Burejospermum seymourense, 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 : 7-9

Burejospermum seymourense sp. nov.

Figures 4.1-3 ; 5 ; 6

zoobank.org/ 9745DE12-F572-4117-9300-56E7EC77A240

Holotype. NRMS089727 ( Figures 4.1-3 ).

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. NRMS089730 – NRMS089732 (from the type locality).

Etymology. After Seymour Island.

Diagnosis. Reddish-brown barrel-shaped cocoons <4 mm long. Hapsine over most of the cocoon wall comprising disorganized threads more densely amalgamated in gently sunken equatorial girdle. Plate-like secretions locally developed on the outer surface of hapsine. Alytine dense, consisting of tightly amalgamated threads, and locally containing blind tubular cavities parallel to cocoon surface.

Description. Cocoon reddish-brown in reflected and transmitted light ( Figure 4.1 ), ellipsoidal, 2.4 mm long from the anterior terminus to the level of the operculum detachment (estimated total length ~ 3 mm); diameter 2 mm; essentially circular in cross-section except where crushed by burial compaction. Anterior end broadly rounded. Posterior operculum absent from studied specimens ( Figure 4.2 ). Hapsine ~ 5–75 μm thick ( Figures 5.5 , 6.10 ) consisting of moderately dense threads interconnected and interwoven to form a thin, brownish, felt-like covering on the cocoon ( Figures 4.3 , 5.2 , 6.1, 6.9 ; Supplementary Animation 1). Threads typically ~ 3 μm thick, locally reaching 10 μm thick. Hapsine of consistent density throughout except for a 350-μm-wide, slightly depressed, pale (whitish to yellowish in reflected light) girdle ( Figures 4.1 , 5.1 ) bearing more densely spaced and amalgamated threads ( Figure 5.3 ). Isolated, flat, elliptical patches or plates of secreted material, 40–60 μm long, 32–45 μm wide and <3 μm thick distributed sparsely on outer hapsine surface ( Figure 5.4, 5.6 ).

Alytine composed of secreted threads amalgamated into numerous thin sheets to form a continuous densely laminated layer ~ 22 μm thick ( Figures 5.5 , 6.10 ). Inner surface of alytine is smooth or undulated ( Figure 6.2 ). Alytine incorporating sparse tubular cavities> 400 μm long and ~ 25 μm wide, parallel to cocoon surface, mostly in equatorial regions. Tubes lined by threads deflected towards one end of the cavity forming a pseudo-braided or chevron pattern ( Figure 6.3-8 ; Supplementary Animations 2-4).

Remarks. Burejospermum seymourense differs from the two other taxa in the Seymour Island assemblage by its combination of a dense alytine layer and a robust hapsine that stands proud of the alytine surface. It differs from previously described Burejospermum species by its small dimensions, the distinct pale equatorial girdle of dense hapsine threads and the presence of sparse elliptical plate-like secretions on the outer surface of the hapsine. Burejospermum seymourense is the most common cocoon form in the Seymour Island assemblage. All specimens represent hatched cocoons (lacking an operculum).

The tubular cavities oriented parallel to the surface within the alytine layer ( Figure 6.2-8 ; Supplementary Animations 2–4) are of uncertain origin. Manum et al. (1991) devoted considerable attention to equivalent structures in other examples of Burejospermum and, although they did not identify any analogue among modern clitellate annelid cocoons, they hypothesized that such structures may have been produced by individual setae on a clitellar segment that became embedded in the alytine during the process of hirudoin secretion. They proposed that the setae were subsequently extracted from the tubes as the animal's clitellar region contracted before egg-laying. The chevronshaped threads lining the tubes in the Seymour Island specimens, and the flaring of the tube at one end (Supplementary Animation 4), are consistent with the hypothesis of distortion caused by extraction of setae before the cocoon wall had hardened. However, we note that this hypothesis is tempered by the observation that modern true leeches and branchiobdellids (the two groups most commonly invoked as the producers of hapsinerich fossil clitellate cocoons: Manum et al., 1991; Bomfleur et al., 2015) lack setae ( Smith, 2001).

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.