© ROYAL ONTARIO MUSEUM. PHOTOS: JEAN-BERNARD CARON
Trilobites are extinct euarthropods, probably stem lineage representatives of the Mandibulata, which includes crustaceans, myriapods, and hexapods (Scholtz and Edgecombe, 2006).
Pagetia – unspecified, likely from Paget Peak (2565 m) in Yoho National Park, named for the Very Reverend Dean Paget, founding member of the Alpine Club of Canada, who, in 1904, made the first recorded ascent.
bootes – unspecified, probably from the Greek Boötes meaning herdsman or ploughman; name of a northern constellation.
Burgess Shale and vicinity: P. walcotti Rasetti, 1966.
Other deposits: many species worldwide, in Lower and Middle Cambrian rocks.
The Walcott and Raymond Quarries on Fossil Ridge. The Trilobite Beds on Mount Stephen.
Walcott (1916) published only a very brief description when he first named and illustrated this species. A full account finally appeared in Rasetti (1966), along with that of a new Burgess Shale species, P. walcotti.
Hard parts: adult dorsal exoskeletons reach about 10 mm in length (including the pygidial spine). The cephalon is semicircular, with a narrow flattened rim crossed by radiating furrows around the front margin. The glabella is narrow and anteriorly pointed with weak lateral constrictions; a delicate spine (usually broken off and not seen) extends up and back from the occipital ring. Tiny eyes are located well out on short, narrow cheeks bounded by proparian facial sutures. There are two thoracic segments. The pygidium is about the same size and outline shape as the cephalon, with a narrow axis of five rings and a terminal piece bearing a slender rearward projecting spine (often broken off). Faint pleural furrows may be visible on the pygidium.
P. walcotti is very similar, but the dorsal exoskeleton bears fine granules.
Unmineralized anatomy: not known.
P. bootes is very common in the Walcott Quarry. It is the third most common trilobite with at least 1000 specimens observed (Caron and Jackson, 2008), prompting Rasetti (1951) to define the “Pagetia bootes faunule” as the conventional shelly fossil assemblage associated with the exceptionally preserved soft-bodied biota. The co-occurring P. walcotti is very rare.
Adult eodiscine trilobites were members of the mobile benthic epifauna, possibly, like their co-occuring agnostine cousins, micrograzers or deposit (particle) feeders, adapted to colder, deeper, offshore waters.
CARON, J.-B. AND JACKSON, D. A. 2008. Paleoecology of the Greater Phyllopod Bed community, Burgess Shale. Palaeogeography, Palaeoclimatology, Palaeoecology, 258:222-256.
RASETTI, F. 1951. Middle Cambrian stratigraphy and faunas of the Canadian Rocky Mountains. Smithsonian Miscellaneous Collections, 116(5):277 p.
RASETTI, F. 1966. Revision of the North American species of the Cambrian trilobite genus Pagetia. Journal of Paleontology, 40:502-511.
SCHOLTZ, G. AND G. D. EDGECOMBE. 2006. The evolution of arthropod heads: reconciling morphological, developmental and palaeontological evidence. Development Genes and Evolution, 216:395-415.
WALCOTT, C. D. 1916. Cambrian trilobites. Smithsonian Miscellaneous Collections, 64(5):301-456.
2D reconstruction – see: http://www.trilobites.info/galagnostida.htm