The La Boca Formation is a geological formation in Tamaulipas state, northeast Mexico. It was originally thought to date back to the Early Jurassic, concretely the Pliensbachian stage epoch of 193–184 Ma. Later studies found that while the unit itself was likely deposited during the earliest Pliensbachian, as proven by zircon dating 189.0 ± 0.2 Ma, the local vulcanism (related to the aperture of the Atlantic Ocean and the several Rift Events) continued until the Bajocian.
However, the lower section of the fossil taxa deposited on the rocks above the La Boca Formation is likely of Late Pliensbachian-Lower Toarcian age, and the upper section of Late Toarcian-Late Aalenian age. More recent works have correlated this unit with the fault movement along the Motagua-Chixoy-Polochic Transform Fault and the first sea transgression in the Portal del Balsas, producing a floodplain.
Due to successions of Aalenian depositional systems on the upper layers of the Huizachal Canyon, has been delimited the formation to the Toarcian stage. Deposits of Late Triassic Age referred to this unit have been reclassified in a new formation, El Alamar Formation. In North America, La Boca Formation was found to be a regional equivalent of the Eagle Mills redbeds of southern United States, the Todos Santos Formation of southern Mexico and the Barracas Group of the Sonora desert region.
__TOC__
Paleoenvironment La Boca Formation is genetically related to the Nazas volcanic Arc of the same age (Pliensbachian-Aalenian, ~189.5–171.6 Ma), which was created when Mexico evolved in a convergent plate margin, with the Gulf of Mexico remaining as a restricted basin and a passive margin. The influence of this arc is seen on the continental units such as Todos Santos Formation, which deposited volcanic materials in both nonmarine strata and marginal marine red beds of eastern Mexico.
La Boca Formation left its sediments on a basin formed between the Nazas Volcanic Arc center and the so-called Huizachal-Peregrina Anticlinorium, giving the basin layers whose origin is linked with braided river deposits with different flooding levels, channels fills, and channel belts filling valleys.
In locations such as Aramberri, the development of fluvial channels and the flooding of surfaces was restricted due to the presence of flanking volcanic activity, as well the local Paleozoic basement highs. In this outcrop the fluvial system evolved in several ways. Towards the north, it meanders from braided to ephemeral sandy, with the presence of common laminated sands sheets. The latter are likely a local indicator of unconfined flash floods across floodplains, with some sections recovering periods of desiccation thanks to the presence of mudcracks.
Towards the south, in localities such as El Olmo Canyon, the layers show gravelly braided rivers, oriented east-west, which then evolve into high-sinuosity single-thread meandering rivers. Other southern localities, such as the Caballeros Canyon and Huizachal Canyon, have layers that record gravel-bed braided rivers over a floodplain with high-energy flows: an element recorded on the local stratigraphy as older layers which were highly degraded by the increased force of the younger flows. The rock fragments moved by the currents are bigger in upper layers. In the southernmost outcrop, in Miquihuana the sheet sands show greater flooding events than on any other location.
In the main fossiliferous level of the Huizachal Canyon, in which more than 8000 specimens have been found, the preservation of delicate specimens such as Pterosaurs suggests an environment with little transportation and reworking. Yet the fossils were not buried in situ, as most of the smaller specimens show disarticulation.
All data trends suggest a highly unusual debris-flow environment in which local fluvial alluvial bodies were not big enough to sustain large freshwater biota such as fishes, and most of the preserved specimens were fast-buried near the place of death.
Fossil record Ichnofossils Genus Species Location Material Type Made by Notes ImagesMermia M. isp. Valle de HuizachalLocomotion traceDomichnia & Fodinichnia Worms Chironomidae midge larvaeDensely looped grooves or ridges.Scoyenia S. isp. Valle de HuizachalBurrowsDomichnia & Fodinichnia Crustaceans WormsBurrow fossils in lacustrine or fluvial environments, probably made by arthropods.
