Sunday, March 10, 2019

Titanosaur osteoderms: introduction and history of study

One of the most notable aspects of titanosaurs is the presence of bony armor in at least some species. It's where we get Lithostrotia ("inlaid with stones"), after all. Armored titanosaurs have been hinted at since the 1890s and accepted since the 1980s, but despite forty years of publications and many finds across the world, there are still many things we do not know about titanosaur armor. Even the position on the body is mostly a matter of inference.

I'd had the idea of looking at titanosaur osteoderms and ossicles (the technical terms) in more detail since poster Ornithopsis commented on them following the very first "Your Friends The Titanosaurs" post. Originally I thought I could do it in one post, but after fourteen pages of notes and forty-plus references, this seemed perhaps overly ambitious. Therefore, this post will focus on the history of study (and a healthy bibliography, for those of you collecting papers), and a couple of future posts will cover more detailed descriptions, location on the body, distribution in time, space, and the titanosaur family tree, and functions. If you want spoilers...

Aside from ossicles, which are smaller than osteoderms and histologically distinct, most complete osteoderms are elliptical to subcircular irregular pieces vaguely resembling keeled oatmeal cookies, or "bulb-and-root" structures that look something like a slug wearing a beret at a jaunty angle. "Bulb-and-root" osteoderms are larger than scute-type osteoderms. Osteoderms range in length from about 10 to 50 cm (4 to 20 in). Osteoderm-bearing titanosaurs probably had only a few. Some features (symmetry, curvature) suggest placement on the midline or off of it, or on the body core versus perhaps the neck or tail, but no osteoderms have been found in place, undisturbed. Although ossicles contacted each other, the larger osteoderms did not, unlike croc and ankylosaur armor. Therefore, if you're restoring titanosaurs with an ankylosaur-like carpet of scutes, or even with large numbers of them, this appears to be unlikely for most species. Osteoderms have been found so far throughout the span of titanosaur existence, and are confirmed from all continents except Antarctica, Australia (one false alarm), and Asia outside of the Indo-Pakistan landmass. They have been found on large and small titanosaurs. "Bulb-and-root" osteoderms are more widely distributed than scutes, which seem to be limited to Saltasaurus and a few friends. Aside from a possible Indian record, Saltasaurus is also the only titanosaur with confirmed ossicles. Many functions have been suggested for osteoderms, and they likely served more than one. There is increasing support for the idea that an important function was mineral storage.

[There! The highlights of titanosaur armor in fewer than 250 words!]

History of study

The history of human knowledge of titanosaur osteoderms begins, as is typical for popular narratives of science, with a rejection. In 1896, French paleontologist Charles Depéret published a report on dinosaur fossils from Madagascar. The material included a sort-of-biscuit-like piece of bone which he tentatively associated with his new species Titanosaurus madagascariensis. This object was about 25 cm across (about 10 inches) with a central peak; he interpreted it as covered by horn. His report was greeted by a great outpouring of nothing. The idea of armored titanosaurs was dismissed or ignored for eight decades.

Depéret (1896)'s plate illustrating the type material of "Titanosaurus" madagascariensis and some obscure thing he named Megalosaurus crenatissimus. "T." madagascariensis is represented by items 1-1a, 2-2a, and 3-3a, and M. crenatissimus gets the rest. The osteoderm is item 3-3a.

A couple of decades after Depéret, Matley (1923) described a tibia, sacrum, two ilia, and a bunch of osteoderms from the Carnosaur Bed of Bara Simla, India as a new stegosaurian: Lametasaurus indicus. Eventually L. indicus would realize its true destiny, which was to be a chimeric taxonomic pain-in-the-neck (the non-armor part turned out to be the back end of a robust abelisaur, maybe Indosaurus or Rajasaurus), but this was in the future. Matley's material 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). It isn't possible at this time to be certain what everything came from (crocs, of course, have osteoderms, and ceratosaurians are known to have come up with some from time to time), but the little ossicles are kind of suggestive of something that'll show up shortly. (I'd include an image, but the only scans I could find look like art projects on dark and light.)

Matley spent some more time with L. indicus for the enormous Huene and Matley 1933 monograph on Indian dinosaurs, relating that Barnum Brown had collected another piece from the Carnosaur Bed for the American Museum of Natural History. It was on display at the AMNH as (in so many words) part of a tail club. Matley interpreted this as evidence that his L. indicus had a tail club, and lamented that no new bones of Lametasaurus except scutes had been found at the Bara Simla site. The site is known to have produced titanosaur bones, though. D'Emic et al. (2009) later described Brown's specimen (AMNH 1959) as a titanosaur osteoderm.

Von Huene was also no stranger to curious scutes from the Southern Hemisphere. He'd gotten into the act by describing a "polacanthid" from Argentina, Loricosaurus scutatus, based on 26 osteoderms (Huene 1929). Like Lametasaurus it also came from rocks that strangely enough produced no other ankylosaurian bones but had yielded titanosaurs. Bonaparte and Powell (1980) suggested in a roundabout way that the Loricosaurus osteoderms belonged to what is now called Neuquensaurus, and Salgado (2003) reported that at least one Loricosaurus osteoderm had been found associated with Neuquensaurus australis. Given that N. australis is known to be armored from a different specimen (Salgado et al. 2005), this is more likely than the other option presented in the literature, Laplatasaurus (or Titanosaurus) araukanicus, which had been suggested on the basis of size (Powell 1980). Powell did not include osteoderms with this species in a later revision (Powell 2003). 

