Even without the asteroid, dinosaurs were doomed
Dinosaurs ruled the earth for a period of 134 million years. They appeared in the Triassic, but their pseudosuchian cousins, as well as the remaining therapsids, were the dominant animals across the planet. Dinosaurs did expand into large herbivore niches in the form of sauropodomorphs, but only in the Jurassic did they became the dominant fauna, as their competitors were decimated. Thus, we can outright disprove the notion that dinosaurs were special above all other animal clades, that they too were lucky underdogs rather than a superior race.
Most shockingly, however, is that their reign was not as secure as previously thought. No matter how much we worship dinosaurs, the fact is that, by the late Cretaceous, most of their original diversity was lost. Ornithopods, while represented as the widely successful hadrosaurs, were otherwise represented by thescelosaurid relics and one or two iguanodontish clades, also with only localised success. Sauropods were cosmopolitian, but only the specialised titanosaurs remained. Non-coelurosaur theropods were represented by the also highly specialised abelisaurs, their more generalistic noasaur cousins, and one or two allosauroids like Orkoraptor. Coelurosaurs themselves produced the extremely diverse Maniraptora, the only remaining dinosaurs in the Cenozoic in fact, but otherwise only the specialised, hyena like tyrannosaurs and the ratite like ornithomimids survived. And with the exception of Marginocephalia and the ankylosaurs, nearly all non-ornithopod ornithischians were gone.
This is natural among animal clades. Nearly all of the Mesozoic/Cenozoic mammal diversity is gone as well, the remaining clades being a small percentage of the immense radiation of mammals.
However, not only did non-avian dinosaurs lose such a large amount of clades, but by the late Cretaceous, other animals moved into dinosaurian niches. Many clades of previous underdogs quickly filled the void left by the taxa lost, and some even outright outcompeted dinosaurs once they were established into new niches. If dinosaurs had to wait for the Triassic/Jurassic extinction event to become dominant, these animals only had to wait for a much minor extinction event, and some didn’t even had to do so. Indeed, more credit can be given to pseudosuchians as “ruling lizards” than to dinosaurs, no matter what popular culture or even dinosaur specialists like Bakker or Horner believe in.
The Cretaceous Thermal Maximum (aka Turonian Extinctions)
The Mesozoic witnessed several cases of minor extinction events. The most notable of these events was the Cenomanian/Turonian boundary, caused by what is reffered as the Cretaceous Thermal Maximum, an extreme change of global climates towards a Greenhouse Earth.
Previous to the CTM, the global climate was cold and temperate, as Liaoning’s fossil sites show. The exact reasons that led to the CTM are not well understood, but things ranging from asteroid impacts to increased volcanism have been to blame. The result is that the Earth’s temperate drastically increased in a matter of a few thousand years, if not less, replacing the old cold climate with a globally tropical one. This had drastic consequences in the ecosystems around the world, bringing many clades to extinction.
Among the taxa we know perished in the event include:
– Non-titanosaur sauropods
– Most plesiosaurs, the exceptions being Polycotylidae and Elasmosauridae
– Most pterosaurs (to the point that the only taxa we know for sure survived are azhdarchids and ornithocheiroids, with forms like anurognathids being likely, but not confirmed, survivors)
– Stegosaurs (though probably already extinct before it happened; Dravidosaurus‘ status as a possible stegosaur/plesiosaur chimaera instead of just a plesiosaur has been considered)
– Polacanthids (though some nodosaur remains in Appalachia could belong to polacanthids)
– Most theropod clades (sans Tyrannosauria, Ornithomimosauria, Maniraptora, Abelisauria and a few neovenatorids and spinosaurs)
– Pholidosauridae, and possibly also Thalattosuchia (if they survived later than previously thought)
– Most ornithopods, with Hadrosauria, Rhabdodontidae, Thescelosauridae and a few other taxa being the exception
This, of course, accounts for fauna in areas with known post-Turonian fossil reccords; we have little to no evidence of species living in Africa and Australia, which could have preserved several taxa mentioned here.
For the purposes of this article, though, we will focus on the areas we do know that have a decent fossil reccord in the post-CTM Cretaceous. As you can see, the extinction event left enormous niche gaps, which were promptly filled by numerous animal species. Dinosaurs jumped into the vacant niches, but so did many other vertebrates.
