People generally think of the Mesozoic as warmer than the Cenozoic, a notion well spread by BBC’s famous Walking With… documentary series, perhaps most insanely in the behind the series extras for Walking With [Prehistoric] Beasts, which clearly mentions that the Mesozoic was supposedly one big greenhouse for all the Jurassic and Cretaceous periods, while the Cenozoic lived through a much more exciting range of temperature changes. This idea has in fact become so well rooted in the popular understanding of the Mesozoic that it is the number one tactic used by
idiots dumbasses to explain why dromaeosaurs supposedly lacked feathers, as tropical animals tend to lose insulatory integrument once they grow too large. Nevermind that similar sized tropical birds like ostriches and cassowaries retain such intense feather coats that they can even live confortably at sub-zero temperatures thanks to the more efficient temperature control, but then again there is no arguing with idiots dumbasses.
It is perhaps surprising to learn that the Mesozoic went through several extreme temperature changes, most notably during the Cretaceous period. I recall my very first dinosaur related books as a child to mention that the Cretaceous had a temperate climate closer to our own; while the truth is far more complex than that (during the Turonian, for example, the earth was defenitely a greenhouse world), it surprisingly fitting to describe some parts of the Cretaceous.
China: maybe colder than the South Pole
Liaoning, with it’s multiple fossil formations dating from the early Cretaceous, has become one of the most important areas in regards to the study of Mesozoic life. It is from here that the first fossils of unambiguous feathered non-avian dinosaurs had come from (Archaeopteryx and Rahonavis were originally considered true birds, Avimimus‘ feathered integrument was poorly preserved and hence prone to be considered something else, and the other maniraptoriformes found ’till then had no preserved integrument), forever setting in stone that birds are in fact derived theropods (or that all theropods with feathers are birds according to BANDits, although it doesn’t mean much as everyone with a sane mind considers them
idiots dumbasses). There are also many other important discoveries in regards to other Mesozoic clades, like evidence that mammals were far more diverse than previously thought (i.e. Repenomamus and it’s undescribed relative that was roughly twice as big), that pterosaur diversity was high in inland environments and that even with a large diversity of avian taxa they still thrived (a pattern still observable for the rest of the Cretaceous, as terrestrial flying birds still remained small while pterosaurs remained diverse as large flyers), and that choristoderes began diversifying into sauropterygian like niches roughly in the late early Cretaceous, after hundreds of millions of years as just giant salamander/aquatic lizard like forms (in fact, that the oldest known hyphalosaurids and neochoristoderes occur in Liaoning’s fossil sites suggests that choristodere diversity took place in eastern Asia, before neochoristoderes spread across Laurasia).
Thanks to the nature of the basins themselves, as well as their fauna and flora, we know that Liaoning was once a system of humid forests and swamps, punctuated by large, deep lakes, part of a large river system (given the presence of marine ichthyofauna, as well as of pleisiosaur like choristoderes, it is is possible that both the individual lakes and the river system associated with them were vast enough to functional as a small inland sea; see below). Despiste suggestions that the climate was temperate, it has often been considered subtropical if not outright tropical.
However, in 2010 a paper regarding the study on oxygen isotopes in the fossils of Liaoning’s dinosaurs showed a much unexpected truth about the local climate. Here’s an excerpt:
“Early Cretaceous vertebrate assemblages from East Asia and particularly the Jehol Biota of northeastern China flourished during a period of highly debated climatic history. While the unique characters of these continental faunas have been the subject of various speculations about their biogeographic history, little attention has been paid to their possible climatic causes. Here we address this question using the oxygen isotope composition of apatite phosphate (δ) from various reptile remains recovered from China, Thailand, and Japan. δ values indicate that cold terrestrial climates prevailed at least in this part of Asia during the Barremian—early Albian interval. Estimated mean air temperatures of about 10 ± 4 °C at midlatitudes (∼42 °N) correspond to present day cool temperate climatic conditions. Such low temperatures are in agreement with previous reports of cold marine temperatures during this part of the Early Cretaceous, as well as with the widespread occurrence of the temperate fossil wood genus Xenoxylon and the absence of thermophilic reptiles such as crocodilians in northeastern China. The unique character of the Jehol Biota is thus not only the result of its evolutionary and biogeographical history but is also due to rather cold local climatic conditions linked to the paleolatitudinal position of northeastern China and global icehouse climates that prevailed during this part of the Early Cretaceous.”
