Muyelensaurus pecheni
We start off in a familiar area, Rincón de los Sauces in northern Neuquén Province, western Argentina. In 1997, Marcelino Palomo discovered fossils at Loma del Lindero, about 10 km (6 mi) west of Rincón de los Sauces, which led to four years of excavations and approximately 300 titanosaur bones. Yes, we have another titanosaur bonebed, but this one so far appears to be monospecific. The specimens were described in Calvo et al. (2007). "Muyelen" is one of the names of the Río Colorado, just north of the discovery site, and "pecheni" honors Ana María Pechen, who was the head of the National University of Comahue in 2002–2006 and supported the study of Neuquén Province dinosaurs (Calvo et al. 2007). This gives us "Ana María Pechen's Muyelen lizard". Calvo et al. (2007) reported the formation as the Portezuelo Formation, but Garrido (2010) reassigned the site to the younger Plottier Formation. The host rocks were overbank reddish pelites (metamorphosed fine-grained sedimentary rocks) and yellowish sandstones (Calvo et al. 2007).As mentioned, we are dealing with hundreds of bones. Every section of the body is represented by something, even if specific bones are not. The downside is it's a bonebed, so it's not known which bones belong to which individuals. The hindlimb bones indicate at least five individuals are represented, described as four adults and one juvenile. A braincase, MRS-PV 207 (Museo de Rincón de los Sauces), was selected as the holotype. The rest of the titanosaur smorgasbord includes a premaxilla, cervicals, dorsals, a sacrum, caudals, scapulae, a sternal plate, humeri, ulnae, radii, metacarpals, ischia, pubic bones, ilia, femora, tibiae, fibulae, an astragalus, metatarsals, and phalanges, with a few ribs and chevrons darkened on the skeletal figure but not discussed in the text. No finger bones were reported, nor osteoderms (Calvo et al. 2007).
Unfortunately, the description of M. pecheni is decidedly not in compliance with MYDD protocols. As a matter of fact, the anatomical description is short considering the abundance of material, and it could stand a longer monographic treatment. Calvo et al. (2007) described the species as slender, but we only really get to have a sense of that with the photo of the strikingly gracile humerus. Take a look at humerus "c" below; that's M. pecheni. Now look at the other humeri. M. pecheni was probably not impressing the other sauropods with its arm muscles. The scale bar with the humerus indicates a length of a little less than 70 cm (28 inches), which reinforces the impression from the scaled bones in the figure that we're not dealing with a particularly large titanosaur. (Note that the skeletal figure in Calvo et al. 2007 only indicates which bones are known; the skeleton itself is an Alamosaurus from Lehman and Coulson 2002).
 |
| You may remember this from the previous titanosaur post, but it includes two of this post's titanosaurs, so it makes sense to bring it back. Humerus "c" belongs to Muyelensaurus pecheni, and humerus "d" is from Narambuenatitan palomoi (Figure 4 in González Riga et al. 2019). CC-BY-4.0. |
Calvo et al. (2007) performed a phylogenetic analysis and found M. pecheni to be most closely related to Rinconsaurus caudamirus, without the quirky caudal articulations. They coined "Rinconsauria" for the clade formed by the two. Sometimes authors propose a clade and it never shows up again, but in this case, when authors include both Muyelensaurus and Rinconsaurus in an analysis, they usually show up close enough to each other that designating a clade is reasonable (even if it does sometimes swallow Lognkosauria or the aeolosaurs). Gonzaléz Riga et al. (2019) coined Colossosauria for Rinconsauria plus Lognkosauria.
