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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