A "Brief" Overview of the Lords of the Mesozoic, the Sauropods

To be quite frank, while tyrannosaurids are cool, nothing, and I mean, nothing, can hold up to the mighty sauropods. This dynasty of dinosaurs lasted from the late Triassic (210 Ma), to the latest Cretaceous (66 Ma), conquering every continent, and growing to be the largest land animals that ever existed. I find sauropods utterly fascinating, because they are in many ways, absolutely crazy to the point it's unfathomable that these animals ever existed. Obviously their size gets a lot of attention, but that's not it; their ontogeny and growth rates are super crazy, their ecology has by far the largest impact on the environment of any dinosaur group (maybe even any animal group), and their plethora of body plans capture my imagination with a fishing hook tight grip.

Because of my very strong and peculiar interest in these enigmatic animals, sauropods will definitely become the main focus on my blog. Over the last 8-10 months I have been stuck in a whirlwind of sauropod biology and evolution, and it doesn't seem like this will end anytime soon. Everything new about them only makes my attachment to them stronger. I’ll be doing random little sauropod posts here and there, but today I wanted to provide a brief overview of sauropod evolution as a whole to provide background context for later, more in depth posts.

Sauropod Origins and Evolution

Sauropods have been traditionally classified in the group known as saurischia, and to be more specific, put as the sister clade to theropods. Of course, this has been challenged this year by the publication of the “Ornithoscelida hypothesis” of dinosaur classification. Not to go off on a tangent, but for simple purposes let's just use the Ornithoscelida hypothesis. The way we traditionally view dinosaur classification is probably wrong, due to mainly being based on hip shape, and while the Ornithoscelida hypothesis has many hurdles to jump through, it is probably more correct than the traditional classification.

If we follow the Ornithoscelida hypothesis, sauropods are still saurischians, but this time saurischia only includes sauropods and their relatives (the “prosauropods”, and herrerasaurids). Saurischia/sauropodomorpha first originated in the late Triassic during the Carnian epoch, and, as most if not all early dinosaurs, were theropod like. Even back then, when early sauropodomorphs such as Eoraptor lunensis (theropod?) first appeared, they had both carnivorous dentition and herbivorous dentition, suggestive of omnivory. In fact, early dinosaurs as a whole were probably omnivorous given it would allow them to be adaptive to the unstable Triassic climate.

By the Norian epoch (220-215 Ma), the first “prosauropods” evolved, such as Plateosaurus and massospondylids. They increased quite a bit in size, the largest individuals about 10 metres and excess of 3 tonnes or more. They also lengthened their necks, decreased skull size, evolved a large gut, and their mouths were now filled with a completely herbivorous dentition. Why the increase in neck length and decrease in skull size? Well, these animals seemed to be browsers, and a longer neck would help with reaching higher up in the branches, but because of this they had to reduce their head size in order to keep themselves balanced. Smaller skull size means limited to no chewing, and this is where the bigger gut would come in; because they couldn't chew, they would've relied on hindgut fermentation to break down the vegetation, hence a large gut. Their forelimbs were robust, and have three large unguals on their manus. These could not rotate so the palm could face the ground (pronate), which strongly suggests they were still bipedal.

Then came the anchisaurians. These sauropodomorphs were different in several ways; they got larger, and in order to help combat their increasing size, their manus became pronated so they could support their weight on four limbs. To help allow the back end of the animal support the growing weight, they had four sacral vertebrae, which is seen later on in true sauropods, which came immediately after the anchisaurians. Surprisingly, the oldest known true sauropod remains come from Thailand in the late Norian, and this suggests that true sauropods coexisted with the non-sauropod sauropodomorphs for quite a long length of time. In the end, it does seem like other sauropodomorphs died out because sauropods were just better at what they did.

Why did sauropods get so big? The answer is in their anatomy. Sauropods had well distributed air sacs in their body, which lightened the weight of the animal. This is pretty deceptive, sauropods look really big and while they are pretty big, they aren't as big as you might think. This is more apparent in more derived sauropods such as rebbachisaurids and titanosaurs, where they took pneumatization far beyond any other sauropod group. Still, this allowed them to reach their infamously gigantic sizes.

