A Pliocene–Pleistocene continental biota from Venezuela

Abstract

The Pliocene–Pleistocene transition in the Neotropics is poorly understood despite the major climatic changes that occurred at the onset of the Quaternary. The San Gregorio Formation, the younger unit of the Urumaco Sequence, preserves a fauna that documents this critical transition. We report stingrays, freshwater bony fishes, amphibians, crocodiles, lizards, snakes, aquatic and terrestrial turtles, and mammals. A total of 49 taxa are reported from the Vergel Member (late Pliocene) and nine taxa from the Cocuiza Member (Early Pleistocene), with 28 and 18 taxa reported for the first time in the Urumaco sequence and Venezuela, respectively. Our findings include the first fossil record of the freshwater fishes Megaleporinus, Schizodon, Amblydoras, Scorpiodoras, and the pipesnake Anilius scytale, all from Pliocene strata. The late Pliocene and Early Pleistocene ages proposed here for the Vergel and Cocuiza members, respectively, are supported by their stratigraphic position, palynology, nannoplankton, and 86Sr/88Sr dating. Mammals from the Vergel Member are associated with the first major pulse of the Great American Biotic Interchange. In contrast to the dry conditions prevailing today, the San Gregorio Formation documents mixed open grassland/forest areas surrounding permanent freshwater systems, following the isolation of the northern South American basin from western Amazonia. These findings support the hypothesis that range contraction of many taxa to their current distribution in northern South America occurred rapidly during at least the last 1.5 million years.

A historical vertebrate collection from the Middle Miocene of the Peruvian Amazon

“…These teeth are up to 4 mm in height, straight with a conical and pointed crown, distinctly sharp edges, and with a basal section with parallel sulcus. The specimens PIMUZ A/I 4828 from the Iquitos fauna are very similar to those recent and fossil teeth of Hoplias (e.g., Carrillo-Briceño et al, 2021).…”

“…PIMUZ A/I 4810 was preliminarily reported as Myletes by Peyer (1937), andPeyer (1968: p. 107) noted later that the genus was "replaced by the three genera: Colosoma, Metynnis and Mylosoma". The diagnostic concavity in the lingual face of symphyseal mandibular teeth, in which the elevation of the distal edge is different among Colossoma, Piaractus and Mylossoma (see Dahdul, 2004;Aguilera and Machado-Allison, 2013;Carrillo-Briceño et al, 2021), allows the assignment of the two left symphyseal teeth PIMUZ A/I 4812 (10.9 mm in height; Fig. 4B1-B2) and PIMUZ A/I 4813A (10.9 mm in height; Fig.…”

“…It is important note that the tooth PIMUZ A/I 4813A is a cast, not knowing the current repository of the original specimens. On the basis of the diagnosis for symphyseal mandibular teeth determination in Colossoma, Piaractus and Mylossoma (Dahdul, 2004;Aguilera and Machado-Allison, 2013;Carrillo-Briceño et al, 2021), the left tooth PIMUZ A/I 4818 (7.1 mm in height; Fig. 4D1-D2.)…”

“…Specimens such as the operculum (PIMUZ A/I 4811) and large molariform-like teeth likely belong to Colossoma. However, as has been referred by Carrillo-Briceño et al (2021), "pacu clade" species have a combination of molariform-like teeth adapted for crushing hard foods that look very similar and difficult to distinguish among Colossoma, Piaractus, and Mylossoma, especially when the determination is based on isolated elements. Isolated cranial bones of the large Colossoma and Piaractus species appear to be indistinguishable.…”

“…Most species of Doradidae have ridges or spines on the lateral face of the posterior cleithral process, whereas others have deep, rectangular processes, and all have the dorsal process covered by the posttemporal-supracleithrum and unleveled with the ventral and posterior portion. Fossils that constitute the posterior cleithral process were previously described for Doradidae (Sabaj et al, 2007;Carrillo-Briceño et al, 2021), but not for Auchenipteridae. In reference to the isolate pectoral fin-spines, he most complete one is PIMUZ A/I 5050, which is from the left position and of approximately 23 mm in length (Fig.…”