FunctionSo, we've got osteoderms on sauropods. On the surface, one's first response might be "Well, that's armor, right? They're there for protection, of course. That's how they work on other dinosaurs." Let's not be too hasty, though. To begin with, osteoderms can serve a variety of functions, and it is likely that they did so in titanosaurs (Cerda et al. 2015; Vidal et al. 2017). We've also got to contend with the "osteoderms per titanosaur" (OPT) factor: whatever hypothesis we come up with, it should be compatible with the seemingly small number of osteoderms per individual dinosaur, which limit body coverage. In short, how many osteoderms do you need to slap onto your titanosaur before your proposed function works?
About a half-dozen functions have been proposed or evaluated in the literature for titanosaur osteoderms:
- Passive defense
- Active defense
- Display and/or species recognition
- Body strengthening
- Mineral storage
Passive defense is a basic default for any kind of hard outer covering. The existence of probable bite marks on a titanosaur osteoderm (Marinho and Iori 2011) is evidence that osteoderms could have been used for at least passive defense, provided they weren't too hollow. The main challenge facing passive defense as the primary function is the OPT. This was brought up as early as Salgado (2003), who had interpreted the osteoderms as limited to a single row and was unsure if this would be useful. Marinho and Iori (2011) proposed that the osteoderms could have worked for passive defense in smaller and younger titanosaurs, but would have been unnecessary in larger titanosaurs. Curry-Rogers et al. (2011) interpreted Rapetosaurus osteoderms as not useful for defense because they were too thin and didn't imbricate with each other. It is not out of the question, though, that a small number of large osteoderms in vulnerable areas could have provided at least some protective value without burdening the animal too much. Titanosaurs with ossicles could have been much better protected, as well as retaining skin flexibility (Cerda et al. 2015).
No stegosaur-like thagomizers or ankylosaur-like spikes have been reported for titanosaurs. However, Cerda et al. (2015) noted that keeled or pointed osteoderms could have been used for striking if positioned on the tail. We can guess that the distance between a titanosaur's brain and its tail would have made it difficult to have both rapid response time and fine control. Of course, given the size of your average titanosaur and the laws of physics, an osteoderm wouldn't have needed to be specially shaped or wielded with great skill to cause damage. For that matter, an osteoderm-free tail wielded by an unhappy titanosaur would have been something to avoid, so while a titanosaur lashing with an armored tail is a fun mental picture and doesn't run up against the low OPT, it doesn't seem necessary. Interestingly, other sauropods experimented with tail clubs (Shunosaurus and friends especially), but it didn't catch on as far as we know.
Thermoregulation as a significant function of titanosaur osteoderms is considered unlikely due to the low ratio of surface area to volume (Salgado 2003; Curry Rogers et al. 2011; Marinho and Iori 2011; Cerda et al. 2015). In other words, you want your solar panel or radiator to be a big flat thing, rather than a lump. In addition, with a low OPT, the osteoderm coverage probably would not have been enough to make them more effective for gaining or losing heat than the neck, limbs, and tail.
Display and/or species recognition
For any unusual bony structure on a dinosaur, someone eventually considers it in terms of a display feature, and titanosaur osteoderms are no exception. Curry Rogers et al. (2011) found this function unlikely because the osteoderms are nondescript and broadly similar among titanosaurs. Cerda et al. (2015) found the possibility impossible to test from the current data, and suggested that a display function would be limited to large osteoderms. I suppose it's inevitable that any kind of large surficial feature can and does communicate some information about its bearer, but it seems difficult to make this a primary function of the osteoderms as they are currently understood.
It's not the first thing people usually consider when they think about armored animals, but osteoderms can provide significant structural support. This is actually one of the earliest proposed functions for titanosaur osteoderms. Sanz and Buscalioni (1987) suggested that osteoderms strengthened the sacral region, while Le Loeuff et al. (1994) proposed that they compensated for the absence of hyposphene-hypantrum articulations in the dorsal vertebrae, similar to the supporting function of crocodilian scutes. Many titanosaur osteoderms have keels on their basal surfaces that could have been used for the insertions of tendons and/or ligaments, and osteoderms which were at least twice as long as a vertebra could have helped stiffen the back (Vidal et al. 2017). For those titanosaurs with ossicles, a pavement of these would have strengthened the integument, not just in terms of defense (Cerda et al. 2015). Curry Rogers et al. (2011) were skeptical of a strengthening function for titanosaur osteoderms because they do not form pavements or shields, but it certainly seems reasonable that the longer osteoderms were providing some support simply by their size.
