Abstract
Field-based revision and phylogenetic analysis demonstrate that the pliomerid trilobite taxon Ibexaspis Přibyl and Vaněk (in Přibyl et al., 1985), previously known from a single formally named species (I. brevis [Young, 1973]), belongs to a complex of 14 mostly newly discovered, related species from the Early Ordovician (Floian; Tulean and Blackhillsian) of the northern Laurentian margin. The species are known from silicified samples recovered from sections in eastern Nevada, western Utah, and southeastern Idaho. The stratigraphically early Tuleaspis n. gen. (type species: T. jeneki n. sp.; Tulean; low Protopliomerella contracta Zone) includes its type and two species described in open nomenclature. Tuleaspis is sister to the remainder of the clade. Ibexaspis now includes three additional species: I. coadyi n. sp. (Blackhillsian; Carolinites nevadensis Zone), I. leuppi n. sp. (Blackhillsian; Presbynileus ibexensis Zone), and I. rupauli n. sp. (Blackhillsian; "Pseudocybele nasuta Zone"). Ibexapsis is sister to a clade of Millardaspis n. gen. + Deltapliomera n. gen. Millardaspis (type species M. milsteadi n. sp.; Tulean; Heckethornia hyndeae Zone) also includes M. knoxi n. sp. (Tulean; Panisaspis sevierensis Zone). Deltapliomera (type species D. humphriesi n. sp.; Blackhillsian, Carolinites nevadensis Zone) also includes D. inglei n. sp. (Tulean; Heckethornia bowiei Zone), D. heimbergi (Tulean; Panisaspis sevierensis Zone), D. eppersoni n. sp. (Blackhillsian; Bathyurina plicolabeona Zone), and a species described in open nomenclature.
References
Adrain, J.M. & McAdams, N.E.B. (2012) The Lower Ordovician (upper Floian) bathyurid trilobite Aponileus Hu, with species from Utah, Texas, and Greenland. Zootaxa, 3293, 1–67.
Adrain, J.M., McAdams, N.E.B. & Westrop, S.R. (2009) Trilobite biostratigraphy and revised bases of the Tulean and Blackhillsian Stages of the Ibexian Series, Lower Ordovician, western United States. Memoirs of the Association of Australasian Palaeontologists, 37, 541–610.
Adrain, J.M., Karim, T.S. & Westrop, S.R. (2014) The Early Ordovician (Floian) bathyurid trilobite genera Jeffersonia, Cullisonia and Bathyurina. Memoirs of the Association of Australasian Palaeontologists, 45, 305–347.
Adrain, J.M., McAdams, N.E.B. & Westrop, S.R. (2011a) Affinities of the Lower Ordovician (Tulean; lower Floian) trilobite Gladiatoria, with species from the Great Basin, western United States. Memoirs of the Association of Australasian Palaeontologists, 42, 321–367.
Adrain, J.M., McAdams, N.E.B., Westrop, S.R. & Karim, T.S. (2011b) Systematics and affinity of the Lower Ordovician (Tulean; lower Floian) trilobite Psalikilopsis. Memoirs of the Association of Australasian Palaeontologists, 42, 369–416.
Barton, D.C. (1915) A revision of the Cheirurinae, with notes on their evolution. Washington University Studies, 3, 102–152.
Bremer, K. (1994) Branch support and tree stability. Cladistics, 10, 295–304.
https://doi.org/10.1111/j.1096-0031.1994.tb00179.xDemeter, E.J. (1973) Lower Ordovician pliomerid trilobites from western Utah. Brigham Young University Geology Studies, 20, 37–65.
Goloboff, P.A., Farris, J.S., Källersjö, M., Oxelman, B., Ramírez, M.J. & Szumik, C.A. (2003) Improvements to resampling measures of group support. Cladistics, 19, 324–332.
https://doi.org/10.1111/j.1096-0031.2003.tb00376.xGoloboff, P.A., Farris, J.S. & Nixon, K.C. (2008) TNT, a free program for phylogenetic analysis. Cladistics, 24, 774–786.
https://doi.org/10.1111/j.1096-0031.2008.00217.xHarrington, H.J. (1957) Notes on new genera of Pliomeridae (Trilobita). Journal of Paleontology, 31, 811–812.
Hintze, L.F. (1953) Lower Ordovician trilobites from western Utah and eastern Nevada. Utah Geological and Mineralogical Survey Bulletin, 48, 1–249. [for 1952]
Holliday, S. (1942) Ordovician trilobites from Nevada. Journal of Paleontology, 16, 471–478.
