Friday, 24 February 2012

Marsh's "Roofed Lizard"


When I was growing up, and the dinosaur bug had well and truly embedded itself into my system, there was only one purpose to explain the dermal plates of Stegosaurus – they were purely a defensive shield to help protect the animal from its arch enemy, Allosaurus. As a kid this all made sense to me and many a restoration of Stegosaurus was constructed in this fashion. The dinosaur with a brain the size of a walnut, who had a second “brain” in his hip region to help him even exist, needed all the help he could get not to be eaten.
Things have moved on somewhat over the years but Stegosaurus remains an iconic and fascinating dinosaur and the function of the plates is still the subject of intense study. The animal was discovered and named in 1877 by O.C. Marsh who originally thought that the plates covered the back of the animal in an overlapping fashion hence the name – “roofed lizard”.
At the time this was a reasonable assumption and, even as recent as 1975, Halstead also ascribed to this theory. However it soon became obvious, as more specimens were recovered, that the plates were very much vertically positioned and situated over the back in two parallel rows. It was also considered that the plates may have been positioned in one single continuous row but, as the plates came under closer scrutiny, it was apparent that they were likely to be covered in some form of sheath, which would have prevented this formation and may have actually forced the staggered alignment we recognise today.
Nowadays, most palaeontologists accept the now general perception of Stegosaurus with the two parallel rows of plates along the back embedded into the skin. So what about the function of these plates?  Well, as I’ve already mentioned, defence was amongst the first theories that was suggested and this was universally accepted for many years.  However, the plates do not really offer any significant protection when you think about it. The wide flanks of the stegosaur are still fully exposed and even the head and neck are still vulnerable.
That interaction between Stegosaurus and Allosaurus actually occurred is not doubted since there are stegosaur bones displaying bite marks and it must be noted that there is at least one plate with allosaurs bite marks (Carpenter et al 2005). Perhaps the most contradictory evidence against the plates- as-defence theory is the fact that they appear to be of a relatively weak construction as indicated by their histological structure and it would seem likely that a defensive plate would have been much sturdier and constructed of solid bone.
It was James Farlow et al (1976) who suggested an alternative theory about the plates  after the first histological studies revealed that they were permeated by a vast vascular network, both inside and out,  leading to the first suggestions that the plates may have served a thermoregulatory role. The analogy used was that of an elephant’s ear which does indeed perform such a function. However, further studies (eg Main et al 2005) show that the blood vessels on the exterior of the plate do not lead anywhere particular and that the apparently large blood pipes, in the base of the plates, did not appear to supply the volume of blood required for the plates to form a heat exchange system as postulated (Buffrénil et al 1986).
Perhaps the heavily vascularised plates were used for display and intraspecific purposes, just as the heavily ornamented frills of ceratopsians are generally believed to be nowadays.  The plates may have been flushed with blood to attract a mate or, equally, to warn off predators and competitors. However, there is no evidence of sexual dimorphism to back up the attraction theory since animals do tend to be dimorphic when such display structures are utilised and, although this may be changing (eg Redelstorrf and Sander 2009), there is a persistent lack of evidence for dimorphism in Stegosaurus. The defensive “flushing” may warrant consideration, however, and all those blood vessels in the plates must have been doing something.
Perhaps the plates were multi-functional performing a combination of tasks. It’s interesting  that, although the plates are not strong enough as a physical barrier to attack by theropods, they still may have acted as form of deterrent since an adult Stegosaurus, with heavily coloured plates and a swinging tail would have have been an extremely imposing sight. The same technique may have been used for attracting a mate and sorting out territorial disputes – there are so many variables.
It is still possible that the plates may have indeed performed a thermoregulatory function and Farlow et al (2010), using continually improving and sophisticated sectioning and CT scanning, have again suggested  so. Hayashi et al (2012) also agree with these conclusions although still favour display as the primary function. In the end it may simply be something as simple as species recognition.
References

Buffrénil, V. de, J. O. Farlow, and A. J. de Ricqlès. 1986. Growth and function of Stegosaurus plates: Evidence from bone histology. Paleobiology 12:459473.
Carpenter, K. 1998. Armor of Stegosaurus stenops, and the taphonomic history of a new specimen from Garden Park. Colorado. Modern Geology 23:127144.

