Agnostida

Cephalon of Peratagnostus cf. nobilis, from Shergold (1980).

Belongs within: Euarthropoda.
Contains: Lejopyge, Diplagnostidae, Agnostinae.

The Agnostida are a distinctive group of fossil arthropods known from the Cambrian and Ordovician. They were mostly blind, and may have been planktic or pseudoplanktic (Prothero 1998). The affinities of Agnostida have been subject to debate. They have most commonly been regarded as derived trilobites, as indicated by the exoskeletal features of the plesiomorphic Eodiscina. However, preserved appendages of Agnostus are quite different from those known from other trilobites, casting doubt on this relationship (Fortey 2001).

Characters (from Prothero 1998): Isopygous, with button-shaped pygidium equal in size to cephalon; thorax with only two or three thoracic segments. Usually eyeless.

Agnostida [Miomera]
    |--Eodiscina P98
    |    |--Calodiscus P79
    |    |--Serrodiscus bellimarginatus P79, L95
    |    |--Triangulaspis Lermontova 1940 P79, SS84
    |    |    `--T. vigilans L95
    |    `--Hebediscus P79
    `--Agnostina S80
         |--Diplagnostidae S80
         |--Agnostidae S80
         |    |--Agnostinae S80
         |    |--Hypagnostus Jaekel 1909 SS84 [incl. Cyclopagnostus Howell 1937 S80; Quadragnostinae SS84]
         |    |     `--H. parvifrons P79
         |    `--Quadrahomagnostinae S80
         |         |--Phalacroma Hawle & Corda 1847 FT93
         |         `--Peratagnostus Öpik 1967 S80
         |              |--*P. nobilis Öpik 1967 S80
         |              |--‘Cyclopagnostus’ asper Lazarenko 1966 S80
         |              |--‘Leiopyge’ controversa Kryskov in Borovikov & Kryskov 1963 S80
         |              |--P. hillardensis Palmer 1968 S80
         |              |--‘Agnostus (Leiopyge)’ obsoletus Kobayashi 1935 [=Phoidagnostus obsoletus] S80
         |              |--‘Cyclopagnostus’ orientalis Lazarenko 1966 S80
         |              `--‘Phalacroma’ sinica Hsiang in Jegorova et al. 1963 S80
         `--Ammagnostidae [Ammagnostinae] HLC03
              |--Ammagnostus Öpik 1967 HLC03
              |--Proagnostus Butts 1926 HLC03
              |--Hadragnostus Öpik 1967 HLC03
              `--Kormagnostus Resser 1938 SS84

Agnostida incertae sedis:
  Clavagnostidae HLC03
    |--Clavagnostus Howell 1937 SS84
    |    `--C. calensis Rusconi 1950 TS11
    `--Aspidagnostus HLC03
  Peronopsidae HLC03
    |--Peronopsis HLC03
    |    |--P. elkedraensis (Etheridge 1902) [=Agnostus elkedraensis, Diplorrhina elkedraensis] F71
    |    `--P. interstrictus P98
    `--Lisogoragnostus Rozova in Lisogor et al. 1988 HLC03
         |--*L. kalisae Rozova in Lisogor et al. 1988 HLC03
         |--L. circularis (Yang & Liu in Yang et al. 1991) HLC03
         |--L. coreanicus Hong, Lee & Choi 2003 HLC03
         |--L. hybus Peng & Robison 2000 HLC03
         |--L. mictus Peng & Robison 2000 HLC03
         |--L. minor (Kobayashi 1962) [=Phalacromina minor] HLC03
         |--L. rasettii (Pratt 1992) HLC03
         `--L. shergoldi (Pratt 1992) HLC03
  Ptychagnostidae HLC03
    |--Lejopyge HLC03
    `--Ptychagnostus P79
         |--P. atavus P79
         |--P. cassis P79
         |--P. gibbus P79
         |--P. lundgreni P79
         |--P. nathorsti P79
         `--P. punctuosus P79
  Glyptagnostus F01
    |--G. reticulatus F01
    `--G. stolidotus F01
  Geragnostus Howell 1935 [Metagnostidae] SS84
    `--G. subobesus LM05
  Geragnostella tullbergi FO99
  Corrugatagnostus morea FO99
  Litagnostus meniscus FO99
  Strictagnostus Shergold 1975 SS84
  Plurinodus Öpik 1967 [Trinodidae] SS84
  Kormagnostella Romanenko 1967 [incl. Litagnostoides Schrank 1975] SS84

*Type species of generic name indicated

REFERENCES

[FT93] Fensome, R. A., F. J. R. Taylor, G. Norris, W. A. S. Sarjeant, D. I. Wharton & G. L. Williams. 1993. A classification of living and fossil dinoflagellates. Micropaleontology Special Publication 7: i–viii, 1–351.

[F71] Fletcher, H. O. 1971. Catalogue of type specimens of fossils in the Australian Museum, Sydney. Australian Museum Memoir 13: 1–167.

[F01] Fortey, R. A. 2001. Trilobite systematics: the last 75 years. Journal of Paleontology 75 (6): 1141–1151.

[FO99] Fortey, R. A. & R. M. Owens. 1999. Feeding habits in trilobites. Palaeontology 42 (3): 429–465.

[HLC03] Hong, P. S., J. G. Lee & D. K. Choi. 2003. Trilobites from the Lejopyge armata zone (upper Middle Cambrian) of the Machari Formation, Yongwol Group, Korea. Journal of Paleontology 77 (5): 895–907.

[L95] Landing, E. 1995. Upper Placentian–Branchian series of mainland Nova Scotia (middle–upper Lower Cambrian): faunas, paleoenvironments, and stratigraphic revision. Journal of Paleontology 69 (3): 475–495.

[LM05] Lehnert, O., J. F. Miller, S. A. Leslie, J. E. Repetski & R. L. Ethington. 2005. Cambro-Ordovician sea-level fluctuations and sequence boundaries: the missing record and the evolution of new taxa. Special Papers in Palaeontology 73: 117–134.

[P79] Palmer, A. R. 1979. Cambrian. In: Robison, R. A., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt A. Introduction. Fossilisation (Taphonomy), Biogeography and Biostratigraphy pp. A119–A135. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

[P98] Prothero, D. R. 1998. Bringing Fossils to Life: An introduction to paleobiology. WCB McGraw-Hill: Boston.

[S80] Shergold, J. H. 1980. Late Cambrian trilobites from the Chatsworth Limestone, western Queensland. Bureau of Mineral Resources, Geology and Geophysics—Bulletin 186: 1–111.

[SS84] Shergold, J. H., & K. Sdzuy. 1984. Cambrian and early Tremadocian trilobites from Sultan Dağ, central Turkey. Senckenbergiana Lethaea 65 (1–3): 51–135.

[TS11] Tortello, M. F., & N. M. Sabattini. 2011. Totoralia, a new conical-shaped mollusk from the Middle Cambrian of western Argentina. Geologica Acta 9 (2): 175–185.