Dinosaur Tracks of Mesozoic Basins in Brazil

The vertebrate ichnological data from the Brazilian Mesozoic basins represented by the fossil tracks are well-known, in fluvial, lakes and coastal deposits. They are generally recognized by their superficial morphological features, including digits, pedal pads, contour, claws, or as a sequence of deformations in a same bedding plane. There are also some footprints, especially the ones observed in natural cliffs, outcrops in roads and open mines that are recognized through cross sections. They are sometimes interpreted as load casts, convolute or fluidization structures related to the physical processes distinct from the ones produced by the dinosaur trampling. The footprints in this context are 3D structures, not restricted to the superficial bedding plane, but they deform many underlying beds. The Brazilian sedimentary basins where dinosaur footprints are found include intracratonic and rift basins. The main intracratonic basins with footprints are the Paraná, Sanfranciscana and Parnaíba basins, whose geological history spans from Paleozoic to Cenozoic in a syneclise tectonic context. During the Mesozoic, the South Atlantic opening created new basins in the present Atlantic margin and in the interior Proterozoic belts. The structural lineaments of the Brazilian and Pan-African provinces were intensely reactivated, with expressive vertical movements. In the interior of the Borborema Province and along the current Brazilian continental margin there were crustal ruptures and the origin of new sedimentary basins such as the Rio do Peixe, Recôncavo, Sergipe-Alagoas, Potiguar, São Luís and Tacutu basins. The dinosaur footprints are found in outcrops that indicate deposition in a wide variety of geological settings, including fluvial, lacustrine and seashore environments.

The central region of Rio Grande do Sul State, in southern Brazil, has increasing importance due the discoveries of Triassic fossils, including some of the world’s oldest dinosaurs and their tracks. Tridactyl footprints of Carnian age, from the Santa Maria Formation, have been identified as ?Grallator isp., in addition to indeterminate tracks of dinosaurs. These footprints are attributed to basal dinosauriforms and they are considered the earliest examples of dinosaur tracks in Brazil. These discoveries hold significant implications, as they are associated with a diverse fauna whose dating aligns with the Carnian Pluvial Episode. This is a pivotal period marked by substantial environmental changes linked to global warming, leading to significant extinctions and transitions in both flora and fauna. Other tracks, dating back to the Norian or possibly Rhaetian age, were reinterpreted as “Theropod tracks indet.”, and were produced by large theropod dinosaurs. These particular occurrences are connected to the Caturrita Formation and provide valuable insights into a later episode following the climate changes and their resulting effects on faunal and floral composition. Previously registered tracks within the Caturrita Formation at the Novo Treviso site, attributed to prosauropod dinosaurs, have been reinterpreted as belonging to the Guará Formation, ranging from the Late Jurassic to the Early Cretaceous. Thus, the Santa Maria-Caturrita sequence, a sedimentary record shaped by diverse processes in ancient fluvial systems, offers an almost continuous timeline of the transition between the emergence of dinosaurs and their eventual establishment as ecologically dominant elements in continental faunas. These Brazilian Triassic dinosaur tracks, albeit limited to two strata at present, provide valuable supplementary data to the osteological record, further reinforcing these significant events in paleontological history.

The Jurassic Guará Formation (Paraná Basin) was established in the early 2000's and since then dinosaur tracks and trackways have been found at the southwestern Rio Grande do Sul State of Brazil and in its Uruguayan counterpart, the Batoví Member of the Tacuarembó Formation. Sauropod, theropod, ankylosaurs and ornithopod dinosaur tracks have already been described from eolian facies of the Guará-Batoví unit, but some other new findings are reported here for first time. Saurischian tracks are the most common, being represented by narrow- to very wide-gauge sauropod trackways and at least two theropod track morphotypes, one of which comparable to the ichnogenera Iberosauripus and Jurabrontes. Ornithischian tracks are rarer but, up to this point, two lineages have been recorded. Ankylosaur tracks were recently described and compared to Metatetrapous, conjecture corroborated by new material. Ornithopod tracks are underrepresented at the Guará-Batoví. Although the dinosaur track record of the Guará-Batoví unit is mostly composed of undertracks, true tracks can also occur. This work aims to present a concise state of the art about the dinosaur ichnology of the Guará-Batoví unit. Together with invertebrate traces and the body fossil record of this unit, the paleogeographical, paleoenvironmental and paleoclimatic context of the dinosaur tracks are discussed.

