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First dinosaur tracks from the Açu Formation, Potiguar Basin (mid-Cretaceous of Brazil)

Abstract

We record a new ichnosite bearing dinosaur tracks from the mid-Cretaceous (Aptian? -Cenomanian) of Brazil. The Fazenda dos Pingos ichnosite is situated within a sandstone pavement of the Açu Formation (Açu 3 unit) from the Potiguar Basin in the State of Rio Grande do Norte. This is a small ichnofauna, with only a few individual isolated tracks. One specimen is unequivocally attributed to a large sauropod and another to an ornithopod. Two other specimens represent poorly preserved tracks assigned to sauropods. This is the first record of dinosaur tracks from the Potiguar Basin and from the Rio Grande do Norte State.

Key words
tracks; dinosaur; Açu Formation; Potiguar Basin; mid-Cretaceous

INTRODUCTION

Dinosaur trackways are quite common in the Rio do Peixe Basin in the western Paraíba State (Leonardi 1979LEONARDI G. 1979. Nota Preliminar Sobre Seis Pistas de Dinossauros Ornithischia da Bacia do Rio do Peixe (Cretáceo Inferior) em Sousa, Paraíba, Brasil. An Acad Bras Cienc 51(3): 501-516., 1987a, 1994LEONARDI G. 1994. Annotated Atlas of South America Tetrapod Footprints (Devonian to Holocene) with an appendix on Mexico and Central America. Companhia de Pesquisa de Recursos Minerais, Brasília: 248 p., Carvalho 2000 aCARVALHO IS. 2000a. Geological environments of dinosaur footprints in the intracratonic basins of northeast Brazil during the Early Cretaceous opening of the South Atlantic. Cretac Res 21: 255-267., bCARVALHO IS. 2000b. Huellas de saurópodos Eocretácicas de la cuenca de Sousa (Serrote do Letreiro, Estado da Paraíba, Brasil). Ameghiniana 37(3): 353-362., Leonardi & Santos 2006LEONARDI G & SANTOS MFCF. 2006. New dinosaur tracksites from the Sousa Lower Cretaceous basin (Paraíba, Brasil). Studi Trentini Sci Nat Acta Geol 81: 5-21., Leonardi & Carvalho 2002LEONARDI G & CARVALHO IS. 2002. Icnofósseis da Bacia do Rio do Peixe, PB. O mais marcante registro de pegadas de dinossauros do Brasil. In: Schobbenhaus C, Campos DA, Queiroz ET, Winge M & Berbert-Born M (Eds), Sítios geológicos e paleontológicos do Brasil. Brasília: Departamento Nacional de Produção Mineral, Brasília, Brasil, p. 101-111., 2021) and in a number of basins of the south of Ceará (Carvalho et al. 1993 aCARVALHO IS, VIANA MSS & LIMA FILHO MF. 1993a. Bacia de Cedro: a icnofauna cretácica de vertebrados. An Acad Bras Cienc 65: 459-460., bCARVALHO IS, VIANA MSS & LIMA FILHO MF. 1993b. Os icnofósseis de vertebrados da bacia do Araripe (Cretáceo Inferior, Ceará–Brasil). An Acad Bras Cienc 65: 459., Leonardi & Spezzamonte 1994LEONARDI G & SPEZZAMONTE M. 1994. New tracksites (Dinosauria: Theropoda and Ornithopoda) from the Lower Cretaceous of the Ceará, Brasil. Studi Trentini Sci Nat Acta Geol 69: 61-70., Leonardi & Muniz 1995LEONARDI G & MUNIZ GCB. 1995. Observações icnológicas (Invertebrados e Vertebrados) no Cretáceo continental do Ceará (Brasil), com menção a moluscos dulçaquícolas. In: Congresso Brasileiro de Paleontologia, 9., Fortaleza. Resumo das Comunicações, 9, Fortaleza: Sociedade Brasileira de Paleontologia, p. 45., Carvalho et al. 2020CARVALHO IS, LEONARDI G, RIOS-NETTO AM, BORGHI L, FREITAS AP, ANDRADE JA & FREITAS FI. 2020. Dinosaur trampling from the Aptian of Araripe Basin, Brazil, as Tools for Stratigraphic Correlation. Cretac Res 117: 104626.) (Figure 1a). However, they had not been recorded in the adjacent State of Rio Grande do Norte, in the Cretaceous rocks of the Potiguar Basin, up to now. A lot of dinosaur bones and fish scales were found at the western end of that basin, in Ceará State (Santos et al. 2005SANTOS MFCF, FLORÊNCIO CP, REYES-PÉREZ YA, BERGQVIST LP, PORPINO KO, UCHOA AF & LIMA-FILHO FP. 2005. Dinossauros na Bacia Potiguar: o registro da primeira ocorrência. In: XXI Simpósio de Geologia do Nordeste “A Geologia e a Sociedade”, Recife. Boletim de resumos expandidos, Recife: Sociedade Brasileira de Geologia, Núcleo do Nordeste, p. 325-328., Veiga et al. 2019VEIGA IMMG, BERGQVIST LP & BRITO PM. 2019. The fish assemblage of the Cretaceous (?Albian-Cenomanian) Açu Formation, Potiguar Basin, Northeastern Brazil. J S Am Earth Sci 93: 162-173., Pereira et al. 2020aPEREIRA PVLGC, VEIGA IMMG, RIBEIRO TB, CARDOZO RHB, CANDEIRO CRA & BERGQVIST LP. 2020a. The path of giants: a new occurrence of Rebbachisauridae (Dinosauria, Diplocoidea) in the Açu Formation, NE Brazil, and its paleobiogeographie implications. J S Am Earth Sci 100: 102515., bPEREIRA PVLGC, RIBEIRO TB, BRUSATTE SL, CANDEIRO CRA, MARINHO TS & BERGQVIST LP. 2020b. Theropod (Dinosauria) diversity from the Açu Formation (mid-Cretaceous), Potiguar Basin, Northeast Brazil. Cretac Res 114: 104517.), but no tracks till now (Figure 1b).

