There are a few taxa of interest that haven't yet been covered. They include: 1) "historical titanosaurs", the kind you might stumble across in a 1980s dinosaur dictionary; 2) poorly known species regarded as titanosaurs mostly on the circumstantial evidence of time and place, without the anatomical evidence to back up a classification; 3) species that have turned up in Titanosauria once or twice during the cladistic era (mid-1990s to the present) but are not currently or generally regarded as titanosaurs; and 4) species that appear to have been near Titanosauria and sometimes hop the line in analyses, but usually are found outside. This post is for the first three varieties. We've got 15 in the queue, so as you can image I'm not going to go into a great deal of detail.
Curiously, most of McIntosh's "Sauropoda incertae sedis" from the first
edition of The Dinosauria show up on this page: "Pelorosaurus"
becklesii (=Haestasaurus), Mongolosaurus,
Austrosaurus, and Aepysaurus (=Aepisaurus), plus a shout-out to "Apatosaurus" minimus. The only
ones not here are "Morosaurus" agilis (now described as a
rebbachisaurid [2021/04/02: no, dicraeosaurid; do this long enough and they all run together], Smitanosaurus), and
Campylodoniscus, which was previously featured. This says something about quasi-titanosaurs, but
I'm not sure what. The taxa also skew old, with many of Early Cretaceous or even Late Jurassic age, suggesting it's harder to get a handle on putative early titanosaurs. Unsurprisingly, many of them are dubious, albeit in all kinds of ways: from garden-variety causes like too little material, to "unavailable for study due to being destroyed by monsoons", to "the osteoderms turned out to be ribs", to "the original describer thought a pile of caudals from several sites separated by miles belonged to one individual", to "actually filled mollusk borings".
As a reminder, because the terms come up several times, Titanosauriformes is the clade made up of the most recent common ancestor of Brachiosaurus and Saltasaurus plus all of its descendants, and Somphospondyli is the clade made up of all sauropods more closely related to Saltasaurus than to Brachiosaurus.
I thought going in that I'd need to include Parrosaurus missouriensis, but it turns out that it wasn't considered a titanosaur during its sauropod days: no procoelous caudals. (We'll get that name thing straightened out eventually.)
Aepisaurus elephantinus: Aepisaurus is most notable for its spelling variations, the most persistent issues being substitution of "y" for "i" in the genus name, and an "o" sometimes being added after "y" or "i". The problem extends all the way into The Dinosauria, where McIntosh (1990) (one of the only publications since the description to do more than include Aepisaurus in a list, I might add) includes it as "Aepysaurus". The orthography doesn't do it any favors, either: the name was originally spelled Æpisaurus, but "æ" is no longer permitted in scientific names, being converted to "ae". This means that when using a search engine, the oldest literature will not be found unless you use the original spelling.
Anyway, when the most notable thing about a taxon is its conflicting spellings, it's probably dubious. A. elephantinus is based on a relatively slender, 90-cm-long (35 in) left humerus from Albian rocks in southern France, and was described in Gervais (1852, or 1853 in some sources; for that matter, the volume itself says "1848–1852". Make up your minds!). It may be that Gervais intended to revise the name to Æpysaurus, as its second appearance saw it spelled mostly with "y" (Gervais 1859). Whether or not this counts is not for me to decide. Von Huene (1932) put it into Titanosauridae because he thought the humerus was indistinguishable from that of Laplatasaurus. McIntosh (1990) acknowledged there was some similarity, but also noted that the humerus also resembled those of Camarasaurus and brachiosaurids. Le Loeuff (1993) agreed that it had camarasaurid or titanosaurid proportions, but elected to simply regard Aepisaurus as an undetermined sauropod. Tortosa and Buffetaut (2007) considered the specimen to be most similar to camarasaurid humeri, which would be more helpful if Camarasauridae was not, generally, a vanity project for Camarasaurus. Some kind of macronarian, perhaps?
|The humerus that launched a thousand spelling variants, from Gervais (1859).|
Algoasaurus bauri: We've already spent some quality time with this South African sauropod. Suffice it to say that I still suspect it might have been a rebbachisaur, although McPhee et al. (2016) could not find any evidence to confirm this.
