Sunday, May 5, 2019

Titanosaur osteoderms: distribution

My plan had been to wrap up everything in this post. (Well, originally it was to do everything in one post. Ha.) However, again the contents were beginning to sprawl, and I felt guilty about the lengths of a couple of recent posts. After all, this isn't a peer-reviewed journal article, is it? There are different expectations for a blog post. Therefore, we'll just have distribution here, and one more post for the functions and conclusion. For the previous installments, go here (introduction) and here (characteristics).

Distribution

If all you really want to know is which named taxa are associated with osteoderms, it's Aeolosaurus sp., Alamosaurus sanjuanensis, Ampelosaurus atacis, Lametasaurus indicus (for a certain value of "associated"), Lirainosaurus astibiae, Loricosaurus scutatus, Magyarosaurus dacus, Malawisaurus dixeyi, Maxakalisaurus topai, Neuquensaurus australis, Rapetosaurus krausei, Saltasaurus loricatus, and "Titanosaurus" madagascariensis. Of course, Lametasaurus, Loricosaurus, and "T." madagascariensis are of limited usefulness at best, but the others are all well-established.

Now, for those who'd like to dig deeper:

Carrano and D'Emic (2015) included a table summarizing which formations had titanosaurs, with osteoderms indicated. I've extracted just the osteoderm listings from this table, with some additional comments and finds in brackets. In a couple of places where they had "Titanosauria indet.", I substituted the name of an omitted dubious taxon ("Lametasaurus", "Titanosaurus" madagascariensis). Oddly, although the paper was about finding osteoderms with Alamosaurus from the North Horn Formation, this occurrence was not marked on the table, but Javelina and McRae occurrences of Alamosaurus were marked as osteoderm-iferous; I took the liberty of correcting this. (Perhaps at some point osteoderm-bearing taxa were indicated in all formations they were known from, rather than just those where the osteoderms were found for those taxa.)

  • Adamantina: Maxakalisaurus
  • Alcântara: Unnamed
  • Allen: "Aeolosaurus", Bonatitan [osteoderms not mentioned in the description of this taxon], Neuquensaurus [Cerda et al. 2015 report only Aeolosaurus sp. confirmed for osteoderms in this unit; more reported by Zurriaguz 2017]
  • [Anacleto, Cerda et al. 2015]: [Neuquensaurus]
  • Ankazomihaboka: ["Titanosaurus" madagascariensis; horizon of this species generally given as the Maevarano instead]
  • "Continental Intercalaire", Mali: Unnamed
  • Densuş-Ciula: potentially Magyarosaurus [possibly an error reflecting the presence of potentially armored Magyarosaurus in this unit?]
  • Dinosaur Beds: Malawisaurus
  • [Gokwe, Woolley et al. 2015: Unnamed (added 2019/09/01)]
  • [Guichón, Soto et al. 2012: Unnamed]
  • Lameta: ["Lametasaurus"]
  • Lecho: Saltasaurus
  • Lubur Sandstone: Possible, unnamed [I have no other information about this record]
  • Maevarano: Rapetosaurus
  • Malargüe Group: Unnamed
  • Marília: "Aeolosaurus", unnamed [would be more fair to say the osteoderms are similar to Aeolosaurus sp. osteoderms, and A. sp. has been found in the formation; Marinho and Candeiro 2005]
  • [Marnes Rouges: Ampelosaurus]
  • [North Horn: Alamosaurus, Carrano and D'Emic 2015, ironically enough]
  • [Pab or Vitakri: in flux]
  • Plottier: Mendozasaurus
  • Río Colorado Subgroup: Unnamed [this might be Loricosaurus; if so, it would be redundant with the Anacleto report]
  • Sânpetru: Unnamed [attributed to Magyarosaurus dacus by Csiki 1999]
  • Unnamed (Armuña) [Vegas de Matute Formation; this is the Sanz and Buscalioni 1987 report]: Unnamed
  • [Unknown Brazilian unit, Pereira et al. 2018: Unnamed]
  • [Villalba de la Sierra, AKA the Lo Hueco site, Vidal et al. 2014/2017: Unnamed]
  • Vitoria: Lirainosaurus [the Laño site is frequently described in recent literature as "equivalent to the Sedano Formation" without using the Vitoria Formation, as we'll get to when we come to Lirainosaurus]

