Sunday, November 17, 2019

Your Friends The Titanosaurs, part 18: Mendozasaurus, Microcoelus, and Mnyamawamtuka

The three guests for this entry are Mendozasaurus neguyelap from Argentina, Microcoelus patagonicus also from Argentina, and Mnyamawamtuka moyowamkia from Tanzania. Mn. moyowamkia is one of the most recent additions to Titanosauria, Me. neguyelap is a reliable sort that doesn't get a lot of publicity outside of the technical literature, and Mi. patagonicus is more or less a historical afterthought.

Mendozasaurus neguyelap

Mendozasaurus neguyelap is among the best-represented and most thoroughly studied titanosaurs, making it widely cited in the titanosaur literature. Despite its scientific utility, it is not widely known to the public; like many sauropods, it doesn't have an easy hook for publicity. When you dig down, it's actually kind of notable for not being flashy. (I do not say "average" or "generic" titanosaur, because the variation within Titanosauria makes that concept essentially meaningless.) Of course, it's not as if there's something wrong about not being flashy.

An assortment of titanosaur humeri, presented at the same scale (scale bar = 20 cm, or 7.9 in). Mendozasaurus neguyelap is "e", much bigger than something like Neuquensaurus (a), but nowhere near the quartet at the bottom (Dreadnoughtus, Patagotitan, Paralititan, and Notocolossus) (Figure 4 in González Riga et al. 2019). CC-BY-4.0.

M. neguyelap has been the primary focus of several papers (González Riga 2003, 2005; González Riga and Astini 2007; González Riga et al. 2018). Of these, I recommend starting with the most recent. It's the most detailed and best illustrated, and it makes some corrections and updates to the earlier papers. González Riga and Astini (2007) is distinct from the other three because it focuses on taphonomy instead of anatomy.

M. neguyelap was described by Bernardo J. González Riga in 2003 for bonebed remains found in southern Cerro Guillermo in the Malargüe Department of southern Mendoza Province, Argentina. The site is identified as Arroyo Seco in later publications. Fossils were discovered in the area by oil workers and several sites, including the Arroyo Seco site, were excavated beginning in 1998 (González Riga 2003). M. neguyelap was the first non-avian dinosaur described from Mendoza Province, and its name reflects this: "Mendoza" in "Mendozasaurus" refers to the province, and "neguyelap" is a combination of the Millcayac words "neguy", meaning "first", and "yelap", meaning "beast" (González Riga 2003). Literally you'd get something like "first beast, Mendoza Province lizard", but a looser reading more in line with the intent is "first beast from Mendoza Province". The holotype is IANIGLA-PV 065, 30 bones from the tail: 065/1 through 22 are mostly articulated caudals, 065/23 through 25 are chevrons, and 065/26 through 30 are chevron fragments (González Riga et al. 2018; this is slightly different from the original group of fossils reported in González Riga 2003). IANIGLA-PV refers to the Colección de Paleovertebrados of the Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales in Mendoza, Argentina.

M. neguyelap is known from fossils discovered in a nearly monospecific bonebed. The stratigraphic unit has changed several times following refinements in Upper Cretaceous geology in Argentina, and it is now regarded as within the upper Sierra Barrosa Formation, a Coniacian-aged unit (González Riga et al. 2018). The titanosaur bones range from articulated (caudals) to disarticulated but associated (most of the limb bones) to disarticulated. The majority were distributed in two intersecting lines covering an area about 8 m by 9 m (26 ft by 30 ft). The great majority of the bones were weathered before being buried, and have been significantly deformed by compression. The site is interpreted as a type of overbank facies known as a crevasse splay (a deposit that forms when a stream breaks through a levee and deposits onto the floodplain behind it), in a meandering river system. Aside from M. neguyelap, fossils of wood, indeterminate titanosaurs, and maniraptoran theropods were also found. Similar overbank assemblages of disarticulated to partly articulated dinosaur specimens, with flow-oriented bones, are known elsewhere in the Neuquén Basin  (González Riga and Astini 2008).

At least four M. neguyelap individuals of three size classes are known from the site; most of the remains may belong to one intermediate-sized individual, but it's hard to be certain because the intermediate-sized bones must include at least two individuals. The whole lot attributed to M. neguyelap includes five cervicals, two dorsals, twenty-two caudals, a rib, three complete chevrons, chevron fragments, a right scapula, a right sternal plate, left and right humeri, a right radius and ulna, six metacarpals, a pubis fragment, a left and partial right femur, four tibiae (three right and one left), left and right fibulae, a right astragalus, twelve metatarsals, ten phalanges, and four osteoderms. The large bones include a tibia and two femora, and possibly some other hindlimb bones. A smaller individual is only known from a single metacarpal (González Riga et al. 2018). González Riga (2003) gave a length range of 18 to 25 m (60 to 82 ft), but it seems to have gotten a bit smaller with further investigation. The complete femur, attributed to the large individual, is 1,530 mm long (5.02 ft) (González Riga et al. 2018), which is ordinary big for a sauropod.

