versión impresa ISSN 0716-0208
Rev. geol. Chile v.32 n.2 Santiago jul. 2005
Revista Geológica de Chile, Vol. 32, No. 2, p. 293-311, 2 Figs., 3 plates, July 2005.
Conchostracans and insects from the Upper Triassic of the Biobío river ('Santa Juana Formation'), south-central Chile
Conchóstracos e insectos del Triásico Superior del río Biobío ('Formación Santa Juana'), Chile centro sur.
Oscar F. Gallego1, Rafael G. Martins-Neto2, Sven N. Nielsen3
1Paleontología (Facultad de Ciencias Exactas y Naturales y Agrimensura, Universidad Nacional del Nordeste) y Área Paleontología (Centro de Ecología Aplicada del Litoral - CONICET), Casilla de Correo 128, 3400 Corrientes-Argentina firstname.lastname@example.org
Fossil arthropods from Upper Triassic outcrops along the Biobío river, south-central Chile, are reported in this paper. Two species of the order Coleoptera, Ademosyne sp., and Ischichucasyne santajuanaensis gen. et sp. nov. (Permosynidae) are described for the first time for the Triassic of Chile. The conchostracans Menucoestheria terneraensis Gallego and Polygrapta troncosoi (Gallego) comb. nov., are re-examined based on newly collected specimens and scanning electron microscope photographs. A close relation between M. terneraensis and Argentinean and Antarctic species is postulated, with the possibility that Menucoestheria evolved from Euestheria Depéret y Mazerán. Menucoestheria could be the origin of all other Gondwanic eosestherids. The use of Menucoestheria and Polygrapta as fossil guides for continental levels of the Upper Triassic of Argentina and Chile is tentatively proposed. The reported entomofauna, which exhibits close affinities with the Argentinean entomofauna, sharing, for example, the genera Ademosyne and Ischichucasyne, broadens our knowledge of South American arthropod diversity during the Triassic. The presence of the homopteran dysmorphoptilid Bandelnielsenia Martins-Neto and Gallego, the most plesiomorphic of the South American forms, seems fundamental for an understanding of the phylogeny of this important Triassic group.
Key words: Conchostraca, Insects, Upper Triassic, Chile.
En este trabajo se da a conocer el hallazgo de artrópodos del Triásico Superior, en estratos que afloran a lo largo del río Biobío, en el centro sur de Chile. Se describen, por primera vez, para el Triásico de Chile, representantes del orden Coleoptera: Ischichucasyne santajuanaensis gen. et sp. nov. y Ademosyne sp. (Permosynidae). Sobre la base de nuevos materiales recolectados y fotografías de detalle con microscopía electrónica de barrido, se reestudiaron los conchóstracos Menucoestheria terneraensis Gallego y Polygrapta troncosoi (Gallego) comb. nov. Se destaca la estrecha relación entre M. terneraensis Gallego y otras especies de Argentina y Antártica. Se menciona la posibilidad de que este género haya evolucionado a partir de especies de Euestheria Depéret y Mazerán, y que a su vez Menucoestheria Gallego haya dado origen a todos los eosestherideos gondwánicos. Se propone, tentativamente, el empleo de Menucoestheria y Polygrapta como fósiles guías para los niveles continentales del Triásico Superior de Argentina y Chile. La entomofauna registrada muestra estrechas afinidades con las faunas argentinas, con las que comparte los géneros Ademosyne e Ischichucasyne, y amplía el conocimiento sobre la diversidad de artrópodos durante el Triásico en Sudamérica. La presencia del homóptero dysmorphoptilideo Bandelnielsenia Martins-Neto y Gallego, la más plesiomórfica de las formas sudamericanas, resulta fundamental para comprender la filogenia de este importante grupo triásico.
Palabras claves: Conchóstracos, Insectos, Triásico Superior, Chile.
The record of fossil conchostracans and insects from Triassic sedimentary rocks of Chile is poorly known. Recently it was summarized by Gallego and Covacevich (1998) and Martins-Neto et al. (2003). Previous records mainly consisted of brief mentions with geographical and stratigraphical sources. Gallego and Covacevich (1998) cited only two references (Fuenzalida, 1937; Cecioni and Westermann, 1968) mentioning fossil insects in the Triassic of the Punta Puquén and Los Molles areas. Martins-Neto et al. (2003) described the first Triassic insect (homopteran, commonly called bugs or planthoppers) from Chile, Bandelnielsenia chilena Martins-Neto and Gallego, 2003 (in Martins-Neto et al., 2003) (Auchenorrhyncha, Dysmorphoptilidae, Gallegomorphoptilinae), from the Upper Triassic of the Biobío river, near Concepción (south-central Chile) (text-Fig. 2.D; Pl. 3, Fig. 8).