Carpenter, K., F. Sanders, L. McWhinney, and L. Wood,. 2005. Evidence for predator-prey relationships: Example for Allosaurus and Stegosaurus. Pp. 325-350 in Carpenter, K. (ed.) The Carnivorous Dinosaurs. Indiana University Press, Bloomington.
Farlow, J. O, C. V. Thompson , and D. E. Rosner. 1976. Plates of the dinosaur Stegosaurus: forced convection heat loss fins? Science 192 (4244):11231125.
Farlow, J. O, Hayashi, S. and Tattersall, G. 2010. Internal vascularity of the dermal plates of Stegosaurus (Ornithischia: Thyreophora). Swiss Journal of Geosciences, 103, 173–185. 

Halstead, L. B. 1975. The Evolution and Ecology of the Dinosaurs. Peter Lowe, London. 

Main, R. P., A. J. de Ricqlès, J. R. Horner, and K. Padian. 2005. The evolution and function of thyreophoran dinosaur scutes: implications for plate function in stegosaurs. Paleobiology 31:291314.
Redelstorff, R., & Sander, P. M. (2009). Long and girdle bone histology of Stegosaurus: implications for growth and life history. Journal of Vertebrate Paleontology, 29, 1087–1099.

Thursday, 16 February 2012

Is there such a thing as a predator trap?



Tucked away in a suburb of Los Angeles, amongst the sprawl of a modern concrete and steel infrastructure, is a link with North America’s prehistoric past. Rancho la Brea is one of the most famous fossil locations in the world – and with good reason. An astonishing one million bones have been recovered from the tar pits since 1906 representing an incredible 231 vertebrate species (“Brea” is Spanish for tar). The age of these specimens’ span thousands of years but the oldest specimens can be carbon dated to around the fifty thousand year old mark with a margin of error of around five thousand years either way.

Carnivores make up the vast majority of animals recovered. Over 4000 specimens of dire wolves, 2000 specimens of sabre-toothed cats head the carnivorous throng and there are also bears, foxes, coyotes, weasels, badgers and, of course, Panthera atrox – the American lion. The most obvious question is why did so many carnivores become trapped in the tar?

Rancho la Brea is generally considered to be a predator trap. Unsuspecting animals such as mastodons, elephants, horses, bison and antelope may have become mired in the tar pools and their cries of distress, in turn, would have attracted predators eager for an easy meal but actually found themselves to also be trapped in the sticky tar. An interesting aside to this is that a lot of the carnivores are very young, very old or crippled and the sabre-tooths, in particular, often display chronic pathologies such as fused vertebrae around the pelvic region. This could be interpreted that such injured, old or inexperienced animals were drawn to the apparent easy pickings of the tar pools  with often fatal consequences.

Although a predator trap makes perfect sense in this instance, there is still no hard and fast evidence to back it up. And yet all the circumstantial evidence screams that it is a trap and, for me, it is almost certainly the most sensible explanation (I use the present tense because even today the tar is still claiming unsuspecting victims). Possible predator traps are not confined to recent times, however, and they have often been suggested to explain concentrations of carnivores in the Mesozoic. Chief amongst these is the Cleveland-Lloyd Dinosaur Quarry (CLDQ).  

The CLDQ is located 30 miles south of Price, Utah and was first worked in 1929 by a field crew from the University of Utah. The quarry has been incredibly productive ever since and over 12,000 bones have been recovered and, like Ranch la Brea, the predominant fossils recovered are those of carnivores – especially Allosaurus fragilis. Over 70% of all the bones here are from this one taxon alone (Foster 2007) and Gates (2005) suggests at least 46 individual allosaurs are represented although that number is sure to increase (if it has not already done so).

