Ditrupa bartonensis Wrigley, 1951

Buckman, James O., 2020, The tube of Ditrupa bartonensis (Annelida, Serpulidae), from the Eocene of southern England: Observations on microstructure and its significance, Palaeontologia Electronica (a 37) 23 (2), pp. 1-13 : 2-4

Ditrupa bartonensis Wrigley, 1951

Figures 2-7

Description. Simple tusk-like curved tubes, expanding towards aperture, typically 1-2 mm in diameter, up to 15 mm in length. A notable swelling of the tube is commonly developed at the apertural end. Two distinct layers are clearly observed ( Figure 2 ). The tube has a chevron-shaped growth lamellae passing through both the major layers ( Figure 2 ). In the inner layer the chevron is vertical to ‘C’-shaped, whereas in the outer layer it continues at an angle of approximately 10˚ from the interface between the two layers. The outer layer comprises regularly ridged prismatic crystals (RRP) ( Figures 3-5 ), arranged approximately perpendicular to growth lines. Crystals have a typical width of 1.54 to 5.88 µm, by up to 300 µm in length (the thickness of the layer), with a distinct parallel ridge and groove texture on the outer surface of the crystals. Ridges and grooves in the order of 23 to 155 nm wide. Ridge and grooves on adjacent crystal faces (within a single crystal) are arranged in a V-shaped pattern (in which the ‘V’ points towards the interior of the tube). Organic matter rarely observed between crystals ( Figure 3B, D ). Cross-sections of the crystals form a range of triangular, wedge-shaped, polygonal to more irregular shapes ( Figure 4 ), with margins marked by peglike facets ( Figure 4 ), that mark the position of surface ridges and grooves. Pegs range from tightly to loosely interlocking ( Figure 4F ). One cross-section illustrates internal structure, comprising 500-700 nm euhedral, zoned crystals ( Figure 6A, B ). Transverse breaks appear smooth, though closer examination shows that they often display irregular stepped conchoidal fractures, and that these surfaces have a rough undulating nature, with structure in the sub-micron range ( Figure 5 ). Other crystals broken along their length have an apparent ‘laminated’ structure approximately perpendicular to the crystal long axis ( Figures 3 , 5 ), although some breaks illustrate that these are divided into micron-sized packages that are strongly inclined to the tube surface ( Figures 3B , 5D, E ).

The interface between the outer and inner layers is sharply defined ( Figures 2 , 6C, D ), with the regular ridged prismatic (RRP) crystals each initiating from a single point locus ( Figure 6D ). The inner layer comprises long thin acicular crystals, with polygonal, occasionally observed as hexagonal, cross-section, typically 94 to 393 nm by ~1.5 µm to 11.5 µm. Along the inner tube surface crystals are roughly parallel to the circumference of the lumen ( Figure 7A ), perpendicular to the tube long axis, and appear to occur in loose bundles, angled at up to 10˚ to the line of the tube circumference ( Figure 7B, C ). In addition, transverse sections across tubes illustrate similar groupings and orientations ( Figure 7D, E ). Both the latter have a “woven” appearance ( Figures 7 A-D, 8). In the longitudinal section, this layer has a ‘granular’ appearance ( Figures 7 F-H, 8). The outer layer is typically four or five times as thick as the inner layer.

Composition. Powder XRD analysis indicates the occurrence of calcite, with a d 104 spacing of between 3.02-3.04 ( Figure 9 ), which equates to approximately 4-10 mol% MgCO 3 (see Goldsmith and Graf 1958, table 5). EDX analysis of selected areas (approximately 50 µm 2) indicates that the inner layer has a higher Mg content (Ca:Mg ratio ~15:1 to 19:1) compared to the RRP layer (21:1 to 30:1).

Figures

FIGURE 2. Overview, longitudinal cross-sections of Ditrupa bartonensis. A)- B) Optical images. C)-D) Scanning electron microscope, backscattered electron (BSE) images. Illustrating the two layers (1) –inner layer, and (2) –outer layer, and the overall chevron lamella character of the tube. White and black arrows indicate the position of growth lamellae within the outer (2) and inner (1) layers, respectively.

FIGURE 3. Scanning electron images of the upper structural layer from Ditrupa bartonensis. A)-D) Longitudinal views of ridged crystals. Arrows indicate locations where cleavage planes are observed (approximately perpendicular to crystal long axis) – interpreted as growth lines in Vinn et al. (2008c). Areas marked with asterisk (*) are transverse cross-sections with conchoidal fracture. In (B) area within {} is possible remnants of organic framework / membrane, expanded in (D). 1-4 in (B) highly inclined fracture packets (also see Figure 5).

FIGURE 4. Scanning electron images of the upper structural layer from Ditrupa bartonensis. A)-F) Showing crosssections through ridged crystals. Note that crystals commonly have triangular or wedge-shaped cross-sections, although many other highly variable shapes also occur. Also ridges range from tightly to loosely interlocking. Transverse sections appear smooth (but see Figure 5), with occasional conchoidal fractures (*).

FIGURE 5. Scanning electron images of the upper structural layer from Ditrupa bartonensis. A)-C) Showing crosssections through ridged crystals, illustrating the presence of curved (conchoidal) fractures, which have a rough uneven surface. The latter possibly reflecting the occurrence of protein vesicles. (A) shallowly inclined transverse fractures, (B) fractured surface with a fan-like pattern of conchoidal fractures, passing into rod and groove ornament. D)-F) longitudinal broken sections, illustrating irregular fracture surfaces parallel to exterior ridge and groove ornament. (D) and (E) white arrows indicate ridge and groove structure parallel to near vertical rough platey fracture (black arrow); also see Figure 3B. F) Detail of surface of near vertical rough platey fracture surface. Note images taken in low-vacuum mode, on uncoated surfaces.

FIGURE 6. Scanning electron images of Ditrupa bartonensis. A), B) Transverse polished section through a single ridged crystal in the outer layer of D. bartonensis. A) Backscattered electron (BSE) image, B) gaseous secondary electron (GSE) image, of the same area as in (A). Illustrating what appears to be an indication of internal crystal structure. Arrows indicate the edge of the crystal. C), D) Longitudinal sections showing layer 1 (inner layer) and 2 (outer layer). Note in (D) at the interface between the layers that the regular ridged prismatic crystals each initiate from an individual point locus.

FIGURE 7. Details of the microfabric of the inner layer of Ditrupa bartonensis. A)-C) Views of the inner tube surface (arrows indicate tube long axis), with loosely arranged bundles of long slender crystals, oriented perpendicular to tube long axis (with an approximate deviation of 10˚ within the plane of lamellae). D), E) Transverse vertical section (arrow indicates direction to upper surface) with loosely arranged bundles of long slender crystals, oriented parallel to tube circumference (with a maximum deviation of ~15˚ within the plane of lamellae). In (E) prismatic crystals occur associated with granular calcite (*), of possible authigenic origin. F)-H) Longitudinal vertical section (arrow indicates direction to upper surface), with apparent granular fabric.

FIGURE 9. XRD trace of whole powdered tube of Ditrupa bartonensis. Illustrating the occurrence of calcite, with no other carbonate phases.