Sunday, 24 June 2012

Social Sauropods?

Many of my recent posts have been concerned with what could be termed “analogues and inferences” and, keeping the theme going, the recent paper by Salgado et al that was published in the most recent issue of the Journal of Vertebrate Paleontology suggests that a recent find of rebbachisaurid sauropods may be indicative of gregariousness behaviour in these dinosaurs.

The specimens, an adult and two juveniles, were recovered from the Pichi Neuquén Member of the Rayoso Formation in Patagonia, Argentina. The specimens are clearly referable to Rebbachisauridae based upon several characters that were recognised in the recovered elements but are not diagnostic to species level although the authors suggest that the remains are morphologically similar to Zapalosaurus (Salgado et al 2006).  
Since these specimens were recovered together in what is obviously a monotaxic bone bed the authors speculate that it may be suggestive that these sauropods did indeed demonstrate some form of social interaction. There is no evidence in the sedimentology and taphonomy of the site that is suggestive of a catastrophic event or miring - nor signs of transportation and the remains are clearly in situ although there is the usual skeletal displacement. These animals apparently died together.
So, in the absence of any environmental calamity, why did these animals seemingly perish together? One line in this paper proved troublesome to me:

One possible, though purely speculative, explanation for this is that the death of the adult triggered the death of the two juvenile individuals.”

Unfortunately the authors do not quantify this statement without any clear indication of what they mean. If we were talking about mammals, and the juveniles were heavily dependent on the adult, then maybe the juveniles would succumb fairly quickly. Extant dinosaurs, the birds, also suffer such deaths since if the adult birds were to die or were killed then the chicks would soon expire. But sauropods? Perhaps I am missing something here and please feel free to enlighten me if you have any detail you wish to add.
In the meantime, and as mentioned here before, sauropod nesting sites indicate that the eggs were laid in enormous numbers in large sites that covered vast acreage, the most famous of which is Auca Mahuevo in Argentina. There is no evidence, circumstantial or otherwise, that sauropods demonstrated any parental care at the nesting stage and it seems the nesting grounds were deserted once the eggs had been laid and covered over. Once hatched, the hatchlings appeared to have fended for themselves and must have suffered catastrophic losses.
However, those that survived do indeed appear to have formed social groups and the principal evidence for this comes from their track ways. There are hundreds of sites all over the world and as palaeoichnology becomes more sophisticated and modern digital techniques are employed, they are able to help demonstrate that, indeed, some sauropods engaged in gregarious behaviour to a degree.
Interestingly some of these sites reveal that sauropods appear to be age segregated and travelled around in small herds (Myers & Fiorillo 2009). This can be supplemented by finds such as those in the Mother’s Day Quarry which exposes the Upper Jurassic Morrison Formation of Montana in the USA. Here have been recovered the multiple remains of many diplodocoid sauropods but they are all juvenile and sub-adult animals. Not a single adult is represented by as much as one element (Myers & Storrs 2007).
Both track ways and sites such as the Mother’s Day Quarry, and similar sites in China, India and other countries, all point to the possibility of gregariousness in sauropods. Naturally, this all has to be quantified by pointing out that such sites are greatly in the minority, albeit not rare either, and there are many considerations that have to be taken, the most important being taphonomic, but it does seem to be reasonably convincing.

Gregariousness in dinosaurs has featured in these pages many times now and whilst we cannot say with absolute certainty to what degree and form this social interaction may have taken, we can now certainly say that it did occur. There are numerous examples in the fossil record which can be interpreted as demonstrating such behaviour.
Monotaxic bone beds are the classic example and the large hadrosaurian and ceratopsian graveyards of North America expose vast quantities of animals numbering from the tens to hundreds – maybe even thousands. Examination of the majority of these sites indicates these dinosaurs perished in a catastrophic event and since only a single taxa is represented then it would seem reasonable to surmise that they were moving through the environment as a herd of some description when disaster struck. However, for what reason they were moving together we can only guess at but cannot say with any certainty.
As already mentioned with regards to sauropods, the ancient nesting grounds of dinosaurs reveal much. The most famous examples of these include the nesting grounds of hadrosaurs such as Hypacrosaurus and Maiasaura and the fact that multiple remains of juveniles have been found in, what is in effect, a nest mound suggests that they were probably quite helpless when born and required parental help. The site at Egg Mountain, by way of example, appears to have been a large colony of hadrosaurs that nested communally and actively cared for their young and is very reminiscent of an extant bird colony of today.
Track ways, again as with sauropods, are represented by thousands of examples across the globe and these again often indicate that animals moved in the same direction in the same place but it has always been hard to clarify that they moved together at the same time. But today, and as mentioned earlier, digital technology is enabling us to almost section track ways and determine if dinosaurs moved together as a group and this technology is still at a relatively early stage but results are encouraging and will only get better.
I won’t go into the world of gregariousness in theropods since that subject has, and will continue to, feature in this blog. Past and current research into this subject would fill up a library on its own. Suffice to say that different theropod taxa spent time together for reasons unknown and for an unspecified amount of time.  
So the challenge now is not to prove that dinosaurs were gregarious, since they obviously were, but to what degree and why. Unfortunately, this is mostly going to be guided by speculation and inference yet again but that is all we have in some cases – oh and a little bit of science too! 

References

Myers, T.S., and G.W. Storrs. 2007. Taphonomy of the Mother’s Day Quarry, Upper Jurassic Morrison Formation, South-Central Montana, USA. Palaios, 2007, v. 22, p. 651-666.
Myers, T.S., and A.R. Fiorillo. 2009. Evidence for gregarious behaviour and age segregation in sauropods. Palaeogeography, Palaeoclimatology, Palaeoecology 11:96-104.
Salgado, L., I. De Souza Carvalho, and A.C. Garrido. 2006. Zapalasaurus bonapartei, un neuvo dinosaurio saurópodo de la Formación La Amarga (Cretácico Inferior), noroeste de Patagonia, Provincia de Neuquén, Argentina. Geobios 39:695-707.