Figure 1
Map of the Brazilian northeastern basins with the main occurrences of dinosaur tracks (a), map of Potiguar Basin with localization oh the paleontological sites [tracks and bones] (b) and simplified stratigraphic section of the Potiguar Basin (c; modified from Pessoa Neto et al. 2007). Abbreviations: AL, Alagoas; ALG, Alagamar; BRS, Barreiras; CE, Ceará; DpE, Depositional Environment; Fm, Formation (litostratigraphic unit); GMR, Guamaré; Gr, Group (litostratigraphic unit); JD, Jandaíra; Mya, million years; MC, Macau; PB, Paraíba; PCD, Pescada; PD, Pendência; PE, Pernambuco; PI, Piauí; QBD, Quebradas; RN, Rio Grande do Norte; TB, Tibau; UBN, Ubarana.

The very first dinosaur track-site in the Potiguar Basin was localized by the second author in 2002, and it was visited and studied in the field by both the first and second authors together on January 18, 2008 (more dinosaur tracks were found in this ichnosite on this occasion). They have been further studied by the second author and a team of paleontologists from the Museu Câmara Cascudo in December 2018. The purpose of this paper is record and illustrate this material.

GEOLOGICAL SETTING

The Potiguar Basin

The Potiguar Basin (Figure 1b) is situated in the junction between the northern and eastern continental margins of Brazil; it is a rift basin, which began sinking during the Late Jurassic, depending on the separation of Africa and South America. The origin and evolution of the Potiguar Basin, and on some smaller intracratonic basins of the Brazilian North-East, were controlled by the reactivation of preexisting tectonic structures in the basement (Precambrian rocks) during the Jurassic and Cretaceous. This reactivation was closely related to the opening of the South Atlantic Ocean (Leonardi & Carvalho 2021LEONARDI G & CARVALHO IS. 2021. Dinosaur Tracks of Rio do Peixe Basins, Brazil: A Lost World of Gondwana. Indiana: Indiana University Press, 462 p.). The Potiguar Basin includes about 48,000 km2, almost one half of which (45%; about 21,500 km2) is on the continent, and the rest (55%; 26.500 km2) is offshore (Bertani et al. 1990BERTANI RT, COSTA IG & MATOS RMD. 1990. Evolução tectono-sedimentar, estilo estrutural e “habitat” do petróleo na Bacia Potiguar. In: Gabaglia GPR & Milani EJ (Eds), Origem e Evolução de Bacias Sedimentares, Rio de Janeiro: Petrobras, p. 291-310., Pessoa Neto et al. 2007PESSOA NETO O, SOARES UM, SILVA JGF, ROESNER EH, FLORENCIO CP & SOUZA CAV. 2007. Bacia Potiguar. Bol Geoci Petrobras 15(2): 357-369.). However, the total area of this basin would be about 60,000 km2 according to Araripe & Feijó (1994)ARARIPE PT & FEIJÓ FJ. 1994. Bacia Potiguar. Boletim de Geociências da PETROBRAS. Rio de Janeiro 8(1): 127-141.. It is mainly located in the northern portion of the State of Rio Grande do Norte, just a relatively narrow western belt is located in the State of Ceará, where dinosaur bones were found in 2002. Geologically, the basin is limited in the east, south, and west by crystalline rocks of a Precambrian basement (Caicó Complex and Seridó Belt: Jucurutu, Equador and Seridó formations; Borborema Province), intruded by granites (Pessoa Neto et al. 2007).

