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.[3] 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.[4]
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.[7] 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.[7]
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.[7] 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.[7] 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.[7]
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.[8]
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.[8]
Densely looped grooves or ridges. Taxon linked with the major Early Jurassic flooding of the Huizachal Valley, developed locally mostly on a gravel-bed braided fluvial style with paleocurrents oriented west-northwest
Dinosaur 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 braincase
A strange burrowing Lepidosauromorph, probably related with Ardeosauridae. 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.[13]
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.[20]
A 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.
Affinities with Williamsoniaceae inside Bennettitales. Representative of large arboreal to low arbustive Bennetittes. The dominant foliar remain recovered on the formation, with up to 50 specimens
^ abcdeFraser, Nicholas; Sues, Hans-Dieter, eds. (1994). In the Shadow of the Dinosaurs: Early Mesozoic Tetrapods. Cambridge University Press. ISBN0-521-45242-2.
^ abcStiegler, J.; Hernández-Rivera, R.; Clark, James M. (2020). "Two Small Crocodyliformes from the Middle Jurassic La Boca Formation of Tamaulipas, Mexico". The Society of Vertebrate Paleontology 80th Annual Meeting · Virtual 2020. 80 (1): 32.
^Clark, James; Montellano, Marisol; Hopson, James A.; Hernandez, Rene; Fastovsky, David A. (1994). "An Early or Middle Jurassic tetrapod assemblage from the La Boca Formation, northeastern Mexico". In Fraser, N.C.; Sues H.-D. (eds.). In The Shadow of the Dinosaurs: Early Mesozoic Tetrapods. Cambridge: Cambridge University Press. pp. 295–302. ISBN0-521-45899-4.
