The Romualdo Formation is a geologic Konservat-Lagerstätte in northeastern

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

Araripe Basin The Araripe Basin () is a rift basin covering about ,Neto et al., 2013, p.1 in Ceará, Piauí and Pernambuco states of northeastern Brazil. It is bounded by the Patos and Pernambuco lineaments, and is situated east of the Parnaíba Basin, southwes ...

where the states of

Pernambuco Pernambuco () is a state of Brazil, located in the Northeast region of the country. With an estimated population of 9.6 million people as of 2020, making it seventh-most populous state of Brazil and with around 98,148 km², being the 19 ...

Piauí Piaui (, ) is one of the states of Brazil, located in the country's Northeast Region. The state has 1.6% of the Brazilian population and produces 0.7% of the Brazilian GDP. Piaui has the shortest coastline of any coastal Brazilian state at 66&n ...

and

Ceará Ceará (, pronounced locally as or ) is one of the 26 states of Brazil, located in the northeastern part of the country, on the Atlantic coast. It is the eighth-largest Brazilian State by population and the 17th by area. It is also one of the ...

come together. The geological formation, previously designated as the Romualdo Member of the

Santana Formation The Santana Group is a geologic group, formerly included as the middle part of the Araripe Group, in the Araripe Basin of northeastern Brazil. The group comprises the Crato, Ipubi and Romualdo Formations and is dated to the Aptian to Albian stag ...

, named after the village of

Santana do Cariri Santana do Cariri is a municipality in the state of Ceará in the Northeast region of Brazil. It is located in the immediate and intermediate region of Juazeiro do Norte, as well as in the region of influence of Juazeiro do Norte.The municipality ...

, lies at the base of the

Araripe Plateau The Chapada do Araripe, also known as the Serra do Araripe, is a ''chapada'' (plateau) in northeastern Brazil. The chapada forms the boundary of Ceará and Pernambuco states, and forms the watershed between the Jaguaribe River of Ceará, which flow ...

. It was discovered by

Johann Baptist von Spix Johann Baptist Ritter von Spix (9 February 1781 – 13 March 1826) was a German natural history, biologist. From his expedition to Brazil, he brought to Germany a large variety of specimens of plants, insects, mammals, birds, amphibians and fish. ...

in 1819. The

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

were deposited during the

Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...

stage of the

Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma. Geology Pro ...

in a

lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

rift basin In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben wi ...

with shallow marine incursions of the proto-Atlantic. At that time, the South Atlantic was opening up in a long narrow shallow sea. The Romualdo Formation earns the designation of Lagerstätte due to an exceedingly well preserved and diverse

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

faunal assemblage. Some 25 species of

fossil fish The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish ...

es are often found with stomach contents preserved, enabling paleontologists to study predator–prey relationships in this ecosystem. There are also fine examples of

pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to ...

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

s and

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s, and

crocodylomorphs Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. During Mesozoic and early Cenozoic times, cr ...

. Even dinosaurs are represented (

Spinosauridae The Spinosauridae (or spinosaurids) are a clade or family of tetanuran theropod dinosaurs comprising ten to seventeen known genera. They came into prominence during the Cretaceous period. Spinosaurid fossils have been recovered worldwide, includin ...

Tyrannosauroidea Tyrannosauroidea (meaning 'tyrant lizard forms') is a superfamily (or clade) of coelurosaurian theropod dinosaurs that includes the family Tyrannosauridae as well as more basal relatives. Tyrannosauroids lived on the Laurasian supercontinent b ...

Compsognathidae Compsognathidae is a family of coelurosaurian theropod dinosaurs. Compsognathids were small carnivores, generally conservative in form, hailing from the Jurassic and Cretaceous Periods. The bird-like features of these species, along with other d ...

). The unusual

taphonomy Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term ''taphonomy'' (from Greek , 'burial' and , 'law') was introduced to paleontology in 1940 by Soviet scientist Ivan Efremov t ...

of the site resulted in

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

accretions that formed nodules around dead organisms, preserving even soft parts of their anatomy. In preservation, the nodules are etched away with acid, and the fossils often prepared by the

transfer technique Fossil preparation is the act of preparing fossil specimens for use in paleontological research or for exhibition, and involves removing the surrounding rocky matrix and cleaning the fossil. Techniques Acid maceration Acid maceration is a techn ...

.Maisey et al., 1991, pp.99–103 Local mining activities for cement and construction damage the sites. Trade in illegally collected fossils has sprung up from the decade of 1970, driven by the remarkable state of preservation and beauty of these fossils and amounting to a considerable local industry. An urgent preservation program is being called for by paleontologists. In addition, the

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement), ...

of Romualdo Formation rocks has contributed

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...

conditions unlike elsewhere in the region. The

Araripe manakin The Araripe manakin (''Antilophia bokermanni'') is a species of critically endangered bird from the family of manakins (Pipridae). It was discovered in 1996 and scientifically described in 1998. The species epithet commemorates Brazilian zoologi ...