Tetrapod fossil footprints are common in the aeolian sandstones from the Botucatu Formation in many Brazilian states, all around the edge of the Paraná Basin. This unit is represented by well-selected, reddish aeolian sandstones, related to an extensive dune field. Its age is Early Cretaceous (Berriasian-Barremian). The main ichnosite from the Botucatu Formation occurs in the interior of São Paulo State, in the Araraquara region, at the São Bento quarry, where slabs for covering and paving of public sidewalks, were extracted. This quarry presents a section of a large dune, about 20 m high and 100 m long, exhibiting foresets with a dip of 29° in the S-SW direction. Alongside the bedding surfaces there are abundant tracks attributed to mammals, Theropoda and Ornithopoda. These are herein grouped in four morphotypes, in addition to other kinds of isolated footprints difficult to classify. All these occurrences belong to bipedal and functionally tridactyl dinosaurs; no Sauropodomorpha tracks were found. To these theropod and ornithopod footprints and trackways from the eastern side of the Paraná Basin are added those of the western side, in the area of Nioaque, Mato Grosso do Sul State. The Botucatu paleodesert harbored a possibly endemic fauna, of bipedal carnivorous and herbivorous dinosaurs, which makes difficult a faunistic comparison with other localities, in other paleobiogeographic contexts.

The aeolian environments are often very restrictive to the preservation of fossil footprints. Dry sands and the low humidity substrate difficult the processes that allow the early cementation and prevent the destruction of the tracks by the wind erosion. Deflation of loose material from dry, uncemented sediments, transports away the clay and silt particles, and leaves only the sand and pebbles. These deposits, in the context of a hot and dry environment, show a narrow window of environmental conditions, which allow the footprints preservation. In the Early Cretaceous deserts of the central Gondwanaland existed well-adapted theropod communities to highly hot and arid climates. They are recognized, through ichnological data, as well as in the Sanfranciscana Basin, in the Paraná Basin (Botucatu Formation). In the Sanfranciscana Basin the dinosaur footprints occur in a context of humid interdune environment, in which isolated footprints and short theropod tracks were preserved.

Fossil footprints are generally recognized by morphological features from the feet registered on the unconsolidated substrate; therefore, they can also be understood as biogenic primary deformation structures. The footprints can be found in the bedding surface and as cross-section deformations of the strata. Then, the substrate properties and the animal’s behavior allow for a wide range of track morphologies. The main modes of footprint preservation can thus be evaluated as the relationship between the substrate and the lower surface of the feet. In the Araripe Basin, both kinds of preservation are found in the Mauriti, Rio da Batateira, Crato, and Exu formations. In the Mauriti Formation, the footprints are imprints in the bedding surface of fine to coarse-grained sandstones. The cross-section footprints occur in the Rio da Batateira, Crato, and Exu formations. These footprints are produced by foot pressure on a depositional surface, and transmitted downward inside a bed or a bed set. In some cases, they are difficult to recognize, as they can be misinterpreted as load or liquefaction inorganic features related to sediment compaction, usually triggered by earthquakes and not to trampling by terrestrial vertebrates. They are in reality load casts produced by dinosaur trampling, allowing evaluation of substrate consistency besides the potential trackmaker identification. The temporal and environmental contexts of the dinosaur footprints from the Araripe Basin include the dinosaur trampling in fluvial sand bars, floodplains, deltas, and saline-alkaline lake borders.

