Thread by @AdityaBasu2001

Dreadnoughtus!!! 🦕

I like sauropods (long-necked dinosaurs) in general, as these Mesozoic puzzles 🧩 pushed evolutionary innovation to the extreme, & even the simplest aspects of their biology are confounding, oft-debated subjects. A thread 🧵 highlighting few of those.

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https://twitter.com/23luvr/status/1575600974458064897

Dreadnoughtus schrani was discovered (2014) in southern Patagonia, Argentina 🇦🇷 and is thought to be the largest 🏆 terrestrial animal to have ever walked the planet (i.e., it possesses the greatest mass of any land animal that can be calculated 🧮 with...

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...reasonable certainty). This colossal sauropod flourished during the Late Cretaceous Epoch (100.5 million to 66 million years ago). It was about 26 m (85 feet) long 📏 and may have weighed about 59 metric tons ⚖️ (image source: Wikimedia Commons) -...

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...more than a dozen African elephants! It had a 12.2 m long neck and head! Gargantuan body size has its drawbacks, but it also brings enormous benefits. It takes an absurd amount of resources to grow this large and power the organs needed to support life.

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However, if enough food is present to sustain this growth, predators 🦖 are no longer an issue. The word "Dreadnoughtus" literally means "fear nothing", and it is thought that the immense size of this herbivore left it with few - if any - predators as an adult.

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It is a reference to the armored battleship 'Dreadnought' and a tribute to the dinosaur's perceived fearlessness. Their skulls were lightweight, without molars 🦷. These herbivores must have gulped down vast quantities of greens 🌿 unchewed, which were then slowly...

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...digested in their cavernous guts. Not pausing to chew gave the animals more time to consume the calories needed to fuel their fast-growing bodies. Based on studies of the microscopic 🔬 internal structure of its bones, it's possible that the already-immense...

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...name-bearing specimen ("holotype") wasn't even done growing before it died! This holotype specimen is among the most complete giant titanosaur skeletons 🦴 ever found. It's notoriously hard to find complete sauropod skeletons. As their bodies and bones were so large,...

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...they tended to break apart and to be at least partially destroyed before they could be buried and preserved. In that sense, the holotype is truly remarkable.

Dreadnoughtus belongs to the clade Titanosauria. Titanosauria, which is a group of sauropods that includes...

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...other gigantic, heavy, stocky dinosaurs. Other titanosaurs, such as Argentinosaurus, are thought to have been even larger, but as their remains are fragmentary, it is impossible to get good estimates of their sizes.

Members of the family Diplodocidae...

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...are some of the longest of dinosaurs too, but are not the largest as they weigh less. Amphicoelias fragillimus, unearthed from the famous Morrison Formation by Edward Drinker Cope, might represent one exception. It is estimated to have been 190 feet (58 m) long...

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...and weighed around 122,400 kg! It was a period of ruthlessly competitive fossil hunting. In the pursuit of paleontological supremacy, Cope was fighting the Bone Wars (a.k.a. the Great Dinosaur Rush; 1877 - 1892) ⛏️⚔️ against Othniel Charles Marsh at that time!

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Even though Amphicoelias is reputed to be the largest dinosaur that ever lived, it is based on a single drawing (1878) ✏️ of a partial vertebra made by Cope. Amphicoelias has recently been renamed as Maraapunisaurus fragillimus and reclassified as a rebbachisaur.

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Reconstruction of Maraapunisaurus as a rebbachisaur gives it a length of 'only' 32 m (104 feet). To his credit, Cope tried to preserve the remains but despite his efforts, the remains went missing and now we are left wondering what it was that he found.

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Multiple lines of evidence have indicated that extinct dinosaurs had physiological profiles more like those of avian dinosaurs (🐦) and mammals than any reptile, but maintaining an active metabolism and high body temperature (endothermic/warm-blooded lifestyle) 🌡️...

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...came at a cost for gigantic dinosaurs. The bigger the dinosaur, the more difficult it would have been to dump excess heat. If a hot-running sauropod had to hoof it to catch up with a mate or escape a stalking theropod (mostly carnivorous dino, characterized...

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...by three-toes and claws on each limb), it could run the risk of overheating through exercise 🚨. What was possibly the solution then? Sauropods had birdlike respiratory systems that combined lungs with a system of air sacs🎈. In addition to lightening the skeletons...

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...of sauropods and boosting their breathing efficiency, this complex system may have played a role in allowing sauropods to dump heat through evaporative cooling 🌬️ in the same way that large birds do today!

Gratuitously long sauropod necks are just fascinating
...

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...on so many levels - mechanically, biologically, physiologically, ecologically. They're arguably the most extreme body parts evolved by anything that's ever lived. Important to note however that neck-length measurements 📏 in sauropods are not straightforward.

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Some of their
neck vertebrae attach to the ribs, making it hard to see where the neck ends and the trunk
begins.

Extraordinarily long necks can cause a lot of problems to the animal. We can gain insight to the cardiovascular problems of sauropods...

