Monday, May 01, 2023

No Stegosaur Comeback

Some researchers believe a stegosaur will be found at Doellings Bowl, an early Cretaceous site in Utah. Forming the base of the Yellow Cat, the lowest member of the Cedar Mountain formation, Doellings Bowl has yielded iguanodonts, dromaeosaurs, polacanthid ankylosaurs, sauropods, therizinosaurs and a carcharodontosaur. Currently, no stegosaurs are known from the site. Their presence is thought probable, however, because Doellings Bowl formed only about a stage after the stegosaur heyday of the late Jurassic. In addition, stegosaurs persisted into the early Cretaceous on other continents. Inasmuch as Morrison sauropods appear to have vanished by the Cretaceous, replaced by European forms, Stegosaurus could have suffered the same fate. Perhaps its niche was taken by European taxa such as Dacentrurus or Miragaia. The latter, in fact, was already present in the late Jurassic (Morrison formation) of America.

Survival of Stegosauria into Yellow Cat times is not likely, however. The Jurassic-Cretaceous transition witnessed a wetter climate, which proved inimical to plated dinosaurs. With the exception of Hesperosaurus mjosi from the base of the Morrison, local stegosaurs were adapted to a dry climate. This appears to have been true of Stegosaurus. Occuring higher in the Morrison (or later) than Hesperosaurus, Stegosaurus would've been vulnerable to wetter conditions. It wouldn't be surprising if the iconic stegosaur, like coeval sauropods (also dry-adapted) vanished at the end of the Jurassic.

Based on the record of the Yellow Cat, no stegosaur, local or European, persisted into the Berriasian stage. Doellings Bowl taxa are essentially the same as higher (Valanginian) taxa. Thriving under humid conditions, therizinosaurs, iguandonts and polacanthids predominated. Inasmuch as no stegosaur was ever found in the Valanginian aged deposits, they seem equally unlikely at Doellings Bowl, where similar taxa suggest a similar environment. And this environment excluded plated dinosaurs.

Even in Cretaceous environments highly reminiscent of the Jurassic, stegosaurs apparently never returned to regions where they had vanished. Africa was one example.

The Cenomanian Baharije environment had big sauropods, carcharodontosaurs and spinosaurs. The much older (late Jurassic) Tendaguru had a similar fauna suggestive of similar habitats. The Baharije looks like the Tendaguru revived, but with an important difference. Stegosaurs had vanished. Not only had they disappeared, for some reason they couldn't reappear despite prima facie evidence for a favorable environment.

The same appears true in the middle Cretaceous of America. At the time there was faunal exchange between Asia and America. Caenagnathids, hadrosauroids, ceratopsians, tyrannosauroids etc radiated across beringia and entered both continents. Surprisingly, though, stegosaurs--still present in Asia down to at least Aptian--did not recolonize America. There is no sign of a stegosaur in the Antlers formation or the middle Cedar Mountain (Ruby ranch) member. Both have yielded big allosauroids and sauropod giants, yet even in habitats which seem to mimic the Morrison to a considerable degree, plated dinosaurs remained defunct. Although allosauroid and sauropod remains in the Doellings Bowl suggest persistence of some favorable habitat, hence the possibility of a stegosaur, the record elsewhere argues this is illusory.

Why were stegosaur habitats ruined permanently, in one region after another? Wetter conditions probably undermined the key vegetation of stegosaurs--cycadopohytes and bennitatales. Stomach contents of Isaberrysaura confirm that stegosaurs required such plants, yet they became increasingly rare in the Cretaceous. Angiosperms proliferated at their expense. As narrow-snouted, selective feeders, stegosaurs could not easily adapt to other plants, or if they could they faced too much competition from ankylosaurs and iguanodonts which could more efficiently process it.

7 Comments:

Anonymous Neal Robbins said...


Stegosaurs obviously could not survive in the Cretaceous environments. Lack of the right types of foods must have been a major factor. Predation from theropods would not have caused their extinction. The Jurassic period had many powerful carnivorous theropods, including Allosaurus, but stegosaurs thrived during that period.

5:10 AM  
Blogger starman said...

I agree absolutely, Neal. Ever since they evolved, stegosaurs were adapted to deal with predators. In addition to thagomizers, their large dorsal osteoderms intimidated many predators, preventing attacks in the first place. Stegosaurs obviously withstood big theropods for the first four stages of their documented existence--from early Bathonian to early Tithonian. Loss of preferred vegetation due to climate change seems the best explanation for their demise.


May 3, 2023

12:43 AM  
Anonymous Anonymous said...

Caenegnathid oviraptoriforms may have entered North America from Europe based on a partial cervical vertebra from the Wealden. I had thought it was therizinosaur, but Darren Naish corrected me as to its Caenignathid affinities. Also no big sauropods in the Mussentuchit...just tiny teeth going out? as part of the Asian immigration induced extinction at base of Upper Cretaceous?

7:42 AM  
Blogger starman said...

Thank you Anonymous for your input!
No big sauropods in the Mussentuchit, or just its uppermost part, which produced Siats and Eolambia? I thought the Mussentuchit had Abydosaurus.
I was under the impression the sauropod hiatus in North America resulted from a highstand or two. This would've led to a more humid climate favorable to the ornithopod Eolambia--possibly descended from the asiatic Choyrodon--as opposed to sauropods.


May 4, 2023

12:56 AM  
Blogger Martin wong said...

I don't enough about the survivability of the stegosaurus so I poached a link from Google



Answer to Can Stegosaurus live and survive in the Cretaceous period? by David M. Prus https://www.quora.com/Can-Stegosaurus-live-and-survive-in-the-Cretaceous-period/answer/David-M-Prus?ch=15&oid=1477743631617540&share=3f91348f&srid=6Ierq&target_type=answer

1:58 AM  
Blogger Martin wong said...

Can Stegosaurus live and survive in the Cretaceous period?
Well, in the early Cretaceous some stegosaurs did survive- Mongolostegus , Paranthodon , Wuerhosaurus , Craterosaurus, and Regnosaurus

However, they were rare and smaller than Stegosaurus. By this time, ankylosaurs had become far more common, and most low browsers were ornithopods. Ornithopods were far more efficient in processing food and lived in much larger groups. By the end of the Cretaceous, we have ankylosaurs the size of Stegosaurus, and the fauna are overwhelmingly big hadrosaurs and ceratopsians.

The vegetation changed with cycadoids in decline and angiosperms dominant, the climate changed, and even the predators changed to more intelligent, agile Tyrannosaurs.

Stegosaurus might have a chance in earlier Iguanodont-dominated faunas if only in small numbers, but by the late Cretaceous they were outmoded

2:00 AM  
Blogger starman said...

Thanks for your input, Martin. :)

Even in the sauropod habitats of the Cretaceous, stegosaurs apparently disappeared by about 100 m.y.a. They seem to have disappeared in Europe in Barremian time or earlier, and in Gondwana around early Aptian at the latest. In Asia they held out at least til late Aptian and may have vanished entirely at the end of the Albian.
Stegosaurs seem to have done best in sauropod dominated environments and waned in areas with many iguanodonts and ankylosaurs.

May 8, 2023

2:17 AM  

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