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Ptychagnostus praecurrens
Ptychagnostus praecurrens (USNM 116212). Complete individual originally interpreted as the holotype of Triplagnostus burgessensis by Rasetti (1951). Specimen length = 8 mm. Specimen dry – direct light. Walcott Quarry.
© Smithsonian Institution – National Museum of Natural History. Photo: Jean-Bernard Caron
Taxonomy:
Owing to their distinctive appearance, agnostids have either been classified as trilobites, related to Eodiscina, or as stem group “crustaceans” (Müller and Walossek 1987; Cotton and Fortey 2005; Haug et al. 2009). The most recent phylogenetic analysis finds that agnostids form a grouping with trilobites, supported by shared features of the dorsal exoskeleton, such as mineralization, the expression of segmental boundaries, and the form of the thoracic joints (Moysiuk and Caron 2019). More taxonomically inclusive analyses will be needed to determine whether they belong inside or outside the group of true trilobites.
Ptychagnostus – from the Greek ptychos, “pleated” (some species have pleat-like furrows on the cephalon), and agnostos, for “unknown” or “unknowable.”
praecurrens – from the Latin prae, “before,” and currens, “to run,” in reference to the old age of this fossil.
Burgess Shale and vicinity: none.
Other deposits: other species occur throughout the world in Middle Cambrian rocks.
Age & Localities:
Most abundant in the Walcott Quarry on Fossil Ridge, but also present in most other localities.
History of Research:
Burgess Shale fossils assigned to this genus and species have been historically described under several name combinations. Originally, Rasetti (1951) described these as Triplagnostus burgessensis, but subsequently (1967) considered T. burgessensis to be a synonym of Ptychagnostus praecurrens (Westergård 1936), a name retained by Peng and Robison (2000), despite numerous interim variations. Naimark and Pegel (2017) proposed to reassign the species to the genus Pentagnostus based on a study of Russian material, but we maintain it here under Ptychagnostus pending taxonomic restudy of Burgess Shale specimens. Moysiuk and Caron (2019) recently described the appendages, digestive tract, and other soft tissues from exceptionally preserved specimens.
Description:
Adult dorsal exoskeletons reach about 10 mm in length. The semicircular cephalon has a narrow marginal rim around the front and sides and rounded genal angles. There are no dorsal eyes and no facial sutures. The narrow glabella comes to an ogival point, with a median furrow extending across the short preglabellar field to the anterior margin; a transverse furrow crosses the glabella just in front of a low tubercle located behind the midpoint. Two short thoracic segments carry lateral nodes on the axial rings. A narrowly rimmed pygidium, the same size and general shape as the cephalon, has abruptly angled anterolateral corners. The pygidial axis is broader than the glabella, but of similar outline, with a median tubercle between two transverse furrows. The pointed tip of the axis reaches almost to the rim posteriorly, without a median furrow. A saddle-shaped hypostome is present ventrally, unfused to the headshield. Unmineralized anatomy: The head probably bears six pairs of appendages, including one pair of elongate sensory antennules, two pairs of appendages with oar like outer branches, and probably three pairs of stout walking limbs with a row of club-like projections. Additional walking limbs were present beneath the thorax (2) and pygidium (probably 4). The digestive tract curves dorsally from the mouth before emitting two pairs of branching gut glands, the first of which is the largest and occupies much of the space below the headshield. Behind this, the cylindrical midgut extends back to the pygidium. The hindgut begins roughly below the pygidial tubercle, and narrows considerably before reaching the anus below the tip of the pygidial axis.
Very common in the Walcott Quarry on Fossil Ridge, where it is the most abundant artiopodan (Caron and Jackson 2008). Ptychagnostus also occurs in notable numbers at the Raymond Quarry, Mount Stephen, Tokumm Creek, the lower Marble Canyon Quarry and a few smaller localities (Caron et al. 2014; O’Brien and Caron 2016; Nanglu et al. 2020).