Other carnivorous taxa represented include Ceratosaurus, Torvosaurus, Marshosaurus and Stokesosaurus, as well as multiple species of sauropods, Stegosaurus and Camptosaurus highlighting what a productive, and very important, quarry the CLDQ is. The same question can be asked here as was asked about Ranch la Brea – why is there such a high percentage of predators recovered from this quarry?
The initial thoughts, again, were that animals became mired, this time in mud at a dwindling water hole, attracting large numbers of carnivores who in turn became, themselves, trapped. Of course this appears the most obvious explanation and, coincidentally, around 80% of the Allosaurus individuals are juveniles, again mirroring a lot of the fauna at Ranch la Brea. Unfortunately, these things are never that simple.

The bones are found in a mudstone of the Brushy Basin Member of the Morrison Formation and are indicative of a floodplain ephemeral-pond deposition. They are found in a horizontal or semi-horizontal position which is of interest since similar quarries, such as the Howe Quarry, have exposed limb bones in an upright position which is highly suggestive of a miring scenario.

Another proposal to explain the unusually high proportion of predators in the quarry is that it was a drought-induced assemblage – a theory proposed by Terry Gates (2005).  At some water holes today, during times of drought, it is a documented fact that, in Africa, predators have been known to prevail over the dwindling water source safe in the knowledge that any herbivore approaching the hole for a drink was almost certainly desperate, near death or just plain mad.

There is a possibility that the allosaurs may have behaved in a similar fashion. Like today’s extant predators, they may have been reluctant to leave their only source of water and, if the prey animals had all been consumed in the immediate area, the carnivores would certainly have been susceptible to starvation or disease. This could also account for the high percentage of juveniles in the quarry since they would have been more vulnerable.

Similarly, there is the suggestion that animals that died around the dwindling water hole would have been scavenged by the predators who in turn would have succumbed to disease, especially botulism, since rotting carcasses are full of such nasties. In such a scenario, large numbers of allosaurs could have easily died (eg see Varricchio 1995).

Of course there are counter arguments. It is very difficult to recognise drought and disease in the fossil record. There is the thought that, in a situation such as this, that carcasses may have been spread over a much wider area – it’s hard to believe that so many animals would congregate in a relatively small area and there are sauropod remains in this quarry. But if it was a dwindling water source then why not?

Other things of note are predation marks on the bones – not only on prey animals but on the allosaur bones themselves. Foster (2007) also reports that many foot bones are chewed by carnivores but this is not so easily explained. So there are still many questions to be answered about the CLDQ regardless of what theory you subscribe to. Of course there are other examples of quarries with a high predator prey ratio such as the Albertosaurus bone bed (featured many times in this blog) and Ghost Ranch but they too attract comparable discussion and their taphonomic origins remain debatable.

References 

Foster, J.R. 2007. Jurassic West: The Dinosaurs of the Morrison Formation and Their World: pp 93-95. Indiana University Press, Bloomington, Indiana.

Gates, T.A. 2005. The Late Jurassic Cleveland-Lloyd Dinosaur Quarry as a drought-induced assemblage. Palaios 20:363-375.

Varricchio, D.J. 1995. Taphonomy of Jack's Birthday Site, a diverse dinosaur bonebed from the Upper Cretaceous Two Medicine Formation of Montana. Palaeogeography, Palaeoclimatology, Palaeoecology 114:297-323

Thursday, 9 February 2012

To Chew or not to Chew?


In my recent post about Daspletosaurus I made reference, in my imaginary scenario, that tyrannosaurs were almost certain to swallow their food whole once they had torn away the flesh and bone from a carcass. Tyrannosaurs obviously did not chew and so this is a perfectly logical assumption but hadrosaurs, on the other hand, did indeed utilise a form of chewing with their unique jaw hinge arrangement and their impressive array of grinding teeth that lined the jaws.