Liaoning’s average climate was therefore at the very least temperate, similar in terms of temperature ranges to that of the north of the Iberian Peninsula and temperate eastern USA; the prevalence of this type of climate across Asia indicates that the early Cretaceous was generally considerably colder than the greenhouse world that it is usually depicted as. The chinese forests were not tropical rainforests, but temperate humid forests, with clear winter/summer schisms, and while snow was likely absent in all but the most harsh of winters, frost and cold mysts were probably present during the coldest months, a reality much different from the tropical world associated with dinosaurs.
Perhaps most notably, however, is that Liaoning’s temperature ranges were possibly lower than those of the South Pole in the same time period. The early Cretaceous South Pole, then formed of the joined landmasses of Antarctica and Australia, is represented by the iconic Dinosaur Cove fossil site in Victoria. It preserved an ecosystem with no modern parallel: polar forests that went through long periods of darkness during winter months, but the climate was temperate, like in Liaoning.
The estimated temperature ranges in Dinosaur Cove are not precise. The technique used to determine Liaoning’s climate, by using oxygen isotopes, originally revealed a temperature range between 0 and 8 ºC (32 and 46 °F), making it apropriately colder, but floral evidence places the mean temperatures above 10 ºC (50 ºF), and indeed some of the latest studies in determining the temperatures of Dinosaur Cove indicate similar results. Thus, at the very least the Dinosaur Cove region experienced warmer average temperature ranges than eastern Asia.
The exact reason why Asia was colder than Australia/Antarctica is perhaps due to the main factor determining unusual temperature distributions: oceanic currents. Australia was split by an inland sea during at least the early Cretaceous, allowing warmer currents from the tropical oceans to the north to reach the southern areas, bringing monsoon winds. Indeed, Dinosaur Cove has evidence of intense raining as well as vegetation well adapted to monsoon climates. Even well after the australian inland seas disappeared (the exact date not being clear given that late Cretaceous Australia has no fossil record), warm currents were still forced southwards by the western australian and eastern south american coastlines, allowing the South Pole to remain at warm temperate climates until the Oligocene.
By contrast, the closest marine fossil sites to Liaoning show relatively cold waters, and overall asian climates seem to had been influenced by winds derived from the interior of the continent; Asia remained the largest laurasian landmass across the whole of the Cretaceous, giving enough room for colder continental climates to develop. This situation seems to have reached it’s logical extreme in the post-Turonian Cretaceous; even after global thermal maximums, Asia retained a dry, cold climate, likely not as extreme as the modern temperature range of the central asian deserts but still the closest Mesozoic counterpart.
Implications for Liaoning’s biota
Even before Liaoning was proven to be under a temperate climate, the endemic fauna gave away it’s climatic nature. The extensive lake system lacks several species found in Mesozoic tropical aquatic environments, such as crocodyllians and several fish taxa; even Dinosaur Cove had them, although they weren’t very common. The Liaoning lake system produced thus a very unusual ecosystem; the lakes that became the fossil beds were considerably large and deep, and being connected the whole system functioned as essencially an inland sea. Isolated both by cold temperatures and by actual geographical position, it developed an unique ichthyofauna, many species actually descending from marine ancestors, suggesting a connection to the sea at least for a brief period of time. The most common genera were the benthic feeding Peipiaosteous, a stem sturgeon, and the pelagic Lycoptera, a ray-finned fish with no modern descendents, although being an open water filter feeder it was essencially a freshwater sardine in ecological terms. The basal paddlefish Protosephurus was a larger-scale pelagic filter-feeder, while Yanosteus was another benthic feeding stem-sturgeon. The bowfin Sinamia was likely an opportunistic predator like it’s modern relative, while the lamprey Mesomyzon was obviously a parasite and benthic feeder. An undescribed hybodont shark was likely the top predator of this lacustrine environment, although perhaps also opportunistically an occasional benthic feeders.
