Bathyuriscus rotundatus (USNM 116232b) – Plesiotype. Nearly complete individual with right free cheek in place. Specimen length = 14 mm. Specimen dry – direct light. Trilobite Beds on Mount Stephen.
© Smithsonian Institution – National Museum of Natural History. Photo: Jean-Bernard Caron
Trilobites are, by a large margin, the most diverse of extinct arthropod classes, encompassing more than 2000 species, although the biomineralization of their cuticle is likely largely responsible for their much better-known diversity. A recent probabilistic analysis of trilobite relationships suggests that many of the diagnostic traits used for the taxonomy of the main groups remain highly variable across lineages (Paterson, Edgecombe & Lee, 2019), which explains long-standing difficulties in resolving the trilobite evolutionary tree. The kinship of trilobites relative to other arthropod groups has been a historical dilemma also (Cotton & Braddy, 2004; Aria, Caron & Gaines, 2015; Paterson, 2020). Antennules and some other characters, on the one hand, would ally them with mandibulate arthropods (the Gnathomorpha or Antennulata hypothesis), while others would place them within the chelicerate lineage (the Arachnomorpha hypothesis). Recent phylogenetic results have brought increased support to Arachnomorpha, but there is still no consensus on the question (Aria, 2022).
Bathyuriscus – a variation of the earlier trilobite genus name Bathyurus, originally based on the Greek bathys, “deep,” and the Greek oura, “tail,” thus, a trilobite with a deep tail.
rotundatus – from the Latin rotundus, “round,” presumably alluding to the rounded outline of the dorsal shield.
Burgess Shale and vicinity: Bathyuriscus adaeus Walcott, 1916, from several localities higher in the Bathyuriscus-Elrathina Zone on Mount Stephen, Mount Odaray, and Park Mountain.
Other deposits: other species of Bathyuriscus have been described from numerous localities elsewhere in the Cambrian of North America.
The Trilobite Beds and other localities on Mount Stephen. Fossil Ridge in sections stratigraphically below the Walcott Quarry.
Bathyuriscus rotundatus was first described in the same 1887 publication as several other important Mount Stephen trilobites. Carl Rominger initially used the name Embolimus rotundata for partial specimens of this trilobite, and named a second similar species in his collection Embolimus spinosa (now known as Zacanthoides romingeri). In 1908, Walcott revised Rominger’s original species name to yield the combination Bathyuriscus rotundatus, still in use today (Walcott, 1908). Along with the co-occurring Elrathina cordillerae, B. rotundatus is a signature fossil for the Middle Cambrian Bathyuriscus-Elrathina Zone in the southern Canadian Rockies.
Hard parts: adult dorsal exoskeletons may be up to 5 cm long and are narrowly oval in outline, with a semicircular cephalon, a thorax of nine segments ending in blade-like tips with short spines, and a semicircular pygidium without spines. The long glabella reaches almost to the anterior cephalic border; the posterior portion is narrow and parallel-sided, while the anterior third expands rapidly forward. There are four pairs of lateral glabellar furrows, with the two front pairs angled forward and the posterior pair directed obliquely back. The eyes are relatively long and lie close to the glabella. Broad free cheeks are extended back into short genal spines. The pygidium is slightly smaller than the cephalon, with a well-defined narrow axial lobe of five rings and a terminal piece; four pairs of pygidial ribs are usually visible. The exoskeleton is mostly smooth externally, but very well-preserved specimens may show faint anastomosing ridges on the free cheeks. Unmineralized anatomy: not known.
Extremely common in the Mount Stephen Trilobite Beds, where it rivals Ogygopsis klotzi in abundance.
Bathyuriscus rotundatus was a mobile epibenthic trilobite. Because we have no direct evidence of limb structure, its feeding habits are uncertain. It may have been a deposit feeder and opportunistic scavenger. Like Ogygopsis, Bathyuriscus may occur as fully intact individuals (probably carcasses), with the free cheeks missing, inverted, or rotated (presumed moults), and as scattered pieces. Some show evidence of healed injuries that may be predation scars (Rudkin, 2009).