Gallego and Covacevich (1998) described three new species of Triassic conchostracans: Menucoestheria terneraensis Gallego (in Gallego and Covacevich, 1998), from the La Ternera Formation (Upper Triassic) and the La Coipa Beds (?Lower Triassic sensu Hutter in Suárez et al., 1995, based on a palynomorph assemblage; or Upper Triassic, sensu R. Herbst, oral communication, 1997, based on the a plant assemblage, and M. Suárez, oral communication, 2004, see also Gallego and Covacevich, 1998, p. 129); Menucoestheria puquenensis Gallego (in Gallego and Covacevich, 1998), from the Pichidangui? and Profeta? formations (Upper Triassic); and Liograpta troncosoi Gallego (in Gallego and Covacevich, 1998), from the La Coipa Beds (for age assignation see above).
In recent fieldwork, one of the authors (SNN) and Klaus Bandel (University of Hamburg, Germany) recovered several samples with an abundant conchostracan fauna and insect remains from a road-cut outcrop near the village of Santa Juana, near Concepción (southern-central Chile) (text-Fig. 1B). This material comes from the Upper Triassic sediments of the Biobío river. In the present work, the authors identify the record of Menucoestheria terneraensis Gallego (in Gallego and Covacevich, 1998) and Polygrapta troncosoi (Gallego in Gallego and Covacevich, 1998) comb. nov. among the conchostracan fauna. This new material and its differing stratigraphical and geographical source permit a more detailed study of these previously known conchostracan taxa. Scanning electron microscope (SEM) studies and new available characters allow us to offer an extended diagnosis of both taxa. Two insect taxa, Ademosyne sp. and Ischichucasyne santajuanaensis gen. et sp. nov. (Order Coleoptera), are identified and comments on the paleogeographic distribution, dispersal paths and relationships of the described insects and conchostracans are given.
The Upper Triassic sediments from the Biobío river were deposited in a basin surrounded by Paleozoic metamorphic and granitoid rocks (González-Bonorino and Aguirre, 1970; text-Fig. 1A, B). The name Santa Juana Formation was introduced by Ferraris (1981) for Triassic sedimentary rocks along the Biobío river, near Santa Juana. Ferraris (1981) did not clearly define this formation. Accordingly, the term Santa Juana Formation is interpreted in this work as including all Triassic sedimentary rocks exposed in the mentioned area, the extent of which is indicated on text-Fig. 1A. While this is not satisfying, it is not within the scope of this paper to give a clearer definition of the Santa Juana Formation. Comprehensive accounts on the sedimentology and paleontology have been conducted by Felsch (1910), Steinmann (1921), Tavera (1960), Hervé et al. (1976), Charrier (1979), Leppe and Moisan (2003), Pérez-Barría (2004) and, more recently, by Nielsen (in press).
The exclusively clastic sedimentary rocks of the Upper Triassic from the Biobío river have been interpreted as the deposits of rivers, lakes, playas and alluvial fans (Nielsen, in press). Material described in this article comes from black mudstones exposed by a road-cut just outside the village of Santa Juana (text-Fig. 1A, B) at S37°10.270'/W 072°57.406' (GPS data). These mudstones have been interpreted 'as the product of settling from suspension in relatively deep water' of lakes (Nielsen, in press). However, some of the black shales may have been deposited in abandoned channels or lakes leading to still conditions preferred by conchostracans (Nielsen, in press). The lower fossiliferous level (FL 1, text-Fig. 1B) consists of a dark grey silty mudstone with abundant conchostracans, M. terneraensis Gallego and P. troncosoi (Gallego), while the upper fossiliferous level (FL 2, text-Fig. 1B) consists of a finer black mudstone yielding abundant conchostracans, M. terneraensis Gallego and P. troncosoi (Gallego), rare insect remains (Bandelnielsenia chilena Martins-Neto and Gallego, Ischichucasyne santajuanaensis gen. et sp. nov. and Ademosyne sp.), and plant fragments.
