Thursday, 28 June 2012

Big Game Hunter


This is one of my favourite pictures of Barnum Brown. Taken in 1934, this is almost certainly the Howe Quarry in Wyoming. He really does look the classical big game hunter, with his pith helmet in place, and holding the rib as if it was taken from an animal that he has just despatched himself. A true trophy shot and so typical of the man. Awesome.

Sunday, 24 June 2012

Social Sauropods?

Many of my recent posts have been concerned with what could be termed “analogues and inferences” and, keeping the theme going, the recent paper by Salgado et al that was published in the most recent issue of the Journal of Vertebrate Paleontology suggests that a recent find of rebbachisaurid sauropods may be indicative of gregariousness behaviour in these dinosaurs.

The specimens, an adult and two juveniles, were recovered from the Pichi Neuquén Member of the Rayoso Formation in Patagonia, Argentina. The specimens are clearly referable to Rebbachisauridae based upon several characters that were recognised in the recovered elements but are not diagnostic to species level although the authors suggest that the remains are morphologically similar to Zapalosaurus (Salgado et al 2006).  
Since these specimens were recovered together in what is obviously a monotaxic bone bed the authors speculate that it may be suggestive that these sauropods did indeed demonstrate some form of social interaction. There is no evidence in the sedimentology and taphonomy of the site that is suggestive of a catastrophic event or miring - nor signs of transportation and the remains are clearly in situ although there is the usual skeletal displacement. These animals apparently died together.
So, in the absence of any environmental calamity, why did these animals seemingly perish together? One line in this paper proved troublesome to me:

One possible, though purely speculative, explanation for this is that the death of the adult triggered the death of the two juvenile individuals.”

Unfortunately the authors do not quantify this statement without any clear indication of what they mean. If we were talking about mammals, and the juveniles were heavily dependent on the adult, then maybe the juveniles would succumb fairly quickly. Extant dinosaurs, the birds, also suffer such deaths since if the adult birds were to die or were killed then the chicks would soon expire. But sauropods? Perhaps I am missing something here and please feel free to enlighten me if you have any detail you wish to add.
In the meantime, and as mentioned here before, sauropod nesting sites indicate that the eggs were laid in enormous numbers in large sites that covered vast acreage, the most famous of which is Auca Mahuevo in Argentina. There is no evidence, circumstantial or otherwise, that sauropods demonstrated any parental care at the nesting stage and it seems the nesting grounds were deserted once the eggs had been laid and covered over. Once hatched, the hatchlings appeared to have fended for themselves and must have suffered catastrophic losses.
However, those that survived do indeed appear to have formed social groups and the principal evidence for this comes from their track ways. There are hundreds of sites all over the world and as palaeoichnology becomes more sophisticated and modern digital techniques are employed, they are able to help demonstrate that, indeed, some sauropods engaged in gregarious behaviour to a degree.
Interestingly some of these sites reveal that sauropods appear to be age segregated and travelled around in small herds (Myers & Fiorillo 2009). This can be supplemented by finds such as those in the Mother’s Day Quarry which exposes the Upper Jurassic Morrison Formation of Montana in the USA. Here have been recovered the multiple remains of many diplodocoid sauropods but they are all juvenile and sub-adult animals. Not a single adult is represented by as much as one element (Myers & Storrs 2007).
Both track ways and sites such as the Mother’s Day Quarry, and similar sites in China, India and other countries, all point to the possibility of gregariousness in sauropods. Naturally, this all has to be quantified by pointing out that such sites are greatly in the minority, albeit not rare either, and there are many considerations that have to be taken, the most important being taphonomic, but it does seem to be reasonably convincing.

Gregariousness in dinosaurs has featured in these pages many times now and whilst we cannot say with absolute certainty to what degree and form this social interaction may have taken, we can now certainly say that it did occur. There are numerous examples in the fossil record which can be interpreted as demonstrating such behaviour.
Monotaxic bone beds are the classic example and the large hadrosaurian and ceratopsian graveyards of North America expose vast quantities of animals numbering from the tens to hundreds – maybe even thousands. Examination of the majority of these sites indicates these dinosaurs perished in a catastrophic event and since only a single taxa is represented then it would seem reasonable to surmise that they were moving through the environment as a herd of some description when disaster struck. However, for what reason they were moving together we can only guess at but cannot say with any certainty.
As already mentioned with regards to sauropods, the ancient nesting grounds of dinosaurs reveal much. The most famous examples of these include the nesting grounds of hadrosaurs such as Hypacrosaurus and Maiasaura and the fact that multiple remains of juveniles have been found in, what is in effect, a nest mound suggests that they were probably quite helpless when born and required parental help. The site at Egg Mountain, by way of example, appears to have been a large colony of hadrosaurs that nested communally and actively cared for their young and is very reminiscent of an extant bird colony of today.
Track ways, again as with sauropods, are represented by thousands of examples across the globe and these again often indicate that animals moved in the same direction in the same place but it has always been hard to clarify that they moved together at the same time. But today, and as mentioned earlier, digital technology is enabling us to almost section track ways and determine if dinosaurs moved together as a group and this technology is still at a relatively early stage but results are encouraging and will only get better.
I won’t go into the world of gregariousness in theropods since that subject has, and will continue to, feature in this blog. Past and current research into this subject would fill up a library on its own. Suffice to say that different theropod taxa spent time together for reasons unknown and for an unspecified amount of time.  
So the challenge now is not to prove that dinosaurs were gregarious, since they obviously were, but to what degree and why. Unfortunately, this is mostly going to be guided by speculation and inference yet again but that is all we have in some cases – oh and a little bit of science too! 

