Diplodocus: A history of reconstructions - Part 2

Here we are back again with a history of Diplodocus reconstructions! Last part we looked at the very first reconstructions of the genus, its predecessors and the fame and controversy that came with the various Dippy mounts. Today we will look at what happened after, as well as the many, sometimes weird ideas that have been made about the genus.

From Water to Land

Fig. 1

Starting from the 1920s onward, Diplodocus, like all sauropods, was interpreted as an erect-legged, but tail-dragging and mostly aquatic animal, with some researchers, such as William Diller Matthew, even going as far as saying that they never left the water by giving birth to live young. The most common interpretation about their diet was that they fed on soft aquatic plants, as their teeth seemed highly unsuited for chewing anything tougher. It was also thought that the head was too small to gather enough food to feed the giant body in bulk, hence why they were restricted to a very slow metabolism. But even back then, some questions about their diet came up, especially in relation to the bizarre, pencil-like teeth of Diplodocus with their blunt tips. In 1924, William Jacob Holland, the same man who supervised the Dippy mounts, proposed that Diplodocus may have actually primarily fed on mussels and other shelled animals, using the teeth to pluck them from rocks. The poor clams were then swallowed whole and crushed in a gizzard by gastroliths. Although he was the one proposing it, Holland himself was skeptical of the idea, as he correctly observed that no one has ever found mollusc shells in the stomach region of any sauropod skeleton. The idea still interestingly foreshadows some modern suggestions that Nigersaurus may have been a freshwater filter-feeder (Hallett & Wedel 2016).

Fig. 2

In the 30s, new specimens were being discovered, such as the Smithsonian’s USNM V 108655, adding to our knowledge about the genus. This specimen was originally assigned to the type species D. longus, but seems to more likely have been part of Diplodocus hallorum (Tschopp et al. 2015), which you might know better as “Seismosaurus”.

Fig. 3 & 4

The view of Diplodocus and other sauropods as aquatic grazers prevailed well into the 40s and 50s, as can be seen by these two paintings, the top one made by Mathurin Méheut for the French University of Rennes and the bottom one by none other than Zdeněk Burian.

Fig. 5

Already in the 50s, doubt began to appear about the classic watery sauropods. Kenneth Kermack’s (1951) studies showed that the laws of physics would have prevented them from using their long necks as a snorkel, like here in this Burian reconstruction of Brachiosaurus, as the water pressure on a fully submerged sauropod would have compressed its lungs so much that it would have been impossible for it to draw in air through its windpipe. If you want to test this yourself (though mind you that this is pretty unsafe), drop to the bottom of a pool and try drawing in air through a two-metre straw. Unfortunately, Kermack’s conclusions were either ignored by the paleontological community or dismissed by Edwin Colbert with the argument that whales can breathe while in the water just fine (Desmond 1975), ignoring the fact that whales do not have long necks and have to come very close to the water surface with nearly the whole body in order to take in air.

Fig. 6

In the late 60s, much change was on its way. In his influential paper The superiority of dinosaurs, Robert Bakker (1968) readdressed Kermack’s results and found various other flaws with the idea of sauropods having hippo-like lifestyles. Their hollow bones meant that they would have awkwardly floated on the water’s surface, whereas hippos and whales have heavy, solid bones in order to better sink. The feet of sauropods were also not adapted for muddy environments and, perhaps most importantly, many of their fossils were found in sediments that implied a very arid environment. Bakker reinterpreted the sauropods not only as fully terrestrial animals that ate the leaves of tall plants like giraffes do today, but also as metabolically highly active creatures, which used their bizarre teeth to rake off coniferous plant matter in massive bulk to then digest it in a gizzard (if sauropods truly used gastroliths to break down their food has come into question over the years (Wings & Sander 2007) and pure hindgut fermentation through a specialized caecum may be more viable (Hallett & Wedel 2016)). While not Diplodocus, Bakker used close relative Barosaurus to provocatively illustrate this new vision of sauropods, showing the animals with a proudly high-held neck, the tail well above ground, striding into the prehistoric savannah like giraffes. This roughly still remains the default interpretation of sauropod lifestyle, as many subsequent studies have confirmed its validity. But it was certainly not the end of mystery and debate around the life appearance of these animals.

