Putting the life back in science fiction


A Shocking Explanation for the Long Necks of Plesiosaurs and Others

I’m venturing back into the land of speculative paleontology with a modest suggestion about the reason why two groups of aquatic Mesozoic animals had ridiculously long necks. Some of these animals are very familiar: plesiosaurs. Some Plesiosaurs, members of the Plesiosauroidea, had ridiculously long necks. This trait was shared with the lesser known Triassic Tanystropheids, such as Tanystropheus longobardicus. Their necks are typically relatively stiff and weakly muscled, which gives rise to real questions about how the animal used them. Plesiosaurs, for example, could not raise their necks out of the water in the classic Loch Ness Monster or “swan” pose, nor could they sinuously retract their necks as if they were snakes’ bodies. Tanystropheus’ neck was even more limited, being compared to the stiff tails of hadrosaurs.

How did they use these necks? Proposals include Elasmosaurus “conceal[ing] itself below the school of fish. It then would have moved its head slowly and approached its prey from below” (from Wikipedia) to Tanystropheus fishing from marine shores as some sort of dipsy-diver, dropping its head down into the water from above. More bizarrely, taphonomic evidence in the form of fossilized sea floor gouges suggests that Plesiosaurids with long stiff necks were benthic feeders like rays or grey whales, grabbing their prey out of the mud (from Tet Zoo 2.0). It is hard to image an animal less adapted to such a hunting mode.

This doesn’t even get into the exquisite vulnerability of this body shape. Long, thin, stiff necks are very vulnerable to aquatic predators. Indeed, multiple artists have illustrated elasmosaur necks as the chew toys of large pliosaurs, and it is hard to imagine Tanystropheus surf fishing without getting its neck dislocated.

I’d like to suggest a different hypothesis, that these long, stiff necks were perfectly functional, and that, indeed, there are animals today that have similarly constrained morphologies. They aren’t tetrapods though, they’re fish. Electric fish, to be precise. Electrogenic organs have evolved at least four separate times in fish (Gymnotiformes, Mormyridae, Malapteruridae, Torpediniformes), and occur in both salt and freshwater. The South American knifefish (Gymnotiformes) are a particularly good example. As a group they have linear, fairly stiff, poorly muscled bodies. The apparent explanation for their shapes relates to the complexity of interpreting information from electric fields, and simpler body shapes make for more unambiguous signals. It appears that most electrogenic animals (animals that actively generate an electrical field for sensory purposes) have stiffer bodies with simpler shapes than do their less shocking relatives. This is also true for manmade electrogenic sensors, as a simple shape makes for a simple, easily interpretable field.

If the long stiff necks of Plesiosaurids and Tanystropheids are electrogenic organs, the weaknesses of the necks become strengths. Their necks’ main job is to be held stiff and straight in the water, and they appear well-built for this task. Moreover, electrogenic organs are built from stacks of electrocytes, which were the inspiration for batteries. The longer the neck, the more “batteries” it can hold, the bigger a field it can create, and the higher a voltage it can generate. The advantages don’t stop there. Electrogenic organs have three potential functions: sensing, electrofishing, and defense, and I will explore each in turn.

Active Electrolocation

Many animals can detect electrical fields, with or without special organs. Humans can detect sufficiently strong electric fields, while everything from catfish to sharks and rays to platypuses and river dolphins have structures specialized in passively detecting weak electrical fields. Electrogenic animals all actively use their electrical organs to sense their environments, feeling differences in the field due to the presence of things that either are either more or less conductive than the surrounding water. They can also detect the electrical fields innately given off by all animals through things like muscular exertion, heartbeats, or (in fish) the gill area. All of this adds up to a sophisticated electrolocation sense.

This is particularly important for animals that hunt in waters where vision is limited, either through turbidity or at night. It is also quite useful for hunting animals buried in the sediment, which is an explanation for the Jurassic sea-floor gouges caused by Plesiosaurids.

In an attempt to illustrate this, I chose the small (30 cm long) tanystropheid Tanytrachelos. This species was found in the Triassic, in the Solite Quarry in Virginia. It was apparently amphibious, for it was found in the sediments of a highly seasonal lake, and its webbed footprints are found fossilized in lake mud. Its main prey were apparently insects, and it apparently co-occurred with the fish Turseodus, of approximately the same size. Given the description of the lake sediments (alternating layers of mud and decayed vegetation), I suspect that the water wasn’t terribly clear, as it has been illustrated. The lake water may have been stained tea-brown by tannins, or it may have been muddied by rain and animal activity. Either way, I would suggest that Tanytrachelos was something like a platypus, an aquatic insectivore that found its prey using their electrical fields instead of eyesight.

