Sunday, 25 November 2012

SVP 2012 - Theropods Part 1


 
There is no doubt that theropods are probably the most popular dinosaur group of all and, knowing most of you like I do, then it is the theropods that are of the greatest interest whenever there is new research and information revealed. So without further ado, we can proceed. I always feel that ornithomimosaurs do not get as much attention as they deserve but they are bang in the news at the moment because of the newly revealed feathered ornithomimosaurs from Alberta and they also received some attention at SVP this year.
Ornithomimosaurs have long be known as “ostrich mimics” but how true a comparison is this with today’s extant palaeognaths – especially when it comes to the crania? By digitally reconstructing three ornithomimosaur skulls (Garudimimus, Struthiomimus and Ornithomimus), Andrew Cuff and Emily Rayfield, both of the University of Bristol, have produced some interesting results.
By utilising a combination of techniques including virtual muscle attachment data, adding hypothetical beaks to the rostra and studying finite element models and then, by comparing them to both ostrich and other theropods, has indicated that the sutures of the skull were key in minimising pressure to the skull. The beaks too also help in reducing strain and the different morphologies, especially of those beaks that are more extensive, help compensate for the loads.
More derived ornithomimosaurs reveal lesser muscle loads but still strain similarly to more basal forms. Compared on a like for like basis, the skulls of the advanced forms buckle quicker than the more primitive forms. The authors take this as indication that derived ornithomimosaurs may have been changing or varying their diets.
Not surprisingly, the skulls of ornithomimosaurs differ greatly from other theropods – in this study with Allosaurus and Coelophysis. The authors speculate that this may simply be a feature of herbivory evolving in ornithomimosaurs and in other theropods and the repeated evolution of beaks in Theropoda would simply have been a natural progression.
I mentioned earlier the feathered ornithomimosaurs from Alberta and a bone bed from the Upper Cretaceous Horseshoe Canyon Formation in the same province has been providing interesting histological data to Thomas Cullen, of Carleton University in Ottawa, and colleagues. The bone bed, the first of its kind in North America, contained the remains of three individuals and multiple leg elements from these specimens were histologically examined.
As is often the case with histological studies of non avian dinosaurs, indications are that growth was rapid, lines of arrested growth are equispaced and there is fibrolamellar tissue present.  At time of death, however, the larger individual reveals growth to be slowing up thus indicating the onset of maturity. Age for the two smaller individuals is estimated to be between two and three years old whilst the older specimen may be approaching four years of age.
Because the lines of arrested growth are constant throughout growth the authors suggest that postcranial long bones may be used as good indication of age – particularly in smaller theropods and, if they are correct in their hypothesis, then we may already have a wealth of information in our collections that enables us to determine growth patterns in different taxa - especially if the overall body size of a specimen can be ascertained.
Now I have mentioned on this blog on more than one or two occasions that the Upper Jurassic Morrison Formation of North America has many surprises still left up its sleeve and that there were many new taxa still to be described. However, this next one may seem surprising since it appears that we have yet another large bodied allosauroid in residence in what already appears to be an ecosystem already well represented in that department.
The specimen was recovered from McElmo Canyon in Montezuma County, Colorado and is late Tithonian in age. There are multiple elements represented including cranial, axial and appendicular and these have been examined by Sebastian Dalman, of Fort Hays State University, and colleagues with particular attention made to the cranial bones which were CT scanned and then digitally reconstructed three dimensionally.
This has enabled the authors to highlight characters in the premaxilla, maxilla and quadratojugal that warrant the erection of a new taxon. This animal is a large allosauroid comparable with other sympatric theropods in the same formation such as Allosaurus and Saurophaganax although, when compared with elements of similar sized Allosaurus specimens, is clearly a more massive animal.
Early phylogenetic analysis indicates the specimen is well placed within Allosauroidea and is closely related to Allosaurus, Fukuiraptor and Neovenator.   
 
