For this post, I’m going to go back to the dawn of the Cretaceous period, and exterminate the ceratopsians, hadrosaurs and lambeosaurs, and all but a few protoceratopsids....and see what new things arise.
(Yes, I know I’m not spelling everything perfectly – one or two genera names might be misspelled….this is an off-the-cuff effort for me, though I am not doing this randomly – everything is considered, decided based on what I know of each clade)
Nodosaurs and Anklyosaurs grab up more of the low-to-the-ground feeding niches which otherwise would’ve gone to the hadrosaurs. Both clades – nodosaurs and anklyosaurs – grew bigger and, in a number of cases, ornately-armored. The largest species, Anklyotaurus gigans, is believed to have bullied predators away from the carcasses of herbivores, whereupon it scavenged.
The Therizinosaurs diversify, taking niches which otherwise would have fallen to the ceratopsians and lambeosaurs. Its easier to diversify with so much competition removed, though now they have rivals within the therapod camp for the leafy foodstuffs. The biggest challenge comes from the flightless birds such as Patiagoniatryx. What we would call avian maniraptorans (as opposed to “non-avian”) take advantage of the windfall of foods in the trees that they don’t have much competition for, at least not initially. Fortunately, evolution is not a zero-sum game…even so, flight has been hindered a little, what with so many of the birds developing larger guts to process the food.
One exception is the Opposite Birds of China and similar areas, where Gansus and its relatives find themselves the prey of harpy- and hawk-like Opposite Birds.
Volaticotherium diversifies not a great deal, but a respectable amount, giving rise to a few genera. The volaticotherids have a relationship with the arboreal maniraptorans that is rather like the caiman and the giant otter of our modern day here in OTL: unless they’ve specialized to eat only leaf buds and shoots, volaticotherids eat maniraptorans that’re smaller than they are; likewise, non-specialized maniraptorans eat volaticotherids that are smaller than they are.
On the ground and under it, the mammals are dominated by the marsupials who are the majority of the furry carnivores, preying upon the placentals and insectivorous marsupials.
The last protoceratopsids are Diceratus mus spp, measuring only a foot long (1/3rd of a meter) tail included, and endemic to a single equatorial archipelago. Not even a single subspecies of this survives to the K-T Event.
I know, I left a number of questions unanswered….what is the fate of the oceans while the land has this vast change? What is happening to the Tyrannosaurids and non-arboreal maniraptors? Do Spinosaurus and Oviraptor exist in this timeline? Do marsupials and opposite birds dominate the post-Cretaceous world?