Ok no talk of that movie, or Spinosaurus cuz it gets everyone including myself into a tizzy - no let's talk about something nice and non-controversial: plesiosaur feeding ecology. That is a pretty hum-drum topic no?
Of course your sarcasm detector should be buzzing and plesiosaur (for our purposes here just assume I am referring to the long necked variety when I say plesiosaur so I don't have to say "plesiosauromorph plesiosaurs" every time, ok) feeding ecology is just one of those topics in paleontology that continues to stifle us. And with such a strange and unparalleled morphology there are no shortage of foraging techniques, and otherwise, that have been suggest for dat neck. Adam S. Smith has compiled a good overview of many of these, check it out at your one stop shop for all things plesiosaurian: The Plesiosaur Directory. And hey, it included my twist feeding idea (not just for ammonites though but for any carcass, scavenged or otherwise, too large to swallow), among others. Now in this post I am not going to focus on twist feeding but actually incorporate that style of foraging - with several others listed there - into a more holistic interpretation of plesiosaur feeding ecology.
Before we begin - some qualifiers. Plesiosaurs were not as "samey" as often portrayed. Some were possibly straining small food particles out of the water column or substrate; some were equipped with especially heavy bones that may have allowed specialization on bottom foraging; some had extremely long, almost comical necks and some had necks that were rather conservative. I would argue further that what we are really seeing is a spectrum of species ranging from the plesiosauromorph bauplan to the pliosauromorph bauplan. Guys like Rhomaleosaurus (pictured below) traditionally called a pliosaur, actually fit nicely between the two extremes. Although big headed, it also has a long neck, especially by todays standards. So keep in mind that some species may have varied quite a bit from the picture I am going to paint depending on neck, teeth, skull etc. etc.
|Rhomaleosaurus. A "pliosaur" yes, but still pretty "necky". wiki. Niki Odolphie|
In short more like this:
|Thomas Hawkins' "Demonic Plesiosaur Battling Other Sea Monsters (Temnodontosaurus) in Eternal Darkness"|
And less like this:
|Dmitry Bogdanov. wiki|
Today's post is on foraging strategy. In order to do so I have to put the cart ahead of the horse a bit here. I am assuming a high level of social unity and group foraging in these sauropterygians. I will address why this is a very defensible position to take with these creatures in a later post and I am sure some of you already know why I am making this general assumption. But for now please allow a little bit of "suspension of disbelief" if you are a doubter.
Of course asserting "group foraging" is a veritable land mine in paleo-interpretation. Let me qualify this a bit. I want to draw a distinct line between cooperative hunting emphasizing a high division of labor, foresight, and usually associated with high intelligence - the kind seen in humans, chimps, wolves, and killer whales and a more of "mob" foraging strategy which I favor for plesiosaurs. Like looters in a riot each individual is acting on their own selfish best interests but have learned that their rate of capture increases when moving and foraging together as a group. This method of feeding is well known in various birds, fish, and sharks and I see no reason that plesiosaurs, though likely limited intellectually, could not have fit into this model. Of course if you have been paying attention we are in the midst of a bit of a revolution when it comes to reptilian intellect and social behavior - planning, social strategy, tool use, and dare we say emotion - so the door might still be open for truly cooperative behavior in plesiosaurs. But for now I am going to take the more cautious approach and invoke the "mob - looters in a riot" model.
And to prime you I want to draw your attention to two examples of group foraging that offer much utility here, white pelicans and white-tip reef sharks.
Now the videos, by Jack Polanen (please like, subscribe, and leave a comment not enough views of these awesome clips), above and below are highly demonstrative of the efficacy of leaderless, opportunistic group foraging of white-tipped reef sharks (Triaenodon obesus). I will forewarn you it gets pretty graphic and violent at the end of the top video but I would encourage you to watch it all the way through. One thing I would hope you pay attention to: it is often not the first shark to find and/or flush a fish out of hiding that gets it, it is the second or third shark that opportunistically grabs the harried fish. This is an important point that will be revisited later. But it becomes obvious that as a whole this style of foraging creates more opportunity and better efficiency than foraging alone.
And now for another video, again via youtube and from an amateur birder/wildlife documentarian Michael Descamps, which depicts a quite large congregation of American white pelicans (Pelecanus erythrorynchos) group foraging at the Nygren wetlands preserve in Rockton, Illinois. Like the white-tipped reef shark videos above there is so much here to look at and find visually stunning. Amidst the action it is sometimes hard to pick out individual feeding events but Descamps does a good job of slowing down the video at times and narrating the events. He even took the time to note a frog seeking refuge from the feeding swarm. Again not enough views (something this cool should have more views than the "gangnam style" video lolz) so please watch, like, comment, and subscribe to his videos. Link below pic, for some reason blogger will not link direct to this video ?!?
|Michael Descamps (c)|
Also, on white pelicans check out the video below of a pretty impressive swallowing feet of a quite large fish by a bird at the same locality by Michael Descamps:
And if you are further intrigued by pelicans check out this amazing video below of how Australian pelicans (Pelecanus conspicillatus) utilize the arid interior of Australia. Highly recommended but a little longer.