Chondrosteosaurus gigas: Chondrosteosaurus is one of seemingly several dozen quaint Victorian-era Wealden sauropod taxa based on whatever was at hand (including previously named specimens), in this case a couple of cervical centra notable for their pneumaticity (Owen 1876). After spending much of the 20th century as a synonym of Pelorosaurus, because nobody cared about sauropods at the time and every genus sunk into Pelorosaurus meant more time for useful pursuits like, I don't know, rodent evolution or something, it briefly appeared in Titanosauridae in 1980s–early 1990s field guides (Lambert 1983, 1990). This does not seem to have been very popular. Camarasauridae was generally the more preferred option of the 1990s following a tentative classification in McIntosh (1990), and it was included as such in Naish and Martill (2001), although not with any great enthusiasm. The more recent review of Wealden sauropods by Upchurch et al. (2011) found it was instead more likely to be a basal titanosauriform.
|The two cervicals of C. gigas, from Owen (1876) (Wikimedia Commons scan).|
Janenschia robusta: Janenschia is a name created during the long disentangling of "Gigantosaurus" Fraas (1909), part of which I covered in the Malawisaurus dixeyi entry. To put it briefly, Fraas introduced the name Gigantosaurus robustus for suitably robust sauropod limb bones from the famous Upper Jurassic Tendaguru Beds of southeastern Tanzania. Fraas's choice to use a genus name already in use for an English sauropod led to slowly unfolding taxonomic wrangling not resolved until Wild (1991) put robustus in the new genus Janenschia. For the 1990s into the early 2000s, Janenschia was the number-one contender for the "World's Earliest Titanosaur" crown. A funny thing happened on the way to the coronation, though: J. robusta fell apart. We can see the seeds of its downfall in McIntosh (1990). Although McIntosh placed the species in Titanosauridae, he noted that there was only circumstantial evidence connecting the various remains that had been assigned to it over the years, and was not entirely convinced it was a titanosaurid anyway. Essentially, J. robusta had grown up under the premise that there were only four sauropod genera at Tendaguru, and whatever didn't belong to the others surely must belong to it. The primary offender was a string of caudals in which the first few are procoelous. Although the genuine uncontested Janenschia robusta bones are not wholly incompatible with Titanosauria, the story boils down to "Do these caudals belong to it" and "Are they titanosaurian caudals?" Mannion et al. (2019), finding there wasn't any convincing evidence that tail and limbs went with each other, and uncovering different positions for the two in a phylogenetic analysis, put the tail into new genus and species Wamweracaudia keranjei. Wamweracaudia, in turn, comes out as a mamenchisaurid (euhelopodid of Moore et al. 2020), mamenchisaurids also having a few procoelous caudals. The "Jurassic titanosaur" is a false alarm for the time being. Janenschia is hard to pin down; Mannion et al. (2019) kept finding it in a funky little clade with East Asian Bellusaurus and European Haestasaurus (see below) outside of Neosauropoda, sometimes in Turiasauria.
Lametasaurus indicus: Lametasaurus has come up a couple of times, most thoroughly in the introduction to titanosaur osteoderms. It has a delightfully checkered past, for a certain value of "delightful". Matley (1921) first reported the remains as theropod, then a couple of years later (Matley 1923) formally named it as a stegosaurian, based on a pair of ilia, a sacrum, a left tibia, and various scutes and spines. Chakravarti (1935) pointed out that the postcranial remains were indeed from a theropod, and Walker (1964) opted to remove those theropod bones from the type specimen, leaving behind only the osteoderms on the grounds that Matley had intended to describe an armored dinosaur. This seems to have been missed by some later researchers, who have pointed out that the "Lametasaurus" postcrania probably represents one of the numerous named abelisaurs of the Lameta Formation, perhaps Rajasaurus, but seem to think that Lametasaurus itself is the theropod remains. Nope, they're removed per Walker*.