This basement is covered by three supersequences: Rift, Post-Rift and Drift (Figure 1c). The Rift supersequence was deposited during the Early Cretaceous and includes the fluvial-deltaic and lacustrine Pendência and Pescada formations, which are not cropping out. The Post-Rift Supersequence is also composed of fluvial-deltaic sediments and includes the upper part of the Areia Branca Group. Among its units is the Alagamar Formation, which is not cropping out either, and showing some records of the beginning of the great marine ingression, defined globally as the largest marine transgression in the last 250 million years (Costa et al. 2014COSTA ABS, CÓRDOBA VC, NETTO RG & LIMA FILHO FP. 2014. Registro faciológico e paleoambiental da transgressão que marca a passagem do Cenomaniano para o Turoniano na Bacia Potiguar, NE do Brasil. Com Geol 101, Especial I: 415-420.), and the Drift Supersequence, deposited since the Albian and consisting of the siliciclastic and then carbonate Apodi Group, (Açu, Ponta do Mel, Quebradas, Jandaíra and Ubarana transgressive formations), and the Agulha Group, with its regressive Ubarana, Tibau and Guamaré formations (Pessoa Neto et al. 2007, Costa et al. 2014COSTA ABS, CÓRDOBA VC, NETTO RG & LIMA FILHO FP. 2014. Registro faciológico e paleoambiental da transgressão que marca a passagem do Cenomaniano para o Turoniano na Bacia Potiguar, NE do Brasil. Com Geol 101, Especial I: 415-420.). Only the Açu and Jandaíra formations (mid and Upper Cretaceous, respectively) crop out at the surface.

Along the coast, these Cretaceous terrains are covered by Paleogene terrains (Barreiras Formation) and by Quaternary alluvial sediments (Pessoa Neto et al. 2007). The Pendência, Pescada and Alagamar formations are well known by means of the study of the cores of four wells located in the onshore portion of the basin (Costa et al. 2014COSTA ABS, CÓRDOBA VC, NETTO RG & LIMA FILHO FP. 2014. Registro faciológico e paleoambiental da transgressão que marca a passagem do Cenomaniano para o Turoniano na Bacia Potiguar, NE do Brasil. Com Geol 101, Especial I: 415-420.).

The Açu Formation

The Açu Formation (Figure 2) is composed of sandstones intercalated with shales, mudstones, siltstones and conglomerates of alluvial, fluvial and estuarine facies of Aptian? - Cenomanian age (Maraschin et al. 2010MARASCHIN AJ, MIZUSAKI AM, VASCONCELOS PM, HINRICHS R, DE ROS LF & DOS ANJOS SMC. 2010. Depositional age definition of the Açu Formation (Potiguar Basin, northeastern Brazil) through 40Ar-39Ar dating of early-anthigenic K-feldspar overgrowth. Pesqui Geocienc 37(2): 85-96.). The formation has been subdivided into four informal units identified from bottom to top as Açu 1, Açu 2, Açu 3 and Açu 4 (Vasconcelos et al. 1990VASCONCELOS EP, LIMA NETO FF & ROOS S. 1990. Unidades de correlação da Formação Açu, Bacia Potiguar. In: SBG/NÚCleo Nordeste, Congresso Brasileiro De Geologia, 36., Natal, Anais, 1, p. 227- 240.). Costa et al. (2014)COSTA ABS, CÓRDOBA VC, NETTO RG & LIMA FILHO FP. 2014. Registro faciológico e paleoambiental da transgressão que marca a passagem do Cenomaniano para o Turoniano na Bacia Potiguar, NE do Brasil. Com Geol 101, Especial I: 415-420. identified eight facies for Açu 3 Unit (Early Cenomanian), mainly including gravelly/sandy bars, sandflat deposits and floodplain deposits, and sixteen facies in Unit Açu-4 (Late Cenomanian), fourteen being siliciclastic and two hybrids. The siliciclastic facies were grouped into nine associations, including mainly channel fill deposits, floodplain deposits, abandoned channel deposits, sandflat, and mudflat deposits. As for the hybrid facies, the landscape of meandering fluvial systems with tidal influence was so being replaced by an estuarine complex dominated by tides, and then by a shallow platform adjacent to an estuary.

Figure 2
Partial view of a sandstone section with make-up characteristic of the Açu Formation, which occurs close to the new ichnosite bearing dinosaur tracks. Arcosean composition, medium to coarse texture, well selected, showing large tabular cross-stratifications (tangential at the base and truncated at the top). It is interpreted as braided river channel (a); This cavern, called Gruta dos Pingos is a good reference mark to find the rocky pavement with the dinosaur tracks (b); The tracks are located on the surface of a plateau more or less 15 m over the “ceiling” of the cavern of Pingos (c); An aspect of the structure (trough cross stratification) of the Açu Formation layers on the wall between the Gruta dos Pingos and the track bearing surface (d); and the environment of the Pingos farm is the typical Brazilian northeastern “caatinga”, with xique-xique cacti, (Pilocereus gounellei), pereiro trees (Aspidosperma pyrifolium) and the macambiras (Bromeliaceae, Bromelia laciniosa); and a lot of stone (e). The hammer for a graphic scale = 28 cm.