Or, to look at it another way:
  • Adamantina: Campanian–Maastrichtian of Argentina, South America
  • Alcântara: early Cenomanian of Brazil, South America
  • Allen: late Campanian–early Maastrichtian of Argentina, South America
  • Anacleto: early Campanian of Argentina, South America
  • "Continental Intercalaire": Early Cretaceous of Mali, Africa
  • Densuş-Ciula: Maastrichtian of Romania, Europe
  • Dinosaur Beds: Aptian of Malawi, Africa
  • Guichón: early? Late Cretaceous of Uruguay, South America
  • Gokwe: Early? Cretaceous of Zimbabwe, Africa
  • Lameta: Maastrichtian of India, Indo-Pakistan landmass
  • Lecho: Maastrichtian of Argentina, South America
  • Lubur Sandstone: Early Cretaceous of Kenya, Africa
  • Maevarano: middle Maastrichtian of Madagascar
  • Malargüe Group: Campanian–Maastrichtian of Argentina, South America
  • Marília: Maastrichtian of Brazil, South America
  • Marnes Rouges: late Campanian of France, Europe
  • North Horn: Maastrichtian of United States, North America
  • Pab or Vitakri: late Maastrichtian of Pakistan, Indo-Pakistan landmass
  • Plottier: late Coniacian–early Santonian of Argentina, South America
  • Río Colorado Subgroup: Santonian–early Campanian of Argentina, South America
  • Sânpetru: early–middle Maastrichtian of Romania, Europe
  • Unknown Brazilian unit: approximately Early–Late Cretaceous boundary age
  • Vegas de Matute: late Campanian of Spain, Europe
  • Villalba de la Sierra: late Campanian–early Maastrichtian of Spain, Europe
  • Vitoria/Sedano equivalent: late Campanian–early Maastrichtian of Spain, Europe

Can we have that presented in a graphical format?

Instances of titanosaur osteoderms, per stage. Click to embiggen.

The above chart shows the distribution of the records by landmass and stage, with the undivided "Early Cretaceous" and "Late Cretaceous" reports omitted. You may notice that stage-straddling records are counted twice, so perhaps you'd prefer this?

Figure 9 from Vidal et al. (2014), with colors corresponding to different types of osteoderms (red = bulb-and-root, blue = scute, green = controversial, gray = too fragmentary to tell). CC-BY-4.0.

Vidal et al. (2014) and I have the occasional difference in unit age, and their chart includes the Australian record that was later reinterpreted, but overall we come up with similar results. From a quick glance at the figures and bulleted lists, one might be tempted to suggest that osteoderms arose in African titanosaurs during the Early Cretaceous and spread relatively quickly to South America, appearing only later in Europe, Indo-Pakistan, Madagascar, and North America. However, I think it's more likely that the osteoderm record is a rough proxy for the presence of titanosaurs in general. I'm not saying that the above evolutionary scenario is wrong, just that deriving it from the known distribution of osteoderms would be a case of getting the right answer by accident. (It *would* be compatible with recent suggestions that titanosaurs evolved in Africa.)

There is one interesting hole in the record, though, which is the absence of osteoderms from non-Indo-Pakistan Asia. At this point, I'm hesitant to suggest it's a real absence as opposed to the luck of the draw. I suppose there could be something endemic going on, but there isn't currently a firm case that Asian titanosaurs make a clade, and "It's an endemic Chinese/east Asian clade" is such a cliché in sauropod study anyway.

It seems likely that the ability to form bulb-and-root osteoderms appeared fairly early in titanosaur evolution. Bulb-and-root osteoderms have been found in Africa, Europe, the Indo-Pakistan landmass, Madagascar, South America (Vidal et al. 2014), and North America (Carrano and D'Emic 2015), and they go back at least to Malawisaurus, which usually (but not always) turns up as a basal titanosaur. As mentioned in the previous installment, scute osteoderms are only known from Madagascar and South America, and may be limited to saltasaurs (Vidal et al. 2014).

