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

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.

Interest in Lotagnostus has greatly increased in recent years because Lotagnostus americanus (Billings, 1860) is proposed as an index species for the definition of the youngest stage (Stage 10) within the Furongian Series; a subject that is currently being considered by the International Subcommission on Cambrian Stratigraphy (ISCS).Peng and Babcock (2005) and Peng et al. (2015) regard L. americanus as a morphologically variable taxon and proposed an extensive synonymy for it, including species [e.g., L. trisectus (Salter, 1864a), L. asiaticus Troedsson, 1937, L. punctatus Lu, 1964, among others] from numerous localities of most continents.However, this synonymy was questioned by Westrop et al. (2011), who, in addition, consider that L. americanus can be identified with confidence only at its type locality of Quebec, Canada.Westrop et al. (2011) and Peng et al. (2015) use dissimilar criteria for establishing limits between species of Lotagnostus.In any case, large collections of uncompacted sclerites of different sizes are particularly useful in systematic analysis, since they allow a complete evaluation of taxonomically important characters such as the original convexity and profile of the gena, and the degree of expression of dorsal furrows, lobes, and sculpture.
Carbonate rocks of the Precordillera Terrane of western Argentina contain abundant and well-preserved material of two distinct Lotagnostus species, L. peladensis (Rusconi, 1951) from Cerro Pelado and L. atenuatus (Rusconi, 1955a) from Quebrada San Isidro (Fig. 1).Although Lotagnostus peladensis was originally described on the basis of a few specimens (Rusconi 1951), further collections from the type locality led to deeper knowledge of its morphology and its stratigraphic significance (Borrello 1971;Shergold et al. 1995;Tortello 2014a).Lotagnostus peladensis is a variably effaced species that is closely allied with L. obscurus Palmer, 1955 and L. "sp. indet."from western and northwestern North America (Tortello 2014a).It largely dominates the trilobite assemblages from Cerro Pelado, associated with the olenid Mendoparabolina pirquinensis Rusconi 1951 (Rusconi 1951;Tortello 2014a).
On the other hand, knowledge about Lotagnostus atenuatus is still incomplete.The original description of Rusconi (1955a, b) is fairly concise, and since then, the species has only been briefly quoted by Jago (1972) and Bordonaro (1985), who discussed its generic affinity, and by Shergold et al. (1995: pl. 1, figs. 10, 11) and Tortello and Bordonaro (1997: fig. 3.4), who photographically illustrated the holotype cephalon and one paratype pygidium.With the aim of providing a full description of the morphology of Lotagnostus atenuatus, the complete type series at the Museo de Ciencias Naturales y Antropológicas "Juan Cornelio Moyano" (Mendoza) is revised herein.The material studied includes several three-dimensionally preserved sclerites of different holaspid sizes, allowing for an emended diagnosis of the species and comprehensive comparisons with other forms of Lotagnostus.Mendoparabolina brevicauda Rusconi, 1955a, which is recorded in association with L. atenuatus, is also redescribed here.Lotagnostus and Mendoparabolina proved to be representative elements of open ocean facies of the late Furongian of the Argentinian Precordillera, and their systematics are updated below.

Geological setting
The Cambrian rocks of the Precordillera of Mendoza consist mainly of limestones, marls, marly shales, and shales representing outer platform to upper slope environments (Bordonaro 2003a and references therein).The most notable fossil localities were discovered by C. Rusconi and M. Tellechea (Museo de Historia Natural de Mendoza) in the late 1940s and early 1950s, and comprise Cerro El Solitario, El Totoral, Cerro Pelado, and San Isidro (Fig. 1).With the exception of the Cerro Pelado locality, which represents an autochthonous succession (Cerro Pelado and El Relincho Formations, Furongian;Heredia 1996Heredia , 1999;;Keller 1999), the Cambrian of Mendoza is characterized by allochthonous blocks (olistoliths) of different sizes, ranging from centimeters to hundreds of meters in thickness (= La Cruz Limestones sensu Keller 1999), which occur chaotically within the Ordovician shales of the Estancia San Isidro and Empozada Formations (Bordonaro et al. 1993;Bordonaro and Banchig 1996;Bordonaro 2003a, b;Heredia and Beresi 2004).Olistoliths from Cerro El Solitario and El Totoral contain only trilobites of the Lejopyge laevigata Zone (late middle Cambrian) (e.g., Bordonaro and Liñán 1994;Tortello 2009Tortello , 2011)), whereas the fossil record of San Isidro is more diverse.

