Sunday, October 28, 2018

Your Friends The Titanosaurs, part 5: Argyrosaurus, Atacamatitan, and Atsinganosaurus

We're almost beyond the letter "A" in the alphabet of titanosaurs (for whatever reason titanosaur researchers have given a disproportionate number of genera names that begin with "A"). For this entry we have a historic name (Argyrosaurus superbus), the first non-avian dinosaur named from Chile (Atacamatitan chilensis), and our second well-represented smallish titanosaur from from western Europe (Atsinganosaurus velauciensis).

Argyrosaurus superbus

Historically, Argyrosaurus superbus is one of the few titanosaurs to have gotten much press. It has a 19th century description date to thank for this; it got to enjoy being one of the earliest "titanosaurids" and for many years was one of the few sauropods known from the Southern Hemisphere. Unfortunately, most of what has been written about A. superbus and Argyrosaurus in general is based on material with tenuous connections to the actual holotype of A. superbus, at best. Mannion and Otero (2012) tell the story of A. superbus and the various faux Argyrosaurus.

A. superbus got off to a rough start. The holotype is most of a left arm of a large sauropod, but apparently there was more to the skeleton before inadequate collecting practices reduced it to its current state. It was originally located in 1888 by pioneering Argentine vertebrate paleontologist Carlos Ameghino and then collected without his assistance (Mannion and Otero 2012). British geologist Richard Lydekker described what was left in 1893, along with several other titanosaur species (if you're familiar with any of them, it's probably his Titanosaurus australis, now known as Neuquensaurus). Lydekker chose the name Argyrosaurus as a reference to Argentina; "argyros" being silver in Ancient Greek and "Argentina" from the Italian word for "[made] of silver". The specimen was found on the northwest side of the Río Chico near Pampa Pelada in Chubut Province, southern Argentina (Mannion and Otero 2012). Opinion on the source rocks has changed over the years; it is now considered to be from the Lago Colhué Huapi Formation (Casal et al. 2015), also the source of Aeolosaurus colhuehuapensis.

As described by Lydekker, the type specimen, MPL 77-V-29-1 (Museo de La Plata, La Plata, Argentina), included the left humerus, ulna, radius, a carpal, and five metacarpals; per Mannion and Otero (2012), there were probably two carpals, but they are missing. The presence of bony carpals at all is very unusual for titanosaurs (the only known occurrence per Mannion and Otero 2012), so it's rather unfortunate they are missing. (Incidentally, because derived titanosaurs had no manual phalanges, I suppose that means they were some kind of obligate knuckle-walkers, if you count knuckles when there are no fingers.) The humerus measures a whopping 1,370 mm (53.94 in) long, the ulna 965 mm (38.0 in), and the longest metacarpal 513 mm (20.2 in) (Mannion and Otero 2012). All together, they get us within shouting distance of a 3-m (10-ft) arm, and also show that A. superbus had relatively long metacarpals.

Plate V of Lydekker (1893) shows the holotype arm of Argyrosaurus superbus, plus one of those femora that kept ending up with it.

So far, so good. Lydekker didn't stop there, though; he also had a partial left femur and a couple of caudal centra from other locations, and tentatively assigned them to A. superbus, seemingly on the basis that they couldn't belong to his other species. 36 years later, von Huene got into the act, and tossed a few more humeri, caudals, and femora at Argyrosaurus, including one just over 2 m (7 ft) long (Huene 1929). It should go without saying that it's kind of difficult to justify assigning femora and caudals to a taxon where the type includes neither femora and caudals and there are no overlapping specimens, and I don't recommend it. Powell (2003) took specimens previously assigned to Antarctosaurus wichmannianus for Argyrosaurus. These included the Field Museum femur and tibia (FMNH 13019 and 13020) used by Mazzetta et al. (2004) to estimate the size of A. wichmannianus, and the partial skeleton PVL 4628 (Colección de Paleontología de Vertebrados de la Fundación Instituto Miguel Lillo, Tucumán) and MACN-CH 217 (Museo Argentino de Ciencias Naturales Bernardino Rivadavia, Buenos Aires). Mannion and Otero (2012) found that none of the additional specimens could be assigned to Argyrosaurus in general or A. superbus in particular. Furthermore, none of them were diagnostic at the species level except for PVL 4628/MACN-CH 217, which they named Elaltitan lilloi. This leaves us, and A. superbus, with a very large left arm and not much else. (There's also an Argyrosauridae, but there's not much call for it at the moment.)

