Feeding Strategies: Grazer



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Pagetia bootes

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

Taxonomy:

Kingdom: Grazer
Phylum: Grazer
Higher Taxonomic assignment: Artiopoda, Order Agnostida
Species name: Pagetia bootes
Remarks:

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).

Described by: Walcott
Description date: 1916
Etymology:

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.

Type Specimens: Syntypes (P. bootes) – USNM 62855-61; Holotype (P. walcotti) – USNM 146310; in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Other species:

Burgess Shale and vicinity: P. walcotti Rasetti, 1966.

Other deposits: other species occur throughout the world in Lower to Middle Cambrian rocks.

Age & Localities:

Age:
Middle Cambrian, Wuliuan stage, Burgess Shale Formation (approximately 505 million years ago).
Principal localities:

The Walcott and Raymond Quarries on Fossil Ridge. The Trilobite Beds on Mount Stephen. Mount Odaray.

History of Research:

Brief history of research:

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).

Description:

Morphology:

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.

Abundance:

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.

Maximum Size:
About 10 mm.

Ecology:

Life habits: Grazer
Feeding strategies: Grazer
Ecological Interpretations:

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).

References:

  • CHATTERTON, B. D. E., COLLINS, D. H. and LUDVIGSEN, R. 2003. Cryptic behaviour in trilobites: Cambrian and Silurian examples from Canada, and other related occurrences. Special Papers in Palaeontology, 70: 157–173.
  • 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.
  • JELL, P. A. 1975. The abathochroal eye of Pagetia, a new type of trilobite eye. Fossils and Strata, 4: 33–43.
  • LIN, J. P. and YUAN, J. L. 2009. Reassessment of the mode of life of Pagetia Walcott, 1916 (Trilobita: Eodiscidae) based on a cluster of intact exuviae from the Kaili Formation (Cambrian) of Guizhou, China. Lethaia, 42: 67–73.
  • 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.
  • 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.
  • ÖPIK, A. A. 1952. The hypostoma of Pagetia. Journal of Paleontology, 26: 272–274.
  • PATERSON, J. R., EDGECOMBE, G. D. and LEE, M. S. Y. 2019. Trilobite evolutionary rates constrain the duration of the Cambrian explosion. Proceedings of the National Academy of Sciences of the United States of America, 116: 4394–4399.
  • RASETTI, F. 1951. Middle Cambrian stratigraphy and faunas of the Canadian Rocky Mountains. Smithsonian Miscellaneous Collections, 116: 1–277.
  • RASETTI, F. 1952. Revision of the North American trilobites of the family Eodiscidae. Journal of Paleontology, 26: 434–451.
  • RASETTI, F. 1966. Revision of the North American species of the Cambrian Trilobite genus Pagetia. Journal of Paleontology, 40: 502–511.
  • WALCOTT, C. D. 1916. Cambrian geology and paleontology, III, No. 5; Cambrian trilobites. Smithsonian Miscellaneous Collections, 64: 303–456.
Other Links:

http://www.trilobites.info/ordagnostida.htm

2D reconstruction – see: http://www.trilobites.info/galagnostida.htm



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Aysheaia pedunculata

Reconstruction of Aysheaia pedunculata.

© Marianne Collins

Taxonomy:

Kingdom: Grazer
Phylum: Grazer
Higher Taxonomic assignment: None
Species name: Aysheaia pedunculata
Remarks:

Aysheaia is one of a variety of lobopodian taxa from the Cambrian, which are early members of the lineage that gave rise to arthropods, and whose only modern survivors are onychophorans (velvet worms) and tardigrades (water bears). Lobopodians characteristically have annulated, unjointed bodies and bear soft limbs after which they are called: the lobopods. Although the finding is not consensual, the type of claws and orientation of rear limbs suggest that Aysheaia could be the earliest member of the water bear lineage.

Described by: Walcott
Description date: 1911
Etymology:

Genus – after the nearby Aysha peak (since renamed Ayesha peak) in the Wapta icefield (3,065 m); original meaning unknown.

species – from the Latin pedunculus, “foot.”

Type Specimens: Holotype –USNM57655 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Other species:

Burgess Shale and vicinity: none

Other deposits: A.? prolata from the Middle Cambrian of Utah (Robison, 1985).

Age & Localities:

Age:
Middle Cambrian, Wuliuan stage, Burgess Shale Formation (approximately 505 million years ago).
Principal localities:

The Walcott Quarry on Fossil Ridge.