References

Myers, T.S., and G.W. Storrs. 2007. Taphonomy of the Mother’s Day Quarry, Upper Jurassic Morrison Formation, South-Central Montana, USA. Palaios, 2007, v. 22, p. 651-666.
Myers, T.S., and A.R. Fiorillo. 2009. Evidence for gregarious behaviour and age segregation in sauropods. Palaeogeography, Palaeoclimatology, Palaeoecology 11:96-104.
Salgado, L., I. De Souza Carvalho, and A.C. Garrido. 2006. Zapalasaurus bonapartei, un neuvo dinosaurio saurópodo de la Formación La Amarga (Cretácico Inferior), noroeste de Patagonia, Provincia de Neuquén, Argentina. Geobios 39:695-707.

Salgado, L.,J.I. Canudo, A.C. Garrido, and J.L. Carballido. 2012. Evidence of gregariousness in rebbachisaurids (Dinosauria, Sauropoda, Diplodocoidea) from the early Cretaceous of Neuquén (Rayoso Formation), Patagonia, Argentina. Journal of Vertebrate Paleontology Vol.32, (3), pp.603-613.

Friday, 15 June 2012

From the Tethys to the Niobrara


About 70 million years after the muds and shales of the Oxford Clay Sea were being deposited, the continents began to shift toward their present day configuration. As the Atlantic Ocean widened the once mighty super-continent of Gondwanaland broke up and the continent of North America was subject to volcanism, mountain building and particularly subduction. This, in turn, led to higher sea levels and at least a third of the continent that we are familiar with today was flooded and the great Western Interior Seaway was born.
One of the most famous fossil bearing deposits laid down during this time was the Niobrara Formation that extends from south western Kansas to southern Manitoba. The deposit sweeps across in a broad arc across the continent and is best exposed in the north west of Kansas and consists of a soft yellow chalk that is hundreds of feet thick. At this point it is worth remembering how this chalk was formed.
It is still a matter of fascination to realise that the chalk is made of billions of microscopic plants known as coccoliths. Coccolithophores (to give them their full title) were part of the planktonic foraminifera that flourished and diversified during this part of the Cretaceous. Conditions were perfect for them as the climate was warm and seasonal change negligible. Coccoliths were algae that shed their platelets via cell division which fell to the sea floor and this formed the basis for the Niobrara chalk.
The algae was also consumed in copious amounts by a vast array of marine life which ranged from other planktonic life forms right through to the largest filter feeding fish and those very same platelets were then excreted and also fell to the sea bed. The Niobrara chalk, therefore, is a combination of coccoliths, salt and vertebrate remains. Indeed, chalk deposits worldwide represent 70% of the total carbonate sediment deposited over the last 100 million years (Pollastro and Scholle 1986).
The chief outcrop for vertebrate fossils in the Niobrara is the Smoky Hill Member. This chalk deposit is underlain by the Fort Hays Limestone Member and overlain by the Pierre Shale. The Smoky Hill Member contains a rich profusion of vertebrate marine life such as mosasaurs, plesiosaurs, turtles, pterosaurs, sharks and other predatory fish. Early birds are also represented by well known taxa such as Hesperornis and Ichthyornis.
Because the chalk is soft it is constantly under attack from the elements and erodes readily. Not only are new fossils being constantly exposed but the terrain is always being transformed and over the years the formation of structures such as spires and hoodoos gives the Niobrara a sometime eerie and other world-like aspect.
Naturally, such a rich fossil bearing strata was sure to attract attention and the Niobrara has been worked by some of the biggest names in palaeontology such as Marsh, Cope, Williston and the Sternbergs.  They, and other workers, have amassed a vast collection of specimens over the years and we have a real decent understanding of the life in the Niobrara Sea.
Fish dominated the ancient sea and are represented by multiple genera. Chief amongst these were the sharks and the cretoxyrhinids were apex free swimming carnivores and reached lengths of over twenty feet. Nearly as big, and just as formidable, were the primitive bony fish such as Xiphactinus audax which was a fast tarpon-like pelagic hunter with powerful skulls and a mouthful of razor sharp teeth.  
Xiphactinus audax

