The black volcanic rocks are related to the basalts of the Serra Geral Formation. The Bauru Group is subdivided into several formations and members Fig. The most commonly used terminology stems from the work of Soares et al. Additionally, there is a seventh highly localized formation, the Uberaba Formation, which crops out only in Minas Gerais and interdigitates with the Adamantina Formation Souza Furthermore, other units may occur only in the subsurface, as shown by well log and core data. According to Paula e Silva et al. Age assessments have been based on a complex interplay of stratigraphical correlations, vertebrate and microfossil biostratigraphy, and magnetostratigraphy.
Unfortunately, there are no radioisotopic dates that constrain the ages of the upper portions of the Bauru Group, which makes it difficult to determine how much of the formation is genuinely latest Cretaceous Campanian-Maastrichtian in age. This, in turn, precludes a more robust timeline of local dinosaur evolution during the latest Cretaceous and makes it impossible, at present, to determine how close to the K-Pg boundary which has not yet been identified in the Bauru Group the Brazilian dinosaur record extends.
Recent works on the geology of the Bauru Group have upheld this view. More recently, a critical clue from magnetostratigraphy has helped refine when in the middle-Late Cretaceous some of the Bauru Group rocks must have formed. Tamrat et al.
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This is because a large swathe of the preceding Early-mid Cretaceous, from about million years ago, was a long-stand of normal polarity the so-called Cretaceous Normal Superchron or Cretaceous Quiet Zone: Cande and Kent , He et al. By extension, portions of the Adamantina Formation correlative with the Uberaba Formation, and the portions of the Presidente Prudente Formation that sit above or interfinger with the Adamantina Formation, must also be latest Santonian-Maastrichtian in age.
Magnetostratigraphy also helps define which portions of the Bauru Group are not Late Cretaceous in age. This confirms the initial stratigraphic interpretations of Soares et al. This unconformity has also been identified in well logs by Paula e Silva et al. Within the unequivocally latest Cretaceous portion of the Bauru Group, there are three fossiliferous formations that may date, in part, to the Maastrichtian, the final stage of Cretaceous Fig. Therefore, these formations may yield some of the last-evolving dinosaurs in South America. We briefly review the ages and depositional environments of these formations here.
The Adamantina Formation comprises sandstones with rare conglomerates and mudstones, formed in channels, thin overbank floods, and ponds. There are also some desert lithologies, which decrease upwards through the formation. The lacustrine and fluvial clays and sandstones begin to appear in the middle-to-upper part of the unit, which were deposited in warm and humid conditions Soares et al.
It is also worth noting here that another unit often considered to be a formation-the Uberaba Formation, which interfingers with the Adamantina Formation-is composed of limestones, sandstones, and conglomerates, often cemented by calcite with volcaniclastic sediments [ Hasui , Barcelos ]. It is often stated in the literature that the Adamantina Formation is Turonian-Santonian in age, based on its ostracod and charophyte assemblages Dias-Brito et al.
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However, vertebrate biostratigraphic data suggests that parts of the formation, particularly its upper interval, may be younger. Santucci and Bertini identified specimens of the titanosaurian sauropod Aeolosaurus in the Adamantina Formation, and because this dinosaur genus is known from late Campanian-early Maastrichtian aged rocks in Argentina, they proposed a similar age for the upper part of the Adamantina Formation in Brazil.
A follow-up study by Martinelli et al. The lower-level taxonomic affinities of these specimens is the subject of ongoing work, and it may be that none of them belong to the genus Aeolosaurus proper Simbras et al. However, it is widely recognized that they are all very closely related sauropods in a restricted subclade, Aeolosaurini, regardless of what genera they are assigned to. Therefore, if Aeolosaurus or Aeolosaurini is a good index taxon that ranged across South America at roughly the same time, this would indicate that the upper part of the Adamantina Formation dates to sometime close to the Campanian-Maastrichtian boundary.
