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

Abstract

Lotagnostus is a biostratigraphically important late Furongian (late Cambrian) agnostoid trilobite whose interspecific and intraspecific variability is currently under discussion. In this context, type material of Lotagnostus atenuatus Rusconi, from an open-marine black limestone allochthonous block (La Cruz Olistolith) of Quebrada San Isidro, Precordillera of Mendoza, western Argentina, is revised herein. It includes several three-dimensionally preserved sclerites, allowing for an emended diagnosis of this species and comprehensive comparisons with other forms of Lotagnostus. Although to date L. atenuatus can be identified with confidence only at Quebrada San Isidro, it strongly resembles fragmentary material of Lotagnostus sp. indet., from late Furongian dysoxic upper slope facies of the Central Appalachians in eastern North America. The olenid Mendoparabolina brevicauda Rusconi, which is recorded in association with L. atenuatus, is also redescribed here. Mendoparabolina is closely allied with Wujiajiania Lu and Lin and Bienvillia Clark, differing by having a proportionately long and transversely bilobate pygidium with a posteriorly truncate axis and a postaxial median ridge.

Taxonomy and stratigraphic distribution of Lotagnostus (Agnostida: Agnostidae) and associated trilobites and conodonts in the Upper Cambrian (Furongian) of Laurentia

“…Yet the range of intraspecific variation within any of the synonymized species has not been objectively established with large, stratigraphically constrained collections from a single bed or thin interval. In fact, such documentation provided in recent studies of other species of Lotagnostus confirms only modest amounts of intergrading morphologic variation (Rushton, 2009, for Avalonian Britain;Tortello, 2014, 2018Westrop et al, 2011, andLanding, 2016, for Avalonian Canada). These observations reinforce concerns expressed by some (e.g., Westrop & Landing, 2016, p. 8) that the exceptionally broad range of variation claimed for L. americanus sensu Peng et al (2015) was actually the product of a biostratigraphically motivated consolidation of multiple species to create a single taxon with nearly global distribution.…”

“…The range for Lotagnostus hedini in Figure 8 is based on all reported occurrences of the species, particularly those in Asia Apollonov et al, 1988), where associated conodont faunas facilitate precise correlation into the Laurentian biostratigraphy. Thorough summaries of the areas and stratigraphic intervals where it has been reported were provided by Westrop et al (2011), and more recently by Tortello et al (2018), the latter utilizing the relatively narrow range for L. hedini to more precisely constrain the age of the Parabolina frequens argentina Zone in the Cordillera Oriental of Argentina. Only one correction is required to those summaries; they mistakenly place the range of L. hedini in Kazakhstan in the Euloma limitaris-taoyuania Zone of that region, the base of which is confirmed by associated conodonts to coincide with the base of the Cordylodus proavus Zone.…”

Elviniid trilobites from the Elvinia Zone (late Cambrian, Furongian) of Mendoza, western Argentina

“…There, the carbonate-dominated San Martín, San Isidro, and La Cruz olistoliths (= La Cruz Limestones sensu Keller, 1999), as well as other minor exotic blocks, contain trilobites of middle and late Cambrian age. The oldest olistoliths yield trilobites of the Glossopleura Zone (early middle Cambrian), whereas the youngest blocks are assignable to the late Furongian Saukia Zone (e.g., Rusconi, 1956a and references therein; Borrello, 1971; Shergold et al, 1995; Bordonaro and Banchig, 1996; Keller, 1999; Bordonaro, 2003a, 2014; Bordonaro and Fojo, 2011; Tortello, 2014, 2017, 2018).

Figure 1.Map of the San Isidro area, Mendoza Province, Argentina (after Bordonaro et al, 1993 and Tortello, 2014).

…”

Trilobites from theCedaria prolificaZone (Cambrian, upper Guzhangian) of the Precordillera of Mendoza, western Argentina

“…It is now recognized that Cambrian trilobites from the Argentinian Precordillera have clear Laurentian affinities. Recent systematic revisions of the Rusconi collections provided updated information on several standard North American trilobite zones such as the Glossopleura walcotti , Ehmaniella , Lejopyge laevigata , Elvinia and Saukia zones (Tortello and Bordonaro, 1997; Bordonaro, 2003, 2014; Bordonaro and Fojo, 2011; Pratt and Bordonaro, 2014; Tortello, 2018, 2020, 2022).

Figure 1.Map of the San Isidro area, Mendoza Province, Argentina (after Bordonaro et al, 1993 and Tortello, 2022).

…”