Sauropods, of course, present another conundrum. I mentioned in my previous post that sauropods were almost certainly perpetual eaters and it has always fascinated how so small a head, mounted on a long neck, was capable of consuming enough food to maintain their giant bodies. Well, like tyrannosaurs, it appears that sauropods also swallowed without chewing and, since so much fodder would have been required, then this simple action would have been a necessity.
This actually makes a lot of sense since, simply put, large bodies demand large amounts of food and it seems a reasonable assumption that, since sauropods survived until the very end of the Cretaceous, they had obviously developed a very efficient feeding and digestive system.
Hadrosaurs were also big animals and some, such as Shantungosaurus, grew to around fifty feet in length but these animals employed a much more complex chewing and swallowing technique. They were able to process vegetation into much smaller bits of matter; indeed the process of cutting the plants with the horny beak began the process before the dental battery more or less ground the fodder to pulp. This processed mulch was then broken down in the gut much quicker and easier than if it had been swallowed whole.
To generate the force needed to operate the hadrosaur jaw mechanism required significant power and the skull was quite robust and muscled accordingly.  Sauropod skulls, on the other hand, were small and kinetically weak.  Sauropods were so big that they did not have the time to utilise such a process like that used by the hadrosaurs and this is the primary reason, I suspect, why hadrosaurs had virtually reached their size limit with animals like Shantungosaurus 
What sauropods did have, however, was a remarkably long neck and this appears to have enabled the animals to consume vast amounts of fodder in various positions and angles without actually having to move. Physiologically this all makes good sense. If you need to consume copious amounts of vegetable matter, you are better off with a very simple head that swallows matter whole, utilising minimal effort with only the neck moving whilst the body remained virtually motionless. Perfect.
Also of interest is the fact that a lot of Mesozoic flora actually appears to be very nutritious. You may recall during my SVP recap that a recent reappraisal of the Jurassic flora  found in the Morrison Formation and a nutritional analysis of their equivalent extant varieties reveal that many species of plant were very nutritious and ideal for the sauropods (Gee 2011). Horsetails were one of these likely to have been consumed and today’s examples are very tough and fibrous but if you are swallowing your food whole then this does not represent a problem (Sander et al 2011). Plants such as horsetails would also have been tough on teeth, and recovered sauropod teeth are often heavily worn, but they would have been replaced frequently so this would not have been an issue. 
Of course, large amounts of fodder require a large stomach for digestion and the vat of gastric juices, comprising of billions of bacterial microbes needed to break down such tough fibrous plants, would have been very large indeed. It was thought that this may have been supplemented by a stone filled gizzard to help the digestive process, something akin to birds that utilise grit in the same way, but, contrary to popular belief, there is no hard evidence to support this theory (Wings & Sander 2007). In any event the process of digestion would have taken a long time but this would have not caused a problem for sauropods since the sheer size of the gut alone would have virtually ensured a continual release of energy to the animal.
Also of use to sauropods were their unique skeletal pneumaticity and an extremely sophisticated avian-like respiratory system. The cervical vertebrae were full of air sacs that ran in tandem with what was obviously a sophisticated series of valves that controlled and regulated both blood flow and air exchange. So not only was the neck lightened considerably, which aided the feeding process, but they were also able to breathe much more efficiently and this also made the task of supporting their entire body structure much easier. In fact this has also just been alluded to in a very recently published paper (Sookias 2012).
There are obviously various feeding techniques in dinosaurs that are yet to be fully determined and perhaps oviraptorosaurs, ornithomimids and therizinosaurs represent those that are amongst the most fascinating and challenging to describe fully.
References
Gee, C. 2011. Sauropod Herbivory During Late Jurassic Times: New Evidence for Conifer-Dominated Vegetation in the Morrison Formation in the Western Interior of North America. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp115.
Sander, P. M., Christian, A., Clauss, M., Fechner, R., Gee, C. T., Griebeler, E.-M., Gunga, H.-C., Hummel, J., Mallison, H., Perry, S. F., Preuschoft, H., Rauhut, O. W. M., Remes, K., Tütken, T., Wings, O. and Witzel, U. (2011), Biology of the sauropod dinosaurs: the evolution of gigantism. Biological Reviews, 86: 117–155. doi: 10.1111/j.1469-185X.2010.00137.x
Sookias, R.B., Richard J. Butler, and Roger B. J. Benson  2012 Rise of dinosaurs reveals major body-size transitions are driven by passive processes of trait evolution Proc R Soc B 2012 : rspb.2011.2441v1-rspb20112441.
 Wings, O & P. Martin Sander 2007. No gastric mill in sauropod dinosaurs: new evidence from analysis of gastrolith mass and function in ostriches Proc Biol Sci. 2007 March 7; 274(1610): 635–640. Published online 2006 December 19. doi: 10.1098/rspb.2006.3763