However, terrestrial vertebrate fossils may not always be good biostratigraphic indicators Rayfield et al. With that said, it is also worth noting that Gobbo-Rodrigues et al. The Presidente Prudente Formation consists of sandstones and mudstones deposited in a shallow meandering fluvial system, representing channel and overbank settings Fernandes and Coimbra , Simbras There has been little work on the geochronology of the formation, but it is often considered to be Campanian-Maastrichtian in the literature e. This is largely based on lithological correlations that indicate lateral gradational contacts between the Presidente Prudente Formation and the Adamantina Formation, whose age has been determined as latest Santonian-Maastrichtian through more intensive study see above Zaher et al.
Furthermore, Azevedo and Simbras described an aeolosaurin caudal vertebra from the Presidente Prudente Formation; if aeolosaurins are reliable biostratigraphic indicators see above , then this would support a Campanian-Maastrichtian age of the formation.
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It is also worth noting that Gobbo-Rodrigues et al. However, this area was recently mapped as the Presidente Prudente Formation by Fernandes and Coimbra Therefore, the ostracods lend support for the Campanian-Maastrichtian age of the Presidente Prudente Formation. For this reason, they are sometimes mined locally as a source of lime for agricultural fertilizer. These rocks were formed in alluvial fans associated with the uplift of the eastern border of the Bauru sedimentary basin Riccomini The coarse-grained alluvial sediments were later reactivated and redeposited by an anastomosing fluvial system rich in calcareous minerals Barcelos and Suguio The age of the formation is usually considered to be Maastrichtian, based on its charophyte and ostracod assemblages Gobbo-Rodrigues et al.
The Formation has four depositional sequences, and the mudstones that provided the microfossils dated by Dias-Brito et al. Dinosaur fossils extend into the fourth, and youngest, sequence. The location of the microfossils dated by Dias-Brito et al. There is a growing record of dinosaurs and other vertebrate fossils from the unequivocally latest Cretaceous latest Santonian-Maastrichtian aged formations of the Bauru Group Figs.
Many of these specimens are isolated teeth and vertebrae Bertini et al. We here focus on dinosaurs and summarize the most important aspects of their fossil record. This is not an exhaustive survey, but a chronicle of the most salient fossils that give insight into how the major dinosaur groups were evolving during the end-Cretaceous. For more exhaustive reviews of the Brazilian dinosaur record as a whole, please consult Kellner and Campos and Bittencourt and Langer Abbreviations of d: cpol, centropostzygapophyseal lamina; cprl, centroprezygapophyseal lamina; eprl, epipophyseal-prezygapophyseal lamina; pcdl, posterior centrodiapophyseal lamina; pcpl, posterior centroparapophyseal lamina; podl, postzygodiapophyseal lamina; posl, postspinal lamina; prdl, prezygodiapophyseal lamina; prsl, prespinal lamina; spd, spinodiapophyseal lamina; spol, spinopostzygapophyseal lamina; sprl, spinoprezygapophyseal lamina; tpol, intrapostzygapophyseal lamina; tprl, intraprezygapophyseal lamina.
More details in Machado et al. Adamantina Formation : A number of theropod and sauropod fossils have been reported from the Adamantina Formation.
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The vast majority of the theropod remains found in this unit can be assigned to Abelisauridae, one of the main groups of mid-to-large-sized predatory dinosaurs that ranged across Gondwana during the middle-Late Cretaceous, often filling niches at or near the top of the food chain. Most of these specimens are isolated teeth that do not give much insight into the phylogenetic relationships or paleobiology of these carnivores e. However, a few specimens preserve portions of the skeleton. Bertini described a fragment of a right premaxilla and Brum et al. These latter specimens were estimated to belong to separate individuals that were each approximately meters long in total body length, making them relatively small abelisaurids compared to giants such as Carnotaurus that were approximately 8 meters long Grillo and Delcourt and had body masses in excess of 1.
There is one Brazilian abelisaurid, however, that was considerably larger. Kellner and Campos described Pycnonemosaurus based on an associated collection of teeth, caudal vertebrae, and parts of the pelvis and hindlimb. The identity and correlations of these conglomerates are somewhat uncertain. This unit is covered by the conglomerates and sandstones of the Cambambe Formation.
