Mesozoic Coleopteran Faunas from Argentina: Geological Context, Diversity, Taphonomic Observations, and Comparison with Other Fossil Insect Records

1 Entomoloǵıa, Departamento de Bioloǵıa, Facultad de Ciencias Exactas y Naturales y Agrimensura, Universidad Nacional del Nordeste and Área Paleontoloǵıa, Centro de Ecoloǵıa Aplicada del Litoral (CONICET), Casilla de Correo 128, 3400 Corrientes, Argentina 2 Micropaleontoloǵıa, Departamento de Bioloǵıa, Facultad de Ciencias Exactas y Naturales y Agrimensura, Universidad Nacional del Nordeste and Área Paleontoloǵıa, Centro de Ecoloǵıa Aplicada del Litoral (CONICET), Casilla de Correo 128, 3400 Corrientes, Argentina 3 Área Paleontoloǵıa, Centro de Ecoloǵıa Aplicada del Litoral (CONICET), Casilla de Correo 128, 3400 Corrientes, Argentina


Introduction
Continental invertebrate communities from the Mesozoic Era are represented principally by two phyla: Arthropoda and Mollusca. Arthropods constitute the most abundant and diverse fossil record in lacustrine sequences of Argentina with "conchostracans", insects and ostracods as most representative groups. The mollusks, represented by bivalve and occasionally gastropods, have a low diversity and restricted distribution [1].
The recent insects constitute the richest class in terms of species diversity with estimates ranging from 3 to 50 million species [2][3][4][5]. There are 1275 families of known insects in the fossil record and 967 presently existing, of which 70% are also known as fossils [6,7]. The data from insect fami-lies/genera indicate that the group's growth model follows an exponential curve of evolution, contrary to the occurrence of new orders, which declines [8,9].
The Coleoptera represents the most diverse order within the Class Insecta, both taxonomically and ecologically. In addition, it is the most specious group in the Animal Kingdom with the number of described modern species exceeding 350,000, representing about 40% of the known insect fauna [10]. This diversity is probably related to certain features that allow adult living in restricted niches while retaining their ability to fly [10].
The systematics and phylogeny of fossil beetles is a very complex area, with old and new proposals and numerous publications which surprisingly still not clear this question. Most studies use different methodologies and are based Psyche mainly on adult morphology and rarely use immature stages of ontogenetic development. The basis of modern classification of fossil coleopterans was initially established by Crowson [10,11].
Mesozoic beetles were much more common and diverse than Paleozoic; geological information shows that beetles have had a dominant record among the group of insects since early Jurassic [16]. However, further information still remains elusive as the Mesozoic beetles are less informative, because of isolate elytra, than those of the Paleozoic [11]. The Mesozoic associations consisted mainly of xylophagous forms and larval stages have been found in numerous localities, except in the Upper Cretaceous.
The aim of this paper is to present the record of abundance and diversity of Mesozoic beetles in Argentina. The information comes from a bibliographic compilation and study of materials found in recent paleontological expeditions conducted by our research group. At the same time, we provide information on the geology and age of localities where the material was collected and present a comparison with other Mesozoic groups of insects collected in Argentina and contemporary faunas from southern Hemisphere. Finally, it provides paleobiogeographic information and highlights the importance of beetles in understanding the evolution of the Mesozoic after the Permian Triassic extinction event.
Our analyses are based primarily on elytra in the Upper Triassic localities from Argentina, which provide a significant amount of information for Coleoptera and allow to have a vision of the composition of the assemblages about 252 million years ago. Cretaceous and Jurassic materials are mentioned in passing, and their study is in progress.