Ok now that you have brushed up a bit with group foraging techniques among white pelicans and white tip reef sharks (in actuality I could have went further with loads of examples, especially among fish) before we get to the plesiosaur bit one final rejoinder per popular plesiosaur depictions and the constant push/pull of science/art/opinion. Go google plesiosaur pictures. Ok did you do it? Now among the depictions you are looking at how many show the plesiosaur swimming in a horizontal plane? Most if not all of them, right?
Well here I am going to depart from that convention and suggest that, when group foraging in the water column a vertical orientation was the dominant swimming position. By assuming a vertical orientation this would minimize the likelihood of prey detecting plesiosaurs visually and through pressure waves when attacking from below; and minimize the chance of detection via casting a shadow when approaching from above. The vertically orientated eyes would naturally align to target prey in this position and allow the head maximum chance of getting in biting range without being detected.
And now without further ado and because a picture is worth a thousand words blah, blah, blah: Introducing the Plesiosaur Phalanx Attack!!
Above I depicted a foraging group of Hydrotherosaurus alexandrae from the Maastrichtian of what is now Fresno, California (being a Cali boy gots to rep) but what was then ocean. Much in the way the military phalanx could crush an enemies defense, the plesiosaur phalanx attack offered little recompense to prey in its sight. The plesiosaurs are rising, silently, cryptically, and slowly from the depths and picking off small cephalopods and fish as they move up the water column. If a prey item is detected the plesiosaur begins a slow, stealthy approach. If the first strike fails and the prey evades capture it is not out of the woods yet as the next nearest plesiosaur swoops its neck and head in to snatch it right up. If that second plesiosaur misses and the poor prey item in question manages to evade it there might still be a third plesiosaur attracted to the commotion which targets said prey. Small fish, crustaceans, and cephalopods could likely move quick enough to evade the first and even second strikes - but facing a full phalanx of lunging plesiosaurs they would quickly tire and be snatched up. Again very similar to the situation with the white tip reef sharks I discussed above. In this manner we can see how the neck of the plesiosaur - while not snake like per se - offers much utility in terms of covering a wide arc lunging ventrally, dorsally, and laterally after available feeding opportunities and leaving prey little chance of escape.
|Range of motion as estimated by Zammit et. al. illustrated & used w/permission via Adam S. Smith|
The mesopelagic critters I depicted are putative members of the "deep scattering layer", so named because when this biological phenomena was discovered using by sonar during World War II sound would literally bounce, or scatter, off of the dense accumulation of life that congregated at around 300-500 meters below the surface creating essentially a "phantom bottom". Of course every evening, when the largest biological migration on earth occurs - the vertical migration - this deep scattering layer would disappear as multitudes of creatures seeking refuge in the depth from predators, temperature, and likely other factors rise to the surface at night and then return again in the morning.
So yes, I do imagine that a number of plesiosaurs, especially many elasmosaurids, took advantage of foraging in a phalanx formation on this bountiful and dependable food source. It is worth noting that the famed western interior seaway, a shallow continental ocean usually suggested to be about 100 meters deep, is noted for a sparsity of elasmosaurid plesiosaurs. However once you get off the west coast of North America, where you find deeper, colder and possibly more productive oceans (and a deep scattering layer presumably) - the amount of elasmosaurid plesiosaurs goes up in numbers and diversity. Robert Bakker noted this in his book Dinosaur Heresies on page 430. Furthermore there is a detailed study on evidence of decompression syndrome i.e. "the bends" across sauropterygia. What did they find? Avascualar necrosis was found across all families but significantly lower in cryptoclydids. Best known for the species Cryptoclidus eurymerus (that wikipedia page has got to be changed, 8 tons I think not) and also famously featured in the original Walking With Dinosaurs, these beasts were likely near shore hunters of small prey and even infaunal sifting has been suggested. They are a bit of an outlier with regards to most plesiosaurs with weak lower jaws, small meshing teeth, and a fairly conservative neck.
Just compare the top pic, Cryptoclidus (Brown, 1981) to my boy, Hydrotherosaurus bottom pic (Weles, 1943) both pics used courtesy of Adam Stuart and from the Plesiosaur Directory.
To familiarize with what a much smaller, shorter, weaker toothed, and weaker jawed "small prey specialist" can do with its chompers check out these moray eel pics/videos.
Now the video above has been making the rounds for a bit now but it goes to show the impressive way a moray can utilize its body and jaws to subdue a small white tip reef shark. The video below is actually more interesting and startling actually - you might want to skip to about the 2:10 mark where a large green moray disturbs a lobster which seems to jump out of the water and never reappears (?!), and then the moray just charges the diver and does a small nip, but not really a dedicated bite per se. Did the moray see the human as possible predator? competitor? Startling video none the less.
Another scary video (below) about a diver who habituated a green moray (Gymnothorax tunebris) to feed off of little sausages. Unfortunately the eel mistook the divers thumb for a sausage and off went the thumb!! On the bright side at least the diver was not skinny dipping!!
