Sunday, February 16, 2020

Your Friends The Titanosaurs, part 21: Nullotitan, Opisthocoelicaudia, and Overosaurus

It seems like getting two short entries and one long entry is how these are going lately. This time Opisthocoelicaudia skarzynskii gets the feature spot, with Nullotitan glaciaris and Overosaurus paradasorum as co-stars. N. glaciaris was just published a couple of months ago and is based on fragmentary material, so there isn't a lot to say about it yet. O. paradasorum includes most of a vertebral column, making it an important point of comparison for other titanosaurs.

Nullotitan glaciaris

Until the announcement of Abdarainus barsboldi as I was writing this (Averianov and Lopatin 2020), Nullotitan glaciaris was the most recently described titanosaur. It was one of three dinosaurs named in a lengthy report on the Chorrillo Formation of Patagonia (the others being the basal iguanodont Isasicursor santacrucensis and the bird Kookne yeutensis) (Novas et al. 2019). As is not uncommon, the history of the fossils goes back a number of years. Dinosaur remains had been reported from the Chorrillo Formation as far back as the mid-1940s. Francisco Nullo, a geologist of the Argentine Geological Survey, found a partial sauropod skeleton in 1980 and alerted José Bonaparte to the find. Bonaparte in turn collected a partial cervical vertebra from the site, which was cataloged as MACN-PV 18644 (Museo Argentino de Ciencias Naturales "Bernardino Rivadavia", Buenos Aires). Other remains were left in place. In January and March 2019, this find was followed up with larger collections of a variety of fossils, including titanosaur remains from the original site (Novas et al. 2019).

Figure 1 in Novas et al. (2019) lays out the geography. Locality 1 in the lower map is Nullo's original locality, and the type locality for N. glaciaris. Locality 2 is another titanosaur locality, which includes eggshell fossils. CC-BY-3.0.

The genus name honors geologist Francisco Nullo, and the species name is a reference to the Perito Moreno Glacier, which could be seen from the excavation site (Novas et al. 2019). This gives us something like "Francisco Nullo's titan of the glacier". So far N. glaciaris is known from rather fragmentary remains. At Nullo's original site, six associations of titanosaur bones representing multiple individuals were found over 100 square meters on a slope. The exact horizon is not certain for all of these associations, because some were found as float, but they all appear to have come from the lower third of the Chorrillo Formation, probably dating to somewhen around the Campanian–Maastrichtian boundary. Additional titanosaur remains were found in the middle but not the upper part of the formation. The holotype is designated as MACN-PV 18644 (Bonaparte's cervical centrum) and MPM 21542 (fragments of cervical and dorsal ribs, caudal verts, a fragmentary left scapula, proximal and distal ends of the right femur, and the right tibia, fibula, and astragalus; Museo Padre Molina, Río Gallegos). Other specimens add a humerus, additional caudals (including a sequence of five articulated caudals with chevrons), another tibia, and a partial rib (Novas et al. 2019).

Figure 14 in Novas et al. (2019) shows N. glaciaris itself, admittedly a bit thin on the ground to date. CC-BY-3.0.

N. glaciaris was a good-sized titanosaur, estimated at 20 m (67 ft) long, with the holotype tibia 105 cm (41.3 in) long and the referred humerus 114 cm (44.9 in) long (Novas et al. 2019). The type tibia and fibula are robust, but the humerus is gracile (Novas et al. 2019), which could just mean the animal was more heavily built in the legs than the arms, or, in the light of the recent extensive discussion of Neuquensaurus variation, that N. glaciaris was dimorphic, but let's not hop onto that train yet.

In terms of classification N. glaciaris has had a fair amount of history despite only being named in December. The first collected cervical, MACN-PV 18644, had been cataloged as cf. Antarctosaurus. Later, Bonaparte et al. (2002) thought it had aeolosaur affinities, while Powell (2003) considered it to belong to an indeterminate titanosaur. In the absence of more complete remains, Novas et al. (2019) did not prepare a phylogenetic analysis, but regarded it as a colossosaurian rather than an aeolosaur or saltasaur. One feature the authors drew attention to is the presence of numerous blind pits in the caudals, which they tentatively interpreted as attachment points for tendons, perhaps pointing to unsuspected anatomy for "tail support and motion control".

Opisthocoelicaudia skarzynskii

First off, we went over the Nemegtosaurus/Opisthocoelicaudia controversy back with N. mongoliensis, and there hasn't been any great advance on that subject in the past two months.

It's been a couple of decades now since Opisthocoelicaudia skarzynskii became a titanosaur, but from the time it was described until well into the '90s it was widely thought to be a camarasaurid. Borsuk-Białynicka (1977) described it as a camarasaurid. It was a potential camarasaurid in Norman (1985), a camarasaurid in McIntosh (1990), and a camarasaurid as late as Glut (1997) and Maryańska (2000). Then it was a titanosaur (e.g., Upchurch et al. 2004). Obviously the transition wasn't quite as simple as that (the idea goes back to the early 1990, as in Salgado and Coria 1993 and Upchurch 1994), but unlike Nemegtosaurus, there was not an extended back-and-forth in the literature. Rather, thanks to the magic of the Dinosaur Mailing List archives, you can see that the back-and-forth took place in more informal channels. At the time, I was one of the camp who was strongly against O. skarzynskii as a titanosaur.

