Paleontological discoveries in the Chorrillo Formation (upper Campanian-lower Maastrichtian, Upper Cretaceous), Santa Cruz Province, Patagonia, Argentina

Abstract

The first fossil remains of vertebrates, invertebrates, plants and palynomorphs of the Chorrillo Formation (Austral Basin), about 30km to the SW of the town of El Calafate (Province of Santa Cruz), are described. Fossils include the elasmarian (basal Iguanodontia) Isasicursor santacrucensis gen. et sp. nov., the large titanosaur Nullotitan glaciaris gen. et sp. nov., both large and small Megaraptoridae indet., and fragments of sauropod and theropod eggshells. The list of vertebrates is also composed by the Neognathae Kookne yeutensis gen. et sp. nov., two isolated caudal vertebrae of Mammalia indet., and isolated teeth of a large mosasaur. Remains of fishes, anurans, turtles, and snakes are represented by fragmentary material of low taxonomical value, with the exception of remains belonging to Calyptocephalellidae. On the other hand, a remarkable diversity of terrestrial and freshwater gastropods has been documented, as well as fossil woods and palinological assemblages. The Chorrillo Formation continues south, in the Las Chinas River valley, southern Chile, where it is called Dorotea Formation. Both units share in their lower two thirds abundant materials of titanosaurs, whose remains cease to appear in the upper third, registering only elasmarians (Chorrillo Formation) and hadrosaurs (Dorotea Formation). Above both units there are levels with remains of invertebrates and marine reptiles. It is striking that the dinosaurs of the lower two thirds of the Chorrillo and Dorotea formations are represented by large basal titanosaurs and Megaraptoridae coelurosaurs, being the Saltasaurinae and Aeolosaurinae sauropods and Abelisauridae theropods totally absent. In contrast, these taxa are dominant components in sedimentary units of central and northern Patagonia (e.g., Allen, Los Alamitos, La Colonia formations). Such differences could reflect, in part, a greater antiquity (i.e., late Campanian-early Maastrichtian) for the Chorrillo fossils, or, more probably, different environmental conditions. Thus, knowledge of the biota of the southern tip of Patagonia is expanded, particularly those temporarily close to the K-Pg boundary.

Giants Among Dwarfs: The Arrival Of A New Clade Of Southern Dinosaurs In The Latest Cretaceous Of Europe

“…The deltopectoral crest is robust and strongly expanded mediolaterally to reach the central axis of the shaft, as in several titanosaurs, but differs from the unexpanded or moderately expanded crests of all the Ibero-Armorican taxa, and other titanosaurs. It projects unambiguously to medial, similarly to Gondwanatitan 32 , Nullotitan 33 , and Jainosaurus 34 . Unlike the Ibero-Armorican titanosaurs, Abditosaurus humerus exhibits a distally expanded deltopectoral crest, a synapomorphy of Saltasauridae 46,48 shared with Opisthocoelicaudia 41 , Saltasaurus 49 , N. australis 50 , and Alamosaurus 51 .…”

“…1). humerus with a medially projected deltopectoral crest (shared with Gondwanatitan 32 , Nullotitan 33 , and J. septentrionalis 34 ) and articular surface of radial condyle facing anterodistally (shared with Saltasaurus 35 and Paralititan ); radius with an oblique small ridge on the posterodistal surface*; tibia with (21) an anteriorly projected cnemial crest (shared with Gondwanatitan 32 ), and a small prominent bulge at the posterior margin of the proximal end (shared with Neuquensaurus 37 ); fibula with a distal end beveled posteriorly 20º with respect to the long axis of the shaft*.…”

A Late Cretaceous dinosaur and crocodyliform faunal association–based on isolate teeth and osteoderms–at Cerro Fortaleza Formation (Campanian-Maastrichtian) type locality, Santa Cruz, Argentina

“…3 , 5 – 8 ]. A recent sedimentological analysis [ 2 ] included the Cerro Fortaleza, La Anita, La Irene, and Chorrillo (another dinosaur-bearing unit [ 9 , 10 ]) formations as lithologically similar beds under the denomination “Uppermost Cretaceous Continental Deposits”. Regardless of their names, the dinosaur-bearing units were generally considered as Campanian-Maastrichtian [ 4 , 6 , 11 – 16 ].…”

“…However, the association observed in the sample is rich in terms of archosaur taxonomic diversity for the Austral Basin; particularly, peirosaurid teeth are reported here for the first time. Interestingly, the other highly diverse dinosaur-bearing unit in the basin, the Chorrillo Formation, located 160 km south from the Cerro Fortaleza locality, has been recently considered as late Campanian-early Maastrichtian based on its faunal content [ 9 , 10 ]. The correlation between the two formations is not clear, and there are differences in the faunas documented in the Chorrillo and the probably underlying Cerro Fortaleza formations.…”

Palynoflora from the La Anita Formation (Maastrichtian), Austral-Magallanes Basin, Argentina

“…Dettmann & Thomson, 1987, Askin, 1990bCantrill & Poole, 2005). In southern South America terrestrial palynological records are sparse (Papú, 1988a(Papú, , b, 1989Baldoni, 1992;Baldoni & Askin, 1993;Papú, 2002;Povilauskas et al, 2008;Povilauskas, 2010Povilauskas, , 2011Povilauskas, , 2012Povilauskas, , 2013Povilauskas, , 2016Vallati, 2010;Novas et al, 2019), with several geologic units with no information on their spore-pollen content. For this reason, the study of new assemblages is essential for evaluating the evolution of austral floras and will also serve as a basis to better evaluate the magnitude of the drop in plant diversity and abundance throughout the K/Pg boundary (Barreda et al, 2012).…”