Ecology:
The mode of life of agnostids has been extensively debated (Fortey and Owens 1999). With the oar-like appendages capable of protruding while the animal was partially enrolled, agnostids certainly appear well-adapted for swimming (Müller and Walossek 1987). Together with their occurrence in mass mortality beds with wide geographic range, this evidence has been proposed to support a pelagic lifestyle (Fortey 1985). However, most specimens at the Burgess Shale are found in unrolled position, suggesting they did not live permanently enrolled. Further, Ptychagnostus is sometimes found in groups, associated with the remains of other Burgess Shale organisms, where it was potentially feeding on carrion or bacterial films, providing evidence for a benthic habitat. The huge, branching gut glands in the head likely acted as a food storage organ, enabling a feast-and-famine lifestyle. The club-like outgrowths on the walking legs may have functioned in respiration (Moysiuk and Caron 2019).
References:
- CARON, J.-B. and JACKSON, D. A. 2008. Paleoecology of the Greater Phyllopod Bed community, Burgess Shale. Palaeogeography, Palaeoclimatology, Palaeoecology, 258: 222–256.
- CARON, J.-B., GAINES, R. R., ARIA, C., MÁNGANO, M. G. and STRENG, M. 2014. A new phyllopod bed-like assemblage from the Burgess Shale of the Canadian Rockies. Nature communications, 5: 1–6.
- COTTON, T. J. and FORTEY, R. A. 2005. Comparative morphology and relationships of the agnostida. In KOENEMANN, S. and JENNER, R. (eds.) Crustacea and Arthropod Relationships, CRC Press, 95–136 pp.
- FORTEY, R. A. 1985. Pelagic trilobites as an example of deducing the life habits of extinct arthropods. Earth and Environmental Science Transactions of The Royal Society of Edinburgh, 76: 219–230.
- FORTEY, R. A. and OWENS, R. M. 1999. Feeding habits in trilobites. Palaeontology, 42: 429–465.
- HAUG, J. T., MAAS, A. and WALOSZEK, D. 2009. †Henningsmoenicaris scutula, †Sandtorpia vestrogothiensis gen. et sp. nov. and heterochronic events in early crustacean evolution. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 100: 311–350.
- MOYSIUK, J. and CARON, J. B. 2019. Burgess Shale fossils shed light on the agnostid problem. Proceedings of the Royal Society B: Biological Sciences, 286: 20182314.
- MÜLLER, K. J. and WALOSSEK, D. 1987. Morphology, ontogeny, and life habit of Agnostus pisiformis from the Upper Cambrian of Sweden. Fossils and Strata, 19: 1–124.
- NAIMARK, E. B. and PEGEL, T. v. 2017. Revision of the Cambrian Agnostina (Trilobita?) from Russia. Paleontological Journal, 51: 1167–1248.
- NANGLU, K., CARON, J.-B. and GAINES, R. R. 2020. The Burgess Shale paleocommunity with new insights from Marble Canyon, British Columbia. Paleobiology, 46: 58–81.
- O’BRIEN, L. J. and CARON, J. B. 2016. Paleocommunity analysis of the Burgess Shale Tulip Beds, Mount Stephen, British Columbia: Comparison with the Walcott Quarry and implications for community variation in the Burgess Shale. Paleobiology, 42: 27–53.
- PENG, S. and ROBISON, R. A. 2000. Agnostoid biostratigraphy across the middle-Upper Cambrian boundary in Hunan, China. Memoir ( The Paleontological Society ), 53: 1–104.
- RASETTI, F. 1951. Middle Cambrian stratigraphy and faunas of the Canadian Rocky Mountains. Smithsonian Miscellaneous Collections, 116: 1–277.
- RASETTI, F. 1967. Lower and Middle Cambrian trilobite faunas from the Taconic Sequence of New York. Smithsonian Miscellaneous Collections, 152: 1–112.
- WESTERGÅRD, A. H. 1936. Paradoxides oelandicus beds of Oland: with the account of a diamond boring through the Cambrian at Mossberga. Sveriges Geologiska Undersökning. Series C, no. 394, Årsbok 30: 1–66.
