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Even without the asteroid, dinosaurs were doomed

February 17, 2012

A group of Baurusuchus killing a titanosaur with inaccurate feet. Terrestrial crocodyllians were slowly replacing theropods as the dominant predators in Cretaceous Gondwanna; baurusuchids are dominant in Brazil's and Pakistan's fossil sites, while theropods are more common in southern areas, where the crocodyllians weren't s common.

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)

Cretaceous period. Between the periods known as Cenomanian and Turonian, a series of extinction events occured, appearently based on a phenomenon known as Cretaceous Thermal Maximum.

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

– Ichthyosaurs

– 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.

Crocodyllians

Araripesuchus rattoides, or "RatCroc". Small terrestrial crocodyllians like these appearently began replacing small ornithopods.

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.

Post-Turonian Cretaceous South America. The dotted areas are zones where theropods (red) and baurusuchids (green) are known to have occured; as the pic indicates, baurusuchids seem to have been dominant in northern areas, while theropods were only predominant in the colder south. Should dinosaurs have survived to see the Paleocene Thermal Maximum, chances are that baurusuchids would have replaced the south american theropods almost completly.

Pterosaurs

Hatzegopteryx as depicted in Planet Dinosaur. In Late Cretaceous Europe, azhdarchids were the dominant predators, with only a few abelisaurs as competitors.

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.

Mammals

Didelphodon. Stagodontids were badger and otter like metatherians prevalent in Late Cretaceous North America; mammals like them were becoming increasingly common as the Cretaceous went by.

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.

Veredict

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.

 

Refferences:

DinosaurMailingList

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

29 Comments leave one →
  1. That guy from spec and deviantart that won permalink
    February 18, 2012 10:07 pm

    So which groups do you think would go extinct Faa?

    • February 18, 2012 11:17 pm

      Theropods would defenitely suffer tremendous blows; I think only the more specialised and unorthodox ones would survive.

      Ornithopods have competitors on all sides, so they too would disappear, although at a slower pace, and I think some hadrosaurs could still be alive by the present time.

      Ankylosaurs would most certainly be gone by the Paleocene Thermal Maximum.

      Pachycephalosaurs and ornithomimids both are noted to have laid proportionally large eggs, which might indicate specialisation, but they were fairly generalistic, so maybe they could live on.

      In the end, ceratopsians (aside from the huge ceratopsids, obviously) and maniraptors are the ones with the brightest futures.

      • That guy from spec and deviantart that won't stop pestering you permalink
        February 19, 2012 1:09 am

        While its true barusuchids would do well in the eocene they would have major setbacks once Antartica freezes. So I guess its really a question of which theropods could survive the Eocene…

        As you said before the ornithopods could do well in Australia but the non-hadrosaurs may be doomed in the Northern hemisphere. I could also see abelisaurs surviving in Australia as well considering that during the thermal maximum it was so close to south pole

        Why would anklyosaurs die out? I mean I can see what would replace them but they seemed to be doing fine when the end came

        Ornithomimids might replace therzirinosaurs should the latter die out. I could see something like specs jackalopes arising from the pachycephalosaurs.

        Agreed

  2. February 19, 2012 1:19 am

    “While its true barusuchids would do well in the eocene they would have major setbacks once Antartica freezes. So I guess its really a question of which theropods could survive the Eocene…”

    Well, the way I invision things:

    – Paleocene Thermal Maximum: end of the line for Gondwannan theropods.
    – Eocene: baurusuchids get to Laurasia, end of the line for tyrannosaurs.
    – Miocene onwards: baurusuchids get restricted to the tropics, while dromaeosaurs and predatory mammals begin filling the empty niches.

    “As you said before the ornithopods could do well in Australia but the non-hadrosaurs may be doomed in the Northern hemisphere. I could also see abelisaurs surviving in Australia as well considering that during the thermal maximum it was so close to south pole”

    Do remember that, with the Paleocene Thermal Maximum, even Antartica has a subtropical climate (a really weird one, what with the polar nights, but I digress), though abelisaurs could co-exist with baurusuchids anyway. I however have doubts if abelisaurs were present in Australia, and if baurusuchids were either for that matter; transit of taxa in Antartica could had been stalled due to the polar nights. That’s why we don’t have sloths in Oz, for instance.