Narambuenatitan palomoi
Narambuenatitan palomoi is one of those species which is relatively well represented but not widely discussed in the literature. The genus name refers to the discovery area Puesto Narambuena, and the species name honors Salvador Palomo, the Museum Municipal "Argentino Urquiza" technician who discovered the type specimen, giving us "Salvador Palomo's Narabuena titan." (Coincidentally, Salvador Palomo was also part of the story of Muyelensaurus pecheni, bringing the find to the attention of the authors; Calvo et al. 2007.) The locality is in northeastern Neuquén Province, west-central Argentina. Stratigraphically, the site is in the lower third of the Anacleto Formation, in a stack of decimeter- to meter-scale mudstone, silty sandstone, and sandstone beds. Plant remains and gastropods are also found in the beds, which are interpreted as low-sinuosity sandy channels and surrounding overbank deposits. The titanosaur fossils were excavated in 2005 and 2006 (Filipp et al. 2011).The holotype and only described specimen is MAU-Pv-N-425 (Museo Argentino Urquiza, Rincón de los Sauces, Neuquén, Argentina). It includes a little bit of everything, found associated in an area of about 15 square meters (about 160 square feet; if that sounds like a lot, note that this corresponds to a square less than 13 feet on a side). Bones include a premaxilla and maxilla, braincase, both quadrates, a cervical vertebra and cervical rib fragments, a dorsal vert, three dorsal ribs, 17 caudals plus a couple of neural arch fragments and two chevron fragments, a sternal plate, a coracoid, a humerus, an ulna, both pubis bones, an ilium and ischium fragments, and an incomplete femur. Because the caudal vertebrae have incomplete fusion of the neural arches, MAU-Pv-N-425 is thought to have been a subadult (Filippi et al. 2011).
The handful of skull bones are well preserved. Four teeth were present in the premaxilla and eight in the maxilla, although no functional teeth were found in the sockets, only replacement teeth (Filippi et al. 2011). The cervical looks weird but has been heavily crushed. The dorsal is also skewed and includes a deep pleurocoel. The anterior caudals have tall neural arches and the middle caudals have slender arches. Filippi et al (2011) described the humerus as gracile and the ulna as relatively robust; however, González Riga et al. (2019) considered the ulna relatively gracile. Filippi et al. (2011) gave the length of the humerus as 92 cm (3.0 ft) and the ulna as 60.5 cm (1.98 ft). Although this individual was not fully grown when it died, it probably wouldn't have reached the upper echelon of titanosaurs had it lived to a ripe old age.
 |
| A few postcranial bones of Narambuenatitan palomoi: ulna (A), humerus (B), coracoid (C), and sternal plate (D) (Figure 10 in Filippi et al. 2011; see for full caption). CC-BY-4.0. |
Filippi et al. (2011) ran a phylogenetic analysis and found N. palomoi to be in the middle of their Titanosauria, in a polytomy with Epachthosaurus and Eutitanosauria. It hasn't gotten much play since then. I have a file of transcribed titanosaur phylogenetic results going back into 2015, which allows me to come up with deep insights such as "Muyelensaurus tends to show up in the vicinity of Rinconsaurus." Currently the file includes 28 analyses from 23 publications. Care to guess how many N. palomoi shows up in? Not a one. Go figure. It has made brief anatomical appearances in Wilson et al. (2016) and González Riga et al. (2019).
Nemegtosaurus mongoliensis
Nemegtosaurus mongoliensis is one of the most familiar titanosaurs, being based on one of the few nearly complete skulls known for this group. It also has a nose for controversy, in terms of classification and for what exactly constitutes the species. Let's begin, though, with the facts in the case of N. mongoliensis.Nemegtosaurus mongoliensis was named in Nowinski (1971) for a largely complete skull with mandible. This specimen, Z. PAL MgD-I/9 (Palaeobiological Institute of the Polish Academy of Sciences, Warsaw), is missing only what we might call "the bridge of the nose" (which has allowed for some notably different interpretations of the profile of the skull) and bits of the mandibles. It was discovered in 1965 at the Nemegt locality, Ömnögovi, Mongolia. The name refers to the Nemegt locality and Mongolia (Nowinski 1971), translating to something like "Nemegt lizard of Mongolia". It is usually ascribed to the Nemegt Formation, but per Currie et al. (2018), the find was made near the base of the Nemegt in an interval of interfingering with the underlying Barun Goyot Formation, and Figure 2B in that article places it in a Barun Goyot bed (BG-II). Having relocated the discovery site, Currie et al. (2018) reported finding a caudal centrum, femur (124 cm long, or 48.8 in), tibia, fibula, astragalus, and toe claw of a sauropod, which they interpreted as belonging to the same individual as the skull. Another skull has been mentioned in the literature (Maryańska 2000), but has never been described and thus cannot be evaluated (Wilson 2005); per Currie et al. (2018) it is MPC D100/402 (Institute of Paleontology and Geology of the Mongolian Academy of Sciences, Ulaanbaatar).