As evident, the present ichthyofauna was low in terms of genera, but the individual species were generally very common; this is a feature of cold water environments, where the few present taxa thrive in large numbers. As to be expected, an enormous variety of aquatic invertebrates was present, large in both species and individual numbers, supporting the large numbers of fish and aquatic reptiles; cold water environments are more productive, as phytoplankton thrives better in colder waters.
Besides fish, the basal cryptodire turtle Ordosemys was also common, competing with benthic feeding fish for crayfish and molluscs, if not actively predating on small fish. However, the most common aquatic reptiles were choristoderes, represented in various fossil beds as either the hellbender like monjurosuchids (Monjurosuchus and Philydrosaurus), the nothosaur like Hyphalosaurus (it’s closest relative Shokawa likely lived in a similar environment) and the metriorhynchid/polycotylid like neochoristoderes, represented by Ikechosaurus and Liaoxisaurus (both are generally considered simoedosaurids; it is not clear if champsosaurids were present in Liaoning or even any other part of the Jehol area, but since all derived choristoderes evolved here it is likely that they were). The early Cretaceous obviously witnessed a choristodere adaptative radiation, as previously the known choristoderes were small, lizard like animals. It is pretty clear that the cold freshwater ecosystems of Liaoning played a role in choristodere diversity; in the absence of other aquatic reptiles like crocodyllians, choristoderes were free to occupy as many ecological niches as possible. This event was essencially an aquatic version of the insular diversity of Galapagos finches and Hawaii’s honeycreepers, both groups also exploiting the absence of competitors. Eventually, choristodere diversity expanded outside of Liaoning’s lake system, with simoedosaurids and champsosaurids expanding across Laurasia’s waters.
To exploit the thriving aquatic vertebrate fauna, large fish eating pterosaurs were present. Liaoningopterus was an ornithocheirid, pterosaurs more associated with marine ecosystems, and therefore a testament to the productivity of the lake system; with a wingspan of five meters and powerful, almost crocodile like jaws, it was big enough to pose a threat to even the simoedosaurids and hybodont sharks; the smaller ornithocheirid Haopterus, with a wingspan of 1.35 meters, migth had been a juvenile of this animal, or a competing species. A much more bizarre pterosaur was Boreopterus (this article treats the other two boreopterids, Feilongus and Zhenyuanopterus, as the “teenager” and adult phases of Boreopterus, as most pterosaur experts consider the three animals to be just different growth stages of the same species), a slightly smaller pterosaur with longer, thinner jaws and teeth. The thinner teeth clearly suggest that it wasn’t specialised in preying on large aquatic prey like Liaoningopterus, but was instead specialised to trap small fish and crustaceans, making it sort of a pelican analogue, being a facultative filter feeder. While Liaoningopterus might had captured prey on the wing like a sea eagle, Boreopterus foraged on the surface like most modern aquatic birds; it might even had dived to feed on benthic prey, as most facultative filter feeders like river dolphins begin as benthic foragers, but this is not certain.
Other pterosaurs included several ctenochasmatid taxa, almost certainly dedicated filter feeders like modern waterfowl and flamingoes; over five genera occured in Liaoning’s Yixian beds, a number far superior to any other lagerstattën, further evidence of the richness of Liaoning’s lake systems. The gallodactylid Gladocephaloideus was probably a small rail like opportunist, prey on small animals by the lacustrine shores; the same likely also aplied to the undetermined pterodactyloid Ningchengopterus. Four chaoyangopterids are known from the Jiufotang beds, more so than anywhere else on earth, as well as the tapejarids Huaxiapterus, Sinopterus and possibly Nemicolopterus (nowadays generally recognised as a baby tapejarid); these pterosaurs were terrestrial omnivores, not unlike modern fowl, and might had been adept tree climbers as well. The istiodactylids Nurhachius and Liaoxipterus, possibly the same animal, were scavengers, or possibly even active predators of terrestrial tetrapods like small dinosaurs. Two anurognathids, Dendrorhynchoides and possibly Jeholopterus, were present, occupying the niches of nightjar/bat like nocturnal aerial insectivores.
