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The San Gregorio Formation is a lithostratigraphic unit dating back to the Pliocene to Pleistocene of Venezuela. The formation is split into three members, the older Vergel Member (Late Pliocene to earliest Pleistocene), the Cocuiza Member (Pleistocene) and the Río Seco Member. During this time the region is thought to have been covered by a mixed environment of open grassland and forested areas surrounding a permanent freshwater system. The diverse fauna of the San Gregorio Formation, including a variety of freshwater fish, crocodilians, turtles and snakes, also includes many mammals interpreted to have been part of the first major wave of the Great American Interchange in addition to native clades such as glyptodonts, ground sloths and caviomorph rodents.[1]
| San Gregorio Formation | |
|---|---|
| Stratigraphic range: Zanclean-Chibanian (Chapadmalalan-Lujanian) ~ | |
| |
| Unit of | Urumaco stratigraphic section |
| Sub-units | Vergel Member Cocuiza Member |
| Overlies | Codore Formation |
| Thickness | ~570 m (1,870 ft) |
| Lithology | |
| Primary | Sandstone, mudstone |
| Other | Limestone |
| Location | |
| Country |  Venezuela |
| Type section | |
| Named for | San Gregorio, Venezuela |
 | |
The San Gregorio Formation overlies the Codore Formation and forms the most recent sequence of the stratigraphic section. Outcrops of this formation are found in Falcón State, northeast of the town of Urumaco. The formation is named for the town of San Gregorio, Venezuela.[1]
Robert Masterman Stainforth divided the San Gregorio Formation into three subsections. The oldest of these is the Vergel Member, composed of 260 meters of sandstone, mudstone and sparse conglomerates. In their study on the formation Carrillo-Briceño and colleagues determined a late Pliocene to earliest Pleistocene age. Overlying the Vergel Member is the Cocuiza Member, a sequence of 80 meters composed of sandstone, mudstone and limestone. Using strontium dating, an age between 0.709100 and 0.709342 Ma was determined for the Cocuiza Member. Some samples utilized suggested that they had been chemically altered, while two localities yielded an early Pleistocene age. The third member recognized by Stainforth is the Rio Seco Member, however it has not been dated by the 2021 study by Carrillo-Briceño and colleagues.[1]
The fish fauna of the Vergel Member, showcasing a great abundance of catfish in particular, clearly shows that permanent rivers played a vital part in the environment of the Pliocene San Gregorio Formation. The stingray Potamotrygon in particular is commonly associated with flowing waters and sandy banks, although it can also be found in still waters. Overall the depositional environment may have been consisting of floodplains and braided rivers alongside a series of other environments. Swamps, ponds and flooded areas for instance are also possible and may have been home to armored catfish and swamp eels, whose modern members are known to inhabit oxygen-poor bodies of water. Aside from freshwater fish, some fossils also indicate the presence of animal's capable of tolerating a wider range of salt (such as Sciades), possibly indicating a proximity to marine environments. The presence of foraminiferans with low tolerance for saltwater supports this, suggesting that the region also featured estuaries. Frogs, podocnemidid turtles and caimans are all indicative of freshwater environments of varying nature, while Crocodylus falconensis could have easily inhabited areas closer to the coast. Matamata turtes meanwhile are generally associated with slow moving waters, marshes and swamps. Other animals found in the Vergel Member associated with bodies of water include freshwater molluscs and crabs, the False Coral Snake which requires humid soil and prehistoric relatives of the Capybara. This highly diverse fauna indicates that the Vergel member heavily featured permanent bodies of freshwater, likely rivers, as well as other slower moving bodies of water such as swamps, marshes and/or ponds.[1]
