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(66,000,000 BC)
The article is recent so I do not think that dang would update the title ;)
I would suspect that imperator may stand but regina will not.

Two million years (oldest to youngest Tyrannosaurus specimens) is potentially long enough to delineate chronospecies, but two very similar apex predators living side by side violates expectations re: niche partitioning.

In Jurassic Morrison, they had Allosaurus and Ceratosaurus but those were from very different families and probably lived/hunted very differently.

it's not such a crazy idea.

as a not-so-scientific back-of-the-envelope: assume a poissonian mutation rate and a genome length; how long till a measurable taxonomic difference?

Is there a number on how often a split in species happens?
To paraphrase some of the paleontologists on twitter, "Another taxonomic argument. Yay."
"T. rex has many substantially complete specimens, which is unusual for dinosaurs."

Are there theories as to why? It seems odd since apex predators are usually much more limited in numbers than their prey.

It's because fossilisation is a rare process. For it to occur the organism needs to be covered and preserved soon after death.
More interest and resources put into recovering them could counts also, as is a coveted trophy and much easier to sold to Museums. If resources are scarce, many efforts will focus on T-Rex even if there are more species in the area. Is much more difficult for a museum to survive displaying only the sparrow size dinosaurs.

And smaller animals have fragile bones.