    “Why would anklyosaurs die out? I mean I can see what would replace them but they seemed to be doing fine when the end came”

    They were fairly specialised, and critters like Armadillosuchus were more adaptable, so…

    “Ornithomimids might replace therzirinosaurs should the latter die out. I could see something like specs jackalopes arising from the pachycephalosaurs”.

    I like this, very much so. I’ve always preffered to work with ornithomimids over therizinosaurs.

    • That guy from spec and deviantart that won't stop pestering you permalink
      February 19, 2012 2:01 am

      Couldn’t noasaurs survive as well? After all they weren’t competing directly with barusuchids.

    • Random Guy permalink
      April 20, 2012 8:37 am

      I wouldn’t say the Eocene would be the end of the line for tyrannosauroids. Some dryptosaurs could survive as spinosaur, cheetah and even dromaeosaur analogues.

  3. Duane permalink
    February 20, 2012 6:41 am

    In defense of theropods

    http://blogs.smithsonianmag.com/dinosaur/2010/11/cretaceous-croc-bones-were-dinosaur-table-scraps/
    http://www.ameghiniana.org.ar/index.php/ameghiniana/article/view/166

    Although the author does not posit a known theropod, they propose a large carcharodontosaurid type theropod. Is it possible there were actually barasuchids of t-rex proportions that caused the bite marks?

    Anyways, good article. Looking at S.America in particular that continents tenure for various large predator dynasties seems exceptionally short lived throughout geologic history.

    • February 20, 2012 3:38 pm

      Since it took place in Argentina, where theropods were more common than baurusuchids (which were prevalent in Brazil, hence the hypothesis that theropods held on on colder zones while baurusuchids replaced them in northern latitudes), I’m not surprised, and peirosaurids were not top predators anyway, being coyote/jackal analogues.

      I suppose the critter who did that was more likely a neovenatorid like Orkoraptor, because carcharodontosaurs were rare if not outright extinct after the Turonian.

    • February 20, 2012 3:40 pm

      Of course, with the discovery of critters like Fasalosuchus, gigantic crocodyllians are not out of the question either, specially because allosauroids like carcharodontids had weak bites.

  4. March 3, 2012 5:22 am

    I’m quite sure that some of the abelisaurs would survive as cursorial predators (as shown by the Pakistani Vitakridrinda and Pabweishi), and no crocodylian would ever replace the sauropods and the bigger ornithopods of

    And if baurusuchids did invade Laurasia, the rather polar and boreal ecosystems would survive (along with it’s tyrannosaurids, burrowing ornithopods, dromaeosaurs, and those animals you would find in Maastrichtian Canadian deposits)

    The polar Australian environments would also survive, as is the Mongolian and Chinese environments

    Also the European insular environments would survive unscathed (except if those omnivorous crocodylians develop aquatic locomotion)

    The only place of enigma is Africa, we really need non-marine Maastrichtian African deposits, yes? 😄

    • March 3, 2012 9:00 am

      Well, the basic ecological role of sauropods and titanosaurs could easily be replaced with time by crocodyllians and “ungulates”, though giant ungulates co-existing with sauropods and hadrosaurs would be more awesome.

      Also, the boreal environments, I think, would become warm enough to allow sebecians in them, though they’d have to conquer them before the Azolla Event.

      Also, sebecians already existed in Africa and the Iberian Peninsula, so they pretty much were advancing in on all angles.

      • That guy from spec and deviantart that won't stop pestering you permalink
        March 10, 2012 3:52 pm

        Thats the thing though. The Global Rainforests of the Eocene didn’t last very long in Geologic time scales and while its true that barrusuchids would rule South America, I doubt they could of spread out of India fast enough to take over the Arctic. I am not saying they wouldn’t do well I am just saying I don’t see them having enough time to go truly global before the Azolla event.

