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Description

I'm sure you have all heard of the newly described tail fin of Spinosaurus aegyptiacus and hopefully aren't sick to death of seeing it yet- the man has been plastered all over the paleo art side of the web for a while now.

Here he is dragging the monster coelocanth Mawsonia out of the water- yes, out of the water with those stumpy little legs.

It's clear to me, from the glut of art coming out, that people have finally made peace with the proportions outlined in the 2014 paper. But be careful! We're not "out of the water" yet. Just because Spino has all this extra junk behind the trunk doesn't mean that you can just throw it in a regular theropod pose and call it a day- As a definitely aquatic creature there were some necessary divestments made from the terrestrial, long-distance, hunter-stalker body plan of other theropods. More specifically the ossified tendons that pin each vert against its neighbors, turning the entire axial skeleton into a ramrod-straight shock absorber that teeters over the pelvis like a seesaw. This is, obviously, directly at odds with turning into a huge and slonky salamander. Basically the damn thing was too floppy to walk like that, even if it weren't the longest theropod dinosaur of all time.

So what? A knuckle-walking Gorillazillasaurus then? NO WAY. Spinosaurus, of course, walked like every other torpedo-shaped semi-aquatic dinosaur: like a penguin!

Or a cormorant, or a grebe, or a loon, a pelican... I don't know why people have it in their heads that this is the first marine dinosaur. Don't say "non-avian" either, you look Halszkaraptor in the eye and tell me you can hurt his feelings like that.

Obviously it's not that simple. We have no trackways and only a vague idea of the mass distribution in the body, and being half tail changes things a lot. But even in the age of maybe-normal-legs-spino I had seen differences in the pelvic girdle and vertebrae of Spinosaurus compared to other theropods and even the closely related Icthyovenator that would have made it difficult or even impossible to walk in a horizontal pose. I'm going to be going into a lot of detail on this (pending what little time and motivation I have that isn't being bulldozed by Coronageddon ofc) because frankly it's too awesome to be true, and I don't want to be one of those people. Even an honest-to-godzilla toho style dinosaur needs to stand up to rigorous logic.

While i'm here I am also firmly of the belief that the primary selective pressure on the sail is as a hydrodynamic element, not heat regulation or sexual display. The profile of the fin is remarkably similar to those of the more rotund fast-swimming gamefish such as bass, tuna, and marlin, as infamous paleo provocateur Duane Nash once brought up on da' Salad . He also puts the spotlight on some more-or-less throwaway comments by Ibrahim that elements associated with vertical stability were reduced in favor of horizontal propulsion... but I ask, is everyone clear about what vertical and horizontal mean to this diving dinosaur? Mr. Nash explored this idea mainly in support of a dense-boned hippo-like Spino that punted off the riverbed instead of swam. While that idea is dear to my heart it is unfortunately played out by that shockingly mosasaur-like tail.

Anyways the neural spines seem too... important? just to go enlarging them for the purpose of being large alone. Why not just evolve a bigger antorbital fossa and supraorbital crests like a normal dinosaur? There must be a biomechanical reason.
HOWEVER! that doesn't mean that the feature can't be co-opted conveniently for those purposes.

Yards at a time, the great dragon of the Kem Kem wetlands crests the water surface. Over each laborious footfall, the bones in its sail torque in rhythm like the great ivory keys of an ancient piano, stayed only by the creature's rippling back muscles as it overcomes the surface tension of pondwater against a cavernous body cavity. Without warning, the sail's two lobes of pallid soda-glass green list to either side, exploding into brilliant electric blues and infernal reds. Spiderwebbing between the blobs of color are osteoderms that glitter between the brightest white and deepest indigo.

The resemblance of the display to the Spinosaur's own pointed, recurved toothrow is no coincidence. At these sizes, eyespots have long gone out of vogue simply because there is no creature with eyes large enough for them to mimic. All dinosaurs, however, have the image of a theropod's pearly whites burned into their genetic memory; after all, it is the first thing many hatchling dinosaurs will see. When living with thieving dromaeosaurs and bully megaraptorans there is nothing like good old psychological manipulation to remind everyone that one fish is for one hunter, without even leaving the water. Trusting his reputation the drake strides as triumphantly from the shoreline as an emperor would.

To potential mates, the sail pattern sends out an honest signal of the animal's health. Ironically, the striking blues are the easiest to produce, since they are a structural iridescence inherent to the scales. The yellow undertones and blood-red rim, on the other hand are derived from carotenoid pigments which cannot be produced by reptiles and must be obtained from food. As hunters of fish, Spinosaurs proudly display this rare luxury among hypercarnivores- but only if they are doing well for themselves. Without secondary pigment the iridescent patches appear washed out against the blue sky and water, even at optimum angle, and are less likely to attract a mate... or prevent a rival from encroaching on their territory. Unlike many Cenozoic birds dinosaurs do not seem to have much in the way of sexual dimorphism and must have co-parented if they cared for their eggs at all. Both sexes would have equally vibrant sails- indicating egg-laying ability in the female, and value as a babysitter in the male. This drake's billboard reads, "my teeth look like this, so your hatchlings don't look like this fish i'm holding"

Description

I'm sure you have all heard of the newly described tail fin of Spinosaurus aegyptiacus and hopefully aren't sick to death of seeing it yet- the man has been plastered all over the paleo art side of the web for a while now.

