropshire, UK. Holotype, NMW 2019.21G.2.3, bryozoan colony encrusting gastropod shell and developing ramose form at shell aperture.

Fig. 5. Thin sections of trepostome bryozoan Homotrypa cochlea sp. nov. from Upper Leintwardine Formation, Ludfordian, upper Ludlow, Silurian; Delbury Quarry, Shropshire, UK. A. Holotype, NMW 2019.21G.2.3, ramose colony form (A1), thick mesozooecia walls connected by diaphragms, continue cystose shape of earlier growth (A2). B. NMW 2019.21G.2.4, autozooecia rounded in cross-section, some irregularly shaped.

Fig. 2. Trepostome bryozoan Homotrypa cochlea sp. nov. from Upper Leintwardine Formation; Ludfordian, upper Ludlow, Silurian; Delbury Quarry, Shropshire, UK. A. NMW 2019.21G.7.1, bryozoan encrusting the surface of a gastropod shell. B. NMW 2019.21G.33, bryozoan encrusting the surface of a gastropod shell but not growing across aperture.