Tests preserved in calcite (Figs. 2, 3)
Eight tests, NNM RGM 617 801-617 808, that vary from specimens showing little more than surface abrasion to those that have been extensively altered by bio- and physical erosion. The illustrations are arranged to show this taphonomic gradient.
NNM RGM 617 801 is the most complete test (Fig. 2a, b). Primary tuberculation is moderately well preserved on the oral surface, with some secondary tubercles apparent in the depressed area surrounding the peristome. The apical surface is smooth and worn free of tubercles. Some plate sutures have opened up and there are cracks that cross plates (Fig. 2a, posterior and left, respectively). There are no encrusting organisms and irregular holes are probably produced by physical impacts (Fig. 2a, close to apex). There are numerous small, circular pits close to the apex of the sort referred to as putative Oichnus isp. by Donovan et al. (2010b, p. 52).
Other specimens have either had part of the test removed by physical erosion or breakage, more or less assisted by weakening produced by borings, particularly Entobia isp. NNM RGM 617 802 has a complete oral surface which has lost surface detail (Fig. 2d). Apically the calcite of the test has been breached and eroded away, particularly in the left anterior region where the Chalk infill is exposed down to the ambitus. Some test is preserved anteriorly and on the right lateral side of the apical surface (Fig. 2c); the junction with the Chalk infill is chamferred, suggesting physical abrasion. Puncture marks in, particularly, the Chalk are irregular and no undoubted bioerosional structures are identifiable. In this and other specimens described below, remnants of the test are essentially continuous between oral and apical surfaces.
NNM RGM 617 805 has some similarities to the previous specimen, the test being worn thin on the right side of the apical surface, but exposed on the left. However, in this Chalk infill and, to a lesser extent, the test of the oral surface the specimen is infested by Entobia isp. Posteriorly, the Chalk exposes numerous small Caulostrepsis isp. borings.
NNM RGM 617 806 repeats this pattern of exposure of the Chalk infill with a more completely preserved oral surface and Chalk infill large exposed apically. The test and Chalk are intensely bored on the left apical surface by Entobia isp., with some apertures puncturing the test of the oral surface. The internal chambers of Entobia isp. are well exposed in the Chalk. Slender, gently curved borings on this surface are juvenile parts of the Entobia network. More Entobia chambers, less well preserved, are apparent in the right lateral position. Modern serpulid worm tubes encrust the oral surface of NNM RGM 617 806 adjacent to the peristome and are best preserved in the depression surrounding this region. The anterior part of the oral surface has been depressed mechanically, leading to preservation of both primary and secondary tubercles adjacent to the serpulids. Chalk exposed anterior to the peristome includes small, slot-like openings of the borings of Caulostrepsis isp.; similar borings are found above the Entobia on the right posterior apical surface. Below this same Entobia infestation are Caulostrepsis isp. close to C. taeniola Clarke 1908, as illustrated by Bromley and d’Alessandro (1983, pl. 21, fig. 2).
A small specimen, NNM RGM 617 808, preserves a disharmonious combination of features. The oral surface is near-complete and preserves most of the primary tubercles, with secondary tubercles apparent adjacent to the sunken peristome. Yet most of the supra-ambital test is lost and the highest remnant on the apical surface only extends for 14.4 mm above the ambitus. Nevertheless, the specimen is 40.1 mm high because much of the Chalk infill is preserved. Small borings are not uncommon, but only Caulostrepsis isp. is identifiable.
Some specimens are particularly heavily infested by Entobia. NNM RGM 617 803 (Fig. 3a, b) preserves some test on the right side of the specimen as illustrated, although its true orientation is indeterminate (primary tubercles are rarely preserved at the front of the oral surface as oriented, which may indicate the position of the sunken peristome). The rest of the specimen is a Chalk internal mould with a dense Entobia isp. infestation. The residual test is weakly bored, mostly on the oral surface. NNM RGM 617 807 retains more of the test, including most of the oral surface, but the apex is planed off. Where the Chalk infill is exposed it is densely infested by Entobia isp. and Caulostrepsis isp. Caulostrepsis occurs posteriorly, on the planed apical surface and in the left lateral test of the apical surface; Entobia infestation is dense on the right side of the apical surface, both test and infill, and the test of the oral surface is perforated by numerous apertures, mainly on the same side. Other perforations of the test appear to be common mechanical damage, in part controlled by plate sutures and the distribution of borings. Primary and secondary tubercles are preserved in the depression adjacent to the peristome.
The final recognisable state of test breakage is in specimens where it is reduced to the oral surface and little else. NNM RGM 617 804 has an entire oral surface, worn smooth, although primary and secondary tubercles are still apparent around the depressed peristome (Fig. 3d). The apical surface is largely planed away (Fig. 3c); the greatest preserved height is 29.0 mm, although it may originally have been 60–70 mm high. The Chalk infill has some rare Caulostrepsis isp. apertures and other indeterminate borings or mechanical damage. The prominent, circular depression is the distal termination of a Gastrochaenolites isp. that must have penetrated the infill when it was more complete, but has otherwise been lost with the planing off of the apex. No borings penetrate the test.
Tests preserved in flint (Fig. 4)
Four tests are preserved in flint, NNM RGM 617 809-617 812. Although all show at least some physical damage, none preserves definite evidence of modern biological interactions such as encrustation or boring. NNM RGM 617 809 is the most complete specimen preserved in flint, a steinkern with a large conchoidal fracture anteriorly (best seen in oral view; Fig. 4b) and some of the calcite test remaining although eroded through in places. More or less elongate structures in the calcite test, but not penetrating the steinkern, are concentrated on the apical surface (Fig. 4a) and are most probably the result of physical abrasion, not showing the geometry of any bored tests preserved in Chalk. A U-shaped groove just posterior to the peristome (Fig. 4b) is infilled with limonite, as is the periproct, and is possibly part of an ancient, albeit indeterminate boring, although its origin is equivocal. NNM RGM 617 812 preserves a less complete test in a flint nodule. The steinkern lacks part of the apical surface, the calcite test is less well preserved than the figured specimen and there is no evidence for biological infestation.
Two specimens are very incomplete. NNM RGM 617 810 (Fig. 4c) is an external mould of parts of four plate columns in a flint cobble. As illustrated, the apex is towards the top of the page. The two centre plate columns are part of an interambulacrum, flanked on either side by single plate columns derived from contiguous ambulacra and each preserving single central pits that are poorly preserved pore pairs. Some small and discontinuous calcite fragments shaped by dissolution adhere to the surface of some plates. There is no evidence of boring and encrustation. NNM RGM 617 811 is a triangular, reworked flint nodule preserving a test fragment seen in internal view, retaining parts of two interambulacra and two ambulacra, and the apical system. This test has not been punctured by any borings; the external surface is not seen.