Stratigraphic implications of the faunas
Lotagnostus atenuatus clearly dominates the low-diversity assemblage of the olistolith studied, and is associated with a few sclerites of Mendoparabolina brevicauda.Although to date both species can be identified with confidence only at Quebrada San Isidro, L. atenuatus is closely allied with cephala of the poorly known "Lotagnostus sp.indet."(= L. cf.trisectus of Rasetti 1959), from dysoxic upper slope facies of the Lime Kiln Member of the Frederick Formation, Maryland, eastern USA (Rasetti 1959: fig. 51.8-9;Westrop and Landing 2016: fig. 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).

Systematic palaeontology
The material is housed in the Museo de Ciencias Naturales y Antropológicas "Juan Cornelio Moyano" (Mendoza, Argentina) with the prefix MCNAM.Slabs containing more than one specimen are labeled with both an original collection number and additional letters.Many of the types remain partly buried by matrix because permission was not granted for additional preparation.Before photography, the specimens were coated with magnesium oxide.The morphological terms used below have been mostly defined by Robison (1964), Shergold et al. (1990), and Whittington and Kelly (1997).
Remarks.Shergold and Laurie (1997 and references therein) and Westrop et al. (2011) recently discussed in detail the diagnosis of this genus.Westrop (1995) and Westrop et al. (2011) regarded the partially effaced taxon Distagnostus Shergold (1972) as a junior synonym of Lotagnostus, a conclusion that is followed herein.
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.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.
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].
Based on a global analysis of collections from different parts of the world, Peng and Babcock (2005) and Peng et al. (2015) recognize an extremely wide variation in cephalic scrobiculation for L. americanus, including almost imperceptible to strongly impressed furrows; a gena that in some cases is inflated and shows evenly curved flanks in anterior view, while in other cases becomes flat adaxially and is separated from the glabella by a clear break in slope; a glabella of different lengths (occupying about 68-78% of the total cephalic length); an ogival or a subquadrate anteroglabella; and a posteroaxis having a rounded or an ogival end.
However, this range of variability seems to be higher than that expected for an agnostoid species.When large and well-preserved collections are available, samples of Lotagnostus from individual regions of the world usually show lower levels of morphologic variation.For example, all late holaspides of L. americanus from the type locality of Quebec, Canada, exhibit an inflated cephalon with weak, uniformly developed scrobiculae, a rounded anteroglabella, and a proportionately wide (tr.), non-ogival pygidial M3 (Rushton 2009 5G-J).All large holaspides of L. asiaticus from South China (see the discussion of this species in Westrop and Landing 2016), on their part, share an inflated gena, faint genal scobiculae, a proportionately long (sag.)glabella, a subquadrate anteroglabella, and an ogival posteroaxis.
The Lotagnostus collection studied herein is in line with the abovementioned observations.As described below, all large specimens of L. atenuatus consistently have weakly developed scrobiculae, a proportionately short glabella which occupies barely 68-72% of the cephalic length, a subrounded to ogival anteroglabella, a gena that becomes relatively flat as it approaches the axial furrow, and an ogival posteroaxis.Intraspecific variability regarding all these features proved to be relatively low.This material mostly resembles topotype specimens of L. americanus (Quebec, Canada) and L. asiaticus (Xinjiang, Hunan and Zhejiang, China), but differs from the former in having a less-inflated gena which shows a more pronounced change of slope with the flanks of the glabella, as well as an ogival, rather than a well-rounded, posterior end of the posteroaxis; and it is distinguished from L. asiaticus in having a flatter gena, a noticeably shorter glabella, and a subrounded to ogival, rather than subquadrate, anteroglabella.
The cephalic convexity and profile of the gena (Westrop and Landing 2016), as well as the size and proportions of the axis, are features of specific significance in agnostoid systematics.Following narrow criteria for discriminating species of Westrop et al. (2011), L. atenuatus can be considered a valid taxon which, to date, is restricted to open-marine facies of the Southern Precordillera.Besides, as stated by Westrop et al. (2011), L. americanus is a species that is known with confidence only from Laurentia, a fact that reduces its value as an index fossil for the upper Furongian.(Rusconi, 1955a) Figures 2a-v Description.Cephalon en grande tenue, moderately convex, subcircular to subelliptical in outline in dorsal view, subequal in length (sag.) and width (tr.) or slightly longer than wide (maximum width at the posterior part of the acrolobe).