Particularly notable anatomic features of M. neguyelap include its beefy cervical neural arches and neural spines, and its osteoderms, which lack a cingulum and are kind of egg-shaped to more strongly flattened. The cervicals helped link it with Futalognkosaurus dukei, for which we have Lognkosauria (Calvo et al. 2007). Other sauropods come and go in Lognkosauria depending on the analysis, but as noted in the earlier post the pairing of these two species has been quite durable. No wrist bones or phalanges for the hands were found, and there is no evidence that it had either (González Riga et al. 2018). The limb bones are gracile for titanosaurs (González Riga et al. 2019). González Riga (2005) reported that it had unusually short presacral vertebrae, but this was later shown to be a misinterpretation due to crushing (González Riga et al. 2018). Crushing, compression, and other distortion make it difficult to interpret some bones (González Riga et al. 2018).

Microcoelus patagonicus

Richard Lydekker named four titanosaur species in 1893. One, Argyrosaurus superbus, has already showed up in this series. Another, Titanosaurus australis, has appeared in cameos in other entries and will get its own turn as Neuquensaurus. The other two were not much to write home about. One was Titanosaurus nanus, which is a bit of piffle (Wilson and Upchurch 2003). The other was Microcoelus patagonicus, not to be confused with the cyanobacterium Microcoleus. As far as I know, Microcoelus is the only dinosaur that can be confused with a cyanobacterium, which is about as good an epitaph for M. patagonicus as any. The name means "small cavity from Patagonia", the "small cavity" presumably being the depression in the side of the centrum which can be seen in the photos.

Lydekker based M. patagonicus on an anterior dorsal vertebra. Today the specimen is cataloged as MLP-Ly 23 (Museo de La Plata in La Plata, Argentina), with a provenance of the Bajo de la Carpa Formation on the right bank of the Río Neuquén, near Neuquén (Otero and Reguero 2013). Lydekker also tentatively assigned a small (54.6 cm or 21.5 in long) robust left humerus to the species, but given that the species was based on a dorsal there's no particular reason to add a humerus to it. For what it's worth, the humerus is of the same basic form as humeri of Neuquensaurus australis (Powell 2003; Otero and Reguero 2013). There was also half of an amphicoelous or platycoelous posterior caudal which Lydekker thought might belong to the same species, and which has attracted little comment.

The type specimen of M. patagonicus as illustrated in Lydekker (1893; Plate III, part 2).

The specimen as illustrated by Otero and Reguero (2013) (scale bar 100 mm). CC-BY-4.0, apparently.

M. patagonicus has rather understandably been ignored since 1893. Lull (1910) couldn't even spell the name correctly, which later led to Microsaurops after further misunderstandings. Friedrich von Huene (1929) assigned the type specimen and other referred material to Lydekker's Titanosaurus australis, observing no differences. More recent reviews by McIntosh (1990) and Upchurch et al. (2004) have followed suit, just changing the genus name of T. australis as appropriate (the 2004 publication accidentally making it a synonym of both Saltasaurus and Neuquensaurus australis, presumably due to an editing oversight from use of the 1990 table). There's something of a problem in this, in that Microcoelus has quite a bit of priority over Neuquensaurus or Saltasaurus, so this should lead to Microcoelus australis. While you could go to the effort of formally suppressing M. patagonicus, in practical terms it's easier to just consider M. patagonicus an indeterminate saltasaur, even if that means you don't get to show off your "synonymizer" merit badge at the next meeting of Penitent Vertebrate Paleontologists. Powell (2003) went this route, finding the holotype to lack any features that could definitely place it in another titanosaur taxon, but at the same time being most likely from some kind of saltasaur. (For what it's worth, M. patagonicus and N. australis are known from different formations.)

Microsaurops Kuhn 1963 is an unnecessary replacement name for what is given as Microsaurus Hatcher 1900, or Hatcher 1900 vide Lull 1910. This is an obvious typo for Microcoelus that isn't even in Hatcher (1900). It is actually in Lull (1910) and was unfairly attributed to Hatcher, as a close reading of Lull's publication (p. 29) will show: "The last stand of these huge creatures, so far as our present knowledge goes, and again excepting Deperet's Titanosaurus (vide supra, p. 26), was in Patagonia, where the remains of three genera, Titanosaurus, Argyrosaurus and the small aberrant Microsaurus [sic] are found in the Guaranitic beds correlated by Hatcher (1900, p. 95) with the Laramie (Danian) of North America." Lull is not saying that Hatcher named any of these genera, he is merely saying that Hatcher correlated the source beds with rocks in North America. Hatcher's article nowhere mentions any of the genera. Lull simply got the name wrong and neglected to cite the original source for the genera. There was no need for Microsaurops; it definitely deserves to be forgotten.

