Mosasaurs achieving thunniform bauplan?

[cj227]

If the K/T extinction had no happened, would mosasaurs eventually develop the thunniform bauplan like derived ichthyosaurs, lamnid sharks delphinids?

Or were there selective pressures in favor of them retaining the (sub-)carangiform swimming mode and relatively elongated bauplan?

Moreover, did they really have the same general shape of carcharhinid sharks? These sharks seem more truncated...

[Infinity Blade]

Oddly enough, there's a paper that speculates the answer to your question is 'yes', with regards to the Plotosaurus lineage.

Contrary to most other mosasaurs that inhabited shallow, epicontinental seas (Russell 1988; Bardet & Pereda Suberbiola 1996), Plotosaurus lived in the deeper water off the continental coast of what is now California. By the time mosasaurs had colonized the marine realm (about 92 Ma), rising sea levels had globally drowned immense low-lying areas in present day Europe, Africa and North America (Haq et al. 1987), forming shallow seas teeming with potential food resources. Although mosasaurs obviously were capable of crossing open bodies of water (Mulder 1999; Lindgren & Siverson 2004), most taxa recorded to date apparently inhabited these food-rich, nearshore environments. Hence, they retained elongated, lizard-like body forms, optimal for ambush predation (see e.g. Massare 1994), but of limited use when it came to sustained cruising. In stark contrast to this, the generally low frequency of food resources in open-water, deep-sea environments probably increased selective pressure for optimization of body design for energy-efficient locomotion. Accordingly, the bodies of ancestral plotosaurids gradually became stiffer and more piscine, limiting propulsive movements to the semilunate tail. With time, the Plotosaurus lineage might even have yielded fully thunniform animals, had it not been for the aftermath of a bolide impact that brought an abrupt end to the reign of mosasaurs 65 Ma.

www.idunn.no/doi/pdf/10.1111/j.1502-3931.2007.00009.x


However, as to why mosasaurs might have opted for a sub/carangiform swimming mode, remember that the anguilliform to thunniform spectrum is also a spectrum in the lateral mobility of the vertebral column. Thunniform swimmers have stiff vertebral columns in order to remain stable like a torpedo, lest they waste energy trying to swim fast for extended periods of time. But as you go increasingly from thunniform to anguilliform swimming, the vertebral column becomes increasingly more mobile. Accordingly, anguilliform swimmers are more maneuverable/agile than subcarangiform swimmers, subcarangiform swimmers are more agile than carangiform swimmers, and carangiform swimmers are more agile than thunniform swimmers. It would seem that mosasaurs weren't as concerned with swimming at high speeds for long periods of time as modern cetaceans, but favored agility (though not quite to the extent of true anguilliform swimmers).

[Supercommunist]

According to prehistoric planet, mosasaurs could swim 30 miles. Assuming this is correct they would have been approaching the maximum speed an animal can swim before it incurs injury due to cavitation. At that point becoming faster wouldn't be beneficial.

[Infinity Blade]

Yeah, I'm surprised the consultants went out of their way to calculate a giant mosasaur's speed like that. 3/4 of their body length in one second, damn.

[cj227]

Sept 4, 2023 5:01:35 GMT 5 Supercommunist said:

According to prehistoric planet, mosasaurs could swim 30 miles. Assuming this is correct they would have been approaching the maximum speed an animal can swim before it incurs injury due to cavitation. At that point becoming faster wouldn't be beneficial.

Well, even if PP is right, becoming more thunniform doesn't necessarily entail an increased top speed, but more energy efficiency and/or maintaining the top speed for longer.

[cj227]

Sept 4, 2023 4:53:51 GMT 5 Infinity Blade said:

Oddly enough, there's a paper that speculates the answer to your question is 'yes', with regards to the Plotosaurus lineage.

Contrary to most other mosasaurs that inhabited shallow, epicontinental seas (Russell 1988; Bardet & Pereda Suberbiola 1996), Plotosaurus lived in the deeper water off the continental coast of what is now California. By the time mosasaurs had colonized the marine realm (about 92 Ma), rising sea levels had globally drowned immense low-lying areas in present day Europe, Africa and North America (Haq et al. 1987), forming shallow seas teeming with potential food resources. Although mosasaurs obviously were capable of crossing open bodies of water (Mulder 1999; Lindgren & Siverson 2004), most taxa recorded to date apparently inhabited these food-rich, nearshore environments. Hence, they retained elongated, lizard-like body forms, optimal for ambush predation (see e.g. Massare 1994), but of limited use when it came to sustained cruising. In stark contrast to this, the generally low frequency of food resources in open-water, deep-sea environments probably increased selective pressure for optimization of body design for energy-efficient locomotion. Accordingly, the bodies of ancestral plotosaurids gradually became stiffer and more piscine, limiting propulsive movements to the semilunate tail. With time, the Plotosaurus lineage might even have yielded fully thunniform animals, had it not been for the aftermath of a bolide impact that brought an abrupt end to the reign of mosasaurs 65 Ma.

