Question: How did Therizinosaurs evolve?

[Dilopho]

I don't even know if this is the right section to post this in, but...
I've been watching Planet Dinosaur yesterday and today- it's a really good show, may I add- and the part on Nothronychus says that it is a relative of tyrannosaurs and that it switched to eating plants. Right, right, I get that- but how? How did the theropods just...switch diets? Was it like ornithomimids, omnivorous, but they just decided to stick with plants? 

[Nanuqsaurus]

I don't even know if this is the right section to post this in, but...
I've been watching Planet Dinosaur yesterday and today- it's a really good show, may I add- and the part on Nothronychus says that it is a relative of tyrannosaurs and that it switched to eating plants. Right, right, I get that- but how? How did the theropods just...switch diets? Was it like ornithomimids, omnivorous, but they just decided to stick with plants? 

Well, of course they didn't really decide to stick to eating plants. There was probably an empty niche for herbivores in the ecosystem, or competition with another omnivore (or both).

[stargatedalek]

To be fair, it's never actually been proven that Therizinosaurs weren't omnivores.

[soft tissue]

therizinosaurids were specialised for herbivory in their most basal forms (Zanno, 2006)--they were herbivorous very early on. therizinosauroids are the basalmost maniraptorans (Zanno et al., 2009) and possibly had a ghost lineage extending back to the early Jurassic, so it is possible that therizinosaurs were exploiting relatively pristine niches of all kinds when hadrosauroids didn't exist. remember that therizinosauroids likely exploited many different niches considering how functionally and morphologically diverse they were (Lautenschlager, 2014).

[Halichoeres]

It does seem to be uncommon that carnivorous lineages produce herbivorous descendants, but it had to happen at least once or there'd be no herbivorous vertebrates at all.

I study a group of fish that includes piranhas, and was basally carnivorous (like most fish). Several lineages within the group have evolved herbivory, usually in wet tropics, namely, places with very high productivity where it is feasible to get enough food by eating fruit (tambaquí) or algae (flannelmouth) or both (some of the trout-like characins). It probably involves an omnivorous intermediate form (some of my fish eat both plants and animals). You could think of bears and pandas as sort of an analogous situation. They're nested within a pretty thoroughly carnivorous clade, but most bears are opportunistic feeders, and pandas managed to evolve themselves into a specialized corner eating >90% plant matter (read: evolutionary dead end).

[Flaffy]

Does the example of pandas changing their diet from carnivorous to herbivorous apply to therizinosaurs? (although they still do eat meat once in a while)

[Halichoeres]

Does the example of pandas changing their diet from carnivorous to herbivorous apply to therizinosaurs? (although they still do eat meat once in a while)

I don't think that's known with any certainty (see the references posted by soft tissue above), I'm just using bears as an example of how it can happen. I might be mistaken, but I think the closest things we have to intermediates for therizinosaurs are Falcarius and Beipiaosaurus, whose dentition shows signs of at least not being committed to hypercarnivory.

[Dinoguy2]

In most cases of diet changes there's usually some kind of intermediate stage that is omnivorous, and then certain omnivorous lineages begin to rely more and more on plants until they are strict herbivores. Zanno's work also suggests that the ancestors of all maniraptorans were already omnivores, so it makes sense that some of those would evolve into strict herbivores (and of course others evolved into strict carnivores, like dromaeosaurids and hawks).

Therizinosaurs therefore probably evolved from a small Ornitholestes type animal that was adapted for eating both small animals and plant matter like seeds and berries, like modern bears.

[Newt]

Halichoeres brought up hypercarnivory, and I think it's useful to expand on that a bit.

The line between herbivores and carnivores is not so hard and fast as we sometimes think. Hypercarnivores prey on animals and that's it. Other carnivores are generally more open, both to scavenging and to eating plant matter; not necessarily foliage, which requires specialized digestive adaptations to process well, but more digestible plant parts such as fruits, flowers, seeds, and even taproots and tubers are exploited by many "carnivores". Equally, many "herbivores" will eat small animals, eggs, and carrion when available.

Even some lineages typically considered hypercarnivores can stray outside the lines; domestic cats often develop a taste for fruits and vegetables, and some snakes will eat eat fruit.