The terrestrial fauna features a variety of endemic South American mammals ranging from notoungulates, ground sloths, glyptodonts to smaller types of armadillos. These animals would suggest a forested-grassland environment, with the presence of the genus Corallus, a tree boa, clearly indicating the presence of forested areas. This is supported by the palynoflora, which shows the presence of plants associated with rainforests. However, the general size of seeds and fruits found in the Norte Casa Chiguaje locality is more in-line with open environments, ultimately suggesting an environment featuring both forested areas as well as more open grassland.[1]
The Cocuiza Member meanwhile represents a mostly marine environment following marine incurions in northern South America. The presence of sharks, echinoderms, oyster colonies and marine gastropods suggests a coastal setting. Fossils of terrestrial and freshwater fauna discovered in the San Gregorio Oeste del Pueblo (SGOP) Locality has been interpreted to represent carcasses that were washed into the ocean by streams and rivers of the backshore, supported by the disarticulated and fragmentary nature of most remains found in the Cocuiza Member. However, the fauna discovered in this member suggests that an environment similar to that of the older Vergel Member (forested grassland mixed with wetlands or permanent freshwater habitats) was found further inland, providing the locality with its freshwater and terrestrial fossil remains.[1]
The rainforest taxa recovered from the Vergel Member's flora are absent from the Cocuiza Member, suggesting that drying climate during the Pliocene-Pleistocene transition caused a great turnover in the flora of the northern neotropics.[1]
The composition of the Rio Seco Member suggests a fluvial environment.[1]
Color key
|
Notes Uncertain or tentative taxa are in small text; |
| Name | Species | Member | Material | Notes | Image |
|---|---|---|---|---|---|
| Amaranthaceae | Vergel Member | ||||
| Asteraceae | Vergel Member | ||||
| aff. Cleomaceae | Vergel Member | ||||
| Cucurbitaceae | Vergel Member | ||||
| Poaceae | Vergel Member | grasses resembling Chloroideae | |||
| aff. Vitaceae | Vergel Member | ||||
| Name | Species | Member | Material | Notes | Image |
|---|---|---|---|---|---|
| Bivalvia indet. | Vergel Member | ||||
| Gastropoda indet. | Vergel Member | ||||
| cf. Planorbidae | Vergel Member | ||||
| Ostreidae | Cocuiza Member | ||||
| Name | Species | Member | Material | Notes | Image |
|---|---|---|---|---|---|
| Trichodactylidae indet. | Vergel Member | ||||
| Name | Species | Member | Material | Notes | Image |
|---|---|---|---|---|---|
| Agassizia | A. excentrica | Cocuiza Member | |||
| Arbacia | A. punctulata | Cocuiza Member | |||
| Encope | E. cf. emarginata | Cocuiza Member | |||
| Encope | E. secoensis | Cocuiza Member | |||
| Mellitella | M. falconensis | Cocuiza Member | |||
| Moira | M. atropos | Cocuiza Member [2] | |||
| Name | Species | Member | Material | Notes | Image |
|---|---|---|---|---|---|
| Actinopterygii indet. | Vergel Member | abundant isolated material | |||
| cf. Amblydoras | cf. A. sp. | Vergel Member | a partial cleithrum | the first recorded fossil of the genus | |
| Callichthyidae indet. | Vergel Member | fin spine fragments and bony armor plates | one or more species of armored catfish | ||
| Cichlidae indet. | Vergel Member | fin spines | |||
| Doradidae indet. | Vergel Member | fin spines, scutes and skull remains | while some elements may represent Amblydoras and Scorpiodoras, other fossils may represent a third genus was present in the region | ||
| Hoplias | H. sp. | Vergel Member | isolated teeth |  | |
| Hypostominae indet. | Vergel Member | fin spines fragments and dermal teeth | some specimen are similar to Hemiancistrus, but are too fragmentary to be referred to the genus |  | |
| Loricariidae indet. | Vergel Member | parts of the pectoral spine and armor plates | the elements are too fragmentary to safely refer to Hypostominae that are present in the formation | ||
| cf. Megaleporinus | cf. M. sp. | Vergel Member | two symphyseal premaxillary teeth | ||
| Myliobatiformes indet. | Vergel Member | a single isolated vertebra | |||
| Mylossoma | M. sp. | Vergel Member | dentary teeth | a "pacu clade" serrasalmid |  |
| cf. Pimelodella | cf. P. sp. | Vergel Member | incomplete pectoral fin-spine |  | |
| Pimeloidea indet. | Vergel Member | pectoral spines | |||
