The Los Rastros Formation is a mid-Carnian fossiliferous formation of the Agua de la Peña Group, in the Ischigualasto-Villa Unión Basin of northwestern Argentina. Fossil archosaur tracks,Leonardi, 1994, p.20 as well as many insects, fish, bivalves, crustaceans and plants have been reported from the formation.
Geology The formation overlies the Chañares Formation (and/or the Ischicuca Formation), and is overlain by the Ischigualasto Formation.Aceituno Cieri, 2015, p.60 The formation belongs to the Agua de la Peña Group of the Ischigualasto-Villa Unión Basin, where it is exposed in the Ischigualasto Provincial Park, a World Heritage Site in Argentina. The up to thick formation of claystones, mudstones and sandstones was deposited in a deltaic to lacustrine environment during the first post-rift sequence in the basin.
A CA-TIMS U-Pb zircon age of 234.47 ± 0.44 Ma from the lower Los Rastros Formation was published in 2020. The acquired age demonstrates that most of the overlying lacustrine strata within the basin were deposited during or after the Carnian Pluvial Event (CPE).
Paleobiota The only tetrapod body fossil reported from the Los Rastros Formation is a poorly-preserved temnospondyl amphibian.
Ichnofossils Tetrapod footprints are a conspicuous type of ichnofossils (trace fossils) found in the Los Rastros Formation, particularly at the Quebrada de Ischichuca locality near Cerro Bola. Most of the trackways are symmetrical prints created by animals with a narrow bipedal stance, three toes contacting the ground, and a raised fourth toe. They were presumably early dinosaurs or dinosauromorphs, with a hip height exceeding 75 cm. These prints are found in deltaic sandbars, parallel to the lakeshore. While some sources tentatively interpret these as theropod footprints,Weishampel et al., 2004, pp. 528 the generalized nature of Triassic dinosaurs and their relatives prevent a precise referral. Some of the tridactyl trackways also preserve hand prints, with quadrupedal pseudosuchians or other archosaurs as likely trackmakers. Named reptile ichnotaxa from the Los Rastros Formation include Rhynchosauroides sp. (sphenodonts or other small reptiles) and Rigalites ischigualastianus (large quadrupedal pseudosuchians).
Another form of trackway consists of numerous non-overlapping sets of broad rounded depressions on a heavily trampled wackestone surface. They may have been left by a herd of dicynodonts walking down to the lakeshore. Palaeophycus tubularis invertebrate burrows are also abundant.
Fish Actinopterygians (ray-finned fish) of the Los Rastros FormationGenusSpeciesLocalityMaterialNotesImagesChallaiaC. elongataAgua de la Peña Creek (San Juan Province)A very large (>70 cm) specimen which is mostly complete, missing the tail and the front of the skull.A late-surviving acrolepid, also known from the Cuyo Basin. Previously referred to Myriolepis.GualolepisG. carinaesquamosaCañón del Gualo (La Rioja Province)Two specimens, each representing portions of the skull and anterior torso.A probable chondrostean (distantly related to modern sturgeons and paddlefish) with similarities to the family Peipiaosteidae.RastrolepisR. latipinnataCañón del Gualo (La Rioja Province)A large (>39 cm) specimen which is mostly complete, missing the rear of the tail and the front of the skull.An indeterminate actinopterygian similar to redfieldiiformsR. riojaensisCañón del Gualo (La Rioja Province)A large (>54 cm) specimen which is mostly complete.An indeterminate actinopterygian similar to redfieldiiforms
Invertebrates The Los Rastros Formation has produced many insect fossils, with most new species belonging to blattopterans (cockroaches and kin), coleopterans (beetles and kin), or hemipterans (true bugs). Among Triassic formations of South America, the Los Rastros Formation's insect diversity is only rivaled by the Potrerillos Formation in the Cuyo Basin. Other invertebrate fossils from the Los Rastros Formation include Lioestheria ("conchostracans" or clam shrimp) and Paleomutella (freshwater mussels).