The Upper Triassic age of the sequence was established by Steinmann (1921) based on the fossil flora (see Hervé et al., 1976; Leppe and Moisan, 2003). Steinmann (1921) also reported the presence of fresh water invertebrates, including Esthena Mengolinesis (sic. in Hervé et al., 1976, probably 'Estheria' mangaliensis Jones) and Unio sp. Tavera (1960) gave a Carnian to Rhaetian age for all of these Triassic series (see Hervé et al., 1976) based on the fossil flora and marine mollusk fauna (Arcestes, Myophoria and Palaeoneilo). Leppe and Moisan (2003) gave an Upper Carnian to Lower Rhaetian age based on the paleoflora, while Pérez- Barría (2004) proposed an Upper Carnian to Lower Norian age based on an abundant marine mollusk fauna (Clydonautilidae, Nuculidae, Malletidae, Nuculanidae, Halobiidae, Limidae and Kalenteridae).
Here, the Carnian age proposed by Nielsen (in press) is used, which is based on known stratigraphic ranges of the paleoflora.
MATERIAL AND METHODS
The material studied was collected by Sven N. Nielsen and Klaus Bandel during field trips in 1997 and 2000 near Concepción, south-central Chile. The samples are kept in the Paleontology Section of the Museo Nacional de Historia Natural, Santiago (Chile), with sample numbers prefixed by SGO.PI.
Supplementary material of the conchostracans are kept in the Paleozoological Collection of the Facultad de Ciencias Exactas y Naturales y Agrimensura (Universidad Nacional de Nordeste, Corrientes, Argentina) under the collection number CTES-PZ.
Well-preserved specimens of conchostracans were described and measured. The morphometric parameters used were those of Defretin-LeFranc (in Tasch, 1987; Gallego and Covacevich, 1998): L: valve length; H: valve height; Ch: hingeline length; Cr: distance from beak to anterior end of the valve; Av: distance from anterior end of the dorsal margin to the anterior end of the valve; Arr: distance from the posterior end of the dorsal margin to the posterior end of the valve; a: distance from maximum anterior bulge to dorsal margin; b: distance from maximum posterior bulge to dorsal margin; c: distance from maximum ventral bulge to the anterior end of the valve. All measurements are given in mm.
SEM (JEOL JSM-5800LV Scanning Microscope, SECyT-UNNE, Corrientes, Argentina) photographs provide more detailed microscopic evidence on the morphologic characteristics of the conchostracan species described here.
The terminology adopted in this paper for the insects conforms mainly to that of Ponomarenko (1969).
Chen and Shen´s (1985) conchostracan systematic scheme is followed for the infraordinal level, because it is the best adapted to South American Triassic conchostracan faunas.
The genus Menucoestheria was erected by Gallego (in Gallego and Covacevich, 1998) for two new species of the family Eosestheriidae from northern Chile, M. terneraensis Gallego from La Ternera Formation (Upper Triassic) and La Coipa Beds (Lower Triassic, see above) and M. puquenensis Gallego from the Pichidangui ? and Profeta? formations (Upper Triassic). Gallego (1999) recognized for the first time the family Eosestheriidae in the Triassic of Argentina. The name of the genus comes from its first record in the Vera Formation, which is part of the Los Menucos Group (early Late Triassic age, previously known as Los Menucos Formation) in the area of Los Menucos (Río Negro Province, Argentina). The fauna recovered from the Upper Triassic of the Biobío river, also includes Polygrapta troncosoi (Gallego) comb. nov. This species was described by Gallego and Covacevich (1998), belonging to the genus Liograpta, from La Coipa Beds (Lower Triassic, see above) from northern Chile.
Order Conchostraca Sars, 1867
Type species: Menucoestheria terneraensis Gallego in Gallego and Covacevich, 1998, Upper Triassic, Chile.
Diagnosis (emend.): carapace valve small to moderate in size; subovate, elongated ovate, elliptical to subcircular in outline. Posterior margin strongly convex, elongated posteriorly. Growth lines 14 to 45. Narrow growth bands ornamented with rounded areolar reticulation near anterior and medium part of valve, thin radial striae near ventral and postero-ventral sides. Meshes with 0.01 to 0.05 mm diameter and 30 to 80 radial striae per millimeter.
