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
Domain: Eukaryota
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]

Related Research Articles

<i>Opabinia</i> Extinct stem-arthropod species found in Cambrian fossil deposits

Opabinia regalis is an extinct, stem group arthropod found in the Middle Cambrian Burgess Shale Lagerstätte of British Columbia. Opabinia was a soft-bodied animal, measuring up to 7 cm in body length, and its segmented trunk had flaps along the sides and a fan-shaped tail. The head shows unusual features: five eyes, a mouth under the head and facing backwards, and a clawed proboscis that probably passed food to the mouth. Opabinia probably lived on the seafloor, using the proboscis to seek out small, soft food. Fewer than twenty good specimens have been described; 3 specimens of Opabinia are known from the Greater Phyllopod bed, where they constitute less than 0.1% of the community.

<i>Marrella</i> Extinct genus of Arthropods

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<i>Sidneyia</i> Extinct genus of arthropods

Sidneyia is an extinct arthropod known from fossils found from the Early to the Mid Cambrian of China and the Mid Cambrian Burgess Shale of British Columbia, Canada.

<i>Yohoia</i> Extinct genus of arthropods

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<i>Anomalocaris</i> Extinct genus of cambrian radiodont

Anomalocaris is an extinct genus of radiodont, an order of early-diverging stem-group arthropods.

<i>Canadaspis</i> Extinct genus of arthropods

Canadaspis is an extinct genus of bivalved Cambrian arthropod, known from North America and China. They are thought to have been benthic feeders that moved mainly by walking and possibly used its biramous appendages to stir mud in search of food. They have been placed within the Hymenocarina, which includes other bivalved Cambrian arthropods.

<i>Waptia</i> Cambrian arthropod

Waptia is an extinct genus of arthropod from the Middle Cambrian of North America. It grew to a length of 6.65 cm (3 in), and had a large bivalved carapace and a segmented body terminating into a pair of tail flaps. It was an active swimmer and likely a predator of soft-bodied prey. It is also one of the oldest animals with direct evidence of brood care. Waptia fieldensis is the only species classified under the genus Waptia, and is known from the Burgess Shale Lagerstätte of British Columbia, Canada. Specimens of Waptia are also known from the Spence Shale of Utah, United States.

<i>Leanchoilia</i> Extinct genus of arthropods

Leanchoilia is a megacheiran arthropod known from Cambrian deposits of the Burgess Shale in Canada and the Chengjiang biota of China.

<i>Odaraia</i> Extinct genus of crustaceans

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<span class="mw-page-title-main">Habelia</span>

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A number of assemblages bear fossil assemblages similar in character to that of the Burgess Shale. While many are also preserved in a similar fashion to the Burgess Shale, the term "Burgess Shale-type fauna" covers assemblages based on taxonomic criteria only.

<i>Actaeus armatus</i> Extinct genus of arthropods

Actaeus is a genus of leanchoiliid megacheiran arthropod, containing the single species Actaeus armatus. It is known from a single specimen recovered from the Middle Cambrian Burgess Shale of British Columbia, Canada, and it may be actually a poorly preserved specimen of Alalcomenaeus. The specimen is over 6 cm long and has a body consisting of a head shield, 11 body tergites, and a terminal plate. It was named after Actaeus, first king of attica, and armatus a combination of greek and latin words to describe the frontal appendages of the species.

<i>Sarotrocercus</i> Extinct genus of Cambrian organisms

Sarotrocercus is a small Cambrian arthropod known from Burgess shale, reaching a centimetre or two in length (0.39–0.79 in). Sarotrocercus is only known from 7 specimens. It may lie in the arthropod crown group, and a recent study has revised some points of its original description.

<span class="mw-page-title-main">Megacheira</span> Extinct class of arthropods

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<i>Plenocaris</i> Extinct genus of arthropods

Plenocaris plena is a genus of extinct bivalved hymenocarine arthropod that lived in the Cambrian aged Burgess Shale and Chengjiang. Originally described as a species of Yohoia by Walcott in 1912, it was placed into its own genus in 1974.

<i>Schinderhannes bartelsi</i> Extinct species of radiodont

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<i>Isoxys</i> Genus of extinct arthropods

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<span class="mw-page-title-main">Radiodonta</span> Extinct order of basal arthropods

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<i>Erratus</i> Extinct genus of Cambrian arthropod

Erratus is an extinct genus of marine arthropod from the Cambrian of China. Its type and only species is Erratus sperare. Erratus is likely one of the most basal known arthropods, and its discovery has helped scientists understand the early evolution of arthropod trunk appendages. Some of the stem-arthropods like radiodonts did not have legs, instead they had flap like appendages that helped them swim. Erratus on the other hand had not only flaps but also a set of primitive legs. It also supported the theory that the gills of aquatic arthropods probably evolved into the wings and lungs of terrestrial arthropods later in the Paleozoic.

References

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  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.