(''Antilophia bokermanni'') is a very rare bird that was discovered only in the late 20th century; it is not known from anywhere outside the characteristic forest that grows on the

Chapada do Araripe The Chapada do Araripe, also known as the Serra do Araripe, is a ''chapada'' ( plateau) in northeastern Brazil. The chapada forms the boundary of Ceará and Pernambuco states, and forms the watershed between the Jaguaribe River of Ceará, which fl ...

soils formed ultimately from Romualdo Formation rocks.

Geology and dating

Romualdo Formation - outcrop and interpretation

The

Crato Formation The Crato Formation is a geologic formation of Early Cretaceous ( Aptian) age in northeastern Brazil's Araripe Basin. It is an important Lagerstätte (undisturbed fossil accumulation) for palaeontologists. The strata were laid down mostly duri ...

was previously considered the lowest member of the then Santana Formation, but has been elevated to a formal formation. The Crato Formation is the product of a single phase, where complicated sequence of sediment

reflect changeable conditions in the opening sea. The age of the Romualdo Formation, formerly known as the Romualdo Member of the Santana Formation, has been controversial, though most workers have agreed that it lies on or near the

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0.9 M ...

boundary, about 112 million years ago. Nevertheless, a

Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in the s ...

age cannot be ruled out.Martill et al., 2007Martill, 2007 The extent of the Crato unit and its relationship to the Romualdo Formation had long been ill-defined. It was not until a 2007 volume on the unit by Martill, Bechly and Loveridge that the Crato Formation was given a formal type locality, and was formally made a distinct formation separate from the Romualdo Formation, which is about 10 Ma younger.

Fossil content

Archosaurs

Indeterminate remains of non-avian

theropods Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...

avialans Avialae ("bird wings") is a clade containing the only living dinosaurs, the birds. It is usually defined as all theropod dinosaurs more closely related to birds (Aves) than to deinonychosaurs, though alternative definitions are occasionally use ...

ornithischia Ornithischia () is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek s ...

ns, and possibly

oviraptorosaurs Oviraptorosaurs ("egg thief lizards") are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot-like skulls, with or ...

have been found in Ceara state, Brazil. The oviraptorosaurian remains have been re-identified as

megaraptora Megaraptora is a clade of carnivorous Tetanurae, tetanuran theropod dinosaurs with controversial relations to other theropods. Its Derived (phylogenetics), derived members, the Megaraptoridae are noted for their elongated hand claws and proporti ...

n fossils.

Dinosaurs

Crocodylomorphs

Pterosaurs

Turtles

Fish

* ''

Araripelepidotes ''Araripelepidotes'' is a genus of ginglymodian fish. Habitat ''Araripelepidotes'' was probably endemic to the Araripe Basin, and was commonly found in Santana formation, and rare in the Crato Formation, mostly in carbonate concretions, but ...

'' * '' Beurlenichthys ouricuriensis'' * ''

Brannerion ''Brannerion'' is an extinct genus of prehistoric bonefish. Fossils of the genus were found in the Romualdo Formation of the Santana Group, Araripe Basin, northeastern Brazil.Calamopleurus ''Calamopleurus'' is an extinct genus of bowfins, comprising three species: ''C. cylindricus'' and ''C. mawsoni'' from Brazil and ''C. africanus'' from Africa Africa is the world's second-largest and second-most populous continent, af ...

''Fara et al., 2005, p.152 * ''

Cladocyclus ''Cladocyclus'' (derived from the Greek κλάδος/kládos ("branch") and κύκλος/kýklos ("circle")) is an extinct genus of Ichthyodectidae. It was a predatory fish of about in length, found in the Albian Romualdo and Crato Formations ...

'' * '' Enneles audax'' * '' Iemanja palma'' * '' Lepidotes wenzai'' * '' Microdon penalvai'' * '' Notelops'' * '' Obaichthys'' * '' Oshunia brevis'' * '' Placidichthys bidorsalis'' * ''

Rhacolepis ''Rhacolepis'' is an extinct genus of ray-finned fish from the Cretaceous Santana Formation of Brazil. Complete fossilised heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circ ...

'' * '' Rhinobatos beurleni'' * '' Santanaclupea silvasantosi'' * ''

Tharrhias ''Tharrhias'' is an extinct genus of prehistoric bony fish that lived during the Aptian stage of the Early Cretaceous epoch. The type species ''T. araripis'' is named after the Araripe Basin, in which it was found in sediments of the Santana Fo ...

'' * '' Tribodus limae'' * '' Vinctifer''Ze Gomes
at

Fossilworks Fossilworks is a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database The Paleobiology Database is an online resource for information on the distribution and classification of fossil animals ...

.org

References

Bibliography

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