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Sunday, March 14, 2021

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.

6 comments:

  1. Given your experience with various phylogenetic analyses of titanosaurs, IS Ninjatitan the earliest named one?

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    1. Heh, heh...
      It's got a reasonable case, although it would be nice if one of these times we could get more than a few caudals and miscellaneous pieces for the "world's oldest titanosaur". I'd be curious to see it in some other analyses, to see how close it is to the metaphorical velvet rope maintained by Andesaurus. (Which, of course, is an arbitrary thing, and that just gets magnified when we're dealing with the base of a clade.)

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    2. The "caudals and miscellaneous" bit is not to pick on Ninjatitan; the pre-Aptian titanosaur record as a whole is frustratingly sparse. Then we get to the question of how did titanosaurs go from players in an ensemble cast of sauropods to practically the only thing going in the Cenomanian (barring the uber-specialized rebbachisaurs and a couple of oddities). What happened in the middle Cretaceous to knock out the other sauropod clades, and how did the titanosaurs avoid it? Better suited for the mid-Cretaceous climate? Better equipped to deal with the spread of angiosperms? Something else? (Osteoderms as mineral dump for egg-laying giving them an advantage in reproduction?) Pure dumb luck?

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    3. I suppose it would more or less depend on whether Australodocus is a titanosaur or not, right?

      Personally I sometimes feel that (at least if Euhelopus turns out to be a mamenchisaur) Titanosauria ought to be the "closer to Titanosaurus than Brachiosaurus" clade. Somphospondyli was coined in the context of a titanosaur-Euhelopus clade and Andesaurus hasn't done a very good job of defining Titanosauria...

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    4. It would make things simpler in some ways, although in other ways it pushes the problem to a different part of the tree. (It also appeals because I hate Somphospondyli as a word. It feels like a mouthful of chewing gum that's lost its flavor.) I do think there's some value in recognizing "that sauropod clade that went bonkers in the Middle Cretaceous and took over the world", and at the moment the best name for it appears to be Titanosauria, for better or worse.

      (Honestly, poor Andesaurus is doing about the best that it can. If only it could get over its tendency to hang out in epic basal polytomies in certain matrices...)

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  2. I firmly believe that titanosaurs first appeared in the Jurassic or at least 150 Million years ago in laurasia and the study of ninjatitan did not include volgatitan which was discovered in 2018, 3 years before ninjatitan. Volgatitan was known from Russia & was 130 million years old. Studies have proved that it couldn't have been an immigrant from South America & that titanosaurs had a long history in laurasia & may have even originated there possibly in the late Jurassic

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