With exception of Duarte & Santos (1962)DUARTE L & SANTOS RS. 1962. Fósseis do Arenito Açu. Coleção Mossoroense, 21 p., who described body fossils of crustaceans, mollusks, fish scales and plant fragments, the Açu Formation has been considered non-fossiliferous. However, this work remained the only report of fossils in the Açu Formation for four decades, when Fernandes et al. (2002)FERNANDES ACS, BORGHI L, CARVALHO IS & ABREU CJ. 2002. Guia dos Icnofósseis de invertebrados do Brasil. Rio de Janeiro: Interciência, 260 p. identified escape structures in the core samples, which represent the displacement of an invertebrate organism in response to an environment disturbance.

In the early 2000s, the first body fossils in the Potiguar Basin were collected in the sediments of the Açu Formation (Açu-4 unit) by Francisco Pinheiro Lima Filho, from the Geology Department of the Universidade Federal do Rio Grande do Norte (UFRN) at Natal. These materials included dinosaur vertebrae and long bone fragments, herbivorous and carnivorous dinosaur teeth, crocodylomorph teeth and fish scales. A preliminary analysis attributed the dinosaur teeth to Carcharodontosauridae, Abelisauridae, Sauropoda and Diplodocoidea (Santos et al. 2005SANTOS MFCF, FLORÊNCIO CP, REYES-PÉREZ YA, BERGQVIST LP, PORPINO KO, UCHOA AF & LIMA-FILHO FP. 2005. Dinossauros na Bacia Potiguar: o registro da primeira ocorrência. In: XXI Simpósio de Geologia do Nordeste “A Geologia e a Sociedade”, Recife. Boletim de resumos expandidos, Recife: Sociedade Brasileira de Geologia, Núcleo do Nordeste, p. 325-328.).

More recently, new fossil findings in the Açu Formation have significantly increased the knowledge of the paleofaunistic composition of this lithostratigraphic unit. Currently, there are at least eight taxa of identified freshwater fishes (Veiga et al. 2019VEIGA IMMG, BERGQVIST LP & BRITO PM. 2019. The fish assemblage of the Cretaceous (?Albian-Cenomanian) Açu Formation, Potiguar Basin, Northeastern Brazil. J S Am Earth Sci 93: 162-173.). In addition, different taxa of dinosaurs have been identified, including Rebbachisauridae, Spinosauroidea, Carcharodontosauria, Maniraptora and Megaraptora (Pereira et al. 2020aPEREIRA PVLGC, VEIGA IMMG, RIBEIRO TB, CARDOZO RHB, CANDEIRO CRA & BERGQVIST LP. 2020a. The path of giants: a new occurrence of Rebbachisauridae (Dinosauria, Diplocoidea) in the Açu Formation, NE Brazil, and its paleobiogeographie implications. J S Am Earth Sci 100: 102515., b).

Fazenda dos Pingos ichnosite

The new dinosaur ichnosite is located at Fazenda Pingos (= Pingos Farm; Figure 2) in the municipality of Assú. One can arrive there by leaving the town of Assú and following along the BR 304 highway toward the Mossoró city (north-westward) for 16 km – 1 km NW from the CONPASA (Construtora Paraibana S.A.) and then entering a dirt road on the left untill Fazenda Pingos (5°34’10” S, 37°02’20” W, datum WGS 84; Figure 2). In that site, there is an interesting cavern, called Gruta dos Pingos, which can be used as a geomorphological reference mark (Figure 2b). In fact, the main tracks are located on the surface about 15 m over its “ceiling” of it (Figure 2c-d). The cavern is inhabited by tejú lizards (Tupinambis teguixin Linnaeus, 1758) and by swarms of dangerous “African” bees living in natural honeycombs. The natural environment of the Pingos farm – as almost all of the Northeast of Brazil – is typical “caatinga” (Figure 2e), that is, an environment consisting of xeric shrubland and thorn brush, with a lot of cacti and other leathery and thorny plant, adapted to endure living in semi-arid and really hot stony soil.