The possibility of titanosaur osteoderms being used for mineral storage was mentioned as far back as Salgado (2003), but at the time titanosaur osteoderms with hollow spaces were not known. The first lengthy discussion of this hypothesis was published in Curry Rogers et al. (2011), a description of a largely hollow Rapetosaurus osteoderm, which the authors reported was not pathological or the only such occurrence from the Maevarano Formation. Since this time, other osteoderms with cavities have been found elsewhere (Alamosaurus: Carrano and D'Emic 2015; Spain: Vidal et al. 2017; Brazil: Pereira et al. 2018). The basic idea behind the hypothesis is that egg-laying takes a lot out of an animal, and a reservoir of calcium and phosphorus would have been advantageous, especially if the animal was growing rapidly and living in a stressed environment, such as the seasonally semi-arid to arid depositional setting that became the Maevarano Formation (Curry Rogers et al. 2011), or if the animal had heavily pneumatic bones (saltasaurs) and thus could not get away with losing a lot of minerals from the rest of the skeleton (Cerda et al. 2015). It is known that modern female crocodilians take minerals from their osteoderms during reproduction (Chinsamy et al. 2016; Vidal et al. 2017), and armored titanosaurs also show a lot of remodeling of long bones (García et al. 2015).
Vidal et al. (2017), looking at a group of Spanish osteoderms, found some but not all to have internal voids. They decided that the hollows were consistent with use for minerals during reproduction, but not with aging (because a modified osteoderm was found with the skeleton of an individual that was not old) or seasonal stress (because the modifications were only found in a minority of osteoderms). Modified and dense osteoderms were of similar size, implying they were all part of the same population. The presence of modified osteoderms would also imply the presence of nesting or gravid females.
A couple of unanswered questions come up with the mineral storage hypothesis. If this was the main function of osteoderms, and it's already known that titanosaurs didn't have oodles of the things, is it possible that male titanosaurs would have had fewer or lacked them entirely? (Perhaps the unarmored skeletons of Epachthosaurus and Opisthocoelicaudia represent males, using this idea.) Also, how much osteoderm volume is necessary for the storage function to be viable? Titanosaurs already had a ton (or more) of bone; how much could another few lumps help? Is it that the osteoderm bone is in a place where it has few other biological responsibilities, making it advantageous as a non-structural mineral source?
Function ConclusionsAt this time, I think it's reasonable to think that titanosaur osteoderms served multiple functions. The presence of large void spaces in some is a clear indication that they were being cannibalized for one reason or another, and it is known that other osteoderm-bearing archosaurs dipped into their own osteoderms for minerals, so mineral storage looks like a likely function. The evidence of a bitten osteoderm shows that a defensive function was also possible, albeit limited by low OPT. We can also assume that titanosaur osteoderms naturally had some potential for structural support and visual recognition, although there's no indication that there was strong selection for either. It is also possible that the importance of different functions changed over time; Marinho (2007) proposed that the osteoderms were primarily for defense when the animal was young, then became used for mineral storage when it was older.
Speculation and Final ThoughtsIt's going to be fun to get to Neuquensaurus and Saltasaurus. What's not to like about sauropods that 1) were lucky to exceed a large Allosaurus in length, 2) yet had tubby torsos and hips, 3) and had short stocky forearms and shins, so walked around on stubby legs, 4) and were coated in mm-scale nubbins of bone, 5) and finally, like rebbachisaurids, were made up in no small amount by air? They're about as close as you can get to actual dinosaur parade balloons. (Also, I wonder if intensively pneumatic sauropods such as saltasaurs and rebbachisaurids would have been good for distribution to nearby islands via swimming, just as elephants are good at dispersing to nearby landmasses.)
|Neuquensaurus australis, the perfect sauropod for around the yard. From Otero (2010:Figure 1C). CC-BY-4.0.|
Since it's my blog and "if you can't be crazy on your own blog, where can you be crazy?", let's take the mineral storage hypothesis farther. Titanosauria took over from the other various clades of sauropods in the mid-Cretaceous and never looked back. We saw in the previous installation that osteoderms appeared early in titanosaur history, and are known from a variety of taxa across the continents. It's safe to guess that osteoderms are a fairly basal trait. Is it possible that osteoderms are part of the secret of titanosaur success, not because they were great at protecting them from malicious teeth and claws, but because they helped titanosaurs make lots and lots of little titanosaurs? The mineral storage hypothesis would give titanosaurs advantages in producing eggs and recovering from egg-laying, which are certainly useful adaptive features, whereas a few lumps of bone can only protect so much.
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