Lotagnostus atenuatus
Excluding basal lobes, glabella nearly parallel-sided to gently tapered forward, faintly constricted at F2 and F3 and expanded (tr.) at M3, bluntly pointed anteriorly, defined by narrow, well-impressed axial furrows, occupying about 68-72% of the cephalic length (sag.) and 31-33% of the cephalic width (tr.) at M3; in lateral view, the glabella is faintly convex, with maximum elevation at M2, and M3 and anteroglabella weakly raised above level of gena (Fig. 2d, o); basal lobes large, entire, subtriangular in outline, longer than wide, placed well below level of adjacent median body of posteroglabella, length (exs.)equal to about 19-20% of glabellar length (sag.), connected medially by an occipital band (Fig. 3b), delimited by basal furrows which are generally less distinct than axial furrows; glabellar rear obtusely angulate, with faint indications of a terminal glabellar node (Figs.2k, 3b); F1 weakly represented by a faint, short (tr.) groove near anterior tip of basal lobes (Fig. 2a, g, r); median node distinct, elongate, slightly posterior to the glabellar midpoint, more conspicuous anteriorly; some specimens exhibit a pair of rounded anterolateral inflations on M2 (Figs. 2g, p, r, 3b); F2 better developed than F1, directed inward from the axial furrows and then curved strongly forward exsagittally, delimiting two distinct lateral lobes which are separated from each other by a forward extension of the axial glabellar node; transglabellar furrow (F3) narrow (sag.) but well-defined, transverse to slightly bowed forward medially and curved forward exsagittally; anteroglabella subrounded to ogival, occupying about 34-36% of the glabellar length.Acrolobe evenly rounded anteriorly, unconstricted, lacking anterior indentation (sag.);gena subequal in width anteriorly and laterally, moderately convex, becoming flatter adaxially and separated from the glabella by a clear break in slope (Fig. 2b, i), with weak to moderately expressed scrobiculation on both the external surface of the exoskeleton and internal molds; median preglabellar furrow distinct, extending fully from the anterior tip of the glabella to the border furrow.Cephalic border narrow and gently convex, becoming narrower posterolaterally, separated from the gena by a shallow and narrow border furrow, lacking deltoid depression; sagittally, the border and border furrow combined occupy about 4-6% of the total length of the cephalon.Pygidium en grande tenue, convex, subelliptical in outline in dorsal view, evenly rounded posteriorly, subequal in length and width; axis long, convex, trilobed, occupying about 74-81% of the total pygidial length (excluding articulating half-ring), delimited by narrow, well-impressed axial furrows; M1 and M2 divided into a pair of subquadrate lateral lobes and a central ridge, which is low at M1 and highest posteriorly (Fig. 3u); M1 a bit wider (tr.), with length subequal to slightly shorter (exs.)than M2, occupying about 44-50% of the maximum pygidial width; F1 and F2 of similar width and depth; F1 directed inward and slightly forward from the axial furrow, and then strongly deflected anteriorly to the articulating furrow; F2 continuous across axis and nearly transverse, but gently deflected forward in front of the intranotular axis; M3 long, ogival in outline, occupying 58-60% of the total length of the axis (excluding articulating half-ring), showing apparent but variably impressed notulae, a gently inflated intranotular axis and a delicate terminal node.Acrolobe gently constricted; pleural fields convex, confluent, somewhat narrower (sag.)behind the axis, smooth or with weak indications of scrobiculation consisting of delicate pits, which are much more evident in small holaspides, and short radiate furrows around M3. Pygidial border narrow and slightly convex, minimum width (tr.) at anterior corner of pygidium, becoming wider backward until level of F2, then maintaining nearly even width, separated from the pleural fields by a shallow border furrow; sagittally, the border and border furrow combined occupy about 7-10% of the total length of the pygidium (excluding articulating half-ring); posterolateral spine vestigial, with base opposite or slightly behind tip of axis; articulating halfring prominent, convex, transversely subelliptical in outline, separated from the axis by a nearly transverse, well-defined articulating furrow.
