Marrella

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Marrella
Marrella fossil

Marrella fossil

Temporal occurrence
Cambrian
Locations
Systematics
Primordial mouths (protostomia)
Molting animals (Ecdysozoa)
Arthropod (arthropoda)
Marrellomorpha
Marrelliina
Marrella
Scientific name
Marrella
Walcott , 1912

Marrella is an extinct arthropods , which as Fossil initially of a single layer of the Middle Cambrian Burgess Shale ( Burgess Shale ) of British Columbia has been known. Further discoveries indicated that Marrella is one of the most common Burgess slate fossils. Only the type species M. splendens has been scientifically described . Fossils assigned to the genus, poorly preserved and not described down to the species were later found in China.

Along with other unique fossils such as Opabinia and Yohoia , Marrella shows the diversity of the Burgess slate.

anatomy

Marrella was a small animal from 2.5 to about 20 millimeters, up to a maximum of 24.5 millimeters in length. The body consisted of two sections ( tagmata ), the head with a head shield and the torso section made up of about 26 segments , some of which overlapped slightly, and a tiny end section ( telson ).

The head shield is conspicuous by two pairs of very long, curved, backward-directed thorns, a lateral pair and a middle pair protruding upward (dorsal). The middle pair could be longer than the torso and the end of the body protruding backwards, it was keeled on the upper side in some (not all) specimens and finely serrated. In addition, there was a pair of shorter thorns on the underside of the posterior corners (posteroventral). As attachments can be seen two pairs of long, einästiger (uniramer) antennas, evidence of mouth parts are missing. The first antennae were long, with more than thirty limbs, with short, whorled bristles at the limb boundaries, they inserted in the rear half of the underside of the head shield. The second antennae were wider, six-segmented with a long, cylindrical basal limb and five shorter, flattened limbs, which had a hem of long bristle hairs ( setae ) on the outer edges . The trunk consisted of cylindrical segments without widened side lobes (pleurs), each of which consisted of a tergite and a sternite . A split bone pair sat on each segment . The inner branch (endopodite) consisted of six limbs, the outer branch, which was located near the base of the first limb, was finely feathered and probably served as a gill.

The nature and function of the second head attachments at Marrella is interpreted differently. Some researchers thought they were homologous to the mandibles , but most consider them homologous to the second antennae of crustaceans . Their most likely function is as a swimming leg-like extremity for locomotion, but some also suspect a function as a pusher towards the mouth when eating.

Structures that correspond to an optical grating have been demonstrated on well-preserved specimens . The animals could therefore have presented a colorful, shimmering sight.

Way of life

Reconstruction of Marrella

There are two different hypotheses about the Marrella way of life . According to Whitington, the animal probably lived as a benthic scavenger , feeding on the remains of other animals and other organic matter. A swimming ( pelagic ) way of life was reconstructed in a later version, as was the case by Charles Walcott, who first described it . According to this hypothesis, Marrella rowed with the large second antennae that served as swimming legs. In this case, a filtering diet would be more plausible.

Phylogeny and Kinship

Marrella is one of the Marrellomorpha, a group usually regarded as a separate class of the Arthropoda, to which many bizarrely shaped representatives belong , for example the genera Mimetaster and Vachonisia from the Devonian Hunsrück slate . There is no agreement on the family classification of the Marrellomorpha. According to their combination of characteristics, they belong as a basal branch to the "higher" or Euarthropoda. Whether they belong to the other trunk group of the Chelicerata (referred to as Arachnomorpha or Artiopoda) or (more likely) to the trunk group of the Mandibulata is disputed between different studies. A closer relationship with the arachnids in the narrower sense or the trilobites seems unlikely today.

Research history

Marrella was the first fossil that the best known among the early editors of the now famous site, the geologist Charles Walcott (1850-1927), found in the Burgess slate. He named the genus after John Edward Marr . Walcott understood Marrella informally as a type of crab, and formally described it as a strange species of trilobite , but changed this himself in a posthumous publication in 1931 by assuming close kinship with Burgessia , which he interpreted as gill pod . Later it was assigned to the class of Trilobitoidea described by Leif Størmer in the Treatise on Invertebrate Paleontology . In 1971 Harry Blackmore Whittington revised the genus from the ground up and decided on the basis of the training and number of legs, the type of gills and head appendages that the animal belonged neither to the trilobites, the chelicerates nor the crustacea .

Marrella is probably the most common species of Burgess slate arthropod, with thousands in museum collections, including around 15,000 at the Smithsonian and more than 9,000 at the Royal Ontario Museum .

Individual evidence

  1. ↑ James W. Hagadorn: Burgess Shale: Cambrian Explosion in full Bloom. In David J. Bottjer (editor): Exceptional Fossil Preservation: A Unique View on the Evolution of Marine Life. (Series Critical Moments and Perspectives in Earth History and Paleobiology). Columbia University Press, New York 2002. ISBN 0-231-10255-0
  2. ↑ Qing Liu (2013): The First Discovery of Marrella (Arthropoda, Marrellomorpha) from the Balang Formation (Cambrian Series 2) in Hunan, China. Journal of Paleontology, 87 (3): 391-394. doi : 10.1666 / 12-118.1
  3. ↑ Stephen Jay Gould: Wonderful Life: Burgess Shale and the Nature of History. WW Norton & Co., New York 1989. ISBN 0-393-02705-8 . Also OCLC 44058853 German translation: Chance man. The miracle of life as a game of nature. Carl Hanser Verlag, 1991. ISBN 3 446 15951 7
  4. ^ A b c Harry B. Whittington: Redescription of Marrella splendens (Trilobitoidea) from the Burgess Shale, Middle Cambrian, British Columbia . In: Geological Survey of Canada (Ed.): Bulletin - Geological Survey of Canada . 209, 1971, pp. 1-24.
  5. ↑ a b c d Diego C. García-Bellido & Desmond H. Collins (2006): A new study of Marrella splendens (Arthropoda, Marrellomorpha) from the Middle Cambrian Burgess Shale, British Columbia, Canada. Canadian Journal of Earth Sciences 43: 721-742.
  6. ^ AR Parker: Color in Burgess Shale animals and the effect of light on evolution in the Cambrian . In: Proceedings of the Royal Society B: Biological Sciences . 265, No. 1400, 1998, pp. 967-972. doi : 10.1098 / rspb.1998.0385 .
  7. ↑ Jonathan R. Hendricks & Bruce S. Lieberman (2008): New Phylogenetic Insights into the Cambrian Radiation of Arachnomorph Arthropods. Journal of Paleontology Vol. 82, No.3: 585-594. doi : 10.1666 / 07-017.1
  8. ^ GD Edgecombe & DA Legg (2014): Origins and early evolution of arthropods. Palaeontology 57: 457-468. doi : 10.1111 / pala.12105
  9. ^ Charles D. Walcott (1912): Cambrian Geology and Paleontology. Volume 2: Middle Cambrian Branchiopoda, Malacostraca, Trilobita, and Merostomata. Smithsonian Miscellaneous Collections 57 (6). Full text in Wikisource

Web links

Commons : Marrella  - collection of images, videos and audio files