Large Herbivores -- Have I Found an Error Here?

Factual Questions
1

In the course of setting up the parallel to mammalian extinctions for the Dinosaur Extinction thread, I came to a realization of something that I think exposes an error in the popularizations.

Typically, books on mammalian evolution suggest that the artiodactyls (land-living cetioartiodactyls, used without excuse hereafter, as whales don’t enter into this discussion) outcompeted the other large herbivores during the later Tertiary, mostly thanks to the advantages rumination provided, to take over most of the large-herbivore niches. The other orders, when exposed to direct competition from advanced artiodactyls, either died off (notoungulates, litopterns) or survived in some specialized niches (perissodactyls, proboscideans). Probably the glyptodons and ground sloths need to be taken into account as well.

But when I started looking at the extinction data, what seemed to be the case was that the other orders held their own until the Ice Age, and mostly even during it, dying off or back only at or near its end. There were mammoths and mastodons, chalicotheres, rhinos, and horses all over Eurasia (and North America except for chalicos and rhinos). The Great American Interchange was more complex than “South American native forms got replaced.”) True, artiodactyls were widespread, but they weren’t the sole dominant large herbivores in most of the world until post-glacial times

Am I correct in presuming that the glib generalization of the popularizations is in fact untrue? Is the dominance of the artiodactyls an artifact of the Holocene?
++++

For those who hate taxonomy but are interested in the discussion, a quick summary:

(Cetio-)Artiodactyls: A group often described as “even-toed hoofed animals” that includes most modern large herbivores, including the pigs and peccaries, hippos, camels and llamas, giraffes, deer, pronghorns, antelope, sheep, goats, and cattle, among other forms, and also, strangely, the whales and dolphins.

Perissodactyls: “odd-toed hoofed mammals”, including horses, asses, zebras, tapirs, and rhinos, and the extinct brontotheres and chalicotheres

Proboscideans: elephants in the broadest sense, including mammoths, mastodons, deinotheres, and gomphotheres as well as the modern forms

Notoungulates: Extinct South American forms filling many of the artiodactyl and perissodactyl niches as well as some bizarre unique forms prior to about 5 million years ago.

Litopterns: Another order of extinct South American forms that filled the camel/llama and horse niches there up until about 2 million years ago.

Hope that’s useful.

2

Not entirely. Don’t forget that a heck of a lot of artiodactyls, including all but one antilocaprid, all but two genera of giraffids, and many camelids, were also wiped out during the Holocene. So I’m not sure that the artiodactyls and the perissodactyls were affected disproportionately.

The perissodactyls have been decreasing in diversity relative to the artiodactyls since probably the Oligocene of Miocene. So there may be some long term competitive advantage due to the more efficient digestive system of the artiodactyls.

As for the notoungulates and litopterns, they began to decline soon after the artiodactyls and perissodactyls entered South America during the Great American Biotic Interchange. While a few species survived up until the end of the Pleistocene (toxodonts and Macrauchenia, the latter the only litoptern to survive the interchange), the Holocene extinction was just a coup de grace.

3

I think you are using far too technical language if you are looking for a response from the type of people likely to be generalizing about that.

4

ETA: That probably sounds slightly snarky. But it’s not meant to. I realize you put up definitions and all that. I’m just saying, people who are already that specifically knowledgeable about that period of evolution probably aren’t going to be prone to such specific generalization.

5

Article on notungulates. If you’re familiar with them, there’s nothing new, but apparently someone has a sense of humor. The precision on the longeivity of the clade reminds me of this old joke:

Museum visitor: “How old is this dinosaur fossil?”
Tour guide: “70,000,008 years!”
Visitor: “Wow! How do they know exactly?”
Tour guide: “Well. . . It was seventy million years old when I first started working here. . . and I’ve been working her for eight years.”