Q. Does Economos’ 1981 paper, The Largest Land Mammal, prove that gravity was the same 23 to 34 million years ago?
A. This old paper by Angelos Economos is sometimes quoted as positive proof that gravity was the same when the largest land mammal existed.  A typical use of Economos’ paper to confirm the belief that there has been no variation in ancient gravity is given in Biology of the sauropod dinosaurs: the evolution of gigantism when the authors state “we have to assume that there were no secular variations in Earth’s gravity in the Phanerozoic geologic past (Economos, 1981)” and science writers, such as Brian Switek on National Geographic, have used this to claim, "paleontologist [have] debunked the proposal that Jurassic gravity was weaker."  In practice, a more rigorous examination of Economos' paper indicates that it does not prove gravity is unchanged, so it would be unwise to offer it as firm proof that the Earth’s ancient gravity was the same as now.
In his 1981 paper Economos first presents the evidence that all life is limited in size based on hyper-gravity experiments on a mouse, rat, chicken and dog and then extrapolates the maximum upper size possible for a mammal based on this data. He concludes that the maximum upper size limit for a mammal is 20,000 kg, which he notes is about the same weight estimated for the largest known mammal Baluchitherium (also known as Indricotherium or Paraceratherium) so assumes that gravity was the same 23 to 34 million years ago.
Economos advocates that gravity imposes a metabolic cost on all life that is higher for larger animals due to their larger mass. This increasingly higher metabolic cost is due to the scale effect although it is not explained in these terms within his paper. Using experiments on small animals subjected to higher gravity in a centrifuge the gravitational tolerance of a mouse was 7g (7 times normal gravity), a rat was 5g and a dog and chicken 3g. He then proposes that as the gravitational tolerance “decreases with increasing body mass, an upper limit for body size would be reached, that “largest” mammal having a gravitational tolerance equal to terrestrial gravity”. In this he has presumed that gravity was the same in the past. This assumption is then verified by plotting three of these hyper-gravity results on a graph using a double-logarithmic plot and extrapolating the largest mass that any mammal could achieve as 20,000 kg. A graph is shown below using the original data given by Economos.