Z. PAL MgD-I/9 is described in detail in Nowinski (1971) and Wilson (2005), although you've probably seen photos and reconstructions a fair few times before. Looking at it now, one difference from the diplodocid model stands out immediately: the jog in the lower margin of the upper jaw, as if the cheek region has been tugged up and back from a straight line. Similar excursions are present in other titanosaur skulls (Bonitasaura, Quaesitosaurus, Rapetosaurus, Sarmientosaurus, Tapuiasaurus). The skull otherwise has an elongate profile, slightly concave (accentuated by the missing bones) and featuring that nice sclerotic ring on the right side. In dorsal/ventral view, the skull is slightly wider at the ends. The teeth are concentrated at the front of the jaws. There are four in each premaxilla, eight or nine in each maxilla, and thirteen in each dentary. The teeth are peg-like and taper. They do not differ within a row, but compared to the uppers the dentary teeth are not as broad, have elliptical cross-sections, and curve slightly to the outside; the uppers are broader, D-shaped in cross section, and slightly curved toward the tongue. The jaws and tooth rows are U-shaped, not squared off as in Bonitasaura.
 |
| Nemegtosaurus mongoliensis, slightly restored (D), and some titanosauriform friends (Giraffatitan [A], Abydosaurus [B], Sarmientosaurus [C], Rapetosaurus [E], and Tapuiasaurus [F]). Figure 33 in Martínez et al. (2016). CC-BY-4.0. |
Those are the facts. Now, the controversies:
When N. mongoliensis was named in 1971, there were a little more than 100 recognized sauropod species, about 50 of which are considered valid today. Of that group, 12 or so were from Upper Cretaceous rocks, and outside of N. mongoliensis they included a couple of braincases and the ever-popular Antarctosaurus wichmannianus to illustrate what the skulls were like. Virtually every known sauropod skull came from Upper Jurassic rocks or was thought to come from Upper Jurassic rocks (Euhelopus zdanskyi). Described skulls were basically Antarctosaurus (for a very meager value of "skull"), African "Brachiosaurus", Camarasaurus, Dicraeosaurus, Diplodocus, and Euhelopus. Out of these options, Nowinski found N. mongoliensis to be most like Dicraeosaurus, so Nemegtosaurus became a dicraeosaurine, the first Upper Cretaceous diplodocid (well, atlantosaurid, but close enough. Atlantosauridae used to be a well-regarded family name; nowadays Atlantosaurus can't get arrested). That's where you'll find it if you go into any pop dinosaur book worth its salt published between approximately the mid-1970s and early 1990s (e.g., Norman 1985), and that's where it is in the first edition of "The Dinosauria" (McIntosh 1990).
Indications that N. mongoliensis was really a titanosaur began in the early 1990s (Salgado and Calvo 1992; Calvo 1994). Restorations of a very specific late-1990s vintage may show heavily brachiosaurized faces for N. mongoliensis and fellow Mongolian "dicraeosaur" Quaesitosaurus orientalis following Salgado and Calvo (1997), brachiosaurs at the time being the only group with good skulls known to be close to titanosaurs. Upchurch (1999) struck back with an analysis finding nemegtosaurids as diplodocoids ("diplodocids" by this time having sprawled into several clades), and the sauropod chapter of the second edition of "The Dinosauria" continued the Upchurchian position (Upchurch et al. 2004). By now, though, undoubted titanosaur skulls were becoming more abundant, and the consensus now has N. mongoliensis and Q. orientalis placed firmly within Titanosauria.
The other controversy concerns the eternal question of whether or not the famous tongue-twister Opisthocoelicaudia skarzynskii is the same thing as N. mongoliensis. In this corner we have a head (plus a leg nowadays), and in the opposing corner an inconveniently headless (and neckless) skeleton of comparable geology. The issue goes back to the description of O. skarzynskii (Borsuk-Bialynicka 1977); obviously, Borsuk-Bialynicka argued that her new genus and species was not N. mongoliensis. As long as N. mongoliensis insisted on remaining a head and O. skarzynskii insisted on not having one, there wasn't much in the way of solid evidence to argue one way or the other. One's position was based on A) whether they thought the two species were anatomically compatible, which was less evident back when Nemegtosaurus was a diplodocid and Opisthocoelicaudia was a camarasaurid, and B) whether they thought the Nemegt Formation could have supported two sauropod species. The Nemegt is not thought to have been notably lush, but it did also support hadrosaurids (Saurolophus did quite well) and giant omnivorous to herbivorous coelurosaurs (Deinocheirus and Therizinosaurus) as well as titanosaurs, and it's hardly unusual for a titanosaur-bearing formation to have more than one species of titanosaur. (For that matter, it's not unusual for a titanosaur-bearing *site* to have more than one species of titanosaur.)
