Burejospermum punctatum, McLoughlin, Bomfleur, Mörs & Reguero, 2016

McLoughlin, Stephen, Bomfleur, Benjamin, Mörs, Thomas & Reguero, Marcelo, 2016, Fossil clitellate annelid cocoons and their microbiological inclusions from the Eocene of Seymour Island, Antarctica, Palaeontologia Electronica 11 (1), pp. 1-27 : 9

Burejospermum punctatum sp. nov.

Figures 4.4-6 ; 7

zoobank.org/ D51D3E3F-C6F1-498C-BBDA-960761C7E2D6

Holotype. NRMS089728 ( Figures 4.4-6 ; 7 ).

Type Locality, Stratum, and Age. Locality IAA 2/ 95; ‘ Natica horizon’ within the Cucullaea I Allomember (Telm 5) of the La Meseta Formation; early Eocene.

Additional Material. NRMS089733 (from the type locality) .

Etymology. After the locally punctate nature of the outer surface of the alytine.

Diagnosis. Reddish-brown cocoons <4 mm long. Hapsine very weakly developed, consisting of irregularly interconnected slender threads that may be depressed locally into the outer layer of the alytine. Alytine composed of numerous densely amalgamated laminae forming a solid wall with irregularly spaced circular pits on its outer surface. Weakly developed, densely granulate girdle positioned slightly to the posterior of cocoon equator.

Description. Cocoon deep reddish-brown in reflected ( Figure 4.4 ) and transmitted light; ellipsoidal ( Figure 7.1 ) with broadly rounded anterior end. Posterior end lacking operculum in all cases ( Figure 4.5 ). Cocoon 3 mm long between anterior terminus and the line of operculum detachment (estimated length ~ 3.5 mm including operculum); diameter 2.4 mm; essentially circular in cross-section except where crushed. Hapsine weakly developed ( Figure 4.6 ) or locally absent ( Figure 7.3 ); where present it is composed of slender threads interconnected to form an irregular reticulum that is commonly depressed into the outer layer of the alytine ( Figure 7.2 ). Hapsine threads <4 μm (typically ~ 2 μm) thick.

Alytine composed of numerous (>25) thin (0.5–1 μm thick) laminae densely amalgamated to form a solid layer ~ 24 μm thick ( Figure 7.6 ). Outer surface of alytine commonly bearing irregularly spaced, circular pits up to 10 μm in diameter and <10 μm deep ( Figure 7.3 ); surface otherwise relatively smooth except where it is densely granulate in a weakly developed 400-μm-wide girdle ( Figure 7.4 ) positioned sightly to the posterior of the equator. Outer alytine surface with minute, short (80 μm long, ~ 20 μm wide), parallel cleft-and-fold structures ( Figure 7.5 ); inner alytine surface smooth.

Remarks. Burejospermum punctatum differs from all other species in the genus by its relatively smooth outer surface, i.e., by having a very weakly developed hapsine that, to varying degrees, is depressed into the outer surface of the alytine. Cocoons of this type are similar in size, shape and colour to B. seymourense , so it is conceivable that they represent variability of cocoon development by a single cocoon maker. However, we did not observe any intermediate forms between these two cocoon types, so we propose that they were produced by different species of clitellate annelids. Only sparse examples of spermatozoa entrapment were identified between the alytine laminae of this species (described below).

Figures

FIGURE 4. Light micrographs of early Eocene clitellate annelid cocoons from Seymour Island. 1–3, Burejospermum seymourense sp. nov. (NRMS089727). 4–6, Burejospermum punctatum sp. nov. (NRMS089728). 7–9, Pegmatothylakos manumii sp. nov. (NRMS089730). 1, 4, 7, Lateral views of cocoons (posterior to left); g = equatorial girdle. 2, 5, 8, Views of posterior end of cocoons showing opening left by detachment of operculum. 3, 6, 9, Enlargements of cocoon wall exterior showing pale felt-like hapsine covering in B. seymourense (3), relatively smooth surface in B. punctatum (6) and reticulate pegmatine of P. manumii (9). Scale bars represent 1 mm in 1, 2, 4, 5, 7, 8; 100 µm in 3, 6, 9.

FIGURE 5. Scanning electron micrographs of Burejospermum seymourense sp. nov. (NRMS089727). 1, Cocoon in lateral view. 2, Details of typical hapsine threads. 3, Detail of dense hapsine threads over girdle region. 4, Elliptical secreted plate-like structures on exterior of hapsine. 5, Broken cocoon wall in cross-section showing solid alytine (lower) and thread-like hapsine (upper). 6, Detail of plate-like secretion on exterior of hapsine. Scale bars represent 1 mm in 1; 100 µm in 2; 50 µm in 3, 4; 25 µm in 5, 6.

FIGURE 6. Computed tomographic images of a Burejospermum seymourense cocoon wall (NRMS089729) generated from Synchrotron-radiation-based X-Ray tomographic microscopy (SRXTM) at the TOMCAT beamline of the Swiss Light Source at the Paul Scherrer Institute, Switzerland. 1, Isosurface rendering of cocoon’s external surface showing mineral grains embedded in hapsine. 2, Isosurface rendering of cocoon’s internal surface showing creasing caused by alytine threads. Arrow indicates the position of a hollow tube within the alytine. 3, 4, Longitudinal orthoslice sections, at different focal planes, of a tube within the alytine showing pseudosegmentation. 5–8, Translucent volume rendering of a tube within the alytine imaged at different focal planes and magnifications to highlight the transverse to chevron-shaped thread arrangement lining the tube wall. 9, Isosurface rendering of hapsine outer surface showing details of the complex arrangement of hirudoin threads. 10, Transverse orthoslice section of the cocoon wall showing the near-solid alytine layer (bottom) and mesh-like hapsine layer (top). 11, Isosurface rendering of interior alytine wall showing an embedded spermatozoal nuclear region. 12, Colour-inverted rendering of Figure 6.11. 13, Volume rendering of the cocoon wall showing numerous mineral (white–yellow) inclusions. 14, Oblique translucent volume rendering of the cocoon wall showing the distribution of mineral grains mainly in the outer hapsine layer. Scale bars represent 100 µm in 1, 2, 10, 13, 14; 50 µm in 9; 25 µm in 3–7; 10 µm in 8; 5 µm in 11, 12.

FIGURE 7. Scanning electron micrographs of Burejospermum punctatum sp. nov. (NRMS089728). 1, Cocoon in lateral view. 2, Details of sparse hapsine threads embedded in alytine surface. 3, Detail of typical cocoon surface showing pitting. 4, Detail of girdle region showing granular microtexture and a single surface pit. 5, Detail of outer alytine surface showing cleft and fold structures (arrowed). 6, Cross-section of cocoon wall showing laminated architecture. Scale bars represent 1 mm in 1; 100 µm in 3, 5; 10 µm in 2, 4, 6.