What big neural spines you have…

2 08 2007

While Tyrannosaurus rex may always be the unofficial “ambassador” of dinosaurs, there is scarcely a large theropod more intriguing than Spinosaurus. As a child it was one of my favorites, and it seemed to make at least some appearance in every dinosaur book, but I had never actually seen any fossils of it. If there was some huge sail-backed carnosaur, where are all the fossils? Most of the depictions looked like a Megalosaurus or Allosaurus with a fin tacked on its back, and I was very surprised when I saw it “brought back to life again” in Jurassic Park III; it was even weirder and more wonderful than I could have imagined.

By now the story is relatively well-known, thanks to headlining discoveries made in Egypt over the last decade and some popular books/documentaries like The Lost Dinosaurs of Egypt (which is ok, but it doesn’t hold a candle to Sternberg’s Life of a Fossil Hunter). In 1912 Richard Markgraf, under the direction of German paleontologist Ernst Stromer, collected the first known remains of Spinosaurus aegyptiacus near the Bahariya Oasis in western Egypt. The fossil material that pointed to a new, large theropod unlike any other known at the time was lost, however, as in April 1944 the skeletal elements (a lower jaw, some teeth, and some of the vertebrae with extended neural spines) were destroyed in an Allied bombing raid on Munich.

Spinosaurus
The Spinosaurus (prior to its destruction) on display at the Palaontologische Staatssammlung Munchen (from Smith, et al., 2006). Note how the vertebrae are likely not in the correct order, but were sorted by neural spine length (Stromer recognized this problem with the mount as well).

Given the strange lower jaw shape of Spinosaurus, I wonder why it was interpreted as essentially a big Allosaurus by some illustrators, toy-makers, etc. Granted, some might not have seen any photos of the remains (simply knowing that it was a big theropod with a sail), but lower jaw that Stromer examined is clearly different from that of any theropod known at the time, causing Stromer to give it it’s own group, the Spinosauridae.

Stromer's Spino
Stromer’s reconstruction of the material shown above from his monograph on Spinosaurus (via Wikipedia: Spinosaurus)

Reconstructions aside, in subsequent years some paleontologists thought they had found more Spinosaurus material, but none seemed to be conclusive. Then, in 1996 remains of another Spinosaurus (attributed to Spinosaurus marocannus, but this is probably a nomum dubium, based upon inadequate fossil material) were found, and in 1998 more material from Spinosaurus aegyptiacus was recovered, giving us a much better look at this long-lost dinosaur. In 2005 even more material from the huge upper jaw of Spinosaurus was described, showing that this animal had one of the longest skulls of all theropod dinosaurs (although certainly not as robust as that of Tyrannosaurus). What is interesting, however, is that we still don’t know much about Spinosaurus. We have a good idea what the skull and vertebrae look like, but the limbs have yet to make any appearance, and even though the Jurassic Park version used its huge claws to break the neck of Tyrannosaurus execution-style, we can only infer that it had long-powerful arms with huge claws from its close relatives like Suchomimus and Baryonyx.

Despite the lack of appendicular skeleton elements, the skull of Spinosaurus has been highly diagnostic and helped determine its relationship to other similar theropods that have been found only within the last few decades. Looking at the skulls of the smaller Baryonyx and Suchomimus, there is a substantial bump where the premaxilla and maxilla meet on the upper jaw. Indeed, the premaxilla has a bit of a convex bump to it, smaller teeth bordering at least two larger teeth, and then the maxilla has a concave shape downwards, with one or two small teeth before the teeth get larger, then becoming smaller as you move backwards in the jaw. In Spinosaurus and Angaturama/Irritator, however, it almost looks as if the snout has been pulled out a bit, the concave/convex features still being apparent, but not as deep as in the earlier forms. The differentiation of tooth size is also apparent, but the teeth in the convex portion of the Spinosaurus premaxilla are much smaller than the relatively huge teeth at the crest of the concave portion of the maxilla. It should also be noted that the nose moved backward in this lineage, in the earlier forms being just behind the concave bump in the maxilla, but in Spinosaurus it is much further back towards the eye. These differences, (among others) are enough to put these spinosaurs into two groups, the Baryonychinae (Baryonyx and Suchomimus) and Spinosaurinae (Spinosaurus, Irritator, and Angaturama).(All these observations are based off of a comparison drawing made in Sasso, et al., 2005, which I do not reproduce here for fear of copyright issues).