However, recent discoveries seem to suggest that the CTM was not as influencial to the ascension of non-dinosaurian critters; many were appearently already expanding before the massive hit.
By the late Cretaceous, crurotarsans clearly got their revenge after being killed in the Triassic extinctions. Post-CTM Cretaceous is abundant with crocodyllian taxa, and unlike the modern aquatic crocodiles, Mesozoic forms were far more diverse, the majority of which being terrestrial animals. New discoveries show that terrestrial crocodyllians were not only successful, but actually very mammal like: they were clearly omnivorous, endothermic critters with heterodont dentitions, with some some forms having teeth easily mistaken for that of therian mammals, and some might even have had whiskers or even lips. It is thought that these were the true competitors to mammals in the Mesozoic; where mammals were smaller, crocodyllians were present, and only grew larger when the terrestrial pseudosuchians were absent.
At the time of the CTM, however, crocodyllians were no mere mammal competitors. Africa’s fossil reccord shows an immense variety of land crocodyllians, from the omnivorous Araripesuchus and Anatosuchus to the giant, dragon like Kaprosuchus. This strange procession of crocodyllians was most defenitely preventing mammals from diversifying, but they were also jumping at dinosaur niches; small ornithopods are pratically unheard off not only in Africa, but also in all of post-CTM Gondwanna, with the exception of Gasparinisaura and probably the thescelosaurids (which are thought of being of Gondwannan ancestry). In contrast, small to medium sized omnivorous notosuchians like Araripesuchus and Armadillosuchus are prevalent, heavily suggesting that ornithopods were replaced by these animals. This seems to coincide with the disappearence of small ornithopods in Laurasia, where they were replaced by marginocephalians, and possibly also oviraptors.
A more extreme case, however, occured with baurusuchids. These crocodyllians not only ventured into large macropredator niches, they actually also replaced their theropod competitors. Large theropods are virtually absent from post-Turonian Brazil, the exceptions being one or two abelisaur genera (south american abelisaurs are thought to have been cheetah like predators, thus being so specialised as to offer little competition with other theropods); by contrast, several baurusuchid genera occur in Brazil, having clearly replaced the local theropods. In fact, the only theropods known in post-Turonian South America occur in Patagonia, having been pushed into the colder zones. Likewise, abelisaurs occur in India/Madagascar, but they are only common in what was thought to have been the southern zones of this landmass, with baurusuchids dominating what is now Pakistan. Other terrestrial crocodyllians like Mahajangasuchus occured in Madagascar, where the only small non-avian theropods are Masiakasaurus and Rahonavis, both of which not competing with the crocodyllians.
Pterosaurs, as previously discussed, not only produced several clades of terrestrial predators, but in fact competed viciously with theropod dinosaurs. Already by the early Cretaceous, azhdarchoids and dsungaripteroids occupied niches associated with those of small theropods, hunting prey on the ground; some clades went even further. Dsungaripteroids and thalassodromids might have ventured in macropredatory niches, while tapejarids and possibly also chaoyangopterids were terrestrial omnivores, not unlike coelurosaurs like ornithomimids or ceratosaurs like Limusaurus. Unlike birds, pterosaurs could easily exploit terrestrial niches without sacrificing flight capacities, and if alive today, they’d probably far more efficient competitors towards mammals than birds are.
The CTM killed off many pterosaur clades, and indeed the fact that pterosaurs were efficient competitors to theropods becomes evident when ornithomimids and oviraptors were clearly “cheap” replacements for tapejarids, chaoyangopterids and dsungaripterids, having occupied the niches left vacant by terrestrial, omnivorous pterosaurs. However, the CTM also killed off many theropod clades, and this opened new paths for the surving pterosaur taxa. Azhdarchids rapidly increased in size, producing the iconic giraffe sized forms. Indeed, in North America, medium sized theropods were absent; between the dog sized dromaeosaurs and the enormous tyrannosaurs, azhdarchids occupied the intermediary predator niches, akin to pterosaurian canines. The only other medium sized predators, troodontids, were omnivorous, and only in Alaska did they both grow to large sizes and actually displayed macropredatory tendencies. In particular, Quetzalcoatlus seems to be associated with Alamosaurus, and it is possible that it was a titanosaur specialist, feeding on the juveniles and weak adults; no other predator seems to be associated with these titanosaurs, making Quetzalcoatlus not only a macropredator and one of the two largest flying vertebrates, but the only consistent threat to one of Maastrichtian North America’s largest animals.
