Tag Archives: Dinosaurs

Mystery of upside-down dinosaurs resolved

Remains of ankylosaurs, a popular family of heavily armoured dinosaurs, occur in sedimentary sequences that range in age from early Jurassic to the close of the Cretaceous. Their defences seem almost impregnable, being constructed of thick, fused scales, often bearing formidable spines, which covered them completely. They bore a crude resemblance to modern armadillos apart from the fact that they were unable to roll-up defensively. In some species the rigid tail bears a large, knob of scale tissue. At up to 3 m long, were the tail to be swung it would have packed bone-cracking momentum. Interestingly, to a poorly sighted predator the club may have been mistaken for the animal’s head at the end of a long neck, so perhaps it lured a potential assailant within range of its devastating power. The largest ankylosaur, from the Cretaceous of western Canada, was the size of a small bus, up to 8m long, 1.5 m wide, standing 1.7 m high and weighing in at around 5 to 8 t. Such dimensions would have made it almost impossible to be bitten, even by the largest predatory dinosaurs, and difficult to turn over. Their teeth show that ankylosaurs were herbivorous, and their somewhat bulbous bodies almost certainly contained a massive digestion system.

Polski: JuraPark Bałtów - Park Dinozaurów - An...

Recfonstruction of Ankylosaurus at the JuraPark Bałtówin Poland (credit: Wikipedia)

The mystery lies in the fact that most ankylosaur fossils are found lying on their backs. Early dinosaur aficionados suggested a tendency for the lumbering beasts to tumble down slopes and become stranded on their backs, so to die miserably. Such clumsiness is hardly a positive characteristic of evolutionary fitness for such a long-lived group, and, besides, the sedimentary formations in which they are found indicate very gentle slopes. So, were they flipped by dextrous predators, as imagined in some early films purporting to look to the distant past? Probably not, for most well preserved fossils show no sign of bites or gnawing. From a study of 36 late-Cretaceous ankylosaurs from Alberta four Canadian and US palaeontologists (Mallon, J.C. et al. 2018. A “bloat-and-float” taphonomic model best explains the upside-down preservation of ankylosaurs. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 497, p. 117-127; doi:10.1016/j.palaeo.2018.02.010 support the idea of their carcases or even living animals having been picked up by flood waters when their high centre of gravity would have flipped them upside down. Bloating through decay might then allow them to be transported large distances. Unsurprisingly, their conclusions rest on model simulations.

The winter of dinosaurs’ discontent

Under the auspices of the International Ocean Discovery Program (IODP), during April and May 2016 a large team of scientists and engineers sank a 1.3 km deep drill hole into the offshore, central part of the Chicxulub impact crater, which coincided with the K-Pg mass extinction event. Over the last year work has been underway to analyse the core samples aimed at investigating every aspect of the impact and its effects. Most of the data is yet to emerge, but the team has published the results of advanced modelling of the amount of climate-affecting gases and dusts that may have been ejected (Artemieva, N. et al. 2017. Quantifying the release of climate-active gases by large meteorite imp-acts with a case study of Chicxulub. Geophysical Research Letters, v. 44; DOI: 10.1002/2017GL074879).  . From petroleum exploration in the Gulf of Mexico the impact site is known to have been underlain by about 2.5 to 3.5 km of Mesozoic sediments that include substantial amounts of limestones and evaporitic anhydrite (CaSO4) – thicknesses of each are of the order of a kilometre. The impact would inevitably have yielded huge volumes of carbon- and sulfur dioxide gases, as well as water vapour plus solid and molten ejecta. The first, of course, is a critical greenhouse gas, whereas SO2 would form sulfuric acid aerosols if it entered the stratosphere. They are known to block incoming solar radiation. So both warming and cooling influences would have been initiated by the impact. Dust-sized ejecta that lingered in the atmosphere would also have had climatic cooling effects. The questions that the study aimed to answer concerns the relative masses of each gas that would have reached more than 25 km above the Earth to have long-term, global climatic effects and whether the dominant effect on climate was warming or cooling. Both gases would have added the environmental effects of making seawater more acid.