Much noise about a nose

Fig. 7

One of the first questions trying to be unravelled during the Dinosaur Renaissance was that of the sauropod face. Most sauropods, especially Diplodocus, have their nares (the bony holes for the nostrils) on top of their skull right above the eyes. Classically, the nostrils were therefore placed right there as well, giving these animals a whale-like blowhole, which of course perfectly lined up with their original aquatic interpretation. With the knowledge that sauropods were actually land dwellers, the position of the nostrils became an intriguing mystery during the Dinosaur Renaissance. A sober take by McLoughlin (1979) was that the “blowhole” instead developed to more easily breathe while the mouth was submerged deeply in the spiky canopies of conifer trees. Giraffes, gerenuks and other high-browsing mammals of today also have retracted nostrils to not get stung in the nose by tree needles and in recent times this has even been put forth as an explanation for the sauropod-like skulls of litoptern mammals like famous Macrauchenia (Croft 2016).

Fig. 8

However, the most infamous take was of course that there never was a blowhole. Coombs (1975) was the first to argue that the retracted nares were actually evidence for a proboscis, based on the fact that animals like tapirs or elephants also have retracted nares to give a strong base for their trunks. Coombs himself did not illustrate this, but many after him did, such as Bakker (1986) above, who was open to the idea, but seems to have preferred the classic blowhole, with the explanation that the on-top nostril-position may have instead been useful for sound production. The sauropod trunk became a recurring phenomenon throughout many 70s and 80s books, mostly aimed at general audiences (read: children) to illustrate the degree of uncertainty in paleontological reconstructions. A trunked alternate history sauropod even appeared in Dougal Dixon’s The New Dinosaurs. Today the idea of the sauropod trunk is not taken seriously anymore, for good reason. No reptile group ever had the facial musculature required for such an organ and various details of the skull anatomy also speak against it.

Fig. 9

Nonetheless, researcher John Martin proposed a variation if it in 1996 with this 3D model of Diplodocus with prehensile lips. This never went anywhere and the exact reasoning and methods behind it remain obscure, though as Darren Naish noted, the position of the nostrils in this model is somewhat prophetic.

Fig. 10

For in 2001, Lawrence Witmer released an influential study, wherein he compared the fleshy nostril positions of various living reptiles and came to the conclusion that the position of it in sauropods and other dinosaurs was much more forward on the skull, close to the snout-tip, as in most other terrestrial vertebrates. In this view, the bony nostrils were just the base for an elaborate flesh-and-cartilage structure, not too dissimilar from what is seen in the noses of modern monitor lizards. This probably could have served a variety of functions, like sound production, thermoregulation and/or maybe housing a rete mirable, the same type of organ giraffes use to soften blood pressure when lowering their heads.

Fig. 11

Witmer’s hypothesis has become widely accepted among modern paleontologists and has now become a standard in paleoart. It should be mentioned, however, that not everyone has been on-board with this. Though open to the fleshy nose reconstructions, Hallett & Wedel (2016) still prefer the classic placement, for a rather succinct reason. Many of the giant, erect-necked sauropods would have had to bow their neck and head down to drink water at such an angle that, if the nostrils were truly at the front of the snout, they would have been submerged in the water, while if they were atop the head, the animal could have breathed more easily. In some ways this seems to go full circle to the old blowhole-interpretation, though it seems like a valid point to consider. The idea that retracted nostrils also made bulk-feeding on thorny trees easier could also still hold some water.

