Bone histology suggests Hadrosaurs endured a polar winter

Fossil bones provide us with a wealth of information, which is just as well, since bone constitutes the overwhelming majority of the material that vertebrate palaeontologists are able utilise in their pursuit of trying to describe the animals of the past. Skeletal remains provide us with the basic body plan of the animal, and vertebrate evolution has provided us with a remarkably stable blueprint to follow from fish right through to Homo sapiens.

Mechanically the skeleton provides the support and scaffolding for the body and, of course, provides vital protection for the organs. Increasingly though, the actual make up of fossil bone is continually providing new data and on-going histological study appears to be maintaining this steady flow of new information.  Histological studies of fossil bones from dinosaurs first came to the fore in the 1970’s, with Armand de Riqlès leading the way, and his work was very much part of the Dinosaur Renaissance. 

Dinosaur bone is heavily vascularised and, since it is remarkably similar to extant mammalian bone in histological make up, it is fair to assume that bone received anything between ten and twenty per cent of the total blood supply pumped around the body by the cardiac system. Bone can also sometimes reveal whether the bone is from a juvenile or adult. Woven bone is a primary bone tissue that is temporary and coverts throughout ontogeny to lamellar and/or fibrolamellar bone and is indicative of maturity. This kind of data has been used extensively recently and particularly within the continuing ceratopsian ontogeny debates.

Bone is also remarkable in that, although you may think that it has stopped growing once maturity has been reached, it continues to develop in various ways and this is a process known as remodelling. Throughout adulthood remodelling is a continuous process as any damaged bone is repaired and can also be reinforced to cope with any additional stresses put upon the animal. The result of this remodelling is the pathologies that are frequently found on dinosaur bones and this is another area of intense interest in dinosaur palaeontology.

Darren Tanke, of the Royal Tyrrell Museum in Alberta,   is currently compiling masses of data on hadrosaur pathologies and some of the images of remodelling that have taken place have to be seen to be believed. In mammals, a remodelling episode may complete in around six months and it is likely that dinosaurian bone took around the same amount of time but, of course, this is purely conjecture.

Fibrolamellar bone is indicative of an animal whose bones need to grow fast and expand in diameter rapidly to cope with the stress of a large body. Large extant mammals of today also have fibrolamellar bone for the same reasons, for it can be laid down extremely rapidly by a complex process that utilises interwoven layers of both woven and lamellar bone.

However, fibrolamellar bone is not always consistent in its development and, in some cases, there are definite interruptions and these are known as lines of arrested growth (LAGS).  These LAGS are layers of bone that are indicative of what can be described as a slowing up of rapid development and are interpreted as a result of some form of environmental stress. Drought, environmental change, food shortage and migratory pressure have all been proposed as possible agents of growth arrest.

This brings us nicely to a recently published paper by Anusuya Chinsamy et al (2012)describing the bone histology of Edmontosaurus  bones from the Prince Creek Formation in northern Alaska and the Horseshoe Canyon Formation in Alberta. Both formations are Late Cretaceous and were temporally very close and were certainly contemporaneous for at least two million years.

The bones of the northern hadrosaur clearly display interrupted fibrolamellar growth whilst those of the southern type do not. However, in the northern type, these interruptions are not LAGS but rather a slowing down of bone deposition whilst the southern type displays no such interruption. This, therefore, poses a rather intriguing scenario.

Northern Edmontosaurus (left) displaying numerous growth interuptions whilst the southern type (right) does not.
From Chinsamy et al 2012.

 The authors pose the possibilities that the differences between the two hadrosaurs may be the result that both types were migratory, that Edmontosaurus may have included both migratory and non-migratory populations and, finally, that the northern type was, in fact, resident all year round. The first suggestion is unlikely since the southern type displays no interrupted fibrolamellar growth.

The second suggestion is a possibility since the northern type does indeed display histological differences that can be associated with migration. The third option is the authors preferred hypothesis since an overwintering scenario would demonstrate a histological sequence exactly as found in northern Edmontosaurus. They suggest that the darkness of a polar winter and a reduction in the quality of fodder that resulted would manifest themselves in differing bone textures.

Southern Edmontosaurus may display the odd signal in bone textures which are symptomatic of occasional stress, perhaps a food shortage, whereas the northern type displays a record of consistent, probably annual, events. The authors stress that histological evidence alone cannot substantiate the overwintering scenario but additional data does appear to support it. For example, a lot of the edmontosaur remains from the Alaskan North Slope are from juvenile animals that were almost certainly too small to migrate.

They also appear to have perished in floodwater events as result of the winter melt which is highly indicative that these animals had overwintered in the North. Further comparison with the polar dinosaur fauna of New Zealand also lends support to the overwintering scenario – for they too endured a polar darkness lasting for perhaps six months.  

Although this paper is short (barely 5 pages), it contains some nice data and adds more substance to the  growing belief that dinosaurs were well equipped to deal with the rigours of the cold and darkness that only a polar winter could throw at them and, again, is highly suggestive of endothermian dinosaurs as I have discussed here previously.

Incidentally, this obviously does not mean that all dinosaurs did not migrate – far from it. It does suggest, however, that not all dinosaurs did either. Besides, there were large numbers of herbivores, including hadrosaurs and ceratopsians, ploughing through enormous amounts of fodder each day and it is almost certain they would have to keep on the move to find new pasture on a pretty regular basis.

Finally, there is yet another histology paper that has just been published (Cerda & Chinsamy 2012) looking at the bone microstructure of the basal ornithopod  Gasparinisaura cincosaltensis , from the Late Cretaceous of Patagonia, which again may be indicative of either sexual dimorphism, suspension of plant growth or localised stressed conditions. Bone histology continues to provide significant information regarding dinosaurian physiology and the environments they frequented and looks like doing so for a long time to come.

References

Ignacio A. Cerda & Anusuya Chinsamy (2012) Biological implications of the bone microstructure of the Late Cretaceous Ornithopod Dinosaur Gasparinisaura cincosaltensis. Journal of Vertebrate Paleontology 32(2): 355-368 DOI:10.1080/02724634.2012.646804

Chinsamy, A., Thomas, D. B., Tumarkin-Deratzian, A. R. and Fiorillo, A. R. (2012) Hadrosaurs Were Perennial Polar Residents. The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology (advance online publication) doi: 10.1002/ar.22428