*And also missing per Mohabey et al. (2013)
The freshly shorn Lametasaurus toddled along as an ankylosaur, perhaps a nodosaur, until Saltasaurus loricatus established that osteoderms were also part of the accoutrements of titanosaurs. "Aha! Then Lametasaurus must be a titanosaur!" you cry. Not so fast! While at least one osteoderm previously attributed to Lametasaurus is now known to be titanosaurian (D'Emic et al. 2009), the original Lametasaurus material is... interesting (again, for a certain value of "interesting"). Matley collected an astonishing 5,000 bits and pieces. To repeat the earlier post, these included "thousands of small ossicles (2 cm or less, or smaller than an inch), with a smaller number of larger osteoderms (to about 6 cm, or a little over 2 in), and a few big pieces up to about 12 cm long (about 5 inches)." That's... an awful lot of material, honestly, and it raises some questions about the taphonomy of the site, such as "with thousands of sub-centimeter ossicles, where's the rest of the skeleton?" Matley himself wasn't sure the specimens all belonged to the same animal, based on differences in form. I suppose we could be looking at the disintegration of a sheet of ossicles like the patches found with Saltasaurus. There's also the possibility that Matley's suspicion is correct and there is more than one animal represented by the osteoderms and ossicles, with leading suspects for other contributors being crocs and abelisaurs. There are also persistent rumors of true ankylosaurs in the Late Cretaceous of India, most notably a partial skeleton from Rahioli mentioned in Chatterjee and Rudra (1996) as under description. (Well, and maybe whatever poor Brachypodosaurus actually is, too.) Pending further revision of the Lametasaurus armor, wherever it may be, I'm leaving it out of Titanosauria. If you'd like to try your hand at Remote Internet Taxonomic Expertise, or just have a thing for trying to interpret terrible scans of plates, you can view the osteoderms Matley chose to figure here and here.
Macrurosaurus semnus: Macrurosaurus as a titanosaur goes back, more or less, to the time
when paleontologists first started to attach special importance to procoelous
caudals. It actually predates
as a name, being described in Seeley (1869). Why has it not been included
here? The problems with M. semnus go back to the beginning; it's a pile
of caudals from multiple locations. Seeley named it from caudal vertebrae
discovered in the "middle" Cretaceous Cambridge Greensand. There was one
"series" from Coldham Common and a second group from Barton, several miles
away, which he thought "may be part of the same individual" as the Coldham
Common caudals. (This would be, shall we say, an unprecedented coincidence.) He then added another group of three vertebrae, which he
conceded belonged to a different individual. He provided additional details on
the anatomy and provenance of the specimens in Seeley (1876), at which point
he was, if anything, even more convinced that the 25 Coldham Common caudals
came from the same tail as the 15 Barton caudals, and also assigned the
metapodials of his Acanthopholis platypus to the species due to size,
despite the notable handicap of there being no overlapping material. (Wow.)
No one seems to have been much troubled by all of this until Le Loeuff (1993) pointed out the obvious: what Seeley really had was a pile of caudals from multiple locations. They didn't even go together all that well morphologically, with Le Loeuff suggesting that the specimens included a true titanosaur and something else. Le Loeuff (1933) recommended that the first procoelous caudal of the Coldham Common series should be designated the lectotype following Seeley's usage; unfortunately, this specimen, B55630, honestly doesn't even look like it would make a good doorstop, let alone a lectotype. (Maybe a paperweight.) Mannion et al. (2013) went even further, dismissing the caudals as probably mostly pertaining to indeterminate titanosauriforms. Most of them are amphicoelous, only a few have the neural arch placed anteriorly, and none of the centra have the typical ventral longitudinal hollows or ventrolateral ridges of somphospondylan sauropods. The punch line is that the "Acanthopholis platypus" metatarsals include a titanosauriform feature, so Seeley was on the right track in linking them to the same general type of animal as the caudals, only he came to this conclusion for the wrong reasons.
|A Macrurosaurus caudal, from Seeley 1876.|
Succinodon putzeri: Succinodon is not one of Friedrich von Huene's finer moments. In 1941 he described the new genus and species Succinodon putzeri from some tubular fossils collected by a German military geologist from the gorge of the Vistula River near Nasiłów, Poland. The rocks were thought to be Campanian and the fossils were thought to be cylindrical sauropod teeth in jaw fragments. Later collections and research from the area found no dinosaur remains, but abundant calcareous tubes produced by shipworms (attributed to Kuphus sp.) in early Paleocene rocks. Despite the disappearance of the original S. putzeri material, von Huene's illustrations were good enough to show that the supposed sauropod teeth were also shipworm tubes or tube molds (Pozarskya and Pugaczewska 1981). (Despite the name, shipworms are not worms but bivalve mollusks.) Given the background of why there should happen to be a German military geologist working in Poland in 1941, the whole thing seems like an appropriate fate.
