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This is a skeleton of a titanosaur, not quite the longest of the dinosaurs, but most certainly the heaviest! As far as we have found, we'd found no species exceeding 75 tons.

This post focuses on a neat piece of evolution called "convergent evolution", in which an unrelated species evolves similar features to deal with similar if not identical circumstances. But before we get to the one sole candidate I have in mind, let's look at the bones that made sauropods both famous and infamous among the human culture:

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Skull

Small (at least in proportion to the rest of the body); Peg-shaped teeth used to strip down leaves; No mechanism to make chewing possible

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Tail

Long; Usually tapering into a whip

In this alternate scenario, dinosaur evolution came later than back home, which means that when the comet slammed the Earth 65 million years ago, geology and biology were Tithonian, not Maastrichtian. (It also helps that the comet worked alone in this scenario--which means no Deccan eruptions and no sea-level regression.)

The sauropods became extinct in this scenario, but at the time, there was a group of dinosaurs small enough to survive the catastrophe and, once the ash had been cleared, become the giants of South America, Africa, India, Australia and Antarctica:

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Dryosauridae.

The most notable example of this family, Dryosaurus, may be fleet-footed and bipedal, but recent evidence has found that early sauropods were just like that before the Jurassic gave them the chance to grow out of bounds.

However, because Dryosaurus occupied a different niche, it was anatomically different. The skull had a horny beak and teeth situated on the cheeks, which would be useful for chewing. The tail was stiff, useful for balance.

And yet, in this alternate scenario, it was the dryosaurids that would give rise to the alternate titanosaurs, complete with elongated necks and tails. Would they need to change their skulls and loosen up their tails to satisfy this given niche?

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    $\begingroup$ Are you wanting your sauropods 2.0 to still be alive today in the modern world? And how important to you is it that they are complete duplicates of the original sauropods? For instance could they eat grass instead of ferns? Or be similar but not identical, the way a Paraceratherium is not an elephant, but is a giant mammalian herbivore. $\endgroup$
    – DrBob
    Commented Sep 8, 2016 at 16:52

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To my knowledge no, while they would need long necks, the tails and skulls are not really necessary, there are alternatives.

  • The skull of sauropods were just one of many skull designs related to the herbivore dinosaurs, really an design could theoretically work, from the triceratops beak, to the duck-billed dinosaurs, well, duck bill.
  • The tail of sauropods had two primary uses, counterweight and defense. but there was another tail design that worked just as well, if not better;

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The tail dawned by the Ankylosaurus was impressive, but not unique, Gastodons also had it and while not the same body as Dryosauridae, theoretically, evolution could favor an adaptation of this iconic design.

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  • $\begingroup$ except the dryosaur skull is directly related to its feeding mechanism, changing the feeding mechanism is going to drastically change the skull. combining a bulk feeding neck and a high processing head will not work. $\endgroup$
    – John
    Commented Jan 2, 2017 at 17:59
  • $\begingroup$ @John According to what? $\endgroup$
    – JohnWDailey
    Commented Jan 2, 2017 at 18:09
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    $\begingroup$ functional anatomy specifically how feeding mechanisms in question work. The super long neck of sauropods is a highly specialized highly costly adaptation, it is not going to arise is another mechanism already exists, high processing is such a mechanism. sauropods evolve the super long neck to focus exclusively on collecting food becasue the processing is handled elsewhere (crop) and they could shrink the skull and reduce its weight. The skull of a 180ft long sauropod is the same size as a horses skull. $\endgroup$
    – John
    Commented Jan 2, 2017 at 18:19
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    $\begingroup$ Chewing teeth are heavy as are the muscles that work them, I mentioned hadrosaurs becasue they are what you end up with with large size and chewing. the skull gets heavy by the time you get to sauropod size the skull would be massive, if chewing is still happening. evolution is only going to favor one or the other. $\endgroup$
    – John
    Commented Jan 2, 2017 at 18:22
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It has already happened in dinosaurs, titanosaurs are the second group of sauropods to evolve ridiculous size.

but for sauropods, they don't need chewing teeth because they have gastroliths to do the chewing for them, which is why they could evolve the long hoover necks to reach into trees. maximizing intake because processing food is handled on the crop, so you won't have the long necks with chewing. if you want to know what big dryosaurs look like look at hadrosaurs like Parasaurolophus, because that's basically what they are. they whole group are dedicated chewers so no super long necks sorry.

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    $\begingroup$ Sorry, but that does not answer the question. $\endgroup$
    – JohnWDailey
    Commented Nov 5, 2016 at 17:20
  • $\begingroup$ Sorry I was explaining that nature has already answered that question, you can;t turn a hadrosaur into a sauropod, the feeding mechanisms are just too different, they either need to give up chewing or give up having super long necks. The tail depends on this choice so I did not want to get into it. $\endgroup$
    – John
    Commented Nov 14, 2016 at 4:53
  • $\begingroup$ Who said I was targeting a hadrosaur? $\endgroup$
    – JohnWDailey
    Commented Nov 14, 2016 at 5:06
  • $\begingroup$ There were no hadrosaurs at the time of Dryosaurus. $\endgroup$
    – JohnWDailey
    Commented Jan 2, 2017 at 5:53
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    $\begingroup$ feel free to explain, they question is if dryosaurus evelve long necks like sauropds does the skull have to change, the answer is yes. if you want the skull shape you need a different feeding mechanism which means a much shorter neck. you can have the super long neck or the chewing skull not both. $\endgroup$
    – John
    Commented Jan 2, 2017 at 18:06
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In today's nature animals that can eat from trees and grass normally are four legged like a giraffe or elephant and have teeth for tough vegetation. What would the teeth be good for and would vegetation alone be enough to sustain a animal of that size. The shape of the head and only having front teeth seem that it to pull soft water vegetation in water, swallows its food mostly whole while using it teeth for sifting or catching fish. Flamingos have a similar head structure and sifts water. I imagine land was lush including the swamps would be teaming with life. It would help to know its environment.

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On the other hand it would not be very fast but its size could run off anything smaller and use the tail offensively to take predictors food and eat soft dead food swallowing it whole a huge buzzard. That maybe this why it has a small head in the picture below.

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  • $\begingroup$ This is a little hard to read. $\endgroup$
    – JohnWDailey
    Commented Sep 16, 2018 at 11:02

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