S B Crofts, R Shehata, B Flammang, Flexibility of heterocercal tails: what can the functional morphology of shark tails tell us about ichthyosaur swimming?, Integrative Organismal Biology, , obz002, https://doi.org/10.1093/iob/obz002

1. What are (were) ichthyosaurs?

A: Ichthyosaurs were large swimming reptiles that were abundant during the Triassic and early Jurassic, the last of which died out some 90 million years ago. Ichthyosaurs broadly resembled a mash-up of dolphin- and shark-like traits: streamlined bodies, fins instead of fingers, vertically-arranged tails (like sharks), but lungs instead of gills (like dolphins). However, the earliest ichthyosaurs were more lizard-like, with long-lobed tails and less specialized appendicular fins and only midway through their history did they evolve into the more shark-like shapes we popularly see in books and media. While the exact reasons for their decline aren’t entirely known, the oceans at the time were filled with other groups of large marine predators like pliosaurs, plesiosaurs, sharks, and the like: a competitive ocean quite a bit toothier than what we see today.

1. Heterocercal, homocercal – why do some tails have longer lobes?

A: While sharks and ichthyosaurs have more similar-looking derrieres than say, ichthyosaurs and dolphins, their tails differ in one major way: sharks have heterocercal tails, while ichthyosaurs have hypocercal tails. What does that mean? In fish and reptiles with large, forked tails the spinal column either extends into the upper (heterocercal) or lower (hypocercal) lobe of the tail, never both. Why does this happen? Well, sharks are negatively buoyant despite a fatty liver while ichthyosaurs were positively buoyant, stemming from the air in their lungs. Contradictorily, it’s thought that hypocercal tails help to generate downwards momentum in ichthyosaurs while heterocercal tails create upward torques in sharks. So, internal tail skeletons help keep their critters on a level course. But what do tails do in the first place? They provide forward thrust while other fins counteract roll (dorsal fins) or provide lift (pectoral fins), although these stories get more complicated when animals actively re-position their fins while swimming.

1. Can we learn what ichthyosaurs were like, by studying modern predators?

A: Crofts et al. studied the mechanical properties of different sharks’ tails and then relate what they learned to inferring how tails worked in ichthyosaurs (essentially by imagining them upside down). Specifically, Crofts measured the flexural stiffness or bendiness of shark tails and what aspects of internal anatomy lent tails either greater flexibility or rigidity. Stiffer, crescent-shaped (or lunate) tails are found in highspeed marathon swimmers like tunas, makos, and some Cretaceous ichthyosaurs. Conversely, floppier, asymmetrically-shaped heterocercal tails (i.e. tails with one lobe larger than the other), are seen in more languorous-swimming animals like sandtigers, eels, and earlier ichthyosaurs – presumably an adaptation for maneuverability and efficient swimming at low-speeds in the fish, and a hold-over from their terrestrial origins in the ichtyosaurs. This gradient, from more flexible, heterocercal tails to stiffer, lunate tails is a pattern we see repeated again and again in the history of sharks and ichthyosaurs alike, and Crofts et al got to the bottom of it.

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1. Calling on collagen; a tendon-cy for stiffer tails…

A: Crofts et al found that sharks like makos with lunate tails, also have stiffer tails. Stiffer tails are more efficient at displacing water and producing thrust. Alternatively, they found more flexible and heterocercal tails in slower swimmers like sandtigers (with a couple exceptions, see below). Several factors played into explaining why some tails are stiffer than others and it has to do with several considerations: (1) the shape, number, and arrangement of the supporting fin elements, (2) the arrangement of stiffening collagen fibers along the side of the tail, and (3) the shape and size of individual vertebrae in the tail. Shark fins are supported by cartilage rods called ceratotrichia and Crofts et al found that arranging these at smaller angles relative to the spinal column made the tail stiffer. They also found that fast-swimmers with lunate tails had more ceratotrichia relative to heterocercal tails. A similar story was found for the tiny tendon-like collagen fibers that wrap around tail tissues – the angle formed with their attachment to the spine was smaller and the overlap of these fibers on adjacent vertebrae, like cables on a suspension bridge, keep lunate tails stiffer. Essentially, increasing connectivity among skeletal bits by bundling them together, in this case the spine, makes for a more rigid tail.

1. So what about ichthyosaur tails? Did we learn how they work?

A: Sharks and ichthyosaurs have tail skeletons arranged asymmetrically: the spinal component of the tail skeleton projects into either the upper or lower lobe of their forked tails. Crofts et al found differences in stiffness between these lobes and differences in stiffness overall among different species with different tail shapes (more lunate vs. more heterocercal). Sharks and ichthyosaurs with more lunate tails were presumably much faster, with stiffer tails able to displace water more efficiently during swimming. We also know that sharks and ichthyosaurs share a similar problem of counteracting buoyancy – sharks are negatively buoyant while ichthyosaurs are positively buoyant – and their tail shape helps correct these problems. Crofts et al also found similarities in spinal anatomy among sharks and ichthyosaurs, with more laterally-compressed vertebrae in the most-posterior parts of the tail, which helped make tails stiffer. It turns out that the similarities between sharks and ichthyosaurs is just too neat to miss and we can learn a lot about how ancient predators swam through the oceans by studying their modern analogs.