Synapsida Genus Species Location Stratigraphic position Abundance Notes Images Bocaconodon B. tamaulipensisJim's Joy, Huizachal Canyon Lower Part TeethA basal Mammaliaform Bocatherium B. mexicanumHuizachal Canyon Lower PartIGM 3492, SkullA Tritylodontid HuasteconodonH. wibleiHuizachal Canyon Lower PartTeethA Gobiconodont(?) VictoriaconodonV. inaequalisRene's Roost, Huizachal Canyon Lower Part TeethA Triconodontid(?)Mammaliaforms indet.IndeterminateCementario, Huizachal Canyon Casa de Fidencio, Huizachal CanyonLower Part IGM 6855, partial right dentary; IGM 6856, left dentary; IGM 6622, partial right dentary
Lepidosauromorpha Genus Species Location Stratigraphic position Abundance Notes Images TamaulipasaurusT. morenoiDinosaur National Monument South, Huizachal Canyon Lower Part IGM 6620, a nearly complete skull, missing the tip of the rostrum, with articulated mandible and eight articulated vertebrae IGM 6621, a nearly complete skull with articulated mandible IGM 6623, an isolated braincaseA strange burrowing Lepidosauromorph. Described originally as a "Burrowing diapsid representing a heretofore unknown clade", was later found to have a compact skull similar to that of Dibamidae and Amphisbaenia by convergent evolution.
Sphenodontia Genus Species Location Stratigraphic position Abundance Notes Images Clevosaurusaff. C. sp.Puente de Piedra JMC 85-A Rene Roost Tierra Buena del Sur at Huizachal CanyonLower Part IGM 6565, left lower jaw IGM 6566, split left lower jaw IGM 6567, broken lower jaw IGM 6568, almost complete lower jaw IGM 6569, split right? lower jaw IGM 6570, split right? lower jaw.A Sphenodontidae Rhynchocephalian of the family Clevosauridae. CynosphenodonC. huizachalensisHuizachal Canyon Lower Part IGM 6652 Dental portion of the right dentary with a complete tooth row IGM 6653, posterior right dentary IGM 6654, anterior portion of a right maxilla IGM 6655, middle portion of a right maxilla IGM 6656, anterior part of a right dentary IGM 6657, anterior portion of a right dentary IGM 6658, symphysial region of a right dentary IGM 6659, anterior part of a left dentary IGM 6660, posterior part of a left dentary with coronoid processA sphenodontine rhynchocephalian closely related to the living tuatara. Opisthiasaff. O. sp.Rene's Roost and Puente de Piedra Fidencio Hideway Oeste de DNMS in Huizachal CanyonLower Part IGM 6571, middle portion of a left lower jaw IGM 6572, partial right lower lower jaw IGM 6573, complete eroded right lower jaw IGM 6574, left lower jaw ?IGM 6748, partial right lower jaw IGM 6749, partial left lower jaw IGM 6750, almost complete right maxilla IGM 6751, anterior portion of a right lower jaw IGM 6752, partial right? lower jaw IGM 6753, partial right lower jaw IGM 6754, partial left? lower jaw IGM 6755, very small partial left lower jaw IGM 6756, partial left lower jaw IGM 6757, partial right mandible, right maxilla, two incomplete vertebrae, a humerus epiphysis and some not recognized long bones IGM 6758, partial right maxilla.A Sphenodontidae Rhynchocephalian of the family Opisthodontia. SphenoviperaS. jimmysjoyiJim's Joy, Huizachal Canyon Lower Part IGM 6076, an almost complete right lower jaw with teeth A possible venomous Sphenodont ZapatadonZ. ejidoensisTierra Buena, western part of the Huizachal Canyon Lower PartIGM 3497, crushed skull, missing part of the skull table and roofing bones A dwarf Sphenodont
Pterosauria Genus Species Location Stratigraphic position Abundance Notes Images "Dimorphodon""D." weintraubiHuizachal Canyon Lower Part Fragmentary skeleton of a large rhamphorhynchoid that includes a remarkably preserved pes.A Pterosaur of uncertain phylogenetic placement, being originally proposed as a member of the genus Dimorphodon, although, has some great differences with the original holotype, and Dimorphodon is a lower Liassic Genus. Most recent analyses place it on different positions on Novialoidea.