Chicxulub2

3-D simulation of the Chicxulub crater based on gravity data (credit: Wikipedia)

Such estimates depend on a large number of factors beyond the potential mass of carbonate and sulfate source rocks. For instance: how big the asteroid was; how fast it was travelling and the angle at which it struck the Earth’s surface determine the kinetic energy involved and the impact mechanism. How that energy was distributed between atmosphere, seawater and the sedimentary sequence, together with the pressure-temperature conditions for the dissociation of calcite and anhydrite all need to be accounted for by modelling. Moreover, the computation itself becomes extremely long beyond estimates for the first second or so of the impact. Earlier estimates had been limited by computer speeds to only the first few seconds of the impact and could not allow for other than vertical impacts. The new study, by supercomputers and improved algorithms, used a likely 60° angle of impact, new data on mineral decomposition and simulated the first 15 to 30 seconds. The results suggested that 325 ± 130 Gt of sulfur and 425 ± 160 Gt CO2 were ejected, compared with earlier estimates of 40-560 Gt of sulfur and 350-3,500 Gt of CO2.  The greater proportion of sulfur release to the stratosphere pushes the model decisively towards global cooling, probably over a lengthy period – perhaps centuries. Taking dusts into account implies that visible sunlight would also have been blocked, devastating the photosynthetic base of the global food chain, in the sunlit parts of oceans as well as on land.

But we have to remember that these are the results of a theoretical model. In the same manner as this study has thrown earlier modeling into doubt, more data – and there will be a great many from the Chicxulub drill core itself – and more sophisticated computations may change the story significantly. Also, the other candidate for the mass extinction event, the flood basalt volcanism of the Deccan Traps, and its geochemical effects on the climate have yet to be factored in. The next few lines of Shakespeare’s soliloquy for  Richard III may well emerge from future work

… Made glorious summer by this sun of York;
And all the clouds that lour’d upon our house
In the deep bosom of the ocean buried …

See also: BBC News comment on 31 October 201

 

The dinosaur they could not kill: Brontosaurus is back

It would be pretty safe to say that everyone has heard of Brontosaurus, but in the 1970s the genus vanished from the palaeobiology lexicon. The ‘Bone Wars’ of post-Civil War US palaeontology stemmed from the astonishing prices that dinosaur skeletons fetched. The frenzy of competition to fill museums unearthed hundreds of specimens, but the financial enthusiasm did not extend to painstaking anatomy. Finding a new genus meant further profit so a slapdash approach to taxonomy might pay well. So it did with the dinosaur family Diplodocidae for Othniel Marsh, one of the fossil marauders. He along with his main competitor, Edward Cope, was a wizard fossicker, but lacked incentive to properly describe what he unearthed. In 1877 Marsh published a brief note about a new genus that he called Apatosaurus, then hurried off to for more booty. Two years later he returned from the field with another monster reptile, and casually made a brief case for the ‘Thunder Lizard’, Brontosaurus. Unlike his usage of ‘Deceptive Lizard’ for Apatosaurus, the English translation of Brontosaurus caught the public imagination and lingers to this day as the archetype for a mighty yet gentle, extinct beast. Yet, professional palaeontologists were soon onto the lax ways of Marsh and Cope, and by 1903 deemed Brontosaurus to be taxonomically indistinguishable from Apatosaurus, and as far as science was concerned the ‘Thunder Lizard’ was no more.

Illustration of a Brontosaurus (nowadays calle...

Artist’s impression of a Brontosaurus . The idea that it was wholly or mostly aquatic is now considered outdated. (credit: Wikipedia)

But, the legacy of frenzied fossil collecting of a century or more ago is huge collections that never made it to display, which form rich pickings for latter-day palaeontologists with all kinds of anatomical tools now at their disposal: the stuff of almost endless graduate studies. Emanuel Tschopp of the New University of Lisbon with colleagues took up the challenge of the Diplodocidae by examining 49 named specimens and 32 from closely related specimens as controls, measuring up to 477 skeletal features (Tschopp, E. et al. 2015. A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ, v. 3, doi10.771/peerj.857). An unintended consequence was their discovery that 6 specimens of what had become Apatosaurus excelsus (formerly Marsh’s Brontosaurus) differed from all other members of its genus in 12 or more key characteristics. It seems to taxonomists a little unfair that Brontosaurus should not be resurrected, and that looks likely.

Had this been about almost any other group of fossils, with the exception perhaps of the ever-popular tyrannosaurs, the lengthy paper would have passed unnoticed except by specialist palaeontologists. In a little over a week the open-access publication had more than 17 thousand views and 3300 copies were downloaded.

See also: Balter, M. 2015. Bully for Brontosaurus. Science, v. 348, p. 168