Previous Coleopteran Records
Contributions involving the study of beetles in Mesozoic continental sediments of Argentina began with Frenguelli [17,18], who observed small curculionid elytra in shale samples from southwestern Mendoza Province (stratigraphical levels unknown) and other beetle elytra originating from various levels of the Ischigualasto Formation (probably levels of the Los Rastros Formation sensu Stipanicic and Bonaparte [19]), San Juan Province. Fossa-Mancini [20] reported the presence of galleries related to burrowing larvae of certain xylophagous beetles in silicified trunks from Upper Cretaceous of Patagonia. Feruglio [21] mentioned fossil beetles represented by silicified incomplete elytra, thorax, and abdomen remains, probably attributable to the Elateridae, from Laguna del Molino locality (Gran Bajo de San Julián) in Santa Cruz Province. Subsequently, Genise [22] presented a description of different ichnofossils found in fossil trunks and fructifications from the Upper Cretaceous of Rio Negro Province, assigned to the probable activity of beetle and termites larvae. Further, Genise and Hazeldine [23] described insect traces in fossil wood from La Matilde Formation from the Jurassic Petrified Forest of Jaramillo in Santa Cruz Province, probably assigned to the activity of buprestid larvae. Martins-Neto and Gallego [24] disclosed the remains of beetles consisting of isolated elytra and body parts from the La Matilde Formation of Gran Bajo de San Julián in Santa Cruz Province, assigned mainly to Caraboidea. Gallego et al. [25] reported a second discovery of fossil beetles as Argentinocupes and Argentinosyne and other insects as blattids and hemipterans in the Bermejo Basin in San Juan Province and the first record from the upper Los Rastros Formation of this basin.
The Order Coleoptera was also treated in the works of Martins-Neto et al. [26][27][28][29][30] and Martins-Neto and Gallego [31], where new Triassic species assigned to Permosynidae, Schizocoleidae, Cupedidae, and Elateridae were described.  It is important to emphasize that material collected in the Potrerillos-Cacheuta sequences during an expedition occur-Psyche 3 ring in April of 2010 was included in our analyses, as was Jurassic specimens from Chubut Province, collected during a 2009 fieldtrip and material from the Los Rastros Formation. This newly discovered material is important for understanding beetle evolution in Argentina during the Mesozoic because it is so diverse, abundant and well preserved.

Material and Methods
The material cited in the literature is deposited in the paleontological collection with the acronyms PULR-I (Invertebrate Paleontological Collection, Universidad Nacional de La Rioja, La Rioja Province), CTES-PZ (Paleozoological Collection of the Universidad Nacional del Nordeste, Corrientes Province), MCNAM (Museo de Ciencias Naturales y Antropológicas "J. C. Moyano", Mendoza Province), MHIN-UNSL-GEO (Museo de Historia Natural de la Universidad Nacional de San Luis, San Luis Province), MLP (Museo de La Plata, Invertebrate Paleontology, La Plata, Buenos Aires Province), and CORD-PZ (Palaeozoological Collection, Universidad Nacional de Cordoba, Córdoba Province).

Mesozoic Insects Record from Argentina
Most information on the Mesozoic insect faunas of Argentina comes from the Triassic Period and, above all, from collections made of the Los Rastros Formation, Bermejo Basin (La Rioja), and Potrerillos Formation, Cuyo Basin (Mendoza). Both provide 90% of the total abundance of insects in Argentina, constituting one of the most important records of continental life developed in Gondwana. In addition, these basins are known for their large extent, exceptional outcrops, well-developed stratigraphy columns, and the wealth of their taphofloras and vertebrate faunas.
In Argentina, knowledge of Mesozoic insects has made remarkable strides in recent years, having as a background the works published by Wieland [34,35], Tillyard [36], and Cabrera [37] on fossil insects from the Cacheuta and Potrerillos Formations of early Late Triassic age. So far, 86 species in 27 families of 12 orders have been described from 526 samples collected ( Figure 1). The fossil insect fauna of Argentina comes from stratigraphic levels of continental sediments assigned to the interval between the upper Middle Triassic to Lower Cretaceous. (Figure 2 miomopterans, orthopterans, trichopterans, plecopterans, odonatans, and grylloblattids originate from the Cerro de las Cabras Formation (1 specimen), the Potrerillos Formation (21 described species and 229 collected specimens), and the Cacheuta Formation (2 described species and 27 specimens) [29,30,[39][40][41][42][43].

