Pagetia bootes (ROM 60756). Complete individual. Specimen length = 4.5 mm. Specimen dry – direct light (left) and coated with ammonium chloride sublimate to show details (right). Walcott Quarry.
© ROYAL ONTARIO MUSEUM. PHOTOS: JEAN-BERNARD CARON
Pagetia is a member of Agnostida, small arthropods characterized by a pygidium (tail plate) similar in size and shape to the head and only two or three thoracic segments in adults. Unlike some other agnostids, Pagetia has dorsal eyes and moulting sutures. In this respect, its morphology is intermediate between that of presumably more derived agnostids (e.g. Peronopsis) and other trilobites (Cotton and Fortey 2005). A phylogenetic analysis recovered the group including Pagetia and related species to have been one of the first to branch off among trilobites (Paterson et al. 2019).
Genus – unspecified, presumably 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 made the first recorded ascent in 1904.
species – 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: other species occur throughout the world in Lower to Middle Cambrian rocks.
The Walcott and Raymond Quarries on Fossil Ridge. The Trilobite Beds on Mount Stephen. Mount Odaray.
Pagetia bootes was first named by Walcott (1916). Öpik was the first to recognize the hypostome (1952). Rasetti later provided the first detailed description (1966). The species has been featured in numerous works discussing trilobite biogeography, biostratigraphy, and phylogeny, e.g. (Rasetti 1951, 1952; Cotton and Fortey 2005; Paterson et al. 2019). Chatterton and colleagues described several Burgess Shale specimens preserved within Selkirkia tubes (2003).
Adult dorsal exoskeletons reach about 10 mm in length (including the pygidial spine). The semicircular cephalon has a narrow marginal rim around the front and sides and acute genal angles. The border is divided perpendicularly by a series of shallow furrows. The cephalon laterally bears a pair of proparian moulting sutures and compound eyes, connected to the axis by narrow, arcing eye ridges. The eyes of Pagetia and related genera are structurally unique among trilobites, having relatively few lenses separated by gaps (Jell 1975). The narrow glabella comes to an ogival point, with a median furrow extending across the short preglabellar field to the anterior margin. Faint traces of segmentation are visible on the glabella. The occipital lobe at the posterior or the cephalic axis emits an elongate spine, directed posteriad. The thorax contains two short segments with pleural furrows. A narrowly rimmed pygidium, the same size and general shape as the cephalon, has abruptly angled anterolateral corners. The pygidial axis shows six well-developed segments. The first five each bear a median tubercle while the terminal segment emits a long spine, directed posteriad. An elongate, convex hypostome is present ventrally, unfused to the headshield. Appendages are unknown.
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.
The mode of life of agnostids has been extensively debated (Fortey and Owens 1999). Their small size, degenerate eyes, adaptations for enrollment, and wide geographic occurrence have been proposed to support a pelagic lifestyle (Jell 1975; Fortey 1985). However, most specimens at the Burgess Shale are found in unrolled position, suggesting they did not live permanently enrolled. Further, Pagetia is sometimes found in groups, occasionally entombed inside the tubes of Selkirkia, which suggests the animal was spending at least part of its life cycle near the benthos (Chatterton et al. 2003; Lin and Yuan 2009). Due to the absence of appendage preservation, less is known about the mode of life of Pagetia than other Burgess Shale agnostids (Moysiuk and Caron 2019).
http://www.trilobites.info/ordagnostida.htm
2D reconstruction – see: http://www.trilobites.info/galagnostida.htm