Your Friends The Titanosaurs, part 33.5: Arackar and Ninjatitan

Two titanosaurs were announced within a few days of each other at the end of February–beginning of March: Arackar licanantay and Ninjatitan zapatai. They're both from the South American stronghold of the group, but at opposite ends of the titanosaurian geologic time frame. Neither is currently known from a great deal of material. Will these be the last species to sneak in under the line before the end of this series?

Arackar licanantay

Back in the entry for Atacamatitan chilensis, I mentioned that A. chilensis was based on the second-best titanosaur specimen from Chile, with the best specimen being undescribed at that time. That specimen, SNGM-1 (Servicio Nacional de Geología y Minería, Santiago, Chile), has now received the name Arackar licanantay (Rubilar-Rogers et al. 2021), although not without some controversy over less-than-helpful aspects of how the preprint has been presented.

Genus and species: Arackar licanantay is translated as "in reference to 'bones of the Atacamenians' in Kunza, the language of the original indigenous people of the Atacama region" (Rubilar-Rogers et al. 2021), but the exact breakdown of the genus and species names is not specified. [2021/03/19: a breakdown by Ben Creisler can be found here. Short answer: "arackar" for bones, "Lican Antay" for the Atacamenian people.]

Citation: Rubilar-Rogers, D., A. O.Vargas, B. Gonzalez Riga, S. Soto-Acuña, J. Alarcón-Muñoz, J. Iriarte-Díaz, C. Arévalo, and C. S. Gutstein. 2021. Arackar licanantay gen. et sp. nov. a new lithostrotian (Dinosauria, Sauropoda) from the Upper Cretaceous of the Atacama Region, northern Chile. Cretaceous Research. doi:10.1016/j.cretres.2021.104802.

Stratigraphy and Geography: The type and only known specimen of A. licanantay was discovered in 1993 at Quebrada La Higuera, approximately 75 km (47 mi) south of Copipaó in Atacama Region, northern Chile. The site is in the lower to middle Hornitos Formation, described as Campanian–Maastrichtian in age. The type specimen was found in lacustrine mudstone within a lacustrine mudstone–fluvial sandstone sequence (Rubilar-Rogers et al. 2021).

Holotype: SNGM-1/1–23, consisting of two cervical centra, two anterior and one posterior dorsal neural arch, three dorsal centra, the right humerus, the left ischium, the left femur, and fragments, found over an area of about two square meters (22 square feet). These bones are well-preserved and represent a partially grown individual (Rubilar-Rogers et al. 2021).

A. licanantay is one of a small number of titanosaurs known from the Pacific side of South America, joining Atacamatitan chilensis and Yamanasaurus lojaensis as the only named species, and is the most completely represented of this group. Most of the diagnostic features pertain to the various laminae of the vertebrae; perhaps the most obvious feature from a distance is the strong posterior angle of the dorsal neural spines. The limb bones are on the gracile side. The humerus, at 590 mm long (23.2 in), is about 4/5ths the length of the femur, at 740 mm (29.1 in) (Rubilar-Rogers et al. 2021). Rubilar-Rogers et al. (2021) ran a phylogenetic analysis and found their new species to group with an Indo–Madagascar group consisting of Isisaurus colberti and Rapetosaurus krausei, interestingly enough.

Ninjatitan zapatai

Our next guest was announced a few days before A. licanantay. Not only does it come with a memorable name, it is also a contestant in the ongoing "world's oldest titanosaur" competition.

Genus and species: Ninjatitan zapatai contains references to two people. The genus name honors paleontologist Sebastián Apesteguía by his nickname "Ninja"; if you've been looking at the citations in this titanosaur series, you'll have noticed his name numerous times. The species name honors Rogelio Zapata, a technician at the Museo Municipal Ernesto Bachman, and by extension the work of the rest of the museum's technician team (Gallina et al. 2021). The name might be translated loosely as something like "titan of Sebastián Apesteguía and Rogelio Zapata".

Citation: Gallina, P. A., J. I. Canale, and J. L. Carballido. 2021. The earliest known titanosaur sauropod dinosaur. Ameghiniana 58(1):35–51. doi:10.5710/AMGH.20.08.2020.3376.

Stratigraphy and Geography: Lower Cretaceous Bajada Colorada Formation (late Berriasian–Valanginian age), Bajada Colorada locality, approximately 40 km (24 mi) southwest of Picún Leufú in Neuquén Province, Argentina. We visited this locality a couple of years ago for Bajadasaurus pronuspinax. In this case, the bones came from a level 4 m (13 ft) below the previously described specimens (Gallina et al. 2021).

Holotype: MMCh-Pv228 (Museo Municipal Ernesto Bachmann, Villa el Chocón, Neuquén), which includes a partial anterior/middle dorsal, a middle dorsal centrum, an anterior caudal centrum with a bit of neural arch (first caudal?), the left scapula, distal femur, and nearly complete left fibula. The bones came from an area 6 m square (about 65 square feet, which might sound like a lot but is basically 8 feet by 8 feet) and are regarded as representing one individual (Gallina et al. 2021).

We can tell that N. zapatai is not a diplodocoid and thus is neither Bajadasaurus nor Leinkupal, the other named sauropod from the Bajada Colorada Formation. Three features indicate it is a titanosaurian: a slightly procoelous caudal, pneumatization of the caudal's neural arch, and the position of a process on the scapula (Gallina et al. 2021). (If the caudal in Figure 3.2 seems backwards, note that it's in right-lateral view rather than left-lateral view [threw me for a moment before I read the caption].) When analyzed phylogenetically, it came out as either just within Titanosauria or nested in Colossosauria, in fact within Lognkosauria (Gallina et al. 2021). N. zapatai as the earliest named titanosaur would push the origin of the group at least near the Jurassic–Cretaceous boundary. We also get the interesting coincidental association of the most recent named diplodocid (Leinkupal) with the earliest named titanosaur.

References

Gallina, P. A., J. I. Canale, and J. L. Carballido. 2021. The earliest known titanosaur sauropod dinosaur. Ameghiniana 58(1):35–51. doi:10.5710/AMGH.20.08.2020.3376.

Rubilar-Rogers, D., A. O.Vargas, B. Gonzalez Riga, S. Soto-Acuña, J. Alarcón-Muñoz, J. Iriarte-Díaz, C. Arévalo, and C. S. Gutstein. 2021. Arackar licanantay gen. et sp. nov. a new lithostrotian (Dinosauria, Sauropoda) from the Upper Cretaceous of the Atacama Region, northern Chile. Cretaceous Research. doi:10.1016/j.cretres.2021.104802.