A late Furongian trilobite assemblage from the eastern Cordillera Oriental ( Santa Rosita Formation ; Jujuy , Argentina ) and its biostratigraphic significance

The trilobite Parabolina frequens argentina Zone is widely represented in the upper Furongian of northwestern Argentina. A fossil assemblage from the lower Santa Rosita Formation at Quebrada de Abra Blanca, Jujuy Province, is described herein. Parabolina frequens argentina (Kayser) and Parabolinella coelatifrons Harrington & Leanza have previously been reported from other localities of the biozone, whereas Lotagnostus hedini (Troedsson) is here described from South America for the first time. The latter provides high resolution data on the age of the P. frequens argentina Zone in the eastern Cordillera Oriental, where conodont-bearing strata have not been found yet. L. hedini exhibits a short biostratigraphic range, restricted to the latest Furongian of China (Xinjiang, Bulbolenus Zone; western Zhejiang, Lotagnostus hedini Zone), Kazakhstan (Euloma limitarisTaoyuania Zone) and ?Canada (western Newfoundland, Phylacterus saylesi Fauna). In China and Kazakhstan, it occurs in association with, or a few meters below, the first appearance of the conodont Cordylodus proavus; a fact that increases the potential for global correlation.

In addition, there are still some outcrops that require further sampling and comprehensive systematic and biostratigraphic studies.In this regard, the Huacalera region is of particular importance (Fig. 1.B).In Quebrada de La Huerta, Manca (1992) reported the occurrence of P. frequens argentina in association with fragmentary material of Lotagnostus Whitehouse, 1936, a key fossil of Furongian strata of different continents.In addition, Di Cunzolo (2006) and Zeballo (2010, unpublished) mentioned the presence of P. frequens argentina in Quebrada de Abra Blanca, although detailed systematic study was not achieved.Based on new collections, the present paper includes a description of the trilobite assemblages from Quebrada de Abra Blanca and a discussion on their biostratigraphic implications.The material studied provides precise information on the age of the P. frequens argentina Zone, which correlates with late Furongian successions of China, Kazakhstan and Canada.

GEOLOGICAL SETTING
The Cordillera Oriental is a high relief thrust system that is delimited to the east by the Sierras Subandinas and to the west by the Puna.Lower Paleozoic rocks are well represented in the mountain ranges of this geological province, including Cambrian quarzites of the Mesón Group and late Furongian-Early Ordovician shales and sandstones of the Santa Victoria Group (Santa Rosita and Acoite formations and equivalents), which are juxtaposed on Neoproterozoic-early Cambrian slates and meta-graywackes of the Puncoviscana Formation (e.g., Harrington & Leanza, 1957;Turner, 1960;Turner & Mon, 1979;Moya, 1988;Ramos, 1999Ramos, , 2008;;Astini, 2003Astini, , 2008)).
The trilobites studied herein come from the discoloured siltstones and shales of the Casa Colorada Member exposed at the Abra Blanca creek (Fig. 2.A, B), about 3 km southeast of Huacalera village, Jujuy Province (Fig. 1.A, B).There, a 110 m-thick section displays thinly bedded yellowish and light brown shales with intercalations of sandstones, which overlie the sandstones of the Tilcara Member and are overlain by Quaternary deposits.
The first evidence of the genus Lotagnostus in the Cordillera Oriental was provided by Manca (1992), who described one cephalon and one fragmentary pygidium (Lotagnostus sp.) associated with Parabolina frequens argentina from Quebrada de la Huerta (Fig. 1).Because Lotagnostus is a cosmopolitan key fossil of late Furongian age, (e.g., Shergold & Laurie, 1997 and references), its finding in the P. frequens argentina Zone provided strong evidence in favor of a pre-Tremadocian age for this unit.Subsequently Esteban & Tortello (2007) described new material of Lotagnostus (Lotagnostus shergoldi Tortello; L. llamaensis Tortello), associated with P. frequens argentina and other species of the biozone, from different localities of the western Cordillera Oriental (Lampazar Formation; Sierra de Cajas, Angosto del Moreno, Quebrada de Jueya).
The record of Lotagnostus hedini (Troedsson, 1937) in the Quebrada de Abra Blanca provides new high resolution data on the age of the Parabolina frequens argentina Zone within the late Furongian.This information is novel for the eastern part (Eastern Belt sensu Astini, 2003) of the Cordillera Oriental (lower Santa Rosita Formation, Casa Colorada Member), where conodont-bearing strata have not been found yet (Zeballo & Albanesi, 2013).Lotagnostus hedini is one of the youngest species of Lotagnostus and exhibits a short biostratigraphic range, restricted to the latest Furongian.It is known from the Bulbolenus Zone of Xinjiang, Northwest China (Troedsson, 1937), the Lotagnostus hedini Zone of western Zhejiang, China (Lu & Lin, 1980, 1981, 1983a-c, 1984, 1989), the Euloma limitaris-Taoyuania Zone of the Malyi Karatau Range of Kazakhstan (Apollonov et al., 1984), and, probably, the Phylacterus saylesi Fauna of western Newfoundland (Ludvigsen et al., 1989; see Westrop et al., 2011).In Zhejiang and Kazakhstan, L. hedini is recorded with, or a few meters below, the first appearance (FAD) of Cordylodus proavus.
The occurrence of L. hedini in the P. frequens argentina Zone is in line with the global upper Cambrian correlation chart of Shergold (1988, fig.2).Strata with L. hedini correlate with the Acerocare Zone of Avalon and the Baltic region.Rushton (1982) and Żylińska (2001) recognized olenid faunas including Parabolina frequens in Wales and Poland, respectively, below the record of the graptolite Rhabdinopora (Rushton, 1982).Fragmentary material of P. frequens has also been described from the late Furongian of Mexico (Tiñu Formation), from levels of the Cordylodus proavus Zone (Robison & Pantoja-Alor, 1968;Landing et al., 2007).