The interesting thing is, O. skarzynskii could have gone either way at the time of its description; Borsuk-Białynicka (1977) decided that the camarasaurid traits were the informative traits and the titanosaur traits were convergent. It wasn't that long ago that sauropod classification was handled in ways that bring to mind how people described the whys of mountain building before they came up with plate tectonics. Only a few features were considered when assigning relationships. Two of the most important were the articulations of the vertebrae and whether or not the neural spines of the cervicals and dorsals were single or split. O. skarzynskii has opisthocoelous caudals as stated in the name (ball in front, socket behind) and split neural spines, which ruled it out as a titanosaur at the time. To make a long story short, there are a lot of potential problems with relying on a small number of key features. Later discoveries and research would show that titanosaurs weren't necessarily limited to procoelous caudals, and split neural spines evolved several times within sauropods. If O. skarzynskii wasn't a titanosaur, it was sure going out of its way to come across as one, except for those couple of notable features.

The tail that launched a tongue-twister and controversy. The chevrons are fused to the vertebrae. Photo taken by Adrian Grycuk of the mount of O. skarzynskii at the Museum of Evolution in Warsaw, Poland, CC BY-SA 3.0 PL. Found on Wikimedia Commons, where there are a number of other photos of this mount as well.

The type specimen of O. skarzynskii was discovered during the 1965 Polish-Mongolian Palaeontological Expedition to the Gobi Desert, at a locality Borsuk-Białynicka (1977) identified as Altan Ula IV but which is usually called Altan Uul IV these days (e.g., Currie et al. 2018 and Averianov and Lopatin 2019). This quarry was relocated in 2009, but had been filled in by blowing sand over the past 40+ years, making further exploration for additional bones impossible (Currie et al. 2018). The type specimen of Nemegtosaurus mongoliensis was found during the same expedition at a different locality. The Opisthocoelicaudia type was found in early June, and excavated late June into early July, while the Nemegtosaurus type was found on June 15th on a side trip to Nemegt (Currie et al. 2018). All in all, a good month's work. Stratigraphically, the Altan Uul IV quarry is from the lower Nemegt Formation, slightly higher than the type Nemegtosaurus horizon (Currie et al. 2018).

The famously complicated genus name refers to the opisthocoelous caudal vertebrae, and the species name honors the preparator, Wojciech Skarżyński (Borsuk-Białynicka 1977), making this something like "Wojciech Skarżyński's posterior-socket tail". The type specimen was initially cataloged as ZPAL MgD-I/48 (Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland) (Borsuk-Białynicka 1977), but has returned to Mongolia, where it is known as MPC-D100/404 (Institute of Paleontology and Geology, Academy of Sciences of Mongolia, Ulaanbaatar, Mongolia) (e.g., Currie et al. 2018). Borsuk-Białynicka (1977) referred a scapula and coracoid from a juvenile to the species as well (ZPAL MgD-I/25c).

The type specimen was found as an associated and mostly articulated skeleton of a fully grown individual, missing the skull, almost all of the neck, some of the ribs, and part of the left foot (Borsuk-Białynicka 1977; and you really ought to download a copy of this monograph, which extends beyond simple anatomical description into reconstruction of the limb muscles, paleobiological interpretations, and numerous measurements). This makes it not only one of the most complete titanosaur individuals known, but also one of the most complete sauropods known. The vertebral column (except for the missing neck) was articulated and the limbs and ribs were closely associated. The skeleton was situated belly-up with minor fluvial disruption. Tooth marks are present on some of the bones, such as the pelvis and right femur, and offer some idea of what might have happened to the neck and head. However, we can safely assume that this otherwise complete and well-associated specimen was not long exposed to scavengers or the elements (Borsuk-Białynicka 1977).

A longer view of the O. skarzynskii mount (with the head of Nemegtosaurus mongoliensis). Photo taken by Adrian Grycuk, CC BY-SA 3.0 PL. Found on Wikimedia Commons.

The immediate impression given by the skeleton is of a low-slung and stocky animal. It was not exceptionally large for a sauropod, despite its advanced age. The preserved portion of the skeleton is about 8.5 m long (28 ft), and the femur is 139.5 cm long (54.9 in). The dorsal centra and neural arches are relatively long anterior-posterior. The neural spines of the vertebrae near the base of the neck are deeply split into a U-shape. The complete tail is relatively short, with only 34 vertebrae. The first few are strongly opisthocoelous, transitioning to flat articular surfaces by the 16th caudal and simple biconvex caudals by the 28th. Chevrons are only present back to the 19th caudal and are fused to the posterior surface of their caudals. The fusion of the chevrons to the caudals would have limited the tail's downward flexibility (Borsuk-Białynicka 1977), which is part of why early illustrations always gave O. skarzynskii a ramrod-straight tail (the other part being the fusion of several of the centra after the chevrons disappear). (Given that this was an aged individual, the question comes to mind that maybe some of the fusion was a function of age.)