Remarks: Shen (oral communication, 2004) suggested the possibility that Menucoestheria 'should belong to the family Polygraptidae rather than Eosestheriidae based on sculpture and size of the valves' and concludes that 'Menucoestheria is very similar to Polygrapta Novojilov, 1946'. According to comparisons made between both forms and their original diagnoses, the authors concluded that Menucoestheria belongs to the family Eosestheriidae due to its ornamentation, which differs from that of Polygraptidae (radial striae with cross bars, constituted by the alignment of small reticulate sculpture, see Pl. 3, Figs. 3-5) because it has rounded areolar reticulation near the anterior and medium part of the valve, changed into thin radial striae near the ventral and postero-ventral sides (Pl. 2, Figs. 2, 4). With respect to the carapace size, Menucoestheria varies from 3 to 8 mm in length, and therefore overlaps with the dimensions of many species referred to Polygrapta by Novojilov (1958). Also, it shares this size range with the eosestheriid Carapacestheria Shen, 1994 (Early to Middle Jurassic, Antarctica). Considering Shen's different opinion in this paper, the authors included a question mark at the family level, waiting to resolve this nomenclatural problem with new material.
Menucoestheria differs markedly from all other genera of the family Eosestheriidae (see Gallego and Covacevich, 1998, Table 2). Carapacestheria (Early to Middle Jurassic, Antarctica) is the most similar genus, but differs slightly in the smaller dimensions of the carapace, polygonal reticulation with 0.02 to 0.036 mm in diameter and about 40 radial striae per millimeter, and with minute punctae filling in meshes and between striae which are lacking in Menucoestheria (Pl. 3, Figs. 1-2). Due to their close similarities in carapace size, outline and mainly in the type of ornamentation, both genera are regarded as phylogenetically closely related.
'Estheria' Steinmann in Solms-Laubach and Steinmann, 1899, p. 590.
Estheria mangaliensis Jones, 1862; Steinmann, 1921, p. 352.
Estheria forbesii Jones, 1862; Tavera, 1960, p. 343.
Dimensions: (in mm)
Material: 77 specimens.
Type locality: 'Quebrada del Carbón' (Cerro La Ternera), Atacama Region, Chile. La Ternera Formation, Upper Triassic.
Other localities: La Coipa mine (La Coipa area), Atacama Region, Chile; La Coipa Beds, ?Lower Triassic (probably Upper Triassic, see Introduction). Santa Juana, Biobío Region, Chile; Upper Triassic from the Biobío river.
Type material: holotype SGO.PI. 5715 (L: 4.7, H: 3.3), paratypes SGO.PI. 5716-5719, 5723-5732.
Additional material: CTES-PZ 7289 (MEB-3), 7294 (MEB-4), 7325 and SGO.PI. 6126, 6128-6133, 6137, 6138 (from Upper Triassic levels from the Biobío river). CTES-PZ: 7269, 7289 (MEB-3) (from La Ternera Formation), 7270 (from La Coipa Beds).
Diagnosis (emended): carapace valve moderate in size, subovate, elongated ovate to subcircular in outline. Straight to slightly convex dorsal margin.
Posterior margin strongly convex and elongated posteriorly. Growth lines 14 to 42. Growth bands ornamented with rounded areolar reticulation near anterior and medium part of valve, and thin radial striae near antero-ventral and postero-ventral sides. Meshes with 0.01 to 0.05 mm diameter and 30 to 75 radial striae per millimeter. Transitional area of two types of ornamentation in last growth bands.
Description: carapace valve moderate in size, between 3.6-8.4 mm length and 2.8-5.4 mm height, subovate, elongated ovate to regularly rounded subcircular in outline. Dorsal margin straight or slightly convex, postero-dorsal angle marked or slightly joined. Posterior margin more convex and larger than anterior one in subovate forms. Elongated ovate forms with subparallel antero-ventral to postero-dorsal margins. Ventral margin slightly convex. Highest at anterior region. Umbonal region located subcentrally to subterminally, raised above dorsal margin. Growth lines 14 to 42. Growth bands ornamented with rounded areolar reticulation distributed nearly on the whole valve surface, and thin radial striae in last five growth bands near antero-ventral and postero-ventral sides. Isodiametrical meshes with 0.01 to 0.05 mm diameter and 30 to 75 radial striae per millimeter. Antero-ventral area with 30 radial striae and medium-ventral and postero-ventral region with 50 to 75 radial lines. Transitional area of two types of ornamentation in last growth bands. Radial striae bifurcated upward and anastomoused forming small meshes in dorsal area of each growth band.
