Wikipedia reports, “Cope’s rule, named after American paleontologist Edward Drinker Cope, postulates that population lineages tend to increase in body size over evolutionary time. It was never actually stated by Cope, although he favoured the occurrence of linear evolutionary trends.”
On the other hand, there is no Wikipedia category for Phylogenetic Miniaturization, which postulates that population lineages tend to decrease in body size over evolutionary time.
Both have their time and place.
For instance, if you take a look at the large reptile tree (LRT, 1026 taxa) in the mammal subset you’ll find the following taxon pairs representing Cope’s Rule:
- Toxodon is a giant Dromiciops
- Panthera (lion) is a giant Procyon (raccoon)
- Homo (human) is a giant Ptilocercus (tree shrew)
- Physeter (sperm whale) is a giant Tenrec (tenrec)
- Balaenoptera (blue whale) is a giant Ocepeia
- Giraffa (giraffe) is a giant Cainotherium
- Elephas (elephant) is a giant Procavia (hyrax)
- Ceratotherium (rhino) is a giant Hyracodon
- Paraceratherium is a giant Mesohippus.
- And in pterosaurs…Pteranodon is a giant Germanodactylus
- Quetzalcoatlus is a giant TM10341
these are general trends, not always direct lineages. We’ll never find the exact ancestors of living or fossil taxa, though we can get very close! Employed taxa represent evolutionary stages, sets of derived characters mixed with some small or large number of autapomorphic traits not shared by the unknown common ancestor of the small and large taxon pairs.
you’ll also find in the LRT the following taxon pairs representing examples of phylogenetic miniaturization, some from the large pterosaur tree:
- Gephyrostegus is a tiny Proterogyrinus
- Terrapene (box turtle) is a tiny Elginia
- Cosesaurus is a tiny Macrocnemus
- Hypuronector is a tiny Jesairosaurus
- Bellubrunnus is a tiny Campylognathoides
- TM 13104 is a tiny Scaphognathus
- Tetrapodophis is a tiny Adriosaurus
- Hadrocodium is a tiny Haldanodon.
- Elaschistosuchus is a tiny Proterosuchus
- Gracilisuchus is a tiny Vjushkovia.
- Archaeopteryx is a tiny Sinornithoides.
What goes down (gets smaller), usually goes up (gets bigger)
And sometimes what gets bigger gets smaller. Case in point: the Pygmy or Channel Islands mammoth.
Anything can happen at any time in evolution
given enough time. As noted earlier, phylogenetic miniaturization was present at the origin of several major clades of tetrapods and in clades of pterosaurs in particular. And this appears to occur during times of survival stress for several reasons. On the other hand, apparently it takes an epoch filled with plenty of food and other resources to produce giant animals. As you know, various parts of the Earth have created stress and bounty throughout its long prehistory.