Hyolitha (Order: Hyolithida, stem group molluscs)
Haplophrentis belongs to a group of enigmatic cone-shaped to tubular fossils called hyoliths that are known only from the Palaeozoic. Their taxonomic position is uncertain, but the Hyolitha have been regarded as a separate phylum, an extinct Class within Mollusca (Malinky and Yochelson, 2007), or as stem-group molluscs.
Haplophrentis – from the Greek haploos, “single,” and phrentikos, “wall,” in reference to the single wall within the shell.
carinatus – from the Latin carinatus, “keel-shaped,” referring to the morphological similarity to the bottom of a boat.
Lectotype – ROM 8463a in the Royal Ontario Museum, Toronto, Canada.
Burgess Shale and vicinity: none
Other deposits: H. reesei Babcock & Robinson, 1988 (type species), from the lower Middle Cambrian Spence Shale and elsewhere in Utah; H.? cf. carinatus from the Middle Cambrian Kaili deposit in China (Chen et al., 2003).
Middle Cambrian, Albertella Zone to Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott, Raymond and Collins Quarries on Fossil Ridge, the Trilobite Beds on Mount Stephen and Stanley Glacier in Kootenay National Park.
Brief history of research:
Matthew described Hyolithes carinatus from the Trilobite Beds in 1899 based on five incomplete specimens. Babcock and Robison (1988) reviewed the original fossils, along with additional specimens collected by the Royal Ontario Museum from various Burgess Shale localities. They concluded that the species carinatus didn’t belong in Hyolithes, and established a new genus, Haplophrentis, to accommodate it.
Like all hyoliths, Haplophrentis had a weakly-mineralized skeleton that grew by accretion, consisting of a conical living shell (conch), capped with a clam-like “lid” (operculum), with two slender, curved and rigid structures known as “helens” protruding from the shell’s opening. The function of these helens is still debated, but one possibility was to allow settlement and stabilization on the sea floor. Haplophrentis had a wiggly gut whose preserved contents are similar to the surrounding mud.
H. carinatus usually grew to around 25 mm in length, although some specimens reached as much as 40 mm; the species is distinguished from H. reesei, its cousin from Utah, by the faint grooves on its upper surface, the more pronounced net-like pattern on its “lid” (operculum), and its wider, more broadly-angled living shell (conch).
Haplophrentis can be distinguished from the similar hyolith genus Hyolithes because it bears a longitudinal wall running down the inner surface of the top of its living-shell.
Haplophrentis is relatively common on Fossil Ridge and in the Walcott Quarry in particular, accounting for 0.35% of the community there (Caron and Jackson, 2008).
Deposit feeder, herbivorous?
Haplophrentis probably moved very little; its helens appear unsuited for use in locomotion (See Butterfield and Nicholas, 1996; Martí Mus and Bergström, 2005; Runnegar et al., 1975). Whilst Haplophrentis feeding mode remains somewhat conjectural, it probably consumed small organic particles from the seafloor. Numerous specimens have been found in aggregates or in the gut of the priapulid worm Ottoia prolifica suggesting Haplophrentis was actively preyed upon and ingested (Conway Morris, 1977; Babcock and Robison, 1988).