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...by examining giraffes 🦒. The heart-head vertical distance is 3.4 m (a column with hydrostatic pressure of 260 mmHg) in the tallest (5.88 m) giraffe on record,requiring a MAP (mean systemic arterial blood pressure) of 310 mmHg at the heart. MAP is the average...

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...arterial pressure throughout one cardiac cycle: systole (ventricular contraction) & diastole (ventricular filling). From the value given for giraffes, it is not hard to imagine how large an MAP is required for the Dreadnoughtus! A ridiculously high MAP means that...

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...the titanic dinosaurs would need an unusually heavy and large left ventricle! Aside from the heart being too large to fit easily inside the body, a thicker-walled ventricle becomes progressively inefficient in pumping blood due to interfascicular tension. 📉

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Osteological 🦴 evidence indicates that some sauropods had rather straight, horizontal necks, but others had enough cervical vertical flexion to raise the head a few meters above the shoulders. The high cardiac 🩸 costs associated with the upright neck of sauropods...

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...are also reinforced by the unreasonable foraging 🌿 cost. The volume of the semi-spherical feeding envelope is maximal at shoulder level, but it decreases as the animal raises its head. Thus, it would be energetically better to use the neck as a vacuum cleaner hose...

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...near the ground and not move the bulk of the body.

Undaunted by skeletal rigidity or blood pressure problems, some paleobiologists still reconstruct
sauropods in a rearing pose. To conceptually address the exorbitant circulatory cost, a number of solutions...

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...have been proposed:
1) Accessory hearts in the long neck, in which one heart pumps to the next, and so on, right up to the head. The advantage of this is that the rest of the body and the tissues between the accessory hearts can be at reasonable pressures.

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However, such hearts are unknown in the arterial system of any vertebrate. Accessory hearts would also have to occur in each artery passing through the neck, not just the carotids.
2) Siphon mechanism. An attractive solution, because the circulation to the neck...

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...is a loop. In a siphon, the potential energy used by raising the blood in one arm of the loop is reclaimed by the descending blood in the other arm, so the pressure at the heart does not have to be high. However, this mechanism would lead to...

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...a very low-pressure region at the topmost part of the loop. Even if soft structures of the face, mouth, eyes, and ears could be protected,
severely sub-atmospheric pressures in the head 🧠 are
impossible. If the vessels were effectively rigid,...

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...even a tiny wound would aspirate air, filling the upper circulation with gas. Wounds would not bleed or clot. In addition, sub-atmospheric blood pressure would cause degassing of the blood in the head (just like the deep-sea divers experiencing...

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...decompression sickness due to N2 bubbles). 🥴
3) Every neck muscle "paying for itself". In sauropods, as the cervical ribs flexed, they would have compressed towards the neck, and the muscle would have pushed on air sacs wrapped around the vertebral artery.

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In effect, the motion acts as an accessory pump to the heart. This way, the blood pumping would get a boost any time the dinosaurs moved their necks. The neck doing even a little work would have been a great relief to the heart. However, no solid evidence...

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...for the same exists.
This list goes on and on.

Interestingly, one can think of a much simpler solution! Blood flow in the sauropod's body could occur without difficulty if gravity were eliminated by immersion of the whole body in water 🌊. Aquatic or amphibious...

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...habits of sauropods were generally accepted throughout most of the 20th century, with some depictions of them on the bottom of water bodies and using their neck as a snorkel 🤿. As air pressures in the lungs are necessarily slightly sub-atmospheric during...

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...inspiration, it would be difficult
to expand the lungs against the hydrostatic pressure of the water column (of the water body). The sauropods possessed air-filled vertebrae, so they could have easily floated in the water with the lungs, neck, and nostrils...

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...at the surface 🏊 where lung inflation would not be a problem. It is certainly not a problem in whales 🐳 (📏 up to 98 feet; ⚖️ up to 180 metric tons; the largest animal known to have ever existed), where the distance from the blow-hole to the mid lungs...

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...can exceed 6 m, but they bring the lungs close to the surface during inspiration. The sauropod neck could be used to reach deep aquatic vegetation without any problems involving blood pressure (image source: R. S. Seymour, Physiology, 2016, 31, 430-441).

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This would have freed them from the energy 🔋 required to support a large body against gravity. In fact, this lifestyle is consistent with the limited upward flexion of sauropod necks, but ability to flex it down well below the level of the feet.

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All terrestrial features of the sauropod skeleton can be explained by the requirement of all
dinosaurs to lay eggs 🥚 on land, because the eggshell pores had to be air-filled 🐣. Breath-holding duration provides a strong selective pressure for larger body size,...

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...which can explain the sauropod gigantism very simply.

However, nobody has a direct evidence in support of this explanation. The 'story' of titanosaurs is still murky. This is precisely what makes these prehistoric behemoths so intriguing!

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#dreadnoughtus #titanosaur #sauropod #dinosaur #jurassicpark #paleontology #paleobiology #evolution #evolutionarybiology #physiology #circulatorysystem #fossil #SciComm