Blattoptera Blattoptera (cockroaches and relatives) of the Los Rastros FormationGenusSpeciesLocalityMaterialNotesImagesArgentinoblattaA. herbstiRío Gualo (La Rioja Province)ArgentinoblattulaA. revelataRío Gualo (La Rioja Province)CondorblattaC. lutzaeRío Gualo, Los Chañares (La Rioja Province)HermosablattaH. crassatellaRío Gualo, Picos Gemelos (La Rioja Province)H. pectinataRío Gualo (La Rioja Province)LariojablattaL. chanarensisLos Chañares (La Rioja Province)MancusoblattaM. pulchellaRío Gualo (La Rioja Province)PulchellablattaP. nanaRío Gualo (La Rioja Province)SamaroblattaS. corrientesinaRío Gualo (La Rioja Province)S. gualoensisRío Gualo (La Rioja Province)TriassoblattaT. argentinaPicos Gemelos (La Rioja Province)
Coleoptera Coleoptera (beetles and relatives) of the Los Rastros FormationGenusSpeciesLocalityMaterialNotesImagesAdemosyneA. arcucciaeAn ademosynidA. elongatusAn ademosynidA. hexacostataAn ademosynidA. punctuadaAn ademosynidArgentinocupesA. abdalaiA cupedid (reticulated beetle)A. pulcherRío Gualo, Los Chañares (La Rioja Province)A cupedid (reticulated beetle)ArgentinosyneA. bonaparteiLos Chañares (La Rioja Province)A permosynidA. frenguelliiA permosynidA. gonaldiaeA permosynidA. gualoensisA permosynidA. losrastroensisA permosynidA. rugosaA permosynidCardiosyneC. elegansA possible elaterid (click beetle)C. obesaA possible elaterid (click beetle)GemelinaG. triangularisA possible elaterid (click beetle)
Hemiptera Indeterminate corixids (water boatmen) are also known from the Gualo locality. Hemipterans (true bugs) of the Los Rastros FormationGenusSpeciesLocalityMaterialNotesImagesApheloscytaA. mayaeGualo (La Rioja Province)A scytinopterid cicadomorph (a type of true bug related to leafhoppers and cicadas). Previously given the name Gualoscytina mayae.DysmorphoptiloidesD. losrastroensisGualo (La Rioja Province)A dysmorphoptilid auchenorrhynchanFulgoboleF. arcucciaeGualo (La Rioja Province)A mesojabloniid cicadomorph. Previously given the name Australocicada arcucciae.GallegomorphoptilaG. acostaiGualo (La Rioja Province)A dysmorphoptilid auchenorrhynchan, formerly referred to the genus Dysmorphoptiloides.G. brevipteraGualo (La Rioja Province)A dysmorphoptilid auchenorrhynchanG. giganteaGualo (La Rioja Province)A dysmorphoptilid auchenorrhynchan, formerly referred to the genus Dysmorphoptiloides.G. pulcherrimaGualo (La Rioja Province)A dysmorphoptilid auchenorrhynchan, formerly referred to the genus Dysmorphoptiloides.GualoscartaG. obscuraGualo (La Rioja Province)A dysmorphoptilid auchenorrhynchanMesoscytinaM. forsteraeGualo (La Rioja Province)A scytinopterid cicadomorph. Previously given the name Argentinopheloscyta forsterae.PopovigocimexP. yuriiGualo (La Rioja Province)ForewingA progonocimicid coleorrhynchan (moss bug)SaaloscytinaS. carmonaeLos Chañares, Gualo (La Rioja Province)A saaloscytinid cicadomorph. Fossils of this species were formerly given the names Chanarelytrina nana and Chanarescytina carmonae.YurigocimexY. popoviGualo (La Rioja Province)ForewingA progonocimicid coleorrhynchan (moss bug)
Other insects Other insects of the Los Rastros FormationGenusSpeciesLocalityMaterialNotesImagesArgentinoglosselytrinaA. pulchellaGualo (La Rioja Province)A polycytellid glosselytrodeanArgentinoperlidiumA. rogersiGualo (La Rioja Province)Forewing fragmentA gripopterigid plecopteran (stonefly).FrenguelliphlebiaF. labandeiraiGualo (La Rioja Province)Forewing fragmentAn indeterminate odonatopteran (a relative of dragonflies and damselfies)MiominaM. riojanaLos Chañares (La Rioja Province)A miomopteranNotopamphagopsisN. spp.Gualo (La Rioja Province)Wing fragmentsThree unnamed species of haglid ensiferans (cricket-like insects)
Plants A wide variety of plant fossils are known from the Los Rastros Formation, including both microfossils and macrofossils.