The ichnosite where the dinosaur tracks were found is a surface of the Açu Formation, Açu 3 unit, which is composed of sandstone deposited in a fluvial, high energy environment (Vasconcelos et al. 1990VASCONCELOS EP, LIMA NETO FF & ROOS S. 1990. Unidades de correlação da Formação Açu, Bacia Potiguar. In: SBG/NÚCleo Nordeste, Congresso Brasileiro De Geologia, 36., Natal, Anais, 1, p. 227- 240.). It represents an immature, poorly sorted, siliciclastic track-bearing deposit, with some thin quartzose conglomerate beds and dispersed quartz pebbles. The main sedimentary structures are cross-channel and planar stratification (Figure 2a, d). The color of these sandstones is pink to purple and sometimes reddish. The coarse-grained lithologies of this unit were certainly a restrictive factor for the fossil track preservation, especially for continuous preservation of sequences of footprints, i.e., complete trackways. The lithofacies, sedimentary structures, and geometry of the beds point to sedimentation in braided fluvial environments. The preservation of the few dinosaur tracks was probably favored by the presence of microbialitic mats (Carvalho et al. 2013CARVALHO IS, BORGHI L & LEONARDI G. 2013. Preservation of dinosaur tracks induced by microbial mats in the Sousa Basin (Lower Cretaceous), Brazil. Cretac Res 44: 112-121.), such as it occurs in the coarse sediments of the Antenor Navarro and Rio Piranhas formations in the Rio do Peixe Basin (Leonardi 1984LEONARDI G. 1984. Le impronte fossili di dinosauri. In: Bonaparte JF et al. (Eds), Sulle orme dei dinosauri (Esplorazioni e ricerche), Venezia: Erizzo, p. 161-186., 1987b, Leonardi & Carvalho 2021LEONARDI G & CARVALHO IS. 2021. Dinosaur Tracks of Rio do Peixe Basins, Brazil: A Lost World of Gondwana. Indiana: Indiana University Press, 462 p.). On the other hand, the presence of good fossil tracks is not rare even without their formation and preservation being favored by the presence of microbial mats; even in coarse-grained sediments deposited in high-energy environments (Szewczyk et al. 2020SZEWCZYK L, VENNIN E, MOREAU JD, GAND G, VEROLET M, KLEE N & FARA E. 2020. Tracking Dinosaurs in Coarse-Grained Sediments from the Upper Triassic of Ardèche (Southeastern France). Palaios 35: 447-460.).

In fact, despite the low preservation potential of alluvial fans and braided fluvial systems, some dinosaur tracks are found in this context at Fazenda dos Pingos ichnosite, and this fact invites a search for more ichnosites in Açu Formation outcrops, mainly where – like here – the top surface of the layers is preserved. The presence of mud-cracked surfaces as well as the presence of microbial wrinkles could be a good sign of the possibility of finding fossil footprints.

MATERIALS AND METHODS

We documented, photographed, and measured every track. The vertebrate paleoichnological terminology used here mainly follows Leonardi (1987a)LEONARDI G. 1987a. Glossary and Manual of Tetrapod Footprint Palaeoichnology. DNPM (Serviço Geológico do Brasil). Brasília, 117 p.. The measurements include hind-track length, hind-track maximum width, depth of the track, thickness of the displacement rim and maximum width of the displacement rim (Table I). The code AÇPI (= Açu-Pingos) was given to the specimens of this site.

Table I
Track measurements of specimens identified at Fazenda dos Pingos ichnosite (Açu Formation).

The Ichnofauna

This is a small ichnofauna, with a little number of individual isolated tracks. The better two specimens are sure and fine, although isolated; the other two are of bad quality and collapsed (Figure 3 and 4). However, this small ichnofauna is interesting, because it includes the first fossil footprints of tetrapods in the Potiguar Basin (Figure 5). In addition, these are the first evidence and record of dinosaurs in the State of Rio Grande do Norte. Unfortunately, their preservation quality is not particularly good: they can be set to level 1 (or being pessimistic, between levels 0 and 1) in the Preservation Scale proposed by Marchetti et al. (2019)MARCHETTI L ET AL. 2019. Defining the Morphological Quality of Fossil Footprints. Problems and Principles of Preservation in Tetrapod Ichnology with Examples from the Palaeozoic to the Present. Earth-Sci Rev 193: 109-145.. Nevertheless, this does not detract that the titanosaur footprint AÇPI 1 is impressive.

Figure 3
This is a fine and large sauropod hind-footprint (AÇPI 1), rather deeply imprinted. It produced a large and high displacement rim, especially large in front of the track, and there it probably covered the whole corresponding fore-footprint, which is not seen (a); Interpretative drawing of the same sauropod hind-track AÇPI 1 (b); The solid thick line represents the footprint’s outline: the thin lines indicate anatomical details and the outline of the displacement rim. The hatched surface represents the eroded portion of the footprint’s sole; The probable sauropod hind-footprint AÇPI 2 (c); and the probable sauropod hind-footprint AÇPI 3 (d). All of them from the same ichnosite, at Pingos farm.
Figure 5
Art reconstruction of the trace producers, showing the probable living environment of the animals (Art by Guilherme Gehr).

Sauropod tracks (Figure 3)

AÇPI 1

This is an exceptionally fine and large sauropod hind-footprint, rather deeply imprinted on a mud-cracked surface. The maker produced a large and high displacement rim, which is especially large in front of the track, where it has the characteristic aspect of a wave of lithified sand, that quite probably covered the whole corresponding fore-footprint. Because of this, this latter is not seen. This fact would probably imply that the manus-track was not so large and that the surface-ratio between fore- and hind-tracks in a set would be more probably 1:2 or 1:3 rather than 1:1. (Figures 3a-b).