Cartoon of  electrolocating a Turseodus in Solite Lake

Cartoon of Tanytrachelos electrolocating a Turseodus in Solite Lake

I should note that all the long-necked species probably used electrolocation. It doesn’t take a large electric organ, and in turbid or dark environments it can be critical. It’s also possible that a majority of Plesiosaurids and Tanystropheids were electrolocators only. In modern electric fish, a majority are electrolocators, not active shockers, and there’s no reason to think this was different in the past. Certainly there is a tradeoff between carrying an electric organ and using a neck for something else, and there’s no reason to expect them all to be electrofishers. But some could have been.

Electrofishing

Here, I would like to compare the fish biologists’ standard sampling tool, electrofishing, with the biological versions. While it is not clear that electric eels hunt with their electric organs, marine torpedo rays certainly do. However, the best insight comes from human electrofishing. For those who are not familiar with it, electrofishing involves using a generator, a transformer, and at least two electrodes. When the system is properly tuned, fish are stunned and can be captured for population samples. Most electrofishing rigs work in freshwater, but several research groups practice marine electrofishing. Still, there are a number of complexities.

Human electrofishing works on a simple principle. Many fish, when caught in a pulsed DC field between a cathode and an anode, involuntarily swim towards the anode, a phenomenon called positive electrotaxis that is caused by involuntary muscle contractions in the fish. Fish biologists use this trick to draw fish into the anode area without killing them, so that they can count and measure them. Translating this to electrofishing animals, I propose that the animals used pulsed DC current, with the anode located immediately behind the head. If one looks at the field lines, this would cause fish to swim uncontrollably straight into the predator’s mouth. Additionally, electrofishing rigs are deliberately designed with the anodes as large as possible to avoid damaging the fish (reference). One could easily argue that the long, slender necks with small heads of animals like Elasmosaurus or Tanystropheus are the exact opposite, with small anodes evolved to stun, injure, or even kill the the prey before it reaches the predator’s mouth. I used Tanystropheus in the cartoon below to illustrate the principle, with the anode behind the animal’s head.

Tanystropheus electrofishing, with the anode behind the head.

Tanystropheus electrofishing, with the anode behind the head.

While this is simple in theory, it becomes complex in practice. For one, seawater conductivity varies depending on temperature and salinity. For another, fish catchability varies depending on the ratio of conductivity between the fish and the water, with a maximum efficiency where the fish has the same conductivity as the water. There are other factors, such as the frequency of the pulsed DC current used, which varies by fish targeted (usually determined empirically by biologists), and factors such as the thickness of the fish scales (thick-scaled fish are harder to catch this way) and the size of the fish (larger fish are more vulnerable than smaller fish) (reference). As an aside, it is not clear whether electrofishing works on squid or insects, apparently due more to lack of experimentation than anything else.

Thus, there is no one optimal design for electrofishing animals. Plesiosaurids could not broadly harvest every fish in the water, but would be constrained by how they could adjust to factors like salinity and the fish present, and I suspect that the substantial diversity they show represents adaptations to different electrofishing strategies. Most likely, the biggest plesiosaurids would have to migrate frequently to avoid fishing out local habitats and to take advantage of spawning clusters or feeding congregations, much as large sharks do today. Since a proportionally bigger electrofishing rig is required for oceanic uses, it suggests that freshwater electrofishers should have proportionally shorter necks. This appears to parallel the fossil record, where known estuarine or freshwater species have shorter necks than do marine animals.

As an aside, I get the impression that Mesozoic fish had thick scales compared to those of today. While this may be erroneous, it is possible the Plesiosaurid electrofishing caused adaptive pressure on Mesozoic fish to favor thicker scales than we find today.

Why are there so few electrofishing modern animals? I would suggest that the answer is aerobic capacity. Electric eels reportedly get 80% of their oxygen from the surface. They are air-breathers, more than some amphibians, but torpedo rays (the other electrofishers) are not. While I’m not aware of any physiological studies, large electrical organs have to be metabolically expensive, and being air-breathing does make it easier to power them However, electric eels are stuck morphologically, because they have to cram their all their organs into a shape optimized for electrogenesis, and they have heavily vascularized oral cavities rather than true lungs. Air-breathing reptiles are not so constrained. Better still, their electrical array is physically separated in their necks, away from their heart, lungs, and swimming fins, allowing each system to work separately with fewer morphological constraints. As a result, they could grow much larger than electric eels or any modern electric animal. As for how Plesiosaurids avoided electrocuting themselves with their own voltage, all I can say is that electric eels somehow get away with it, so presumably it’s quite possible. Some electrolocating fish have encephalization quotients close to those of humans, so it’s unlikely that electrogenesis would be a problem for Plesiosaurid nervous systems.