  5. March 10, 2012 3:56 pm

    Depends on the level of impact of the Azolla event. With more herbivores everywhere, it is possible that Azolla covers might not reach the same extensiveness as they did in our world.

  6. Random Guy permalink
    April 20, 2012 5:42 am

    Come on scientists, clone some baurusuchids and put them in the houses of every wolfaboo on Earth!

    • Random Guy permalink
      April 27, 2012 11:51 pm

      And put an azdarchid in the houses of those bastards who got you banned from DA. Then feed the scraps to a hyena.

  7. Random Guy permalink
    May 2, 2012 4:25 am

    It would be hilarious if Dravidosaurus turned out to be a plesiosaur with tail spikes, back plates and a herbivorous diet. 😆

    On a more on topic note, would it be possible for cetaceans, bats, rodents and carnivorans to evolve even if the K-T never happened, considering that dinosaurs and marine reptiles (Except choristoderes and possible palaeophids) were being outcompeted?

    • May 2, 2012 3:05 pm

      They could, but I don’t think they would. We have no evidence of derived laurasitheres or euarchontans, or even Boreotheria as a whole, so evolution could take a totally different path and create totally different mammal groups.

  8. Random Guy permalink
    June 12, 2012 9:36 am

    I just realized that Poposaurus looks like the V.Rex from King Kong. 😯

  9. December 13, 2012 8:11 pm

    Just wondering, even if Baurusuchids are still dominant in Brazil compared to theropods, what do you think about these supposed unenlagiine and megaraptoran remains? Do you think they were just lucky, or were specialised in some other niche (which I can definitely see in the former).

    Also, interesting to note is that after the Eocene, South America was kind of similar – phorusrhacids were dominant predators in the south, yet sebecids were dominant in the north.

    • December 13, 2012 8:17 pm

      Unenlagiinae is easy – they were already somewhat specialised heron like piscivores. As for megaraptorans, they seem fairly rare and unspecialised, compared to other Higher Cretaceous macropredators.

      The last is quite fascinating.

  10. Khalil Beiting permalink
    September 20, 2013 2:57 pm

    MY MIND HAS BEEN BLOWN. I always wondered why there were barely any carnivorous theropods in many parts of Gondwana.

    • September 20, 2013 5:50 pm

      Indeed, only abelisaurs occupied traditional theropod niches in Gondwanna, and even then only a few species show adaptations to deal with large prey. Orkoraptor was pretty much the only other well known non-piscivorous/generalistic omnivorous theropod, and it was restricted to Patagonia.

      Crocodylomorphs, on the other hand, have a variety of predatory groups.

  11. Vasika permalink
    May 8, 2014 3:58 pm

    Do you think a line of gigantic sebecids could survive into Later Tertiary and Quarternary times (long enough to cross over into North America) and then into Eurasia? Look I know a situation of this sort seems absolutely impossible but just out of curiosity….do you think they could live that long out of South America?

    • May 10, 2014 1:06 am

      Yes.

      • Vasika permalink
        May 10, 2014 12:47 pm

        Interesting to know. But how long until they face competition from large mammalian carnivores, do you think? Long past the Miocene? Or would they have to become low-energy niche herbivores like giant sloths?

      • May 10, 2014 9:33 pm

        Well, in HE they did co-exist with sparassodonts and whatever caused their extinction doesn’t seem to be related to competition (since sparassodonts were at their prime when Cenozoic sebecids were still common), so I’m in favour of arguing for niche partitioning.

  12. January 3, 2016 5:36 pm

    I don’t remember the source, but I have read that small or medium-sized dinosaurs did not evolve because the young of larger species ocupied this ecological nitche. Seems more plausible. Also, isn’t it now agreed that Quetzalcoatlus was a stalker of small terrestrial prey, rather than a macropredator? Didn’t spinosaurus become a successful marine animal as well?
    It seems crocodilians spread only when temperatures rose significantly. Does it mean that they had less efficient thermoregulation, even if they were warm-blooded? And what could happen if then the Earth cooled, and their distributions contracted?? Dinosaurs would take over again.

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