 
Now we are all familiar with the great nest sites of sauropods and oviraptorids but those of other saurischians, particularly large theropods, whilst not unknown, remain relatively rare. The great nest sites of Portugal are an exception and a talk by Octávio Mateus, of the Universidade Nova de Lisboa, and colleagues have brought some amazing insights.
The nest, from the Upper Jurassic Lourinhã Formation in Paimago, Portugal contains the remains of approximately 100 eggs although erosion and the amount of eggshell present indicates the nest held many  more. Also of interest is that the nest also contains crocodile eggs but we can only speculate as to their presence here.
There appears to be no clear structure to the nest but the best preserved eggs and embryonic bones are preserved in the centre suggesting that these were the first laid and were at a more advanced ontogenetic stage than those on the exterior of the nest.  There are around 300 bones that have been recovered so far and this is despite the fact that the vast majority of the eggs are crushed.
The authors tentatively assign these embryos to Lourinhanosaurus antunesi since, although the bones are very similar to those of this theropod, the autapomorphies required to confirm identification were not preserved. However, there are sufficient differences in the remains to confirm that they are neither Allosaurus, Ceratosaurus nor Aviatyrannis. Nor does it seem that these eggs were laid by Torvosaurus, since the known nest, from the same formation, that was designated as belonging to this taxon are dissimilar in as much as the egg shell ornamentation is completely different and the embryonic remains are larger.
The authors point out that these embryos are very much fully loaded miniature versions of the adults and appear ready to become active predators from the moment they hatched. One other point of interest is that there are some very small patches of skin preserved in the nest and that these particular baby theropods display no evidence whatsoever of feathers or fuzz – only clearly defined scales -  again highlighting the danger of assuming that all theropods were feathered to some degree.
Scott Persons and Phil Currie, both of the University of Alberta, have been continuing their research into the cursorial ability in theropods and this time looked at the relative length of limb bones measured against the enormous variance in overall theropod body size as well as the effects of allometry on limb proportions.
Sixty one different theropod taxa were studied that all had complete hind limbs. The authors confirm that distal limb bones are unaffected by allometry and compensate for this fact. This allowed them to recognise adaptions in limb proportions and, as a result, they were able compare theropods despite the extreme differences in size.
The essential result of this is that there is no evidence that limb proportions in theropods that change throughout ontogeny are different in any way from those that would be predicted by simply observing body size. Highlights of this study include that the relative limb proportions of Nanotyrannus, when compared to other tyrannosaurids including Tyrannosaurus, are so extreme that the authors suggest that Nanotyrannus is indeed a valid taxon (here we go again). There is also evidence that there was continuous adaptivity in improving cursorial ability in many theropod groups including troodontids and tyrannosauroids.
 Of particular surprise, however, is that dromaeosaurids actually demonstrate a reduced ability to run which is quite the shock when you consider how we have all assumed them to be amongst the quickest and most agile of all the theropod dinosaurs.

More in Part Two.

References  
Cuff, A.R. & Rayfield, E.J. 2012. Functional Mechanics of ornithomimosaur crania compared to other theropods. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 82.
Cullen, T.M., Ryan, M.J., Evans, D.C., Currie, P.J. & Kobayashi, Y. 2012. Multi-element histological analysis of an ornithomimid (Dinosauria) bone bed from the Horseshoe Canyon Formation, Alberta. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 82- 83.
Dalman, S., Paulina Carabajal, A. & Currie, P.J. 2012. A new large-bodied theropod dinosaur from the Upper Morrison Formation (Late Jurassic, Tithonian) of Colorado. . Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 84.
Mateus, O., Carrano, M.T. & Taquet, P. 2012. Osteology of the embryonic theropods from the Late Jurassic of Paimago, Portugal.  Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 137.
Persons, W.S. & Currie, P.J. 2012. Adaptive cursorial trends among theropod dinosaurs and an attempt to look beyond allometry. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 156 - 157.

 

 

      

8 comments:

Henrique Niza said...

"... these particular baby theropods display no evidence whatsoever of feathers or fuzz – only clearly defined scales"

Most likely, in my opinion, the small patches of skin preserved are insufficient and filamentous weren't preserved... but I'm biased about it.

"... dromaeosaurids actually demonstrate a reduced ability to run..."