Thursday, 2 February 2012

Dinosaur Conundrums - How did Dinosaurs sleep?


One of the best aspects of palaeontology is that no matter what we think we know there is always so much that we don’t. Dinosaur palaeontology is often dominated by cladistics and phylogenetic analysis and this is quite understandable but, for me, how these animals lived and behaved is much more interesting.


Image © by Jeff Bucchino, "The Wizard of Draws"


How did dinosaurs sleep? Come to think of it, did they actually sleep at all? The first place to start is with the extant phylogenetic bracket (EPB) which, as most of you are aware, places dinosaurs between crocodiles and birds. Crocodiles do sleep, not in the traditional mammalian sense, but in a rather disjointed way by which they tend to grab short sessions of sleep but without falling into a deep slumber. They sleep with one eye open, so to speak, and are able to be instantly awake at the slightest disturbance.
These short sleeping sessions usually occur after bouts of activity and especially after eating although, being poikilothermic, this may not actually be that often although crocodiles also use these short sessions to shut down non-essential biological functions to aid digestion. The other thing to take into account with crocodiles is that, if they were to fall into a deep sleep, they would lose control of their body temperature which the animal regulates by moving in and out of the sun and water to adjust its body temperature. Deep sleep could be potentially fatal to any poikilotherm.
We all know that birds sleep as anybody who has kept a budgerigar or parrot will testify. And yet birds too sleep somewhat lightly but for different reasons. Crocodiles are big apex predators which are able to rest comfortably in the knowledge that it is extremely unlikely that anything will bother them. For birds, the issue of sleep is much more life threatening and for a bird to be caught off guard and in a deep sleep could be disastrous.
Birds employ a number of strategies when it comes to sleeping which are employed to negate the possibility of being predated when at rest. Some birds sleep on the ground using a combination of camouflage and ground cover for protection whilst some water birds will find refuge on predator free islands or, indeed, sleep on the water itself. By far the most common strategy, which includes birds that spend the majority of their time on the ground, is to take cover in trees and shrubs. This prevents the vast majority of ground predators from reaching them and, even if they could climb, chances are that vibration and noise would warn the bird of impending danger.
Safety in numbers is another solid technique for avoiding predation and some roosts may number hundreds, if not thousands, of birds. Interestingly though, despite all of these anti-predator measures, birds, like their crocodilian cousins tend not to sleep very long with an average length of two and a half minutes in some phases while other sessions will be counted in the seconds. Also of note is that birds, particularly those that sleep or perch standing up, manage to keep their muscles taught when asleep and flexor tendons ensure that the toes lock around the perch thus preventing the bird from falling.
So did dinosaurs sleep and, if they did, how? By way of inference, utilising the EPB, then it is almost a certainty that they did sleep but just how this occurred must remain a matter of conjecture but we can make a few educated guesses. Theropods ranged in size from very small to very large and would have employed various resting and sleeping techniques. Since nearly everyone agrees that most dinosaurs were much more than ectothermic, and probably nearer endothermic, then it follows that theropods spent a lot of their time resting or sleeping since hunting and feeding would take up only a small proportion of their time.
Small theropods may have slept in a squatting position, similar to ground dwelling birds of today, perhaps with the head tucked in against the body. Indeed, the high profile discovery Mei long, from Early Cretaceous sediments in Liaoning Province in China, was found in just such a position and the name translates as “soundly sleeping dragon”. Although Mei represents the best evidence of an avian rest posture to date, it is not the only example and a specimen of Saurornithoides recovered in the early nineties from Mongolia was found preserved in a similar posture. We must remind ourselves, however, that this is still an untestable assumption of an avian sleep posture and that these animals may have simply died in this position.