There are surely other possibilities why the forefoot print in this sauropod track is missing: Pes-only sauropod trackways are not rare, when the hind-foot sinks in deeper than the forefoot. Overstepping of the forefoot by the hind-foot is also another option (Jens N. Lallensack, personal communication, May 31, 2021). In the case of the Pingos tracksite, the first option is unlikely as the large, wide, and high displacement rim seems to testify that this sauropod footprint is not an underprint. The maker was walking right on this surface and seems to have displaced the sand of this layer, giving origin to this large rim. However, the second option (the overstepping of the forefoot by the hind-foot) is indeed possible in this case, as an alternative.

The bottom of this track is partially eroded and irregular; the true and original sole of the print can be seen only in the distal portion of it. Maximum pes width (60 cm) is the most reliable measure, as the margin of the heel is not so clear, thus creating an uncertain interpretation of the pes length (probably ~82 cm). However, the width of the proximal portion of the footprint is less (~ 40 cm). The displacement rim is about 40 cm wide in the front of the track; it is wider on the left side than on the right, and less on the rear. The maximum thickness of the displacement rim is 5-7 cm; the depth of the track is in 3-5 cm, if compared to the substrate level (8-12 cm to the top of the displacement rim).

The whole width of the track (latero-medially), including the displacement rims, is about 100 cm, and the length, also including the displacement rims, is about 140 cm.

The track is wider in the front portion and narrower in the rear, and it has the characteristic outline of a bell; the heel outline is roundish; on the front margin, one can observe at least four claw impressions.

Unfortunately, the internal width of the trackway cannot be measured or inferred, because this, like the other tracks of this ichnosite, is an isolated footprint. It is a pity, because this parameter would be an important characteristic for the classification of the track and of the maker, as either wide-gauged or narrow-gauged. The long axis of this footprint points exactly to the south.

The footprint AÇPI 1 looks in many ways like the sauropod tracks from Toro-Toro (Campanian, Toro-Toro Formation, Department of Potosí, Bolivia) illustrated by Leonardi (1984)LEONARDI G. 1984. Le impronte fossili di dinosauri. In: Bonaparte JF et al. (Eds), Sulle orme dei dinosauri (Esplorazioni e ricerche), Venezia: Erizzo, p. 161-186., figure 13, and in plate 113; Leonardi, 1994, plates VIII and XXI (3-4); and cf. also Lockley et al. (2002)LOCKLEY M, SCHULP AS, MEYER CA, LEONARDI G & MAMANI DK. 2002. Titanosaurid trackways from the Upper Cretaceous of Bolivia: evidence for large manus, wide-gauge locomotion and gregarious behavior. Cretac Res 23(3): 383-400., fig. 12. Note that there are currently many new ichnosites with several hundreds of tracks, even sauropods, around Toro Toro (Meyer et al. 2019MEYER CA, MENEGAT R, GARCIA G, ALEM A & JALDIN M. 2019. New dinosaur tracks from the Late Cretaceous El Molino Formation of Toro Toro (Dep. Potosi, Bolivia). Hallesches Jahrb Geowiss Beiheft 46: 63-64., 2021MEYER CA, MARTY D, THÜRING B, THÜRING S & BELVEDERE M. 2021. The Late Cretaceous dinosaur track record of Bolivia – Review and perspective. J S Am Earth Sci 106: 102992.; and personal communication, 6 June 2021). AÇPI 1 is also similar in shape to the sauropod tracks from Floresta dos Borba ichnosite (probably Berriasian; Sousa Basin, Paraíba, Brazil; cf. Leonardi & Santos 2006LEONARDI G & SANTOS MFCF. 2006. New dinosaur tracksites from the Sousa Lower Cretaceous basin (Paraíba, Brasil). Studi Trentini Sci Nat Acta Geol 81: 5-21., Leonardi & Carvalho 2021LEONARDI G & CARVALHO IS. 2021. Dinosaur Tracks of Rio do Peixe Basins, Brazil: A Lost World of Gondwana. Indiana: Indiana University Press, 462 p.), mainly because of the bell-shaped outline. Therefore, AÇPI 1 can be likely attributed to a titanosaurid. As for the rear foot, the only foot of which we have record at Fazenda Pingos, it is completely different from the two morphotypes present at Cal Orqo (middle Maastrichtian, Sucre, Bolivia), i.e., it is different from either morphotype E or the morphotype called Calorckosauripus lazari Meyer, Marty & Belvedere, 2018 (formerly morphotype F) (Meyer et al. 2018MEYER CA, MARTY D & BELVEDERE M. 2018. Titanosaur trackways from the Late Cretaceous El Molino Formation of Bolivia (Cal Orck’o, Sucre). Ann Soc Geol Pol 88: 223-241.).

