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.
Stratigraphy 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.
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.
Paleoenvironment
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.
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.
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.
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.
The composition of the Rio Seco Member suggests a fluvial environment.
Paleoflora
Name Species Member Material Notes ImageAmaranthaceae Vergel MemberAsteraceae Vergel Memberaff. Cleomaceae Vergel MemberCucurbitaceae Vergel MemberPoaceae Vergel Membergrasses resembling Chloroideaeaff. Vitaceae Vergel Member
Paleofauna Molluscs Name Species Member Material Notes ImageBivalvia indet.Vergel MemberGastropoda indet.Vergel Membercf. PlanorbidaeVergel MemberOstreidaeCocuiza Member
Crustaceans Name Species Member Material Notes ImageTrichodactylidae indet.Vergel Member
Echinoderms Name Species Member Material Notes ImageAgassizia A. excentricaCocuiza MemberArbacia A. punctulataCocuiza MemberEncope E. cf. emarginataCocuiza MemberEncope E. secoensisCocuiza MemberMellitella M. falconensisCocuiza MemberMoira M. atroposCocuiza Member
Fish Name Species Member Material Notes ImageActinopterygii indet.Vergel Memberabundant isolated materialcf. Amblydorascf. A. sp.Vergel Membera partial cleithrumthe first recorded fossil of the genusCallichthyidae indet.Vergel Memberfin spine fragments and bony armor platesone or more species of armored catfishCichlidae indet.Vergel Memberfin spinesDoradidae indet.Vergel Memberfin spines, scutes and skull remainswhile some elements may represent Amblydoras and Scorpiodoras, other fossils may represent a third genus was present in the regionHopliasH. sp.Vergel Memberisolated teethHypostominae indet.Vergel Memberfin spines fragments and dermal teethsome specimen are similar to Hemiancistrus, but are too fragmentary to be referred to the genusLoricariidae indet.Vergel Memberparts of the pectoral spine and armor platesthe elements are too fragmentary to safely refer to Hypostominae that are present in the formationcf. Megaleporinuscf. M. sp.Vergel Membertwo symphyseal premaxillary teethMyliobatiformes indet.Vergel Membera single isolated vertebraMylossomaM. sp.Vergel Memberdentary teetha "pacu clade" serrasalmidcf. Pimelodellacf. P. sp.Vergel Memberincomplete pectoral fin-spinePimeloidea indet.Vergel Memberpectoral spinescf. Platysiluruscf. P. sp.Vergel Memberpectoral spinesPotamotrygonP. sp.Vergel Memberisolated teeth and caudal spinesSchizodoncf. S. cortiVergel Memberteeth of the premaxilla and dentarycf. Sciadescf. S. sp.Vergel Membera fragmented dorsal spinecf. Scorpiodorascf. S. sp.Vergel Membera partial cleithrumthe first recorded fossil of the genusSerrasalmidae indet.Vergel Memberisolated teethpossibly one or several species of the "pacu" or "Myleus clade"Siluriformes indet.Vergel Membereroded and isolated elements of the skull and postcraniaSynbranchusS. sp.Vergel Memberdentaries, a pharyngeal bone with teeth and vertebraea genus of swamp eelBagreB. marinus Cocuiza Member
Amphibians Name Species Member Material Notes ImageAnura indet.Vergel Memberisolated cranial and postcranial elementscf. Pipacf. P. sp.Vergel Membersacral vertebrae fused to a partial urostyle
Reptilians Name Species Member Material Notes ImageAniliusA. scytaleVergel Memberincomplete trunk vertebrathe extant American Pipe Snake or False Coral SnakeBoidae indet.Vergel Memberisolated vertebraesimilar to the genus EpicratesBoidae/Aniliidae indet.Vergel Memberincomplete trunk vertebraCaimaninae indet.Vergel Member Cocuiza Membercranial and postcranial remainsChelonoidisC. sp.Vergel Memberincomplete right femurChelusC. sp.Vergel Memberplastron fragmentA type of Mata mata turtleColubroides indet.Vergel Memberfragmentary trunk vertebraCorallusC. sp.Vergel Memberincomplete trunk vertebraa moderately large tree boaCrocodylusC. falconensisVergel MemberPodocnemididae indet.Vergel Membervarious postcranial remainsSerpentes indet.Vergel Memberfragmentary trunk vertebraeSquamata indet.Vergel Memberfragmentary skullsnon-snake squamatesTestudines indet.Vergel Memberfragmentary skulls, shell and assorted other postcranial remainsTupinambis s.l.T. sp.Vergel Memberdentary fragmenta large sized teiid lizardCaimanaff. C. yacareCocuiza Membermaxillary fragmentCrocodylia indet.Vergel Member Cocuiza Membercoprolites, teeth, osteoderms and other bonesEunectesE. sp.Cocuiza Membertrunk vertebraa large anaconda
Mammals Name Species Member Material Notes Imageaff. Boreostemmaaff. B. sp.Vergel Membertwo osteoderms?Caviomorpha indet.Vergel Memberisolated teeth and tooth fragmentsCricetidae indet.Vergel Membercurrently undescribedCyonasuaC. sp.Vergel Membercf. Didelphiscf. D. sp.Vergel Memberpartial humerusa type of OpossumFalcontoxodonF. sp.Vergel Memberteeth, ankle bones and a metatarsalaff. Holmesinaaff. H. floridanusVergel Member Cocuiza MemberosteodermsHydrochoerinae indet.Vergel Memberfragmentary teethHydrochoeropsis?Hydrochoeropsis wayuuVergel Membera dentary and molarsan extinct relative of the CapybaraInteratheriidae indet.Vergel Memberan isolated tooth crownMammalia indet.Vergel Member Cocuiza Membervarious postcranial remainsMariselaMarisela gregorianaVergel Memberan isolated molarMylodontidae indet.Vergel Memberright humerusNeoepiblemidae indet.Vergel Memberdistal phalanxaff. Plainaaff. P. sp.Vergel MemberosteodermPliodasypus Pliodasypus vergelianuVergel Membercf. Proeremotheriumcf. P. sp.Vergel Member Cocuiza Membera nearly complete skull and an isolated toothProterotheriidae indet.Vergel Membermetacarpal remainsTypotheria incerta sedisVergel Membera partial jawCamelidae indet.Cocuiza Membera fragmentary femurChapalmalaniaC. sp.Cocuiza Memberpartial hemimandiblea large bodied procyonidSirenia indet.Cocuiza MemberToxodontidae indet.Cocuiza Memberteeth and a partial mandibleXenarthra indet.Cocuiza Memberincomplete caudal vertebra
References