The question was raised from these abstract considerations to something more concrete with the rediscovery of the N. mongoliensis type locality, and the recovery of additional postcranial bones thought to have come from the holotype (Currie et al. 2018). The most important new piece of evidence was a femur that Currie et al. (2018) described as essentially matching that of O. skarzynskii. That's it, now, right? Surely we've got it? Not so fast! Averianov and Lopatin (2019) noted that the "O. skarzynskii" femur used for comparative purposes by Currie et al. was not really from the O. skarzynskii holotype, because the holotype femur has a larger lateral condyle (the outer of the two rounded bits at the knee end of the femur), but the femur used for comparison had a larger medial condyle (the inner rounded bit), as does the N. mongoliensis femur. They also presented evidence that some Nemegt Formation sauropod vertebrae represent a titanosaur that is not O. skarzynskii, which would establish the presence of a second form regardless of the status of the two named species and the two femur types. Once again we are left looking for just a few more bones.
 |
| The crux of the issue for Averianov and Lopatin (2019; Figure 4). "A" is the femur of the type specimen of Opisthocoelicaudia skarzkynskii, "B" is a less famous femur from the Nemegt, "C" is the femur found at the Nemegtosaurus mongoliensis type locality (Currie et al. 2018), and "D" is the femur Currie et al. depicted as O. skarzynskii, with the arrow indicating the dominant condyle. My own question is if it is possible for some kind of non-taxonomic variation (age, sex, geologic age, geographic, individual, etc.) to produce the difference. CC-BY-4.0. |
There is also a second species of Nemegtosaurus nobody ever talks about, N. pachi (Dong 1977), and I don't see why we should do differently. It's based on a skinny tooth (IVPP V4879, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China) from the Upper Cretaceous Subashi Formation of Xinjiang, China. The tooth is not diagnostic (Wilson 2005).
References
Apesteguía, S., J. E. Soto Luzuriaga, P. A. Gallina, J. Tamay Granda, and G. A. Guamán Jaramillo. 2019. The first dinosaur remains from the Cretaceous of Ecuador. Cretaceous Research, Article 104345. doi:10.1016/j.cretres.2019.104345.Averianov, A. O., and A. V. Lopatin 2019. Sauropod diversity in the Upper Cretaceous Nemegt Formation of Mongolia— a possible new specimen of Nemegtosaurus. Acta Palaeontologica Polonica 64(2):313–321.
Borsuk-Białynicka, M. M. 1977. A new camarasaurid sauropod Opisthocoelicaudia skarzynskii gen. n., sp. n. from the Upper Cretaceous of Mongolia. Palaeontologia Polonica 37(5):5–64.
Calvo, J. O. 1994. Jaw mechanics in sauropod dinosaurs. Gaia 10:183–193.
Calvo, J. O., B. J. González Riga, and J. D. Porfiri. 2007. A new titanosaur sauropod from the Late Cretaceous of Neuquén, Patagonia, Argentina. Arquivos do Museu Nacional, Rio de Janeiro 65(4):485-504.
Currie, P. J., J. A. Wilson, F. Fanti, B. Mainbayar, and K. Tsogtbaatar. 2018. Rediscovery of the type localities of the Late Cretaceous Mongolian sauropods Nemegtosaurus mongoliensis and Opisthocoelicaudia skarzynskii: Stratigraphic and taxonomic implications. Palaeogeography, Palaeoclimatology, Palaeoecology 494:5–13. doi:10.1016/j.palaeo.2017.10.035.
Dong Z. 1977. On the dinosaurian remains from Turpan, Xinjiang. Vertebrata PalAsiatica 15(1):59–66.
Filippi, L. S., R. A. García, and A. C. Garrido. 2011. A new titanosaur sauropod dinosaur from the Upper Cretaceous of North Patagonia, Argentina. Acta Palaeontologica Polonica 56(3):505–520.