But what about the “sail”? Many prehistoric creatures have been known to be fin-backed, including Arizonasaurus, Dimetrodon, Edaphosaurus, Ouranosaurus, and possibly even a sauropod, Rebbachisaurus [and Playhystrix too, thanks Zach!], and there have been just as many different attempts to explain why neural spines of the vertebrae would have become so elongated. In the case of Edaphosaurus, E.D. Cope initially proposed that it used it’s fin to literally “sail,” the enigmatic cross-bars of the extended spines reminding him of aspects of sailing ships, although this idea has not been taken seriously for some time. More reasonable explanations deal with body temperature/heat, either too much or too little. Growing up I remember explanations that sail-backed dinosaurs and other animals needed to “warm-up” in the morning to get the best of their prey, so while other cold-blooded animals were still sluggish they could be active. Later, sexual selection came into play, the sails possibly becoming flushed with color or serving as signals (and this idea does not exclude others). Debate will still go on, but in the case of Spinosaurus and Ouranosaurus, it’s possible that the sails served multiple roles. During the day it would be exceedingly hot, so the sails may have been used to get rid of excess heat (which is hard to do if you’re a large animal). Desert nights are known to be cold, however, so the sails could have collected more heat towards the end of the day before the sun went down. These suggestions actually remind me of some modern termite mounds in Africa, built in a way that they capture heat in the morning and evening, but in midday the run is right above them so there is less surface area, allowing the mound to remain relatively cool.

But what about sexual selection? The membranes between the spines would likely have been filled with blood vessels (especially if they were used in heat gathering/loss), and may have been able to become flushed with color if the capillaries expanded or contracted. Indeed, why should such a huge organ only have one use? Sails did not just pop up out of the backs of fin-backed dinosaurs overnight, however. If we look at Suchomimus (and even the American Acrocanthosaurus) we see the beginnings of neural-spine extension, but certainly not to the same degree as in Spinosaurus. Could sexual selection have played a role here? What function elongated neural spines would have had at such stages, I don’t really know, but could the production of such big sails have resulted from one sex preferring a bigger “hump” or sail on a mate? Such sexual selection would allow for the growth of a sail over evolutionary time relatively quickly, and I certainly think it’s an avenue that should be considered when thinking about the evolution of sails in whatever group we might be talking about (especially if changes in the rest of the animal are relatively minor over time). Once sails got big enough, they could then be co-opted for heat gathering/loss purposes, thus having a double usage for the animals. All this is inference/hypothesis, however, and does not prove anything just because I can think of it. Still, fin-backed dinosaurs are amazing creatures, and fins seem one of those morphological attributes that has shown up over and over again in the history of life. I’m sure as we continue to dig and open up new areas for exploration, more of them will start coming out of the ground.

References:

Sasso, et al. “NEW INFORMATION ON THE SKULL OF THE ENIGMATIC THEROPOD SPINOSAURUS, WITH REMARKS ON ITS SIZE AND AFFINITIES.” Journal of Vertebrate Paleontology 25(4):888–896, December 2005

Sereno, et al. “A Long-Snouted Predatory Dinosaur from Africa and the Evolution of Spinosaurids.” Science, 13 November 1998

Smith, et al. “NEW INFORMATION REGARDING THE HOLOTYPE OF SPINOSAURUS AEGYPTIACUS STROMER, 1915.” J. Paleont., 80(2), 2006, pp. 400–406


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14 responses

2 08 2007
Zach Miller

I think, in gigantic dinosaurs, sails were used for display. In surely ectothermic synapsids, they were probably used for heat collection. And you forgot to mention Playhystrix, the sail-backed Permian amphibian! It probably used its sail for…well…don’t know yet. It’s neural spines are a bit different than reptiles, and there wasn’t as much room between the individual spines, but I’m ranting now.

Great post! Nowadays, depending on who you ask, Suchomimus has been sunk into Baryonyx, but given a distinct species (B. tenerensis). Angaturama is most likely synonymous with Irritator, and I’m extremely skeptical of S. marocannus (as a distinct species).

My question is: From what group did spinosaurs evolve? Sereno claims they are related to African “torvosaurus,” but “torvosaurs” are essentially synonymous with “megalosaurs,” a possibly paraphyletic grouping of poorly-understood Middle Jurassic theropods. I remember reading a suggestion awhile back (forget the author) that Pelecanimumus is actually a primitive spinosaur! The idea is hogwash, of course, but it just means we need more specimens!

