Agnostida

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Agnostida
Temporal range: Cambrian Stage 3 –Late Ordovician
Itagnostus interstrictus
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
(unranked): †Artiopoda
Subphylum: †Trilobitomorpha
Class: †Trilobita (?)
Order: †Agnostida
Salter, 1864
Families

Suborder

Agnostina

Suborder Eodiscina

Synonyms

Isopygia Gürich, 1907
Miomera Jækel, 1909

Agnostida are an

extinct in the Late Ordovician.[2]

Systematics

The Agnostida are divided into two suborders —

cephalon
. Most agnostid species were eyeless.

The systematic position of the order Agnostida within the class Trilobita remains uncertain, and there has been continuing debate whether they are trilobites or a

artiopodans like nektaspids, and their placement as stem-crustaceans was unsupported. The study recovered agnostidans as the sister group to other trilobites within the Artiopoda.[1]

Ecology

Scientists have long debated whether the agnostids lived a

fossil record is uncharacteristic of benthic animals, suggesting a pelagic existence. The thoracic segment appears to form a hinge between the head and pygidium allowing for a bivalved ostracodan-type lifestyle. The orientation of the thoracic appendages appears ill-suited for benthic living. Recent work suggests that some agnostids were benthic predators, engaging in cannibalism and possibly pack-hunting behavior.[6]

They are sometimes preserved within the voids of other organisms, for instance within empty

hyolith conchs,[7] within sponges, worm tubes and under the carapaces of bivalved arthropods,[8] presumably in order to hide from predators or strong storm currents; or maybe whilst scavenging for food.[8] In the case of the tapering worm tubes Selkirkia, trilobites are always found with their heads directed towards the opening of the tube, suggesting that they reversed in; the absence of any moulted carapaces suggests that moulting was not their primary reason for seeking shelter.[8]

References

  1. ^
    PMID 30963877
    .
  2. S2CID 233184869
    .
  3. ISBN 8200075117
    .
  4. ISBN 978-1-4200-3754-8
    .
  5. ISBN 978-1-4200-3754-8
    .
  6. ^ McMenamin MA (October 2010). "Cambrian cannibals: agnostid trilobite ethology and the earliest known case of arthropod cannibalism". Geological Society of America Abstracts with Programs. 42 (5): 320.
  7. ^ Fatka O, Vokáč V, Moravec J, Šinágl M, Valent M (January 2009). "Agnostids entombed in hyolith conchs". Memoirs of the Association of Australasian Palaeontologists. 37: 481–489.
  8. ^
    ISBN 978-0-901702-81-4
    .

External links
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