Sharing the oceans with the fish were, of course, the famous marine reptiles and these too formed a diverse and spectacular community. The ichthyosaurs had vanished during the Cenomanian-Turonian extinction event and this accounted for over 25% of all marine life that swam in the seas at this point. Soon large land living reptilian squamates returned to the shallow seas to take advantage of predatory niches left open by the extinction event. In a short period of time they had evolved to become the dominant predators in the chalk sea - they were the mosasaurs.
Animals such as Platecarpus, Globidens, Clidastes and Tylosaurus are all well known from the Niobrara, were all large taxa and were only vulnerable to the equally large sharks and probably to larger mosasaurs. They were superbly adapted predators that flourished throughout the Late Cretaceous only to disappear at the K-T boundary.
Mosasaurus hoffmanni
Plesiosaurs are also represented in the chalk but are nowhere near as common as the mosasaurs. The long necked elasmosaurids are particularly impressive but their scarcity in the chalk is likely to be due to environmental considerations and it is thought they may have stayed in very shallow waters to avoid predation or maybe that they were only migrating through these waters.
Turtles are well represented in the chalk and there are many different taxa. These were all fully marine and some displayed conical shaped defensive structures mounted dorsally along the mid-line of the carapace which earnt them the very apt nickname of “saw turtles”.
Pterosaur remains are abundant in the chalk which probably represents the greatest pterosaur graveyard in the world. Naturally, though, many of the fossils are crushed and some elements are very incomplete which is due, in no uncertain manner, to the fragility of the pterosaurian skeletal structure. Having said that, many specimens are truly spectacular with the most famous pterosaur of all, Pteranodon, represented by multiple specimens and some of these had a wing span approaching 9 metres.
Birds are represented by the hesperornithorms and ichthyornids and are well known, even to the palaeontological layman, since the beaks were partly lined with teeth betraying their obvious reptilian ancestry. Birds such as Hesperornis demonstrate a dense skeletal structure which would have helped these flightless creatures overcome their natural buoyancy as they dived under water for food. Ichthyornis, on the other hand, was an extremely agile flyer and appears to have been very tern-like which is probably just as well considering the need to avoid predation, both in the air and on land, which was filled with dangers.
It is also worth noting that dinosaur remains have also been recovered from the chalk but these are exceptionally rare (as they are in the Oxford Clay) and probably only represent the odd bloated carcass that may have drifted out to sea. Hadrosaurian elements have been recovered represented by Claosaurus but the majority of dinosaurian elements are from the armoured nodosaurs. It is apparent now that armoured dinosaurs from all over the world must have preferred coastal environments since their remains are often found in fully marine deposits such as those near Lyme Regis in the UK as well as the chalk in the Niobrara Sea.
I have only skimmed the surface of this fascinating world but for a wealth of data can I point you to Mike Everhart’s superb website, Oceans of Kansas and also to check out Anthony Maltese’s blog over at the RMDRC Paleo Lab where he blogs about both fieldwork in the chalk and highlights their preparation work on some of the truly spectacular specimens they have recovered.
Reference

Pollastro, R. M. and Scholle, P. A. (1986) Diagenetic relationships in a hydrocarbon-productive chalk--The Cretaceous Niobrara Formation: in Studies in Diagenesis, F. A.Mumpton, ed., Geological Survey Bulletin 1578, 219-236.

Thursday, 7 June 2012

Palaeochat: 2


Another brief post this week since, although I have managed to stay in touch with the palaeo-blogosphere throughout, I have been away from the office, as it were, and finding time for a blog post has been difficult. Hopefully I will get back to normal posting shortly.

At the back end of May this blog clocked up over 50,000 page views and I have to say that I am absolutely thrilled at reaching such a milestone and really am very grateful to all of you who have taken the time to peruse these pages. When I first started out, this was really something I wanted to do for me in as much that it was nice to be able to record some of my experiences that were swimming around in my head and it was an opportunity to share them.
I had been writing the originally named Saurian’s Field Journal for a few years which was a record of field trips, meetings and other palaeo-related paraphernalia that I had recorded and it was this that prompted me to start the blog. Since then things have moved on and this blog has evolved into something that is part of the bigger palaeoworld and I am delighted to be part of it and long may it continue.
One of the best things now is that there is a definite communal spirit amongst all of us that want the best for our science and the recent high profile episodes concerning the auction of the alleged illegally imported Tarbosaurus specimen in New York City and the on going struggle in support of open access have highlighted that a relatively small but highly motivated group of likeminded people can help to make a difference.
For now, I share an image of the skull of Liopleurodon – again from my time in Lisbon and what a magnificent beast it is. Normally we recognise rare isolated elements from this taxon on occasion so to come face to face with a skull such as this is was a rare treat. It is with a tinge of sadness that we recall that a specimen of this pliosaur was slowly coming to life in Quarry 4 before it was closed and that this part of the quarry now resides under hundreds of tons of spoil and will remain buried for all eternity.