2 08 2007
laelaps

Thanks for the compliment and the comments, Zach! I definitely think there’s a good case for sexual selection in dinosaurs with sails, although this is much harder to pin down.

Even though I haven’t seen it submitted scientifically, I am intruiged by the idea that Gregory S. Paul floated in Prehistoric Dinosaurs of the World that close relatives of Dilophosaurus might have given rise to the spinosaurs. I can definitely see the correlation in the jaw (and maybe even the crests that are sometimes inferred in reconstructions of various spinosaurs), but like you said, we really need more specimens if we’re ever going to figure this one out.

Thanks for your submission to the Boneyard, too! I’m definitely looking forward to this next edition.

2 08 2007
Zach Miller

I love Greg Paul. He draws beautiful dinosaurs, does incredible work without a degree in geo or paleo, and most of his conclusions from “Theropods of the World” turned out, twenty years later, to be true. Examples: He posited a relationship between Carnotaurus and Ceratosaurus before Carnotaurus had been fully described. He realized that Archaeopteryx had a hyperextendable second toe decades before the Thermapolis specimen was found. He separated Acrocanthosaurus and Spinosaurus, realizing that long neural spines do not a family tie imply. He realized that Baryonyx and Spinosaurus were sister taxon, and that both were more basal than tetanurines.

It actually bothers me that so many paleontologists write him off. His “Dinosaurs of the Air” book is mind-blowing. He really convinced me that dromaeosaurs and maybe troodontids are secondarily flightless and probably represent basal members of Aves, just after the Archaeopteryx condition. But even today, you’ll see science bloggers and professionals turn their noses up at the idea that dromaeosaurs are birds–Pondering Pikaia and MicroEcos both kind of irritated me when they described Microraptor as being “not a BIRD, but a non-avian theropod.” There’s hardly a line in the sand when it comes to Microraptor on whether its a bird or not.

But that’s hardly the point. Greg Paul’s been right about so many things before, I’m just confused as to why he gets so much flack for suggesting that maybe bird origins aren’t as neat ‘n’ tidy as we’d like to think. What IF dromaeosaurs are secondarily flightless? Does the entire science of paleontology crumble to the ground? Why are people so against that idea?

It’s a sore spot for me. I’ll shut up now.🙂

2 08 2007
laelaps

I’m a GSP fan as well, and even though some of the taxonomy Paul proposed didn’t work out, he’s been incredibly prescient when it’s come to theropods and birds. I absolutely love the “dueling” Archaeopteryx in Predatory Dinosaurs (then again there’s hardly a GSP illustration I don’t like).

When I received my copy of Dinosaurs of the Air I was a little skeptical of the secondarily-flightless hypothesis at first, but as I thought more about it, it made more and more sense. It had always bothered me that the fact that feathered dinosaurs being touted as so close to birds came after Archaeopteryx, not enough to make me disbelieve that birds are descended from dinosaurs, but it was awfully confusing because it never seemed to be fully addressed. If nothing else, I think Paul has definitely struck something important in the “secondarily flightless” hypothesis, and the origin of birds seems to be a lot more complex than first realized (then again, so is much of evolution!). I always found it hard to believe that there was just this straight line of evolution, resulting in Archaeopteryx on a few islands but never any other forms that were close. Like you also said Zach, the Thermopolis specimen has definitely helped cement this relationship, although I think calling Microraptor a bird or non-avian theropod seems to be a matter of preference. It seems to be just as much “bird” as Archaeopteryx is, but I guess some people would call it a “non-avian theropod” because it comes after Archaeopteryx and may not be ancestral to birds. Regardless of the name game, it is interesting that Microraptor somewhat confirmed the hypothesis of Heilmann and others who thought that there could be Archaeopteryx-like creatures with wings on all four limbs, even if it wasn’t a direct progression from four wings to two.

Anyway, I’m sure that bird evolution isn’t entirely neat and tidy, and there was probably a much larger early diversity than we now realize. I’d love to combine evolution with stratigraphy to try and determine the best spots to look for ghost lineages. All the specimens coming out of China are great, but I would really want to look back through the Jurassic to see if the earlier history couldn’t be turned up.