Going Down the Tree

I'm now going to briefly introduce each sauropod group and give a basic run down on them. The first are the vulcanodonts, the most basal sauropod group. Sadly no known skull of vulcanodontids have been found, so to give an idea of their ecology is a bit hard. We do however have a complete foot of a vulcanodontid, and the pes is rather long, which hints both as a basal trait and that they were probably not large. Indeed, vulcanodontids probably did not exceed 9-11 metres and 3.5 tonnes. The feet might also hint at an ability to still rear up and perhaps even walk on two legs.

The eusauropoda are the next, slightly more derived group, which include sauropods such as the club tailed Shunosaurus. The eusauropods in turn also contained the cetiosauridae, which might or might not be a monophyletic clade. Some studies have also found the super long necked Chinese mamenchisauridae to be within cetiosauridae, in the subfamily mamenchisaurinae. Cetiosaurus is the first named sauropod, described after Megalosaurus bucklandii, in 1842. At the time however, Richard Owen believed them to be the remains of a massive marine crocodile. Another known cetiosaurid is Patagosaurus in Argentina, with a rather long tail to go with it. Mamenchisaurines are without a doubt high browsers, due to their incredibly long necks.

Nigersaurus is also known as the best biological example of a vacuum cleaner to date.

Then there are the turiasauria, a poorly known radiation of sauropods seemingly endemic to Europe, and also seem to hav shorter necks than other sauropods. They are still very big and might have reached lengths of 30 metres with an estimated weight of 40-50 tonnes. We then enter into the more derived neosauropoda, which contain two groups; diplodocoidea and macronaria. Diplodocoidea are my personal favourite group of sauropods containing three families; diplodocidae, dicraeosauridae, and rebbachisauridae.

Diplodocidae are one of the more recognizable sauropods, which include the near universally known Brontosaurus, and the also well known Diplodocus, the family known to having long, whip like tails, peg like teeth restricted at the anterior, and raised nares located a top the skull. Dicraeosaurids mainly have short necks, some taxa, like Brachytrachelopan, have the shortest necks of any sauropod and seem to be filling a niche similar to some ornithopods. They also include the mysterious Amargasaurus, which have keratinous covered spikes on the neck, possibly for defence. Rebbachisaurids are endemic to South America and Africa from the early to late Cretaceous, and include the bizarre Nigersaurus, a sauropod with a broad, possibly keratinized snout. Rebbachisaurids seem to be grazer, unusual among sauropods.

Then there are the macronarians, the most derived group of sauropods. At the base lie camarasauridae and brachiosauridae. Camarasaurids are known almost exclusively from North America (some possible species from China are known) and exclusively during the late Jurassic. They had large box shaped skulls with spoon shaped teeth (typical of most macronarians), and also might've had the ability to masticate, unlike other sauropods. The brachiosaurids are also one of the most iconic sauropods, containing the fairly poorly known Brachiosaurus, and the very well known Giraffatitan. Their forelimbs are obviously longer than their hindlimbs, also unlike other sauropods, and this suggests there was no need for rearing and that they were very well adapted at plucking branches from the tops of trees.

Yes, Hallett shrink-wraps and shit, but that doesn't mean it ain't porty right?

Further down we have somphospondylia, which includes sauropods like Sauroposeidon porteles, which might look like brachiosaurids but are far slimmer. Within somphospondyls, there are two groups of sauropods. One are the euhelopidae, a sauropod group only found in the early Cretaceous of China. They seem to be high browsers, like brachiosaurids, and have longer forelimbs, but this evolved independently from brachiosaurids. They are also pretty gracile, with average weight estimates for these animals being around 3-6 tonnes.

The other group of somphospondyls, and also the last and most derived of all sauropods, are the titanosauria. While they are some of my favourite sauropods, they are absolutely a bitch when it comes to good remains and phylogeny. Only the more derived titanosaurs like Opisthocoelicaudia, Nemegtosaurus, Rapetosaurus, and Saltasaurus are well known, the more basal titanosaurs like Argentinosaurus and Puertasaurus being very fragmentary and thus their ecology is up in the air. The more derived titanosaurs, which include aeolosaurinae, nemegtosauridae, and saltasauridae (which might all be the same group anyways), we know a bit more about, thanks to well preserved skulls from Rapetosaurus and Nemegtosaurus, which show a very diplodocid like skull and teeth, suggesting similar habits to them.