AÇPI 2

It is a bad quality sauropod hind-footprint, with no anatomical details, however, with clear, although low, displacement rim, and an oval outline, rather narrow, probably collapsed. It is not classifiable in detail. The surface of the rock around it is mud-cracked (Figure 3c).

AÇPI 3

This is also a bad quality sauropod hind-footprint, with no anatomical details, however, with a small displacement rim, and an oval outline, narrower than the former, probably collapsed. It is not classifiable in detail. The trowel is 28 cm long (Figure 3d).

Ornithopod track (Figure 4)

AÇPI 4

This is an isolated, very deep, left hind-footprint, longer than it is wide, with three roundish and short hooves and a rounded (monolobed) interdigital pad (Figure 4a-b). The hooves have blunted distal end, they are short (especially toes II and IV, which are much shorter than toe III); the III digit is spatulate. Digit II (on the right side) is separated from the heel pad by a rather typical notch or incision. There is no handprint, perhaps because the portion of rocky pavement where the handprint could be found is completely broken and lacking, or perhaps the maker was bipedal. There is also a large and high displacement rim, 15-20 cm wide around the footprint. The longitudinal axis of the track points to N20°E.

Figure 4
The deep ornithopod footprint AÇPI 4, probably a left one (a); Interpretative drawing of the same ornithopod hind-track (b); Solid thick lines indicate footprint outline. Thin lines indicate anatomical details and the outline of the displacement rim; and for comparison, some unnamed tracks attributed to ornithopods from the Early Cretaceous (Hauterivian-Barremian, local stage Aratu) of the Sousa Basin, from the Mãe d’Água ichnosite, Rio Piranhas Formation, Western Paraíba, Brazil (Redrawn from Leonardi, 1987b) (c).

Besides of the length and width measurements of this footprint (data provided in the Table I), we can also provide some probable measurements of the three toes: (i) digit II; length ~23 cm (length of the phalangeal portion of the digit), width ~16 cm; (ii) digit III; length ~19 cm (free digit), length ≥30 cm (probable length of the phalangeal portion of the digit), width ~24 cm; and (iii) digit IV; length ~29 cm (probable length of the phalangeal portion of the digit), width ?.

In the beginning, we had some doubt on this track, it could seem an erosion structure; however, the outline corresponds well to an ornithopod track and especially both hypexes are clearly convincing.

Probably, the erosion had here and there made the footprint deeper than when it was originally, and it had largely destroyed and obscured even the walls of the deep footprint. In fact, only parts of the original surface of the footprint walls can be seen with confidence: medially at toe II, in its deepest portion, laterally at digit IV, even in the deepest part; and can be clearly seen at the lower part of the two hypexes. The rest of the footprint wall was eroded. The exact original position of the footprint wall on the front of toe III is questionable. It had to be more protruding and overhanging. The surface of the footprint, on the front of the outer toes (toes II and IV) is much less concave and less deep than the situation of the toes of Sousaichnium pricei Leonardi, 1979; in this latter form the toes often entered deeply into the substrate and were then extracted, leaving deep cavities. It is possible that the maker of the AÇPI 4 footprint had a lower velocity than the ornithopod maker of the Sousa Basin’ trackway. However, the original surface of this footprint AÇPI 4 had to have overhangs even more protruding inward. Because of the erosion, one cannot distinctly observe impressions of the tarsus and metatarsals on the proximal walls of the footprint.

It is possible that presence of some overhangs is related to the fact that the track-maker, resting its foot in the muddy sand, had enlarged its toes; and retracting the foot, it had gathered them, and then, overall, it reduced the surface and the volume of the foot. Despite the abundant erosion, especially in the lateral and medial side, the certainty that this is a dinosaur tridactyl footprint was given by the presence and the characteristic and inescapable shape of the two hypexes, clearly preserved and observable; and it is an ornithopod footprint because of the rounded hoof shape, of the three toes.

It is evidently impossible to classify exactly this isolated footprint; it is different from all the other known ornithopod tracks. However, comparing it with the great number of ornithopod tracks (34 ichnogenera and 44 ichnospecies) illustrated and discussed by Diaz-Martínez et al. (2015), we can find some similarity with the following forms:

  • Caririchnium lotus Xing, Vang, Pan and Chen, 2007; from the mid-Cretaceous of China.

  • Hadrosauropodus (Caririchnium) kyoungsookimi Lim, Lockley and Kong, 2012; from the ‘mid’-Cretaceous of Korea; however, to characterize this form the presence of the hand-print would be important.

  • Hadrosaurichnoides igeensis Casanovas, Ezquerra, Fernández, Pérez-Lorente, Santafé and Torcide, 1993: from the Early Cretaceous of Spain.

  • Ornithopodichnus masanensis Kim, Lockley, Kim, Lim and Kim, 2009, Early Cretaceous of Korea; nomen dubium according to Diaz-Martínez et al. (2015).