"Titanosaurs" of provenance
I was going to include Arkharavia heterocoelia, but it was never actually considered a titanosaur in any formal sense, just a basal titanosauriform (Alifanov and Bolotsky 2010). Then it turned out to be a chimera (Alifanov 2012; Mannion et al. 2013). The holotype caudal actually does appear to be from a sauropod, but can only be potentially placed as far as Somphospondyli; a cylindrical tooth that had been questionably associated does appear to be titanosaurian (Mannion et al. 2013).
Bruhathkayosaurus matleyi: Ah, Bruhathkayosaurus. What hast thou wrought? (More importantly, what is thou about to wring out of me?) Let's start by dealing with the petrified wood thing. Some sources, such as Mickey Mortimer's detailed entry, attribute this urban legend to a 1995 Thomas Holtz post on the Dinosaur Mailing List, in which he stated "The drawings and photographs of the fossil are horrible, and I see no evidence that these aren't petrified wood!" (I've only seen copies of the drawings myself, but suffice it to say that they are not particularly informative.) However, there is actually an older post on the DML referring to the possibility, by John Schneiderman in 1994, and because the text of the post is discussing an older publication (Olshevsky 1993/1994), the idea may go back that far. The wording in the post is "Some of the material is very poor at best. The tibia may even belong to a sauropod [B. matleyi was described as a theropod] or may be a petrified log."
B. matleyi was named in Yadagiri and Ayyasami (1987), in which it
famously staked its claim as The World's Biggest Theropod. Remains including a
partial caudal, an ilium, an ischium, a radius, a distal femur, and a tibia (2
m or 7 ft long) were attributed to it. Unlike most of the Upper Cretaceous
dinosaurs of India, it hailed not from the Lameta Formation, but from the
Kallamedu Formation of Tamil Nadu in southern India. By the mid-1990s, chatter
was in circulation that B. matleyi was not a super-ginormous theropod,
but a ginormous sauropod, probably a titanosaur; you can find it as a dubious
sauropod in Upchurch et al. (2004). Further evaluation cannot be made, on
account of the specimens ceasing to exist. Per Galton and Ayyasami (2017), the
rugged climate conditions of the area (monsoon rains coupled with hot dry
seasons) had so taken their toll on the fossils that they began to
disintegrate in their field jackets, which places any further analysis firmly
in the realm of speculation. Leaving aside hand-waving about size,
B. matleyi has never been firmly established as a titanosaur on anatomical grounds, although
admittedly that option is most consistent with time and place. If it *was* a
titanosaur, it seems to have been rather odd. The caudal was reportedly not
procoelous but platycoelous (flat in front, concave in back). The ilium as
illustrated looks wrong in several ways (it frankly looks drawn more like a
theropod ilium). The tibia (or, as has been suggested, fibula) is wildly
disproportionate to the ilium and femur compared to known titanosaurs: quoting
from Mickey Mortimer's entry, "tibio-ilial ratio 167% compared to 71% in
Opisthocoelicaudia; tibia length compared to distal femoral width 267%
vs. 188% in Opisthocoelicaudia". It's enough to make one wonder if
B. matleyi was in fact a chimera, and/or if just maybe that tibia was a
beat-up femur (Paul 2019) or a log after all.