One other comment: Although it had been suggested that armored titanosaurs tended to be smaller than unarmored titanosaurs (D'Emic et al. 2009), the addition of Alamosaurus to the ranks of osteoderm-bearers shows that this is not necessarily true (Carrano and D'Emic 2015). There may be some validity to this hypothesis if considered for a subset of osteoderms, if it can be shown that scute osteoderms were confined to the saltasaurs. It may also be worth noting that several super-titanosaurs with reasonably well-represented torsos and anterior tails (Dreadnoughtus, Futalognkosaurus, Patagotitan) are not currently known to have had osteoderms. (If true in general, this wouldn't prove that armored titanosaurs tended to be small, but the subtly different proposition that unarmored titanosaurs tended to be large.) The best unarmored specimens remain the skeletons of Epachthosaurus sciuttoi and Opisthocoelicaudia skarzynskii, both of moderate size.

References

Carrano, M. T., and M. D. D'Emic. 2015. Osteoderms of the titanosaur sauropod dinosaur Alamosaurus sanjuanensis Gilmore, 1922. Journal of Vertebrate Paleontology 35(1):e901334. doi:10.1080/02724634.2014.901334.

Cerda, I. A., R. A. García, J. E. Powell, and O. Lopez. 2015. Morphology, microanatomy, and histology of titanosaur (Dinosauria, Sauropoda) osteoderms from the Upper Cretaceous of Patagonia. Journal of Vertebrate Paleontology 35(1):e905791. doi:10.1080/02724634.2014.905791.

Csiki, Z. 1999. New evidence of armoured titanosaurids in the Late Cretaceous–Magyarosaurus dacus from the Hateg Basin (Romania). Oryctos 2:93–99.

D'Emic, M. D., J. A. Wilson, and S. Chatterjee. 2009. The titanosaur (Dinosauria: Sauropoda) osteoderm record: review and first definitive specimen from India. Journal of Vertebrate Paleontology 29(1):165–177.

Marinho, T. S., and C. R. A. Candeiro. 2005. Titanosaur (Dinosauria: Sauropoda) osteoderms from the Maastrichtian of Uberaba, Minas Gerais State, Brazil. Gondwana Research 8:473–477.

Pereira, P. V. G. d. C., T. d. S. Marinho, C. R. d. A. Candeiro, and L. P. Bergqvist. 2018. A new titanosaurian (Sauropoda, Dinosauria) osteoderm from the Cretaceous of Brazil and its significance. Ameghiniana 55(6).644–650. doi:10.5710/AMGH.26.08.2018.3168.

Sanz, J. L., and A. D. Buscalioni. 1987. New evidence of armored dinosaurs in the Upper Cretaceous of Spain. Pages 199–204 in P. M. Currie and E. H. Koster, editors. 4th Symposium of Mesozoic Terrestrial Ecosystems, Drumheller, Alberta, 10–14 August 1987. Short Papers. Royal Tyrrell Museum, Paleontology, Drumheller, Alberta.

Soto, M., D. Perea, and A. Cambiaso. 2012. First sauropod (Dinosauria: Saurischia) remains from the Guichón Formation, Late Cretaceous of Uruguay. Journal of South American Earth Sciences 33:68–79.

Vidal, D., F. Ortega, and J. L. Sanz. 2014. Titanosaur osteoderms from the Upper Cretaceous of Lo Hueco (Spain) and their implications on the armor of Laurasian titanosaurs. PLoS ONE 9(8):e102488. doi:10.1371/journal.pone.0102488.

Vidal, D., F. Ortega, F. Gascó, A. Serrano-Martínez, and J. Luis Sanz. 2017. The internal anatomy of titanosaur osteoderms from the Upper Cretaceous of Spain is compatible with a role in oogenesis. Scientific Reports 7, article number 42035. doi:10.1038/srep42035.

Woolley, H., J. Sertich, C. A. Forster, D. Munyikwa, S. D. Sampson, K. Curry Rogers, and R. Rogers. 2015. Titanosaurian and other vertebrate remains from the Cretaceous Gokwe Formation, central Zimbabwe. Journal of Vertebrate Paleontology, Program and Abstracts, 2015:241.

Zurriaguz, V. 2017. New record of titanosaurian (Dinosauria: Sauropodomorpha) osteoderms from the Upper Cretaceous of North Patagonia. Cretaceous Research 74:175–180.

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