Thursday, 31 January 2013

Dinosaur Diversity Knew No Limits


 
Following on from my feathered tyrannosaurs post, it seems reasonable to consider how the incredible diversity of the dinosaurs may have led to many different ecological niches being filled. It would be churlish to think of dinosaurs as a group of animals that were only simplistic herbivorous or carnivorous animals and, because of today’s rapid improvements in research techniques and the ever increasing amounts of new dinosaur species announced each year, we really are starting to gain further insight into what was a truly remarkable and successful group of animals.
Of course, many dinosaurs were indeed humble herbivores being preyed upon by equally humble carnivores and yet these are not the taxa we are particularly interested in on this occasion. What specialist dinosaurs were there and how diverse were they? What ecological niches could some of them have filled?
One thing that many of the supposed specialist dinosaurs have in common is that they are all a little odd looking. Take the alvarezsaurid Mononykus for example – here is an animal only about a metre long which is best known for its ridiculous little forelimbs armed with only a single claw. The whole forelimb only amounted to about 75mm in total length and it has been speculated that this specialist forelimb may have been useful in breaking into termite mounds.
 
Mononykus by Thomas Cowart
 
I am not so sure about this. Surely it would have been more advantageous to have a longer and stronger forelimb to perform such actions? Check out the forelimbs of anteaters if you want to see proper equipment for dismantling ant hills and the like. I am not suggesting that the forelimbs of Mononykus did not have a specialist use but I am not convinced of their mound busting ability. More fossils will perhaps help to solve the riddle.
Mononykus is also described as having large eyes which have led some palaeontologist’s to suggest that they were nocturnal.  Troodontids and Leaellynasaura are also considered animals of the night due their enlarged eye sockets. This makes a lot of sense anyway and no ecosystem today is comprised of strictly day or night active creatures (cave dwellers not withstanding). Lions today feed almost exclusively at night contrary to their regular appearance on television documentaries as animals that feed during the day although, of course, they are not restricted and do indeed feed during the day.
A study published in 2011 (Schmitz & Motani 2011) looked at the eyes of extant birds and crocodiles and specifically concentrated on whether the sclerotic ring was a good indication of pupil size. Results were encouraging and indicated that, after careful examination of 33 dinosaur specimens, that dinosaurs were active throughout the 24 hour cycle including nocturnal, diurnal and both. For me this is completely what I expected from this successful and adaptable group of animals.
 In the study small theropods were largely nocturnal or day and night active whilst large herbivores were active 24/7 which again makes sense since they needed copious amounts of vegetation to fill their large guts. I suspect that many dinosaurs were active at night including some larger theropods which would clearly have taken the advantage of the dark to ambush unsuspecting prey. This is also indirect confirmation that dinosaurs were certainly more endothermic than ectothermic being able to keep active during the night as it were.
The recently published heterodontosaurid Pegomastax africanus is another bizarre dinosaur that may or may not have led a strange life. Paul Sereno described the animal as “…..a bird, a vampire and a porcupine.” I wish he had not mentioned the word  ”vampire” since the media, of course, latched onto this and we were deluged with headlines about the “vampire dinosaur”.
Pegomastax was recovered from the Lower Jurassic Elliot Formation in Cape Province, South Africa many years ago and remained hidden from view until Sereno rediscovered the remains in 1983. Pegomastax was another small dinosaur that was characterised by an unusual parrot-like predentary combined with two large caniniform teeth at the anterior of the dentaries which, in turn, combined with other tall teeth that cut through vegetation.
Pegomastax africanus
There is debate whether this peculiar arrangement may have also been an adaption for catching insects or eating meat but this would appear unlikely since the teeth of Pegomastax clearly show wear from grinding vegetation although it has often been suggested that heterodontosaurs, as a group, may have been omnivorous. It is also conceivable that the fangs may have also been used for intraspecific conflict and defence.