The Neck Wars

Fig. 12

A more well-known issue that arose in the 90s is the question of neck-posture. As a counter-movement to the increasingly more giraffe-like interpretation of sauropod lifestyle, paleontologists such as John Martin (1998) or Kent Stevens (1999) used computer model studies to argue that the sauropod neck was quite stiff and predominantly held in the osteologically neutral posture, meaning horizontally straight forward and largely unable to raise the head above shoulder-level, with the musculature actually being better adapted towards bending the neck down. In this view, sauropods such as Diplodocus were actually low-browsers, who evolved their necks to more easily forage the ground like living vacuum cleaners or giant geese without having to move much. This interpretation was famously immortalized by documentaries such as Walking with Dinosaurs.

 

Fig. 13

Although quite popular throughout the 90s and 2000s and still repeated in some popular sources here and there, this idea has come under quite a lot of criticism. Not only is the vertical lifestyle blatantly obvious in the skeleton of sauropods such as Giraffatitan, but almost all living tetrapods do not hold their necks in the osteologically neutral posture. Instead, muscles, ligaments and especially cartilage give a great deal more flexibility than would be expected from just the bones, with the neck more often than not being actually held diagonal curving upward when neutral (Taylor et al. 2009). A horizontally held, stiff neck would have also been a prime unprotected target for various predatory dinosaurs (Hallett & Wedel 2016). Even independently of the low-browsing hypothesis, the idea that sauropods evolved their long necks so they could just feed a lot without having to walk (as still repeated in recent popular media like Brusatte’s The Rise and Fall of the Dinosaurs) makes little sense, for no living large animal functions by this strategy (Hallett & Wedel 2016). The energy expended by walking up to a close food source is trivial, especially for large animals, as their larger steps alone mean they need to walk less, they actually use fewer calories relative to their size and need less food per kilogram than smaller animals (Hallett & Wedel 2016). So, if you feed from the ground, simply using your legs will always stay the more viable option rather than evolving a new hyperspecialized organ, which makes it far more likely that the sauropod neck instead evolved to reach hard-to-access food sources, such as tree canopies. Of course, one might point to ostriches, being long-necked grass-eaters, but their neck length evolved to compensate for their long legs, which they need to run away from predators (Bakker 1986), something which sauropods did not do.

Fig. 14

A perhaps final blow was dealt to the beam-necked sauropod idea with the 2020 computer-model study done by Vidal et al. on Spinophorosaurus. This study showed that the vertebrae of the pelvic area of this sauropod articulated into a concave wedge, which naturally lifted the spinal column of the animal diagonally upward and also meant the front limb girdle sat lower than usually reconstructed. This means that even in the osteologically neutral posture proposed by Stevens and others, the head and neck would have been pointing upward, quite ideal for high-browsing. That the musculature of the neck was adapted more for bending down makes even more sense in this light, as the ligaments and bones were already doing a great job holding it upright, meaning the animal only needed to exert muscular force when needing to bow down to drink. This has some rather far-reaching consequences, as Spinophorosaurus is generally classified as a basal eusauropod or at least a close relative of that group and the authors reason that this skeletal configuration would have applied to most if not all members of that clade. Since Eusauropoda comprises the Mamenchisauridae, the Diplodocoids (such as Diplodocus or Brontosaurus) and the Macronaria (Brachiosaurids and titanosaurs), this would mean that we have been reconstructing the majority of sauropod postures not diagonal enough (though some are already on their way to correct that).

Fig. 15

In general, it has therefore become popular again to depict Diplodocus and relatives (the animal depicted here seems to be a brontosaur) as high browsers. Sometimes by even using the double-beam chevron bones in their tails that Dippy derives its name from to prop themselves up onto their hindlegs in order to reach even higher into the treetops. This is by far not a new idea, though if this is something they did only occasionally or were specialized for doing regularly has in itself become a minor debate (see Foster 2020).