Many of the girdle elements are large and/or heavily built. The pelvis is unusual in that the ischia and pubic bones meet in long aprons rather than being more rod-like bones. Borsuk-Białynicka (1977) proposed that O. skarzynskii was a habitual rearer, like the extinct giant sloth Megatherium. This was suggested by the flaring sloth-like hips, the deep ilium supporting the hip socket, and the fused chevrons making it easier to flex the tail up than down (and thus suited for a prop). The arm bones are quite chunky (see SVPOW! for an analysis of the humerus). As with most other titanosaurs, there are no bony carpals or distinct finger bones, just humerus, radius, ulna, and five stocky metacarpals (with some kind of cartilaginous carpal or carpals between them). A "small rounded body fused with the distal surface of the metacarpal IV" might be a phalanx. Borsuk-Białynicka (1977) regarded the well-developed arms and shoulders as incongruous with rearing, but on the other hand, they could have made shifting to a tripodal stance faster, as well as been useful for knocking things over (like trees) with more precision than using the whole body.

Short strong arms, big sternal plates, metacarpals with no phalanges, pubic bones fused along their lengths. Photo taken by Adrian Grycuk, CC BY-SA 3.0 PL. Found on Wikimedia Commons.
As a titanosaur, O. skarzynskii tends to show up in the company of Alamosaurus and/or the saltasaurs, except, of course, when it doesn't (e.g., Gorscak et al. 2017; one analysis in Tykoski and Fiorillo 2017; Sallam et al. 2018, Gorscak and O'Connor 2019).

Overosaurus paradasorum

You may remember that Baurutitan britoi has carved out a scientific niche for itself by being known from 19 vertebrae, comprising a last sacral and much of a tail. Overosaurus paradasorum goes twice as far: it's known from an articulated series of 40 vertebrae, including the last four cervicals, ten dorsals, six caudals, and twenty caudals, plus ribs and a good ilium (Coria et al. 2013; Leonardo Filippi has posted a nice photo of the type on Twitter). What that means is like B. britoi, O. paradasorum shows up a lot in passing in the titanosaur literature.

The type and only known specimen of O. paradasorum was collected from the Cerro Overo site in northern Neuquén Province, west-central Argentina, about 40 km (25 mi) of Rincón de los Sauces. This locality was originally attributed to the lower Anacleto Formation (Coria et al. 2013), but has since been reassigned to the Bajo de la Carpa Formation (see for example Filippi et al. 2018). More recently, it has also yielded the type specimen of the peirosaurid croc Kinesuchus overoi (Filippi et al. 2018). The specimen was excavated in November 2002 by a joint expedition of the Museo Carmen Funes (Plaza Huincul), Museo Argentino Urquiza (Rincón de los Sauces), and the Natural History Museum of Los Angeles County. It had been discovered earlier by Carlos Parada and his family, who also provided logistical support for the field work (Coria et al. 2013). They are recognized in the species name of the dinosaur, with the site itself included in the genus name (Coria et al. 2013), resulting in something like "the Parada family's Cerro Overo lizard". The type specimen is reposited at the Museo Argentino Urquiza as MAU-Pv-CO-439.

As is always the case with these potential model titanosaurs, O. paradasorum comes with caveats. One is that the anatomy of the ilium and sacrum indicate the back rose noticeably from hips to shoulders, giving it longer arms than legs and a brachiosaur-like profile (Coria et al. 2013). The orientation of sauropod shoulder bones has long been a topic of debate, but in this case the sauropod had high shoulders anatomically enforced by the hips. Interestingly, the ilium is also rather brachiosaur-like (Coria et al. 2013). Another caveat is that O. paradasorum was quite small for a titanosaur, not quite in the "dwarf sauropod" class but not too far off. Coria et al. (2013) suggested a length of 10 m (33 ft), making it reasonably comparable to your average hadrosaur. Sacral and neural arch/centrum fusion is well developed, so it's not simply an age thing (the apparent absence of caudal neural arches in the skeletal reconstruction in Coria et al. 2013 is in part an artifact of illustration; they're there if you go farther into the paper).

O. paradasorum is pretty comfortably an aeolosaur. The original analyses by Coria et al. (2013) found it to be an aeolosaur, and over the past few years every analysis that includes both it and Aeolosaurus shows them close enough to each other that you don't have to include more than the same few usual suspects (usually some combination of Argyrosaurus, Gondwanatitan, Panamericansaurus, Shingopana, and Trigonosaurus) to get the two of them in a clade.


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