Titanosaurs are also thought to be the all time record breakers for dinosaur size, but we must take into account the fact that titanosaurs are the most pneumatic of sauropods, and thus, while having a very wide body, probably weighed a lot less than we would expect. Titanosaurs are also distinguishable for the osteoderms adorned on their bodies, as well as in addition, being the only known sauropods left at the end of the Mesozoic. And that folks, is a super brief rundown of the sauropod groups. There is so much material I skipped, but it's better I save those for another day and another post.

Biology and Ecology of the Sauropods

This next section is far more speculative than the first, as it deals with biology and ecology, something is harder to tell from the fossil record. Also, it might feel like I’m just becoming lazy but that's because I really want to discuss this part, so let's begin.

Sauropods; the clowns of the Mesozoic?

We’ll start with a pet hypothesis of mine, more of a suggestion for artists really. When you see sauropod reconstructions, they are mostly a singular dull colour, which is mainly based from looking at other large extant mammals. But there is an issue with this; these aren't mammals. Sauropods are of course, reptiles, which as we know from birds and lizards, can be extremely colourful, and it's almost certain sauropods can see in the same spectrum of colours as extant reptiles, and probably also produce a lot more colours than mammals can. At the very least, I encourage artists to provide sauropods with some complex patternings, since it seems almost universal in extant archosaurs. I would even go a step further and propose that sauropods were brightly coloured.

This is based off the fact that large sauropods in the ecosystem don't really have a need to hide, they're going to make lots of sounds and a lot of other signals which will make it obvious to predators they are around, and in addition they're going to be too large to be hunted anyways. So, I propose that larger sauropods were extremely colourful and extremely display oriented. Obviously, I might add that this does not necessarily discount the idea of dull coloured sauropods; I’d imagine hatchlings and smaller species being more dull. And even then, larger sauropods don't necessarily have to be colourful, it's just possible that some of them were.

So no, I’m not saying sauropods are only super colourful, it's mainly due to the fact that it is a possibility and also a great subject of interest in paleoart. We are running amok with hundreds of illustrations of super dull sauropods, it's about time to at least produce more colourful sauropod reconstructions. There is a lot of leeway and creativity in this idea. My only fear is that with this idea, people will begin to make it into a meme, which I don't want to happen. This is more of a challenge for any paleoartists out there, to come up with their own ideas of sauropod display structures and colours, and I really hope I’ll see more of this in the future.

Reproduction; sauropods are really just baby making machines

Sauropods do get very big, but these individuals are far and away very rare. Why is that so? In extant ecosystems which are mainly dominated by mammals, the adult population might actually be larger than the non-adult population. With sauropods and other dinosaurs it is the opposite. From all the fossil dinosaur nesting sites we have, dinosaurs are more like crocodylians in this aspect, laying a high number of eggs, and the young hatching already being well developed and thus, having limited amount of parental care. This is in contrast to most birds and mammals, where they produce relatively few young, who are born defencless, and have a much more extended period of parental care. The reproductive style employed by mammals and birds is called K-selection, and the reproductive strategy employed by crocodylians and dinosaurs is called r-selection.

Sauropods are very good examples of r-selective animals. While few juvenile and hatchling sauropods have been found, we know enough to say that they were born ready to live life on their own. It should also be noted that sauropods larger size and nesting strategy makes it more and more likely that sauropods provided no parental care to their young. The only hatchling and nests of sauropods found thus far, are from titanosaurs, which are already a pretty derived sauropod group. If this is to be extrapolated, they lay anywhere from 15-50+ eggs in a nest, many the size of footballs. The eggs were laid in a nest dug up by the adults, and buried under dirt. Estimates of the incubation period of sauropod eggs are anywhere from 65 to 82 days, on par with ratites such as emus and ostriches.

When born, sauropods had a shorter neck and a larger head with larger eyes, and were precocial. What makes sauropods so impressive is the rate at which they grow. The largest ones would have hatched out already weighing 2-3 kilograms, and growing to be 20-25 tonne giants in less than ten years. This extremely quick growth rate is the fastest in the animal kingdom, only outdone by rorqual whales. How did baby sauropods achieve such a quick growth? This is pretty suggestive of them ingesting high protein and high energy plants, but what kinds of plants were those in the Mesozoic?