Qinlingosaurus luonanensis: Qinlingosaurus is possibly the most obscure dinosaur named during the 1990s. Practically nothing has been written about it apart from its brief description in Xue et al. (1996); if you have the second supplement to Glut's encyclopedia series (Glut 2002), you can see the figures, and this post by Mickey Mortimer on the Dinosaur Mailing List summarizes it about as thoroughly as can be done. Q. luonanensis is based on an ilium, ischium fragment, and three vertebrae discovered in the Qinling Mountains of Shanxia, China. The formation is not entirely clear; it was apparently first reported from the Hongtuling Formation but more recently has been placed in the Shanyang Formation (e.g., Glut 2002). In either case the age is thought to be late Late Cretaceous, which introduces the temptation to include it in the Titanosauria (as the Wikipedia entry suggests). The ilium as figured looks typically sauropodan; the reduced ischial peduncle (the process where the ischium attaches) indicates we've at least got a neosauropod, which slightly narrows the field of possibilities. The somewhat different profiles of the ilium in internal and lateral views suggest that the crest of the ilium is arched laterally. One of the figured vertebrae appears to be mildly procoelous with an anteriorly-placed neural arch, both of which are consistent with titanosaurian caudals but are not exclusive to that group. It also has a neural spine that flares fore-aft at the tip, and what appears to be an enormous process rising up and laterally from the centrum. As noted in the DML post, this looks suspiciously like a sacral rib, but the photo is not the greatest, and trying to interpret three-dimensional objects from a single two-dimensional view can lead to errors. (In other words, it looks like an anterior caudal with a sacral rib, and it looks off, but I don't want to tie myself down.) There's no obvious reason it couldn't be a titanosaur, but it would be nice to have someone who knows what they're doing have a look.
Before we go in, a tip of the cap to "Apatosaurus" minimus, the Morrison species that likes to make you think it might be a titanosaur, then laughs at you for falling for it.
Agustinia ligabuei: Celebrated as a wildly spiky sauropod, immortalized in toys and innumerable illustrations on the Internet, A. ligabuei has turned out to be more humbug than humdinger. "Couldn't you say the same thing about Bruhathkayosaurus?" you may ask. It's a somewhat different case, mostly because B. matleyi had the good sense to disappear before it could be further interrogated. A. ligabuei is based on a significant chunk of the vertebral column (18 incomplete dorsal, sacral, and caudal vertebrae), the right tibia and fibula, five metatarsals, and nine objects initially described as osteoderms, from the Lower Cretaceous Lohan Cura Formation of Neuquén, Argentina (Bonaparte 1999). A brief description in an abstract had been written a year before, in which the genus was given as Augustia (Bonaparte 1998), but that name had already been claimed for a beetle. Perhaps this should have been seen as a bad omen.
Bonaparte divided the apparent osteoderms into four types: 1) leaf-shaped; 2)
sheet-like and wide with lateral projections; and 3) and 4), elongate, flat or
cylindrical projecting objects. These were all supposed to be positioned along
the spine, beginning anteriorly as unpaired type 1 osteoderms and eventually leading into
the spiky types 3 and 4 posteriorly. He didn't think the new genus and species belonged to
any known sauropod clade and created the family Agustinidae for it (Bonaparte
1999). Upchurch et al. (2004), on the basis of presumed osteoderms and
presumed six sacral vertebrae, provisionally placed it in Titanosauria. By now
its place in pop culture was firmly cemented, but its scientific standing
would not last. D'Emic et al. (2009), reviewing titanosaur osteoderms, noted
that the Agustinia pieces did not have the pitting, crosshatched
texture, or nutrient foramina of other osteoderms, and could be age-related
ossifications. Furthermore, they could not be sure that the sacrum really had
the titanosaurian six vertebrae, and "tentatively consider[ed]
Agustinia to be an unarmored neosauropod of uncertain affinities."
D'Emic (2012) placed A. ligabuei among the dubious titanosauriforms.
Mannion et al. (2013) noted that the type 1 and 3 osteoderms appeared to be
partial ribs and proposed that the type 2 specimen might be fused, distorted,
and incomplete ischia. They also kept the taxon out of Titanosauria,
preferring to identify it as an indeterminate somphospondylan. A recent
histological analysis by Bellardini and Cerda (2017) confirmed the previous
observations, finding that the "osteoderms" of types 1, 3, and 4 did not have
osteoderm histology, but were instead more consistent with cervical and dorsal
ribs. The type 2 pieces was too poorly preserved to be assessed, but was
tentatively considered to be part of the ilium.
