Demospongea (Order: Monaxonida)
Leptomitus is considered a primitive demosponge (Rigby, 1986). Demosponges, the same group that are harvested as bath sponges, represent the largest class of sponges today.
Leptomitus – from the Greek lept, “slender,” and mitos, “thread.” This name refers to the overall shape of the sponge.
lineatus – from the Latin lineatus, “streaked.” This refers to the wrinkle appearance of this sponge.
Lectotype –USNM66448 (L. lineatus) in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Holotype –ROM53558 (L. undulatus) in the Royal Ontario Museum, Toronto, Canada.
Burgess Shale and vicinity: L. undulatus Rigby and Collins 2004 from the Walcott Quarry.
Other deposits: L. zitteli Walcott, 1886 from the Middle Cambrian Parker Slate in Vermont; L. metta Rigby, 1983 from the Middle Cambrian Marjum Formation of Utah; L. conicus García-Bellido et al., 2007 from the Middle Cambrian Murero Formation of Spain; L. teretiusculus Chen, Hou and Lu, 1989 from the Lower Cambrian Chengjiang biota in China (see Rigby and Hou, 1995); unidentified species from the Lower Cambrian Niutitang Formation in China (Yang et al., 2003).
Middle Cambrian, Glossopleura Zone to Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott Quarry on Fossil Ridge. The Tulip Beds (S7) and the Collins Quarry on Mount Stephen.
Brief history of research:
Leptomitus was originally described by Charles Walcott (1920) as a new genus “Tuponia” along with several species (T. lineatea, T. flexilis, T. flexilis var. intermedia). This genus was later synonymized by Resser and Howell (1938) with Leptomitus, a genus named by Walcott in 1886. Ribgy (1986) redescribed the Burgess Shale sponges including Leptomitus and considered L. flexilis to be a junior synonym of L. lineatus. Rigby and Collins (2004) added a second species L. undulatus based on new material collected by the Royal Ontario Museum.
L. lineatus is an elongate tubular sponge with a double-layered skeleton. The outer layer is composed of long monoaxial spicules (simple spicules with pointed ends) arranged vertically along the length of the sponge. The varying thicknesses of these elongate spicules give the sponge a distinctive wrinkly appearance in the fossils. The inner layer is composed of tiny horizontal spicules that form an unclumped thatch; these tufts can be seen at the oscular margin (opening at the top of the sponge). The base of the sponge is rounded in shape and would have had a small holdfast structure. L. undulatus has the same wall structure as L. lineatus but has a rounder goblet shaped skeleton.
L. lineatus is relatively common in the Walcott Quarry and represents 0.26% of the community (Caron and Jackson, 2008). L. undulatus is known from a single specimen.
Leptomitus would have lived attached to the sea floor. Particles of organic matter were extracted from the water as they passed through canals in the sponge’s wall.