SYSTEMATIC PALEONTOLOGY
The material is housed in the Museo de Paleontología, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba (CORD-PZ), and the Museo de La Plata, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata (MLP).Specimens were whitened with magnesium oxide vapors before photographing.The morphologi- The morphological terms used below have been mostly defined by Robison (1964), Shergold et al. (1990) and Whittington & Kelly (1997).Abbreviations: sag sagittally; exs exsagittally.
Remarks.The diagnosis of Lotagnostus was discussed in detail by Shergold & Laurie (1997 and references) and Westrop et al. (2011).Westrop (1995) and Westrop et al. (2011) regarded the partially effaced taxon Distagnostus Shergold, 1972, as a junior synonym of Lotagnostus, a decision that is accepted herein.

.E).
As stated above, Manca (1992) described scarce material of Lotagnostus sp. from Quebrada de La Huerta, consisting of one cephalon and one incomplete pygidium.The latter apparently differs from L. hedini in having a discontinuous F1 and a much shorter (sag.)posteroaxis; however, the deficient state of preservation of this specimen prevents a proper comparison.Lotagnostus shergoldi Tortello in Esteban & Tortello (2007, fig.8A-I, L) and L. llamaensis Tortello in Esteban & Tortello (2007, fig.8J-K, M-Q), from the western Cordillera Oriental, are clearly distinguished from L. hedini because the former have a partially effaced exoskeleton, a more anteriorly positioned glabellar node, a discontinu-ous F1 pygidial furrow, an unconstricted pygidial acrolobe, and a border lacking posterolateral spines.Lotagnostus shergoldi has, in addition, a less constricted pygidial M2, and L. llamaensis exhibits a semicircular cephalon and a more delicate pygidial axial node.
Remarks.Parabolinella coelatifrons Harrington & Leanza is well represented in the Parabolina frequens argentina Biozone (Harrington & Leanza, 1957;Tortello & Esteban, 2003;Esteban & Tortello, 2007).This species is characterized by the presence of a well-developed, faintly striated preglabellar field; a sub-squared glabella having three pairs of lateral glabellar furrows; a facial suture with sub-parallel anterior branches and straight posterior branches, defining sub-triangular fixigenae; and a pygidium with two axial rings.Specimens from Quebrada de Abra Blanca combine all of these characters, but also display some intraspecific variability which broadens the scope of P. coelatifrons.Variations involve the degree of development of the preglabellar field (Fig. 3.I, L), the shape of the anterior glabellar margin (Fig. 3.D, G, J), and the width of the posterior margin of the fixigenae (Fig. 3.D, F, I).Esteban & Tortello (2009, fig. 11.C, D, H, J) assigned to Parabolinella cf.coelatifrons several cranidia from the Iruya area, pointing out that their preglabellar furrows are more curved than those of the typical representatives of P. coelatifrons.However, according to the variability reported above, these cranidia are now regarded confidently as belonging to this species.Some specimens from Quebrada de Abra Blanca show the librigenae displaced backwards below the axial shield (Fig. 3.C, F, G).This configuration is common among olenids (e.g.Henningmoen, 1957;Clarkson et al., 2003;Tortello & Clarkson, 2003) and is interpreted as the result of the molting process (Harrington, 1959;Henningsmoen, 1975).
Remarks.Parabolina frequens argentina is an emblematic late Cambrian trilobite of northwestern Argentina and southern Bolivia (Harrington & Leanza, 1957;Waisfeld & Vaccari, 2003).As stated by Harrington & Leanza (1957) and Tortello & Clarkson (2008), this olenid exhibits a relatively high morphologic variability.In the material from Quebrada de Abra Blanca, variations are especially evident in the width (sag.) of the cephalic anterior border (Fig. 4.A, G), and the degree of expression of the thoracic axial nodes, which are better developed in small adults (e.g., compare Fig. 4.F with Fig. 4.K).It is interesting to note that some late holaspid specimens studied herein reach large sizes (largest observed exoskeleton -excluding 12 th thoracic axial spine-: 43mm in length).

Fig. 2 .
Fig. 2. Lower part of the Santa Rosita Formation at Quebrada de Abra Blanca, Huacalera area, Jujuy.A, stratigraphic section showing distributions of trilobites identified.B, photograph showing part of the outcrop.