Amargatitanis macni: This one is one of the most recent names here, and had one of the briefer stays in Titanosauria. A. macni was described in Apesteguía (2007) for six caudals, a scapula, a femur, and an astragalus collected from the Lower Cretaceous La Amarga Formation in Neuquén, Argentina, representing a very early titanosaur that was also notably derived. The material was described as associated, collected by Jose Bonaparte in 1983 (Apesteguía 2007). Large-scale reviews of basal titanosauriforms were not as impressed. D'Emic (2012) noted that the specimens were actually collected as much as hundreds of meters apart, attributed some of the titanosaurian features to breakage, and suggested that some of the specimens belonged to diplodocoids. Mannion et al. (2013) considered A. macni to be a dubious somphospondylan. Clearly further review was in order, which came via Gallina (2016). Based on re-evaluation of Bonaparte's field notes, Gallina revised the type specimen to include the original femur and astragalus, and two of the caudals, plus previously unpublished specimens (partial right ischium, tibia, fibula, and metatarsal I). The revised holotype was then found to represent a valid dicraeosaurid distinct from the ever-popular Amargasaurus, also known from the La Amarga Formation (Gallina 2016).
Australodocus bohetii: Janenschia is not the only Tendaguru sauropod of relevance here. In 2007, we got Australodocus bohetii (Remes 2007) for a couple of cervicals that had been stashed in Tornieria africana (aka Barosaurus africanus). At this time, it was thought to be a diplodocid distinct from the well-represented Tornieria. However, Whitlock (2011) reinterpreted it as a titanosauriform, and Mannion et al. (2013) found it within Titanosauria. Within a few years, Australodocus descended from these dizzying heights, and as of Mannion et al. (2019) it is most likely a non-titanosaurian somphospondylan.
Austrosaurus mckillopi: Austrosaurus is one of those sauropods that never fit gracefully into pre-cladistic classifications of sauropods, where it was generally put into "Cetiosauridae" with a shrug (Olde-Tyme Cetiosauridae itself being the classification equivalent of a shrug). A. mckillopi was named in Longman (1933) from what was eventually shown to be the last cervical plus the first five dorsals (Poropat et al. 2017) found in the Allaru Mudstone of Queensland. By the early 1980s, a variety of other sauropod remains of broadly comparable age were known from the area, and Coombs and Molnar (1981) described them as Austrosaurus sp. This is when things became more complicated for Austrosaurus, because now not only were there "cetiosaur"-type vertebrae, there were brachiosaur-like limb bones. McIntosh (1990) reflects this uncertainty. Molnar (2001) reexamined the complex and concluded that some of the material, including the holotype, should be classified as titanosaurian, which is where you'll find it in the second edition of The Dinosauria (Upchurch et al. 2004). Continued work on the Queensland sauropods, and more material, resulted in the additional material being removed, some becoming the holotype of Wintonotitan wattsi (Hocknull et al. 2009). Hocknull et al. (2009) regarded Austrosaurus as dubious. A few years later, following rediscovery of the original locality and recovery of a few ribs, Poropat et al. (2017) found it to be valid and potentially a somphospondylan, so the door is open a crack to its being a genuine titanosaur.
Haestasaurus becklesii: This taxon has a long history, starting off its public career as Pelorosaurus becklesii Mantell (1852). Mantell based P. becklesii on a left humerus, radius, and ulna with associated skin impressions from the Lower Cretaceous Hastings Beds Group of somewhere near Hastings in southeastern England. Nowadays we'd take one look at the humerus and know it had nothing to do with Pelorosaurus proper, true Pelorosaurus being noted for a long, slender humerus and P. becklesii being equipped with something McIntosh (1990) compared favorably to that of Apatosaurus (i.e., short and chunky). In 1852, though, nobody knew beans about sauropods, much less their humerus proportions. Redescribing obscure Victorian sauropods not being a huge priority, "P." becklesii remained an open issue for more than 150 years. Upchurch (1995) was the first to propose it was actually a titanosaur, and this possibility was maintained into the 2010s (Upchurch et al. 2011), making it one of the earliest titanosaurs. At about this time, other reviews found a more cautious position, D'Emic (2012) considering it a titanosauriform of uncertain position and Mannion et al. (2013) describing it as a non-titanosaurian somphospondylan. The formal redescription of this species, which transferred it to the new genus Haestasaurus, took it even farther from Titanosauria, finding it most likely to be a basal macronarian. It has traveled farther again in recent years, outside of Neosauropoda (Mannion et al. 2019). At the moment, it and fellow chunky-limbed sauropod Janenschia (see above) look much more likely to have converged on some titanosaurian limb anatomy than to have been titanosaurs themselves.