Holaspid ontogenetic variation.The collection studied is composed of holaspid specimens of different sizes, allowing evaluation of some aspects of the ontogenetic variability.Although late holaspides show weak but distinct scrobiculae on the gena, sculpture seems to be fainter in small cephala (Fig. 3e); a variation that was also documented in other species of Lotagnostus (e.g., L. asiaticus Troedsson, 1937; L. salteri Westrop and Landing, 2016).
Small pygidia of L. atenuatus (Fig. 3i) are characterized by having distinct pits on the pleural fields, which are barely outlined or obscure in larger specimens.The largest pygidia (Fig. 3g, q) show, in addition, a more clearly constricted acrolobe, a wider (tr.) border furrow, and a proportionately longer (sag.)axis.On the other hand, indications of notulae are present in both early and late holaspides of L. atenuatus.
Remarks.Although the original description of Lotagnostus atenuatus was based mainly on one cephalon (MCNAM 18208b, holotype) and one associated pygidium, Rusconi (1955a, b) also reported the existence of numerous additional specimens in his collections, pointing out the presence of faint, uniformly developed scrobiculae on all the cephala examined.Rusconi (1955b: p. 28) also noted that the glabellar segmentation of the species is best illustrated by the holotype, which shows particularly well-differentiated lateral lobes M2 and M3, as well as rounded inflations in the anterior part of M2 (Fig. 2r-v).The transglabellar furrow of this specimen exhibits lateral branches which are more strongly curved forward than those of other cephala (e.g., compare Fig. 2r with Fig. 2g); a specific variability that was also documented, for example, in L. americanus (Billings, 1860) (Westrop et al. 2011: figs. 5D, 6F) and L. germanus (Matthew, 1901) (Westrop et al. 2011: fig.4I, L).
Two Lotagnostus cephala from a loose block of limestone of the Frederick Formation of Maryland, eastern USA ("Lotagnostus cf.L. trisectus" of Rasetti 1959: pl. 51, figs. 8, 9;Westrop and Landing 2016: fig.12a-g) are characterized by having moderately developed scrobiculation, well-incised axial and glabellar furrows, and a gena that is subequal in width anteriorly and laterally; the gena shows a distinct flattening as it approaches the glabella.This material is extremely similar to L. atenuatus, although further specimens are needed to improve this comparison.
Like L. atenuatus, L. asiaticus Troedsson, 1937, from the Furongian of Xinjiang, Hunan and Zhejiang, China (see discussion in Westrop and Landing 2016 and references therein), combines weak to moderate genal scrobiculation, a gently constricted pygidial acrolobe, a long, semiovate to ogival posteroaxis, and a distinct intranotular axis, which is well expressed in both small and large specimens.However, the Chinese species differs by having a longer (sag.)glabella in dorsal view, and consequently a shorter (sag.)preglabellar genal field, which occupies about 19-22% of the total cephalic length (versus 25-27% in L. atenuatus).Besides, the gena of L. asiaticus is evenly inflated (e.g., Peng et al. 2015: figs. 1I, J, 2F, M, 5D) and does not flatten near the glabella as in L. atenuatus, and the anteroglabella is generally more subquadrate in outline.
Partially effaced species of Lotagnostus differ more widely from L. atenuatus.The former constitute a set of taxa that are characterized by partially obliterated dorsal furrows as well as an inflated cephalon with evenly curved flanks in anterior view (Westrop et al. 2011).Lotagnostus peladensis (Rusconi, 1951), from the Saukia Zone of Cerro Pelado (Shergold et al. 1995: pl. 1, figs. 1-9;Tortello 2014a: figs. 2.1-2.28, 3.1-3.7),as well as the closely allied Lotagnostus obscurus Palmer, 1955 and Lotagnostus sp. from western and northwestern North America (see Westrop et al. 2011;Tortello 2014a), exhibit dorsal furrows that are effaced to varying degrees (from en grande tenue to almost completely effaced) and lack cephalic and pygidial scrobiculation.
Remarks.The presence of a subquadrate glabella with conspicuous, backwardly directed lateral furrows; a noninflated preglabellar field; a distinct, oblique ocular ridge; caecate librigena; and a proportionately long (sag.),transversely bilobate pygidium with a posteriorly truncate or slightly indented axis and a postaxial median ridge are characteristic features of Mendoparabolina Rusconi, 1951. Wujiajiania Lu andLin, 1980, is a common genus in deep water facies of the late Furongian of North America, Kazakhstan, Australia  2007) which has a cephalon that compares closely with that of Mendoparabolina.However, the pygidium of Wujiajiania is proportionately shorter (sag.), exhibits a smaller number of pleural furrows and a posteriorly rounded axis, and lacks a postaxial median ridge and a pronounced posterior medial indentation.