History of Research:

Brief history of research:

Walcott originally described Aysheaia as an annelid worm (Walcott 1911). It was later re-described as a velvet worm (or a close relative) (Brues 1923; Walton 1927; Hutchinson 1930; Walcott 1931), although it lacked features such as jaws and slime glands. Certain authors advocated a position in its own phylum (Tiegs & Manton 1958). A morphological reinterpretation based on photographs (Delle Cave & Simonetta 1975) prompted a detailed re-study of the fossil specimens (Whittington 1978), and relationships were suggested with the water bears (tardigrades) (Bergström 1978). Although some more recent phylogenetic approaches resolve Aysheaia as a basal lobopodian (Smith & Ortega-Hernandez 2014; Yang et al. 2015), other find support for a more derived affinity and a close relationship with tardigrades (Caron & Aria 2017, 2020).

Description:

Morphology:

Aysheaia is a worm-like animal, 1 to 6 cm in length and about 5 mm broad, bearing ten pairs of clawed, spiny limbs on the lower part of its body. It did not have a separate head, but a mouth occupied the very front of the body, accompanied by a pair of appendages and a circlet of bumps (papillae). The animal had a soft, flexible, non-mineralized cuticle, which had a corrugated, accordion-like form. Each stubby limb had ten corrugations, some of which bore a spiny projection. The terminal pair of limbs was oriented towards the front of the animal. A suite of claws also adorned the end of each stub-foot. A faint line running down the axis of the organism is interpreted as its gut.

Abundance:

Aysheaia is rare in the Walcott Quarry representing less than 0.04% of the specimens counted in the community (Caron and Jackson, 2008).

Maximum Size:
60 mm

Ecology:

Life habits: Grazer
Feeding strategies: Grazer
Ecological Interpretations:

Aysheaia is frequently associated with the remains of sponges, and an ecological association has been posited. Considering recent evidence that other lobopodians lived anchored to a substrate for suspension-feeding (Caron & Aria 2017), it is possible that Aysheaia also used sponges as substrate for its lifestyle, although the animal lacked the morphological specializations of a suspension feeder, and more likely grazed or predated walking on their surface.

References:

  • BERGSTRÖM, J. 1978. Morphology of fossil arthropods as a guide to phylogenetic relationships. In Arthropod Phylogeny, Van Nostrand Reinhold Co., New York, 1–56 pp.
  • BRUES, C. 1923. The geographical distribution of the Onychophora. American Naturalist, 57, 210–217.
  • CARON, J. and ARIA, C. 2020. The Collins’ monster, a spinous suspension‐feeding lobopodian from the Cambrian Burgess Shale of British Columbia. Palaeontology, 63, 979–994.
  • CARON, J.-B. and ARIA, C. 2017. Cambrian suspension-feeding lobopodians and the early radiation of panarthropods. BMC Evolutionary Biology, 17, 29.
  • DELLE CAVE, L. and SIMONETTA, A. M. 1975. Notes on the morphology and taxonomic position of Aysheaia (Onycophora?) and of Skania (undetermined phylum). Monitore Zoologico Italiano, (N.S.), 67–81.
  • HUTCHINSON, G. E. 1930. Restudy of some Burgess Shale fossils. Proceedings of the United States National Museum, 78, 59.
  • SMITH, M. R. and ORTEGA-HERNANDEZ, J. 2014. Hallucigenia’s onychophoran-like claws and the case for Tactopoda. Nature, 514, 363–366.
  • TIEGS, O. W. and MANTON, S. M. 1958. The evolution of the Arthropoda. Biological Reviews, 33, 255–333.
  • WALCOTT, C. 1911. Cambrian Geology and Paleontology II. Middle Cambrian annelids. Smithsonian Miscellaneous Collections, 57(5), 109–145.
  • WALCOTT, C. 1931. Addenda to descriptions of Burgess Shale fossils. Smithsonian Miscellaneous Collections, 85(3), 1–46.
  • WALTON, L. B. 1927. The polychaete ancestry of the insects. American Naturalist, 61, 226–250.
  • WHITTINGTON, H. B. 1978. The lobopod animal Aysheaia pedunculata Walcott, Middle Cambrian, Burgess Shale, British Columbia. Philosophical Transactions of the Royal Society of London B, 284, 165–197.
  • YANG, J., ORTEGA-HERNANDEZ, J., GERBER, S., BUTTERFIELD, N. J., HOU, J., LAN, T. and ZHANG, X. 2015. A superarmored lobopodian from the Cambrian of China and early disparity in the evolution of Onychophora. Proceedings of the National Academy of Sciences of the United States of America, 112, 8678–8683.
Other Links: