This is an Earth Day special!  Today we are talking about this really cool study that came out last week in the scientific journal Science (one of the top journals in all of ‘science’). It is by Charles Marshall and others from Berkeley, and the title is “Absolute abundance and preservation rate of Tyrannosaurus rex”https://science.sciencemag.org/content/372/6539/284The authors also wrote this very cool Conversation (name of the publication) article where scientists and journalists co-write a summary of research.  https://theconversation.com/how-many-tyrannosaurus-rex-walked-the-earth-159041Well the title pretty well describes what this paper is about, but let’s go through the arguments here, and hopefully we will get a chance to talk to one of the authors of this study at some point in the future! So, here’s what ‘we’ as the field of science knew before this paper:T. rex mated ~15 years oldEstimates of how long each dinosaur livedT. rex walked the Earth for 1.2 to 3.6 million yearsRough estimate of metabolic rate, somewhere between mammals and komodo dragonArmed with this information, Marshall et all performed this cool calculation.  First they figured that each generation of T. rex took ~19 years (average time between consecutive lineages, humans are between 22 and 33)They then needed population density to figure out how many dinosaurs there were at any one time.  So they applied this relationship between body mass (size) and population density for modern animals (Damuth’s law).  Square mile of grassland supports more rabbits than elephants…Damuth’s law is also dependent on metabolic rate, so apply that data. They get an average of 1 T. rex every 42.4 square miles, or 109.9 square km.  This is two individuals in Washington DC!  Ottawa county is 583 square miles, ~13-14 T. rex. This calculation can then go one stage further and estimates that ~20,000 individual T. rex were alive at any one time, and 2.5 BILLION T. rex ever existed on Earth!  That’s amazing!  So, there may be some debate about the actual numbers involved here (and these have a massive uncertainty, between 130 million T rex and 42 billion, using conservative uncertainty stats), but for us it teaches us two important points: There is a method to uncertainty calculation that is a good lesson here: These calculations are all pretty uncertain, how do we know how reliable these final numbers are?Fossils, especially land fossils, are a very very rare thing!