Liaoning’s avifauna was equally diverse, descrediting the notion that avian diversity undermined the pterosaur one, as like in modern birds and bats there is/was a complex niche differentiation, which appearently kept existing for the rest of the Mesozoic, given the conservative sizes of late Cretaceous flying birds. Among the endemic avifauna there were several basal groups that presumably were replaced by more derived birds as the Cretaceous progressed. Confuciusornithids were the most iconic; with soft keratinous bills, most were jay-like omnivores and carnivores, with the exception of the extremely common Confuciusornis, which was a spectacular tropicbird like piscivore, hunting fish and arthropods from the surface – possibly even on the wing – and might had harassed piscivorous pterosaurs like modern terns harass pelicans. Several clades of arboreal frugivores and granivores, like omnivoropterygids, counted among the largest Mesozoic flying birds, with wingspans over 1 meter, while several bizarre fowl like species like Hongshanornis foraged on the ground. The most diverse native birds were enantiornithes, which account for most avian taxa present in the fossil beds.
Both the native birds and pterosaurs were likely migratory, abandoning Liaoning during winter months, when the productivity of the forests and lakes decreased. A number of birds like Jeholornis were poor flyers, and likely stayed year round, while better flying pterosaurs like istiodactylids probably remained to take advantage of corpses pilling up during the harsher winter months.
Among terrestrial tetrapods, mammals were very common in these forests, and reached enormous proportions, like the wolverine like Repenomamus, and it’s undescribed relative that was twice as big. Dinosaurs were also very common, so the reason why mammals grew larger has perhaps to do with the absence of competing sauropsids like sphenodontians and notosuchians, which have recently been heralded as the true Mesozoic competitors of mammals. Hibernation was presumably common among the native synapsids.
Amidst non-avian theropods, other coelurosaurs are the most common, indicating that the development of feathers was an advantage in Liaoning’s colder climate. Tyrannosaurs seem to have outcompeted other large theropods, with compsognathids occupyng smaller predatory niches; dromeosaurs were almost exclusively represented as microraptoriines, which were specialised gliders/flyers that occupied ecological niches not unlike those of modern owls, the exception by the terrestrial, possily rail-like opportunist Tianyuraptor. Troodontids were represented by three genera from the Yixian beds, which were probably small omnivores not unlike tinamous or rails. Oviraptors were represented as both the basal “protarchaeopterygids” and the pheasant like caudipterygids. These animals, alongside the therizinosaur Beipiaosaurus and the ornithomimosaur Shenzhousaurus, were clearly herbivorous, and the evolution of these herbivorous coelurosaurs likely occured due to the absence of several ornithischians.
The bird hipped dinosaurs were still present, however; the ankylosaur Liaoningosaurus, the ornithopods Bolong, Jeholosaurus and Jinzhousaurus and the ceratopsians Hongshanosaurus, Liaoceratops and Psittacosaurus are known from Liaoning’s Yixian beds, with the latter being the only ornithischian present in Jiufotang, further suggesting ornithischian decline due to the colder temperature ranges.
Both Bolong and Jinzhousaurus were large iguanodontians, with the latter being an hadrosauroid. Their size was presumably their main protection against the cold, and they likely migrated southwards during winter months. Liaoningosaurus was unique in that it was among the smallest known ankylosaurs, and likely spent the winters hiernating. The remaining ornithischians could had also hibernated, but more likely spent the winters actively foraging. Jeholosaurus has been suggested to had been an active omnivore, meaning it could possibly had exploited as many food sources as possible during winter months, and derived ceratopsians are suspected of active omnivory as well indicating that the basal forms in Liaoning’s forests were also capable of eating animals. This would had been a rather ironic situation, with theropods specialised towards herbivory being targetted by ornithischians more specialised towards carnivory.
Most interestingly, despiste the long quills on the tail, Liaoning’s ceratopsians don’t appear to have more insulatory integrument. It is possible that they were lost post-mortem, or that these animals relied exclusively on fat for insulation, for whatever reason.