Atacamatitan chilensis

As a recently named species, Atacamatitan chilensis has been playing a role like "Antarctosaurus" brasilensis—mostly featuring in review articles or discussions of Chilean dinosaurs (Rubilar-Rogers and Gutstein 2012; Rubilar-Rogers et al. 2012; Soto-Acuña et al. 2015), reminding everyone that it's there. The major claim to fame for A. chilensis is as the first non-avian dinosaur named from Chile, but it's been rather overshadowed by the second, one Chilesaurus diegosuarezi (which has made a strong recovery after getting shown up by Yi qi at the time of its announcement). A. chilensis was known informally as "Domeykosaurus chilensis" in the early 2000s before acquiring its current name. You can probably guess what Atacamatitan chilensis means, so we don't need to belabor that point.

A. chilensis was described in 2011 by Kellner et al. on the basis of a partial skeleton recovered from the Tolar Formation near Conchi Viejo in the Antofagasta Region of northern Chile. The age of the Tolar is poorly constrained; about all that can be said is "Late Cretaceous". The type specimen is SGO-PV-961 (the vertebrate paleontology collection of the Museu Nacional de Historia Natural in Santiago, Chile). It includes two dorsals, posterior caudals, ribs, what is interpreted as a partial sternal bone, the proximal end of a humerus, the right femur, and other incomplete bones, recovered over two expeditions carried out by a joint Brazilian–Chilean team in 2000–2001 (Kellner et al. 2011). The femur is 1.10 m (3.61 ft) long and on the gracile side (Kellner et al. 2011); the relatively gracile build is the one aspect that Rubilar-Rogers et al. (2012) chose to highlight when briefly describing the species in a longer discussion of Chilean dinosaur. Rubilar-Rogers et al. (2012) found it to be a lithostrotian in a phylogenetic analysis, but as we've seen, your mileage may vary.

The most informative parts of the holotype of A. chilensis, from Soto-Acuña et al. (2015). A and B are the humerus; C, D, and M are various views of the femur; E and F are the possible sternal plate; G and H are a rib; I and J are a dorsal; K and L are another dorsal; O is an associated centrum fragment; N and P are a caudal. The bones share the same scale (10 cm) except for N and P, which use a 5 cm scale. CC BY-SA 4.0.

Because we're here, I ought to comment on the other Chilean titanosaurs. Titanosaurs have been reported from Chile since at least 1969, when Casamiquela et al. discussed remains from the Viñita Formation. A. chilensis is actually based on the second-best titanosaur specimen from Chile (Kellner et al. 2011); the most complete specimen is SNGM-1 (Servicio Nacional de Geología y Minería, Santiago, Chile), which is represented by two cervical centra, three dorsal neural arches, two dorsal centra, the right ischium, the right humerus, and the left femur (Rubilar-Rogers and Gutstein 2012). It was in press at one point, per Rubilar-Rogers et al. (2012), but has not yet made an official appearance. [edit, 2021/05/21: this is now Arackar licanantay.] Rubilar-Rogers et al. (2012) and Soto-Acuña et al. (2015) also referred to several other occurrences of less complete titanosaur finds, so if you're a completist about this sort of thing, I recommend raiding their bibliographies. Rubilar-Rogers et al. (2017) recently described a caudal from an adult titanosaur from the Hornitos Formation, which had previously only produced fossils from juvenile titanosaurs.

Atsinganosaurus velauciensis

For our last guest, we return to France for the first time since Ampelosaurus atacis in the first post, where we get to meet Atsinganosaurus velauciensis. Like Ampelosaurus atacis, Atsinganosaurus velauciensis is well-represented by bones from across the skeleton. The genus name is derived from a Byzantine Greek word meaning "gypsy", in reference to the original describers' interpretation of the species as showing migrations between western and eastern Europe, and the species name comes from the Latin name for Velaux (Velaucio) (Garcia et al. 2010). As a recently described species, A. velauciensis hasn't had time to accumulate many publications. Aside from brief mentions and inclusions as part of an ensemble (e.g., Vila et al. 2012 and Csiki-Sava et al. 2015), the major references for Atsinganosaurus are the original description (Garcia et al. 2010) and the 2018 revision by Díez Díaz et al. (thank you to Verónica Díez Díaz for supplying me with a copy).