This extreme jaw development is  more likely to be a specialist feeding adaption and I suspect that all heterodontosaurids were variants based upon a similar evolutionary trait that were highly dependent on specific plants and foliage for their nourishment.  This is exactly what I see when I look at Pegomastax – it really does look like a niche species with a highly derived feeding technique.
Stomach contents can reveal a lot about an animal’s behaviour and ecology. A PLoS One paper last year highlighted two specimens of Sinocalliopteryx with preserved stomach contents which indicate that they fed on primitive birds and other feathered dinosaurs.
Sinocalliopteryx gigas was a compsognathid around the two metre length mark which was relatively large for one of these small theropods. Named in 2007, its remains were recovered from the Lower Cretaceous Jianshangou Beds of the Yixian Formation in Liaoning Province, China. According to Xing et al (2012), one specimen contains the remains of a dromaeosaurid (attributed to Sinornithosaurus) whilst the other specimen contains the multiple remnants of the primitive avialan Confuciusornis.
The authors, quite reasonably, suggest that two separate specimens of Sinocalliopteryx both containing similar prey may be indicative that these dinosaurs actively preyed on other feathered dinosaurs as well as flying primitive birds. This should not be a surprise really since a large proportion of animals in this biota were small and feathered and it figures that they would be actively hunting each other.
Of course, evidence of feeding is not necessarily evidence of hunting as other palaeontologists’ have already pointed out but this will always be the same counter argument whenever stomach contents of this nature are discovered and, indeed, holds true for bones that display marks of predation. I suggest that, since all available physical and physiological evidence leads to this conclusion, it is probably likely that Sinocalliopteryx was an agile and active hunter.
From Xing et al 2012
The authors also speculate that this could be the case and hypothesise that Sinocalliopteryx could have been an ambush predator, perhaps concealed in the underbrush waiting for its unsuspecting prey to fly unwittingly past before rapidly striking out and grasping the now doomed prey animal. And, of course, this leads to conjecture just what other niches were being filled by all these fascinating feathered dinosaurs.  
Other dinosaurs of note that were likely to be specialist animals include the therizinosaurs and the enigmatic Deinocheirus. Contrary to the previous animals we have been looking at, these dinosaurs were much larger taxa but both had similar adaptions in their forelimbs since they were large robust appendages armed with not insignificant claws.
Therizinosaurs were certainly unusual theropods by anyone’s standard. Their forelimbs, however, are enormous and were obviously of some significance to the animal. The most common theory to describe what these forelimbs were used for is tied in with the fact that therizinosaurs were herbivorous animals and that they used these large limbs to gather in vast tracts of foliage rather like a rake. This is quantified by their possession of a large “pot-belly” which would have  been a large fermenting vat to help digest the tough fibrous plant tissue.
This is interesting in a number of ways. Certainly I subscribe to the theory that therizinosaurs were essentially herbivorous but I am more inclined to think that they were actually more omnivorous than that. Those arms were very capable of tearing down termite mounds (as I alluded to earlier when mentioning anteaters) so I feel certain that insects perhaps may have figured on their diet on occasion. Equally those arms could have easily ripped into a rotting carcass so meat may have also figured on their menu. All this and they could have been used for defence as well – therizinosaurs were certainly specialist animals.
Deinocheirus mirificus is actually very well known in the palaeoworld mainly due to the fact that its remains consist of an enormous pair of forearms which measure around eight feet long. Just what kind of animal Deinocheirus was is still under scrutiny although latest phylogenetic analysis and hypotheses suggest that it is a rather primitive ornithomimosaur albeit extremely large.
 