For me there is little doubt that a 13 meter long, several ton plesiosaur - with a much heavier and stronger frame to spin off of - could do much more damage than 2 meter long moray eels. Just in case you are not convinced just look (graphic warning) at the bone scraping damage a green moray can incur on a humans arm. My point is not to shock with these clips but show that an animal with non-serrated, fish catching jaws can still do a good number on larger sized items.
Ok small digression there, just trying to chip away at the notion that all plesiosaurs did was bite into small stuff. Let's get back to deep diving plesiosaurs. And I think it worth mentioning that there are still loads of questions regarding how deep diving modern predators, especially air breathing tetrapods, work and operate at such depths. Never mind the fact that the sperm whale, an animal whole industries were founded upon, is still a huge mystery. Forgetting about the good old cachelot, let me point you in the direction of an interesting study of short-finned pilot whales (Globicephala macrorynchus) which contrasts their foraging ecology with that of the various species of beaked whales that they share deep sea habitat with off the Canary islands. Full paper here.
|Short-finned Pilot Whale w/large squid tentacle. (c) Pablo Aspas|
In comparing these two deep diving foraging strategies with regards to plesiosaurs I definitely would hedge more towards the "berry picking" of small prey and slow and leisurely foraging style of beaked whales than the "all or nothing" style of short-finned pilot whales. In plesiosarus the cryptic head at the end of a long neck allowed stealthy approach and lack of high velocity swimming/maneuvering capability further speak to beaked whales as a better analog than short-finned pilots - at least among the plesiosaurs that habitually dove into the mesopelagic/abyssal realms after prey. Of course I also think that such plesiosaurs, although having a predilection for small prey - would have opportunistically taken medium sized prey/scavenged carcasses as well. Let's look at a family of deep diving predators, that emphasizes cryptic, stealthy predation to the nth degree and which opportunistically catch prey from the abyssal realm all the way to shallow water - sharks of the genus Somniosus of the dogfish family - Squaliformes.
Somnio: "a dream, sleepy". Sharks of the genus Somniosus, which may compete with the great white shark for title of largest extant predatory fish, are truly creatures out of a dream - or, more precisely, a nightmare. The Greenland shark (Somniosus microcephalus) holds the title of the world's slowest swimming fish (when corrected for size). Paradoxically there is evidence that this shark hunts and catches several species of seal as well as numerous fast, bony fish. It has increasingly become apparent that the slow cruising speed allows this shark to approach undetected towards sleeping or resting marine mammals and fish. The Greenland shark and other sharks of this genus occupy a unique niche of an exceptionally long lived - low metabolism, cryptic, slow cruising, opportunistic predator of a wide variety of prey types and sizes from throughout the water column. More detailed studies of the closely related Pacific sleeper shark (Somniosus pacificus) by the Alaskan Department of Fish and Game further support this adaptation. Investigating whether a marked increase in Pacific sleeper shark population was responsible for Stellar sea lion declines the researchers performed stomach analysis and PAT tagging work revealed a conspicuously wide ranging and catholic predator.
|Pacific Sleeper Shark. wiki|
All right so I think, in a necessarily piece meal fashion, many plesiosaurs that foraged in the open ocean would fit nicely somewhere between the "cherry picking" leisurely foraging of beaked whales and the slow, cryptic, vertically oscillating, and opportunistic foraging of sharks belonging to the genus Somniosus. Combined with the group foraging strategy outlined above, moving in a vertically orientated matter to further hide the body and diminish hydrodynamic disturbance, and the benefits of a long neck to cover a large feeding envelope and, for me, this is a pretty diabolically efficient foraging adaptation. As I mentioned at the beginning of this post plesiosaurs certainly were more diverse than usually portrayed and several specialized in shallow water/lagoonal settings (Crypotclydidae and Leptocleididae come to mind) and some potentially engaged in lots of benthic, demersal foraging similar to what the white tipped reef sharks were doing. But in all cases; deep water, shallow water, or sea bottom foraging or any combination there of - a cryptic approach combined with the benefits of group foraging and the unique physical proportions of plesiosaurs suggest a very successful foraging adaptation.
Anyways hope you stayed with me for the whole post. I know it was a little all over the place but with a difficult group of critters with no exact analogues it has to be a little chaotic and piece meal. Maybe you are not swayed with the idea of some plesiosaurs biting into bigger stuff than generally presumed or you think the whole group foraging hypothesis is a little too tenuous and speculative. And as many have noted the preserved stomach contents point towards small stuff. I want to explore this further in future posts but let me suggest that there can be fossil bias in stomach contents as well. If plesiosaurs are assiduously avoiding large bones due to obvious choking hazards and flesh does not fossilize (as well) we simply won't see evidence of larger animals in their diet. We now know from field observations that pinnipeds will kill good size cetaceans and sharks and strip high energy blubber and liver off of these respective animals. Yet the majority of their diet is small stuff. If these animals were fossilized there likely would be no evidence of these episodic moments. I will revisit these topics in future posts and hopefully can sway you.
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