Insects Records and Geological Context
The Malargüe Basin of southern Mendoza Province includes Choiyoi volcanic and overlain siliciclastic deposits. The insect fauna was collected from the upper section of the Llantenes Formation (Late Triassic) which is built of two coarsening-upward cycles reflecting a deltaic progradation of a fluvial into a lacustrine environment (lower part), succeeded by repeated progradation into a floodplain dominated environment (upper part; with insects, conchostracans, fish, and plants remains). The insect remains includes 2 coleopteran elytra and 1 mecopteran, and these news finds represent the youngest Triassic occurrence from Argentina and South American and the second youngest record from the Southern Hemisphere [33].
The Los Rastros and Ischichuca Formations (Agua de la Peña Group, La Rioja Province) are part of the Bermejo Basin (Ischigualasto-Villa Unión Basin), an extant basin located in northwestern Argentina. The Los Rastros Formation of early Late Triassic age is a lacustrine-deltaic that consists of several sedimentary cycles of black shales, siltstones, and sandstones. The succession is characterized by record of five plants, four invertebrates, and four vertebrates taphofacies [44]. The  insect record consists of 44 described insect species and 236 collected specimens and is comprised of coleopteran elytra and wing remains of blattids, plecopterans, miomopterans, orthopterans, glosselytrodeans, and odonatans [26,27,42,45] The Ischichuca Formation of late Middle Triassic to early Late Triassic age is predominantly composed of shallow and deep lacustrine facies with progradational deltaic successions, that starts to begin as fanglomerates with sandstones and tuffitic intercalations and continue with dark carbonaceous shales alternating with sandstones and pelitic and tuffaceous strata. In particular, insect remains are found in dark brown to olive-green claystones with abundant plant remains from the middle to the basal upper parts of the unit. This interval is interpreted as a shallow, partly saline lake (interpreted from a perennial playa lake association facies, L2 from Melchor [46,47]) that ranges to a deep freshwater lake (deep freshwater lake association facies, L3 from Melchor [46,47]).

5
The fossil insects, consisting of 11 described species and 14 specimens, comprise a blattid wing and coleopteran elytra [31,41,48]. (Table 1) is less well known than the Triassic one and also sparse when compared with other Jurassic localities of the world. This rarity is due to its restricted geographic distribution and low species diversity and is related to the low number of collections and studied localities.

Jurassic. The Jurassic insect fauna from Argentina
The first contribution probably was made by Frenguelli [18] who mentioned and illustrated the "rare insect remains" of a paleohemipteran wing from the "Estancia El Malacara" from Bahia Laura, in Santa Cruz, Province. In the 1990s one of us (O. F. Gallego) and Rafael Martins-Neto restudied this specimen and concluded that it is probably not an insect. Later, Feruglio [21] reported the presence of silicified coleopteran remains from the Laguna del Molino locality of Gran Bajo de San Julian, in Santa Cruz Province) associated with plant fragments, bones, freshwater mollusks, and conchostracans. These previous contributions of insect reports were summarized by Gallego and Martins-Neto [49], Martins Neto and Gallego [24,50], Monferran et al. [32,51,52], Genise et al. [53], and Gallego et al. [54] who reported new finds of insects from the orders Coleoptera (elytra and body remains), Heteroptera (fragmentary wings), and Trichoptera (wing fragments and larval cases) from both the La Matilde Formation of the Laguna del Molino locality and Cañadón Asfalto Formation of Cerro Cóndor and Estancia Fossati localities. These records are associated also with gastropods, bivalve mollusks, ostracods, conchostracans, plants, and fish remains. Other Jurassic insect records such as the presence of chironomid head capsules and larval cases [55] and mecopterans (Bittacidae) with complete bodies [56] come from Gan Gan locality of the Cañadón Asfalto Formation in Chubut Province.
The Jurassic fossil insect record from Argentina comes from two geological units: the La Matilde Formation (late Middle Jurassic, Santa Cruz Province) and the Cañadón Asfalto Formation of late Middle Jurassic to Late Jurassic age, Chubut Province.
The late Middle Jurassic La Matilde Formation comprises a volcaniclastic sequence that bears silicified woods (ferns and gymnosperms), a taphoflora, invertebrates, and tetrapods. The sediments show a typical continental sequence of a low-energy fluvial system, with lentic water bodies under reduced conditions within a floodplain environment, influenced by intensive volcanism evidenced by pyroclastic deposits [57].
The Cañadón Asfalto Formation of Middle-Late Jurassic age is a thick sedimentary sequence with volcanic intercalations that constitutes one of the most important nonmarine Jurassic records from South America. Two members can be distinguished, the lower one, the Las Chacritas Member, and the upper one, the Puesto Almada Member [58]. The insects were found in the upper member and particularly from the locality "Estancia La Sin Rumbo", an assemblage of "con- chostracans", ostracods, bivalves, and caddisfly cases. This assemblage occurs in the upper part of a volcaniclastic lacustrine sequence consisting of yellowish tuffs and tuffites providing evidence of dry climatic conditions. The assemblage recorded from the Estancia Fossati locality came from three levels consisting of shales, limestones, and tuffs, associated with invertebrates such as bivalve mollusks, insect larval cases, ostracods, conchostracans), and fish scales. The Estancia Fossati faunas represent low-energy shallow freshwater environments within associated plant communities [54,59].

