Romualdo Formation
Stratigraphic range: Early Albian ~111–108 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Fossils of Anhanguera (top) and Santanadactylus (bottom) from the Romualdo Formation
Type	Geological formation
Unit of	Santana Group
Underlies	Exu & Arajara Formations
Overlies	Crato & Ipubi Formations
Thickness	2–10 m (6.6–32.8 ft)
Lithology
Primary	Mudstone
Other	Limestone, shale
Location
Coordinates	7°12′S 39°18′W / 7.2°S 39.3°W / -7.2; -39.3
Approximate paleocoordinates	12°12′S 10°42′W / 12.2°S 10.7°W / -12.2; -10.7
Region	Pernambuco, Piauí & Ceará
Country	Brazil
Extent	Araripe Basin
Extent of the Santana Group, to which the Romualdo Formation belongs, in blue

The Romualdo Formation is a geologic Konservat-Lagerstätte in northeastern Brazil's Araripe Basin where the states of Pernambuco, Piauí and Ceará come together. The geological formation, previously designated as the Romualdo Member of the Santana Formation, named after the village of Santana do Cariri, lies at the base of the Araripe Plateau. It was discovered by Johann Baptist von Spix in 1819. The strata were deposited during the Aptian stage of the Early Cretaceous in a lacustrine rift basin 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 faunal assemblage. Some 25 species of fossil fishes are often found with stomach contents preserved, enabling paleontologists to study predator–prey relationships in this ecosystem. There are also fine examples of pterosaurs, reptiles and invertebrates, and crocodylomorphs. Even dinosaurs are represented (Spinosauridae, Tyrannosauroidea, Compsognathidae). The unusual taphonomy of the site resulted in limestone 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.^[1]

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.^[2]

In addition, the weathering of Romualdo Formation rocks has contributed soil conditions unlike elsewhere in the region. The Araripe manakin (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 soils formed ultimately from Romualdo Formation rocks.

Geology and dating

The Crato Formation 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 strata 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 Aptian-Albian boundary, about 112 million years ago. Nevertheless, a Cenomanian age cannot be ruled out.^[3][4]

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.^[3]

Fossil content

Archosaurs

Indeterminate remains of non-avian theropods, avialans, ornithischians, and possibly oviraptorosaurs have been found in Ceara state, Brazil.^[5] The oviraptorosaurian remains have been re-identified as megaraptoran fossils.^[6]

Dinosaurs

Dinosaurs
Genus	Species	Presence	Notes	Images
Angaturama[5]	A. limai	Ceará	Spinosauridae. Partial skull (rostralmost portion).[7] Possible junior synonym of Irritator challengeri.
Aratasaurus[8]	A. museunacionali	Ceará	A coelurosaur. Partial right hindlimb.
Irritator[5][9]	I. challengeri	Ceará	Spinosauridae. Partial skull (posterior half); one of the most complete spinosaurid skulls known.
Mirischia[5]	M. asymmetrica	Pernambuco	Compsognathidae. Pelvis and partial left hindlimb.
Santanaraptor[5]	S. placidus	Ceará	A possible tyrannosauroid. Some caudal vertebrae, partial pelvis, most of both hindlimbs.[10]

Crocodylomorphs

Crocodylomorphs
Genus	Species	Presence	Notes	Images
Araripesuchus	A. gomesii	Romualdo Formation	Type specimen 423-R is a single skull articulating with part of a lower jaw. A more complete specimen, AMNH 24450, is at the American Museum of Natural History.
Caririsuchus	C. camposi	Romualdo Formation	A peirosaurid crocodyliform.

Pterosaurs

Pterosaurs
Genus	Species	Presence	Images
Anhanguera	A. blittersdorffi A. santanae A. araripensis A. robustus A. piscator A. spielbergi
Araripedactylus	A. dehmi
Araripesaurus	A. castilhoi
Barbosania[11]	B. gracilirostris
Brasileodactylus	B. araripensis
Cearadactylus	C. atrox
	C. ligabuei
Kariridraco	K. dianae
Maaradactylus	M. kellneri
Santanadactylus	S. brasilensis ?S. pricei S. spixi
Tapejara	T. wellnhoferi
Thalassodromeus	T. sethi
Tropeognathus	T. mesembrinus
Tupuxuara	T. deliradamus T. leonardii T. longicristatus
Unwindia	U. trigonus

Turtles

Turtles
Genus	Species	Presence	Notes	Images
Santanachelys	S. gaffneyi		[12]
Cearachelys	C. placidoi		[13]
Araripemys	A. barretoi		[14]
Euraxemys	E. essweini		[15]
Brasilemys	B. josai		[16]

Fish