Wednesday, 17 August 2011

Hadrosaurs - Underrated Yet Spectacular (Part 1)

The International Hadrosaur Symposium takes place next month at the Royal Tyrrell Museum of Palaeontology in Alberta and is an eagerly anticipated event. It’s a busy time for the conference circus with the SVPCA in Lyme Regis running from the 12th to the 17th of September, the Hadrosaur Symposium on the 22nd to 23rd and, of course, the 71st SVP meeting is in LA from November 2nd to 5th.

There is going to be so much material and data floating around from these conferences that there will be no shortage of discussion, hotly contested debates and surprises for the palaeoworld to digest for the weeks and months that follow. The early word from SVPCA is that it will be an excellent event this year and SVP is always full of superb material but, I have to confess, that the Hadrosaur Symposium is the one I’m looking forward to more than most.

Hadrosaurs are fascinating dinosaurs. For me, they are some of the most interesting and underrated dinosaurs that have ever evolved. So they didn’t have sharp teeth, spikes or horns but, in the case of lambeosaurines, they have an amazing array of crests, grow extremely large and are extremely well represented in the fossil record with some species represented by a full ontogenetic life history from egg to adult.

Hadrosaurs can be defined by a number of unique traits and are split into two sub-families known as the Hadrosaurinae and Lambeosaurinae. For example, both groups have lost the first digit of the manus, the antitrochanter on the ilia is pronounced and robust for attachment of large muscle and the skulls display a suite of specialist characteristics.

Hadrosaurids have a worldwide distribution thus emphasising their spectacular success as a group. They are also recovered from various formations that demonstrate their adaptability to survive in different environments. These include, but are not restricted to, delta and flood plains and both lower and upper coastal plains.

In size, hadrosaurs varied enormously but the biggest such as Shantungosaurus were huge animals and were the size of the largest tyrannosaurids. Other recently recovered hadrosaurs from China are equally massive. Take a look here at Dave Hone’s recent images of Shantungosaurus – awesome animal, very impressive. Some recent work by Cooper (2008) again demonstrates that hadrosaurs grew extremely fast – at mammalian-like endothermic rates and attained adult size within 8 to 10 years.

But it is the superb dental arrangement that enabled the hadrosaurs to attain what was virtually global domination of the Late Cretaceous forests and flood plains. Vegetation was cropped by a highly efficient and rugose premaxilla that was covered by a horny sheath – or rhamphotheca to give it its proper name. This was then passed to the dental batteries for processing.


The dental battery of Edmontosaurus 

Hadrosaurs employed a unique method of chewing that was a remarkably efficient mechanism, grinding vegetation to pulp. Jaw movement was multi directional made possible by a hinge that was located between its upper jaws and the rest of its skull. This enabled the maxillae to be pushed outwards and sideways, while the dentary teeth ground against the upper maxillary teeth. This grinding action of tooth against tooth ground and shredded the vegetable matter before being swallowed for digestion. This process of oral mastication appears totally unique to hadrosaurs

Interestingly, this process seems to confirm that hadrosaurs had cheeks because if the maxillae hinged outward during chewing, it would seem likely that food would have simply fallen out of the mouth without them! It appears they may have looked a little like they were chewing the cud when eating but, of course, not using the same chewing arrangement as mammals do today. Not for nothing are they known as the cows of the Cretaceous (a rather unflattering description in my opinion).

The tails of hadrosaurs are remarkable feats of structural engineering. A latticework of ossified tendons stretched from the thorax to the anterior tail section and these were held in place by sections of soft tendon which, in tandem with the muscles from the trunk, provided rigidity and stiffness.

This enabled hadrosaurs to be able to walk bipedally by being able to hold the tail horizontal, anchored almost directly from the extremely strong sacral region. The ossification of tendons was an extremely efficient mode of support and acted like a form of cantilever bridge because it was this inflexibility that supported both the tail and torso when the animal moved.

Next time I’ll look at hadrosaurid crests and also discuss the old belief that they were semi-aquatic and how this theory just might be making a bit of a comeback.

Reference

Cooper, LN., Lee, AH., Taper, ML. & Horner, JR. (2008) Relative growth rates of predator and prey dinosaurs reflect effect of predation. Proc R Soc Lond B Biol Sci 275:2609–2615.



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