Remarks: both populations of Menucoestheria terneraensis Gallego recovered from the La Ternera Formation and the Upper Triassic levels from the Biobío river, differ in having a slightly larger size and more elongated ovate outline of the last mentioned specimens. The eosestheriid from Vera Formation (Río Negro, Argentina) (see this paper, p. 4) differs from M. terneraensis in having smaller dimensions and smaller size of the areolar meshes and larger spaces between radial striae (Pl. 3, Figs. 1-2).
Family Polygraptidae Novojilov, 1954
Type species: Polygrapta chatangensis Novojilov, 1946, Upper Permian-Tatarian, Khatanga, Russia.
Dimensions: (in mm)
Material: 33 specimens.
Type locality: 'Quebrada La Pelada' (La Coipa area), Atacama Region, Chile; La Coipa Beds, ?Lower Triassic (probably Upper Triassic, see Introduction).
Other localities: Santa Juana, Biobío Region, Chile; Upper Triassic levels from the Biobío river.
Type material: holotype SGO.PI. 5720 (L: 5.9, H: 4.9), paratypes SGO.PI. 5721, 5722, 5733 - 5741
Additional material: CTES-PZ 7268, 7287 (MEB-1), 7289 (MEB-3), (from La Coipa Beds); SGO.PI 6127, 6128, 6134-6136; CTES-PZ 7289 (MEB-3), 7324 (from Upper Triassic beds from the Biobío river).
Diagnosis (emend.): carapace valves ovate to subcircular in outline. Dorsal margin straight and strongly joined with posterior margin, forming an angle of 140º in subcircular specimens. Posterior margin longer and more convex than anterior one. Ventral margin regularly convex. Umbo sets subterminal and not rised above dorsal margin. Maximum heigth in medial region of valve. Growth lines 14 to 40. Ornamented with radial lines, separated by 0.005 to 0.01 mm interspaces and with cross bars in between. Number of radial striae around 80 per mm.
Description: conchostracans with ovate to subcircular outline. Straight dorsal margin, projecting before umbonal region, strongly joined with posterior margin forming angle of 140º in subcircular specimens. Posterior margin longer and more convex than anterior one. Anterior margin slightly convex and perpendicular, abruptly joined with dorsal one. Ventral margin regularly convex. Umbo sets subterminal and not rised above dorsal margin. Maximum height in medial region of valve. Ovate specimens with maximum height in anterior region of valve, posterior margin more convex and shorter than anterior margin. Growth lines 14 to 40. Ornamented with radial lines separated by 0.005 to 0.01 mm interspaces and with cross bars in between. Number of radial striae around 80 per mm. It resembles areolar ornamentation in middle growth bands of valve.
Remarks: the conchostracans from La Coipa Beds and the Upper Triassic beds of the Biobío river belong to the same species. Here, both are reassigned to the family Polygraptidae and to the genus Polygrapta based upon new micromorphological evidence provided by SEM microphotographs, namely, that the ornamentation in the growth bands is constituted by radial striae with cross bars between them. This ornamentation is characteristic of the genus Polygrapta, rather than Liograpta Novojilov, 1954, the genus to which this material was assigned previously by Gallego and Covacevich (1998), and in which the ornamentation is constituted by dendritic striae (sensu Shen, oral communication, 2003).
Polygrapta was described from the Upper Permian (Tatarian) from the Gulf of Khatanga in the Laptev Sea (Russia). This taxon also includes species from the Upper Permian of China and Russia (Siberia).
Polygrapta troncosoi shares some characteristics of the carapace with many species of the genus (P. chatangensis, P. sibirica, P. evenkorum, P. necta, P. laptewi and P. multinstita; Novojilov, 1958, Figs. 4-10) described by Novojilov (1946, 1958) from the type locality of the genus. These characteristics are: straight dorsal margin, strongly joined with the posterior margin forming a notorious angle, and subcircular to ovate outline. But P. troncosoi differs in many other characteristics, such as an umbo rising above the dorsal margin (only in five of the mentioned species) and a dorsal margin projecting downward to the umbonal region (only in three of the mentioned species). The radial ornamentation of P. troncosoi resembles the ornamentation present in P. sibirica in length and width of the striae (Novojilov, 1958, Pl. 1, Fig. 1e), but differs markedly from that of P. chatangensis, which has narrower striae (Novojilov, 1958, Pl. 1, Figs. 1a-c). Polygrapta hongluoxianensis Shen (in Shen and Li, 1986) from the Late Permian from Liaoning (China), has an ornamentation similar in length and width of the striae to P. troncosoi, but the former differs in its generally ovate carapace outline, and the rounded dorsal margin.