I believe it isn't the fist time that is demonstrated dromaeosaurids were actually in the slowest spectrum of theropod cursorial.

"... differences in the remains to confirm that they are neither Allosaurus, Ceratosaurus nor Avityrannis."

You meant Aviatyrannis.

Mark Wildman said...

Octavio was very clear in his assertion that the skin remnants, whilst small, were clearly defined by scales but, of course, it may be a preservation issue. However, we now do have feathers preserved in sandstone (the aforementioned ornithomimosaurs)so we must not assume that feathers were a given in all theropods.

Thanks for the spot on Aviatyrannis - missed that one and now corrected.

Hadiaz said...

Good hearing from you again. It's been too long since last time. As usual, I have questions/comments.

"Ornithomimosaurs have long be known as “ostrich mimics” but how true a comparison is this with today’s extant palaeognaths – especially when it comes to the crania?"

What do Cuff & Rayfield's results mean as far as the above question goes?

"At time of death, however, the larger individual reveals growth to be slowing up thus indicating the onset of maturity. Age for the two smaller individuals is estimated to be between two and three years old whilst the older specimen may be approaching four years of age."

How does that compare to living animals ( http://bio.fsu.edu/~gerick/dinogrowth.htm )? I'm thinking marsupials b/c, IIRC, Erickson et al. 2009 showed that non-avian eumaniraptorans reached maturity btwn 2 & 3 years of age.

"The authors point out that these embryos are very much fully loaded miniature versions of the adults and appear ready to become active predators from the moment they hatched."

Did Mateus mean that hatchlings ran around hunting small prey for themselves &, if so, did he present any evidence for that? Otherwise, I'll wait for the paper b/c I'm doubtful about that, given the evidence for parental feeding in Ceratosaurus & (to a greater extent) Allosaurus (See Bakker & Bir 2004). As for the skin impressions, that was to be expected IMO, given that Sciurumimus was probably a basal coelurosaur ( http://theropoda.blogspot.com.br/2012/07/sciurumimus-albersdoerferi-rauhut-et-al.html ).

"The essential result of this is that there is no evidence that limb proportions in theropods that change throughout ontogeny are different in any way from those that would be predicted by simply observing body size."

Would you mind rephrasing that? Just making sure I follow.

"Of particular surprise, however, is that dromaeosaurids actually demonstrate a reduced ability to run which is quite the shock when you consider how we have all assumed them to be amongst the quickest and most agile of all the theropod dinosaurs."

You don't necessarily have to be a fast runner to be very agile. Indeed, based on what I've read (See "DROMAEOSAURIDAE": http://www.geol.umd.edu/~tholtz/G104/lectures/104eumani.html ), eudromaeosaurs were both slower runners & more agile than other similarly-sized theropods. Also, as Holtz has pointed out ( http://dml.cmnh.org/1994Nov/msg00148.html ), they (like large cats) still would've been quick accelerators. Just a little nit-pick.

Mark Wildman said...

Cuff and Rayfield's main thrust of comparison was with other theropods but ostriches were included due to their apparent cranial convergence. Both ornithomimosaurs and ostriches have light skulls, large orbits, were toothless and have keratinous beaks.

Ostriches provided an extant comparison that helped to determine whether their convergence with ornithomimosaurs was superficial or functional - or both.

I believe that direct growth rate comparisons between extant mammal/marsupials with non avian dinosaurs are valid up to a point. I think that we can be very happy with the processes of rapid growth in both groups but that direct comparison when one group is extinct is problematic. But I am happy that latest research confirms that dinosaurs are certainly on a par with extant mammals regarding rapid growth and there are hints now that some dinosaurs even surpassed mammalian rates so watch this space.

Mateus made it quite clear that the hatchling theropods were more or less exact copies of the adults and that they were likely capable of catching their own prey - albeit on a miniature scale. However, this does not mean that adults could not have demonstrated parental care as you suggest and I, like you, believe this is more likely than not.

Limb proportions - my bad! Better to quote Persons and Currie:

"Results show that ontogenetic changes in limb proportions observed in many theropods (including Albertosaurus and Gorgosaurus )do not differ from the limb proportion changes that would be predicted based strictly on the changes in body size."

