Are We Shrink-Wrapping Ichthyosaur Tails?

(Don't start with a disclaimer... DON'T START WITH A DISCLAIMER!)

Disclaimer: I'm not trained in palaeontology or fluid mechanics, but after recently illustrating a few ichthyosaurs for a project, I wondered if I was reconstructing their tails too conservatively. I had a poke around the internet and tried to translate it into some coherent thoughts. A water tunnel, tame engineer, and unlimited access to ichthyosaurs would have been useful, but in the absence of all of that, I just had fudge it. And fudge it I did.

The Current Popular Look For Ichthyosaur Tails

If you look at palaeoart depicting ichthyosaurs (including six of the seven I just did... pfft!), a good chunk of it shows animals with tails which are more-or-less cylindrical, following the form of the vertebral column under the surface, skimmed with some muscle and skin, and terminating in a thunniform ('tuna-esque') caudal fin, the lower lobe of which displays a prominent ridge where the vertebrae continue beneath the fin's surface. The top lobe is generally depicted as skinnier than the bottom. But is this the most likely look for ichthyosaurs, and is it worth taking a peak at modern aquatic vertebrates to see how they're doing it?

A horribly-shrink-wrapped Ophthalmosaurus, with a stupidly-long tail. By me. Illustration: copyright © 2003 OUMNH/Gareth Monger

Caudal Fins

The caudal fins of aquatic vertebrates vary greatly in form, reflecting the locomotive styles and ecological niches of their owners. Ocean-going predators, including cetaceans, sharks, and billfish (sailfish, marlins, etc.) have evolved caudal fin shapes which allow them to reach the speeds necessary to run down swift prey and there are broad similarities brought about by convergence. The differences in the orientation of the caudal fin of fishes and reptiles, and mammals, reflect the evolutionary origins of those fins. The ancestors of aquatic reptiles presumably walked with a sprawling gait, their vertebral columns flexing from side to side, resulting in the same undulating motion in water and, therefore, a vertically oriented caudal fin. Cetaceans' terrestrial ancestors walked with an erect gait and cetaceans swim with a vertical undulation and developed a horizontally oriented caudal fin.

Predatory marine vertebrates: A. Atlantic sailfish (Istiophorus albicans); B. tiger shark (Galeocerdo cuvier); C. harbour porpoise (Phocoena phocoena); D. the Jurassic ichthyosaur, Opthalmosaurus. Image: Gareth Monger.

Streamlining Peduncles

Some of these animals also bear modified structures which improve the efficiency of their stroke. The part of the body after the anal fin (broadly speaking, the tail) is called the caudal peduncle, and contains the muscles which drive the caudal fin. It also includes the bony or cartilaginous skeleton, depending on the group to which it belongs. (In cetaceans, the peduncle is also called the tail stock.) In order to generate forward thrust, the caudal fin beats laterally in fish and reptiles, and vertically in mammals. The peduncle must also displace water during the stroke, but pushing the peduncle through water can reduce the efficiency of the caudal fin. Drag created by the peduncle during the stroke is energy wasted which could be converted to forward thrust by the caudal fin. In addition to this, water made turbulent having passed over the animal's body and fins then flows to the caudal fin. The caudal fin is less efficient in this disturbed, turbulent water than in smooth, laminar water.

Many species improve upon these inefficiencies by having peduncles which are streamlined to cut down hydrodynamic drag during the swimming stroke. For example, many sharks' peduncles are dorsoventrally-flattened to ovals when viewed in cross-section, which might be expected anyway because the muscles are grouped either side of the vertebral column – though the overlying tissues produce more-angular apexes to the oval than is achieved by the muscle mass alone.  This produces a lower profile that cuts through the water more easily during lateral beating of the tail. If the stroke generates less turbulence, the animal can transfer more of its energy to the caudal fin to be turned into forward thrust. The cross-section of the cetacean peduncle is similar, except that its oval is oriented vertically.

The peduncle and caudal fin of the harbour porpoise. The cross-section through the peduncle shows the streamlined dorsal and ventral surfaces. Image: Gareth Monger.

Caudal Keels as Laminar Flow Generators

Caudal keel as a possible laminar flow generator.
Image: Gareth Monger.

An additional feature of some fish peduncles is a 'caudal keel' situated on the outermost margins. This is sometimes formed by harder structures such as scales in animals which possess scales – a bit like ridge tiles on a roof. The keels' locations towards the distal end of the peduncle may also partially stabilise the flow of turbulent water as it passes from the animal's body and over its caudal fin. It's less efficient for the caudal fin to push against turbulent water during its stroke, but a longer caudal keel, as seen in some sharks, might convert some of the turbulence to laminar flow. This presumes that a given ichthyosaur's integument didn't sufficiently produce laminar flow on its own.

Caudal Keels as Boundary Layer Fences

The keels might also function as 'boundary layer fences', which serve to reduce slippage of water passing across the caudal fin towards the lobes of the fins. In other words, if the water flows in any other direction not associated with the forward thrust, thrust is lost and the animal must work harder. Imagine balancing a tray on one hand. If the tray is loaded with marbles and it leans slightly, it's fairly easy for all of the marbles to roll together, and the tray will tip, spilling all of the marbles at one end. If there's a small ridge at the centre, it will help to prevent each half of the tray's marbles from slipping to the other side, and it will be easier to control the tray.

Locations of caudal keels for the Atlantic sailfish (Istiophorus albicans) and the porbeagle shark (Lamna nasus). NB: The cross-section for the sailfish is an extrapolated from available photos of live animals. Image: Gareth Monger.

