Unranked clade Coeloscleritophora (Order: Chancelloriida)
Two main hypotheses exist for the affinity of the chancelloriids: they may form a group with Halkieria and relatives, nested close to the base of the bilaterian tree (Bengtson, 2005), or they may simply represent a sponge-grade organism with an unusual mode of spicule formation (Sperling et al., 2007).
Chancelloria – from the nearby Chancellor Peak (3,280 m), which was named to honour the Ontario Chancellor Sir John Boyd for his role in resolving an 1886 dispute between the Canadian Pacific Railway and the Canadian Government.
eros – unspecified; either from the Latin erosus, “gnawed off” or “consumed,” or the Greek erotikos, “pertaining to love.”
Lectotype –USNM66524 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA
Burgess Shale and vicinity: none.
Other deposits: Walcott (1920) described C. drusilla, C. libo and C. yorkensis from Middle Cambrian deposits in the Conasauga shales of Georgia, the Conasauga shales of Alabama, and the York formation of Pennsylvania, respectively. Other workers have described C. maroccana Szduy, 1969 from the Lower Cambrian Campo Pisano Formation, Sardinia, Italy; C. pentacta Rigby, 1978, from the Middle Cambrian Wheeler Shale, Utah, USA (Rigby, 1978); C. sp., from the Cambrian Bright Angel Shale of Arizona (Elliott and Martin, 1987); C. cf. eros from the Early Cambrian (Branchian) Sekwi Formation, Mackenzie Mountains, Northwest Territories, Canada (Randell et al., 2005); C. sp., from the Elvinia Zone (Upper Cambrian) Collier Shale, Ouachita Mountains, west-central Arkansas (Hohensee and Stitt, 1989); C. sp. from King George Island, Antarctica (Wrona, 2004).
Cambrian Stage 2 (basal Botomian, upper Lower Cambrian) to uppermost Middle Cambrian, Bolaspidella Zone (approximately 525-505 million years ago).
Burgess Shale and vicinity: Chancellorids are known from all Burgess Shale localities, in particular from the Walcott, Raymond and Collins Quarries on Fossil Ridge, as well as on Mount Stephen (Trilobite Beds, and Tulip Beds (S7)), Monarch Cirque and other smaller localities. Work is currently in progress to determine how many of these Chancelloria specimens in fact represent other genera, in particular Allonnia and Archiasterella (see below).
Other deposits: C. eros is globally distributed, and has been reported from the Middle Cambrian Wheeler Shale (and Marjum Formation), Utah, USA (Janussen et al., 2002); Lower Cambrian Comley Limestone, England (Reid, 1959); upper Lower to lower Middle Cambrian La Laja Formation, Argentina (Beresi and Rigby, 1994); Andrarum Limestone and the upper alum shale (Middle Cambrian) of Bornholm, Denmark (Berg-Madsen, 1985); Lower Cambrian of Nevada and California (Mason, 1938); the Lower Cambrian of Cape Breton Island (Landing, 1991); the Early Cambrian Todd River Dolomite, Amadeus Basin, central Australia (Laurie, 1986), the Çal Tepe Formation, Taurus Mountains, Turkey (Sarmiento et al., 2001); the Lower Cambrian Forteau Formation of western Newfoundland (Skovsted and Peel, 2007); the Lower Cambrian Hyolithes Limestone of Nuneaton, England (Brasier, 1984).
Brief history of research:
Walcott (1920) considered Chancelloria to represent a sponge, a position that was followed by the majority of subsequent workers. However its mode of sclerite formation is reputedly unlike anything known in modern sponges: the hollow sclerites are composed of multiple elements that are joined together (Bengtson and Missarzhevsky, 1981), with a structure similar to the sclerites of Halkieria (Porter, 2008). This detail convinced most that the chancelloriids could not belong to the sponges (Goryanski, 1973; Bengtson and Hou, 2001). However, some disagree, pointing out that the organic microstructure does have some similarity to the fibres of horny sponges (Butterfield and Nicholas, 1996), suggesting a position in the sponge total group (see also Sperling et al., 2007). The specimens from the Burgess Shale are currently undergoing a detailed re-study and some specimens will doubtlessly be reclassified into other chancelloriid genera (Bengtson and Collins, 2009).
Chancelloria resembled a cylindrical cactus up to 20 centimetres tall. An assortment of star-shaped spines constitutes a loose and unconnected net arranged in various fashions. These spines formed a tight ring around the top of the organism, which seems to have surrounded a pore. Water would probably have passed through this opening and any organic particles would have been filtered out for food.
The spicules of Chancelloria, which varied from millimetric to about a centimetre in diameter, were composed of hollow rays that were stuck together at a central point to form a three-dimensional structure shaped like an umbrella. A central ray pointed out from the organism, and other rays radiating outwards at an angle closer to the surface of the organism, presumably to aid in defence. The nature of the rays distinguishes between the chancelloriid genera and species; C. eros bears four to seven rays per spicule. The closely related Allonnia is differentiated from Chancelloria by its more globular shape and the details of its sclerite construction, which consists of three main rays. A third genus, Archiasterella, is also represented in the Burgess Shale and differs from the two other genera in sclerite morphology and numbers of rays.
Chancelloria accounts for under 0.5% of the Burgess Shale community (Caron and Jackson, 2008), including specimens that may belong to Allonnia or Archiasterella.
Chancelloria primarily attached itself to organisms, commonly sponges or other chancelloriids, but also on occasion to shell fragments that may have been partially buried in the sea floor. It remained in this anchored position and fed by extracting particles from seawater, which it sucked in and squeezed out through an opening in the top of its body. It spines probably served as a defence against predators.