Most species from the formation have been listed by Lutz et al. (2011) and Pedernera et al. (2020).
Calamitales Coniferales Cycadales Equisetales Osmundales Rhexoxilales
Seed ferns "Seed fern" macrofossils of the Los Rastros FormationGenusSpeciesLocalityMaterialNotesImagesDejerseyaD. lobataIschigualasto Provincial Park (San Juan Province)Frond fragmentsA possible peltaspermD. lunensisIschigualasto Provincial Park (San Juan Province)Frond fragmentsA possible peltaspermDicroidiumD. argenteumIschigualasto Provincial Park (San Juan Province)Frond fragmentsAn umkomasialean (or corystosperm)D. crassumIschigualasto Provincial Park (San Juan Province)Frond fragmentsAn umkomasialean (or corystosperm)D. dubiumIschigualasto Provincial Park (San Juan Province), Ischichuca CreekFrond fragmentsAn umkomasialean (or corystosperm)D. lancifoliumIschigualasto Provincial Park (San Juan Province), Ischichuca CreekFrond fragmentsAn umkomasialean (or corystosperm)D. lineatumIschigualasto Provincial Park (San Juan Province)Frond fragmentsAn umkomasialean (or corystosperm)D. nondichotomaIschigualasto Provincial Park (San Juan Province)Frond fragmentsAn umkomasialean (or corystosperm)D. obtusifoliumIschigualasto Provincial Park (San Juan Province)Frond fragmentsAn umkomasialean (or corystosperm)D. odontopteroidesIschigualasto Provincial Park (San Juan Province), Gualo (La Rioja Province), Ischichuca CreekFrond fragmentsAn umkomasialean (or corystosperm)D. pinnis-distantibusIschichuca CreekAn umkomasialean (or corystosperm)JohnstoniaJ. coriaceaAgua de la Peña Creek, Ischichuca CreekFrond fragmentsAn umkomasialean (or corystosperm)J. dutoitiiIschigualasto Provincial Park (San Juan Province)Frond fragmentsAn umkomasialean (or corystosperm)J. stelznerianaIschigualasto Provincial Park (San Juan Province), Gualo (La Rioja Province) Ischichuca CreekFrond fragmentsAn umkomasialean (or corystosperm)MatatiellaM. rosetaSeed conesA seed fern of uncertain affinities, possibly a peltasperm or umkomasialeanPachydermophyllumP. papillosumAgua de la Peña Creek.Frond fragmentsA peltaspermP. praecordilleraeIschigualasto Provincial Park (San Juan Province)Frond fragmentsA peltaspermPeltaspermumP. monodiscumAgua de la Peña Creek.Seed conesA peltaspermRhexoxylonR. sp.Ischichuca CreekFossil woodAn umkomasialean (or corystosperm)RochipterisR. alexandrianaIschigualasto Provincial Park (San Juan Province)Frond fragmentsA possible petriellaleanUmkomasiaU. macleaniiStrobili (seed cones)An umkomasialean (or corystosperm)XylopterisX. argentinaIschigualasto Provincial Park (San Juan Province), Gualo (La Rioja Province), Ischichuca CreekFrond fragmentsAn umkomasialean (or corystosperm)X. densifoliaIschigualasto Provincial Park (San Juan Province)Frond fragmentsAn umkomasialean (or corystosperm)X. elongataIschigualasto Provincial Park (San Juan Province), Gualo (La Rioja Province), Ischichuca CreekFrond fragmentsAn umkomasialean (or corystosperm)X. rigidaIschigualasto Provincial Park (San Juan Province)Frond fragmentsAn umkomasialean (or corystosperm)ZuberiaZ. browniiIschichuca CreekAn umkomasialean (or corystosperm)Z. feistmanteliiGualo (La Rioja Province)Frond fragmentsAn umkomasialean (or corystosperm)Z. sahniiIschigualasto Provincial Park (San Juan Province), Ischichuca CreekFrond fragmentsAn umkomasialean (or corystosperm)Z. zuberiIschigualasto Provincial Park (San Juan Province), Ischichuca CreekFrond fragmentsAn umkomasialean (or corystosperm)
Other gymnosperms