Another probable South American member of the genus, is an unnamed species described by Gallego (1999) from the Cerro Bayo locality (Mendoza province, Argentina) of the Cerro de Las Cabras Formation (late Middle Triassic). Both forms differ slightly in the angles of the postero-dorsal and antero-dorsal areas and have more convex dorsal margin and stronger radial striae than the Argentinean species.
Previous to this paper, the record of Triassic insect fauna from Chile was poorly known (see Introduction of this paper). In this contribution we report the presence of the genus Ademosyne that was erected by Handlirsch (1906) from the Upper Triassic of Australia. Currently, there are more than twenty species of this genus described from Triassic levels of the Southern Hemisphere. Recently, Martins-Neto et al. (in press) defined four new species from the early Upper Triassic (Los Rastros Formation) from the Bermejo Basin (La Rioja Province, Argentina). Ademosyne is the most common genus registered in the Argentinian Triassic. The new genus Ischichucasyne is also known from the late Middle Triassic (Ischichuca Formation) from Argentina, but this material is unpublished. The insect assemblage also includes a hemipteran inmature stage, still under study.
Order Coleoptera Linnaeus, 1758
Type species: Ademosyne major Handlirsch, 1906, p. 402, Pl. 39, Fig. 14, Upper Triassic, Australia.
Material: SGO.PI. 6139 (2A), and (2B), 2 specimens.
Dimensions (in mm): L 2.08-2.8; W 0.83-1.16; L/W 2.4-2.5.
Locality: Santa Juana, Biobío Region, Chile; Upper Triassic levels from the Biobío river.
Description: material (Fig. 2A), elytron 2.08 mm long as preserved and 0.83 mm wide (relation L/W: 2.5), with narrow lateral border. Six smooth costae, not convergent. Space between costae constituted by small granules homogeneously distributed in the whole elytron surface. Material (Fig. 2B): elytra 2.80 mm long as preserved and 1.16 mm wide (relation L/W: 2.4), with narrow lateral border. Six punctate costae, not convergent. Space between costae constituted by striae homogeneously distributed on the whole elytron surface.
Remarks: these specimens are very similar in all observed morphological aspects to a new species that occurs at the Río Gualo locality (Los Rastros Formation, early Late Triassic) from La Rioja Province (Argentina), described in Martins-Neto et al. (in press).
Genus Ischichucasyne Martins-Neto and Gallego gen. nov.
Type species: Ischichucasyne santajuanaensis sp. nov., designated here; Santa Juana, Biobío Region, Chile; Upper Triassic beds from the Biobío river.
Etymology: alludes to Quebrada de Ischichuca Chica, locality where the Argentinian material comes from, and syne, common suffix for Ademosynidae.
Diagnosis: Ademosyne-like elytron shape with 9-11 well-defined smooth costae. The costae closer to the posterolateral margin are multibranched at the proximal part, all of them convergent to the anterolateral margin. Space between costae striated. Elytron relation L/W around 3.5.
Discussion: the morphological variability of Ischichucasyne gen. nov. is closer to Argentinosyne Martins-Neto and Gallego, reported from the Los Rastros and Cacheuta formations. Some Ademosyne species ocasionally have one or more dichotomous costae, but no specimens were reported with the peculiar multibranching pattern present in Ischichucasyne as proposed herein. Ischichucasyne gen. nov. differs from Argentinosyne by having well-defined costae (weakly marked in Argentinosyne) and striated ornamentation between costae (constituted by granules in Ademosyne as well as Argentinosyne).
Derivatio nominis: alludes to Santa Juana locality (south central Chile), from where the material comes.
Holotype: SGO.PI. 6140
Dimensions (in mm): L 5.08; W 1.25; L/W 4.0.
Locality: Santa Juana, Biobío Region, Chile; Upper Triassic beds from the Biobío river.
Diagnosis: elytron around 5.08 mm long and 1.25 mm wide. Relation L/W 4.0 Six smooth costae, converging to distal border. Costae closer to the posterolateral margin are trichotomous at the proximal part, convergent to the anterolateral margin. Space between costae constituted by small granules homogeneously distributed by the elytron surface.
Description (Holotype SGO.PI. 6140, text. Fig. 2C): elytron 5.08 mm long and 1.25 wide (relation L/W 4.0), with narrow lateral border. Six smooth, divergent costae, proximally multibranched. Space between costae constituted by small granules, homogeneously distributed in the whole elytron surface.