The lower sequences of the Cambambe Formation in the Morro do Cambambe area yielded an aeolosaurin specimen studied by Franco-Rosas et al. Sometimes in the literature, the Cretaceous rocks of Mato Grosso state are said to be correlated with the Adamantina Formation based on the aeolosaurin specimen e. Grillo and Delcourt used bone scaling equations to estimate the body length of Pycnonemosaurus at 8.
It is possible that this size estimate may be overinflated, because the bones used to calculate it are fragmentary, but what is clear is that Pycnonemosaurus is among the largest abelisaurids yet found and would have been a top-of-the-food-chain predator probably quite similar to Carnotaurus in its size and behaviors. One peculiar type of non-avian theropod was reported from the Adamantina Formation by Candeiro et al. These authors described a small dorsal vertebra, with a centrum that was only approximately one centimeter long anteroposteriorly in lateral view, as belonging to a unenlagiine Fig.
Unenlagiines are primitive dromaeosaurids, fairly closely related to iconic species of active, dynamic, feathered predators like Velociraptor and Deinonychus.
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The diets and behaviors of unenlagiines, however, are not clear and it is possible that they were not obligatory carnivores but had a more varied diet. Numerous sauropod fossils have been described from the Adamantina Formation, including some reasonably complete skeletons and other associated remains that have been named as a suite of new species Fig.
Many of these belong to Aeolosaurini, a subgroup of derived titanosaurian sauropods currently known only from the Late Cretaceous of South America. Aeolosaurins from the Adamantina Formation include Gondwanatitan , described from a set of bones that includes parts of the neck, back, sacrum, tail, pectoral and pelvic girdles, and fore and hind limbs Kellner and Azevedo ; Maxakalisaurus , represented by an incomplete disarticulated holotype skeleton Kellner et al.
Compared to well-known sauropod giants such as Brachiosaurus and Diplodocus , as well as celebrated titanosaurs like Argentinosaurus and Dreadnoughtus , the aeolosaurins were relatively small in stature.
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Rinconsaurus , one of the few aeolosaurins or close relatives known from long bones that can be used to estimate body mass, tipped the scales at around 4 tons Benson et al. However, not all aeolosaurins may have been so small: Santucci and Arruda-Campos named the species Aeolosaurus maximus in homage to its large size, and their illustrations depict a total body length estimate of meters. Regardless, what is clear is that aeolosaurins may not have been as large as stereotypical sauropods, but were still massive, bulk-feeding herbivores.
There are also two other named titanosaurs from the Adamantina Formation, whose phylogenetic relationships are less certain. The first is Adamantisaurus , described by Santucci and Bertini based on a series of six articulated anterior caudal vertebrae and two chevrons. The second is Brasilotitan , which Machado et al. It is possible that these sauropods also belong to Aeolosaurini, or they may represent other, more distantly related titanosaurs.
As a quick note, it is important to point out that some of these sauropods may not actually come from the Adamantina Formation as described, but from the Presidente Prudente Formation as defined by Fernandes and Coimbra , due to new geological mapping that has refined lithostratigraphic boundaries and regional correlations. This will be clarified by future geological fieldwork. It is also possible that the lower-level systematics and phylogenetic placements of some of these sauropods will change, pending ongoing work by one of us Simbras.
Presidente Prudente Formation : As with the Adamantina Formation, most dinosaur records from the Presidente Prudente Formation consist of isolated teeth e. These specimens record the presence of two major theropod groups: abelisaurids and carcharodontosaurids. Carcharodontosaurids were a clade of large-bodied theropods closely related to Allosaurus , which ranged across much of the globe during their early history in the Early-middle Cretaceous but then apparently became restricted to the southern hemisphere during the latest Cretaceous, where some of them grew to colossal sizes that rivaled T.
More recent discoveries of skeletal material corroborate the identification of abelisaurids and carcharodontosaurids in this formation Fig. Azevedo et al. This latter specimen was the first non-dental record of a carcharodontosaurid from Brazil, and its measurements indicate that it belonged to an animal with a skull ca. This bone was also the first carcharodontosaurid skeletal fossil from anywhere in the world to be found in post-Turonian deposits.