Alalcomenaeus

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Alalcomenaeus
Temporal range: Cambrian Stage 3–Mid Cambrian
20210422 Alalcomenaeus cambricus.png
AlalcomenaeusCambricus-ROM-June11-10.jpg
Specimen of Alalcomenaeus cambricus on display at the Royal Ontario Museum, Toronto
Scientific classification Red Pencil Icon.png
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 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, [2] and Actaeus is often excluded from such works as it is poorly understood. [1] Currently, it is included within Megacheira. [3]

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. [4]

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>Yohoia</i> Extinct genus of arthropods

Yohoia is an extinct genus of megacheiran arthropod from the Cambrian period that has been found as fossils in the Burgess Shale formation of British Columbia, Canada. It has been placed among the arachnomorpha, a group of arthropods that includes the chelicerates and possibly the trilobites. Fossils range in size from 7 to 23 mm. 711 specimens of Yohoia are known from the Greater Phyllopod bed, where they comprise 1.35% of the community.

<i>Anomalocaris</i> Extinct genus of anomalocaridid (also extinct)

Anomalocaris is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of Anomalocaris were discovered in the Ogygopsis Shale of the Stephen Formation in British Columbia, Canada by Joseph Frederick Whiteaves, with more examples found by Charles Doolittle Walcott in the Burgess Shale unit of the Stephen Formation. Other closely related fossils have been found in the older Emu Bay Shale of Australia, as well as possibly elsewhere. Originally several fossilized parts discovered separately were thought to be three separate creatures, a misapprehension corrected by Harry B. Whittington and Derek Briggs in a 1985 journal article. With a body length close to 40 centimetres, A. canadensis is thought to be one of the earliest examples of an apex predator, though others have been found in older Cambrian lagerstätten deposits.

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

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<i>Waptia</i> Cambrian arthropod

Waptia fieldensis is an extinct species of arthropod from the Middle Cambrian Burgess Shale Lagerstätte of Canada. It grew to a length of 6.65 cm (3 in) and resembled modern shrimp in both morphology and habit. It 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. Other specimens of Waptia are known from the Spence Shale in Utah.

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

Leanchoilia is an megacheiran arthropod known from Cambrian deposits of the Burgess Shale in Canada and the Chengjiang biota of China. It was about 5 centimetres (2.0 in) long and had long, whip-like feelers mounted on frontal arm-like appendages. Its internal organs are occasionally preserved within the substrate in three dimensions.

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

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<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

Megacheira is an extinct class of predatory arthropods that possessed a pair of great appendages, hence the class name as well as the common name "great appendage arthropods". Their taxonomic position is controversial, with studies either considering them stem-group euarthropods, or stem-group chelicerates. Most of them were found in marine environments throughout the world from the lower to middle Cambrian. Megacheirans were important components of several faunas, including the Burgess, Wheeler and Maotianshan Shales Lagerstatten.

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

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

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

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The fossils of the Burgess Shale, like the Burgess Shale itself, formed around 505 million years ago in the Mid Cambrian period. They were discovered in Canada in 1886, and Charles Doolittle Walcott collected over 65,000 specimens in a series of field trips up to the alpine site from 1909 to 1924. After a period of neglect from the 1930s to the early 1960s, new excavations and re-examinations of Walcott's collection continue to reveal new species, and statistical analysis suggests that additional discoveries will continue for the foreseeable future. Stephen Jay Gould's book Wonderful Life describes the history of discovery up to the early 1980s, although his analysis of the implications for evolution has been contested.

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

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<i>Retifacies</i> Species of arthropod

Retifacies is an extinct arthropod, that lived in the lower Cambrian. Its fossil remains have been found in the Maotianshan Shales of Yunnan, China. It is a member of the Artiopoda.

<i>Kylinxia</i> Genus of Ancient Arthropod

Kylinxia is a genus of extinct arthropod described in 2020. It was described from six specimens discovered in Yu'anshan Formation in southern China. The specimens are assigned to one species Kylinxia zhangi. Dated to 518 million years, the fossils falls under the Cambrian period. Announcing the discovery on 4 November 2020 at a press conference, Zeng Han of the Nanjing Institute of Geology and Paleontology, said that the animal "bridges the evolutionary gap from Anomalocaris to true arthropods and forms a key ‘missing link’ in the origin of arthropods," which was "predicted by Darwin’s evolutionary theory." The same day the formal description was published in Nature.

<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

  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, doi:10.1111/1475-4983.00104
  2. Hendricks, J.R.; Lieberman, B.S. (2008), "New Phylogenetic Insights into the Cambrian Radiation of Arachnomorph Arthropods", Journal of Paleontology, 82 (3): 585–594, doi:10.1666/07-017.1
  3. 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. doi:10.1038/nature12520. ISSN   0028-0836.
  4. Caron, Jean-Bernard; Jackson, Donald A. (October 2006). "Taphonomy of the Greater Phyllopod Bed community, Burgess Shale". PALAIOS. 21 (5): 451–65. doi:10.2110/palo.2003.P05-070R. JSTOR   20173022.