Here he is dragging the monster coelocanth Mawsonia out of the water- yes, out of the water with those stumpy little legs.

It's clear to me, from the glut of art coming out, that people have finally made peace with the proportions outlined in the 2014 paper. But be careful! We're not "out of the water" yet. Just because Spino has all this extra junk behind the trunk doesn't mean that you can just throw it in a regular theropod pose and call it a day- As a definitely aquatic creature there were some necessary divestments made from the terrestrial, long-distance, hunter-stalker body plan of other theropods. More specifically the ossified tendons that pin each vert against its neighbors, turning the entire axial skeleton into a ramrod-straight shock absorber that teeters over the pelvis like a seesaw. This is, obviously, directly at odds with turning into a huge and slonky salamander. Basically the damn thing was too floppy to walk like that, even if it weren't the longest theropod dinosaur of all time.

So what? A knuckle-walking Gorillazillasaurus then? NO WAY. Spinosaurus, of course, walked like every other torpedo-shaped semi-aquatic dinosaur: like a penguin!

Or a cormorant, or a grebe, or a loon, a pelican... I don't know why people have it in their heads that this is the first marine dinosaur. Don't say "non-avian" either, you look Halszkaraptor in the eye and tell me you can hurt his feelings like that.

Obviously it's not that simple. We have no trackways and only a vague idea of the mass distribution in the body, and being half tail changes things a lot. But even in the age of maybe-normal-legs-spino I had seen differences in the pelvic girdle and vertebrae of Spinosaurus compared to other theropods and even the closely related Icthyovenator that would have made it difficult or even impossible to walk in a horizontal pose. I'm going to be going into a lot of detail on this (pending what little time and motivation I have that isn't being bulldozed by Coronageddon ofc) because frankly it's too awesome to be true, and I don't want to be one of those people. Even an honest-to-godzilla toho style dinosaur needs to stand up to rigorous logic.

While i'm here I am also firmly of the belief that the primary selective pressure on the sail is as a hydrodynamic element, not heat regulation or sexual display. The profile of the fin is remarkably similar to those of the more rotund fast-swimming gamefish such as bass, tuna, and marlin, as infamous paleo provocateur Duane Nash once brought up on da' Salad . He also puts the spotlight on some more-or-less throwaway comments by Ibrahim that elements associated with vertical stability were reduced in favor of horizontal propulsion... but I ask, is everyone clear about what vertical and horizontal mean to this diving dinosaur? Mr. Nash explored this idea mainly in support of a dense-boned hippo-like Spino that punted off the riverbed instead of swam. While that idea is dear to my heart it is unfortunately played out by that shockingly mosasaur-like tail.

Anyways the neural spines seem too... important? just to go enlarging them for the purpose of being large alone. Why not just evolve a bigger antorbital fossa and supraorbital crests like a normal dinosaur? There must be a biomechanical reason.
HOWEVER! that doesn't mean that the feature can't be co-opted conveniently for those purposes.

Yards at a time, the great dragon of the Kem Kem wetlands crests the water surface. Over each laborious footfall, the bones in its sail torque in rhythm like the great ivory keys of an ancient piano, stayed only by the creature's rippling back muscles as it overcomes the surface tension of pondwater against a cavernous body cavity. Without warning, the sail's two lobes of pallid soda-glass green list to either side, exploding into brilliant electric blues and infernal reds. Spiderwebbing between the blobs of color are osteoderms that glitter between the brightest white and deepest indigo.

The resemblance of the display to the Spinosaur's own pointed, recurved toothrow is no coincidence. At these sizes, eyespots have long gone out of vogue simply because there is no creature with eyes large enough for them to mimic. All dinosaurs, however, have the image of a theropod's pearly whites burned into their genetic memory; after all, it is the first thing many hatchling dinosaurs will see. When living with thieving dromaeosaurs and bully megaraptorans there is nothing like good old psychological manipulation to remind everyone that one fish is for one hunter, without even leaving the water. Trusting his reputation the drake strides as triumphantly from the shoreline as an emperor would.

To potential mates, the sail pattern sends out an honest signal of the animal's health. Ironically, the striking blues are the easiest to produce, since they are a structural iridescence inherent to the scales. The yellow undertones and blood-red rim, on the other hand are derived from carotenoid pigments which cannot be produced by reptiles and must be obtained from food. As hunters of fish, Spinosaurs proudly display this rare luxury among hypercarnivores- but only if they are doing well for themselves. Without secondary pigment the iridescent patches appear washed out against the blue sky and water, even at optimum angle, and are less likely to attract a mate... or prevent a rival from encroaching on their territory. Unlike many Cenozoic birds dinosaurs do not seem to have much in the way of sexual dimorphism and must have co-parented if they cared for their eggs at all. Both sexes would have equally vibrant sails- indicating egg-laying ability in the female, and value as a babysitter in the male. This drake's billboard reads, "my teeth look like this, so your hatchlings don't look like this fish i'm holding"