The Rio do Peixe basins are the set of four associated basins: Sousa, Uiraúna-Brejo das Freiras (also known as Triunfo), Pombal and Vertentes. They are located in the western Paraíba State and in the Ceará State, northeastern Brazil, in a total area of ~1,280 km². The origin of these basins (Lower Cretaceous, Berriasian to lower Barremian) is related to normal and transcurrent fault movements along preexisting structural trends of the Proterozoic basement during the South and Equatorial Atlantic Ocean opening. Dinosaur footprints are the most abundant fossils in the siliciclastic continental sediments. The main tetrapod ichnofauna comprises, in at least 42 individual tracksites through approximately 96 stratigraphic levels, isolate footprints and trackways of large and small theropods, besides ornithopods, sauropods, and rare ankylosaurs. There are also invertebrate ichnofossils. The body fossils are fish scales and bone fragments, but also rare dinosaur and Crocodylomorpha bones, palynomorphs, plant fragments, ostracods and conchostracans. These basins comprise a lot of dinosaur ichnofaunas, in the same stratigraphic-time-paleogeographical context, and they represent segments of a widespread megatracksite. There are 447 theropod tracks; 90 sauropods (Total of 537 saurischians); 2 quadrupedal ornithischians (one of them an ankylosaurian); 6 small ornithopods; 30 graviportal ornithopods (total of 38 ornithischians); one track of a juvenile biped dinosaur; about 53 unclassifiable dinosaur tracks (629 dinosaurs, all together); a hand-foot set of batrachopodid prints; traces of a Crocodylomorpha; an isolate lacertoid print; and a large number of small chelonian half-swimming tracks. The environmental setting indicates an endemic biota living nearby ephemeral rivers and shallow lakes under hot climate conditions. These were preserved in alluvial fans, braided, meandering rivers and shallow lake deposits of Berriasian to lower Barremian ages. Dinosaur faunas (both as bone remains and as ichnofaunas) from the Early Cretaceous are quite rare in the world, and the material discovered in these basins is important to the knowledge of the dinosaur diversity during the Cretaceous.

The Mesozoic rift basins of northeastern Brazil, particularly those at the edge of the Atlantic margin, are small, elongated, narrow basins, usually graben or half-graben, controlled by normal or transcurrent faults. Dinosaur tracks have been founded in some of these basins, mainly during the Lower Cretaceous (Berriasian-Albian). The tracks belongs to different types of dinosaurs, including sauropods (most likely titanosaurids and diplodocoids), theropods (likely Abelisauroidea and spinosaurids), ornithopods, both small and graviportals (iguanodontids), and birds. Besides footprints on the bedding plane surface, some others are cross-section footprints. A total of about 32 dinosaur individuals have been found in these seventeen basins so far and their presence has often helped a better understanding of palaeoenvironments, palaeoclimate, and paleobiogeographic aspects of the dinosaur biota.

The dinosaur footprints and trackways of São Luís Basin are found in six ichnosites of São Luís and Alcântara counties: Ponta da Guia, Ponta do Farol, Praia do Boqueirão, Ilha do Medo, Praia da Baronesa and Praia Prefeitura de Alcântara. There are trackways and isolated footprints attributed to small and large theropods, sauropods and ornithopods. These footprints occur in the Cenomanian fine-grained quartz sandstones of the Alcântara Formation. The track-bearing strata of the São Luís Basin compose the São Luís Megatracksite, with the tracksites of this region located along the early equatorial seashore of the Atlantic Ocean. The environmental scenery during the Cenomanian comprises many sub environments associated with an estuary that occupied a low-gradient coastal plain. Distinct dinosaur clades are found in these environments. Palynological data are consistent with the inference of a tidal flat of a low-gradient coastal plain under dry and hot climate inhabited by the trackmakers.

The Brazilian research on dinosaur tracks is no more than one hundred years. Since the first studies in 1924 in the Rio do Peixe basins there was a great improvement in the use of the ichnological information to provide new insights into the paleoenvironmental and paleoecological interpretations and also into the paleobiological aspects of the dinosaurs from Gondwana. There is also the discovery of new tracksites and the comprehension of how the dinosaur footprints can allow a better understanding of the Mesozoic ecosystems. The use of dinosaur footprints as biostratigraphic and correlative tools in the Brazilian Mesozoic basins open new opportunities to evaluate the classic stratigraphic framework of some intracratonic and rift basin. The discovery of new ichnosites, the use of digital technologies, physical and chemical analysis of the footprint matrix allow a glimpse to the potential use in geology and paleobiology. There are many new challenges to the continuity of the dinosaur tracks researches to the improvement of a new comprehension about dinosaurs, including their life environments, behavior and geographical distribution.