Cambrorhytium fragilis (USNM 57626) – Holotype, part and counterpart. Complete tube in close proximity to an isolated claw of Anomalocaris. Specimen length = 25 mm. Specimen dry – direct light (left), dry – polarized light (middle, left), wet – direct light (top right) and wet – polarized light (bottom right). Trilobite Beds on Mount Stephen.
© Smithsonian Institution – National Museum of Natural History. Photo: Jean-Bernard Caron
On account of its similarity to the genus Byronia and its polyp-like tentacles, Cambrorhytium is thought to be related to a group of cnidarian known as the conulariids. However, no distinctive features that would categorically place it in the Cnidaria are preserved.
Cambrorhytium – from the name of the Cambrian period, and the Latin rhytium, “drinking horn.”
major – from the Latin major, “large.”
Burgess Shale and vicinity: C. fragilis (Walcott, 1911) Conway Morris and Robinson, 1988 from the Mount Stephen Trilobite Beds and the Walcott Quarry.
Other deposits: C. elongatum (Steiner et al., 2005) from Lower Cambrian deposits in Chengjiang County; Haikou and Jingmacun of Kunming district; and the Niutitang Formation of Zhongnan, Zunyi County, Guizhou, China; C. fragilis is also known from the Lower Cambrian Latham Shale (Waggoner and Hagadorn, 2005); C. major from the Marjum Formation (uppermost Middle Cambrian, Utah, Conway Morris and Robison, 1998).
Burgess Shale and vicinity: The Trilobite Beds on Mount Stephen and the Walcott and Raymond Quarries on Fossil Ridge.
Other deposits: C. fragilis is also known from the Lower Cambrian Latham Shale (Waggoner and Hagadorn, 2005); C. major from the Marjum Formation (uppermost Middle Cambrian, Utah, Conway Morris and Robison, 1998).
The genus Cambrorhytium was erected in 1988 by Conway Morris and Robinson to accommodate two problematic fossil species now known as Cambrorhytium major and C. fragilis. Walcott (1908) figured a specimen of C. major from the Mount Stephen Trilobite Bedsin the hyolith genus Orthotheca on account of its triangular shape. Thanks to new material discovered in the Walcott Quarry, Walcott (1911) reconsidered this Mount Stephen specimen to belong to a new genus Selkirkia (S. major) thought to be the tube of a polychaete worm at the time (Walcott, 1911). The original Mount Stephen Selkirkia major specimen first figured by Walcott in his 1908 publication lacked soft-parts, but further specimens of this species from the Walcott Quarry clearly showed an affinity with the priapulid worms (Conway Morris, 1977). On re-examination, the holotype of Selkirkia major, and some specimens of the two other species of Selkirkia erected by Walcott (1911) were demonstrably not priapulids. The non-priapulid Selkirkia species were re-homed in the new genus Cambrorhytium.
The chitinous and probably non-mineralized conical and slightly curved tube of Cambrorhytium major gradually expands from a point to a smooth circular opening that lacked a covering or closure. Five fleshy tentacle-like structures emerged from this opening. The organism grew periodically, adding to the cone at its open end, thus leaving a series of “growth rings.” Although it grew to over six cm in length, the average size is 4 cm and most specimens are at the smaller end of the size range. The earliest stages of growth are less regular than the later, so the base of the cone is usually a little wiggly.
C. major is typically larger and more slender than C. fragilis, whose growth-rings are less pronounced. C. fragilis is usually more reflective than C. major, which may represent a difference in original composition or simply a preservational bias.
Cambrorhytium major is rare, known from only a few dozen specimens, but C. fragilis is more common. In the Walcott Quarry, both species represent around 0.35% of the total counts of fossils (Caron and Jackson, 2008).
The organism appears to have reproduced sexually rather than by budding. Although Cambrorhytium was bottom-dwelling, there is no indication of any anchoring structure, suggesting that it might have sat partially within the sediment. Cambrorhytium is thought to have been either a suspension feeder or a carnivore on the basis of its probable tentacles and presumed cnidarian affinity.
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