A. velauciensis is known from the Velaux-La Bastide Neuve locality in the Aix-en-Provence Basin, Bouches-du-Rhône Department, Provence-Alpes-Côte d'Azur Region, in southeastern France (Garcia et al. 2010; Díez Díaz et al. 2018). (This is the same site that yielded the rhabdodont Matheronodon provincialis.) The geology is described slightly differently in the two cited publications: Garcia et al. (2010) reported glauconitic grainstone limestone of no formal unit, whereas Díez Díaz et al. (2018) reported sandstone of the Argiles et Grès à Reptiles Formation, although both assigned the unit to the upper Campanian. As is fairly common in paleontology, there was a long time between the discovery (1992) and publications; the sauropod fossils themselves were first documented in a thesis (Thouand 2004) and named in Garcia et al. (2010). Aside from the sauropod and rhabdodont material, the site has also yielded shark, coelacanth, turtle, croc, pterosaur (Mistralazhdarcho maggii, published barely a week ago), theropod, and ankylosaur fossils, deposited in a fluvial setting (Díez Díaz et al. 2018).

A. velauciensis is based on four articulated posterior dorsals cataloged as UP/VBN.93.01.a, b, c, and d, in the collections of the Université de Poitiers (VBN refers to Velaux-La Bastide Neuve). There were quite a lot of other bones to go around, and some specimens of A. velauciensis are held by the Musée Moulin Seigneurial and Muséum d'Historie Naturelle d'Aix-en-Provence (Díez Díaz et al. 2018; there seems to have been some consolidation since Garcia et al. 2010). All told, there are a few parts of the skull, several teeth, vertebrae from each section of the vertebral column, ribs, chevrons, shoulder and pelvic girdle elements, most of the bones of the arm, and most of the bones of the leg, from multiple individuals (Díez Díaz et al. 2018). To go along with the other long bone measurements I've given, the longest humerus is 555 mm long (21.9 in), and the longest femur is 709 mm (27.9 in) long (Díez Díaz et al. 2018), so A. velauciensis was not quite in the size class of, say, Argyrosaurus superbus above or "Antarctosaurus" giganteus. Overall, it was a relatively gracile species (Garcia et al. 2010; Vila et al. 2012; Díez Díaz et al. 2018), measuring perhaps 8 to 12 m (26 to 39 ft) long and 3.5 to 5 metric tones (3.9 to 5.5 short tons) (Díez Díaz et al. 2018). [Update, 2020/12/27: Díez Díaz et al. (2020) report that the size was miscalculated. Corrected figures are 5.32 to 8.95 m long (17.5 to 29.4 ft) and 2.46 to 5.26 metric tones (2.71 to 5.80 short tons).] As with other small European Late Cretaceous dinosaurs from otherwise large lineages, the implication is that the relatively small landmasses prevented A. velauciensis from reaching larger sizes.

Garcia et al. (2010) interpreted A. velauciensis as rather basal in anatomy for a late titanosaur, and described it as a basal lithostrotian. They also linked it to Romanian Magyarosaurus rather than other titanosaurs from France or Spain (the Ibero-Armorican Island) based on shared primitive tail features. In contrast, Díez Díaz et al. (2018) conducted a phylogenetic analysis and found A. velauciensis to be most closely related to Ampelosaurus and Lirainosaurus, forming a small clade of moderately derived lithostrotians which they named Lirainosaurinae.


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[Díez Díaz, V., G. Garcia, X. Pereda Suberbiola, B. Jentgen-Ceschino, K. Stein, P. Godefroit, and X. Valentin. 2020. A new titanosaur (Dinosauria: Sauropoda) from the Upper Cretaceous of Velaux-La-Bastide Neuve (southern France). Historical Biology (advance online publication). doi:10.1080/08912963.2020.1841184.]

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