Discussion: the new species belongs to Ischichucasyne, because the proximal part of the costae is multibranched. Ischichucasyne santajuanaensis gen. et sp. nov. differs from undescribed specimens from the Ischichuca Formation (late Middle Triassic, Argentina) by having an elytron four times longer than wide (three and one-half in the Argentinean specimens), and six costae (nine to eleven in the Argentinean specimens).
PALEOBIOLOGICAL, BIOSTRATIGRAPHICAL AND PYILOGENETICAL COMMENTS
This study permits to offer some tentative comments with regard to the paleobiology of fossil conchostracans. The assemblage of Menucoestheria terneraensis Gallego in the Upper Triassic horizons of the Biobío river (south central Chile) shows a ratio of about 4:1 of adults to juveniles. This would indicate a certain stability in the environmental conditions and relative duration of the water body they inhabited which permitted to complete its biological cycle. This is supported by the presence of regularly spaced growth lines that do not reflect stressful environmental conditions (evidenced by numerous and thin growth bands, which are here only observed on the margins of the valves). On the other hand, the presence of morphological differences among adult specimens suggests the existence of male and female in this population. This would indicate that the environmental conditions had become unfavorable for this species towards the end of its time of inhabitation (narrow bands, see above). The presence of males (in parthenogenic species) in some recent conchostracans populations are due to the establishing of unfavorable environmental conditions in the ecosystem. In this moment, the parthenogenic females produce eggs that only give rise to males. Then, resistant eggs will be formed through sexual reproduction that are able to survive conditions of total dessication of the water body for a long time (up to 15 years). Later, these are able to hatch normally with the return of normal conditions (Webb, 1979). The number of growth lines and their spacing is able to provide additional information about the duration of the biological cycle (from 5 to 240 days in modern forms) and their occupation of a water body, as well as information about the environmental conditions (Tasch, 1961,1977; Gallego et al., 2004). The calculation of the approximate time of occupation of the said lake by the conchostracans can have a high probability of error (Webb, 1979) and a very wide range of variation (because among other factors we may be dealing with more than one generation). The fossiliferous level (FL) 1 (text-Fig. 1B) from the Upper Triassic of the Biobío river, where M. terneraensis Gallego is considered an autochthonous element (by the presence of specimens in life position), one can calculate that this life cycle lasted approximately 30 to 460 days (with average values between 56 and 310 days). Data taken by Gallego and Covacevich (1998, p. 128) for M. terneraensis Gallego from the La Ternera Formation and the La Coipa beds show similar parameters (number and spacing in the growth lines) for both localities. One can estimate that the inhabiting lasted between 28 and 365 days (with an average of 46 to 266 days). These relatively minor values of occupation of the water body coincides with interpretations of the environment of Bell and Suárez (1994) and Suárez et al. (1995) , who suggested a distal flood plain environment with vegetated banks and lakes (for the La Ternera Formation) and suspension deposits in deep water and dilute turbidity currents (for the La Coipa beds).
Gallego et al. (2004) concluded for Euestheria martinsnetoi Gallego a shallow and ephemeral lacustrine environment with a hot and humid climate for the Río Mendoza Formation (Cuyana Basin) with a life cycle of 37 to 409 days (mean 50 to 300 days). Of course, these values are relative and much influenced by diverse factors, showing that in the most stable settings (environmentally and temporally) the life cycle, or better, the occupation is prolonged a little more.
From the biostratigaphic point of view, M. terneraensis Gallego and Polygrapta troncosoi (Gallego), from the La Coipa beds and the Upper Triassic of the Biobío river are present in monospecific assemblages. In the La Coipa beds they were recorded in different localities and probably came from distinct stratigraphic levels, according to lithologic evidence (see Suárez et al., 1995). In the Upper Triassic of the Biobío river, both species coexist in the same stratigraphic intervals (FL1, 2), but P. troncosoi (Gallego) occurs in fewer numbers than the other species. In the case of the La Coipa beds, it is possible that this distribution is due to different environmental preferences by each of the species. M. terneraensis Gallego always has been recorded in black shales, an indicator of lakes with abundant organic matter, deep, and with long durations. However, in this case the specimens present in the black shale are para-autochthonous (for other authors, allochthonous), being transported from marginal (oxygenated) areas of the lake to deep and anoxic facies. In the case of the P. troncosoi (Gallego) data are not so conclusive. They probably preferred environments different than that of M. terneraensis Gallego (in Gallego and Covacevich, 1998, p. 128). Due to their short stratigraphic range, both genera and species could be used tentatively as 'local fossil guides' to correlate chronologically the continental Upper Triassic sediments from Chile and Argentina.
