Competition among western interior seaway predators, specifically concerning Cretodus

[Jared C]

Hey all, I'll try my best to be brief but detailed in my question, but I'd like it to be a discussion as well, if there is one to be had.

 

From what I understand, the Western Interior Seaway had what appeared to be too many large, active predators for a similar environment to support, especially when one considers how shallow the seaway was. There were the many species of Mosasaurs, with other large predators like Xiphactinus, with the typical western interior seaway sharks as well. 

 

This would make me think that that there are two possible outcomes - either an absolutely enormous supply of prey at the lower trophic levels, or some serious competition among the predators of the higher trophic levels...or maybe both.

 

Now, I understand niche partitioning plays a role. For example, evidence suggests Squalicorax primarily scavenged, while Cretoxyrhina were likely sight oriented, agile top predators. While that may put these particular sharks away from competing with each other, other predators, for example, did theoretically fill the same niches as Cretoxyrhina.

 

For example, the fact that Xiphactinus, Cretoxyrhina, and some large Mosasaurs can all be found in the same locales sometimes suggests not only that that these large, agile, theoretically "top predators" not only lived at the same time, competing for (probably) the same prey, but were in each other's neighborhood as well. How is this possible, or sustainable?

 

Now, perhaps I have my time lines wrong, perhaps a million years this way or that..So this is where I have a few more species specific questions. For example: Tylosaurus is a genus that can be found, for example, in the north sulfur river, and is late Cretaceous. Mosasaurus maximus is also late cretaceous, and the one specimen I know of that comes to mind was found in Austin, supposedly in the "Navarro" formation, whereas the NSR is Ozan.

While I couldn't find an exact age for the Navarro, both are late Cretaceous, and at first glance there's not much to suggest that M. maximus and Tylosaurs were much different from each other... did they really evolve to be in direct competition?

 

The same could be said with Cretoxyrhina and Cretodus. I found shockingly little about Cretodus on the web, but what I did find was that they were able to determine from that lovely speciman from Kansas that at least Cretodus houghtonorum could reach somewhere around 22 feet in length, whereas it's thought Cretoxyrhina could achieve lengths of 26 feet. Again, these are both large active predators that (I think) lived at the same time, and if looking at the teeth on Cretodus, it does seem reasonable to think that they were active predators - putting them in direct competition with each other and the rest of the active seaway predators.

 

If you made it this far through my ramblings, thank you! If you know more than me please chip in - I'm quite specifically curious about Cretodus's role in the the food web, but have a broader interest too in trying to understand how all these mega sized predators co-existed with one another.

 

[Creek - Don]

From what I read fish diversity was low in the Cretaceous period compared to today. Biodiversity in the oceans exploded after dinosaurs fell after about 10 million years later.  There must been lot of competition among the sharks and mosasaurs searching for food.  That's just my guess.   However, there were lots other ammonites, belemnites, and squids which were vast part of the food chain  Not sure these were part of the sharks food chain, but mosasaurs sure did enjoyed them.  

[Jared C]

2 hours ago, Creek - Don said:

Biodiversity in the oceans exploded after dinosaurs fell after about 10 million years later.

I didn't know that - and I did forget about the enormous amount of ammonites that were present. However, the various Ptychodus species likely had diets that were ammonite heavy, especially when one remembers how large they got. Most mosasaurs would have been better generalists than a relatively niche specific shark like Ptychodus though, so I imagine it probably wouldn't have been much a problem for either of them, from each other at least.

 

Mosasaurs on the other hand must of had that competition on both fronts then, from the fast fish eaters to the ammonite eaters. I wonder if this rich predator environment was a contributing factor to the decline of the ichthyosaurs around the early-middle cretaceous, even outside of the western interior sea.

[bthemoose]

I would highly recommend the book Oceans of Kansas by Michael Everhart. While it won’t answer all of your questions, it’s a great overview of what the fossil record (primarily focusing on the Smoky Hill Chalk in Kansas) reveals about the fauna of the Western Interior Sea, at least within a specific region of the sea during a part of the Late Cretaceous. This includes some “whom ate whom” to the extent that’s preserved in the record.
 