Furthermore, Hatzegopteryx and Bakonydraco seem to be the dominant predators in their island habitats; the latter is actually the only carnivore known from it’s fossil site. Their only competitors seem to have been abelisaurs, and even then european abelisaurs seem to be less common than the azhdarchids. Balaur bondoc has been cited as a potential competitor, but given the dromaeosaur’s reduced predatory adaptations, it was more likely part of Hatzegopteryx‘s menu.
Only in South America did azhdarchids had competitors, in the form of unenlagiines. But like other theropods, these maniraptors only occur in the extreme south, while azhdarchids have a more widespread distribution, meaning that the pterosaurs, like the crocodyllians, replaced their dinosaurian competitors elsewhere.
At the time this post was written, the idea that mammals were sorry little shrews when dinosaurs abominated the Earth was most defenitely a myth. Otter, badger, anteater and even wolverine like mammals are well known from the Mesozoic’s fossil reccord, such as the aquatic Castorocauda, the omnivorous Cimolestes and the quasi-macropredator Repenomamus. Giant monotremates like Steropodon not only roamed Australia’s rivers, but were actually big enough to drag the contemporary dinosaurs to a watery death, while the Cretaceous skies were probably already colonised by flying mammals related to Volaticotherium.
Compared to crocodyllians and pterosaurs, mammals remained fairly unimpressive, but a subtle war is still efficient. Mammals larger than badgers already occured in the Jurassic and Lower Cretaceous, and after the CTM, they quickly became the norm. Metatherians in particular seem to have led the way; in North American, stagodontids like the iconic Didelphodon exploited badger, wolverine and even otter like niches, while in Asia deltatheroids expanded in a similar manner. Likewise, multituberculates seem to have also increased in size, coinciding with the decline of small ornithopods, though admitely multies only truly became large after dinosaurs became extinct.
However, a possible extreme case of mammalian success can probably be expressed in the bizarre mammal known as Kharmerungulatum. While it’s exact affinites are not cleared out, it is probably a basal meridiungulate, a distant relative of the bizarre south american “ungulates” that would evolve in the Cenozoic. Regardless, this mammal already shows evidence of having been dedicated towards herbivory, and should it have survived, it could had fathered an entire lineage of “ungulates” as the Cenozoic arrived. The australian Tingamarra was likely related to it, having evolved from a common ancestor in Cretaceous Antartica.
The also gondwannan gondwanatheres were appearently also specialised herbivores, their teeth similar to those of modern grazing mammals, further indicating that herbivorous niches previously occupied by small ornithopods were being filled by mammals.
Mammals, crocodyllians and pterosaurs were clearly moving into dinosaurian niches even before the advent of the CTM. Should the KT event not have occured, it is possible that dinosaurs could have further been replaced by their more generalistic competitors, specially with the occurence of the Paleocene Thermal Maximum, where the polar strongholds of theropods would have fallen to azhdarchids and baurusuchids.
People tend to forget that dinosaurs are just mere animals, not ancient dragon gods as media depicts them, and thus were subject to normal pressures like competition with other clades. Indeed, the fact that the only dinosaurs that colonised marine ecosystems were seabirds alone demonstrates their status as regular animals; they weren’t “superior” to the plesiosaurs, ichthyosaurs, thalattosuchians or mosasaurs that governed the Mesozoic oceans. Likewise, on land their survival wasn’t taken for granted, and just like Cenozoic mammals, crocodyllians and squamates were replaced by less specialised critters multiple times, so were Mesozoic archosaurs and synapsids.
In particular, Gondwanna seems to have been the hotspot for such drastic faunal changes (a strong contrast with the usual view that Gondwanna’s life forms were more “conservative”), while Laurasia’s dinosaur reign was more stable.
RIFF, D., & KELLNER, A. (2011). Baurusuchid crocodyliforms as theropod mimics: clues from the skull and appendicular morphology of Stratiotosuchus maxhechti (Upper Cretaceous of Brazil) Zoological Journal of the Linnean Society, 163 DOI: 10.1111/j.1096-3642.2011.00713.x