Cretaceous.
The poorly known Cretaceous insect fauna from Argentina is reflected by only 5 described species and only 8 collected specimens (Table 1). These records are based on two Early Cretaceous localities: (a) the Anfiteatro de Ticó Formation, Bajo Grande locality, from Santa Cruz Province, and (b) the La Cantera Formation, Gigante Group of San Luis Province. The Anfiteatro de Ticó Formation of the Baqueró Group is composed by siliciclastic and volcaniclastic deposits [60] and the insect fauna consisting of the species Blattulopsis popovi (Blattida) [61] and Argentinopetala archangelskyi (Odonata) [62].
The La Cantera Formation of the Gigante Group exhibits greenish to gray limonite and arcilite with intercalated red sandstones and shales in the upper part [63]. The La Cantera assemblage is composed of a large, diverse association of ostracods, insects (hemipterans, orthopterans, coleopterans and caddisfly larval cases), palynomorphs, plants (leaves and sphenopsid reproductive organs), and fish fragments [64]. The paleontological content of this deposit suggests a lakeshore environment, attributable to fish taxonomic diversity and the poor state of terrestrial insect preservation [65]. Insect remains comprise of Canteronecta irajae (Naucoroidea) [66], Rhomboidella popovi (Corixidae) [67] and Psyche Notonecta mazzoniae (Notonectidae) [65]. Also are recorded the poorly preserved orders Coleoptera, Orthoptera, and Trichoptera.

Mesozoic Coleopterans from Argentina
The analysis of Coleoptera used information obtained from previous publications [21, 26-33, 39, 44]. Information was also obtained from observation of new unpublished material collected in the 2010 expedition to the Potrerillos Formation of Mendoza Province (Figures 4(f)-4(r)), Los Rastros Formation of La Rioja Province, and Jurassic specimens from Cañadón Asfalto Formation of Chubut Province.
The beetles preserved in the various localities appear as complete impressions, body part impressions (abdomens, thoraces), and as elytra moulds that can be complete or fragmentary, articulated or disarticulated, smooth or striated, with or without ornamentation.
Two analyses were performed. The first considered the state of preservation, namely, if the specimen was complete/incomplete, and its degree of articulation/disarticulation. For these measurements, there were used 262 elytra distributed in the different formations as shown in Table 2.

Jurassic and Cretaceous Coleopterans.
Knowledge of beetles and of insects in general from Jurassic and Cretaceous sediments in Argentina is poorly developed. This absence of data highlights the importance of exploring for new fossil insect localities. In general, beetles are only referenced in works that treat other insect groups. By contrast, information generated for the Jurassic and the Cretaceous of Argentina will allow its characterization and comparison with other continental faunas from the Southern Hemisphere (South America, Antarctica, and Australia), currently which are better known. Therefore, owing to the scarce information about the Jurassic and Cretaceous specimens, the present study has considered only examination of Triassic material.