| cf. Platysilurus | cf. P. sp. | Vergel Member | pectoral spines | ||
| Potamotrygon | P. sp. | Vergel Member | isolated teeth and caudal spines |  | |
| Schizodon | cf. S. corti | Vergel Member | teeth of the premaxilla and dentary | ||
| cf. Sciades | cf. S. sp. | Vergel Member | a fragmented dorsal spine | ||
| cf. Scorpiodoras | cf. S. sp. | Vergel Member | a partial cleithrum | the first recorded fossil of the genus | |
| Serrasalmidae indet. | Vergel Member | isolated teeth | possibly one or several species of the "pacu" or "Myleus clade" | ||
| Siluriformes indet. | Vergel Member | eroded and isolated elements of the skull and postcrania | |||
| Synbranchus | S. sp. | Vergel Member | dentaries, a pharyngeal bone with teeth and vertebrae | a genus of swamp eel |  |
| Bagre | B. marinus [3] | Cocuiza Member | |||
| Name | Species | Member | Material | Notes | Image |
|---|---|---|---|---|---|
| Anilius | A. scytale | Vergel Member | incomplete trunk vertebra | the extant American Pipe Snake or False Coral Snake | _(14112604251).jpg) |
| Boidae indet. | Vergel Member | isolated vertebrae | similar to the genus Epicrates | ||
| Boidae/Aniliidae indet. | Vergel Member | incomplete trunk vertebra | |||
| Caimaninae indet. | Vergel Member
Cocuiza Member |
cranial and postcranial remains | |||
| Chelonoidis | C. sp. | Vergel Member | incomplete right femur |  | |
| Chelus | C. sp. | Vergel Member | plastron fragment | A type of Mata mata turtle | |
| Colubroides indet. | Vergel Member | fragmentary trunk vertebra | |||
| Corallus | C. sp. | Vergel Member | incomplete trunk vertebra | a moderately large tree boa | |
| Crocodylus | C. falconensis | Vergel Member | |||
| Podocnemididae indet. | Vergel Member | various postcranial remains |  | ||
| Serpentes indet. | Vergel Member | fragmentary trunk vertebrae | |||
| Squamata indet. | Vergel Member | fragmentary skulls | non-snake squamates | ||
| Testudines indet. | Vergel Member | fragmentary skulls, shell and assorted other postcranial remains | |||
| Tupinambis s.l. | T. sp. | Vergel Member | dentary fragment | a large sized teiid lizard | |
| Caiman | aff. C. yacare | Cocuiza Member | maxillary fragment | _2.jpg) | |
| Crocodylia indet. | Vergel Member
Cocuiza Member |
coprolites, teeth, osteoderms and other bones | |||
| Eunectes | E. sp. | Cocuiza Member | trunk vertebra | a large anaconda | |
| Name | Species | Member | Material | Notes | Image |
|---|---|---|---|---|---|
| aff. Boreostemma | aff. B. sp. | Vergel Member | two osteoderms | ||
| ?Caviomorpha indet. | Vergel Member | isolated teeth and tooth fragments | |||
| Cricetidae indet. | Vergel Member | currently undescribed | |||
| Cyonasua[4] | C. sp. | Vergel Member | |||
| cf. Didelphis | cf. D. sp. | Vergel Member | partial humerus | a type of Opossum | |
| Falcontoxodon | F. sp. | Vergel Member | teeth, ankle bones and a metatarsal | ||
| aff. Holmesina | aff. H. floridanus | Vergel Member
Cocuiza Member |
osteoderms |  | |
| Hydrochoerinae indet. | Vergel Member | fragmentary teeth | |||
| Hydrochoeropsis | ?Hydrochoeropsis wayuu | Vergel Member | a dentary and molars | an extinct relative of the Capybara | |
| Interatheriidae indet. | Vergel Member | an isolated tooth crown | |||
| Mammalia indet. | Vergel Member
Cocuiza Member |
various postcranial remains | |||
| Marisela | Marisela gregoriana | Vergel Member | an isolated molar | ||
| Mylodontidae indet. | Vergel Member | right humerus |  | ||
| Neoepiblemidae indet. | Vergel Member | distal phalanx | |||
| aff. Plaina | aff. P. sp. | Vergel Member | osteoderm | ||
| Pliodasypus [5] | Pliodasypus vergelianu | Vergel Member | |||
| cf. Proeremotherium | cf. P. sp. | Vergel Member
Cocuiza Member |
a nearly complete skull and an isolated tooth | ||
| Proterotheriidae indet. | Vergel Member | metacarpal remains | |||
| Typotheria incerta sedis | Vergel Member | a partial jaw | |||
| Camelidae indet. | Cocuiza Member | a fragmentary femur | |||
| Chapalmalania | C. sp. | Cocuiza Member | partial hemimandible | a large bodied procyonid | |
| Sirenia indet. | Cocuiza Member | ||||
| Toxodontidae indet. | Cocuiza Member | teeth and a partial mandible |  | ||
| Xenarthra indet. | Cocuiza Member | incomplete caudal vertebra | |||
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