Insects that lived alongside with the last dinosaurs in Patagonia

Chitinous remains of three-dimensional preserved insects, are discovered in 70 million years old rocks in Southern Argentine Patagonia. They shed lights about insect communities that evolved previously to the end of the Mesozoic Era
Published in Ecology & Evolution
Insects that lived alongside with the last dinosaurs in Patagonia

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One of the most attractive topics in Paleobiology concerns with the end-Cretaceous (K-Pg) mass extinction event, occurred 66 million years ago (Ma). Notably, current hypotheses on this biotic perturbation rely almost entirely on information from the northern hemisphere.

With the aim to recover information about the end of the “dinosaur Era”, we explored the far south of Argentina to learn about changes in diversity and abundance of vertebrate taxa (Figure 1). Argentine Patagonia, in the southern region of South America, is famous for its Late Cretaceous vertebrate remains, especially dinosaurs.

Figure 1. Map of southern Patagonia indicating with a yellow spot the location where the Chorrillo Formation crops out
Figure 1. Map of southern Patagonia indicating with a yellow spot the location where the Chorrillo Formation crops out. Credit Damián Moyano Paz

Since 2019 we repeatedly explored outcrops of the Upper Cretaceous Chorrillo Formation, a 500 meters thick rock sequence composed by conglomerates, sandstones and limestones. The age of this sedimentary unit is Maastrichtian, the end Cretaceous interval ranging from 72 through 66 Ma (Figure 2).

Figure 2. Outcrops of the Chorrillo beds widely exposed south of El Calafate, Patagonia, Argentina. At the bottom left, waters of the Argentino Lake, SW Santa Cruz Province. Credit Fernando E. Novas

The results of several field seasons, carried on in joint with Dr. Makoto Manabe and his crew from the National Museum of Nature and Science (Tokyo), were quite satisfactory. We collected bony remains of different dinosaur groups, birds, crocodyliforms, turtles, frogs and fishes, as well as shed teeth of diverse mammals and dozens of freshwater and terrestrial snail shells (Figure 3). Some levels afforded beautifully preserved leaves of angiosperms and gymnosperms.

Figure 3. Reconstruction of main dinosaur clades discovered in the Chorrillo Formation (to the left, the meat-eating megaraptorid Maip macrothorax; at the center, the titanosaurid Nullotitan glaciaris, flanked by a couple of iguanodontian ornithopods Isasicursor santacrucensis). Credit Fernando E. Novas and Sebastián Rozadilla

One of the purposes of the project was to recover rock samples searching for the presence of pollen and fern spores. To do that, palynologist Valeria Perez Loinaze (Conicet-MACN) processed more than 60 rock samples, recovering palynological specimens from 16 of those. However, she observed some microscopic elements calling the attention of paleobotanist Ezequiel Vera (Conicet-MACN). They identified three-dimensionally preserved parts of the chitinous body of different arthropod remains, separating isolated heads preserving mandibular elements, as well as others with a halo of tiny hair-like structures on their margins. Among pollen and spores there were also arthropod limbs of different kinds, compound eyes, and wing scales, constituting a “hidden world” lying along the big bones of the dinosaurs. 

Argentine paleoentomologists Oscar Gallego, Julieta Massaferro, Lara Sabater and Mateo Monferrán and geologist Danián Moyano Paz, all of them researchers of Conicet, joined us to evaluate the significance of the discoveries.

 Fossil record of insects of Maastrichtian age

 The relevance of the new discovery relies on the fact that insect faunas from the latest Cretaceous are poorly known worldwide. Maastrichtian insects from South America mainly consist on nests, pupal chambers, cocoons and feeding activities in central and northern Patagonia, but they constitute the indirect evidence of the existence of a complex community of terrestrial insects. The Chorrillo assemblage, on the contrary, consists of body fossils corresponding to aquatic insects, thus filling the gap in the global record between Early Cretaceous and Paleogene times.