It's entirely possible that ichthyosaurs employed a similar system, combining a dorsoventrally-compressed peduncle and some sort of keel, to improve stroke efficiency. After two weeks of looking over literature and images online, I stumbled over a paper by Theagarten Lingham-Soliar (2016), which I wish I had a fortnight ago. Lingham-Soliar looked at convergence in lamnid sharks and Jurassic ichthyosaurs, and interpreted the soft-tissue remains in a particular ichthyosaur fossil (funnily enough, the photo later on in this article) as the impression of the animal's twisted-over peduncle. It's sometimes hard to interpret these soft tissue remains, not least because some earlier examples may have been enhanced, but if the fossil remains are suggesting chunky peduncles, it would make sense for them to find their way into artistic reconstructions.

Speculative diagram showing sections through the tail of Ophthalmosaurus. Vertebral column is shown in white, against body outline. Positions for possible keel-like structure indicated by arrows and pink dashed line. Image: Gareth Monger

So if peduncles are in, what of the ridge in the lower lobe, as defined by the distal vertebrae within the caudal fin? I can only approximate since I don't have ready access to an ichthyosaur skeleton, and I haven't yet found a detailed diagram of ichthyosaur musculature. That ridge has always been a feature of my ichthyosaur reconstructions, but those vertebrae are relatively small – they're only half the diameter of the smaller vertebrae in the peduncle, just in front of the caudal fin, forming a fairly narrow column. The majority of the caudal fin comprises soft tissue, presumably including some muscle which would be necessary to perform the adjustments to the fin's form during the stroke, i.e., preventing too much flexing which might negate the improvements brought about by the keel (re: boundary layer fence). Cetaceans do this, and their caudal fins are not especially skinny structures. It's feasible that an ichthyosaur's caudal fin vertebrae would have been bound in enough connective fibres, muscle and other tissues that they might not have been discernible in a healthy individual, and the upper lobe might not look too different to the lower.

Two highlighted caudal vertebrae, one just inside, and one just outside, the caudal fin. Note the those in the fin are approximately half the diameter of some of their nearest neighbours in the peduncle. Photo: Daderot. CC0 1.0; Digital overlays: Gareth Monger

So, considering that ichthyosaurs' forms shows them to be powerful, efficient swimmers, it's not totally unreasonable to at least consider that they might have evolved the anatomy to allow them to live as active, effective predators. And whilst the wider, flattened peduncle is likely, it doesn't automatically follow that they would have had keels as sharply defined as those found in sharks and other fish. Without knowing much about the sorts of integuments that various ichthyosaurs possessed, we can't know if specialised integument was used in a similar manner to the scutes of sailfish and their kin. I'm inclined to think scuted/scaled keels are a bit of a stretch. But a bit of definition to the peduncle might be likely.

Different ichthyosaur species were subjected to different selective pressures and, as with extant aquatic vertebrates, we should expect some variation in the external appearances of the myriad ichthyosaur species.

Lateral view of the chunky Ophthalmosaurus (based on Sander 2000), and a dorsal view extrapolated (well, fudged) from an anterior skeletal (McGowan & Motani 2003), and various pics of the great mount at Peterborough Museum. This dorsal view shows off the wider peduncle, but this still might be a tad skinny. Gotta find a decent ichthyosaur muscle reconstruction! Image: Gareth Monger.

Generalised ichthyosaurs, shown from different angles and displaying their chunky peduncles. 'Pedunkies'? Illustration: Gareth Monger).

The ophthalmosaurid ichthyosaur, Nannopterygius, reconstructed with a keeled peduncle. Illustration: Gareth Monger.

References

Bernvi, D. 2016. Ontogenetic Influences on Endothermy in the Great White Shark (Carcharodon carcharias). 10.13140/RG.2.1.2888.5367

Fish, F. E. (<-- seriously?). Biomechanical Perspective on the Origin of Cetacean Flukes. research.net

Lingham-Solia, T. 1999. Rare Soft Tissue Preservation Showing Fibrous Structures in an Ichthyosaur From the Lower Lias (Jurassic) of England. The Royal Society, 266, 2367–2373.

Lingham-Solia, T. 2016. Convergence in Thunniform Anatomy in Lamnid Sharks and Jurassic Ichthyosaurs. Integrative and Comparative Biology, Volume 56, Issue 6, 1 December 2016, Pages 1323–1336, https://doi.org/10.1093/icb/icw125

Martill, D. N. 1995. An Ichthyosaur With Preserved Soft Tissue From the Sinemurian of Southern England. Palaeontology, Vol. 38, Part 4, 1995, pp. 897–903, 1 p1.

Motani, R. 2005. Evolution of Fish-Shaped Reptiles (Reptilia: Ichthyopterygia) in their Physical Environments and Constraints. arjournals.annualreviews.org

Naish, D. 2008. Ichthyosaur Skin Impressions. http://scienceblogs.com/tetrapodzoology/

Sagong, W., Jeon W-P., Choi H. 2013. Hydrodynamic Characteristics of the Sailfish (Istiophorus platypterus) and Swordfish (Xiphias gladius) in Gliding Postures at Their Cruise Speeds. PLoS ONE 8(12): e81223. doi:10.1371/journal.pone.0081323

Veterian Key: Cetaceans. https://veteriankey.com/cetaceans/

Walters, V. 1962. Body Form and Swimming Performance in the Sogmbroid Fishes. Zoologist, 2:143-149.