Electrical Defense
This is a normal outgrowth of electrofishing, although current characteristics probably differ. Indeed, more modern electrogenic animals use these organs for defense than for food gathering. This is the classic electric eel defense, and I suspect that any electrofishing animal could effectively defend its neck from larger predators. A pliosaur attempting to bite down on an electrogenic elasmosaur would be in for a nasty shock. I’ve attempted to illustrate that below, with my cartoon of what might happen when a Pliosaur attacks an Elasmosaurus.

An electrified Elasmosaurus teaching an Pliosaur that it is not a prey item (with apologies to Luis Rey and Robert Bakker)

An electrified Elasmosaurus teaching a Pliosaur that it is not a prey item (with apologies to Luis Rey and Robert Bakker)

In Conclusion
Of course this is all speculative, soft-organ paleontology. I haven’t been able to locate a picture of an electric eel skeleton, so I have no idea how electric organs affect bone shape, or whether it’s possible to determine the presence of an electric organ from any skeleton. Some Plesiosaurid neck vertebrae are described as “odd and asymmetric”, but I have no idea whether this could be due to the presence of an electric organ or anything else.

Still, the strength of this hypothesis is that it presents a good explanation of why both Plesiosaurids and Tanystropheids have long, weak, inflexible necks, and it also accounts for how such an animal could be an efficient aquatic or benthic hunter. As such, it is certainly no worse than the idea that they are stealthy hunters, with their bodies hidden by their long necks so that they appear smaller. In fact, it makes them seem rather formidable. Electric sea dragons, anyone?

Offline References
Bakker, Robert. 1986. The Dinosaur Heresies. Zebra Press.
Fraser, Nicholas. 2006. Dawn of the Dinosaurs: Life in the Triassic. Indiana University Press.

Advertisements

20 Comments so far
Leave a comment

Interesting take on the whole long necked marine reptile dilemma. Seems a long neck might offer some advantage in certain feeding situations but the length just offers a tempting target to predators. I recall some other long necked guys around back then-fresh water little critters from China that are somehow alligned with champosaurs if I remember properly.

Hate to get zapped by a full grown elasmosaurus.

Comment by Duane

Thanks Duane. There’s a Tanystropheid from China, but I didn’t know about any champosaur relatives. If you remember anything else, let me know.

Comment by Heteromeles

This is probably a reference to the choristoderes Hyphalosaurus and Shokawa.

Comment by Cameron

This helps a lot. Thanks for the reference.

Comment by Heteromeles

A very interesting speculation. However I wonder why such an advantageous adaptation has not been found in any modern reptiles.
I suspect the stiff necks have better hydrodynamics than very flexible necks. The animal could then power into a shoal of fish catching them rather like fishing with a spear. The head might well be inside the shoal before the fish were aware of the animal, especially the pressure wave and color of the large body behind it.
This hypothesis could even be tested in a tank.

Comment by Alex Tolley

Thanks Alex. It’s simply not clear why a long neck is necessary for such a charging attack. After all, we have a number of aquatic predators today, including whales which are distant relatives of giraffes. None of them have developed an elongated neck to assist with a lunging attack. This doesn’t even touch Tanystropheus, which was at best a clumsy swimmer. Conversely, there are spearfishing animals (such as the anhinga), and they have strong and flexible necks to assist with the lunge.

Comment by Heteromeles

Trying to find some data I came across this NBC item that references a Science article. Seems to suggest pretty much the same explanation I gave, including tank tests.

I note that the sauropod dinosaurs (a very different lineage than the marine reptiles) are now also believed to have kept their necks fairly rigid as they grazed, sweeping back and forth.

If we examine modern strategies, cormorants extend their necks, but have the advantage or starting above the water. Toothed whales tend to hunt cooperatively in schools, and therefore avoiding the need to be stealthy. Other modern fish hunting mammals like seals and otters display considerable maneuverability. I suspect our large marine reptiles were not such agile swimmers, and resorted to lunging strikes. Sperm whales can get away with this strategy despite their bulk because their targets are large and relatively slow. If sperm whales had to hunt fish, they would probably need to be very different.

If long necked, marine reptiles had electric organs, how might we determine that from the fossil evidence? Would fossil skin be sufficient, or do we need to identify an electric organ in some way?

Comment by alexandertolleyAlex Tolley

[…] information means that paleontology is particularly fertile ground for speculation. One of my favorite bits of paleontological speculation comes from Putting the Life Back in Science Fiction where a post wonders whether plesiosaurs had long necks to allow for electric organs like electric […]

Pingback by Speculative Electro-Paleontology | The Finch and Pea

There are some pictures of Electric Eel skeletons here and here. There’s a diagram of internal anatomy here, and it looks like the trunk is mostly made out of electricity-producing organs.