Crocodylomorpha Genus Species Location Stratigraphic position Abundance Notes ImagesMetasuchia indet.IndeterminateHuizachal Canyon Lower Part IGM 3498 & additional specimens. Partial skulls and postcranial skeletons Preliminary results suggest it may be a stem metasuchian.Protosuchia? indet.IndeterminateHuizachal Canyon Lower Part Skull fragment Found to be sister taxon of Platyognathus from the Lower Jurassic Lufeng Formation.
Dinosauria Genus Species Location Stratigraphic position Abundance Notes ImagesCeratosauria indet.IndeterminateCasa de Fidencio, Huizachal Canyon Lower PartIGM 6625, craneal fragmentary elementsA possible basal ceratosaur related with the African Berberosaurus.Heterodontosauridae indet.cf.H.? sp.Huizachal Canyon Lower PartTeethAn Ornithischian of the family Heterodontosauridae.Neotheropoda indet.IndeterminateHuizachal Canyon Lower PartIsolated teethSeveral morphotypes, maybe related with Coelophysoidea, Dilophosauridae or Tetanurae.Sauropodomorpha? indet.IndeterminateRene's Roost, Huizachal Canyon Lower Part Large bone fragmentsA possible Basal Sauropodomorph."Syntarsus""S." mexicanum Casa de Fidencio, Huizachal CanyonLower Part (IGM 6624) partial twelfth dorsal vertebra, partial thirteenth dorsal vertebra, partial synsacrum, incomplete fused pelvisAn indeterminate Coelophysoidean.
Palynology Genus Species Location Stratigraphic position Material Notes ImagesAraucariacitesA. cf. australisLa Escondida canyon La Boca canyonLower MemberPollenA Pollen Grain, affinites with the family Araucariaceae inside Pinales. Conifer pollen from medium to large arboreal plants.ExesipollenitesE. tumulusLa Escondida canyon La Boca canyonLower MemberPollenA Pollen Grain, affinities with the Hirmeriellaceae in the Pinopsida. DapcodiniumD. priscumLa Escondida canyon La Boca canyonLower MemberCystsA Dinoflajellate of the family Rhaetogonyaulacaceae inside GonyaulacalesEucommiiditesE. troedssoniiLa Escondida canyon La Boca canyonLower MemberPollenA Pollen Grain, afinnities with Erdtmanithecales inside Spermatophytes.ExesipollenitesE. tumulusLa Escondida canyon La Boca canyonLower MemberPollenA Pollen Grain, affinities with the family Cupressaceae in the Pinopsida. Pollen that resembles that of extant genera such as the genus Actinostrobus and Austrocedrus, probably derived from dry environments.DictyophilliditesD. sp. 1La Escondida canyon La Boca canyonLower MemberSporesA Miospore, affinities with Dipteridaceae inside Filicopsida KrausellisporitesK. reissingeriLa Escondida canyon La Boca canyonLower MemberSporesA Miospore, affinities with Selaginellaceae or Lycopodiaceae inside Lycopsida.NannoceratopsisN. gracilisLa Escondida canyon La Boca canyonLower MemberCystsA Dinoflajellate of the family Nannoceratopsiaceae inside NannoceratopsialesOvalipollisO. breviformisLa Escondida canyon La Boca canyonLower MemberPollenA Pollen Grain, afinnities with Caytoniales inside Gymnospermopsida. PareodiniaP. sp.La Escondida canyon La Boca canyonLower MemberCystsA Dinoflajellate of the family Pareodinioideae inside GonyaulacalesQuadraeculinaQ. anallaeformisLa Escondida canyon La Boca canyonLower MemberPollenA Pollen Grain, affinities with Podocarpaceae and Pinaceae inside Coniferophyta.RhaetogonyaulaxR. rhaeticaLa Escondida canyon La Boca canyonLower MemberCystsA Dinoflajellate of the family Peridiniphycidae inside DinophyceaeSpheripollenitesS. spp.La Escondida canyon La Boca canyonLower MemberPollenA Pollen Grain, affinities with the Hirmeriellaceae in the Pinopsida. VitreisporitesV. pallidus V. bjuvensisLa Escondida canyon