A final hurdle for the high-browsing camp is the question of how these giant animals handled their blood pressure, which is already a challenge for the much smaller giraffes. Most studies conclude that, in order to pump enough blood into the head of an erect-necked sauropod such as Diplodocus or Giraffatitan, the animals would have required gigantic hearts rivalling those of the largest cetaceans, which seems unlikely. This is today used as the main argument by beam-neck-supporters against sauropods raising their necks vertically. However, to paraphrase Naish (2021), it would be naïve to use this to discount every other evidence in favour of erect-neck postures without first assuming that these remarkable animals did not find solutions to these problems. Instead of having one giant heart, one proposal has been that sauropods may have had multiple pseudohearts along the neck which helped a more reasonably sized main heart with pumping, though such structures are virtually unknown in modern vertebrates, making it seem unlikely (Ganse et al. 2011). More likely, sauropods employed a wide array of smaller soft tissue adaptations, similar to what is seen in giraffes, to collectively lower the need for a large heart and deal with other blood pressure problems, like edema in the extremities. These adaptations were likely a combination of a rete mirabile, muscular venuous pumps, precapillary vasoconstriction, thicker blood vessel walls, extremely strong connective tissues, blood-cushions in the feet (as seen in horses), as well as blood with a much higher oxygen transport capacity (Ganse et al. 2011). That we will ever find evidence for any of this seems unfortunately unlikely, as such organs rarely fossilize. In the best case scenario, a baby sauropod maybe died and got preserved in a high-quality lagerstätte to the same degree as was the Scipionyx holotype and is now waiting to be uncovered.

The Headless Sauropod?

In terms of classification, a lot has also changed in the world of Diplodocus. First described in 1991, Seismosaurus hallorum soon became Diplodocus hallorum in the early 2000s, being a species even larger and longer than the famous D. carnegii, though suspected by some to be synonymous with the original D. longus. The problem with this is that D. longus has itself become a dubious taxon, on account of its original remains being too fragmentary. An attempt was therefore made in 2016 to strip D. longus off its status as the type species for the genus and instead grant D. carnegii the honour, but the proposal was rejected by the ICZN.

A more distressing revelation was made in 2015 by Tschopp et al., in the same study which also resurrected Brontosaurus as a valid genus. Analysing nearly all known remains referred to Diplodocus, they found that, due to the circumstances in which they were found and assigned, none of the skulls thought to belong to Diplodocus could be conclusively linked to the taxon or the species therein. All former Diplodocus skulls were either actually remains of the genus Galeamopus (as is the case with the original head of Dippy, USNM 2673) or could not be identified further than indeterminate Diplodocines, as is the case with USNM 2672, the skull you saw in Part 1 that was retroactively assigned by Marsh to the D. longus holotype. While there is a good chance that the latter ones do indeed come from Diplodocus (Tschopp et al. 2015), we cannot actually be sure until we find a new one firmly attached to a skeleton that is decisively Diplodocus.

In short, for now we do not know for certain what Diplodocus’ skull really looked like (though it likely did not differ too much from that of other diplodocids), which makes all the earlier nitty gritty debates about soft tissue placements rather funny in hindsight. It goes to show that even taxa we thought we knew well for a long time can still end up surprising us, sometimes even becoming more mysterious with time.

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References:

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Image sources:

• Fig. 1: Holland 1924
• Fig. 2: Gilmore 1932.
• Fig. 3: Lescaze 2017.
• Fig. 4 & 5: Augusta 1956.
• Fig. 6: Bakker 1968.
• Fig. 7 & 8: Bakker 1986.
• Fig. 9: Junk in the trunk by Tetrapod Zoology.
• Fig. 10 & 11: Hallett & Wedel 2016, p. 105 & 98.
• Fig. 12: Stevens & Parrish 1999.
• Fig. 13: Taylor et al. 2009.
• Fig. 14: Vidal et al. 2020.
• Fig. 15: Hallet & Wedel, p. 146.