Due to their close similarities in the carapace size, outline and type of ornamentation, Carapacestheria Shen probably evolved from Menucoestheria Gallego or from other related species from the Jurassic of Patagonia, such as Cyzicus (Euestheria) taschi Vallati, 1986 (and other Argentinean forms) that probably belongs to the family Eosestheriidae. Menucoestheria was probably the genus from which all of the Gondwanian members of the family Eosestheriidae originated. If this is the case, the Jurassic forms subsequently migrated across Patagonia through the Antarctic continent to Asia and Europe (see Martins-Neto et al., 2003).
The presence of Menucoestheria terneraensis Gallego in Upper Triassic sediments of the Biobío river shows that the family Eosestheriidae had a wide distribution in southern South America during the Triassic, and in Argentinean Patagonia (?Yanjiestheria sp. and other related forms are still under study) and Antarctica (Carapacestheria disgregaris (Tasch) Shen, 1994 and C. balli Shen, 1994) during the Jurassic. A possible record in Brazil can not be discarded, because in the abundant Jurassic and Cretaceous faunas it is likely that these forms have been interpreted as euestherid or fushunograptid ('lioestherid') conchostracans.
Until this record (Gallego and Covacevich, 1998, and this paper) the family Eosestheriidae had a biochron from the late Early Jurassic to the Early Cretaceous (Chen and Shen, 1985; Shen, 1994). Its record is here extended with certainty back to the Upper Triassic. Moreover, these findings show its Gondwanian or South American origin and their probable ancestral relationship with other members of the family Euestheriidae, due to the fact that the genus Euestheria Depéret and Mazerán, 1912 is the best represented in the South American Triassic record.
With respect to the paleoentomofauna, the new taxa described here, as well as Bandelnielsenia chilena Martins-Neto and Gallego, 2003 (in Martins-Neto et al., 2003), represent the first records for the Chilean territory. Although these records are only a start when compared with other known findings, they are important in extending the paleogeographic distribution of typical Triassic groups represented by Ademosyne spp., which was described for several Argentinean localities. The genus Ischichucasyne now has one undiscribed species from the Ischichuca Formation (Argentina) and another (described here) from the Upper Triassic of the Biobío river (south central Chile), also extending its paleogeographical distribution. Of particular interest for the phylogeny of the group is Bandelnielsenia (Dysmorphoptilidae), which until now was the most plesiomorphic of the known genera. Judging from this first collecting effort, the potential of the Chilean paleoentomofauna is considerable and fundamental toward our knowledge of the real insect diversity during Triassic times in South America.
Dr. K. Bandel (Universität Hamburg, Germany) is thanked for his help and friendship during field work. The authors are grateful to Licenciada C. Salgado and Ingeniera C. Peichotto (from the SEM staff-Secretaría General de Ciencia y Técnica-Universidad Nacional del Nordeste, Argentina) for their assistance. The authors thank also R.Z. Rodríguez (Universidad Nacional del Nordeste, Argentina) for his work in the photographic laboratory, G. Barrios (Argentina) for his work with digital figures, and Dr. S. Yan-bin (Nanjing Institute of Geology and Palaeontology, China) for his comments and suggestions about conchostracans as one of the reviewers. Thanks also to the Consejo Nacional de Investigaciones Científica y Técnicas (CONICET, Argentina) for a postdoctoral fellowship to OFG, and the Universidad Nacional del Nordeste (Corrientes, Argentina). This research was partly supported by the Consejo Nacional de Investigaciones Científicas y Técnicas (Work plan aid) and by the Secretaría General de Ciencia y Técnica, Universidad Nacional del Nordeste (Grant PI-64/04), both to OFG. Field work of SNN was partly financed by the 'Emmy and A.B. Steffens Memorial Fund' of the University of Hamburg, Germany. The authors thank the reviewers Drs. M. Leppe and S. Palma-H. (Universidad de Concepción, Chile) and the editors M. Suárez and E. Pérez d'A. for their suggestions and helpful comments on the manuscript. Dr. A. Troncoso (Universidad de Talca, Chile) provided important literature and Dr. T. DeVries (Burton, USA) corrected the English language.
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Manuscript received: March 11, 2004: accepted: November 18, 2004.