904
Journal of Paleontology 91(5):902–910
Remarks.—O’Brien and Caron (2012) discussed the possibility that Dinomischus could be closely related to Siphusauctum, but concluded that finding homologous characters between the two genera was difficult, given the limited availability of Dinomischus specimens. We follow this conclusion and do not include Siphusauctum in the family Dinomischidae (see Conway Morris, 1977 for more discussion of this taxon).
Genus Siphusauctum O’Brien and Caron, 2012
Type species.—Siphusauctum gregarium O’Brien and Caron, 2012.
Diagnosis.—Soft-bodied, gregarious, stalked, upright metazoan, divided into three distinct parts from top to bottom: a prominent, chalice-shaped calyx; this is attached to a narrow stem; the stem terminates in a small, bulbous or flat holdfast rarely wider than the stem. Maximum dimensions 223mm high and 48mm wide (at calyx). Calyx box-shaped to oblate, approximately circular in cross-section with a rigid conical lower quarter attached to stem. Maximal width at mid-height of narrow ridge, width tapering or constant towards top. Calyx covered with thin external sheath representing margins of possible filtration chamber. External sheath smooth and unbroken, with exception of small openings at base of calyx; an opening for anus at top,with indistinct openings around anus, and possible openings on protrusions half way up calyx. Internal structures include central and prominent sac-like gut enclosed by tube representing margins of body cavity. Gut is differentiated into three main zones: ovoid lower tract near base of calyx; this grades into bulbous mid-gut; then tapers into straight and thin upper intestine projecting upwards to central terminal anus. Gut enclosed within body cavity surrounded by
transverse comb-like grooves (>30? pairs per segment) occupies most of width and height of
segments.Grooves taper to thin point towards body cavity and connect to larger groove extending from top of calyx to level of lower gut along outer edge of segments. Stem flexible but generally straight; of uniform width, and divi- ded into inner and outer layer. Length (sag.) of stem one to three times length of calyx (modified from O’Brien and Caron, 2012, based on new information provided by S. lloydguntheri n. sp.).
Occurrence.—Tulip beds locality (see O’Brien and Caron, 2012), Campsite Cliff Shale Member, Burgess Shale Formation onMount Stephen, British Columbia, Canada, middle Cambrian, Series 3, Stage 5, lowermost Bathyuriscus Biozone; and Langston Formation, Spence Shale Member, Antimony Canyon area, west flank of the Wellsville Mountains, Utah, (41.561 N, 112.006 W), middle Cambrian, Series 3, Stage 5, Glossopleura Biozone.
Remarks.—The new species from the Spence Shale possesses the diagnostic characteristics of the genus, including directly comparable stem and holdfast structures, the placement of the digestive tract in the lower part of the calyx, the presence of sheath protrusions in the outer wall of the calyx, the placement of the anus, and the shape of the upper part of the calyx.
Siphusauctum lloydguntheri new species Figures 2.1, 2.2, 3.1–3.4, 4.1–4.4
2012 Siphusauctum sp.; O’Brien and Caron, p. 6.
Holotype.—KUMIP 135150, one laterally compressed speci- men, preserved as both part and counterpart.
Diagnosis.—Soft-bodied stalked metazoan with chalice-like body, obovate calyx with thickened external sheath, pair of small lateral protrusions half way up calyx, narrow internal stem mantled by outer stem, and flattened holdfast growing out of inner stem at base. Stem-length to calyx-length ratio 1.3:1.
Occurrence.—Langston Formation, Spence Shale Member, Antimony Canyon area, west flank of the Wellsville Mountains, Utah, (41.561 N, 112.006 W), middle Cambrian, Series 3, Stage 5, Glossopleura Biozone.
Description.—The specimen is laterally compressed and com- posed of an obovate calyx-like upper part, with maximum length 35.1mm and maximum width 25.9mm, and a slim stem, which is 45.6mm long, has a maximum width of 5.5mm, and ends in a flattened basal holdfast that is 5.7mm wide and 0.9mm high. The stem is composed of an inner and outer stem
hexaradial filter-feeding segments arranged longitudinally and filling most of calyx. Outer surface of each segment consists of fine, diagonally oriented, parallel striae. Internally, pair of thick
(Fig. 4.1–4.4). The slender inner stem terminates in the holdfast and extends upward at least into the base of the calyx, perhaps slightly higher, is 2.4mmwide just above the holdfast, reaches a minimum width of 1.5mm near the center of the stem, and has width of 1.7mm at the base of the calyx. The outer stem is preserved from the base of the calyx to ~ 16.8mm above the holdfast. It appears to have continued past the point of preservation and likely extended all the way to the holdfast. Where the outer stem is preserved, it has a width of ~5.5mmand appears to have mantled the inner stem. The outer stem has wrinkles or grooves near the base of the calyx, but is smooth from 10mm beneath the calyx to the limits of preservation. The inner and outer stem were likely made of different tissues, because the inner stem has a greater preservation potential than the outer stem. The holdfast appears to be a direct continuation of the inner stem, and was likely made of the same tissue. It is 5.7mm wide and 0.9mm high. The stem smoothly extends into the conical base of the
calyx (Fig. 3.1–3.4) and through the bottom 15.9mm of the calyx, which is approximately half way up the calyx, roughly opposite a pair of small lateral protrusions from the calyx outer wall. The angle between the outer wall of the calyx and the stem is ~ 125˚. Because the outer stem is preserved in the same way as the lower part of the calyx, it suggests that the calyx and the stem may have formed a continuous functional unit. There is a direct connection between the carbonaceous part of the stem that is preserved at the base of the calyx and the ovoid structure above it, which likely represents part of the digestive tract. The obovate calyx seems to have been covered by a thick,
likely compressed, outer sheath. Half way up the outer sheath of the calyx, the specimen has small (0.8mm) protrusions present on both sides of the calyx; each protrusion could be an opening in the calyx, as it is not preserved as prominently as the
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