Salgado, L.,J.I. Canudo, A.C. Garrido, and J.L. Carballido. 2012. Evidence of gregariousness in rebbachisaurids (Dinosauria, Sauropoda, Diplodocoidea) from the early Cretaceous of Neuquén (Rayoso Formation), Patagonia, Argentina. Journal of Vertebrate Paleontology Vol.32, (3), pp.603-613.

9 comments:

Stephen M said...

“One possible, though purely speculative, explanation for this is that the death of the adult triggered the death of the two juvenile individuals.”

I don't necessarily agree, and you have to continue with the speculation, but if the juveniles and adult were separated from a herd. It is possible that the juveniles depended on the adult to lead them to a water/nutrition source.

Alessio said...

How social were dinosaurs?

It's the classic 1 million dollars question, but i'd bet it, like many other aspects of animal behaviour, depended on the species.
Looking at modern archosaurs we can see many degrees of sociality, both in birds and crocs, so i guess it was the same in good ol' Mesozoic; for what we know there could have been 'pods organized in elephant-like herds and other just in lil' family gropus... Speculation, speculation everywhere!


Alessio

Hadiaz said...

"Perhaps I am missing something here and please feel free to enlighten me if you have any detail you wish to add."

Maybe the adult keeled over w/a heart attack & crushed the juveniles under its weight. ;)

BTW, I'm surprised you didn't mention the Davenport Ranch tracksite ( http://www.amnh.org/exhibitions/dinosaurs/trackways/mesozoic.php ).

"However, for what reason they were moving together we can only guess at but cannot say with any certainty."

True, although migration seems to make the most sense, given what we know about their modern counterparts (See the Bell & Snively quote). I know that migration has already been featured, but it seemed too important not to mention.

"Maiasaurus"

Maiasaura. ;)

-Herman Diaz

Quoting Bell & Snively ( http://www.ohio.edu/people/es180210/Snively%20pdfs/bell_polar_dinos.pdf ): "Perhaps the best evidence for dinosaur migration are the enormous bonebeds of hadrosaurs and ceratopsians across western North America (e.g. Currie & Dodson 1984, Fiorillo & Gangloff 1999). These bonebeds can number in the hundreds to thousands of individuals. Herds of many thousands of individuals would represent serious energy (food) drains on the environment. A stand- ing herd of such numbers would not be ecologically sustainable if those animals did not move from place to place (Baker 1980). Modern ungulates such as caribou, wild- ebeest, and reindeer, which form herds of up to 100000 individuals, do so only during their seasonal migrations and occasionally form similar-sized bonebeds to those of their dinosaurian counterparts following mass kills, often as a result of flooding (Capaldo & Peters 1995)."

Mark Wildman said...

Thanks for the comments everyone - they are always welcome.

@Herman - Like I said, there are multiple track sites that indicate gregarious movement in dinosaurs as a whole. The trick is to prove they were moving together at the same time and that the site was not some kind of dinosaurian highway.

You know me - migration works for me too but I tend to be cautious and maintain the neutral stance. However, the recent paper on northern dinosaurs appearing not to migrate south demonstrates an element of caution needs to be taken.

Maiasaura corrected - thanks!

Henrique Niza said...

"... nesting grounds of... Maiasaura... a nest mound suggests that they were probably quite helpless when born and required parental help."

I'm somewhat vague but wasn't there a paper in this last decade or so that concluded the juveniles Maiasaura have more in common with precocial animals rather than altricials?

Mark Wildman said...

It goes on Henrique. Although it appears the majority of palaeontologists' agree with the idea of altricial Maiasaura there is still plenty of counter-argument.

Horner and Makela's paper back in 1979 started it all and maintain its validity whilst work by Geist and Jones (1996), as an example, suggested otherwise.

References

Horner, J. R., and R. Makela. 1979. Nest of juveniles provides evidence of family structure among dinosaurs. Nature 282:296–298.

Geist, N. R., and T. D. Jones. 1996. Juvenile skeletal structure and the reproductive habits of dinosaurs. Science 272:712–714.

San Saurio said...

You can check more data about social sauropods in ichnologic papers like

Castanera, D., Barco, J.L., Díaz-Martínez, I., Herrero-Gascón, J., Pérez-Lorente, F. y Canudo, J.I. 2011. New evidence of a herd of titanosauriform sauropods from the lower Berriasian of the Iberian range (Spain). Palaeogeography, Palaeoclimatology, Palaeoecology 310: 227-237.

Mark Wildman said...

Indeed - I have seen this and thank you for the reference.

Hadiaz said...

"Like I said, there are multiple track sites that indicate gregarious movement in dinosaurs as a whole. The trick is to prove they were moving together at the same time and that the site was not some kind of dinosaurian highway."

Don't get me wrong as I wasn't arguing 1 way or another (although I do agree w/you about gregariousness in sauropods). I was just surprised you didn't mention the Davenport Ranch track site, given how often it's mentioned when discussing gregariousness in sauropods.

"Horner and Makela's paper back in 1979 started it all and maintain its validity whilst work by Geist and Jones (1996), as an example, suggested otherwise."

Don't forget about Horner et al. 2001, which pretty much demolished Geist & Jones' 1996 argument (See "Opposed hypotheses": http://www.ucmp.berkeley.edu/science/eggshell/eggshell_case1.php ). In any case, Geist & Jones are known for "publishing with a hidden agenda" ( http://scienceblogs.com/tetrapodzoology/2009/07/17/birds-cannot-be-dinosaurs/ ).

-Herman Diaz

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