Multitudes of Tyrannosaurus rex in Cretaceous North America

Full-frontal skull of ‘Sue’, the best-preserved and among the largest specimens of T. rex (Credit: Scott Robert Anselmo, Wikimedia Commons)

Long-term followers of Earth-logs and its predecessor Earth-pages News will have observed my general detachment from the dinosaur hullabaloo, which just runs and runs. That is, except for real hold-the-front-page items. One popped up in the 16 April 2021 issue of Science (Marshall, C.R. et al. 2021. Absolute abundance and preservation rate of Tyrannosaurus rexScience, v. 372, p. 284-287; DOI:10.1126/science.abc8300). For over two million years in the Late Cretaceous, just before all dinosaurs – except for birds – literally bit the dust, the authors estimated a lot of the dinosaurian poster-childTyrannosaurus rex lurking in North America. I write ‘lurking’ because ‘tyrant lizard the king’ when fully grown was so big that if it ran and fell over, it would have been unable to get up! Tangible evidence from trackways suggests that it ambled from place to place. The leg bones of a 7-tonner would probably have shattered at speeds above 18 km per hour, and accelerating to the speed of a human jogger would, anyhow, have exhausted its energy reserves, But it was agile enough to be an ambush predator; it could even pirouette! And it could crush bones so well that it was able to consume prey entirely. It has been suggested that T. rex may have been a scavenger, at least in old age. Whatever, how is it possible to estimate numbers of any extinct species, let alone dinosaurs?

The stumbling block to getting a result that is better than guesswork is the fossil record of a species. First, it is incomplete, secondly the chance of finding a fossil varies from area to area, depending on all kinds of factors. These include the degree of exposure of sedimentary rock formed by the environment in which they thrived, as well as the vagaries of preservation due to post-mortem scavenging, erosion and water transport. In life the population density of a particularspecies varies between different ecosystems and from species to species. For instance, more lions can thrive in open rangeland than in wooded environments, whereas the opposite holds for tigers: probably because of different hunting strategies. Many factors such as these conspire to thwart realistic estimates of ancient populations. Studies of living species, however, suggest that population density of an animal species is inversely related to the average body mass of individuals. Take British herbivores: there are many more rabbits than there are deer. On the grasslands of East Africa hyenas and wild dogs outnumber lions. This mass-population relationship (Damuth’s Law) outlined by US ecologist John Damuth also depends on where a species exists in the food chain (its trophic level) as well as its physiology. Yet for living species, populations of flesh-eating mammals of similar mass show a 150-fold variation; a scatter that results from their different habits and habitats and also their energy requirements. Because they are warm-blooded (endothermic), small carnivorous mammals need a greater energy intake than do similar sized, cold-blooded reptiles, which need to eat far less. But not all living reptiles are ectothermic, especially the bigger ones. The Komodo dragon is mesothermic, midway between the two, and uses about a fifth of the energy needed by a similar-sized mammal carnivore. Population densities of dragons in the Lesser Sunda Islands are more than twice those of physiologically comparable mammalian predators.

A number of features suggest that the metabolism of carnivorous dinosaurs lay midway between those of large predatory mammals and big lizards like the Komodo dragon. This is the basic assumption for the analysis by Charles Marshall and colleagues. They did not focus on the biggest T. rex specimens, but on the average, estimated body mass of adults. There are numerous smaller specimens of the beast, but clearly some of these would have been sexually immature. It has been estimated that adulthood would have been achieved by around 15 years. The size data seem to show that achieving sexual maturity was accompanied by a 4 to 5 year growth spurt from the 2 to 3 tonnes of the largest juveniles to reach >7 t in the largest known adults which may have lived into their early 30s. The authors used this range to estimate a mean adult mass of 5.2 t. Taking this parameter and much more intricate factors into account, using intricate Monte Carlo simulations Marshall et al. came up with an estimate of 20 thousand T. rex adults across North America at any one time: but with an uncertainty of between 1,300 to 328,000. Spread over the 2.3 million km2 area of Late Cretaceous North America that lay above sea level their best-estimated population density would have been about 1 individual for every 100 square kilometres. An area the size of California could have had about 3800 adult Tyrannosaurus rex, while there may well have been two in Washington DC. Lest one’s imagination gets overly excited, were tigers and lions living wild today in North America under similar ecological conditions there would have been 12 and 28 respectively in the US capital. Yet those two adult Washingtonian T. rexs would have been unable to catch anything capable of a sustained jog, without keeling over. The juveniles weighing in at up to 3 tonnes would probably have been the real top predators; the smaller, the swifter and thus most fierce. Which leaves me to wonder, “Did the early teenagers catch the prey for their massive parents to chow-down on?”

See also: How many T. rexes were there? Billions. (ScienceDaily 15 April 2021)

2019 Annual Logs added

I have now compiled all the Earth-logs posts from 2019 into PDFs for the categories: Geohazards; Geomorphology; Human Evolution; Miscellaneous; Palaeobiology; Palaeoclimatology; Physical Resources; Planetary Science; Sediments and Stratigraphy, and Tectonics. They are available for download through the Annual logs pull-down in the main menu – just select a category, then scroll down to the 2019 list of contents and click on the link.

I hope this format is useful for reference purposes.

Is there water in the Earth’s core?