Comments on Other Insect Orders from Argentina (Figure 6)
Hemiptera is the second most common order in the Mesozoic from Argentina with 114 specimens collected (22%) and 19 described species from various assemblages. The material is preserved as impressions of wings (fore or hind, complete or fragmentary, with or without a clavus) and occasionally complete bodies of insects, most attributable to the families Dysmorphoptilidae (9 specimens) and Scytinopteridae (7 specimens); the genus Gallegomorphoptila (9 specimens); and the specie Gallegomorphoptila acostai (5 specimens). For the rest of the Mesozoic, there are only three recorded Cretaceous species: Canteronecta irajae (Naucoroidea) [66], Rhomboidella popovi (Corixidae) [67], and Notonecta mazzoniae (Notonectidae) [65]. The Blattida (cockroaches) includes 40 specimens (7%) and 18 species, preserved as disarticulated tegmina (complete or fragmentary, with missing or not the clavus). The assemblage is dominated by the families Mancusoblattidae and Mesoblattinidae (10 specimens) and by the genus Hermosablatta (9 specimens). The species Samaroblatta corrientesina, S. gualoensis, Hermosablatta crassatella, H. pectinata, Lariojablatta neiffi, and Condorblatta lutzae are the most abundant taxa. The Blattulopsis popovi species occurs in the Lower Cretaceous of Santa Cruz Province [61].
The rest of the orders, Plecoptera and Orthoptera with 16 specimens, Odonata with 8 specimens, Glosselytrodea with   3, Miomoptera with 2 specimens, and Mecoptera, Grylloblattida, Trichoptera, and Hymenoptera, all with one specimen each, constitute 9% of the total abundance of the material collected. Each one is represented by a handful of impressions of fore wings and complete or fragmentary bodies of nymphs such as head and abdominal sclerites. The label "indeterminate" record (57 specimens, 11%) includes the complete, incomplete or poorly preserved elements, in which we were unable to be confidently assigned to any particular order or alternatively it is under study. They are represented by impressions of wings (complete or fragmentary), some elements associated with abdominal sclerites and partial bodies.

Paleoecology and Taphonomy
Given the aforementioned information on the Mesozoic insect fauna of Argentina, the material recovered includes disarticulated and fragmentary specimens (evidence of postmortem transportation) approximately 97.5% of which comprises mostly impressions of wings of hemipterans and blattids, elytra of beetles, and other isolated body parts (thoraces, abdomens, appendages) of the Orthoptera, Mecoptera, Grylloblattida, Plecoptera, Trichoptera, Miomoptera, Odonatoptera, Hymenoptera, and Glosselytrodea, in addition to head capsules and dwelling tubes of chironomid and caddisfly cases.
From an ecologic perspective, the analysis of this fauna shows that most groups have a subaerial or terrestrial habit as adults, and some are represented by immature (often aquatic forms), such as the Plecoptera, Grylloblattida, Hemiptera, and Odonata. This implies that the fossil record is biased by the absence of other autochthonous aquatic forms, the causes of which required future inquiry.

Final Considerations
Fossil beetles are one of the most interesting objects in paleontological and stratigraphical research, but they remain poorly understood, as they are difficult to study [68]. The great diversity and abundance that occurs in the fossil record is probably correlated with the composition of the elytra and its shape preservation, appearing as elytra moulds that can be articulated or disarticulated.
In South America, the previous literature reviews and the analyses of unpublished material indicate the relevance of Triassic and Tertiary beetle records. Nevertheless, the group is poorly studied and the intensity of collections is low. Therefore, it requires much more skilled work.
The record (   ages from the Lumbrera Formation (Grupo Salta) in Salta Province [79]; Late Paleocene to Early Eocene age from the Maíz Gordo Formation in Jujuy Province with thirty one species [80][81][82][83][84]; Eocene to Early Oligocene age from the Ventana Formation in Neuquen Province [85]; and Late Miocene age from the Palo Pintado Formation in Salta Province [86]. In Chile the material just comes from Santa Juana Formation of Late Triassic age with one species [87] and Dorothea Formation of Upper Cretaceous [88]. In Peru the specimens come from the Pleistocene tar-seeps of Talara in Piura Province [89].
The Mesozoic localities in Argentina, typically from the Triassic Period, have proven to be rich in fossil insect material; recent findings of insects in the strata of the Cuyo Basin have increased the number of taxa represented to currently 510 specimens. This makes Argentina one of the most important paleoentomologic regions not only in South America but also generally in the Southern Hemisphere, such as the Triassic sequences already known from Australia and South Africa.
All the knowledge provides invaluable information about the composition of the Mesozoic biota, essential for understanding biological processes that different groups of organisms experienced after the great Permian extinction event about 252 million years ago. In addition, time series of taxa within lineages could provide data for understanding beetles evolution.