www.idunn.no/doi/pdf/10.1111/j.1502-3931.2007.00009.x


However, as to why mosasaurs might have opted for a sub/carangiform swimming mode, remember that the anguilliform to thunniform spectrum is also a spectrum in the lateral mobility of the vertebral column. Thunniform swimmers have stiff vertebral columns in order to remain stable like a torpedo, lest they waste energy trying to swim fast for extended periods of time. But as you go increasingly from thunniform to anguilliform swimming, the vertebral column becomes increasingly more mobile. Accordingly, anguilliform swimmers are more maneuverable/agile than subcarangiform swimmers, subcarangiform swimmers are more agile than carangiform swimmers, and carangiform swimmers are more agile than thunniform swimmers. It would seem that mosasaurs weren't as concerned with swimming at high speeds for long periods of time as modern cetaceans, but favored agility (though not quite to the extent of true anguilliform swimmers).

In the recently published Princeton Field Guide to Mesozoic Sea Reptiles, Gregory S. Paul argues that had the K/T extinction not happened, marine reptiles still wouldn't have re-evolved a thunniform body, since that role would be played by fish, both bony and cartilaginous (page 54).

Of course, this flies in the face that cetaceans evolved thunniform bodies despite the existence of thunniform sharks and bony fishes.

Also, this idea seems to be rooted in the now disputed theory that ichthyosaur extinction was in part caused by the loss of ichthyosaurian monopoly on the thunniform bauplan due to the emergence of faster lamnid sharks and teleosts, which Gregory also seem to defend in this book (pages 22 and 24).

What do you think about this?

[Infinity Blade]

Sept 14, 2023 7:03:10 GMT 5 cj227 said:

Sept 4, 2023 4:53:51 GMT 5 Infinity Blade said:

Oddly enough, there's a paper that speculates the answer to your question is 'yes', with regards to the Plotosaurus lineage.

www.idunn.no/doi/pdf/10.1111/j.1502-3931.2007.00009.x


However, as to why mosasaurs might have opted for a sub/carangiform swimming mode, remember that the anguilliform to thunniform spectrum is also a spectrum in the lateral mobility of the vertebral column. Thunniform swimmers have stiff vertebral columns in order to remain stable like a torpedo, lest they waste energy trying to swim fast for extended periods of time. But as you go increasingly from thunniform to anguilliform swimming, the vertebral column becomes increasingly more mobile. Accordingly, anguilliform swimmers are more maneuverable/agile than subcarangiform swimmers, subcarangiform swimmers are more agile than carangiform swimmers, and carangiform swimmers are more agile than thunniform swimmers. It would seem that mosasaurs weren't as concerned with swimming at high speeds for long periods of time as modern cetaceans, but favored agility (though not quite to the extent of true anguilliform swimmers).

In the recently published Princeton Field Guide to Mesozoic Sea Reptiles, Gregory S. Paul argues that had the K/T extinction not happened, marine reptiles still wouldn't have re-evolved a thunniform body, since that role would be played by fish, both bony and cartilaginous (page 54).

Of course, this flies in the face that cetaceans evolved thunniform bodies despite the existence of thunniform sharks and bony fishes.

Also, this idea seems to be rooted in the now disputed theory that ichthyosaur extinction was in part caused by the loss of ichthyosaurian monopoly on the thunniform bauplan due to the emergence of faster lamnid sharks and teleosts, which Gregory also seem to defend in this book (pages 22 and 24).

What do you think about this?

Yeah, this idea doesn't make much sense to me for the reasons you stated. Heck, we know that as cetaceans became fully aquatic (around the time you start getting things like basilosaurids), at least some of them were anguilliform (like Basilosaurus itself) and I doubt any of them were truly thunniform at that point in time. Despite this thunniform cetaceans obviously emerged later. If this could happen with cetaceans, there's no reason to think, at least in my opinion, that mosasaurs couldn't if need be (the presence of thunniform fish be damned).