Mnyamawamtuka moyowamkia

Mnyamawamtuka moyowamkia is one of the most recently described titanosaurs, one of the crop of new African titanosaurs. It was described by Gorscak and O'Connor in 2019 for a partial skeleton excavated between 2004 and 2008 as part of the Rukwa Rift Basin Project. The quarry was along the Mtuka River drainage, south of Lake Rukwa in southwestern Tanzania, and within the Mtuka Member of the Galula Formation. Two other titanosaurs are known from this formation, but from the higher Namba Member (Rukwatitan bisepultus and Shingopana songwensis). The outstanding name, which is derived from Kiswahili (another one that looks difficult to say for the English speaker but comes together with surprising ease), translates best by taking the genus and species separately. The genus name is from "mnyama" for "beast" or "animal" and "wa Mtuka" for "of the Mtuka", giving us "beast of the Mtuka". The species name combines "moyo", meaning "heart", and "wa mkia", "of the tail", for "heart of the tail", referring to the heart-shaped outline of the posterior caudal centra (Gorscak and O'Connor 2019).

M. moyowamkia is based on RRBP 05834 (Rukwa Rift Basin Project collection of the Tanzanian Antiquities Unit in Dar es Salaam, Tanzania). This specimen consists of a number of bones, including: a cervical neural arch and four cervical centra; seven partial dorsals; a sacral neural arch, three partial sacral centra, and three sacral ribs; seven caudal neural arches and seven centra; fragments of dorsal ribs; four chevrons; a right scapula and sternal plate; parts of both humeri; a partial left ulna; a couple of metacarpals; partial left ischium and right pubis; both femora and tibiae, partial; a left fibula; five metatarsals representing both feet; a couple of phalanges and an ungual; and fragments (Gorscak and O'Connor 2019). Once you've run through the inventory, it becomes clear that this is among the most complete individual titanosaur specimens, with most of the skeleton represented by one or more bones.

Within the quarry, the skeleton was disarticulated and dispersed such that the larger bones were mostly found in the western part and smaller or fragmented bones were found in the eastern part. The numerous unfused centra and neural spines suggest it was not fully grown. The femur is estimated as 720 mm (28.3 in) long (Gorscak and O'Connor 2019). Osteoderms were not found (Gorscak and O'Connor 2019); given the presence of many different skeletal elements, including some smaller than typical osteoderms, I think it's reasonable to suppose this individual did not have any, although that doesn't mean the species as a whole lacked them (especially if they were most strongly developed in female titanosaurs for mineral storage, and the type happened to be a male or not mature enough to need them).

The quarry map for RRBP 05834 (Figure 2 in Gorscak and O'Connor 2019). Each square is 1 m on a side. CC-BY-4.0.

The skull is not officially represented in the holotype, but four sauropod teeth were recovered from the quarry. These teeth can be sorted into three categories, which Gorscak and O'Connor (2019) interpreted as representing different positions within the jaws rather than different species. They show a continuum from teeth with D-shaped cross-sections to teeth with circular cross-sections, mimicking in a small way the change of titanosauriform teeth from broader to pencil-like.

The various phylogenetic analyses found M. moyowamkia as a near neighbor to Malawisaurus dixeyi, but never more derived. This works out reasonably well with the geography and geologic ages of the two species. As noted by Gorscak and O'Connor, M. moyowamkia shows a mix of lithostrotian features and ancestral features. The only caudals to have procoelous articulations are the anterior caudals, and procoely of these vertebrae is weak. The caudals also lack certain hollows and ridges, and the sternal plates are relatively small. On the other hand, the neural canal narrows in the cervicals, the dorsals don't have hyposphene–hypantrum articulations, and there is a postero-lateral bulge on the humerus. With this mix of features (and potentially the mix of teeth), M. moyowamkia offers a good look at an early stage of titanosaur evolution.


Calvo, J. O., J. D. Porfiri, B. J. González-Riga, and A. W. Kellner. 2007. A new Cretaceous terrestrial ecosystem from Gondwana with the description of a new sauropod dinosaur. Anais Academia Brasileira Ciencia 79(3):529–541.

González Riga, B. J. 2003. A new titanosaur (Dinosauria, Sauropoda) from the Upper Cretaceous of Mendoza Province, Argentina. Ameghiniana 40:155–172.

González Riga, B. J. 2005. Nuevos restos fósiles de Mendozasaurus neguyelap (Sauropoda: Titanosauridae) del Cretácico Tardío de Mendoza, Argentina. Ameghiniana 42(3):535–538.

González Riga, B. J., and R. Astini. 2007. Preservation of large titanosaur sauropods in overbank fluvial facies: a case study in the Cretaceous of Argentina. Journal of South American Earth Sciences 23(4):290–303.

Gonzàlez Riga, B. J., P. D. Mannion, S. F. Poropat, L. D. Ortiz David, and J. P. Coria. 2018. Osteology of the Late Cretaceous Argentinean sauropod dinosaur Mendozasaurus neguyelap: implications for basal titanosaur relationships. Zoological Journal of the Linnean Society 184(1):136–181. doi:10.1093/zoolinnean/zlx103.

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