San Gregorio Formation, Venezuela

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)
~5.33–0.709100 Ma
Unit ofUrumaco stratigraphic section
Sub-unitsVergel Member Cocuiza Member
OverliesCodore Formation
Thickness~570 m (1,870 ft)
Lithology
PrimarySandstone, mudstone
OtherLimestone
Location
Country Venezuela
Type section
Named forSan Gregorio, Venezuela

Stratigraphy

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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]

Paleoenvironment

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Paleofauna of the San Gregorio Formation

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]

Paleoflora

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Color key
Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon
Notes
Uncertain or tentative taxa are in small text; crossed out taxa are discredited.
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

Paleofauna

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Molluscs

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Name Species Member Material Notes Image
Bivalvia indet. Vergel Member
Gastropoda indet. Vergel Member
cf. Planorbidae Vergel Member
Ostreidae Cocuiza Member

Crustaceans

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Name Species Member Material Notes Image
Trichodactylidae indet. Vergel Member

Echinoderms

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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]

Fish

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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

Amphibians

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Name Species Member Material Notes Image
Anura indet. Vergel Member isolated cranial and postcranial elements
cf. Pipa cf. P. sp. Vergel Member sacral vertebrae fused to a partial urostyle
 

Reptilians

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Name Species Member Material Notes Image
Anilius A. scytale Vergel Member incomplete trunk vertebra the extant American Pipe Snake or False Coral Snake
 
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
 
Crocodylia indet. Vergel Member

Cocuiza Member

coprolites, teeth, osteoderms and other bones
Eunectes E. sp. Cocuiza Member trunk vertebra a large anaconda

Mammals

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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

References

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  1. ^ a b c d e f g h Carrillo-Briceño, Jorge D.; Sánchez, Rodolfo; Scheyer, Torsten M.; Carrillo, Juan D.; Delfino, Massimo; Georgalis, Georgios L.; Kerber, Leonardo; Ruiz-Ramoni, Damián; Birindelli, José L. O.; Cadena, A, Edwin-Alberto; Rincón, ldo F.; Chavez-Hoffmeister, Martin; Carlini, Alfredo A.; Carvalho, Mónica R.; Trejos-Tamayo, Raúl; Vallejo, Felipe; Jaramillo, Carlos; Jones, Douglas S.; Sánchez-Villagra, Marcelo R. (2021). "A Pliocene–Pleistocene continental biota from Venezuela". Swiss Journal of Palaeontology. 140 (9): 9. Bibcode:2021SwJP..140....9C. doi:10.1186/s13358-020-00216-6. PMC 8550326. PMID 34721281.
  2. ^ Mihaljeviü, M.; Klug, C.; Aguilera, O.; Lüthi, T.; Sánchez-Villagra, M.R. (2010). "PALAEODIVERSITY OF CARIBBEAN ECHINOIDS INCLUDING NEW MATERIAL FROM THE VENEZUELAN NEOGENE". Palaeontologia Electronica. 13 (36).
  3. ^ Aguileraa, Orangel; Lopesb, Ricardo T.; Rodriguezc, Felix; dos Santosb, Thaís M.; Rodrigues-Almeidaa, Caroline; Almeidaa, Paulo; Machadob, Alessandra S.; Moretti, Tailan (2020). "Fossil sea catfish (Siluriformes; Ariidae) otoliths and in-skull otoliths from the Neogene of the Western Central Atlantic". Journal of South American Earth Sciences. 101: 102619. Bibcode:2020JSAES.10102619A. doi:10.1016/j.jsames.2020.102619. S2CID 219045960.
  4. ^ Forasiepi, A.M.; Soibelzon, L.H.; Gomez, C.S.; Sánchez, R.; Quiroz, L.I.; Jaramillo, C.; Sánchez-Villagra, M.R. (2014). "Carnivorans at the Great American Biotic Interchange: new discoveries from the northern neotropics" (PDF). Naturwissenschaften. 101 (11): 965–974. Bibcode:2014NW....101..965F. doi:10.1007/s00114-014-1237-4. hdl:11336/32009. PMID 25228347. S2CID 253635703.
  5. ^ Castro, M.C.; Carlini, A.A.; Sánchez, R.; Sánchez-Villagra, M.R. (2014). "A new Dasypodini armadillo (Xenarthra: Cingulata) from San Gregorio Formation, Pliocene of Venezuela: affinities and biogeographic interpretations". Naturwissenschaften. 101 (2): 77–86. Bibcode:2014NW....101...77C. doi:10.1007/s00114-013-1131-5. PMID 24414134. S2CID 253637167.