Turns out the highest value and most easily accessible plants would've been horsetails, which grow well next to bodies of water, as well as in clay soil and soil containing gold. Some low growing ferns like Osmunda also show high overall value, comparable to horsetails. Some non-plant foods which are high value include fungi, which contain folic acid and lots of other vitamins, and even invertebrates. Invertebrates are great sources of protein and fats, and without a doubt could significantly contribute to a sauropods growth.

Sauropods hatch out ready to feed and fend for themselves, but they are still highly vulnerable to predation. It is thought that 80-90% of sauropod hatchlings would have died in the first year a lone, due to being small, very common, and easily accessible. This is rather similar to the mortality rates suffered by baby sea turtles today. Like most dinosaurs, sauropods rarely reached adulthood, and hatchling/juvenile populations would significantly outnumber the adults. There are five basic life stages in a dinosaurs life;

1. Hatchling - Newly hatched dinosaurs, mostly independant, suffered quite high mortality rates.
2. Juvenile - Might be anywhere from a year or more old. More developed than hatchlings but still not yet at the age of reproductive maturity.
3. Subadult - This is when dinosaurs begin to approach adult size and reach reproductive maturity.
4. Adult - Those very close to or at skeletal maturity.
5. Superadult - Skeletally mature individuals which are very old, and of course include the oldest dinosaurs.

Hatchlings are by far the most common life stage of any dinosaur. Beyond a year, the mortality rate of dinosaurs goes down and down, but they are still susceptible to acts of predation by larger carnivores. Such high mortality rates due to predation also explains the rather dull amount of juvenile or hatchling sauropods we have found. Subadults tend to be the most common sauropod fossils found, but their populations would be very low still compared to their early life stages. This is when we start to once again see a large mortality rate, brought on by the stresses of reproduction. Very few adult sauropods have been recovered. Some of the largest sauropods (such as OMNH 1670 and NMMNH 3690) already are 30-35 metres in length and may weigh up to 50+ tonnes, but the OMNH specimen is still only 70-80% fully grown, despite already being 60-65 years old.

In any case, 10 years is the average age of sexual maturity in sauropods. How would mating work? Due to large size, you might think that mounting would be difficult, but not really. Most sauropods can easily rear up on their hindlimbs, and even in those that aren't so well adapted to do so, they still can rear up to get into a mounting position. So, despite the suggestions of backwards mating and laying down mating, mounting is still the most likely option. And because they are huge animals, it would be advantageous for them to have huge sexual organs as well. Because it is hard to get the cloacas to touch, I’d imagine a long, possibly prehensile penis for male sauropods to do their deed.

Going back on old age, it is plausible to imagine the oldest sauropods being sterile. Large, old crocodylians usually tend to have exhausted their sperm and egg resources by the time they reach 40 years of age. Crocodylians (and turtles) will also sometimes lay multiple clutches per year. I’d imagine sauropods to be the same as well, exhausting all their eggs and sperm during their subadult stage of life, leaving the adults and superadults sterile. This might also apply to other dinosaur species and extinct archosaurs as a whole.

Sauropod Sounds; monotone?

In movies (especially Jurassic Park), and documentaries (like Walking with Dinosaurs), sauropods let out these low frequency, almost whale like sounds. And that's what we tend to associate; both are very big animals, so they may have produced similar songs. This however doesn't really take into account the completely different neck and throat anatomy that sauropods. Like whales, sauropods probably had a larynx (voice box), a feature shared in pretty much all amniotes. However, sauropods had very odd nasal anatomy, which can affect how they made sounds. There is a pattern we are starting to see, and that is that longer necks animals tend to produce lower frequency sounds; this is apparent in ratites and giraffes. Giraffes, long thought to be silent animals, produce very low frequency hums, and ratites make deep rumbles.

So, given sauropods have necks proportionally longer than any extant animal, it seems safe to assume that sauropods had a very deep voice. Indeed, a study done on the inner ear of Giraffatitan brancai shows they probably heard mainly or entirely in infrasound. If this is correct, then perhaps sauropods didn't produce the long, whale like songs we usually hear, but instead produced infrasound, which while is undetectable to the human ear, can produce very loud sounds which reverberate into our bodies. And given sauropods extensive size, they almost certainly would have produced incredibly loud sounds, possibly able to detect for kilometers upon kilometers. So while we may not hear them, we could feel them instead.

