Gonkoken nanoi

The Great Dinosaur Drought of 2022–2023 persisted from mid-December 2022 to early June 2023, nearly six months with three new dinosaurs and one silesaur, but with five showing up in the past few weeks, it looks like things have gone back to business as usual. Obviously that calls for some recognition, so this week and next week (pending anything else) I'll look at a couple of new arrivals, starting with the not-quite-hadrosaurid Gonkoken nanoi from the south end of Chile.

Figure 2 in Alarcón-Muñoz et al. (2023), showing a reconstruction of Gonkoken nanoi and an assortment of bones (CC BY 4.0).

Genus and species: Gonkoken nanoi. The genus name is a combination of two Aónik’enk words, "gon" ("same as" or "similar to") and "koken" ("wild duck" or "swan"), and the species name honors Mario "Nano" Ulloa, "who first found dinosaur bones at Río de las Chinas Valley and provided key logistic help during our expeditions" (Alarcón-Muñoz et al. 2023). Together this comes out to something like "wild-duck-like [animal] of Nano".

Citation: Alarcón-Muñoz, J., A. O. Vargas, H. P. Püschel, S. Soto-Acuña, L. Manríquez, M. Leppe, J. Kaluza, V. Milla, C. S. Gutstein, J. Palma-Liberona, W. Stinnesbeck, E. Frey, J. P. Pino, D. Bajor, E. Núñez, H. Ortiz, D. Rubilar-Rogers, and P. Cruzado-Caballero. 2023. Relict duck-billed dinosaurs survived into the last age of the dinosaurs in subantarctic Chile. Science Advances 9(24). doi:10.1126/sciadv.adg2456. [that's a lot of authors!]

Stratigraphy and geography: G. nanoi come from the upper Dorotea Formation, dated to the early Maastrichtian (between 71.7 ± 1.2 and 70.5 ± 5.0 Ma). This formation came up briefly before as a source of unnamed titanosaurs. The fossils of the new species were found at Loma Koken in the Río de las Chinas Valley, Estancia Cerro Guido, Magallanes Region, near the southern tip of Chile (Alarcón-Muñoz et al. 2023).

Holotype: CPAP 3045 (Paleontological Collection of Antarctica and Patagonia, Instituto Antártico Chileno (INACH), Punta Arenas, Chile), a right ilium (Alarcón-Muñoz et al. 2023). (And now, thanks to an acronym with multiple usages, I have yet another post that will receive mistaken visitors.)

The solitary holotype is a bit misleading; G. nanoi was found in a bonebed of multiple individuals, with quite a few other bones. These include bones of the skull and lower jaws; cervicals, dorsals, a partial sacrum, and caudals; ribs; scapula and sternal bones; more ilia and parts of the other pelvic bones; and femora, tibiae, a partial fibula, and a metatarsal. The authors reported at least three individuals based on left humeri and right femora, but there's also a fragment of a left humerus mentioned in the supplementary information, and there's also at least one juvenile, so I suspect at least five. This is a minor quibble, though, as the bonebed has not been completely excavated or evaluated. A similar bone-bearing horizon apparently extends another 5 km (3 mi) to the northeast in the valley, part of a floodplain system (Alarcón-Muñoz et al. 2023).

G. nanoi's species name can also function as a sideways pun, as we are not dealing with a large animal as hadrosaurs go. Alarcón-Muñoz et al. (2023) estimated the body size as 4 m (13 ft) long, and the largest measured femur is 485.1 mm (19.1 in) long. As befits something that is not quite a true hadrosaurid, there are fewer tooth positions and a less elongate skull. Proportionally, the overall impression I have is of something kind of chunky, although we should consider the reconstruction as provisional given how bonebeds work.

G. nanoi is more remarkable in terms of implications than anatomy. If you're reading this post, you've probably got a decent idea of what an almost-hadrosaurid looks like, and G. nanoi is so far not doing anything unusual, although to be fair with the known material there's a chance the nasal has a ridge or there are some exaggerated neural spines or something. Phylogenetically, G. nanoi comes out as the sister taxon to the sister to the sister of Hadrosauridae, which makes it something of an anachronism in an age when true hadrosaurids had almost achieved a large-ornithopod monopoly (and also acts as an inspiration for the use of "gon" ["similar to"] in the name). In fact, if you want a transitional hadrosaur that's reliably dated as younger, the field is basically Telmatosaurus. G. nanoi is also rather more complete than its nearest compatriots, which should be helpful for further studies in this part of the ornithopod tree. An interesting side result of the phylogenetic analysis is that the other included South American taxa (Bonapartesaurus rionegrinus, Huallasaurus australis, Kelumapasaura machi, and Secernosaurus koerneri) form a clade paired with the gryposaurs, which Alarcón-Muñoz et al. designate Austrokritosauria.

This leads to the conclusion that there were at least two distinct lineages of hadrosaurs in South America near the end of the Cretaceous, one of hadrosaurids and another of almost-hadrosaurids. This knowledge complicates any assignment of hadrosaur-like pieces-parts found in South America and down to Antarctica; they can't simply be attributed to Hadrosauridae. Alarcón-Muñoz et al. go further into an analysis of dispersal and distribution. Both lineages most likely originated in North America and dispersed south. G. nanoi is the most southerly hadrosaur to be formally named, and all of the bits of comparable or more southerly latitude are not inconsistent with similar almost-hadrosaurids. The authors propose that G. nanoi's lineage arrived in South America before Austrokritosauria. This would make southern Patagonia a refugium for transitional hadrosaurs in the face of the hadrosaurid advance (G. nanoi being an approximate contemporary of some South American true hadrosaurids). (Now that I think of it, where are the Brazilian hadrosaurids? Is the Brazilian Late Cretaceous so far only represented by settings preferred by titanosaurs?) The authors also comment briefly on the capacity of hadrosaurs for dispersal. While I'm not sure I would go so far as to consider them semiaquatic, I do agree that as large animals with capacious buoyant guts and no obvious anatomical anchors, they were probably well-suited for dispersal by water, much like titanosaurs and elephants.

References

Alarcón-Muñoz, J., A. O. Vargas, H. P. Püschel, S. Soto-Acuña, L. Manríquez, M. Leppe, J. Kaluza, V. Milla, C. S. Gutstein, J. Palma-Liberona, W. Stinnesbeck, E. Frey, J. P. Pino, D. Bajor, E. Núñez, H. Ortiz, D. Rubilar-Rogers, and P. Cruzado-Caballero. 2023. Relict duck-billed dinosaurs survived into the last age of the dinosaurs in subantarctic Chile. Science Advances 9(24). doi:10.1126/sciadv.adg2456.