Wednesday, 7 September 2011

Hadrosaurs - Underrated Yet Spectacular (Part 2)

Back to hadrosaurs then and a closer look at some more interesting points of note about them – especially the lambeosaurines. As mentioned previously, hadrosaurs are split into two sub-families – the Hadrosaurinae and Lambeosaurinae. Hadrosaurines are usually diagnosed by a laterally broad edged premaxillary process that defines the classic “duck bill”; the nasal arches are not indented and some of the more derived taxa have crests that are solid.

Lambeosaurines are diagnosed by vertebrae that have longer neural processes than hadrosaurines; the ischium terminates distally with a flared process and the large nasal cavity often forms crests that are both spectacular and hollow. It is these crests that were once associated with the theory that hadrosaurs were semi-aquatic.

Semi-aquatic dinosaurs have been back in the news again recently (Ford 2010) and hadrosaurids were once considered amphibious, a theory first considered by Leidy way back in 1858 and supported by Cope in 1883. Indeed, growing up with dinosaurs, I was very familiar with hadrosaurs portrayed as aquatic animals and there was hardly any picture of them that didn’t show them as swimming and wading in the Late Cretaceous swamps or fleeing into the water to escape the jaws of an approaching tyrannosaur.

To be fair, at the time, the interpretation of the evidence seemed reasonable. The beak of hadrosaurs was thought to be weak and only suitable for cropping soft vegetation such as that which is found growing in or next to water. The tail, because it was wide, scull-like and obviously strong was imagined to be a powerful propeller which enabled the hadrosaurs to swim very effectively and then, when the famous “Trachodon” mummy was found in 1908, not only were significant amounts of skin preserved, but skin found preserved on the manus appeared to represent webbing situated between the digits, thus appearing to form a kind of flipper. This interpretation of hadrosaurs as semi-aquatic dinosaurs remained intact for many a year.


Image courtesy of Tommy Bradley ©


Since those early theories, we now know that hadrosaurs were highly developed terrestrial bipeds able to alternate between both quadrapedal and bipedal locomotion, depending on what the situation dictated at the time. The tail, as described in the previous post, was consolidated along the dorsal and caudal vertebrae by ossified tendons which stiffened the tail considerably and, despite still being immensely powerful, was not suitable for aquatic propulsion.

It also turns out that that horny beak of hadrosaurs was anything but weak and was a highly efficient cropping tool that comfortably cut through plant material ready for transferral to the dental batteries. A reassessment of the manus also reveals unguals that were obviously performing the same function as hooves, seen in a multitude of extant animals today – an exclusively terrestrial adaption.

Finally, the webbing on the dinosaur mummy that seemed to so support a semi-aquatic lifestyle proved to be a bit of a red herring since closer study revealed the webbing to be highly constricted between the digits and they could not be spread far apart enough to form a sculling paddle anyway. Other taphonomic processes had also distorted the preservation process causing interpretation of some skin remnants to be incorrect.

Despite this, there are still some voices suggesting that perhaps we should not totally discount the possibility of semi-aquatic hadrosaurs. It seems likely that they may indeed have been partial to the odd paddle especially since they seem to have spent so much of their time in both freshwater and coastal habitats.

Interestingly, it was thought that the crests of hadrosaurs, and lambeosaurines in particular, were also part of the adaption to an aquatic life. Because many of the crests were hollow, it was deemed that they could be used like a snorkel or, at the very least, act as an oxygen tank to enable the animal to dive under water. However, the nostrils of hadrosaurs are located anteriorly on the skull and positioned low and are certainly of no use to a snorkelling animal. And as for acting as an oxygen reservoir, well the amount of air that could be retained in the crests was so small as of to be no help to an animal weighing perhaps three and a half tons with big lungs. No, there are better explanations to describe the functionality of crests in hadrosaurs.

One theory is that they were a cooling mechanism to help dispel excess heat around the brain. Respiratory turbinates in dinosaurs have been considered intensely over the last few years and whether they were, indeed, part of dinosaur physiology. It is possible that the crests created more surface area within the nasal network, thus being able to dispel a greater amount of heat because the amount of blood that was being cooled by air during breathing would be greater.

The problem with this theory is that it is awfully hard to find physical evidence for respiratory turbinates on fossil bone although a recent specimen seems to suggest that there may have been (but for the life of me I cannot find the reference). Hadrosaurines may also have been able to utilise the same utility since they also have large nasal cavities.

Image courtesy of Tommy Bradley ©

Another suggestion was that the crests enhanced the sense of smell, again due to the greater surface area of the crest that was infused with olfactory epithelium. However, further research and comparisons with the nasal chambers of extant reptiles suggest this was most unlikely and that the crests played no part in olfactory sensory ability (Evans 2006).

So it seems likely that the crests had other functions and now appears likely that they were an essential part of the complex social behaviour in hadrosaurs. The variation in size and shape suggests that some crests are probably sexually dimorphic and specimens of Lambeosaurus, Corythosaurus and others have been identified as probably being male or female (Evans 2006). It is also likely that the crests were highly coloured which may have helped to identify sexually mature animals and also to ward off potential rivals when competing for mates or territory.

Of course, low frequency sound, when amplified by a suitable resonating chamber, has the ability to travel long distances and the hollow crests of hadrosaurs were well suited to the task. Some very impressive experiments with the skull of Parasaurolophus have produced some startling, almost imperceptive sounds that would have undoubtedly aided intraspecific communication (Diegert 1998).


Image courtesy of Aquaimages

Although hadrosaurines generally lacked raised cranial ornamentation, they, never the less, may have possessed a resonating sac that was located in the deep depression that surrounded the external nares. This could also have been brightly coloured and the combination of sound and inflated air sac would have made for an impressive display – something similar to the frigatebird of today.

So it can be seen that hadrosaurs provide the best overall window into the complex social structure and interaction of any group of dinosaurs. Nesting sites, the amazing array of crests, the production of sounds and the probable use of colour all demonstrate that these amazing animals were complex animals with a surprisingly intricate infrastructure culminating in the remarkable nursery care of the young hatchlings. Cows of the Cretaceous? Don’t believe a word of it – hadrosaurs are uber-cool.


References

Cope, E.D. 1883. On the characters of the skull in the Hadrosauridae. Proceedings of the Academy of Natural Sciences of Philadelphia 35:97–107.

Diegert, Carl F.; and Williamson, Thomas E. (1998). A digital acoustic model of the lambeosaurine hadrosaur Parasaurolophus tubicen. Journal of Vertebrate Paleontology 18 (3, Suppl.): 38A.

Evans, David C. (2006). Nasal cavity homologies and cranial crest function in lambeosaurine dinosaurs. Paleobiology 32 (1): 109–125.

Ford, Tracy L.; and Martin, Larry D. (2010). A semi-aquatic life habit for Psittacosaurus. In Ryan, Michael J.; Chinnery-Allgeier, Brenda J.; and Eberth, David A. (editors.). New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium. Bloomington and Indianapolis: Indiana University Press. pp. 328–339. ISBN 978-0-253-35358-0.

Leidy, J. 1858. Hadrosaurus foulkii, a new saurian from the Cretaceous of New Jersey, related to Iguanodon. Proceedings of the Academy of Natural Sciences of Philadelphia 10: 213–218.

2 comments:

Albertonykus said...

Cattle aren't awesome now? http://blogs.scientificamerican.com/tetrapod-zoology/2011/09/08/frankly-cattle-are-awesome/

Okay, they are synapsids after all...

Mark Wildman said...

heh heh! Well they are awesome in a extant mammalian kind of way but give me a hadrosaur any day of the week!

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