Another World's Lijialaowu
A small experiment at a no KT world on a Paleocene fossil site.
In a world where the KT event didn't happen, all bets truly are off. Even in the placid climate of the Paleocene - just before the Eocene Thermal Maximum and its ensuing turnover, long before the drastic changes of the Cenozoic - entire biotas were collapsing. Laramidia's fauna entered Appalachia and South America, Europe's fauna found itself flanked by Appalachia and North Africa and Asia. This was a period of extensive interchange, and few examples were as dramatic as the Late Paleocene of China.
The Wanghudun Formation spans the Late Paleocene and Early Eocene represents a flood plain environment, likely seasonal in nature. Vegetation was similar to older Cretaceous sites, dominated by conifers and angiosperms, but with a few new additions: grass, and cycads. Starting as a more wet forest environment, it became increasingly more arid towards the end of the Paleocene, resembling a savanna by the time the Eocene Thermal Maximum came.
Earlier Asia fossil sites closely mirrowed those of the Late Cretaceous, but not here. Of the iconic non-avian dinosaur groups in the Nemegt and others, few were present: only six species of pachycephalosaur, two troodontids, four alvarezsaurids, one halzkaraptorine, five ornithomimids and a titanosaur. A new dinosaur had nonetheless arrived on the area: Luosaurotherium inexpectatum, a stegosaur. Of the known avifauna, raptorial avisaurid and crow-like gobipterygid enantiornithes co-existed with flightless hollandids, pigeon-like apsaraviids, spoonbill-like presbyornithids, mesite-like neognaths and crane-like palaeognaths.
By contrast, synapsids and terrestrial crocodylomorphs were much more diverse. The former were represented by a rich diversity of djadochtatherioidean, taeniolabidoid and neoplagiaulacid multituberculates, accompanied by herbivorous zhelestids and gondwanatheres, insectivorous zalambdalestids and anagalids and carnivorous deltatheroideans and sparassodonts while the latter were represented by armoured, herbivorous simosuchids, omnivorous gobiosuchids, mid-sized carnivorous araripesuchids and an enormous sebecian, Shenlong imperator, which at 6 meters in length was one of the largest terrestrial carnivores of the continent and likely of the entire world in the younger strata of the formation. Several squamate taxa are known, the largest at 3.5 meters in length, both as predatory varanids and as herbivorous, multicusped polyglyphanodontians. The water ways were populated by alligatoroids, neochoristoderes, small lizard like choristoderes, meiolaniiforme, baenid, trionychid, carettochelyid and pleurodire turtles and an abundant lissamphibian and ichthyofauna. Pterosaurs were present both as azhdarchids, chaoyangopterids, ctenochasmatids and lonchodectids, with possible anurognathid remains.
The reason for this highly atypical fauna, which quickly replaced the older ceratopsian/oviraptor/trodoontid/ornithomimosaur/therizinosaur/titanosaur/ankylosaur/hadrosaur/tyrannosaur/dromaeosaur complex, lies on the collision between India and Asia. India and Asia had interchanged fauna since the Late Cretaceous, allowing some degree of adaptation on the part of the older Indian endemics. When the two landmasses finally collided, the introduced grass and the monsoon climate favoured both the local crocodylomorph, zhelestid and gondwanathere herbivorous guilds as well as the Asian multituberculate, zhelestid and polyglyphanodontid guilds, while most herbivorous dinosaurs were incapable of dealing with these changes. Those that did were cycad specialists (stegosaurs), generalists (titanosaurs, pachycephalosaurs) or also substantially foraged on aquatic plants (ornithomimosaurs). The extinction of many herbivores and the arrival of competing sebecians also killed off local tyrannosaurs and dromaeosaurs, while several established carnivorous mammal groups expanded as their prey grew larger.
This unique fauna, the Lijialaowu Biota, was one of the first great turnovers of the Cenozoic and a harbinger for things to come. Those hadrosaurs and ceratopsians would eventually spread around across the Eocene, retaking their lost territories in Asia, herbivorous mammals, polyglyphanodontians, ornithomimids and pachycephalosaurs got the first of many adaptive radiations. Stegosaurs would continue on the Eocene of Asia and North America, the last species being Wyomingostega latens from the Late Eocene of the USA. And the old maniraptorans would start on a long decline, their niches taken by flying and flightless ornithothorace birds.