It wasn't until Bonaparte and Powell (1980) described the dinosaurs of El Brete in Argentina that everything clicked. This site produced a theropod they named Noasaurus leali (which spent the 1980s as a weird dromaeosaur from the Southern Hemisphere before its true affinities became clear) and a small titanosaur they named Saltasaurus loricatus. Titanosaur bones were the primary find of the site (around 130 non-osteoderm bones), so titanosaurs were the obvious candidate to have produced the osteoderms mixed with the titanosaur bones. The osteoderms included not only eight oval to circular pieces about 10 to 12 cm across (about 4 to 5 in), but also four groups of hundreds of associated ossicles, each rounded to polygonal in shape and generally a little less than 1 cm in diameter.

Saltasaurus ossicles from Cerda and Powell (2010). Unless Matley's ossicles were the same kind of thing, ossicles have only been found with Saltasaurus to date. CC-BY-4.0.

Although seemingly nobody bought Depéret's argument in 1896, Bonaparte and Powell's work was accepted with a minimum of fuss. Forty years later it seems to have been an "Oh, so that's what those things are" moment, rather than something that spurred years of indignant arguing that they were really intercostal clavicles or what-have-you. There haven't even been that many false alarms. Except for that business with Agustinia, the most notable misidentification seems to have been when Molnar (2011) reported Australian osteoderms which later turned out to be dorsal neural spines (Poropat et al. 2015). Before this was corrected, they showed up in Vidal et al. (2014) as the genuine thing. Agustinia, you may recall, was supposed to be a titanosaur-ish sauropod with ostentatious spines and plates (Bonaparte 1999), but they later turned out to be misidentified fragments of other parts of the skeleton (Mannion et al. 2013; Bellardini and Cerda 2017).

Since 1980, the knowledge base has grown via a steady stream of new reports and reassessed older fossils from around the world:
  • Argentina (Salgado and Coria 1993; González Riga 2003; Salgado et al. 2005; Zurriaguz 2017)
  • Brazil (Azevedo and Kellner 1998; Marinho and Candeiro 2005; Kellner et al. 2006; Marinho and Iori 2011; Lindoso et al. 2013; Pereira et al. 2018)
  • France (Le Loeuff et al. 1994; Le Loeuff 1995)
  • India (D'Emic et al. 2009)
  • Madagascar (Dodson et al. 1998; Curry Rogers et al. 2011)
  • Malawi (Gomani 2005)
  • Mali (O'Leary et al. 2004)
  • Pakistan (Malkani 2003, 2010, 2015)
  • Romania (Csiki 1999)
  • Spain (Sanz and Buscalioni 1987; Sanz et al. 1989; Díez Díaz et al. 2013; Vidal et al. 2014)
  • United States (Carrano and D'Emic 2015)
  • Uruguay (Soto et al. 2012)
(The flip side of the list is that titanosaur osteoderms are still rare enough that you can write a paper about a new occurrence.)

The histology and anatomy of osteoderms have also been explored in detail (Cerda and Powell 2010; Curry Rogers et al. 2011; Cerda et al. 2015; Chinsamy et al. 2016; Vidal et al. 2017), producing evidence that an important function of titanosaur osteoderms was mineral storage, particularly as a source of calcium during egg-laying. Calcium storage was first brought up in Salgado (2003), but the idea did not receive a lot of attention until Curry Rogers et al. (2011)'s publication of a very hollow osteoderm. There still seems to have been a defensive role, as indicated by the bitten osteoderm of Marinho and Iori (2011).

References


Azevedo, S. A. K., and A. W. A. Kellner. 1998. A titanosaurid (Dinosauria, Sauropoda) osteoderm from the Upper Cretaceous of Minas Gerais, Brazil. Boletim do Museu Nacional (Río de Janeiro) 44:1–6.

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. 1999. An armoured sauropod dinosaur from the Aptian of northern Patagonia, Argentina. National Science Museum Monographs 15:1–12.

Bonaparte, J. F., and J. E. Powell. 1980. A continental assemblage of tetrapods from the Upper Cretaceous beds of El Brete, northwestern Argentina (Sauropoda-Coelurosauria-Carnosauria-Aves). Mémoires de la Société Géologique de France 139:19–28.

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., and J. E. Powell. 2010. Dermal armor histology of Saltasaurus loricatus, an Upper Cretaceous sauropod dinosaur from northwest Argentina. Acta Palaeontologica Polonica 55:389–398.

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.

Chinsamy, A., I. Cerda, and J. Powell. 2016. Vascularised endosteal bone tissue in armoured sauropod dinosaurs. Scientific Reports 6, article number 24585. doi:10.1038/srep24858.

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

Curry Rogers, K., M. D. D’Emic, M. Vickaryous, and A. Cagan. 2011. Sauropod dinosaur osteoderms from the Late Cretaceous of Madagascar. Nature Communications 2:564. doi:10.1038/ ncomms1578.

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.

Depéret, C. 1896. Note sur le dinosauriens sauropodes et théropodes du Crétacé Supérieur de Madagascar. Bulletin de la Société Géologique de France 24:176–194. [translated]

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Dodson, P., D. W. Krause, C. A. Forster, S. D. Sampson, and F. Ravoavy. 1998. Titanosaurid (Sauropoda) osteoderms from the Late Cretaceous of Madagascar. Journal of Vertebrate Paleontology 18:563–568.

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