Garrido, A. C. 2010. Estratigrafía del Grupo Neuquén, Cretácico Superior de la Cuenca Neuquina (Argentina): nueva propuesta de ordenamiento litoestratigráfico. Revista del Museo Argentino Ciencias Naturales, n.s. 12(2):121–177.
González Riga, B. J., M. C. Lamanna, A. Otero, L. D. Ortiz David, A. W. A. Kellner, and L. M. Ibiricu. 2019. An overview of the appendicular skeletal anatomy of South American titanosaurian sauropods, with definition of a newly recognized clade. Academia Brasileira de Ciências 91(Supp. 2): e20180374. doi:10.1590/0001-3765201920180374.
Lehman, T. M. and A. B. Coulson. 2002. A juvenile specimen of the sauropod Alamosaurus sanjuanensis from the Upper Cretaceous of Big Bend National Park, Texas. Journal of Paleontology 76(1):56–172.
Martínez, R. D. F., M. C. Lamanna, F. E. Novas, R. C. Ridgely, G. A. Casal, J. E. Martínez, J. R. Vita, and L. M. Witmer. 2016. A basal lithostrotian titanosaur (Dinosauria: Sauropoda) with a complete skull: implications for the evolution and paleobiology of Titanosauria. PLoS ONE 11(4):e0151661. doi:10.1371/journal.pone.0151661.
Maryańska, T. 2000. Sauropods from Mongolia and the former Soviet Union. Pages 456–461 in M. J. Benton, M. A. Shishkin, D. M. Unwin, and E. N. Kurochkin, editors. The age of dinosaurs in Russia and Mongolia. Cambridge University Press, Cambridge, United Kingdom.
McIntosh, J. S. 1990. Sauropoda. Pages 345–401 in D. B. Weishampel, P. Dodson, and H. Osmólska, editors. The Dinosauria. University of California Press, Berkeley, California.
Norman, D. 1985. The illustrated encyclopedia of dinosaurs. Crescent Books, New York, New York.
Novas, F., F. Agnolin, S. Rozadilla, A. Aranciaga-Rolando, F. Brissón-Eli, M. Motta, M. Cerroni, M. Ezcurra, A. Martinelli, J. D'Angelo, G. Álvarez-Herrera, A. Gentil, S. Bogan, N. Chimento, J. García-Marsà, G. Lo Coco, S. Miquel, F. Brito, E. Vera, V. Loinaze, M. Fernandez, and L. Salgado. 2019. Paleontological discoveries in the Chorrillo Formation (upper Campanian-lower Maastrichtian, Upper Cretaceous), Santa Cruz Province, Patagonia, Argentina. Revista del Museo Argentino de Ciencias Naturales 21(2):217–293.
Nowinski, A. 1971. Nemegtosaurus mongoliensis n. gen., n. sp. (Sauropoda) from the uppermost Cretaceous of Mongolia. Palaeontologia Polonica 25:57–81.
Salgado, L., and J. O. Calvo. 1992. Cranial osteology of Amargasaurus cazaui Salgado y Bonaparte (Sauropoda, Dicraeosauridae) from the Neocomian of Patagonia. Ameghiniana 29:337–346.
Salgado, L., and J. O. Calvo. 1997. Evolution of titanosaurid sauropods. II: the cranial evidence. Ameghiniana 34: 33–48.
Upchurch, P. 1999. The phylogenetic relationships of the Nemegtosauridae (Saurischia, Sauropoda). Journal of Vertebrate Paleontology 19(1):106-125.
Upchurch, P., P. M. Barrett, and P. Dodson. 2004. Sauropoda. Pages 259–322 in D. Weishampel, P. Dodson, and H. Osmólska, editors. The Dinosauria (2nd ed.). University of California Press, Berkeley, California.
Wilson, J. A. 2005. Redescription of the Mongolian sauropod Nemegtosaurus mongoliensis Nowinski (Dinosauria: Saurischia) and comments on Late Cretaceous sauropod diversity. Journal of Systematic Palaeontology 3(3):283–318.
Wilson, J. A., D. Pol, A. B. Carvalho, and H. Zaher. 2016. The skull of the titanosaur Tapuiasaurus macedoi (Dinosauria: Sauropoda), a basal titanosaur from the Lower Cretaceous of Brazil. Zoological Journal of the Linnean Society 178(3):611–662.
No comments:
Post a Comment