The pygidium of Mendoparabolina is distinctive and, so far, only two species have been confidently assigned to this genus: M. pirquinensis and M. brevicauda (see below).Parabolina? naomi Pratt, 1992, from the Rabbitkettle Formation of northwest Canada, was regarded as a Mendoparabolina species by Tortello (2014a); however, it has a proportionately short pygidium with a posteriorly rounded axis (Pratt 1992: text-fig. 29, pl. 10, fig. 7), and thus it is more correct to place it in Wujiajiania (see Chatterton and Ludvigsen 1998).
The pygidium of Plicatolinella Robison and Pantoja-Alor, 1968, from the upper Furongian of Oaxaca, Mexico, is exceptionally similar to that of Mendoparabolina, but Plicatolinella is easily distinguished by its much deeper lateral glabellar furrows S3 and S4, and its enormous eye ridges and palpebral lobes.Rusconi, 1955a Figures 4a-i (Rusconi 1955a, b).
Description.Cranidium slightly convex, with broadly rounded anterior margin and downsloping fixed cheeks.Glabella large, subquadrate, somewhat elevated above fixed cheeks, width equal to a little more than 90% of length, subparallel sided to slightly tapered forward, broadly rounded anterolaterally and anteriorly, delimited by narrow and deep axial and preglabellar furrows, showing distinct lateral furrows; it occupies about 80% of the total length of the cranidium; occipital ring with rounded posterior margin and a delicate median node, occupying about one quarter of the total glabellar length; occipital furrow deep and narrow (sag.), marked throughout, slightly bowed forward medially and somewhat oblique forward laterally; lateral glabellar furrows S1 and S2 subequal in development, deep and narrow (exs.), slightly bowed forward, oblique backward, disconnected medially and reaching axial furrows; some specimens show faint indications of a transverse line joining lateral branches of S1; S3 much shallower than S1 and S2.Preglabellar field moderately wide (sag.), flat to faintly convex, occupying about 15% of the total cephalic length; anterior cephalic border short (sag.),slightly upturned, delimited by a shallow but distinct border furrow; anterior facial suture subparallel; eye ridge faint but visible, short, oblique backward; palpebral lobe short (exs.),elevated above fixigena, forward of glabellar midpoint, surrounded by a very faint palpebral furrow; posterior fixigena not preserved for description.Librigena with a densely caecate dorsal surface and a delicate genal spine (Fig. 4c, top left).Pygidium transversely bilobate, maximum width about twice sagittal length.Axis broad, convex, subparallel-sided to slightly tapered backward, truncate posteriorly, anterior width 35-40% maximum width of pygidium, with three axial rings and a subrectangular terminal piece.Pleural field slightly convex, crossed by four sets of distinct, oblique pleural furrows, delicate interpleural lines, and a postaxial median ridge; first two pleural furrows deeper than other furrows of pleural field, with their distal parts strongly curving backward and terminating just inside of pygidial margin; border furrow indistinct; anterolateral corners of the pygidium posteriorly located; posteromedian pygidial margin with a broad indentation.
Remarks.Mendoparabolina brevicauda was originally based on scarce and incomplete material from Quebrada San Isidro, which is first illustrated photographically in Fig. 4a-i.The cranidia examined resemble Bienvillia corax (Billings, 1865), from the upper Furongian of eastern North America (Ludvigsen et al. 1989: pl. 4, figs. 18-22, and references therein), by sharing a relatively long (sag.)preglabellar field, equally developed, backwardly oblique lateral glabellar furrows S1 and S2, and discrete palpebral lobes.However, B. corax is distinguished by having an inflated preglabellar field, a near-circular glabellar outline, and straight lateral glabellar furrows.Although the material studied here exhibits a preglabellar field that is much longer (sag.)than that of the type species of Mendoparabolina (M.pirquinensis Rusconi), it is maintained in this genus based mainly on the presence of a transversely bilobate pygidium with a subrectangular axial terminal piece, a postaxial ridge, and four pleural furrows.Besides having a longer preglabellar field (Rusconi 1955b), M. brevicauda differs from M. pirquinensis in its nonsinuous lateral glabellar furrow S1.