Classification of the Late Cambrian (Steptoean) trilobite genera Cheilocephalus Berkey, 1898 and Oligometopus Resser, 1936 from Laurentia

Abstract

All recent work has assigned the genera Cheilocephalus Berkey, 1898 and Oligometopus Resser, 1936 to the family Cheilocephalidae Shaw, 1956, a small group of largely Laurentian trilobites from the Late Cambrian Steptoean Stage. New data from Nevada and Newfoundland indicate that the Cheilocephalidae is polyphyletic and that Cheilocephalus and Oligometopus are allied with different superfamilies. The Cheilocephalidae, which includes Cheilocephalus and Pseudokingstonia Palmer, 1965, is assigned to the Dameselloidea Kobayashi, 1935, and a pygidium from the Cow Head Group of Newfoundland indicates that Cheilocephalus first appears in the late Marjuman. The Dameselloidea disappeared on other continents at an extinction interval at the end of the Marjuman, but Cheilocephalus survived and underwent modest diversification in Laurentia in the succeeding Steptoean Stage. Oligometopus is transferred to the leiostegioidean family Leiostegiidae Bradley, 1925. The genus shows closest affinities with such Gondwanan genera as Idamea Whitehouse, 1939, and its appearance in mid-Steptoean strata records immigration of the Leiostegoidea into shelf-margin environments of Laurentia.

The mid-Cambrian (Series 3, Guzhangian; Marjuman) trilobite Deiracephalus Resser, 1935, from western Newfoundland

“…Shelf-derived boulders accumulated as debris flows in a continental slope setting (James and Stevens 1986) to be later emplaced on the shelf during the orogeny (Lavoie et al 2003). This paper continues a long-term project to document the diverse trilobites and agnostoid arthropods of the Shallow Bay Formation (Ludvigsen et al 1989;Westrop et al 1996;Westrop and Ludvigsen 2000;Westrop et al 2008;Westrop and Eoff 2012;Westrop and Dengler 2014). Kindle (1982) assigned faunas that include Deiracephalus to his Zone 5, which likely correlates with the late Guzhangian (late Marjuman) Cedaria prolifica and C. brevifrons zones of the Mackenzie Mountains (Pratt 1992; note that this interval is entirely younger than the C. prolifica Zone as proposed by Babcock et al 2014), and the Crepicephalus Zone of the traditional North American zonation (Westrop et al 1996).…”

The Cambrian (Paibian–Jiangshanian; Steptoean) dokimocephalid trilobite Deckera Frederickson, 1949 in Laurentian North America

“…There could be as many as nine species in Steptoean strata of Laurentia, although small sample sizes prevent most of them from being named formally. This paper also continues a long-term project revising the diverse agnostid and trilobite faunas of the Elvinia Zone of the Great Basin and Oklahoma (Westrop et al, , 2008(Westrop et al, , 2010Westrop and Adrain, 2009a, b, 2013, 2016.…”

Biogeographic shifts in a transgressive succession: the Cambrian (Furongian, Jiangshanian; Latest Steptoean–earliest Sunwaptan) agnostoid arthropodsKormagnostellaRomanenko andBiciragnostusErgaliev in North America

“…Other unequivocal immigrants recorded in our collections include the trilobites Comanchia Frederickson, in Wilson andFrederickson, 1950, andAciculolenus Palmer, 1965b. Lower in the transgressive systems tract (upper Corset Spring Shale), the appearances of two genera with Gondwanan affinities, Oligometopus Resser, 1936(Westrop et al, 2008 and Xenocheilos Wilson, 1949 (Westrop andAdrain, 2009), also record immigration, although Oligometopus is known from older (Dunderbergia Zone) strata in shelf margin environments of Alaska and Newfoundland. Moreover, this pattern may extend beyond trilobites.…”