Alalcomenaeus

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Alalcomenaeus
Temporal range: Cambrian Stage 3–Mid Cambrian
USNM PAL 155658 Alalcomenaeus cambricus Wet+Pol Holotype.jpg
Fossil holotype
20210422 Alalcomenaeus cambricus.png
Life restoration
Scientific classification OOjs UI icon edit-ltr.svg
Kingdom: Animalia
Phylum: Arthropoda
Class: Megacheira
Order: Leanchoilida
Family: Leanchoiliidae
Simonetta & Delle Cave, 1975
Genus: Alalcomenaeus
Simonetta, 1970
Species:
A. cambricus
Binomial name
Alalcomenaeus cambricus
Simonetta, 1970

Alalcomenaeus is one of the most widespread and longest-surviving arthropod genera of the Early and Middle Cambrian. [1] Known from over 300 specimens in the Burgess Shale and the Chengjiang biota. It is a member of the family Leanchoiliidae [2] in the group Megacheira.

Contents

Morphology

Alalcomenaeus had three median eyes; two stalked, more lateral eyes; a triflagellate great appendage; and two more head appendages posterior to that. Like its body appendages, these were biramous—their inner branch was spiny, segmented, flexible and leg-like, while the outer portion had a large surface area and resembled a flap.

Alalcomenaeus reached about 6 cm in length, although many smaller specimens are known. [1] Its head was covered with a shield, and its eleven body segments were also covered with an exoskeleton. Its body terminated with a paddle-like telson ("tail") which probably helped to propel the organism; this ended with long flat spikes in the plane of the tail fin. [1]

Ecology

The organism probably swam, wafting its outer flap-limbs in waves along its body to gain propulsion, in the manner of Anomalocaris . [1] Its inner limb branches do not appear to be optimised for walking, although it is possible they helped the organism move along the sea floor. [1] A variety of other functions have been suggested, such as clinging to algal fronds; they seem best suited to grabbing onto, and tearing up, other animals, suggesting that the organism was probably a scavenger. [1] However, its large eyes and the long flagella on its great appendages, combined with its large feeding apparatus and the spines on its inner limb branches, are more consistent with a predatory lifestyle, and the most recent interpretation has it feeding on organisms that lived on or in the surface of the sea floor. [1]

Affinity

Alalcomenaeus was initially thought to fall in the stem group to the crustacea, and was placed in a clade with Leanchoilia , Actaeus and Yohoia . [1] It was subsequently suggested to be related to the opabiniids, mainly on the basis of its great appendage. [1] More up to date analyses place it along with Leanchoilia and Yohoia as well as Sanctacaris, Habelia, Sarotrocercus and Sidneyia , somewhere within the arachnomorpha. The closeness of Sanctacaris and Leanchoilia has been supported by subsequent work, [3] and Actaeus is often excluded from such works as it is poorly understood. [1] Currently, it is included within Megacheira. [4]

Distribution

Alalcomenaeus is very rare in the Walcott quarry of the Burgess Shale, so was described on the basis of half a dozen specimens. As other exposures of the Burgess Shale were unearthed, it became apparent that the creature was, in fact, a dominant member of the fauna. It has also been discovered in the Chengjiang and from Utah, giving it a long stratigraphic range. [1] Including 596 juveniles, 618 specimens of Alalcomenaeus are known from the Greater Phyllopod bed, where they comprise 1.2% of the community. [5]

References

  1. 1 2 3 4 5 6 7 8 9 10 11 Briggs, D.E.G.; Collins, D. (1999), "The Arthropod Alalcomenaeus cambricus Simonetta, from the Middle Cambrian Burgess Shale of British Columbia", Palaeontology, 42 (6): 953–977, Bibcode:1999Palgy..42..953B, doi: 10.1111/1475-4983.00104 , S2CID   129399862
  2. Georg Brenneis (2022). "The visual pathway in sea spiders (Pycnogonida) displays a simple serial layout with similarities to the median eye pathway in horseshoe crabs". BMC Biology. 20 (1): 27. doi: 10.1186/s12915-021-01212-z . PMC   8796508 . PMID   35086529.
  3. Hendricks, J.R.; Lieberman, B.S. (2008), "New Phylogenetic Insights into the Cambrian Radiation of Arachnomorph Arthropods" , Journal of Paleontology, 82 (3): 585–594, Bibcode:2008JPal...82..585H, doi:10.1666/07-017.1, S2CID   86058797
  4. Tanaka, Gengo; Hou, Xianguang; Ma, Xiaoya; Edgecombe, Gregory D.; Strausfeld, Nicholas J. (2013-10-17). "Chelicerate neural ground pattern in a Cambrian great appendage arthropod" . Nature. 502 (7471): 364–367. Bibcode:2013Natur.502..364T. doi:10.1038/nature12520. ISSN   0028-0836. PMID   24132294. S2CID   4456458.
  5. Caron, Jean-Bernard; Jackson, Donald A. (October 2006). "Taphonomy of the Greater Phyllopod Bed community, Burgess Shale". PALAIOS. 21 (5): 451–65. Bibcode:2006Palai..21..451C. doi:10.2110/palo.2003.P05-070R. JSTOR   20173022. S2CID   53646959.