  • Ornithopodichnites magna Llompart, Casanovas and Santafé, 1984, from the Maastrichtian of Spain; nomen dubium according to Diaz-Martínez et al. (2015).

  • Staurichnium diogenis, Leonardi, 1979; Early Cretaceous of the Sousa Basin, Passagem das Pedras and Piau-Caiçara ichnosites (Sousa Formation) and Floresta dos Borba ichnosite (Antenor Navarro Formation), Western Paraíba, Brazil; nomen dubium according to Diaz-Martínez et al. (2015); in any case this form is remarkably similar to AÇPI 4 track.

  • The unnamed tracks, attributed to ornithopods from the Early Cretaceous of the Sousa Basin, from the Curral Velho and Mãe d’Água ichnosites, Rio Piranhas Formation, Western Paraíba, Brazil (Leonardi 1987bLEONARDI G. 1987b. Pegadas de dinossauros (Carnosauria, Coelurosauria, Iguanodontidae) na Formação Piranhas da Bacia do Rio do Peixe, Sousa, Paraíba, Brasil. In: Congresso Brasileiro de Paleontologia, 10., Rio de Janeiro. Anais, Rio de Janeiro: Sociedade Brasileira de Paleontologia, Rio de Janeiro, Brazil, p. 337-351.). These footprints are filled in with material of the overlying contiguous layer (Figure 4c).

These similarities do not permit us to classify exactly the track from the ichnotaxonomic point of view, nor as for the track-maker: we can just attribute it to the morphofamily or ichnofamily Iguanodontipodidae.

We said above that this is an ornithopod track. However, we want to remember that “ornithopod” is a very generic term, that many times was used as a taxonomic wastebasket (Butler et al. 2012). It is perhaps more exact to say that this track could have been imprinted by some kind of iguanodontid (sensu lato). Excluding the small-sized ornithopod families and the Hadrosauria, one could attribute this isolate footprint to an Ankylopollexia, perhaps to a basal member of the Styracosterna clade, although no bones of large members of these clades were yet found in the Cretaceous of Brazil. One could not exclude a track-maker like the contemporaneous Ouranosaurus (Aptian-Albian or perhaps Cenomanian; Styracosterna Sereno 1986) from the Gadoufaoua locality of the Ténéré desert in Niger republic, not so far (about 1,070 km) from Rio Grande do Norte (Brazil) in Albian-Cenomanian, the times of deposition of the Açu Formation. However, the comparison of track AÇPI 4 with a good almost complete left hind-foot of Ouranosaurus nigeriensis, from the Venice specimen (Natural History Museum of Venice, MSNVE 3714; Bertozzo et al. 2017BERTOZZO F, DALLA VECCHIA FM & FABBRI M. 2017. The Venice specimen of Ouranosaurus nigeriensis (Dinosauria, Ornithopoda). PeerJ 5: e3403.), shows that the external toes (II and IV) of the Venice specimen are not as relatively short as in the AÇPI 4 track.

One could not exclude as trackmaker an exceptionally large representative of the clade of the rhabdodontids, larger than Muttaburrasaurus from the Aptian-Albian terrains of Queensland, Australia, at the other end of Gondwanaland.

CONCLUSIONS

This is really a small ichnofauna and its quality is not that good. This fact can be understood because the material is coarse to very coarse sandstones, produced in environments of high energy in the Açu Formation. However, it bears witness that dinosaurs, and especially sauropods and ornithopods, were present and living in the Rio Grande do Norte territory during the mid-Cretaceous (Aptian? - Cenomanian).

As for their presence in the times of deposition of the Açu Formation at Pingos locality, it is something strange and rare that they are represented, till now, only by herbivorous dinosaurs: three sauropods tracks and one ornithopod track.

The discovery of this small ichnofauna suggests searching for others in all the outcrops of the Açu Formation, especially where one can examine large upper surfaces of the layers. Unfortunately, this is a rather rare case in this formation and in this kind of landscape.

ACKNOWLEDGMENTS

We thank Paolo Citton (Universidad Nacional de Río Negro at General Roca, Río Negro, Argentina) for his valuable comments; Leonardo Menezes (PETROBRAS) for showing the locality to the second author, which was fundamental to the discovery of the traces; Manoel Pantaleão de Arruda for his essential support during our field activities; and Francisco Pinheiro Lima Filho (Universidade Federal do Rio Grande do Norte) who provided helpful information about the geological setting. We are also grateful to Lucas Henrique Medeiros da Silva Trifilio and Jasmin Lanker for the help with illustrations. Additionally, the associate editor Juliana Sayão, Jens N. Lallensack and Matteo Belvedere are thanked for their thoughtful comments and suggestions, which greatly improved this paper.

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Publication Dates

  • Publication in this collection
    27 Sept 2021
  • Date of issue
    2021

History

  • Received
    23 Apr 2021
  • Accepted
    30 July 2021
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