I hope that makes it clearer. Thanks for your input as always!

Hadiaz said...

"Ostriches provided an extant comparison that helped to determine whether their convergence with ornithomimosaurs was superficial or functional - or both."

From the sounds of it, the convergence was functional, right? Just making sure I follow.

"Mateus made it quite clear that the hatchling theropods were more or less exact copies of the adults and that they were likely capable of catching their own prey - albeit on a miniature scale."

I'm still gonna wait for the paper b/c, w/all due respect to Mateus, that sounds pretty vague. I mean, if that's really all he said, then we don't know what evidence he based his conclusion on, & thus, whether his interpretation of that evidence is correct. This might be like "The Little Dinosaurs of Ghost Ranch", in which Colbert contradicted himself by claiming that Coelophysis chicks were mini adults lifestyle-wise in Chapter 8 (IIRC, based on the parental behavior of living reptiles) despite having described their cute baby features (I.e. Shortened faces & big eyes) in Chapter 6. Are any skulls & teeth known from adult Lourinhanosaurus?

BTW, this reminded me of another question: Was the Lourinhanosaurus nest what Mateus was talking about in "The Science Of Big Al" ( http://www.220.ro/funny/Walking-With-Dinosaurs-The-Science-Of-Big-Al-1-Din-3/y2wiIjEpoH/ )? I thought so, but wanted to make sure. If you don't mind a minor rant, this also reminds me of the 1 thing that always annoyed me about said doc (despite being my fav Walking With episode): The parental behavior of Allosaurus was based on fossil evidence from a theropod of uncertain relations instead of fossil evidence from Allosaurus itself which not only existed then (See Bakker 1997), but was discussed in ""Walking with Dinosaurs": The Evidence - How Did They Know That?".

"I hope that makes it clearer."

I think it does, but just to make sure, they're saying that the rate at which tyrannosaurid legs grew (from relatively long in youngsters to relatively short in adults) compared to tyrannosaurid bodies doesn't differ from that of other theropod legs compared to other theropod bodies, right?

Mark Wildman said...

With regards to the ornithomimid comparison - yes I understand that convergence was functional since more derived ornithomimosaurs experience similar load strains to ostriches - which kind of makes sense really.

I think Mateus and his co-authors make a reasonable assumption if we use crocodiles as a modern analogy. BUT all the evidence pointing to any behavioural inferance, whether we are talking parental care in theropods or the young looking after themselves as soon as they hatched, remains circumstantial.

As far as Lourinhanosaurus is concerned, I am not sure whether there are skulls and teeth to compare with so I will have to check it out (I am in the process of moving and the library is elsewhere!)

I agree with your assertion regarding theropod limbs. It was hard to put this detail together as I didn't have a lot to work with but I essentially believe that those are the facts.

Hadiaz said...

In reference to ornithomimid beaks & tyrannosaurid legs, many thanks for confirming what I thought. If there's 1 thing I hate, it's not understanding relevant stuff.

As for adult Lourinhanosaurus skulls & teeth, I asked b/c comparing them to those of young Lourinhanosaurus would be a good way of determining the level of parental care shown: If the young had cute baby features & the same teeth as adults, than they were probably fed by adults (like most living birds &, seemingly, most non-avain theropods); If the young had cute baby features & different teeth from adults, then they probably fed themselves w/adult supervision (like some living birds &, AFAIK, all living crocs); If the young lacked cute baby features & had different teeth from adults, then they probably fed themselves w/o adult supervision (like megapodes &, seemingly, pterosaurs).

BTW, good luck w/the move. I know how much of a pain it is.

Mark Wildman said...

The other way of looking at this, of course, is that sauropods laid huge amounts of eggs in enormous nest sites - the so called "R"strategists - whereby the young were almost certainly left to fend for themselves just as soon as they hatched.

Theropods were rare in comparison to herbivores but their nest strategies, by comparison, imply a degree of parental care - but by how much will never be known.The exception here are theropods such as the Oviraptorids which incubated their eggs and that, in itself, lends strength to the parental care theory in other theropods.

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