But, what other sounds can sauropods make? Surely it's not just a simple low pitched hum right? Well, the recurrent laryngeal nerve (the nerve which sends signals from the brain to the larynx to produce vocals), as in all animals, comes from the brain and down the neck, and loops around the heart, then back up the neck to the larynx. Obviously in sauropods, this is on the extreme, with the recurrent laryngeal nerve in some of the longer necked sauropods being 30+ metres long, one of the longest cells of any animal to have lived. The speed at which the signals travel is important. The fastest recorded nerve signals in extant vertebrates is 50-70 metres per second, though it is usually slower. In this case. The long looping recurrent laryngeal nerve is a disadvantage, and thus probably wouldn't allow sauropods to make more complex sounds.

Laryngeal nerve compared to humans, giraffes, and sauropods. From Wedel (2012).

However, there is a loophole around this. The large nare openings in some macronarians could almost certainly be used in amplifications, and also possibly change the pitch of their tones, creating a more wide range of pitches and sounds to produce. This wouldn't have worked in sauropods like diplodocoids, since they don't have large amplification structures. So, rip diplodocoids… guess they just hummed in a frequency we can't hear… or did they?

Diplodocids have very long whip like tails, and it has been heavily suggested that they used their tails as bullwhips, creating loud sonic booms that can be heard for many kilometers. The effect of such a loud sound probably could knock out a person if they stand too close, or even result in permanent hearing damage. But really, do they make these sounds? There have been arguments that suggest this doesn't make biological sense, I mean how can the sauropods not hurt themselves when they produce these loud ass sounds? Pretty simply actually; by reducing the amount of nerves or completely removing them from the end of the tail. Thus, their whip tails may have been “numb” in a way.

One sound however that I see being nearly universal in sauropods is hissing; their closest extant relatives, theropods and crocodylians, do hiss as a threat, and hissing is even found in many mammal species as well (ie., cats). Hissing is a rather simple sound to make, so I can see even the longest sauropods producing deep hisses. I wonder how loud that would be… sauropod vocalization is obviously only under assumptions right now, so how they actually sounded is still up for discussion, which may prove many of the points in this section invalid.

Sauropod Gods; “He said, let there be light”

God is just a ludicrous fantasy for kids. Sauropods are by far and away the largest animals in their environment. The largest extant terrestrial animals, elephants, often shape the environment around them, with herds forming trails in the grass, and knocking down trees which might prevent forests from growing. If they went extinct, the ecosystem would be a very different place. Alligator’s too, shape the world around them. They create small ponds called alligator holes, and in doing so have increased the range of the Everglades in the process. The holes create a sort of oasis during the dry season for many aquatic organisms, and alligators also leave many areas dry and free of water to nest, which in turns allows other reptiles to nest and for plants that are flood intolerant to grow. These animals are ecosystem engineers, keystone species for without their existent, the entire ecosystem would collapse and change.

And given that sauropods are many times heavier than elephant herds, the effect of sauropods on their environment would've been enormous. As sauropods moved around and foraged, which would've trampled up the soil and the undergrowth, an act called dinoturbation (or bioturbation, couldn't resist putting this in, sorry, thanks Wedel and Hallett). The constant trampling would've destroyed the dominant vegetation, allowing for the less dominant to get sunlight and grow. Such movements of sauropods might even have created trails like what elephants do. Sauropods might also have knocked down trees to either reach for their food, or to eat the vegetation on the tree if they cannot reach it, probably creating open lands. In fact, it is interesting to say that rainforests and jungles were nearly non-existent in the Mesozoic, but only when the Cenozoic arrived did they start to appear. Perhaps like elephants, sauropods kept such areas cleared of trees.

Of course, looking at extant dinosaurs, sauropods may have engaged in behaviours such as dustbathing, waterbathing, and sunbathing. If they dustbathed, they might even created lakes. Sauropods were, by far and away, the largest scale ecosystem engineers ever to have lived, the deities of the Mesozoic.

And that will conclude this article! Just a brief (well, “brief”) intro to sauropods. Expect quite a few sauropod posts to come along in the future… actually already have an idea for one! I want to apologize for lack of original photos in this post, I was really in a hurry to do this one. Anyways, buh bye!

References

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