Non avian dinosaur faunas of the Upper Cretaceous Bajo Barreal and Lago Colhue Huapi formations of central Patagonia, Argentina: Paleobiogeographic and biostratigraphic implications

LAMANNA, MATTHEW C.; CASAL, GABRIEL A.; IBIRICU, LUCIO M.; MARTÍNEZ, RUBÉN D.

Congreso; XXI Congreso Geológico Argentino; 2022

Fossil discoveries in recent decades have cast abundant new light on the Upper Cretaceous non-avian dinosaur assemblages of the Golfo San Jorge Basin of central Patagonia, Argentina. These fi nds come from the Chubut Group, specifi cally from the Cenomanian-Turonian Bajo Barreal Formation (BBF) of southern Chubut and northern Santa Cruz provinces and the Coniacian-Maastrichtian Lago Colhué Huapi Formation (LCHF) of southern Chubut Province (Fig. 1A). Collectively, these discoveries provide key insights into central Patagonian dinosaur evolution through the fi nal ~30 million years of the Cretaceous (Casal et al. 2016, Ibiricu et al. 2020).The dinosaur assemblage of the BBF is the better known of the two, due in large part to more intensive study. Non-avian theropods from this Cenomanian-Turonian unit include multiple abelisaurid specimens (belonging to Xenotarsosaurus bonapartei and probably additional taxa), an unnamed early diverging megaraptorid, and the enigmatic early branching coelurosaur Aniksosaurus darwini. The sauropod fauna consists of the rebbachisaurid Katepensaurus goicoecheai, the titanosaurians Drusilasaura deseadensis, Epachthosaurus sciuttoi, and Sarmientosaurus musacchioi, and additional, indeterminate material pertaining to both of these clades (e.g., specimens assigned to the titanosaurians ?Andesaurus sp.? and ?Campylodoniscusameghinoi?; see Ibiricu et al. 2020: table 1). The only ornithischian yet described from the BBF is the probable elasmarian ornithopod Notohypsilophodon comodorensis, known from the partial postcranium of a single immature individual. The LCHF was only recognized as a distinct lithostratigraphic unit in 2015, and, unsurprisingly, knowledge of its dinosaur fauna is limited. Nevertheless, a number of dinosaur specimens have been reported in recent years, suggesting a panorama of species diversity broadly comparable to that of other Upper Cretaceous Patagonian formations. The only theropod clade yet to be unquestionably documented in the LCHF is Megaraptoridae, represented by several fragmentary specimens (e.g., isolated manual unguals) from Campanian-?lower Maastrichtian horizons plus a well-preserved partial cranial and postcranial skeleton pertaining to a new Maastrichtian taxon. Remains possibly belonging to Abelisauridae have also been recently discovered in Campanian ?lower Maastrichtian strata. Titanosaurians are the only known sauropods, represented by the Coniacian Elaltitan lilloi, the Campanian Aeolosaurus colhuehuapensis, the Campanian-?lower Maastrichtian Argyrosaurus superbus, and several generically indeterminate or undescribed specimens. Ornithischians are potentially more diverse than in the BBF and consist of the medium-sized Campanian elasmarian Sektensaurus sanjuanboscoi, Maastrichtian hadrosaurids (Secernosaurus koerneri and indeterminate or undescribed specimens), and indeterminate ornithopod material. The purported ceratopsian ?Notoceratops bonarellii? is based on a partial dentary?most likely that of a hadrosaurid?that cannot currently be located. The dinosaur faunas of the BBF and LCHF hold important paleobiogeographic and biostratigraphic implications. First, more than those of other Patagonian Upper Cretaceous units, the BBF fauna shows remarkable resemblances to similarly aged dinosaur assemblages from eastern Australia. For example, the BBF megaraptorid exhibits closer morphological similarities to penecontemporaneous Australian forms than to other Patagonian members of the clade (Lamanna et al. 2020), and Sarmientosaurus was recently recovered as the sister taxon of several Australian Cenomanian-Turonian titanosaurians within the subclade Diamantinasauria (Poropat et al. 2021). These relationships suggest the existence of a terrestrial dispersal route between southern South America and Australia during the mid-Cretaceous, presumably via Antarctica (Fig. 1B). Second, the respective fossil records of the BBF and LCHF reveal the total or near-total replacement of abelisaurids by megaraptorids as apex carnivores in central Patagonia by the Campanian-Maastrichtian and the persistence of these latter theropods to the end of the Cretaceous in this region. A similar phenomenon may have occurred in the Austral-Magallanes Basin of southern Patagonia, where megaraptorids are, to date, the only latest Cretaceous large-bodied theropods to have been defi nitively recorded. In morenortherly areas of South America (e.g., Neuquén Basin of northern Patagonia, Bauru Basin of Brazil), by contrast, abelisaurids and unenlagiine dromaeosaurids were the dominant large-bodied theropods during the Campanian-Maastrichtian, whereas megaraptorids were seemingly rare or absent (Fig. 1C). These apparent faunal distinctions-megaraptorids in central and southern Patagonia versus abelisaurids/large unenlagiines in more northern areas-may have been driven by paleoenvironmental or paleoecological diff erences (Lamanna et al. 2020). Ongoing studies will continue to characterize the Upper Cretaceous dinosaur assemblages of central Patagonia, enabling more robust evaluations of these and other paleobiogeographic and biostratigraphic hypotheses.