Figure 4. Different morphotypes of chitinous head capsules and mandibles of non-bitting midges (Chironomidae). Credit Ezequiel Vera and Valeria Perez Loinaze

The insect assemblage from the Chorrillo Formation is represented by non-biting midges (Chironomidae dipterans; Figure 4), butterflies (Coelolepida lepidopterans; Figure 5), and mayflies (Ephemeroptera; Figure 6). Chironomids are represented by cephalic capsules of aquatic larvae, and their diversity in the assemblage indicates that modern clades were already dominant and diversified by the end of the Cretaceous. Such diversity is interpreted in concert with the parallel radiation of aquatic angiosperms which became dominant in freshwater habitats and are recorded in the same levels of the Chorrillo Formation. Besides, the presence of Lepidoptera is attested by an array of isolated wing scales, as well as fragments of larval exuviae, also mirroring a diversity of lepidopterans congruent with the evolutionary renewal described above for chironomids. The assemblage recovered in Chorrillo is strikingly modern in taxonomic composition (Figure 7).

Figure 5. Butterfly (Lepidoptera) scales and probable larval exuviae. Credit Ezequiel Vera and Valeria Perez Loinaze

Association with other aquatic organisms, including plants and ornithorhynchids

In particular, the chironomid assemblage from Chorrillo Formation resembles those of the Andean-Patagonian biogeographic region by the absence of Chironominae and abundance and diversity of Orthocladiinae. These austral taxa generally inhabit cool, pristine environments, often upland to montane streams.

Figure 6. Mayfly (Ephemeroptera) naiad head (left) and compound eye of an indetermined arthropod (right). Credit Ezequiel Vera and Valeria Perez Loinaze

Chironomids recorded in the Chorrillo beds are anatomically close to some Australasian taxa, currently present in the Andean and Australasian regions. The strong tie between chironomids from Chorrillo beds and those of the Australasian Region is congruent with the evidence afforded by different fossil vertebrates collected in the same beds, including frequent megaraptorid theropods alongside with abundant elasmarian ornithischians, an assemblage resembling Cretaceous faunas of Australia.

Figure 7. The insect assemblage documented in Chorrillo Formation was taxonomically modern. Credit Mateo Monferran and Darío Larrea

Moreover, in early 2023 we announced the discovery of the monotreme Patagorhynchus pascuali (Figure 8), a Cretaceous relative of the living platypus (Ornithorhynchus), revealing the evolutionary diversification of these egg-lying mammals outside Australia. Discovery of Patagorhynchus clearly demonstrates that the monotremes had already attained a wide paleogeographic distribution, stretching across southern South America and Australia, using Antarctica as a connecting pathway, constituting a clade characteristic of the Weddelian Paleobiogeographical Province (Figure 9).

Figure 8. The egg-lying mammal Patagorhynhus probably fed upon on larvae of mayflies. Credit Fernando E. Novas and Gabriel Lio.

The possibility that Patagorhynchus had already acquired ecological and behavioral characteristics similar to those of the living platypus, which inhabits ponds and lakes, is congruent with sedimentological evidence suggesting that such environments were prevalent during deposition of the Chorrillo Formation, as well as with occurrences of the aquatic Nymphaeaceae and heterosporous ferns, and freshwater snails. It is worthy to note that larvae of chironomid insects and aquatic mollusks form part of the food for the living platypuses.


Figure 9. Paleogeographic map seen from the South Pole, showing location of Patagonia, Antarctica and Australia for the end of the Cretaceous. Credit Fernando E. Novas

Future Insights

 The unexpected discovery of microscopic remains of fossil arthropods in the Chorrillo beds, by employing usual palynological methodologies, encourages paleoentomologists for an active search of insect remains in geological units of different ages, which could provide a unique way of enhancing our knowledge on the past diversity of different insect clades.

 As far as Cretaceous vertebrates and chironomids is concerned, those recorded in southern Patagonia give an insight into the degree of continuity between the continental biotas of western and eastern Gondwana during the Late Cretaceous. Present discoveries imply that an extensive and still unknown southern biota waits to be documented in Late Mesozoic beds of southern South America. Our main purpose is to continue searching for fossils in the Chorrillo Formation to identify patterns in diversity of different clades of animals and plants before the end of the Cretaceous.

 The abundance of chitinous remains in the Chorrillo beds belonging to taxonomically unidentified arthropods, promises new and exciting discoveries in this remote place of southern Argentina. No doubts, the Chorrillo bedrocks constitute a formidable “window” into an ancient ecosystem which may offer novel clues about life crisis at the end of the Mesozoic in the Southern continents.

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