The agnostoid arthropod Lotagnostus Whitehouse, 1936 (late Cambrian; Furongian) from Avalonian Cape Breton Island (Nova Scotia, Canada) and its significance for international correlation

Abstract

New and archival collections from the Chelsey Drive Group of the Avalon terrane of Cape Breton Island, Nova Scotia, Canada, yield late Cambrian trilobites and agnostoid arthropods with full convexity that contrast with compacted, often deformed material from shale and slate typical of Avalonian Britain. Four species of the agnostoid Lotagnostus form a stratigraphic succession in the upper Furongian (Ctenopyge tumida–Parabolina lobata zones). Two species, L. ponepunctus (Matthew, 1901) and L. germanus (Matthew, 1901) are previously named; L. salteri and L. matthewi are new. Lotagnostus trisectus (Salter, 1864), the type species of the genus, is restricted to compacted material from its type area in Malvern, England. Lotagnostus americanus (Billings, 1860) has been proposed as a globally appropriate index for the base of ‘Stage 10’ of the Cambrian. All four species from Avalonian Canada are differentiated clearly from L. americanus in its type area in Laurentian North America (i.e., from debris flow blocks in Taconian Quebec). In our view, putative occurrences of L. americanus from other Cambrian continents record very different species. Lotagnostus americanus cannot be recognized worldwide, and other taxa should be sought to define the base of Stage 10, such as the conodont Eoconodontus notchhpeakensis.

Redescription of a Lotagnostus–Mendoparabolina faunule (Trilobita; late Furongian) from Quebrada San Isidro, Precordillera of Mendoza, Argentina

“…Unfortunately, there is no consensus amongst colleagues about the definitions of several species of Lotagnostus. As mentioned above, Peng and Babcock (2005) and Peng et al (2015) regard L. americanus (Billings, 1860) as a cosmopolitan, morphologically variable taxon with an extensive synonymy and a broad habitat range, whereas Westrop et al (2011) and Westrop and Landing (2016) prefer to restrict it to material from the type area in Quebec, and recognize morphological differences of specific value between this taxon and a plexus of narrowly defined species [e.g., L. trisectus; L. asiaticus; L. germanus (Matthew, 1901); L. matthewi Westrop and Landing, 2016;L. salteri Westrop and Landing, 2016].…”

“…12) (see "Systematic palaeontology" below). The latter occurs in association with trilobites that indicate a correlation with either the Onchonotus richardsoni or Keithia subclavata faunas of the Cow Head Group of western Newfoundland (Ludvigsen et al 1989;Westrop and Landing 2016).…”

“…Species of Lotagnostus are currently defined on the basis of axial characters, the degree of effacement of the exoskeleton, the morphology of the acrolobe, the genal convexity and ornament, and the morphology of the border and border furrows (e.g., Peng et al 2015;Westrop and Landing 2016). Describing the ontogenetic series is particularly important when characterizing species of this genus.…”

“…Describing the ontogenetic series is particularly important when characterizing species of this genus. Because many characters vary ontogenetically (e.g., density and incision of cephalic scrobiculae; length of pygidial axis), comparisons between species must be based on similarly sized specimens (Westrop and Landing 2016). In addition, particular attention must be paid to recognizing variations of taphonomic origin, which can involve the length of the basal lobe, as well as the expression of delicate features such as the glabellar furrow F2, the intranotular axis, and the terminal node.…”

Pseudocryptic species of the Middle Cambrian trilobiteEodiscusHartt, in Walcott, 1884, from Avalonian and Laurentian Newfoundland

“…We paid close attention to all characters in diagnosing and differentiating species, including surface sculpture, and applied a phylogenetic species concept (see also Westrop and Adrain 2016;Westrop and Landing 2017). The emergence of genomic data as powerful tools for species delimitation among modern organisms has been accompanied by the recognition that many species are cryptic and not diagnosable by conventional morphologic criteria (e.g., Ladner and Palumbi 2012).…”