The Burgess Shale

Actaeus armatus

A plump “great appendage” arthropod known from a single specimen

Actaeus armatus (USNM 155597) – Holotype. Complete specimen shown in dorsal oblique view. Specimen length = 66 mm. Specimen dry – polarized light (left), wet – direct light (right). Walcott Quarry.

© Smithsonian Institution – National Museum of Natural History. Photos: Jean-Bernard Caron

 

Taxonomy:

Kingdom: Animalia
Phylum: Arthropoda
Higher Taxonomic assignment: Unranked clade Megacheira? (stem group arthropods)
Species name: Actaeus armatus
Remarks:

Megacheirans are basal true arthropods with a frontal appendage pointing upward and made of multiple claws (the cheira, or “great appendage”). Actaeus and other leanchoiliidae characteristically also bear long filaments on their frontal appendages, called flagellae, and most likely used as sensory devices. Megacheirans are generally considered to be among the first true arthropods (that is, arthropods with both articulated bodies and appendages), and possibly the earliest representatives of the extended chelicerate lineage (Aria, 2022). Relationships within Leanchoiliidae remain largely unresolved.

Described by: Simonetta
Description date: 1970
Etymology:

Actaeus – from the Greek Actaeus, “of the coast”; the first king of Attica, referring to both the antiquity of the genus and, according to Simonetta, the “shallow coastal waters” where the Burgess Shale sediments came from.

armatus – from the Latin armatus, “armored, or armed,” evocating the grasping appendages.

Type Specimens: Holotype –USNM155597 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Other species:

Burgess Shale and vicinity: none.

Other deposits: none.

Age & Localities:

Age:
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
Principal localities:

The Walcott Quarry on Fossil Ridge.

History of Research:

Brief history of research:

Simonetta (1970) described this organism on the basis of a single specimen originally collected by Walcott. Simonetta and Delle Cave (1975) provided the first reconstruction of this animal in a general monograph about Burgess Shale arthropods. The taxon was redescribed by Whittington (1981), and compared to Leanchoilia (Bruton and Whittington, 1983). Although a restudy has been lacking to this date, Actaeus has featured in studies discussing the significance of leanchoiliid morphology (Aria, Caron & Gaines, 2015; Aria et al., 2023).

Description:

Morphology:

Actaeus was a relatively large predator. The body is wide and lacks any biomineralization. Like other leanchoiliid megacheirans, it is characterized by flagellate frontal appendages (cheirae) made of three long claws, and a body divided into two regions (tagmata): the cephalon, covered by a single shield, and the segmented trunk. At the front of the head, leanchoiliids bore a pair of large short-stalked lateral eyes and a pair of smaller, mushroom-shaped median eyes—although the presence of these median eyes in Actaeus is unclear. The megacheiran appendages were of similar simple morphology throughout the body, reflecting the typical arthropod biramous limb: sub-cylindrical basis with teeth for mastication (basipod), relatively strong walking legs (endopods), and paddle-like, semi-rigid flaps (exopods) fringed with lamellae, which in Actaeus are particularly stout. The trunk was composed of about 10 segments. The tail is a single element called a telson, which appears spatulate.

Abundance:

A single specimen is known of this species.

Maximum Size:
6.6 cm.

Ecology:

Life habits: Nektobenthic, Mobile
Feeding strategies: Carnivorous
Ecological Interpretations:

Like other leanchoiliid megacheirans, Actaeus must have used its combined sensing and grasping frontal appendages to detect and catch prey items. Food caught was brought under the body where it might have been rudimentarily masticated between the bases of limbs (basipods), before being channeled back to the mouth. As a leanchoiliid, Actaeus also had large digestive glands atop its gut, suggesting it was either storing food for extended periods of time, or compensating rough mastication with additional enzymes. The megacheiran body appendages, made of relatively strong walking legs (endopods) as well as paddle-like, semi-rigid flaps (exopods), would have allowed for both comfortable locomotion on the sea floor and swimming. The exopods likely served for gas exchanges (like breathing) as well, but recent studies also showed that megacheirans and other Cambrian arthropods possessed dedicated gills (Liu et al., 2021).

References:

  • Aria, C., Caron, J.-B. & Gaines, R. (2015) A large new leanchoiliid from the Burgess Shale and the influence of inapplicable states on stem arthropod phylogeny. Palaeontology 58, 629–660.
  • Aria, C., Vannier, J., Park, T.S. & Gaines, R.R. (2023) Interpreting fossilized nervous tissues. BioEssays, 2200167.
  • Bruton, D.L. & Whittington, H.B. (1983) Emeraldella and Leanchoilia, two arthropods from the Burgess Shale, Middle Cambrian, British Columbia. Philosophical Transactions of the Royal Society of London, Series B 300, 553–582.
  • Caron, J.B. & Jackson, D.A. (2008) Paleoecology of the Greater Phyllopod Bed community, Burgess Shale. Palaeogeography, Palaeoclimatology, Palaeoecology 258, 222–256.
  • Liu, Y., Edgecombe, G.D., Schmidt, M., Bond, A.D., Melzer, R.R., Zhai, D., Mai, H., Zhang, M. & Hou, X. (2021) Exites in Cambrian arthropods and homology of arthropod limb branches. Nature Communications 12, 4619.
  • Simonetta, A.M. (1970) Studies on non trilobite arthropods of the Burgess Shale (Middle Cambrian). Palaeontographia Italica 66 (New series 36), 35–45.
  • Simonetta, A.M. & Delle Cave, L. (1975) The Cambrian non trilobite arthropods from the Burgess Shale of British Columbia. A study of their comparative morphology taxinomy and evolutionary significance. Palaeontographia Italica 69, 1–37.
  • Whittington, H.B. (1981) Rare arthropods from the Burgess Shale, Middle Cambrian, British Columbia. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 292, 329–357.
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