Mei long - Image by Bruce McAdam
It is much easier to imagine that small theropods would have been able to rest and sleep very comfortably and would have been able to regain their footing with some ease. But what do you do if you a big multi-tonned predator such as a spinosaur or tyrannosaur? In theory, there is no reason to suggest that a big theropod could not sleep or rest in the same way that Mei may have done. But it is harder for us to imagine animals approaching 40 feet long (and more) being tucked up in this position and yet – why not?
Could they sleep standing up? It’s hard to comprehend that a big tyrannosaur weighing a few tons would risk falling over because it fell asleep whilst standing up! But, as we have seen, extant birds do possess the mechanisms that prevent just such an accident from occurring so it is not beyond the realms of possibility that perhaps theropods had similar devices that prevented this.
Tyrannosaurus, of course, has been studied intensely since it was first discovered and has had its possible resting positions looked at too. The focus of study centres on the fact that the pubic boot could be utilised to take the weight of the animal as it rested which still left the legs able to assume multiple positions including kneeling, crouching and, of course, standing. Worth popping over to Dinomorph for a fuller explanation.
Whether all large theropods may have utilised the pubic boot is unclear but seems unlikely to me. You would imagine that pubic boots would show signs of stress if being rested on for any amount of time by tons of flesh and bone and yet I am unaware of any study that may have looked into this. Maybe we are complicating things and large theropods were quite capable of lying down to rest and sleep like most extant animals of today. Perhaps it’s time similar studies were performed on other large theropods, apart from Tyrannosaurus, and comparisons made.
Of course, if you have four legs, you are much more stable and are less vulnerable to falling down anyway but what happens if you fall asleep. Taking sauropods, by way of example, they obviously needed copious amounts of food throughout their lives and probably ate almost continually. They are the most likely candidates for sleeping whilst standing up and, again, using birds as the basis here, maybe they had a similar arrangement of tendons that kept everything rigid whilst they slept although there is not a shred of evidence to suggest they did. It’s worth pointing out that today’s mammals that sleep upright also possess a similar mechanism but, although this bears no relevance to sauropods or other dinosaurs, is indicative that any creature that slept upright would almost certainly require counter-measures to prevent a fall.
Perhaps, then, sauropods were able to kneel  or lay down and this is where the human mind can struggle to comprehend such actions since an animal approaching a hundred feet long, weighing upwards of seventy tons, being able to lay down, sleep and get up again almost defies imagination. And yet there is no reason that this should be the case and we have to be constantly aware that Nature is extraordinarily adept at providing animals with the ways and means to exist – and this would include both rest and sleep.
Ultimately, understanding how dinosaurs rested and slept will always be an inferance based on speculation. It’s worth noting that both birds and crocodiles do not spend a lot of their time asleep anyway and it’s entirely possible that dinosaurs only required similar limited amounts of both rest and sleep. As usual we are left with more questions than answers, more dichotomies than you shake a fist at and haven’t even thought about how pterosaurs may have slept either!
From Milner et al 2009

References

Milner, ARC, Harris, JD, Lockley, MG, Kirkland, JI, Matthews, NA (2009) Bird-like anatomy, posture, and behaviour revealed by an Early Jurassic theropod dinosaur resting trace. PLoS One 4 (3): e4591. doi:10.1371/journal.pone. 0004591

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