3 08 2007
neil

Ha! Well slap my hump and call me Irritator. Actually, I wasn’t trying to get political by asserting that Microraptor wasn’t a bird, just trying to distinguish it from her more derived, birdy near-contemporaries like Confuciusornis. I suppose my wording did seem to suggest a clear dividing line betwixt dinosaurs and birds, which is certainly a misguided perception. I wouldn’t be surprised at all to see Microraptor and the rest of the dromeosaurs wind up in Avialae, hopefully with a few new rad arboreal microtheropods. Chalk it up to my poor knowledge of basal bird/theropod phylogeny. Good thing I have you guys to set me straight (feel free to drop me dopeslap comments Zach, at least when I deserve them!).

Speaking of humps (though steering wide of the black-eyed peas), I was hoping you’d give some analysis of Jack Bailey’s ideas about bison-backed spinosaurs. I have to admit that spinosaur vertebrae are pretty unlike Dimetrodon, or even Platyhystix, they look pretty robust to be part of thin sail. I wrote a paper on spinosaur functional morphology way back when, and I think I too came down heavy in the sexual selection camp, although I recently saw Padian rail against the invocation of sexual selection for dino structures without solid evidence for sexual dimorphism. As Zach says, we just need more fossils!

3 08 2007
laelaps

Thanks for the comment, Neil! You and Zach are far better versed/more familiar with much of this than I am, and for my own part I didn’t say much more than “Things are complicated. More fossils, please.”

I also appreciate the steering clear of the Black Eyed Peas (although even their mention gets that infernal song stuck in my head). I’ll have to check out Jack Bailey’s paper and blog about that since it’s the second time it’s come up. I think it could have worked for Suchomimus and Acrocanthosaurus, but the spines of Ouranosaurus and Spinosaurus just seem too high to me to support a “hump” rather than a sail; I would imagine it would make them awfully top-heavy.

As for the sexual selection bit, like everyone has said “We need more fossils!” Spinosaurus and friends are pretty fragmentary as is (Baryonyx being the #1 exception), so we definitely need more and more of them. Still, I’m not so sure that sexual dimorphism has to be the main evidence for sexual selection that has taken place; a characteristic could be selected for in one sex but also be expressed in the other, especially if it also had a functional component to it. If the sails acted only as ornamentation, like a peacocks tail or sparring between males, then I’d say that we’d need to see sexual dimorphism, but if an adaptation benefits the animal outside of such a context, then I think sexual selection might still be in play. Whatever the answer is, though, we’re going to have to look for it among the earlier members of the group because I think that’s more difficult to explain than the later sails (although “ridge-backed” forms could have been the ones with “buffalo backs”). Sounds like I need to get cracking on another post….

3 08 2007
Zach Miller

Oh, next time you make a phylogenetic foul-up, Neil, I’ll let you know!🙂 Also, Suchomimus, Laelaps, is actually far more complete than Baryonyx, unless more specimens of the English spinosaur have come to light since 1988…it’s going to be pretty tough to come up with sexual dimorphism in a group of animals that are represented by a single specimen (or very fragmentary subsequent specimens) per taxon!

I’m betting sexual selection. Dinosaurs were extremely showy creatures, moreso than mammals today (probably because dinosaurs would have been color-sighted, whereas mammals are color-blind) and while most displayed with their heads (crests, horns, etc.), some displayed with other body structures (sauropods, thyreophorans). I don’t think it’s out of line to say that spinosaurs and Ouranosaurus developed eye-catching sails.

You know what really interests me is that Dimetrodon and Edaphosaurus lived in the same place at the same time, yet independantly developed sails. The sails are somewhat different, structurally, but are basically analogous. Spinosaurus and Ouranosaurus lived in the same place at the same time, yet they both developed sails independantly. In both cases, you’ve got a co-habitual carnivores and herbivores evolving the same kind of structure. What environmental or sexual pressure was being put on first those pelycosaurs and then, later, those dinosaurs?

One wonders if, perhaps, the herbivores in both cases developed sails with similar colors/markings to the carnivore’s sail in order to confuse the predator or stop it from attacking the herbivore completely? Defensive mimicking, perhaps. It’s the only real reason I can come up with. I’d say that the two occurrences of sails are completely unrelated, but you never know…

23 03 2008
Amadeus

Im still not convinced spinosaurus did not have a hump but spinosaurus would probably tip over if it did.

Question-since spinosaurus lived by the coast do you think it had webbed feet?

21 09 2009
kiley

this is stupid.

21 09 2009
polly

it is stupid!

21 09 2009
polly

.

15 09 2011
Lani

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1 10 2014
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12 11 2015
- dseggs

[…] *For succinct overviews of Spinosaurus research prior to 2014, check out posts at Tetrapod Zoology and Laelaps. […]

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