Comment by Cameron

Great, thanks! Too bad it looks like the electrical organs don’t make a big impression on the hard skeleton, so we can’t tell the paleontologists what to look for in ancient electrical species.

Comment by Heteromeles

You might want to CT scan some plesiosaur skulls to test this theory.
Electric fish have some parts of the brain dealing with processing electric signals greatly enlarged; in elephant fishes the whole brain is large.
So if you could scan the interior of the cranial cavity you might be able to determine the brain’s shape and see if there are signs that parts of the brain involved with electrosensing are unusually large.

Also if the ability to detect electrical signals was ancestral to the whole plesiosaur clade then pliosaurs might have retained the ability and thus avoided living plesiosaurs; only scavenging their carcasses when the lack of electrical activity signalled that they were safely dead.

The short necked plesiosaurs tend to have very long skulls so perhaps their electrical detecting organs were confined to the long straight jaws, enabling them to sense electrical activity around their head which would help them to feed in low light conditions.

LeeB.

Comment by LeeB.

Brilliant! I’m glad you thought of a way to test this. If there’s a sufficiently intact pliosaur skull out there, it might be even possible to check the jaws for signs of electrical organs or a detector system backed by an anomalously large nervous system.

Comment by Heteromeles

You might want to look at the paper “Revision of Sulcusuchus erraini (Sauropterygia, Polycotylidae) from the upper Cretaceous of Patagonia, Argentina” in Alcheringa v. 37 issue 2 p. 163-176 ; 2013.
The abstract mentions rostral and mandibular grooves which could have housed electrosensitive and/or mechanosensitive structures.

LeeB.

Comment by LeeB.

Plesiosaur skulls are interesting things. The high number of vascular foramina around the snout and along both upper and lower jaws led me three years ago to the idea of electroreception capabilities. There is a small space between the eyes and in front of the Pineal Foramen of many plesiosaurs that I believe could potentially have housed two large nerves. These nerves seem to have been connected with the vascular foramina through a complicated series of neural pathways that run past the external nostrils, and are evident on the skulls, yet hard to spot in some cases depending on the fossil’s condition. There appear to be two more on either side of the lower jaws too. These nerve centres probably received electrical information. The vascular foramina probably worked like the Ampulae of Lorenzini in modern sharks which are filled with small amounts of a conductive gel. Adding further to the circumstantial ‘evidence’ suggesting, Jurassic Pliosaur skulls have been CT scanned. They show that these marine reptiles had brains of roughly the same size and shape as the modern Great White Shark (which uses electroreceptive capabilities). More study would be needed to determine which centre of a Shark’s brain responds to the Ampulae of Lorenzini. We could then look for something similar in Pliosaurs.

Overall :
If my idea is correct, then plesiosaurs and pliosaurs could detect low bio- electrical fields given off by a prey’s movements. It would also be of value in navigation, as the movement of seawater through the Earth’s magnetic field would be distinctive in different places -to them. This would have helped in migratory co-ordination. Also, the electroreceptive capabilities would be of use in hunting and navigation in dark, murky, or compact environments, such as around the bottom of rivers and lakes where there is a lot of tangled vegetation.

However, on the flip-side (it is always good practice to present opposite or opposing views), it is equally possible that the vascular foramina contained pressure sensors- like those in modern crocodyliformes, where they are concentrated specifically around the snout. It is equally possible they had no function. Yet this begs the question- why have many small pits on your skull, and concentrated in strategic areas valuable in hunting and prey capture behaviours, if there was no use for them?

Euan Malpas

Comment by Euan Malpas

There is a new paper here: http://link.springer.com/article/10.1007/s00114-014-1173-3#page-1 discussing a scanned skull of Pliosaurus kevani and showing that the pits on the skull are attached to a system of internal channels belonging to some kind of sensory system.

So it is looking like Plesiosauria as a whole possessing an interesting sensory system on the skull at least.

LeeB.

Comment by LeeB.

Thanks Lee and Euan. I’m glad there are new discoveries coming to light.

Comment by Heteromeles

The full paper is now available here: http://palaeo.gly.bris.ac.uk/Benton/reprints/2014rostral.pdf

LeeB.

Comment by LeeB.

Thnankyou very much LeeB and Heteromeles for the uplinks -always love to check out the latest findings. 🙂

Comment by Euan Malpas

Information like this needs to get out to the public, thank you for it.

Comment by b12 injections

[…] and/or electric defence (to protect from pliosaurs and mosasaurs). This hypothesis comes from here, and was raised to my attention by Darren […]

Pingback by Why did elasmosaurids have such a long neck? at Plesiosaur Bites




Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s



%d bloggers like this: