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Description

We know tetrapods are essentially “highly derived fish”, which switched to a terrestrial lifestyle long time ago, leaving the waters behind and never looking back.

Or nearly so, as a few groups seem to miss the liquid element after a while. And for some of them, once the feeling of fluctuating among waves has been enjoyed again, there's apparently no way back.

We all know cetaceans have evolved from terrestrial ancestors and ended up imitating the body plan of those ancient marine vertebrates all tetrapods have evolved from... but this story has actually happened several times in the history of life on our planet, and every time the final result was a creature which retained the inner anatomy of a terrestrial tetrapod, though hiding it behind a really tricky fish mask.

Here are six creatures representing as many different taxa of terrestrial vertebrates which secondarily decided that water was better than land.

As we can see, their bodies have dramatically changed to a fish-like silhouette; some of them have even lost the hind limbs, but all of them retain the original forelimb structure of tetrapods, that is, the same bones we have in our human hands and arms, although the soft tissues surrounding the bones have turned the limb into a flipper.

It is clear that all of them share some features as a result of convergent evolution, such as the extreme reduction of the long bones of the arm with hyperelongated digits of the manus, but there are significant differences in proportions; also, some of them show extra elements while other don't, and still others have some parts missing.

The first picture (top left) shows Ichthyosaurus (Ichthyosauridae), a marine reptile from Lower Jurassic of Europe.

Though belonging to the oldest clade among those represented here, Ichthyosaurus also shows the highest degree of adaptation, with each bone but the humerus ridiculously shortened and flattened and a dramatic increase of elements in the manus, resembling a 2-D corn cob. Not only this animal shows much more phalanges than normal (hyperphalangy), but also has two extra digits after the 5th (polydactyly).

This animal used to swim like a true fish, with side-to-side movements of its vertically fluked tail, hence the very fish-like appearance of the flippers.

The second picture (top center) features Rhomaleosaurus, a plesiosaur from Lower Jurassic of Great Britain.

In this case the humerus is significantly longer; there are no extra digits but there is hyperphalangia, though less severe than in the previous example. This flipper is suitable for being used for active swimming.

The third animal (top right) is a mosasaur (Tylosaurus), a marine predatory reptile from Late Cretaceous of North America, closely related to monitor lizards and snakes.

In Tylosaurus, the forearm is not that shortened; there are extra phalanges but the fifth digit is somewhat reduced and not in line with the others. The carpals seem to be highly reduced, but it is possible that they were mainly cartilaginous and just partly ossified, as happens in some modern cetaceans. The limb edge is square rather than pointed. This animal did probably swim with side-to-side movements of the fluked tail, but also used the flippers to maneuver underwater.

The first figure in the second row (bottom left) shows Dermochelys, or the extant leatherback turtle.

All the main parts are in place here, with no extra elements. Similarly to what seen in the plesiosaur, the humerus is thick and proportionally longer than in other taxa, which suggests this animal mainly relies on its flipper for locomotion.

The animal at bottom center is Trichechus, or the manatee.

Apart from the first digit reduction, this sirenian's forelimbs are much less derived, if compared to all seen above; in fact this animal relies on its paddle-like tail fluke for propulsion, and the animal uses its flipper mainly for pushing its body along the bottom while foraging and for putting food into the mouth. Mother manatees use the flipper to hold their calves as well.

The creature featuring in the last picture (bottom right) is Orcinus, or the orca, also known as the killer whale.

Cetaceans show a highly modified manus, with hyperphalangy and digit reduction / loss; Orca in particular shows hyperelongation and extra phalanges in digit II and III, reduction in other digits, and very thick cartilage pads (“cartilage phalanges”) between the digit bones.

Although the picture shows a completely ossified carpus, ossification of the carpals in orcas is often incomplete and the structure may appear as a single thick cartilaginous element, with poor distinction between the different parts and just a few dispersed bony cores.

Orcas use their horizontally fluked tail for swimming, and use the pectoral flippers for maneuvering, but also for communication purposes (e.g. water slapping), the latter usage is often asymmetrical, which means each individual has a “favourite” flipper it uses more than the other.

All these examples show that adaptation to an aquatic lifestyle for tetrapods implies some degree of convergent evolution, but also how each taxon's ancestral features and behavioural traits such as diet and locomotion style may lead to important differences.

MAIN REFERENCES:

Scott Hartman – Skeletal drawing of Ichthyosaurus – 2019;

Mark Witton – Plesiosaurs on the rocks: the terrestrial capabilities of four-flippered marine reptiles – in markwitton.com – 2019;

Scott Hartman – Mosasaur Tails: “Teaching the Controversy” – in skeletaldrawing.com – 2015;

Scott Hartman – Skeletal drawing of Tylosaurus pembinensis – 2015;

Lisa Noelle Cooper et al. – Evolution of hyperphalangy and digit reduction in the cetacean manus – 2007.

Description

We know tetrapods are essentially “highly derived fish”, which switched to a terrestrial lifestyle long time ago, leaving the waters behind and never looking back.

Or nearly so, as a few groups seem to miss the liquid element after a while. And for some of them, once the feeling of fluctuating among waves has been enjoyed again, there's apparently no way back.

We all know cetaceans have evolved from terrestrial ancestors and ended up imitating the body plan of those ancient marine vertebrates all tetrapods have evolved from... but this story has actually happened several times in the history of life on our planet, and every time the final result was a creature which retained the inner anatomy of a terrestrial tetrapod, though hiding it behind a really tricky fish mask.

Here are six creatures representing as many different taxa of terrestrial vertebrates which secondarily decided that water was better than land.

As we can see, their bodies have dramatically changed to a fish-like silhouette; some of them have even lost the hind limbs, but all of them retain the original forelimb structure of tetrapods, that is, the same bones we have in our human hands and arms, although the soft tissues surrounding the bones have turned the limb into a flipper.

It is clear that all of them share some features as a result of convergent evolution, such as the extreme reduction of the long bones of the arm with hyperelongated digits of the manus, but there are significant differences in proportions; also, some of them show extra elements while other don't, and still others have some parts missing.

The first picture (top left) shows Ichthyosaurus (Ichthyosauridae), a marine reptile from Lower Jurassic of Europe.

Though belonging to the oldest clade among those represented here, Ichthyosaurus also shows the highest degree of adaptation, with each bone but the humerus ridiculously shortened and flattened and a dramatic increase of elements in the manus, resembling a 2-D corn cob. Not only this animal shows much more phalanges than normal (hyperphalangy), but also has two extra digits after the 5th (polydactyly).

This animal used to swim like a true fish, with side-to-side movements of its vertically fluked tail, hence the very fish-like appearance of the flippers.

The second picture (top center) features Rhomaleosaurus, a plesiosaur from Lower Jurassic of Great Britain.

In this case the humerus is significantly longer; there are no extra digits but there is hyperphalangia, though less severe than in the previous example. This flipper is suitable for being used for active swimming.

The third animal (top right) is a mosasaur (Tylosaurus), a marine predatory reptile from Late Cretaceous of North America, closely related to monitor lizards and snakes.

In Tylosaurus, the forearm is not that shortened; there are extra phalanges but the fifth digit is somewhat reduced and not in line with the others. The carpals seem to be highly reduced, but it is possible that they were mainly cartilaginous and just partly ossified, as happens in some modern cetaceans. The limb edge is square rather than pointed. This animal did probably swim with side-to-side movements of the fluked tail, but also used the flippers to maneuver underwater.

The first figure in the second row (bottom left) shows Dermochelys, or the extant leatherback turtle.

All the main parts are in place here, with no extra elements. Similarly to what seen in the plesiosaur, the humerus is thick and proportionally longer than in other taxa, which suggests this animal mainly relies on its flipper for locomotion.

The animal at bottom center is Trichechus, or the manatee.

Apart from the first digit reduction, this sirenian's forelimbs are much less derived, if compared to all seen above; in fact this animal relies on its paddle-like tail fluke for propulsion, and the animal uses its flipper mainly for pushing its body along the bottom while foraging and for putting food into the mouth. Mother manatees use the flipper to hold their calves as well.

The creature featuring in the last picture (bottom right) is Orcinus, or the orca, also known as the killer whale.

Cetaceans show a highly modified manus, with hyperphalangy and digit reduction / loss; Orca in particular shows hyperelongation and extra phalanges in digit II and III, reduction in other digits, and very thick cartilage pads (“cartilage phalanges”) between the digit bones.

Although the picture shows a completely ossified carpus, ossification of the carpals in orcas is often incomplete and the structure may appear as a single thick cartilaginous element, with poor distinction between the different parts and just a few dispersed bony cores.

Orcas use their horizontally fluked tail for swimming, and use the pectoral flippers for maneuvering, but also for communication purposes (e.g. water slapping), the latter usage is often asymmetrical, which means each individual has a “favourite” flipper it uses more than the other.

All these examples show that adaptation to an aquatic lifestyle for tetrapods implies some degree of convergent evolution, but also how each taxon's ancestral features and behavioural traits such as diet and locomotion style may lead to important differences.

MAIN REFERENCES:

Scott Hartman – Skeletal drawing of Ichthyosaurus – 2019;

Mark Witton – Plesiosaurs on the rocks: the terrestrial capabilities of four-flippered marine reptiles – in markwitton.com – 2019;

Scott Hartman – Mosasaur Tails: “Teaching the Controversy” – in skeletaldrawing.com – 2015;

Scott Hartman – Skeletal drawing of Tylosaurus pembinensis – 2015;

Lisa Noelle Cooper et al. – Evolution of hyperphalangy and digit reduction in the cetacean manus – 2007.