I’ll note that there’s not as much on Cretodus in the book as there is on the others you mentioned as I don’t believe Cretodus is as represented in Kansas as it is in Texas. That fact might reveal something about the different time or habitat ranges of species within the sea, but what specifically is beyond my knowledge.

 

There’s an Oceans of Kansas website from Everhart as well, which is a great resource, but you’ll need the book to get much of the above.

Edited May 8, 2021 by bthemoose

[Jared C]

4 hours ago, bthemoose said:

but you’ll need the book

thanks! I'll be sure to read it

[jdp]

think about it this way: It's not rare to see many species of marine mammal in a single boat ride in many coastal areas. So, we know that numerous large modern marine mammals (as well as smaller marine mammals) can coexist in a single ecosystem, including relatively perturbed modern ecosystems in places like the Mediterranean, Caribbean, or Pacific Coast, and we know that marine mammal diversity dropped over the Pleistocene. So, there's not really much reason to think that the diversity of large marine reptiles in the Mesozoic was "too high." In reality, these are incredibly productive ecosystems which can readily support a diversity of large predators.

[PaleoNoel]

4 hours ago, jdp said:

 In reality, these are incredibly productive ecosystems which can readily support a diversity of large predators.

I agree with @jdp. Considering that a number of these formations are chalk deposits which are formed by high quantities of plankton settling on the seafloor, it makes more sense as to why such a diverse ecosystem, seemingly packed to the brim with predators, can exist. 

Also consider that humans have caused a massive drop off in oceanic diversity in the last several hundred years. Our modern perspective may be skewed over how many predatory species can coexist in a single area.

[pachy-pleuro-whatnot-odon]

  

  

As @jdp and @PaleoNoel said, although the Cretaceous seaways appear packed with large predators, this appears to have been sustainable with evidence pointing towards rich and productive marine ecosystems. Moreover, this wasn't the first time in history that this many predators roamed the seas together. In fact, concerning this matter - though with reference to the European seas around the UK - Fischer et al. (2014, p. 23) notes that

Quote

ichthyosaurs therefore occupied up to three feedings guilds within the single ecosystem of the upper Gault Formation/Cambridge Greensand Member, despite the presence of a diversified plesiosaur assemblage including the gigantic pliosaur Polyptychodon interruptus. The presence of ichthyosaurs at several levels of the trophic chain of one ecosystem has not been previously reported from assemblages dating to after the Early Jurassic, when ichthyosaurs dominated the ecosystems of the European archipelago together with several plesiosaur taxa, including large rhomaleosaurids.

 

 

On 5/8/2021 at 7:15 AM, Jared C said:

Mosasaurs on the other hand must of had that competition on both fronts then, from the fast fish eaters to the ammonite eaters. I wonder if this rich predator environment was a contributing factor to the decline of the ichthyosaurs around the early-middle cretaceous, even outside of the western interior sea.

 

This again doesn't appear to have been the case, as Fischer et al. (ibid.) observes that

Quote

the fact that ichthyosaurs from the late Albian–early Cenomanian deposits of Europe and possibly Russia, like their Early Jurassic ancestors, colonized multiple ecological niches despite the presence of numerous other marine reptile taxa shows that the ‘last’ ichthyosaurs were still a diversified and important component of the marine ecosystems up to a few millions years prior to their extinction, at least in Europe and Russia.

[...]

These high taxonomic richnesses and strong ecological presences occur a few million years prior to the final extinction of ichthyosaurs. This indicates that the ‘last’ ichthyosaurs were diversified and were still a major component of marine ecosystems, contradicting previous views of ichthyosaur evolutionary history. The alpha diversity of ichthyosaur is, however, highly variable between provinces. This new data provides a whole new context to analyze the extinction of ichthyosaurs.

 

In a later publication,   Fischer et al. (2016, p. 7-8) go on to state that

Quote

one major issue of the competition hypotheses are their geographical and temporal discrepancies. The earliest large-bodied mosasauroids, which are the only marine squamates that could have reasonably competed with ichthyosaurs in terms of prey type, prey size and prey location, are Middle Turonian in age, thus appearing about 3 million years after the last appearance of ichthyosaurs [in the Early Turonian] (and likely radiating to fill at least some of their niches).