|We've had a lot of bones illustrated in these posts, so why not something else for a change of pace? Here's the patch of skin impressions found for Haestasaurus. Upchurch et al. (2015):Figure 11. CC-BY-4.0.|
Mongolosaurus haplodon: M. haplodon is one of the better represented species on this page. You get a partial braincase, atlas, axis, third cervical, and five partial tooth crowns. These remains came from the upper Lower Cretaceous On Gong Formation of Inner Mongolia, China, and were described by Charles Gilmore in a 1933 publication that also named the ankylosaur Pinacosaurus grangeri. Pinacosaurus is somewhat more famous. Gilmore recognized the fragmentary nature of the sauropod remains, yet decided to name them anyway because of their unusual nature. Gilmore was particularly struck by the teeth, which had cylindrical roots, but also pointed and faintly serrated crowns. The third cervical also had what he interpreted as the beginnings of a bifid neural spine, on its poorly demarcated neural spine (Gilmore 1933).
Following its description, M. haplodon received almost no useful coverage for about 60 years, the exception being two paragraphs in McIntosh (1990), in which he couldn't find a good place to classify it but suggested that the teeth and possible bifid spine could indicate it went with diplodocids. Barrett et al. (2002) gave it a further review, concluding that it was dubious and observing without much enthusiasm that it might be a basal diplodocoid or titanosaur. Wilson (2005) was kinder, finding it valid and potentially a nemegtosaurid. The thorough redescription in Mannion (2011) concluded that it was a bona fide titanosaur. This was supported in Mannion et al. (2013) but challenged in D'Emic (2012) and Averianov and Sues (2017), which preferred a non-titanosaurian somphospondylan identification. Such an identification would put it in the morass of Cretaceous East Asian sauropods that are kind of titanosaurian and kind of not, which seems fair enough until more material comes along.
Alifanov, V. R., and Y. L. Bolotsky. 2010. Arkharavia heterocoelica gen. et sp. nov., a new sauropod dinosaur from the Upper Cretaceous of far eastern Russia. Paleontologicheskii Zhurnal 1:76–83.
Alifanov, V. R. 2012. [Superorder Dinosauria]. Pages 153–309 in E. N. Kurochkin, and A. V. Lopatin, editors. [Fossil vertebrates of Russia and adjacent countries. Fossil reptiles and birds. Part 2.] GEOS, Moscow, Russia.
Apesteguía, S. 2012. The sauropod diversity of the La Amarga Formation (Barremian), Neuquén, Argentina. Gondwana Research 12:533–546.
Averianov, A., and H.-D. Sues. 2017. Review of Cretaceous sauropod dinosaurs from central Asia. Cretaceous Research 69:184–197. doi:10.1016/j.cretres.2016.09.006.
Barrett, P. M., Y. Hasegawa, M. Manabe, S. Isaji, and H. Matsuoka. 2002. Sauropod dinosaurs from the Lower Cretaceous of eastern Asia: taxonomic and biogeographical implications. Palaeontology 45:1197–1217.
Bellardini, F., and I. A. Cerda. 2017. Bone histology sheds light on the nature of the "dermal armor" of the enigmatic sauropod dinosaur Agustinia ligabuei Bonaparte, 1999. The Science of Nature 104(1). doi:10.1007/s00114-016-1423-7.
Bonaparte, J. F. 1998. An armoured sauropod from the Aptian of northern Patagonia, Argentina. Page 10 in T. Tomida, T. R. Rich, and P. Vickers-Rich, editors. Second Symposium Gondwana Dinosaurs, 12–13 July, 1998. Abstracts with Programs. National Science Museum, Tokyo, Japan.
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