Understandably, the nature of what lies at the centre of the Earth is as much the subject of speculation as tangible evidence. That there must be something very dense within the planet emerged once the Earth’s bulk density was calculated. Because a high proportion of meteorites are dominated by an alloy of the metals iron and nickel, geoscientists adopted that combination as plausible core material. Study of the arrival times around the globe of seismic waves from earthquakes then revealed the actual size of the Earth’s core. Iron-nickel alloy fitted the bill quite nicely. It also fits geochemical evidence, such as the crust and mantle’s depletion in some trace elements that theoretically have an affinity for iron. The fact that seismology showed also that the outer core was molten and able to flow, together with metals’ high electrical conductivity, gave rise to the current concept of the geomagnetic field being generated by a dynamo effect in the core. However the density of Fe-Ni is not ‘quite right’ because the core is somewhat lighter than predicted for the pure alloy under stupendous pressure: it must contain a substantial amount – up to 13% – of lower density materials.  Silicon, sulfur and oxygen have been suggested as candidates, with evidence from a variety of minor minerals in metallic meteorites.

A recent model for core formation (credit Crystal Y. Shi et al 2013; DOI: 10.1038/NGEO1956 Fig. 5)

The world is currently awash with models that attempt to throw light on the course of the Covid-19 pandemic. Many are based on highly uncertain data, leading to suggestions by some people that they have become tools for political elites and a means of helping ambitious scientists into the limelight: a sort of fuel for hubris. In the midst of this unprecedented turmoil there has appeared a suggestion (from modelling) that the core also contains abundant hydrogen (Li, Y. et al. 2020. The Earth’s core as a reservoir of water. Nature Geoscience, v. 13, published online; DOI: 10.1038/s41561-020-0578-1). Yunguo Li and colleagues, from University College London, the Chinese Academy of Science and the University of Oslo, explore the idea that the dominant hydrogen of the pre-planetary Solar nebula, which accreted to form the Earth, may have joined iron during core formation. This had been predicted from the thermodynamics of chemical reactions between water and iron. The team takes this further through the geochemical theory that elements and compounds tend to enter other materials preferentially. For example, during partial melting of the crust alkali metals (Na, K etc) are more likely to enter the granitic melt than to remain in the solid residue. Li et al. have used thermodynamics to predict the partitioning of hydrogen between iron and silicate melts under the very high temperature and pressure conditions at the boundary between the core and mantle.

Their calculations suggest that hydrogen then behaves in much the same manner as, say gold and platinum: it becomes ‘iron-loving’ or siderophile and prefers the molten core, as would H2O. The amount that gets in depends on the water content of the molten silicate that eventually becomes the mantle. If the water now making up Earth’s ocean was ‘degassed’ from the mantle during core formation then the original silicate melt would have been ‘wetter’ than it is now. The implication of such early degassing is that the core may contain 5 ‘oceans worth’ of water! The alternative scenario for Earth’s becoming a watery world is the later accretion of, for instance, cometary material. In that case, the early core would have been drier. Yet, the continual subduction of hydrated oceanic lithosphere into the deep mantle during billions of years of plate tectonics would steadily have added water to the core, in the form of iron oxides and hydrogen. So, the core might, in either case, contain several ‘oceans’ of the components of water. One line of indirect evidence is the deficiency in Earth’s actual water of the heavier isotope of hydrogen (deuterium) relative to the D/H ratio of chondritic meteorites. Theory suggests that D has slightly more affinity for joining iron than does H. Substantial water in the core does help explain the core’s apparent low density, but that notion requires as much faith as politicians seem to have in ‘following the Science’ during the current pandemic …

How does plate tectonics work?

Well, surely we ought to know, 52 years after W. Jason Morgan proposed that the Earth’s surface consists of 12 rigid plates that move relative to each other. But that is not completely true, although most of its mechanisms expressed by external and internal Earth processes are known in great detail. It is still a ‘chicken and egg’ issue: do convective motions in the mantle drive the superficial plates around by dragging at the base of the lithosphere or is it the subduction of plates and slab-pull force that result in overturn of the mantle? Nicolas Coltice of the University of Paris and colleagues from those of Grenoble, Rome and Texas consider that posing plate tectonics in such a manner is an abstraction; rather like the plot for a novel that is yet to be written (Coltice, N. et al. 2019. What drives tectonic plates? Science Advances, v. 5, online eaax4295; DOI: 10.1126/sciadv.aax4295). Instead, all the solid Earth’s vagaries and motions have to be considered as an indivisible whole rather than the traditional piecemeal approach of focussing on the forces that act on the interfaces between plates.

Their approach is to model a combination of mechanisms throughout the Earth as a single, evolving three-dimensional system without the constraint of perfectly rigid plates, which of course they are not. The physical parameters boil down to those involved in relative buoyancy, viscosity, and gradients of temperature, pressure and gravitational potential energy within a spherical planet. Designing the algorithms and running the model on a supercomputer took 9 months to reconstruct the evolution of the planet over 1.5 billion years.

4-whatmakesthe
Still from a movie of simulated breakup of a supercontinent, in bland blue-grey, showing what happens at the surface (left) and, at the same time, in the mantle (right): note the influence of rising plumes (credit: Nicolas Coltice)

The result is a remarkable series of unfolding scenarios. In them, 2/3 of the planet’s surface moves faster than does the underlying mantle, suggesting that the surface is dragging the interior. For the remainder, mantle motions exceed those of the surface. Continents are dragged by the underlying mantle to aggregate in supercontinents, which in turn are torn apart by the sinking of cold oceanic slabs. The model takes on a highly visual form, showing in 3-D, for instance: ocean closure and supercontinent assembly; and example of continental breakup; how subduction is initiated.

It will be fascinating to see the reaction of the authors’ peers to their venture, and the extent to which the technicalities of the paper are translated into a form that is suitable for teaching. My suspicion is that most Earth scientists will be happy to stay with the old conceptions until the latter is achieved, and laptops are able to run the model(!)