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Alalcomenaeus cambricus
Reconstruction of Alalcomenaeus cambricus.
© Marianne Collins
Taxonomy:
Megacheirans are basal true arthropods with a frontal appendage pointing upward and made of multiple claws (the cheira, or “great appendage”). Alalcomenaeus 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).
Alalcomenaeus – from the Greek Alalcomenae, a town in ancient Beotia, birthplace of the goddess Athena.
cambricus – of Cambrian age, derived originally from Cambria in Wales.
Burgess Shale and vicinity: none.
Indeterminate species (Alalcomenaeus sp.) from both the Chengjiang biota (Tanaka et al., 2013; Liu, Scholtz & Hou, 2015) and Utah (Ortega-Hernández, Lerosey-Aubril & Pates, 2019).
Age & Localities:
The Collins Quarry on Mount Stephen, and the Walcott and Raymond Quarries on Fossil Ridge.
History of Research:
Simonetta (1970) described this organism on the basis of a handful of specimens originally collected by Walcott on Fossil Ridge. The taxon was redescribed by Whittington (1981), who accepted only two of Simonetta’s original specimens as Alalcomenaeus. The recovery of over 300 new specimens from Collins Quarry on Mount Stephen allowed for a more detailed description of the taxon (Briggs & Collins, 1999). Although Alalcomenaeus has since not been the object of a dedicated revision, specimens of this genus have been used across studies on early arthropod evolution (e.g., Aria & Caron, 2015). Perhaps most notably, specimens from China (Tanaka et al., 2013) and Utah (Ortega-Hernández et al., 2019) were used to argue for the preservation of central nervous systems, but this was refuted (Aria et al., 2023).
Description:
Alalcomenaeus was a smaller predator compared to its sister genus Leanchoilia. The body 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. In Alalcomenaeus, the most basal segments of the cheirae are shorter relatively to Leanchoilia, resulting in the impression in many specimens that the appendages are shorter and closer to the head shield. 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. 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. Pleurae (lateral extensions of the segments) were enclosing the sides of the body. The trunk was composed of 11 segments, and expressed two strong parallel carinae on its dorsum. The tail is a single element called a telson, here the shape of a spatula bearing long spines at its tip.
Just a few dozen specimens of Alalcomenaeus have been found in the Burgess Shale at several horizons on Fossil Ridge. Alalcomenaeus represents 0.04% of the Walcott Quarry community (Caron and Jackson, 2008). It is more common in the Glossopleura Zone on Mount Stephen, where over 300 specimens have been found in one single locality (Collins Quarry), but in general this species is rare.
Ecology:
Like other leanchoiliid megacheirans, Alalcomenaeus 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, Contrary to larger leanchoiliids, large digestive glands have not been documented in Alalcomenaeus; instead, there seems to be a single large, tubular midgut (Briggs & Collins, 1999). This may point to a different feeding ecology, such as the more frequent ingestion of food or the consumption of more easily digestible prey items. 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), which are observable in Alalcomenaeus (Aria et al., 2023).
References:
- ARIA, C. & CARON, J.-B. (2015) Cephalic and limb anatomy of a new isoxyid from the Burgess Shale and the role of ‘stem bivalved arthropods’ in the disparity of the frontalmost appendage. PLoS ONE 10, e0124979.
- ARIA, C., VANNIER, J., PARK, T.S. & GAINES, R.R. (2023) Interpreting fossilized nervous tissues. BioEssays, 2200167.
- BRIGGS, D.E.G. & COLLINS, D. (1999) The arthropod Alalcomenaeus cambricus Simonetta, from the Middle Cambrian Burgess Shale of British Columbia. Palaeontology 42, 953–977.
- 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.
- LIU, Y., SCHOLTZ, G. & HOU, X. (2015) When a 520 million-year-old Chengjiang fossil meets a modern micro-CT – a case study. Scientific Reports 5, 12802.
- ORTEGA-HERNÁNDEZ, J., LEROSEY-AUBRIL, R. & PATES, S.(2019) Proclivity of nervous system preservation in Cambrian Burgess Shale-type deposits. Proceedings of the Royal Society B: Biological Sciences 286, 20192370.
- SIMONETTA, A.M. (1970) Studies on non trilobite arthropods of the Burgess Shale (Middle Cambrian). Palaeontographia Italica 66 (New series 36), 35–45.
- TANAKA, G., HOU, X., MA, X., EDGECOMBE, G.D. & STRAUSFELD, N.J. (2013) Chelicerate neural ground pattern in a Cambrian great appendage arthropod. Nature 502, 364–367.
- 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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