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.

Conodonts: 520 Million Years Long in the Tooth

Decent conodont fossils are frustratingly rare. Sure, their 'teeth' are so well known they're used as index fossils, id est, the distributions of particular types are used to gauge the age of the rocks in which they're found. Lacking the hard, bony skeletons of 'vertebrates proper', they don't leave so much to fossilise; ergo, only a handful of not-teeth-fossils are known. It's hardly surprising, then, that the arrangement of the hard elements within the head isn't fully understood. The animals are generally pretty small, ranging from 10mm to 400mm, and the teeth are only rarely found associated with the animal which used them. It's not even clear from the remains themselves how they were used, with a variety of feeding methods proposed, including filtration, crushing and actively grabbing hold of small prey. It's not hard to imagine conodonts as analogous to extant eels, and eel-like lampreys and hagfish - after all, they share a broadly similar form - but the feeding methods employed by those animals are disparate to say the least.

Given the poor preservation of the soft tissue elements of conodonts, many reconstructions are understandably pretty basic represented by little more than line art (and there's nothing wrong with that). However, Davide Bonadonna has put together this incredible image, which is probably the nearest anyone is going to get to a face-to-face encounter with our fishy (fishesque? fishish?) friend. Mercilessly terrifying, mercifully small.

Rocking the 'someone stepped on my tail' look: Clydagnathus. (Copyright © Davide Bonadonna.)

So Davide's pop-eyed conodont inspired something a little less scientific from me, in the form of this Alien3-Clydagnathus mash-up, and is available on products at my Redbubble store, here. And if you prefer something a little more scientific, you can buy Jaime Headden's instead.

The conodont Clydagnathus, which, were it alive today, would gestate in your chest and eventually smash through your ribcage. Why? Because pop culture. (Copyright © 2016 Gareth Monger)

Big thanks to Davide Bonadonna for allowing the use of his work in this glorified advert. If you're unfamiliar with his incredible work, correct that immediately!

Blackpool's Place In Illustration History, The Passing Of Wildlife Artist David Johnston And Grabbing Your Reference When You Can

The seaside town they forgot to close down...

BA (Hons) Scientific and Natural History Illustration was a successful degree course with an international reputation and was run at Blackpool and the Fylde College of Art and Design until only a few years ago, when short-sighted management decided to turn an important college with students from all over the world into a very average one which tends only to the needs of the local populace. People hardly need a reason to avoid Blackpool; after all, it's an end-of-the-line seaside town with no pre-tourism industry to speak of (and precious little pre-tourism history), and a local government which has no firm long-term plans. It also finds itself high up in national rankings for deprivation, suicide and low life expectancy.

Two shoppers wait for Primark to open against the stunning backdrop of Blackpool Tower and the Fylde coast. (© Twentieth Century Fox.)

A marriage of science and art

The degree, which we used to refer to as 'Sci Ill', was initially taught by a former Technical Illustration student, Dave Johnston, who would become a world-renowned wildlife artist. Although he left the college the year before I started, I would get to know him at the print shop where I work, printing for him hundreds of reference images of myriad extant dinosaurs, but mainly corvids, larids and sternids. Though in his sixties, Dave still valued fresh reference material, though I was always a little surprised that, given his insatiable appetite for photography, there was still any photographic reference left for him to collect.

Die-hard Dougal Dixon fans may remember Dave as one of the two illustrators (the other being Andrew Robinson) who provided images for Dixon's The Illustrated Dinosaur Encyclopedia which was published by Hamlyn in the late '80s. Although I doubt the artwork blew anybody away, the treatment of many of the dinosaurs, especially the ceratopsians, did make them look 'fuzzy', albeit unintentionally, a long time before most palaeoartists were feathering anything other than Archaeopteryx and the odd segnosaur.

The Illustrated Dinosaur Encyclopedia by Dougal Dixon, illustrated by Dave Johnston and Andrew Robinson. (Not to be confused with The Illustrated Encyclopaedia of Dinosaurs, by David Norman). Section of stolen blue pallet for scale.

Dave Johnston died unexpectedly last month, which ended one chapter in Blackpool's part in the story of British wildlife art - and it was quite a colourful chapter. His humanist service certainly had a 'rock star' vibe and many of those in attendance had that 'lived in' look. Blackpool has its characters; I think most of them were at Dave's funeral.

Sci Ill was set apart from similar courses in that it employed a full-time biologist (Mike Clapham) who was on-hand to tutor students in biological processes, but his main role was to level the playing field by teaching everybody how to effectively research their subject matter. This was combined with photography tuition; the theory went that your illustrations could only be as good as your reference.

This was a time when digital photography hadn't quite kicked film of its perch, so the entire class went out and purchased a tonne of 35mm camera gear. Every photoshoot ended with a trip to the local film developer, and if you didn't get it right, you had to do it all over again. Not really a problem if you're making clay dinosaurs, but if you're shooting something that's more time and location-sensitive, like the annual Fen tiger migration, it can be a real pain in the wallet. You kids don't know how good you've got it.

Cameras, cameras everywhere...

...and still no convincing thylacines or yetis. In 2016, of course, many of us don't go anywhere without at least a basic camera. Most mobile phones come with cameras as standard, and the quality of these has increased dramatically since they became commonplace some time in the '00s. Better lenses, better resolution and camera apps have between them provided people with the digital equivalent of the Instamatic. You don't really need a dedicated point-and-click camera if you own a mobile.

For artists, mobile phone cameras are pretty handy in that should you come across a scene or plant or something else not so easily or ethically brought back into the studio, you can photograph it with minimal fuss and add it to your reference library. You can record compositions and colours, organisms which you may wish to identify later, and, as was suggested to us during a field trip, evidence of illegal poaching and landscape destruction.

The highlight of my day: a dead bird. (Copyright © 2016 Gareth Monger.)

Whilst out on the school run, I noticed this unfortunate infant theropod in the middle of the pavement, tens of metres away from any obvious nest sites.  We can only speculate about how this animal found its way here. It certainly didn't fly itself there. But whilst I did have my trusty phone with me, I didn't have any means to transport the corpse back to my lab open-plan kitchen/lounge where I could take a better set of reference photos, and maybe ID it. From now on I carry a few plastic sandwich bags - just in case.

(I was going to offer a paragraph or two on the possible reasons for the liberal scattering of dead baby birds upon pavements, parks and gardens, but of course the second I searched the net, I see Darren Naish has already done it! - see here.)