Jell, P.A. & Adrain, J.M. (2003) Available generic names for trilobites. Memoirs of the Queensland Museum, 48, 331–553.
Kobayashi, T. (1934) The Cambro-Ordovician formations and faunas of South Chosen. Palaeontology. Part II. Lower Ordovician faunas. Journal of the Faculty of Science, Imperial University of Tokyo, Section 2, 3, 521–585.
McAdams, N.E.B. & Adrain, J.M. (2009) New pliomerid trilobite genus Lemureops from the Lower Ordovician (Ibexian; Tulean, Blackhillsian) of western Utah, USA. Memoirs of the Association of Australasian Palaeontologists, 37, 491–540.
McAdams, N.E.B. & Adrain, J.M. (2010a) A new species of the Lower Ordovician pliomerid trilobite Pseudocybele and its biostratigraphic significance. Zootaxa, 2550, 21–38.
McAdams, N.E.B. & Adrain, J.M. (2010b) Phylogeny of the Early Ordovician trilobites Ibexaspis, Pseudomera, and related taxa: Reconstructing Pliomeridae from the ground up. Geological Society of America Abstracts with Programs, 42 (2), 39.
McAdams, N.E.B. & Adrain, J.M. (2011a) Systematics of the Lower Ordovician pliomerid trilobite Hintzeia, with species from the Great Basin, western USA. Zootaxa, 2910, 1–45.
McAdams, N.E.B. & Adrain, J.M. (2011b) Panisaspis, a new genus of pliomerid trilobites from the Lower Ordovician (Ibexian; Tulean and Blackhillsian) of the Great Basin, western USA. Zootaxa, 2969, 1–68.
McAdams, N.E.B. & Adrain, J.M. (2011c) Revision of the Lower Ordovician (lower Floian; Tulean) pliomerid trilobite Protopliomerella, with new species from the Great Basin, western USA. Zootaxa, 3144, 1–113.
Nixon, K.C. (2002) WinClada. Version 1.0000. Published by the author, Ithaca, New York, USA.
Peng, S.-C. (1990) Tremadoc stratigraphy and trilobite faunas of northwestern Hunan. 2. Trilobites from the Panjiazui Formation and the Madaoyu Formation in Jiangnan Slope Belt. Beringeria, 2, 55–171.
Poulsen, C. (1927) The Cambrian, Ozarkian and Canadian faunas of northwest Greenland. Meddelelser om Grønland, 70, 235–343.
Přibyl, A., Vaněk, J. & Pek, I. (1985) Phylogeny and taxonomy of family Cheiruridae (Trilobita). Acta Universitatis Palackianae Olomucensis Facultas rerum naturalium, 83, 107–193.
Raymond, P.E. (1913) Subclass Trilobita. In: Eastman, C.R. (Ed.), Text-book of Paleontology Vol. 1. 2nd Edition. MacMillan, New York, pp. 607–638.
Ross, R.J., Jr. (1951) Stratigraphy of the Garden City Formation in northeastern Utah, and its trilobite faunas. Peabody Museum of Natural History, Yale University Bulletin, 6, 1–161.
Ross, R.J., Jr, Hintze, L.F., Ethington, R.L., Miller, J.F., Taylor, M.E. & Repetski, J.E. (1997) The Ibexian, lowermost series in the North American Ordovician. United States Geological Survey Professional Paper, 1579, 1–50.
Rozova, A.V. (1960) [Upper Cambrian trilobites from Salair (Tolsochikninsk Suite)]. Trudy Instituta Geologii i Geofiziki. Sibirskoe Otdelenie, Akademiya Nauk SSSR, 5, 1–116. [in Russian]
Schmidt, F. (1881) Revision der ostbaltischen silurischen Trilobiten. Abtheilung I: Phacopiden, Cheiruriden und Encrinuriden. Mémoires de l’Academie Impériale des Sciences de St.-Petersbourg, VII’e Série, 30, 1–237.
Sorenson, M.D. & Franzosa, E.A. (2007) TreeRot. Version 3. Boston University, Boston, Massachussetts.
Swofford, D.L. (2003) PAUP*: Phylogenetic analysis using parsimony (*and other methods). Version 4.0b10. Sinauer Associates, Sunderland.
Young, G.E. (1973) An Ordovician (Arenigian) trilobite faunule of great diversity from the Ibex area, western Utah. Brigham Young University Geology Studies, 20, 91–115.