If it is an ornithomimosaur then we perhaps find an example of parallel evolution with therizinosaurs. It appears generally accepted that ornithomimosaurs were also omnivorous and, that being the case, may have also evolved large forelimbs in Deinocheirus to gather in the various foodstuffs that were available. With the recent announcement of feathered ornithomimosaurs it seems likely that both genera were also feathered.
If this is the case then it would be no surprise that a largely herbivorous omnivore (that’s a new one) may have demanded large body size to sustain such a lifestyle. Within Saurischia, sauropods grew large very early in dinosaurian evolution and developed huge guts to cope with the vast amounts of fodder required to sustain their enormous bodies. Therizinosaurs and ornithomimosaurs are Cretaceous animals, and their origins are a little blurry, but it is interesting that these too evolved to be massive animals.
Deinocheirus really is a fascinating beast and you would love to think that it was a massive super-theropod of some sort if it was built to standard theropod proportions. If this were the case then it would be a theropod to defy the imagination. There has been more material recovered a few years ago but nothing has been published yet but do keep an eye out in Spring of this year when there will likely be news on Deinocheirus that will cause quite a stir!
There must be many dinosaurian taxa yet to be found and it is fascinating to wonder just how many of those will be found with some form of bizarre evolutionary trait and trying to figure out what on earth it was used for.
References
Schmitz & Motani. 2011. Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology. Science http://dx.doi.org/10.1126/science.1200043
Sereno P (2012) Taxonomy, morphology, masticatory function and phylogeny of heterodontosaurid dinosaurs. ZooKeys 226 : 1–225, doi: 10.3897/zookeys.223.2840.
Xing L, Bell PR, Persons WS IV, Ji S, Miyashita T, et al. (2012) Abdominal Contents from Two Large Early Cretaceous Compsognathids (Dinosauria: Theropoda) Demonstrate Feeding on Confuciusornithids and Dromaeosaurids. PLoS ONE 7(8): e44012. doi:10.1371/journal.pone.0044012

6 comments:

Anonymous said...

Thanks Mark.

There's only one way the Deinocheirus mystery could pan-out that would satisfy me. ;-)


Paul W.

Mark Wildman said...

What would be really interesting was if Deinocheirus is NOT an ornithomimosaur. Now that really would rattle the phylogenetic status quo.

Anonymous said...

"I am not so sure about this. Surely it would have been more advantageous to have a longer and stronger forelimb to perform such actions?"

Actually, based on what I've read (Senter 2005: http://www.jstor.org/discover/10.2307/4096941?uid=3739696&uid=2129&uid=2&uid=70&uid=4&uid=3739256&sid=21101796891267 ), the forelimbs of Mononykus are consistent w/the hook-&-pull digging of anteaters.

"There has been more material recovered a few years ago but nothing has been published yet but do keep an eye out in Spring of this year when there will likely be news on Deinocheirus that will cause quite a stir!"

Do you know what the news is or just that there is news? Just curious.

BTW, I'm surprised you didn't mention spinosaurs or unenlagiines. They're some of the 1st dinos to come to mind when I think of specialists. Otherwise, good post (as usual).

-HD

Mark Wildman said...

Hi HD and thanks for your recent comments. I have to disagree with Senter's observations. Biomechanically it does not make any evolutionary sense at all to evolve a highly reduced single digit forelimb to break into a termite mound or ant hill.

But it is obvious that the forelimb of Mononykus was certainly an unusal adaption - far more so, in my opinion, than the reduced forelimbs of tyrannosaurids.

The Deinocheirus issue is very interesting and there are certainly more revelations to come. Whether they will be published during the Spring remains to be seen - publication is notoriously unpredictable. I think I would like to leave it at that for now but I, like every other palaeotype, am busting to know what revelations will be made.

The point of the post was really to make one think about the dinosaurs diversity as a whole and there are probably many other examples of specialists out there - including Spinosaurus. As I said in the post, the fascinating part of all this is how many new specialists there must be still out there to find - it must be mind boggling.

Anonymous said...

"Biomechanically it does not make any evolutionary sense at all to evolve a highly reduced single digit forelimb to break into a termite mound or ant hill."

Sorry to disagree w/you again, but based on what I've read (E.g. http://dml.cmnh.org/2004Dec/msg00118.html ), it does make sense in this case. Besides, as others have pointed out ( E.g. http://dml.cmnh.org/2004Dec/msg00115.html ), Tamandua & Cyclopes have especially short & "mononykine-like forelimbs". Good discussion, though.

-HD

Mark Wildman said...

And yet when you look at the balance of Mononykus it would more or less have to hug a mound to start scratching and digging and even then it would not be able to see what it was doing.

I am not saying I am right but, any way you look at it, it is a bizarre unhelpful adaption for the proposed lifestyle.

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