[...]

[M]any authors have previously suggested that Cretaceous ichthyosaurs were depauperate in taxonomic and/or ecological diversity.Lingham-Soliar linked this decline with the radiation of teleosts and chondrichthyans, which would have slowly outcompeted ichthyosaurs in their niche of fast thunniform swimmers. However, our data demonstrate that Cretaceous ichthyosaurs were actually about as diverse (taxonomically and ecologically) as they were during the Middle–Late Jurassic, and apparently were at their most disparate phase since the Triassic. The scenario of slow but steady replacement is therefore not substantiated by the data.

 

[Jared C]

5 hours ago, pachy-pleuro-whatnot-odon said:

  

  

As @jdp and @PaleoNoel said, although the Cretaceous seaways appear packed with large predators, this appears to have been sustainable with evidence pointing towards rich and productive marine ecosystems. Moreover, this wasn't the first time in history that this many predators roamed the seas together. In fact, concerning this matter - though with reference to the European seas around the UK - Fischer et al. (2014, p. 23) notes that

 

 

 

This again doesn't appear to have been the case, as Fischer et al. (ibid.) observes that

 

In a later publication,   Fischer et al. (2016, p. 7-8) go on to state that

 

Amazing, thank you for sharing such a thorough analysis 

[Crusty_Crab]

When asking questions about the ecology of fossils, I think you need to address the issue of taphonomy. Is the fauna you're seeing in the fossil record an impartial record of the assemblage at any single time? The answer is no for the vast majority of fossil localities, some species fossilize better than others, or some are preferentially collected (i.e. large charismatic macro fauna vs. small invertebrates) which does not reflect their relative abundance at a single point in time.

 

Also, time is a factor. What is the time range the formation represents? For example, if you are comparing animals within the late Cretaceous, this encompasses a timespan of ~30 million years. If someone compressed the current last 30 million years, they might conclude that Megalodon, all transitional species of the great white, the great white, and orcas existed at the same time. In reality, the environment was dynamic with species evolving and dying out and not all species co-existing at the same time and place.  

[siteseer]

2 hours ago, Crusty_Crab said:

When asking questions about the ecology of fossils, I think you need to address the issue of taphonomy. Is the fauna you're seeing in the fossil record an impartial record of the assemblage at any single time? The answer is no for the vast majority of fossil localities, some species fossilize better than others, or some are preferentially collected (i.e. large charismatic macro fauna vs. small invertebrates) which does not reflect their relative abundance at a single point in time.

 

Also, time is a factor. What is the time range the formation represents? For example, if you are comparing animals within the late Cretaceous, this encompasses a timespan of ~30 million years. If someone compressed the current last 30 million years, they might conclude that Megalodon, all transitional species of the great white, the great white, and orcas existed at the same time. In reality, the environment was dynamic with species evolving and dying out and not all species co-existing at the same time and place.  

 

When you consider just the Niobrara Chalk, that formation is said to represent at least 5 million years.  There were likely some large reptiles that existed before that time and lived into that time and might have died out within that time as well as some that appeared during that time and lived on after the deposition of the formation.  It's been a while since I've read the first edition of "Oceans of Kansas."   It's possible there were animals that lived only during that time.  As I recall, assigning Hattin markers was important so that kind of thing could be determined.

 

Some predators are open ocean animals and may or may not have been rare in the CIS.  A Cretodus tooth is uncommon-rare wherever it is known from.  Cretoxyrhina appears to be one of the most common teeth in the Niobrara when you find a tooth.

[pachy-pleuro-whatnot-odon]

@Praefectus just shared a very interesting blog-post by Nick Longrich elsewhere on the forum that, though written with reference to the Moroccan phosphates, deals with the exact same question as has been asked here. Very instructive, and I highly recommend the read.

 

 

Edited May 13, 2021 by pachy-pleuro-whatnot-odon